Org a No Metallic Bonding and Re Activity Topics in Org a No Metallic Chemistry Volume 4

8/2/2019 Org a No Metallic Bonding and Re Activity Topics in Org a No Metallic Chemistry Volume 4

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4T o p i c s i n O r g a n o m e t a l l i c C h e m i s t r y

Edi to r ia l Board :J .M. Br ow n P. Di xn eu f- A. Ffir s tner L.S. Heg edu sP. Ho fm an n P. Kn och el G. va n Kot en T.J. Mar ksS. Mur ai M. Reetz


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SpringerBerlinHeidelbergNew YorkBarcelonaHong KongLondonMilanParisSingaporeTokyo


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O r g a n o m e t a l l i c B o n d i n ga n d R e a c t i v i t yFundam ental Studies

V o l u m e E d i to r s : J.M . B r o w n a n d P. H o f m a n n

W i t h c o n t r i b u t i o n s b yP.B. A rm en tro ut , D. Braga, A. De dieu , P. Gisd akis, A. G6rl ing , E G rep ioni ,E M aseras, N. R6s ch, S.B. Tric key

~ Springer


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T h e s e r i e s Topics n Organometallic Ch emistryp r e se n t s c r i t ic a l o v e r v i e w s o f r e se a r c h r e su l t s in o r g a n o m e t a l -li c c h e m i s t r y , w h e r e n e w d e v e l o p m e n t s a r e h a v i n g a s i g n if i ca n t i n f lu e n c e o n su c h d i v e r se a r e a s a s o r g a n i c sy n -t h e s is , p h a r m a c e u t i c a l r e s e a r ch , b io l o g y , p o l y m e r r e se a r c h a n d m a t e r i a l s s c i e n ce . T h u s t h e s c o p e o f c o v e r a g ei n c l u d e s a b r o a d r a n g e o f t o p i c s o f p u r e a n d a p p l i e d o r g a n o m e t a l l i c c h e m i s t r y . C o v e r a g e i s d e s i g n e d f o r ab r o a d a c a d e m i c a n d i n d u s t r i a l s c i en t if i c r e a d e r sh i p s t a r t i n g a t t h e g r a d u a t e le ve l, w h o w a n t t o b e i n f o r m e da b o u t n e w d e v e l o p m e n t s o f p r o g r e s s a n d t r e n d s i n t h i s i n c r e a s in g l y in t e r d i s c i p l in a r y r i d & W h e r e a p p r o p r i -a te , t h e o r e t ic a l a n d m e c h a n i s t i c a sp e c t s a re i n c lu d e d i n o r d e r t o h e l p t h e r e a d e r u n d e r s t a n d t h e u n d e r l y i n gpr inc ip le s invo lved .T h e i n d i v i d u a l v o l u m e s a r e t h e m a t i c a n d t h e c o n t r i b u t i o n s a r e i n v it e d b y t h e v o l u m e s e d i t o r s .I n r e f e r e n c e s T o p i c s i n O r g a n o m e t a l l i c C h e m i s t r y i s a b b r e v i a te d Top. Organomet. Chem .and i s c it ed a s a jou r -nal .

S p r i n g er W W W h o m e p a g e : h t t p :/ / w w w . s p ri n g e r. d e

ISSN 1436-6002ISBN 3-540-64253-6Spr inger -Ver lag Ber l in H eide lberg New YorkL i b r a r y o f C o n g r e s s C a t a l o g i n g - i n - P u b l i c a t i o n D a t aO r g a n o m e t a ll i c b o n d i n g a n d r e a c ti v i ty : f u n d a m e n t a l s t u d i e s / v o l u m e e d i t o r s, J. M . B r o w n a n d P . H o f m a n n ;w i th con t r ibu t ions b y P . B . Arm ent ro u t . . . [ et a l. ].

p . cm . - - (Topics in o rganometaUic chem is t ry ; 4 )Includ es bib l iographical references.ISBN 3-540-64253-6 (hard cove r : a lk. pap er)1 . Organom etaUic com pou nds . 2 . Chem ica l bon ds . 3 . Reac tivi ty (Che mis t ry ) I . Brown, John M. I I . Hof -m an n, E (Peter), 1947- III . Series.QD411.5.072 1999

547 .05--dc21 99-33293C IP

T h i s w o r k i s su b j e c t t o c o p y r i g h t . A l l r i g h t s a r e r e se rv e d , w h e t h e r t h e w h o l e p a r t o f t h e m a t e r i a l i s c o n c e r n e d ,sp e ci fi c al ly t h e r i g h t s o f t r a n s l a t i o n , r e p r i n t i n g , r e u se o f i l lu s t r a ti o n s , r e c it a t io n , b r o a d c a s t i n g , r e p r o d u c t i o no n m i c r o f i lm o r i n a n y o t h e r w a y , a n d s t o r a g e i n d a t a b a n k s . D u p l i c a ti o n o f th i s p u b l i c a t io n o r p a r t s t h e r e o fi s p e r m i t t e d o n l y u n d e r t h e p r o v i s i o n s o f t h e G e r m a n C o p y r i g h t L a w o f S e p t e m b e r 9 ,1 9 65 , i n i ts c u r r e n t v e r -s i o n , a n d p e r m i s s i o n f o r u se m u s t a l w a y s b e o b t a i n e d f r o m S p r i n g e r - V e r l ag . V i o l at i o n s a r e li a b le f o r p r o se c u -t i on u n d e r t h e G e r m a n C o p y r i g h t La w.' S p r i n g e r- V e r l a g B e r l in H e i d e l b e r g 1 9 99P r i n te d i n G e r m a n yT h e u se o f g e n e r a l d e sc r ip t i v e n a m e s , r e g is t e r e d n a m e s , t r a d e m a r k s , e t c. i n t h i s p u b l i c a t i o n d o e s n o t i m p l y,e v e n i n t h e a b se n c e o f a sp e c if ic s t a t e m e n t , t h a t su c h n a m e s a r e e x e m p t f r o m t h e r e l e v a n t p r o te c t i v e l aw s a n dr e g u l a t io n s a n d t h e r e f o r e f re e f o r g e n e r a l u se .C o v e r : F r i e d h e l m S t e in e n - B r o o , P a u / S p a i n ; M E D I O , B e r l inT y p e se t ti n g : D a t a c o n v e r s i o n b y M E D I O , B e r li nSPIN: 10543767 66/3020 - 5 4 3 2 1 0 - Prin ted on acid-fre e pap er .Compiled by Matt Pretender


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Volum e E ditorsDr. John M . BrownDyson Perrins LaboratorySouth Parks RoadOx ford OX1 3QY,E-mail: [email protected]

Prof. Peter HofmannOrganisch-Chemisches InstitutUn iversit~it HeidelbergIm Neuenh eimer Fe ld 270D-69120 Heidelberg, Ge rma nyE-mail: [email protected]

Ed itorial B oardDr. John M. BrownDyson Perrins La bora torySouth Parks RoadOx ford OX1 3QY,E-mail: [email protected]

Prof. Pierre DixneufCampus de BeaulieuUniversit~ de Rennes IAv. du G1 LeclercF-35042 Rennes Cedex, Fran ceE-mail: Pierre.Dixneuf@univ-rennes 1.fr

Prof. Alois FiirstnerM ax-Planck-lnstitut ftir Ko hlenforschungKaiser-Wilhelm-Platz 1D-45470 Miilheim an de r Ruhr, Ge rm anyE-mail: [email protected]

Prof. Louis S. HegedusD e p a r t m e n t o f C h e m i s t ryColorad o State Univers i tyFo rt Collins, Co lorado 80523-1872, USAE-mail: [email protected] ate.edu

Prof. Peter HofmannOrganisch-Chemisches lnstitutUniversit~it H eidelbergIm Ne uenheim er Feld 270D-69120 Heidelberg, G erm an yE-mail:[email protected]

Prof. Paul KnochelLudwig-Maxim ilians- Universit/ it M iinche nInstitut f i ir Organische Ch em ieButenandstr . 5-13D-81377 Miinchen, G erma nyE-mail: [email protected]

Prof. Gerard van KotenDepartment of Metal-Mediated S ynthesisDebye Research Institu teUtrecht UniversityPadualaan 8NL-3584 CA Utrecht, T he N ethe rlandsE-mail:[email protected]

Prof. Tobin J. MarksDepar tmen t o f Chem is t ryNorthwestern University2145 Sheridan RoadEvanston, I l l inois 60208-3113, USAE-mail: t [email protected]

P r o f. S h i n j i M u r a iFaculty of Eng ineer ingDepar tment o f Applied Chem is t ryOsaka Univers i tyYam adaoka 2-1, Suita-shiOsaka 565, lapa nE-mail: [email protected]

P r o f . M a n f r e d R e e t zM ax-Planck-Institut fiir Koh lenforschungKaiser-Wilhelm-Platz 1D-45470 Mii lheim an d er Ruhr , Ge rma nyE-mail: [email protected]


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Preface

GeneralT he m a k i n g a n d b r e a k i n g o f c a r b o n - m e t a l b o n d s i s f u n d a m e n t a l t o a ll t h e p r o -cesse s o f o r ganom e ta l l ic che m is t r y an d m or eo v e r p l ays a sign if ican t r o l e in ho -m oge neo us a s we l l a s he t e r o gen eou s ca ta lys i s. Th i s r a the r b lun t s t a t em en t em -phas i se s the ex ten t to which a p r o pe r u nde r s t and ing o f t he s t r uc tu r e , ene r ge t ic sand r eac t iv i ty o f C -M bo nd s i s a t t he cor e o f t he d i sc ip line . In o r de r to accep t it,a p r op e r de f in i t i on o f t he t e r m s inv o lv ed i s r equ i r ed . Q ui t e s im ply we de f ine them e t a l - c a rb o n b o n d i n it s b r o a d e s t s e n s e to e m b r a c e c a r b o n l in k e d t o t r a n s i ti o n -m e ta ls , l an than ides and ac t in ides , and m a in g r ou p m e ta ls . W e do n o t d i s t in -g u i sh b e t w e e n f o rm a l l y c o v al e n t s i n g le o r m u l t ip l e b o n d i n g o n t h e o n e h a n d a n dq-b on ding on the o the r . In the s tud ie s to be d esc r ibed in the f o l lowing chap te r s ,t h e e m p h a s i s w i ll b e o n t r a n s i ti o n m e t a l c o m p l e x e s a n d i n s o f a r as t h e f u n d a -m enta l s co m e un de r sc r u tiny , s im ple m e ta l a lky ls o r r e l a ted sp ec ie s (m e ta l a lke -nyl, a lkyny l , a ryl , or a l ly l ) wi l l p lay an em ph at ic par t . Th e ce nt ra l ro le of meta la lky l s and the i r con gene r s a nd e spec ia lly the r o le o f t he i r m e ta l c a r b on l inkagein h o m o g e n e o u s c a ta ly s is m a y b e a p p r e c i a te d b y c o n s id e r i n g s o m e k e y re a c t io ns t eps l ead ing to the i r f o r m a t ion o r b r eakd ow n. The r e f o l lows a few pr om inen tex am ples o f t r ans i t i on m e ta l m ed ia t ed s to i ch iom e t r i c o r ca t a ly t i c p r ocesse s :- I n h o m o g e n e o u s h y d r o g e n a t i o n o f d o u b l e b o n d s , th e s t e p w i s e r e a c ti o n o f a n

q2-coo r d ina ted a lkene w i th d ihy dr oge n g iv es f ir st an a lky l m e ta l hydr ide , andt h e n t h e d e c o o r d i n a t e d a l k a n e b y e li m i n at io n .- I n t h e h e t e r o g e n e o u s c a t al y si s o f h y d r o g e n a t i o n , s u r f a c e - b o u n d m e t a l a l ky l splay a p ivota l ro le in the reac t ion cyc le .- H o m o g e n e o u s o r h e t e ro g e n e o u s d e h y d r o g e n a ti o n r ea c ti o n s o f h y d r o c a r b o n sinv o lv e t r ans i t i on m e ta l a lky l hydr ides , wh ich m ay und e r go f l -e l im ina t iona n d d e c o o r d i n a t i o n o f H 2 a n d a n a lk e n e .- In hyd r of or m yla t ion , a m e ta l a lky l i s f o r m ed in s im i l a r m ann e r b u t i n t e rcep t -e d b y c i s - l i g a n d m i g r a t i o n t o c o o r d i n a t e d C O ; t h e r e d u c t iv e e l i m i n a ti o n t h e ninv o lv es an acy l m e ta l hy dr ide .- Hydr os i ly l at ion and m any m o r e r el a ted add i t ion r eac t ions o f X -H or X-Y un i t sto uns a tura ted organic s ubst ra tes p roc eed v ia meta l a lkyl (or alkenyl, a ryl) in-t e rm e d i a te s , w h i c h a r e p r o d u c e d b y i n s er ti o n s te p s in t o M - H o r M - X , Y b o n d s .Hy dr ocy ana t ion o f a lkenes and d ienes f igur es p r om inen t ly in th i s con tex t.


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VIII ].M. Brown , P. Ho fmann- For tr ans i t i on m e ta l c a t a lysed a lkene am ina t ion , a p r oce ss o f g r ea t i ndus t r i a lpo ten t i a l , t he m os t p r o m is ing ca t a ly ti c cyc le s a r e bas ed up on the in t e r m ed i -

a c y o f a lk y l m e t a l c o m p l e x e s, f o r m e d e i th e r b y a m i n e a d d i t i o n t o a m e t a l -c o o r d i n a t e d o le fi n o r b y o l ef in in s e r t io n i n to M - H a n d M - N b o n d s , re s p e c -tiv ely . M e tal c a t a lysed a lkyne am ina t ion and hyd r a t ion r eac t ions a r e r e l a t edcases .- F o r t h e s i m p l e s t m e c h a n i s m o f a l k e n e p o l y m e r i s a t io n t h e a lk y l c h a in g r o w sthr ough an a lky l m igr a t ion to coor d ina ted a lkene ; t he sam e pr ocess i s r e -s p o n s i b l e f o r C - C b o n d f o r m a t i o n i n a lk e n e d i m e r i s at i o n s a n d o l i g o m e r is a -t ions .- T h e c o p o l y m e r i s a ti o n o f a l k e n e s a n d C O to 1 , 4 - p o ly k e to n e p o l y m e r s i n vo l v ess u c c e ss i v el y a p a l l a d i u m a lk y l a n d a c yl , t h e s e q u e n c e b e i n g c o n t i n u e d b y m i -g r a t ion o f t he acy l t o q2-coor d in a ted a lkene , and f u r the r c i s - l i g a n d m i g r a t i o nt o c o o r d i n a t e d C O .

- I n m e t a th e s i s a n d R O M P p o l y m e r i s a t i o n , th e k e y s t e p s a re a t e m p l a t e c y -c loadd i t ion be tween m e ta l a lky l idene and a lkene , l e ad ing to m e ta l a lky lb o n d s i n a m e t a l la c y c l o b u t a n e s t r u c t u re , a n d t h e r e v e r se p r o c e s s w i t h o p p o -s i te regiose lec t iv i ty .

- Pa l l ad ium and n icke l - ca t a lysed c r oss -coup l ings inv o lv e success iv e add i t iono f a c a r b o n e l ec t ro p h i le a n d a c a r b o n n u c l e o p h i le t o t h e m e t a l a n d t h e n a ne l im ina t ion o f c i s - a d j a c e n t a lky l g r oups ; f o r t he r e l a t ed H eck r eac t ion the k eystep i s the c i s - l i g a n d m igr a t ion o f a pa l l ad ium a lky l o f e lec t r oph i l ic o r ig in toa coor d ina ted a lkene .

- The ca ta ly t i c am ina t ion o r ca r box ya lky la t ion o f ha log ena ted a r enes a s we ll a sthe ca t a ly t i c a r y la t ion o f ca r bo ny l com po un ds us ing pa l l ad ium ca ta lys t s c r e -a te a r y l m e t a l i n t e rm e d i a t e s e n r o u t e t o t h e C - N b o n d f o r m i n g e l im i n a t io ns tep.- In t e r m ed ia t e s o f o l e fin ox ida t ion r eac tions o f t he W acke r - type a r e hyd r ox y-sub s t i tu t ed m e ta l a lky ls o f e .g . pa l l ad ium .- Me ta l q3-a lly ls , o f t en in equ i l ib r ium wi th the i r q l - a l ly l isom er s , hav e a b r oa dbase o f ca t a ly ti c i nv o lv em ent be s t appr ec ia t ed th r ou gh the ex om e ta ll i c r eac -t ion o f ca tion ic pa l l ad ium a lly ls w i th n uc leoph i l e s o r t he in t e r m ediac y o f a lly ln i ck e l c o m p l e x e s i n h y d r o c y a n a t i o n o f b u t a d i e n e . T h e c h e m i s t r y o f q 3 - b e n z y lsys te m s is re la ted.- M i g r a t io n o f a n u n s a t u r a t e d a lk y l g r o u p f r o m i r o n t o c a r b o n i s t h e b a s i s o f t h em os t con v inc ing ex p lana t ion f o r F i sche r -T r opsch t e lom er i sa t ion .- C - H a c t iv a t io n o f a l ka n e s , a f u n d a m e n t a l s t e p f o r C - H f u n c t io n a l i z a ti o n r e -ac t ions in b o th chem ica l and b io log ica l sys t em s g iv es a m e ta l a lky l a s the f i r stf o r m e d i n t e r m e d i a te . C - H f u n c t io n a l i z a ti o n r e a c t io n s o f a lk e n e s a n d a r en e s ,e .g . hyd r ov iny la t ion o r t he M ur a i r eac t ion and r e l a ted p r ocesse s , i nv o lv e m e t -a l a r y l s o r a lkeny l s en r ou te to f unc t iona l i zed h ydr o ca r bo ns .

- L a s t b u t n o t l e as t, n u m e r o u s s t o i c h io m e t r ic r e a c ti o n s o f r e a ct a n t s w h e r e t h et r ans i t i on m e ta l a c t s a s a t em pla t e , pe r m i t t he chem o- and s t e r eose lec t iv es y n t h e s i s o f c o m p l e x o r g a n i c m o l e c u l e s t h r o u g h i n t e rm e d i a t e s w i t h M - Cb o n d s .


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Preface I XNatura lly t h is cons t i t u t e s an i ncom ple t e lis t. G iven t he en orm ou s ly b ro ad scopeo f k n o w n o r p o t e n t ia l t r a n s f o r m a t i o n s o f o r g a n i c s u b s t r at e s i n v o lv i n g M - Cb on ds, h o w t h e n m a y t h e e x p e r i m e n t a l i st o r t h e o r e t i c ia n c o n t r ib u t e t o b a s i c u n -ders tanding? Here i t i s convenient to separa te the cont r ibut ions of these twocomm uni t ie s , a l t hough in p rac t i ce t he re i s cons ide rab l e con ve rgen ce o f e f fo r t .E x p e r i m e n t a l S t u d i e sOn the s t ruc tu ra l s i de t he re i s an ac cum ula t i ng bo dy of r e su lt s f rom X- ray, neu -t ron and e l ec t ron d i f f rac t i on inva luab le fo r deve lop ing a sys t em a t i c corpu s o fd ata o n b o n d l e n g t h s a n d b o n d a n g le s , a n d d e f i n in g t h e t r e n d s w i th r e s p e c t t ova ria tion o f m e ta l and co- l igands . Fas t, h igh ly e f f i c ien t X- ray i ns t ru m enta t i on i nth e fo r m o f a r e a d e t e c to r , C C D , r o t a ti n g a n o d e a n d s y n c h r o t r o n t e c h n o l o g y h a sb r ou g h t ab o u t a r e v o l u ti o n in s p e e d f o r t h e d e t e r m i n a t i o n o f m o l e c u l a r s tr u c -tures o f even the la rges t org ano m eta l l ic sys tems in the sol id s ta te . Duni tz , Bii rgia n d o th e r s m a d e s e m i n a l c o n t r ib u t i o n s t o o u r k n o w l e d g e o f s o li d s t at e s t r u c -tu re / reac t iv i t y re l a ti onsh ips . Now a l a rge bo dy of X- ray and ne u t ro n d i f f rac t i ondata is qu i ck ly and ea s i ly re t r ievab l e f rom s t ruc tu ra l da t abases , and can be w ide -ly u s e d to " m a p o u t " p a r t s o f e n e r g y s u r fa c e s o r o f s p e c if ic r e a c ti o n p a t h w a y s o rto de r ive sub t l e va r i a t ions o f m olecu l a r s t ruc tu r e f ro m l a rge se r ie s o f r e l a tedc o m p o u n d s . T h e a c c u r a c y o f X - r a y d a t a p e r m i t s a n s w e r s to q u e s t io n s a b o u t t h ena ture o f C-M bo nd ing ve rsus Van de r Waa ls con t ac t s . Taken toge the r , i n form a-t ion f rom d i f f rac t ion exp e r im ent s fo rm the bas i s o f e f fo r ts t o t a i lo r t he s t ruc tu reof o rganom eta ll ic com po un ds ( " l i gand des ign" ) fo r spec if ic func t ions i n o rga -nome ta ll ic che m is t ry and cata lysis . So l id s t a t e s t ruc tu re de t e rm ina t io n p rov idest h e t h e m e f o r t h e C h a p t e r b y B r a ga a n d G r e p i o n i " S ta ti c a n d D y n a m i c S t ru c -tu re s o f O rganom eta l li c Mo lecu le s and Crys t a ls " .Desp ite t he h igh level o f p rec i s ion o f con t e m po ra ry so li d s t a te s t ru c tu ra ls tud ie s, m ore de t a i l ed i n form a t ion on ene rge t i c s and reac t i v i t y pa t t e rn s n eed t ob e c o ll ec te d f r o m o t h e r e x p e r i m e n t a l s o u rc e s. T w o a r e a s o f c u r r e n t e n d e a v o u rprovide sig nifican t resul ts .Mass spec t rom e t r i c t e chn iques , w hich a re e l abora t ed i n A rm ent ro u t s Chap-t er " G a s P h a s e O r g a n o m e t a l li c C h e m i s t ry " , p o s se s s t h e p o w e r t o p r o v i d e d i r e c ti n forma t ion on t h e en e rge t i c s o f t r ans i en t spec i e s gen e ra t ed i n t h e gas -phase .Recent r epo r t s have sho wn , t ha t ga s phase i nves t iga t i ons o f r eac t i on pa thw aysand en erge t ics a re feas ible even for " rea l" ca ta lyt ica l ly ac t ive com plexes , as forC-H ac t iva t ing [Cp~Ir(PR3)] 14-e lec t ron inte rm edia te s , for Grub bs ty pe(PR3)zClzRu(carbene) ole f in m eta thes i s a nd Cp2Zr(R) + ole f in p oly m erisa t ioncata lysts . A rm ent ro u t s Ch apte r is l a rge ly co nc e rn ed wi th gu id ed i on be am t an-dem MS, and o the r work e rs have app l i ed FT Ion Cy c lo t ron Resonance [FTICR] .By ana lysis o f t he k ine t i c e ne rgy re lea se d i s tr i bu t ion , ex pe r im enta l bo nd ene r -g ie s m a y b e d e r iv e d , a n d c o m p a r e d w i t h t h e p r e d i c t io n s o f i n c r e a s i n g l y s o p h is -t ic a ted cal cu la t ions . M uch o f t he m ass s pec t ro m e t r i c wo rk i nvo lves ba re m e ta lcations (o r m e ta l ox ide ca ti ons MO +) and pe rm i t s d i rec t com par i son s o f chem -oselec tiv ity , regiose lec t iv i ty an d reac t iv i ty . Fo r ex am ple , the reac t ion of l ight


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X J.M. Brown, E Hofm annm e t a l c a t io n s w i t h h y d r o c a r b o n s c a n r e su l t in s o m e C - C c le a va g e i n c o m p e t i t io nw i t h C - H a c ti v a ti o n . H e a v i e r t r a n s i ti o n m e t a l c a t i o n s le a d t o d e h y d r o g e n a t i o nv ia C -H ac t iv a t ion . MS ex pe r im ent s m ay be ex ten ded to l iga t ed m e ta l i ons ; a sig -n i f ican t r eac t ion be twe en ScMe2+ a n d c y c l o al k a n e s is a s i g m a - b o n d m e t a t h e s i soccu r r ing by a f our -cen t r e t rans i t i on s t at e , i n com pe t i t i on w i th dehydr o gena -t ion so tha t a r ange o f R2Sc+ spec ie s i s obse r v ed . In t e r e s t ing p r op osa l s o f a " tw o-s t a te - r e ac t iv i t y " h a v e b e e n e m p l o y e d t o e x p l ai n t h e g a s p h a s e r e a c t iv i t y o f M O +f r a g m e n t s w i t h o r g a n ic s u b s t ra t e s. I m p o r t a n t q u e s t i o n s c o n c e r n i n g t h e t r a n s -f e ra b i li ty o f g a s p h a s e r e a c t io n p a t t e r n s t o s o l u t i o n c h e m i s t ry r e m a i n t o b e a n -swer ed .In ca t a lys i s it is a fam i l i a r t r u i sm tha t m any o f the m os t i n t e r e s ting spec ie s ar eh igh ly e lus iv e ; t he i r shor t l i f e t im e unde r nor m a l r eac t ion cond i t ions p r ec ludesd e t e c ti o n . T i m e - r e s o lv e d I R s p e c t r o s c o p y h o l d s c o n s i d e r a b le p r o m i s e f o r t h ede f in i ti on o f spec ie s in th i s ca tegor y . G iv en a spec t r om e te r w i th 200 f em tosec -o n d t i m e r e s o l u t io n , i n t er m e d i a t e s o f v e r y s h o r t l if e ti m e m a y b e d e t e c t e d p r o -v ided tha t t he i r t r ans i en t concen t r a t ion i s su f f i c i en t . L ase r pho to lys i s o f t heT p * R h ( C O ) 2 c o m p l e x a t 2 95 n m o c c u r s w i t h a h ig h q u a n t u m y i e ld ( 0 .3 ) f o r C Odissoc ia t ion and C -H ac t iv a t ion f r om hy dr o ca r b on so lv en t . Th i s li es i n con t r a s tt o t h e q u a n t u m y i e l d o f 0 .0 1 f o r th e c o r r e s p o n d i n g C p * c o m p le x . I n t h e p e r i o dof 500 ns a f t e r CO d i s soc ia t ion , sev e ra l i n t e rm ed ia t e s a r e ob se r v ed . F i rs t a m o -l ecu la r a lkane com plex ensues , wh ich d i s soc ia t e s on e o f t he py r azo le un i t s ov e r200 ps . The d i s soc ia t ed spec ie s und e r go es f i rs t C -H inse r t ion and th en r eche la -t ion o f t he p yr azo le , bo th on a 200 ns t im esca le , t o g iv e the s t ab le C -H ac t iv a t ionpr od uc t . The ene r gy ba r r i e r f o r t he c r i ti c a l C -H inse r t ion is a r oun d 35 kJm o1-1 .T h e f a st IR a p p r o a c h i s m a d e m o r e p o w e r f u l w h e n c o u p l e d t o c l as s ic a l m e c h a -n i s t ic p r obes . In a r e l a ted ins t ance wh e r e C p*I r (PMe 3) i s the coor d ina t iv e ly un-s a t u r a te d f r ag m e n t , t h e e x i s te n c e o f a n a l k a n e c o m p l e x e n r o u t e t o t h e C - H a c -t i v a t i o n / i n s e r t i o n p r o d u c t w a s p r o v e d b y t h e p h o t o l y s i s o f a l k y l h y d r i d e i s o -t o p o m e r s a n d s a t is f a c to r y c o rr e la t io n o f th e r e s u l ts w i t h a k i n e t i c m o d e l r e q u ir -ing an a lkane com plex .

P h o t o e l e c t r o n s p e c t r o s c o p y i s a n o t h e r i m p o r t a n t e x p e r i m e n t a l t o o l w h i c hh a s p r o v i d e d d e e p e r i n s ig h t in t o b o n d i n g p a t t e r n s a n d e l e ct ro n i c s t r u c tu r e s o fo r g a n o m e t a l l i c c o m p o u n d s a n d i n t o M - C i n t e r a c t i o n s . H e r e - i n c o n t r a s t t os i m p l e o rg a n i c m o l e c u l es - o n e o b s e r v e s t h e b r e a k d o w n o f K o o p m a n s t h e o r e m .Thi s inev i t ab ly necess it a t e s e ithe r t he spec t r oscop ic com par i son o f se r ie s o f re -l a te d a n d s p e ci fi c al ly m o d i f i e d m o d e l c o m p o u n d s , o r t h e u s e o f a p p r o p r i a tec o m p u t a t i o n a l p r o c e d u r e s i n o r d e r t o i d e n t i fy th e n a t u r e o f o b s e r v e d i o n i s a ti o nev en t s . These can then be r e l a t ed to a qua l i t a t i v e o r quan t i t a t i v e bond ing de -s c r i p ti o n o f th e s p e c ie s in q u e s t i o n . A l ar g e b o d y o f P E s p e c t r o s c o p i c i n fo r m a -t ion on o r ganom e ta l l ic s ha s b een co l l ec ted in the pas t , bu t su r pr i s ing ly i ts d i rec tin f luence and use a s a gu ide l ine f o r syn thes i s and i t s im pac t f o r ex pand ingm e c h a n i s t i c k n o w l e d g e a n d d e v i s in g n o v e l s tr u c t u r e s o r r e a c ti o n p a t h w a y s h a sb e e n s o m e w h a t li m i te d . C e r t a in l y f u r t h e r e f fo r t w i ll b e v e r y i m p o r t a n t h e r e.

M o d e r n s p e c t r o s c o p i c te c h n i q u e s a l s o p r o v i d e i n t i m a t e d e t a il s o f th e s t r u c -t u r e o f s u r f ac e b o u n d g r o u p s . F o r e x a m p l e, h i g h - r e s o lu t i o n e l e c tr o n e n e r g y l o s s


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Preface X I

s p e c tr o s co p y (H R E EL S ) p r o v i d e s a n e q u i v a le n t I R s p e c t r u m o f a d s o r b e n t w h i c hcan be com par ed w i th theor e t i ca l c a l cu la tion (DF ca lcu la tions ) .S o lu ti on th e r m o c h e m i s t r y s h o u l d b e m e n t i o n e d a s a f u r t h e r a r e a o f f u n d a -m enta l s tud ie s , wh ich a r e o f g r ea t im por t anc e , beca use th ey can p r ov ide r e fe r -ence da ta f o r e s t im a t ing r eac t ion en tha lp ie s o r f o r e s t ab li sh ing u se f u l add i t i v e

and inc r em enta l schem es f o r ene r g y ca l cu la tions o f s ing le s t ep o r gan om e ta l l i cr eactions o r ca t a ly t i c cyc le s . U n f or tuna te ly , so l id and r e li ab le th e r m oc he m is t r ydata fo r o r ganom e ta l li c r eac t ions in cond ens ed ph ase a r e r a the r sca rce , and on lya few gr oups a r e o pe r a t ing se r ious ly in th i s f ie ld . The i r r e su l t s f o r m an im p or t an tlink to the r e su l t s o f t heor e t i ca l c a l cu la t ions an d m ay se r v e a s a c r ed ib i l i t y nex u sb e tw e e n t h e o r y a n d e x p e r i m e n t .T h e o r e ti ca l S t u d i e s

All types o f f und am enta l ex pe r im enta l s tud ie s o f o r ganom e ta l l ic s t r uc tu r e s ,s tr uc tu r a l dynam ics , ene r ge ti c s a nd r eac t iv i ty in the so l id s ta t e , i n s o lu t ion o r i nthe gas phase a r e in t im a te ly connec ted to theor e t i ca l chem is t r y w i th i t s l a r gebody o f m od e r n com puta t ion a l t oo l s . I t i s c e r t a in ly adequ a te to s t a t e , t ha t du r -ing the la s t 10 to 15 yea r s we hav e w i tn essed a d r am a t i c change o f t he r o l e tha tis p layed by theor e t i ca l che m is t r y f o r o r gano m e ta l l i c che m is t r y an d ca t a lys i s re -s ea rc h. T h e ra p i d d e v e l o p m e n t o f c o m p u t e r s a n d o f p r o g r a m m i n g t e c h n o l o g yand the con com i tan t com m e r c ia l av a i l ab i li ty o r f r ee access ib i li t y o f o f t en ea sy-to -hand le , g r aph ic s - an d sc r ee n-o r i en ted p r og r am packages hav e caused a r ev -o lu t iona r y change in a t t i tude s towar ds th eo r y am o ng o r ganom e ta l l ic chem is ts .T he 1 99 8 N o b e l P r iz e i n C h e m i s t r y w a s a w a r d e d t o t w o o f th e p i o n e e r s o f t h e o -re tical and com puta t ion a l chem is t r y , John A . Pop le and W al te r Koh n an d n ice lytes ti fies to th is s ta tem ent . Th e exp er im en ta l ch em is t has acce ss to m os t leve ls oftheor y , r ang ing f r om m olecu la r m echan ic s appr oaches and sem iem pi r i ca lquan tum chem is t r y to h igh ly soph i s t i ca t ed , co r r e l a t ed dens i ty f unc t iona l andab initio (m o lecu la r o r b i t a l, v a l ence bon d) ca l cu la t ions .

For th i s r ea son m os t o r ganom e ta l l ic and ca t alys is r e sea r ch l abor a to r i e s hav ecom e to use quan tum chem ica l ca l cu la t ions on a r ou t ine bas i s dur ing th e pas t 10years. I t i s in te res t ing - and to so m e ex tent surpr is ing - to rea li se tha t th e em -p loym ent o f t heor e t i ca l m e tho ds e i the r f o r ana lys ing ex p e r im en ta l r e su l ts o r t op lan o r ganom e ta l li c m olecu la r s t r uc tu r e an d f un c t ion i s an ev en m or e r ou t ine lyes tab l ished too l i n indu s t r i a l R&D labs engaged in o r gano m e taU ic o r ca t a lys i sr e sea r ch , t han in academ ic l abor a to r i e s . Con tem por a r y quan tum chem is t r y a l -lows one to pe r f o r m ca lcu la tions no t o n ly f o r sm a l l m o de l sys t em s , f r om wh ichbas ic e l ec tr on ic s t ruc tu r e p a t t e r ns and un i f y in g conc ep t s can be de r iv ed , bu ta lso allows m o de l l ing o f r ea l sys t em s . M ode l s o f bond ing and e l ec t ron ic s t r uc -tu r e , ba sed upon m or e qua l i t a t i v e o r sem i -quan t i t a t i v e concep t s and m e thodslike l igand and c r ys t a l fi eld theor y , the angu la r ov e r l ap m o de l , PMO the or y an dor b it al i n t e r ac tion r u l e s, a ll v a r i an t s o f Ex tende d Hi i cke l - type ca l cu la t ions a ndthe i r de sc r ip t iv e one -e l ec t r on MO theor y too l s f o r m olecu la r o r ex tended sys -t em s a re use f u l t oo l s fo r ana lys ing an d u nde r s t and ing m an y f ea tu re s o f e l ec t r on-


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XII J.M. Brow n, E Ho fmanni c st r u c t u re , b o n d i n g a n d r ea c ti v it y . C o m p u t a t io n a l c h e m i s t r y wit h first princi-ple ab initio o r d e n s i t y fu n c t i o n a l m e t h o d s m a k e a r el ia b le n u m e r i c a l a s s es s -m ent o f s t r uc tu r e s and ( r el a ti v e ) ene r g ie s inc r ea s ing ly f ea sib le , howev e r . Em -b e d d i n g m e t h o d s , c o m b i n in g ab initio o r d e n s i t y f u n c ti o n a l q u a n t u m c h e m i s t r yf or se l ec t ed sub s t r uc tu r e s w i th an ap pr opr i a t e f o r ce f ie ld o r sem iem pi r i ca l MOt r e a t m e n t o f t h e l i g an d e n v i r o n m e n t e x t e n d t h e u t i li t y o f th e b a s i c m e t h o d s . T h er a n g e o f t h e o r e ti c a l t e c h n iq u e s a v a il ab l e is c o m p l e t e w h e n q u a n t u m d y n a m i c ss tud ie s and the com puta t ion a l m ode l l ing o f so lv en t e f f ec t s a r e inc luded .D e n s i t y f u n c t i o n a l m e t h o d s , d e v e l o p e d o n t h e b a s i s o f t h e H o h e n b e r g - K o h na n d t h e K o h n - S h a m t h e o r e m s h a v e b e e n v e r y s u c c e ss f u ll y f o r m o l e c u l a r q u a n -tum che m is t r y dur ing the la s t decade . The m a in a t t r ac t ion l i e s i n the i r ab i li t y tot r ea t ev en r a the r l a r ge m olecu le s w i th co m p ar ab le a ccur acy bu t m or e eas ily ,f a st e r a n d t h u s m o r e c o s t -e f fe c t iv e l y t h a n b y s t a n d a r d w a v e fu n c t i o n b a s e dm e t h o d s . D F r o u t i n e s a r e i m p l e m e n t e d in , a n d c a n b e c o n v e n i e n t l y u s e d w i t h i n ,m o s t o f t h e s t a n d a rd ab initio pr o gr am packages . The ch ap te r b y G6rl ing , T ri ck-ey, G i sdak i s and R6sch "A Cr i ti c a l Assessm ent o f De ns i ty Fun c t iona l Th eo r ywi th Rega r d to Appl i ca tions in O r ganom eta l li c Ch em is t r y" g iv es a de sc r ip tiv e ,d e t a i le d a n d c r it ic a l s u r v e y o f t h e t h e o r e ti c a l b a c k g r o u n d , t h e h i s t o r y a n d t h ep o w e r o f D F m e t h o d s , d r a w i n g a t t e n t io n a l so t o t h e i r i n h e r e n t l im i t at io n s . T h ee s s e n c e o f th e m o r e w i d e l y u s e d D F a p p r o x i m a t i o n s i s d e s c r i b e d a n d t h e a u t h o r sem phas i se cav ea ts a s we l l a s o f fe r ing pe r spec t iv e s o f t he Ko hn-Sh am (KS) the or yf or m olecu la r qua n tu m chem is t r y . KS or b i t a ls and KS e igenv a lues a r e d i scussedand the i r r e l a tionsh ip to the Har t r ee -Fo ck (HF) desc r ip t ion o f e l ec t ron ic s t r uc -tu r e i s p r e sen ted in a n i ce ly t r ansp a r en t and e luc ida t ing m anne r . The co ncep t o ff u n c t i o n a l s a n d t h e v a r i o u s t y p e s o f l o ca l, a p p r o x i m a t e g r a d i e n t - c o r r e c t e d a n dhy br id f unc t iona l s u sed in DF ca lcu la tions a r e ex p la ined in an app r opr i a t e w ayf or a chem is t r y o r i en ted , non- sp ec ia l i s t reade r sh ip . A ba lan ced v i ew of the t r ea t -m e n t o f e x ch a n g e a n d c o r re l at io n p h e n o m e n a b y D F m e t h o d s is p r e s e n te d a n di s fo l l o w e d b y a s e c t io n , w h i c h p r o v i d e s a c o n c i s e a n d h i g h l y i n f o r m a t iv e b o d yof da ta and r e f e rences a l lowing a quan t i t a t iv e ca l ib ra t ion an d v a l ida t ion o f DFr e su l ts i n c o m p a r i s o n t o t h o s e f r o m conventionalfirst principles w a v e f u n c t i o nb a s e d q u a n t u m c h e m i c a l m e t h o d s . A c ri ti ca l e v a lu a t io n o f t h e g e n e r a l p e r f o r m -a n c e o f D F c a lc u l at io n a l m e t h o d s f o r o rg a n o m e t a l l ic s y s t e m s a n d t h e p r e s e n t a -t i o n o f c as e s tu d i e s o f o r g a n o m e t a l l ic o x o s y s t e m s a n d t h e i r r e a c ti o n s ( O s O 4 ole -f in d ihydr ox y la t ion , CH3ReO 3 o le fin ep ox ida t ion) com ple te th i s chap te r. F r omt h e v i e w p o i n t o f t h e e x p e r i m e n t a l is t w h o i s i n t e re s t e d i n u n d e r s t a n d i n g o r a p -p l y i n g D F c a l c u la t io n s f o r h is o w n r e se a rc h , t h is i s c o m p l e m e n t a r y t o a n d m o r ec h e m i c a ll y o r i e n t e d ( l es s m a t h e m a t i c a l ) t h a n m o s t o t h e r f u n d a m e n t a l r e v ie w so r b o o k s .

T h e t h e m e o f o r g a n o m e t a l li c r e a c t iv i t y a s tr e a t e d b y q u a n t u m c h e m i c a l ca l-c u l a ti o n s is c o n t i n u e d i n D e d i e u s c h a p t e r " T h e o r e t i c a l T r e a tm e n t o f O r g a n o m e -ta ll ic Reac t ion M echan i sm s a nd Ca ta lysi s " wh e r e a t t he beg in n ing a gene r a lo v e r v i e w is g i ve n o f th e t h e o r e ti c a l "t o o l b o x " o f m e t h o d s c u r r e n t l y i n u s e f o rt r ea t ing o r ganom e ta l l i c r eac t ions , r ang ing f r om qua l i t a t i v e m olecu la r o r b i t a lt h e o r y t o ab initio, d e n s i t y fu n c t i o n a l, c o m b i n e d q u a n t u m c h e m i c a l / m o l e c u l a r


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Preface XIII

mechanic s (QM/MM) an d m olecu l a r dy na m ics s im ula t i on (QM/MD, e .g . Ca r r -Pa rine llo ) wi th t he i r m er i t s and shor t com ings . The m a in bo dy o f t h i s Ch ap te rdeals wi th se l ec ted exam ples o f hom og ene ou s ca t a ly ti c p rocesse s wh ich a re o fgreat i ndus tr i al i n te re s t. The au th or f ir s t addre sse s i n dep th t he t op i c o f ea r lyt rans it ion m e ta l (Ti, Z r ) m e ta l l ocen e based o l e fin po lym er i sa t i on , i n pa r t i cu l a rwi th re spec t t o t he m echan i s t i c s i gn i fi cance and the req u i rem ent s fo r a cor re c tt heore t ica l de sc r ip t i on o f ago s t ic M -C -H in t e rac ti ons . A ra the r de t a i l ed rev i ewof quan tum dyn am ics s imula t i on s tud i e s i s g iven . L ike t h i s C hap te r a s a who le ,it is i n t ended to p rov ide t he read e r wi th n o t jus t num er i ca l com pu ta t i ona l re -suits , bu t a lso qua l it a ti ve i n t e rpre t a t i ons and gene ra l concep t s d e r i ved f rom the -oretical findings. D edieus sec on d case s tu d y is l ink ed to th e f i rst , as i t a lso d ealswi th o l e f in po lymer i sa t i on ca t a lys i s . He re repre sen t a t i ve quan tum chemica ls tud ie s o f a m ore re cen t gen e ra t i on o f ca ta lys t sys t ems , ba sed up on l a te t r ans i -tion m e tal (Ni , Pd) d i imin e com plexes , a re o u t l i ned an d d i scussed . The impo r -t ance and in f luence o f so lven t ef fect s , no t t ake n i n to accou nt by mo s t q ua n tu mchemical s t ud ie s o f o rgano m eta l l ic s t ruc tu re an d reac ti v i ty , i s cons id e red i n t helas t sec t ion o f Dedieus chap ter . Poss ible theo re t ica l ap pro ach es to solvent e f -fec ts are co l lec t ed f rom the l i te ra tu re an d f ro m the au thor s ow n re sea rch , o l e finhydroformyla t ion an d the Wacke r p rocess be ing ch osen a s examples . A n ex t en-s ive re fe rence l is t o f t heore t i ca l wo rk on o rgan om eta l l ic r eac t i ons a nd ca t a ly ti ccycles com ple tes the Ch apter .

T h e re is a s tr o n g c u r r e n t i m p e t u s f r o m t h e i n t r o d u c t i o n o f h y b r i d q u a n t u mm e c h a n i c s / m o l e c u la r m e c h a n i c s m e t h o d s , w h i c h p e r m i t c a l c u la t io n s o n l a rg eand rea l i s t i c molecu l a r sys t ems and reac t i on pa thways wi thou t r e sor t i ng t ot ru n c a te d m o d e l s , w h e r e h y d r o g e n a t o m s r e p la c e a c t u a l o r g a n i c s u b s t it u e n t s o fe .g . l arge l igands (e.g . PH 3 s tand s for ptBu3 e tc .) . Such s t ru c tu ra l s impl i f ica tionsrema in mean ingfu l and accep t ab l e on ly i f gene ra l f ea tu re s o f e l ec t ron i c s t ruc -tures and qua l i ta t ive , t ransferable c lass i f ica t ions of organometa l l ic s t ruc tureand reac t iv i ty a re requ i red. If , how ever , s te r ic in te rac t ion s o r th e pre c ise ta ilor -ing o f s t e reoe l ec t ron i c e f fec t s p l ay a dec i si ve ro l e in chem o- and s t e reose l ec ti v i -ty , par t icula r ly in the f ield of en ant iose lec t ive t rans i t ion m eta l catalys is , rea l i s ticm o d e ls h a v e t o b e u s e d i n c o m p u t a t io n a l s tu d i es , a n d t h e Q M / M M m e t h o d o l o g yo ffe rs t h e c h a n c e t o d o s o. In t h e C h a p t e r b y M a s e r a s " H y b r i d Q u a n t u m M e c h a n -ic s/Molecu la r M echanic s M e thod s i n T rans i ti on M e tal Ch em is t ry" t he r ead e rcan ge t an i n s t ruc t i ve f i r s t -han d in t rod uc t ion i n to t h is r ap id ly exp and ing f i eldof com puta t i ona l chemis t ry . The au th or h as ac t i ve ly pa r t i c ipa t ed i n t he d eve lop-ment o f t he In t egra t ed Molecu l a r Orb i t a l Molecu l a r Mechanic s ( IMOMM)method , wh ich is one o f t he p re sen t ly ava il ab l e QM /MM approach es , a s t heyn o w a re a l r ea d y i m p l e m e n t e d in m a n y q u a n t u m c h e m i s tr y p r o g r a m p a ck a ge s.An apprec iab l e pa r t o f t h is C hap te r i s devo ted t o an i n t ro du c t ion i n to t he m e th-odolog i ca l f ea tu re s o f QM/MM mode l s , i ncorpora t i ng enough qua l i t a t i ve de -s c ri p ti o n a n d e x p l a n a t io n o f th e t h e o r e ti c a l b a c k g r o u n d t o m a k e t h e a p p r o a c heas ily compreh ens ib l e w i thou t go ing t oo d eep ly i n to m a them a t i ca l fo rm a l i sms .The ma in p a r t o f Mase ra s con t r i bu t ion focusse s upo n ap p l ica t ions , and s t a r t swi th th ree s t ru c tu ra l s t ud i e s o f s t e r ica l ly h igh ly conges t ed tr ans i t i on m e ta l com -


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XIV J.M. Brow n, E Ho fman nplexes , a l lowing the read e r t o deve lop a fee li ng fo r t he re l iab i li ty o f Q M /MM(IMO MM ) re su lt s wh ich a re show n to be use fu l fo r sepa ra t i ng s t e r ic and e l ec -t ron i c e f fect s up on s t ruc tu re and reac ti v i ty . The spec if ic advan tages o f hyb r idQ M / M M t e c h n i q u e s b e c o m e d e a r l y v i si bl e in t h e t h e o r e t ic a l d e s c r i p t io n o ft rans i t i on m e ta l ca ta lysed o l e fin po lym er i sa t i on , t he m echan i s t i c and ene rg e t i cde t a il s o f which can be com par ed t o t he ana lys is g iven i n Dedieus chap t e r . O the re x a m p l e s c h o s e n b y M a s er a s a r e t h e a s y m m e t r i c d i h y d r o x y l a t i o n o f o l e fi ns b yo s m i u m t e tr o x i d e , w h e r e t h e d e t a il e d a n a ly s is o f t h e a u t h o r s Q M / M M s t u d y c a nb e c o m p a r e d t o t h e r e s u lt f r o m o t h e r c o m p u t a t io n a l s tr a te g i es a s o u t l i n e d i n t h eC hapte r b y G6r l i ng , T rickey, Gi sdak is a nd R6sch. The im po r t anc e o f s t e r i c p re -requ i s it e s fo r agos t ic i n t e rac t i ons i n o rg anom eta l l ic sys t ems , and hen ce t he n eedfor a com ple t e inco rpora t i on o f s te r i c e ffec ts i n t heore t i ca l t r ea tm en t o f com -p o u n d s w h e r e a g o s ti c in t e r a c ti o n s p l a y a n i m p o r t a n t r o le , is e m p h a s i s e d a n dm ad e c lear . Gene ral ly , i t is sh ow n, ho w th e ro l e o f s t e r ic bu lk up on the s t ab il i tyo f o r g a n o m e t a ll ic m o l e c u l a r g e o m e t r i e s c a n b e a d e q u a t e l y d e a lt w i t h i n a q u a l -i ta ti v e a n d e v e n a q u a n t it a ti v e w a y b y u s e o f Q M / M M m e t h o d s . T h e p e r s p e c t iv efor t r ea t i ng l a rge b io inorgan ic complexes i s ou t l i ned i n computa t i ona l mode ls t u d ie s o f p o r p h y r i n c o m p l e x es , f o r w h i c h c o m p a r i s o n a n d e v a l u at io n o f d i ff e r-en t t heo re t i ca l appro aches is g iven .T h i s v o l u m e " O r g a n o m e t a l li c B o n d i n g a n d R e a c ti v it y : F u n d a m e n t a l S t u d ie s "of t he se r i e s "Topic s i n O rganom eta l li c Chem is t ry" p re sen t s a su rv ey by re -n o w n e d e x p e r t s o f i m p o r t a n t e x p e r i m e n t a l a n d t h e o r e t i c al d e v e lo p m e n t s t o u n -de r s t and bas i c a spec t s o f bo nding , ene rge t i c s , r eac ti v i ty , m olecu l a r geom e t r i e sa n d s o li d -s ta t e s t r u c t u r e s o f o r g a n o m e t a ll ic c o m p o u n d s . W e a r e g r a t e fu l t o t h ea u t h o r s f o r t h e i r c o o p e r a ti o n , f o r s h a r in g t h e i r e x p e r t is e a n d f o r c o m m u n i c a t i n gre su lt s o f t he i r ow n a nd of o the r s , w hich p rov ide a f a sc ina t ing ove rv i ew of t hes i t ua t ion a t t he f ron t i e r s o f t he d i sc ip li ne s t r ea t ed i n t h i s vo lum e .Ox ford , Aug us t 1999He ide lbe rg , Au gus t 1999 John M . BrownP e te r H o f m a n n


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Contents

G a s- P ha se O r g a n o m e t a l l ic C h e m i s t r yP.B. A r m e n t ro u t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .S ta tic a n d D y n a m i c S t r u c t u r e s o f O r g a n o m e t a l l i c M o l e c u le s a n d C r y s ta l sD . B ra ga , F. G r e p i o n i . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .T h e o r e ti c al T r e a t m e n t o f O r g a n o m e t a l l i c R e a c t i o n M e c h a n i s m san d C a t a l y s isA . D e die u . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .A C r it ic a l A s s e s s m e n t o f D e n s i t y F u n c t i o n a l T h e o r yw i th R e g a r d t o A p p l i c a ti o n s i n O r g a n o m e t a l l i c C h e m i s t r yA . G 6 r l in g , S .B . T r i ck e y , P. G i s d a k i s , N . R 6 s c h . . . . . . . . . . . . . . . . .H y b r i d Q u a n t u m M e c h a n i c s / M o l e c u l a r M e c h a n i c s M e t h o d s i nT r a n s i ti o n M e t a l C h e m i s t r yF. M a se ra s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .A u th or I n de x . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Gas-Phase Organometallic Chemistry

Peter B. Armentrout

e-mail: [email protected] of Chemistry, University of Utah, Salt Lake City, UT 84112, USA

Studies of organometallic chemistry in the gas phase can provide substantial quantitativeinformation regarding the interactions of transition metals with carbon centers. In this re-view, the techniques associated with such studies are outlined with an emphasis on guided

ion beam tandem mass spectrometry. The use of this technique to measure thermodynamicinformation is highlighted. Periodic trends in covalent bonds between first, second and afew third row transition metals and small carbon ligands are discussed and shown to cor-relate with a carefully defined promotion energy. The bond energies for dative interactionsbetween the first row transition metal ions and ethene, benzene and alkanes are also re-viewed. With this thermochemical background, the reactions of atomic transition metalions with alkanes (methane, ethane and propane) are reviewed and periodic variations inthe reactivity are highlighted. An overview of our results on the effects of ancillary ligands(CO and H

2

O) and oxo ligands on the reactivity of transition metal centers are then provided.

Keywords: Mass spectrometry, Transition metal ions, Thermochemistry, Bond activation,

Bond energies

List of Abbreviations

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1 Introduction

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Experimental Methods

. . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.1 Atomic Metal and Metal Complex Ion Sources . . . . . . . . . . . . 4

2.1.1 Electron Ionization . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.1.2 Laser Vaporization and Glow Discharge . . . . . . . . . . . . . . . . 52.1.3 Surface Ionization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.1.4 Multiphoton Ionization. . . . . . . . . . . . . . . . . . . . . . . . . . 62.1.5 High-Pressure Sources . . . . . . . . . . . . . . . . . . . . . . . . . . 62.2 Mass Spectrometric Methods . . . . . . . . . . . . . . . . . . . . . . 72.2.1 ICR Mass Spectrometry . . . . . . . . . . . . . . . . . . . . . . . . . 72.2.2 Ion Beam Mass Spectrometry . . . . . . . . . . . . . . . . . . . . . . 7

3 Thermochemistry of MetalCarbon Bonds

. . . . . . . . . . . . . . 93.1 Methods of Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.1.1 Reactions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.1.2 Data Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Topics in Organometallic Chemistry, Vol. 4Volume Editors: J.M Brown and P. Hofmann Springer-Verlag Berlin Heidelberg 1999


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2

Peter B. Armentrout

3.2 Covalent MetalCarbon Bonds . . . . . . . . . . . . . . . . . . . . . . 123.2.1 Cations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123.2.2 Bis-Ligated Cations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153.2.3 Neutrals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.3 Dative MetalCarbon Bonds . . . . . . . . . . . . . . . . . . . . . . . 173.3.1 Ethene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173.3.2 Benzene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183.3.3 Alkanes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4 Mechanisms for Alkane Activation

. . . . . . . . . . . . . . . . . . . . 19

4.1 Methane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204.1.1 Early First Row Transition Metal Ions . . . . . . . . . . . . . . . . . . 214.1.2 Late First Row Transition Metal Ions . . . . . . . . . . . . . . . . . . . 234.1.3 Second and Third Row Transition Metal Ions . . . . . . . . . . . . . . 254.2 Ethane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274.3 Propane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314.3.1 Late First Row Transition Metal Ions . . . . . . . . . . . . . . . . . . . 324.3.2 Early First and Second Row Transition Metal Ions . . . . . . . . . . . 344.4 Effect of Ancillary Ligands. . . . . . . . . . . . . . . . . . . . . . . . . 354.5 Effect of an Oxo Ligand . . . . . . . . . . . . . . . . . . . . . . . . . . 374.5.1 Reaction of CoO

+

with Methane . . . . . . . . . . . . . . . . . . . . . 384.5.2 Reaction of FeO

+

with Methane. . . . . . . . . . . . . . . . . . . . . . 40

4.5.3 Reaction of Other Transition Metal Oxide Cations with Methane . . 40

5 Conclusions

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

References

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

List of Abbreviations

AE appearance energy BDE bond dissociation energy CM center-of-mass

E

e energy of an electronEI electron ionizationICR ion cyclotron resonanceIE ionization energy KERD kinetic energy release distributionREMPI resonance enhanced multiphoton ionization

rf radio frequency RRKM Rice-Ramsperger-Kassel-MarcusTS transition stateSI surface ionization


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3

1

Introduction

Why study organometallic chemistry in the gas phase? One of the principle driv-

ing forces behind such studies is to provide a fundamental view of catalytic re-actions that occur at transition metal sites. Yet it is nearly impossible to carry outpractical catalysis in the gas phase [14] because the concentrations involved arepitifully low and it is difficult to obtain the number of reactant encounters need-ed to observe meaningful turnover ratios without competing side reactions andloss mechanisms . Precisely because of this, however, the gas-phase environmentprovides a place where a detailed study of the intrinsic reactivity of a metal sitewith a reagent of interest, unfettered by the influences of solvent and unrestrict-ed by the demands of stability imposed by the 18-electron rule can be carried

out. Such details, which are difficult to obtain in condensed phase systems, in-clude (1) quantitative thermodynamic data of metalsubstrate bonding; (2)characterization of electronic state and spin effects on reactivity; (3) compre-hensive periodic trends (vertical and horizontal) in reactivity; (4) specific mech-anistic insight into reaction pathways; and (5) systematic studies of the effects ofoxidation state, selective ligation, and solvation on reaction energetics andmechanisms. These features of gas-phase chemistry are highlighted in thepresent review. In addition, insights are provided into studies of how reactivitychanges with the number of metal centers, i.e. a gas-phase approach to hetero-geneous catalysis. Such metal cluster studies are an active component of re-search in our laboratories [57] but are beyond the scope of this review.

In addition to providing insight into the chemistry that is observed in con-densed phase systems, there is also the prospect of observing different reactivi-ties than those typical of such media. For instance, one of the early results of gas-phase chemistry was the observation that atomic metal ions could activate theunstrained CC bonds of alkanes [8]. Comparable processes are rarely observedin condensed phase media despite intense efforts. If we can understand what al-lows such unique transformations to be facile in the gas phase, the prospects ofengineering a true catalyst should be enhanced.

Another way of viewing the role that gas-phase studies can play in under-standing organometallic chemistry is the realization that the active reagents inhomogeneous catalysis are coordinatively unsaturated transition metalligandcomplexes. While ligand field theory allows us to organize a tremendous amountof information regarding stable organometallic compounds, the open shell char-acter of the unsaturated complexes makes them less easily organized by theseclosed shell 18-electron rules. Although such stable complexes are the startingmaterials in organometallic reactions and homogeneous catalysis, the key reac-tive intermediates are these unsaturated transition metalligand complexes that

have an open site of reactivity on the metal center formed by loss of one or moreligands from the stable reagent. Precisely because they are reactive (and there-fore good catalysts), such intermediates are transient and difficult to study. Littleis known about the thermodynamics of such reactive species, in contrast to or-


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Peter B. Armentrout

ganic chemistry, where it is straightforward to look up typical CC, CH, CO,etc. bond energies. This enables fairly accurate predictions of the overall ther-mochemistry and thus the feasibility of virtually any reaction desired. Althoughit is impossible to characterize the bond energies of all possible ligands with all

possible unsaturated metal complexes, gas-phase thermochemistry does pro-vide a quantitative handle on the strength of these interactions and the effects ofadjoining ligands on these interactions.

2

Experimental Methods

The gas-phase chemistry of transition metal ions has been examined by manyexperimental techniques. Two complementary methods have dominated the re-

search: ion cyclotron resonance (ICR) mass spectrometry and ion beam massspectrometry. Other techniques that have been used include high-pressure massspectrometry, flowing afterglow methods [9] and collisional activation by tan-dem mass spectrometry [10]. This review will focus on the abilities of and re-sults from ion beam mass spectrometry, the method used in my laboratory.

2.1Atomic Metal and Metal Complex Ion Sources

Atomic metal ions can be produced using a variety of methods: electron ioniza-tion (EI) of volatile organometallic complexes, laser vaporization (or ablation) ofbulk metal samples, a glow discharge where the metal of interest is the cathode,surface ionization (SI, also called thermionic emission) of metal salts and orga-nometallic complexes, and resonance-enhanced multiphoton ionization (REM-PI) of organometallic complexes and gas-phase metal atoms. These variousmethods differ greatly in the distribution of electronic states produced. As anyionization process is an energetic one, it is likely that excited electronic states oftransition metals (which have numerous low-lying levels) are produced. As aconsequence, the experimental generation of specific states of transition metal

ions is difficult and a distribution of states is generally produced. The low-lyingstates of transition metals involve only s and d orbitals and therefore are meta-stable. Optical transitions between these states are parity forbidden such that ra-diative relaxation does not occur on the time scale of most experiments [11].

Organometallic complexes of transition metal cations can also be formed us-ing a variety of techniques. Electron ionization and multiphoton ionization ofstable organometallic precursors can be employed or, alternatively, chemical re-actions of atomic metal ions with suitable reagents can be used to form manycomplexes. A key question in such methods is the internal (vibrational, rotation-

al, and electronic) energy of the complexes. As a consequence, the coupling ofatomic metal ion sources with a high-pressure reaction region for complex gen-eration is a particularly advantageous approach and one that we have utilizedextensively in our laboratories. This is discussed further below.


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5

2.1.1

Electron Ionization

In an electron ionization (EI) source, high energy electrons impinge on a volatile

organometallic compound leading to dissociation and ionization. The efficiencyof this process depends on the energy of the electron (

E

e), which clearly must ex-ceed some threshold for production of the atomic metal ion, called the appear-ance energy (AE) of the atomic metal ion. To provide sufficient intensity, the

E

emust generally be substantially above the AE. A number of experiments havedocumented that excited state populations can exceed 50% for most transitionmetal ions formed under such conditions [1224]. The extent of excitation doesnot vary substantially once the

E

e is 1020 eV above the AE.It is interesting that early experiments failed to find evidence for excited states

of transition metal ions formed by EI, but the reasons for this are subtle. Re-searchers wrongfully assumed that low-lying states would be short-lived, whilethey are actually metastable (see above). They expected to see differences in re-active rates or product branching ratios for metal ions in different electronicstates; however, for exothermic reactions (the most easily observed), the differ-ences in reactivities between states are often small.

As noted above, EI sources can provide a straightforward means of creatingorganometallic complexes given a suitable precursor. For example, we have gen-erated Fe(CO)

5+

ions using such a source [25]. However, even when the electronenergy is set to a value very close to the ionization energy of Fe(CO)

5

, the molec-ular ions produced have considerable internal energy. Hence, the use of suchsources without the addition of a means to thermalize the ions is inappropriatefor quantitative studies of their chemistry and thermodynamics.

2.1.2

Laser Vaporization and Glow Discharge

Laser vaporization and glow discharge sources are intense means for generatingatomic metal ions but produce a multitude of excited states [26, 27]. Strong pri-

ma facie evidence for this is the observation of multiply charged atomic ions [26,2931], species that are even more energetic than excited states of the corre-sponding singly charged ions. However, coupling of such sources with high-pressure devices (such as a flow tube, see below) make these sources particularlypowerful experimental tools. At present, there is no quantitative work character-izing the detailed state populations produced under various laser irradiation ordischarge conditions. This is largely because such studies are difficult and the re-sults would depend on a host of experimental conditions.


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Peter B. Armentrout

2.1.3

Surface Ionization

One of the best controlled sources for producing atomic metal ions is the surface

ionization (SI) source. In such a source, a rhenium or tungsten filament is heatedto 18002300 K and exposed to the vapor of an organometallic compound or ametal salt. Decomposition occurs and atoms with low ionization energies desorbfrom the filament surface with a probability described by the Saha-Langmuirequation [3234]. Consistent with this, there is presently good evidence that thepopulations of electronic states produced are characteristic of the filament tem-perature [23, 35]. Because the available energy is low (

k

B

T

=0.2 eV at 2300 K),only the lowest energy states are formed. This is not a particularly intense source(10

5

10

7

ions s

1

), but it is extremely stable.

2.1.4

Multiphoton Ionization

The most precise means of creating a well-defined electronic state of transitionmetal ions is the resonance-enhanced multiphoton ionization (REMPI) source,as implemented by Weisshaar [36, 37]. Unfortunately, this source is also the mostdifficult (and most expensive) to implement experimentally. Comparison of thestate-specific results obtained with this source are generally in good agreementwith other techniques [38] and provide an absolutely vital check of the statecharacterization of these other methods.

2.1.5

High-Pressure Sources

The EI, laser vaporization, and glow discharge sources are intense sources ofmetal ions but yield many excited states. One way to eliminate the excited statesproduced in these sources is to quench them by introducing a high-pressure gasinto the source region. We have demonstrated that nearly pure beams of the

ground states of many transition metal ions can be produced in this way by cou-pling a flow tube source with EI [39], laser vaporization [27], and glow dischargesources [25]. Studies indicate that thousands of collisions with a species like Aror CH

4

can be needed to efficiently remove the excited states [19]. A typical ICRexperiment, where the ions are produced directly in the cell, has difficulty at-taining this number of collisions, but flow tubes have a sufficiently high pressure(0.51 Torr) of He or Ar that 10

4

10

5

collisions are always present. Further, ad-ditional reagents (e.g. we have used O

2

, NO, and CH

4

) can be added to the flowto enhance the quenching.

Such high-pressure sources are also capable of producing organometalliccomplexes by reaction of the atomic metal ions with suitable reagents or bythree-body condensation. This technique has permitted us to generate largenumbers of unsaturated organometallic complexes without the need for a stable


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7

organometallic precursor [40]. Further, the large number of thermalizing colli-sions cool the internal energies of the complexes. Although there are no unam-biguous probes of the temperatures of such complexes, all evidence indicatesthat the internal energies are well characterized as having reached equilibrium

with the bath gas temperature [4143].

2.2Mass Spectrometric Methods

2.2.1

ICR Mass Spectrometry

Ion cyclotron resonance (ICR) and its derivative technique, Fourier transform

ICR (FT-ICR or FTMS) [44], uses crossed electric and magnetic fields to trapions for further study. This permits the time dependence of the ion populationto be measured, typically over a range of 10

3

to 10

1 s. Thus, rates,

k

(

T

), for ion-molecule reactions at ambient conditions (generally room temperature) andproduct branching ratios of these reactions are routinely measured using thistechnique. One powerful aspect of ICR methods is that sequential reactions canbe monitored easily and reaction pathways identified with double-resonancetechniques.

Both EI and laser vaporization sources have been used routinely in ICR ex-periments. While SI sources have been used for production of alkali metal ions[45], the ion intensities of transition metal ions are too small for this to be aworkable ICR source for these species (although attempts have been made). It istechnically possible to perform REMPI in an ICR cell but this has not yet beenutilized for transition metal ion studies. The more extensive use of external ionsources for ICRs now allows all of the sources discussed above to be used withICRs (including glow discharges).

2.2.2

Ion Beam Mass Spectrometry

Ion beam mass spectrometers, the instruments used in our laboratory [46], in-volve an ion source, a mass spectrometer used to select the ionic reactant, a re-action zone, a second mass spectrometer to analyze ionic products, and an iondetector. The reaction zone is designed so that reactions occur over a well-de-fined path length and at a pressure low enough that all products are the result ofsingle ion-neutral encounters. The sensitivity of the detectors is high; in ourcase, sufficient that individual ions can be detected with near 100% efficiency.An important variant of such instruments is a guided ion beam mass spec-

trometer, which utilizes an octopole ion beam guide, first developed by Teloyand Gerlich [47, 48], in the reaction region. This device utilizes eight rods ar-ranged in an octagonally symmetric array around the ion beam path. Alternatephases of a radio frequency (rf ) electric potential are applied to alternate rods to


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Peter B. Armentrout

create a potential well in the direction perpendicular to the axis of the rods. Thispotential well traps product ions and unreacted metal ions keeping them con-fined until they drift from the reaction region where they are accelerated, massanalyzed, and detected. This greatly enhances the sensitivity of the experiment

and permits routine measurement of the distribution and absolute zero of theion kinetic energy.In an ion beam experiment, one must convert from the laboratory to the cent-

er-of-mass (CM) energy scale. While the kinetic energy of the reactant ion ismeasured in the laboratory frame, some of this energy is tied up in the kineticenergy of the center-of-mass of the reactant system through the laboratory. Be-cause the total mass of the system does not change during reaction, conservationof linear momentum demands that this fraction of the total energy remains con-stant. Therefore, it is not available to induce chemical reactions. The energy that

is available for chemistry is called the center-of-mass energy,

E

(CM), and is eas-ily calculated (in the stationary target limit) as

E

(CM)=

E

(lab)

m

/(

M

+

m

) where

m

and

M

are the masses of the neutral and ionic reactants, respectively. The sta-tionary target assumption does not include the motion of the neutral reactantmolecules or the distribution in the ion beam energy. Rather, the effects of thesedistributions are explicitly included in analysis of the data, as described inSect. 3.1.2.

In addition to converting from laboratory to CM energies, the measured in-tensities of the product and reactant ions must be converted into an absolute re-action cross section, s

(

E

) [46]. This conversion requires knowledge of the neu-tral reactant pressure and the length of the interaction region, both quantitiesthat are straightforward to measure. The cross section can be thought of as theeffective area that the reactants present to one another such that they collide andthen proceed to the desired products. Typical units are 10

16 cm

2

=

2

, properlyreflecting the size of the reactants. Cross sections are a direct measure of theprobability of the reaction at a given kinetic energy and are directly related to amicrocanonical rate constant by

k

(

E

)=

v

s

(

E

) where

v

is the relative velocity ofthe reactants,

v

=(2

E

/

)

1/2

, and

is the reduced mass of the reactants. A temper-ature-dependent rate constant,

k

(

T

), can be obtained by integrating

k

(

E

) over a

Maxwell-Boltzmann distribution of relative velocities (although the distributionof internal energies of the reactants is generally not in equilibrium with their ki-netic energies).

The primary reason for the development of ion beam technology is that thekinetic energy of the reactant ion can be varied easily over a wide range simplyby changing the voltage difference between where the ions are formed andwhere they react. This ability provides the key difference between ion beam andICR technology, although acceleration of ions using resonant excitation is beingincreasingly pursued in ICR mass spectrometers [4951]. The energy range ac-

cessible in ion beam instruments extends from energies as low as thermal,3

k

B

T

/2 (300 K)=0.03 eV=3 kJ mol

1

, to hundreds of volts, 100 eV=3

k

B

T

/2 at750,000 K. Although the organometallic chemistry most relevant to condensedphase systems occurs in the thermal energy regime, hyperthermal kinetic ener-


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9

gies can induce the making and breaking of chemical bonds that cannot occurunder thermal conditions. This enables us to study the activation of manybonds, to probe the potential energy surfaces and thus the mechanisms of reac-tions, and to examine endothermic reactions, thereby acquiring thermodynam-

ic information.Because the ion source is physically separated from the interaction region, ionbeam mass spectrometry can use any of the sources described above. To date,electron ionization, laser vaporization, glow discharge, surface ionization, andhigh-pressure variations of these sources have been coupled with ion beam in-struments. This versatility enables excellent control of the electronic and inter-nal energies of the transition metal reactant cations.

3

Thermochemistry of MetalCarbon Bonds

One of the key abilities of guided ion beam mass spectrometry is the ability toexamine reactions at hyperthermal conditions, thus allowing endothermic reac-tions to be studied. By analyzing the kinetic energy dependence of particular re-actions, the energy thresholds for reaction can be measured and related back tospecific bond energies of interest. We have applied this methodology to a widenumber of systems in order to measure periodic trends in transition metalcar-bon bond energies. Such information has long been available for first row tran-sition metals and has been reviewed several times previously, most recently byArmentrout and Kickel [52] who systematically reevaluated all of our previouswork. These data along with thermodynamic information from other laborato-ries have been tabulated by Freiser [53]. Despite some overlap with previous re-views [52, 5456], it is worth introducing the primary concepts here in order toutilize this information in understanding the mechanisms and potential energysurfaces discussed in later sections. In addition, we compile here for the firsttime reasonably complete information for second row transition metal cations.Although some of these values are still preliminary, trends in the thermochem-istry are quite clear and help guide our discussion of the differences in reactivity

down the periodic table.In this and later sections, energy will be given in either kilojoules per mole,

kJ mol

1

, or electron volts, eV. The latter are the natural units for our experi-ments. The conversion between these units and the pass kcal mol

is 1 eV=96.49 kJ mol

1

=23.06 kcal mol

1

.


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Peter B. Armentrout

3.1Methods of Analysis

3.1.1

Reactions

In many of our studies, the schematic reaction (1) is used to determine the ther-mochemistry of M

+

L bond energies.

M

+

+RL

ML

+

+R (1)

A simple example is formation of metal-methyl cations from ethane (L=R=CH

3

). A slightly more complex case is the generation of methyl-methylidene cat-ions (L=CH

2) which can be formed from methane (R=H2) or cyclopropane (R=C

2H

4). In all these cases, the threshold for reaction, E

0, is related to the desired

bond energy by Eq. (2).

D0(M+L)=D0(RL)E0(1) (2)

Thermochemistry for neutral species can be determined in a related fashionby examination of reaction (3).

M++RLML+R+ (3)

Here the species R is chosen to have a low ionization energy (IE), such thatreaction (3) competes effectively with reaction (1). An example of interest hereis formation of metal-methyls from neo-pentane [L=CH3, R=C(CH3)3]. The ap-propriate thermochemistry is obtained using Eq. (4).

D0(ML)=D0(RL)+IE(R)IE(M)E0(3) (4)

Another general type of reaction that we have used to derive thermodynamicdata is collision-induced dissociation (CID), reaction (5) where Rg is an inertcollision gas. We usually use Xe for reasons described elsewhere [5759]. This re-action provides thermodynamic information straightforwardly as the thresholdfor reaction equals the desired bond energy, E0=D0(M

+L).

ML++RgM++L+Rg (5)

However, accurate thermodynamic information is obtained only when theanalysis of the CID cross sections includes the effects of multiple ion-neutral col-lisions and the lifetime for dissociation. The first effect is handled by extrapolat-ing our data to zero neutral pressure, rigorously single collision conditions [25,60]. The second effect becomes more important as the metal-ligand species be-come larger and more complex, which can eventually lead to a lifetime for dis-sociation that is comparable to the time it takes the ions to travel from the colli-

sion cell to the detector (~104 s). This leads to a delay in the observed onset fordissociation, a kinetic shift. We account for this effect by using Rice-Ram-sperger-Kassel-Marcus (RRKM) theory [61] to calculate a dissociation probabil-ity as a function of the ion internal energy [62, 63].


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Gas-Phase Organometallic Chemistry 11

In all these systems, accurate thermochemistry for the products is obtainedonly if the reactions have no barriers in excess of the endothermicities of the re-actions studied. In contrast to the situation ordinarily found in condensed phas-es, the assumption of no reverse activation barriers is often a reasonable one for

ion-molecule reactions because of the strong long-range ion-induced dipole po-tential [64]. The most obvious illustration of this fact is that exothermic ion-molecule reactions are generally observed to proceed without an activation en-ergy. The converse must also be true, endothermic ion-molecule reactions gen-erally proceed once the available energy exceeds the thermodynamic threshold[65]. We have explicitly tested this assumption in several reactions where thethermochemistry is well established [43, 6671] although the observation of thetrue thermodynamic threshold can require extremely good sensitivity [66]. Ex-ceptions do occur, however, and can result from spin or orbital angular momen-

tum conservation restrictions [65, 72], or the presence of a tight transition state(TS) along the reaction coordinate, as illustrated below for some CH bond ac-tivation steps [7376]. For CID reactions of organometallic complexes, quantummechanics demonstrate that there should be no reverse activation energies, aconsequence of the heterolytic bond cleavages involved [77], although dissocia-tion to an excited state asymptote can occur [25].

The accuracy of the thermochemistry measured in such experiments can beexperimentally verified by using more than one single reaction system. Al-though this is not possible in all cases, such checks have been performed formany of the systems described here. Alternatively, the thermochemistry can beverified by comparison with values from other experiments [7885] and ab ini-tio theory [8694].

3.1.2Data Analysis

Thresholds (E0 values) for reactions (1), (3) and (5) are determined by detailedmodeling of the experimental cross sections using a mathematical expression

justified by theory [95, 96] and experiment [65, 6871, 9799]. This model is giv-

en by Eq. (6).

s(E) = s0 gi(E + Ei E0)n/E (6)Here,s0 is a scaling factor, E is the relative kinetic energy, and n is an adjust-

able parameter. The sum is over the contributions of individual reactant states(vibrational, rotational and/or electronic), denoted by i, with energies Ei andpopulations gi (Sgi=1) The use of this equation to analyze the threshold behaviorof reaction cross sections makes the statistical assumption that the internal en-ergy of the reactants is available to effect reaction. It also presumes that the en-

ergy dependence (as determined byn and s0) does not vary with the state i (al-though this latter assumption can be inaccurate for different electronic states).

Before comparison with the experimental data, the model of Eq. (6) is convo-luted over the explicit distributions of the kinetic energy of the neutral and ion


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12 Peter B. Armentrout

reactants, as described previously [46, 100, 101]. The s0, n, and E0 parametersare then optimized by using a non-linear least-squares analysis to give the bestreproduction of the data. Because Eq. (6) includes all sources of energy for thereactants, the thresholds and bond energies obtained using these methods cor-

respond to 0 K thermochemistry. Uncertainties in E0 reflect the range of thresh-old values obtained for different data sets with different values ofn and the errorin the absolute energy scale. In cases where the internal energy of the reactantsis appreciable and the vibrational frequencies are not well established, the un-certainty also includes variations in the calculated internal energy distributionof the reactants, Ei. Such uncertainties can also influence the lifetimes for disso-ciation in CID experiments. The accuracy of the thermochemistry obtained bythis modeling procedure is dependent on a variety of experimental parametersthat have undergone extensive discussion [52, 54, 65].

3.2Covalent MetalCarbon Bonds

3.2.1Cations

Bond energies for first and second row transition metal cations with CH3, CH2and CH are given in Table 1. Figure 1illustrates the periodic trends in these val-

Fig. 1. Periodic trends in the bond energies (in kJ mol1) of first, second and selected thirdrow transition metal cations with CH3 (solid circles), CH2 (open triangles) and CH (closedsquares) and of neutral first row transition metals with CH3 (open circles)


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ues. Clearly, for a specific metal, M+CH bonds are stronger than M+CH2bonds, which are stronger than M+CH3 bonds. This simply reflects the triple,double and single bond character of these bonds. Across the periodic table, dou-ble-humped trends, common to many transition metal properties, are observed.These are easily rationalized. In the first row, the metal ions having the weakestbond energies are Cr+ and Cu+, which have 3d5 and 3d10 electron configurations.Formation of a bond with any transition metal requires decoupling the bondingelectrons on the metal from the remaining electrons. Because of the stability of

the half-filled and filled shells of Cr+

and Cu+

, the energetic costs of this decou-pling are larger than for other metal ions and hence the bond energies are small-er. In the second row, Ag+ (4d10) behaves the same way, while Mo+ (4d5) is lessstrongly inhibited.

Table 1. Transition MetalCarbon Bond Energies (0 K) in kJ mol1.a

M M+CH3 M+CH2 M

+CH MCH3 M+(CH3)2

Sc 233(10) 402(23) 116(29)* 464(5)

Ti 214(3) 380(9) 478(5) 174(29)* 472(25)V 193(7) 325(6) 470(5) 169(18)* 391(7)

Cr 110(4) 217(4) 294(29) 140(7)

Mn 205(4) 286(9) >35(12)

Fe 229(5) 341(4) 423(29)b 135(29)* 409(12)

Co 203(4) 317(5) 420(37)c 178(8)

Ni 187(6) 306(4) 208(8)

Cu 111(7) 256(5) 223(5)

Zn 280(7) 70(10)

Y 236(5) 388(13)

Zr 244(15)d* 449(5)d* 582(13)d*

Nb 198(28)e* 444(3)e* 597(23)e*

Mo 157(12)e* 329(12)e* 509(10)e*

Ru 160(6)f 344(5)f 494(15)f

Rh 141(6)g 356(8)g 444(12)g

Pd 181(10)h 285(5)h

Ag 67(5)i >107(4)i 134(7)i

La 217(15) 401(7)

Lu 176(20) >230(6)

Ta 196(3)j* >454j* 575(9)j*aValues are taken from [52] unless otherwise noted. bHettich RL, Freiser BS (1986) J AmChem Soc 108:2537. c[75]. d[145]. e[146]. f[148]. g[154]. h[155]. i[156].j[152].*Preliminary values not yet thoroughly evaluated


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14 Peter B. Armentrout

We have previously noted [102, 103] that the periodic trends in these bondenergies can be quantified by correlating them with the promotion energy, Ep,defined as the energy required to prepare a metal ion in a particular electronicconfiguration suitable for bonding. For convenience, the promotion energies we

use here are those calculated by Carter and Goddard [104]. This correlation isshown in Fig. 2. In the case of the MC single bonds, Ep is for promoting to as1dn configuration where the s electron is spin-decoupled from the electrons inthe d orbitals. Clearly, almost all of the M+CH3 bond energies correlate nicelywith this promotion energy and the correlation is very similar for first, secondand the few available third row metal cations. This indicates that there is consid-erable s character in the metal-bonding orbital. On the basis of these results, theintrinsic metalcarbon single bond energy, i.e. the bond energy expected fora metal prepared to bond strongly to a methyl group, is about 240 kJ mol1 (the

y-axis intercept).The two exceptions to this good correlation are PdCH3+ and AgCH3

+, the twometals having the highest promotion energies, 344 and 518 kJ mol1, respectively.

Fig. 2. Transition metal cation ligand bond energies (in kJ mol1) vs the atomic metal ionpromotion energy to a s1dn spin-decoupled state (see text). Results are shown for CH3 (cir-cles), CH2 (triangles) and CH (squares) with data taken from Table 1.Diamonds show re-

sults for neutral metalCH3 bond energies. Closed symbols show results for first row metalcations, open symbols for second row metal cations, and dotted symbols for third row metalcations. Linear regression fits to the data (excluding PdCH3

+, AgCH3+ and YCH2

+, see text)are shown bysolid lines for first row metal complexes, bydashed lines for second row metalcomplexes, and by the dotted line for neutral MCH3 complexes.


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In the case of Pd+ (4d9), the high cost of promoting to the 5s14d8 state means thatit is more favorable to form a bond between the methyl group and the 4ds-orbit-al, even though this bond is not as strong as that with the 5s orbital. Calculations[91] confirm this result and also indicate that other late second row transition

metal cations (Ru and Rh, which also have among the higher Ep values) shouldalso have considerable d character in their bonding orbital. In the case of Ag+

(4d10), calculations [91] indicate that no promotion takes place and the bond issimply a one-electron bond involving the methyl group and the empty 5s orbital.

Similar correlations are found between promotion energies and the M+CH2and M+CH bond energies. In these cases, the promotion energies shown are toa s1dn state where the s and one (for a double bond) or two (for a triple bond) delectrons are decoupled from the remaining electrons. The correlations are allnearly parallel with one another, but there is a striking difference in the magni-

tudes of the correlations for first and second row metals, i.e. in the intrinsic bondenergies. It is clear from Fig. 2 that the M+CH2 and M+CH bond energies are

stronger for the second row transition metals than for the first row metals. Ap-parently, the average p-bond energy for first row metals is about 150 kJ mol1

while it is about 200 kJ mol1 for the second row metals. The YCH2+ bond energy

(Ep=36 kJ mol1) deviates from this pattern because it contains more 4d2 than5s14d1 character [105]. Theory also finds that RuCH2

+ and RhCH2+ utilize pri-

marily 4dn+1 rather than 5s14dn metal character in the bonding; however, thecorrelation with s1dn promotion energies is still quite good, perhaps indicatingthat the sigma bond involves more s character than the calculations suggest. Theenhanced bonding for the second row metals can be

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