162 J. Burgess and F.M. Mckhail Transition Met. Chem. 3, 162-165 (1978) QVerlag Chemie, GmbH, D-6940 Weinheim, 1978

Kinetics of Substitution at Tris-(5-bromo-l,10-phenanthroline)-Iron(II) and -Iron(Ill)*

John Burgess* *

Chemistry Department, University of Leicester, Leicester LE 1 7 RH, U.K.

Fikry M. Mekhail

Nuclcar Chcmistry Departmcnt, Atomic Energy Establishment, lnshas, Cairo, Egypt

(Received December 2Orb, 1977)

Summary

Rate constants arc reported for several substitutions of the tris-5-bromo-1,10-phcnanthroline (5-Brphen) complexes of iron(ll) and of iron(Ill). These reactions include aquation in aqueous solution [iron(fly and iron(Ill)] and in several series of binary aqueous mixtures [iron(fly only], and reaction with hydroxide and with cyanide [iron(ll) only]. Reactivities are compared with those established for analogous reactions of related complexes, and with thermodynamic parameters of the ligands and complexes.

Introduction

A considerable amount of kinetic information is available relating to substitution at tris-l,10-phenanthroline complexes of iron(ll), but very much less so for the analogous iron(liD complexes (t). Although the majority of the results refer to the unsubstituted iron(ll) complex, there are also many for bgand-substituted derivatives. Kinetic data are available for a variety of methyl (electron-releasing) substituted complexes, e.g. [b'e(5-Mephen)3]2~, [I:c(5, 6-Me2phen)3] 2÷, + [Fc(4, 7-Me2phcn)3] 2 ,and [Fe(3, 5,6, 8-Me4phen)3] 2 and for systems containing electron-withdrawing substituents such as chloro-, nitro-, and sulphonato. The negative charges borne by the latter substitucnts introduce an extra variable, while the particularly, strong electron-withdrawing properties of the nitro-group sometimes lead to exceptional kinetic behaviour

(2) involving intermediates of significant lifetime . There are thus few straightforward electron-withdrawing substituents in thcsc complexes, we have therefore investigated the k inctic behaviour of the 5-bromo-derivatives of the iron(l l) and iron(Ill) complexes. Prcparations of the ligand (3) and of

(3) its tris-ligand iron(ll) complex were reported many years ago, but no quantitativc kinetic data appear to be available for the [Fe(5-Brphen)3] 2÷ or [Fe(5-Brphen)3] 3÷ cations. In this paper wc report kinetic data for the aquation (dissocia- tion) of both cations in aqueous solution, for aquation of the iron(ll) complex in several series of binary aqueous solvent mixtures, and for reactions of the iron(ll) complex with hydroxide and with cyanide in water. We find a similarity, be- tween the kinetic behaviour of the 5-bromo- and 5-chloro- compounds, as would be expected from the similarity of li-

* Reprints of this article arc not available. ** Author to whom correspondence should be addressed.

gand pK a values, visible spectra, and redox potentials*. The 5-bromo-complexes provide useful examples of compounds of this type containing moderately electron-withdrawing substituents.

Experimental

5-Bromo-l,10-phenanthroline was obtained from the G. F. Smith Chemical Company, Columbus, Ohio. Solutions of the iron(ll) complex, [Fe(5-Brphen)3] 2+, were obtained by adding a slight excess of ligand to a neutral solution of iron(ll) ammonium sulphate. The optical density and wave- length of maximum absorption of such solutions were checked against the published values (4). Solutions of the iron(Ill) com- plex, [Fe(5-Brphen)s] s÷, were generated by cerium(IV) oxi- dation of the iron(ll) complex as described elsewhere for other phenanthroline iron(liD complexes (s' 6). Other reagents and solvents were AnalaR grade.

Kinetic runs were conducted in 10 mm silica cells in the thermostatted cell compartment of a Unicam SP 800 A or Hilger-Gilford Spectrophotometer. Rate constants were computed using standard unweighted least-mean-squares routines, as were activation parameters where possible. The rapid reaction between the iron(flY complex and cyanide ion at 308.2 K was monitored on the Unicam SP800A spec- trophotometer with the aid of a thermostatted syringe mixing device which permitted optical density monitoring to commence within ca. three seconds of mixing the reagents (v) .

Results and Discussion

Iron(ll): ,4 quation

Formation and dissociation of tris-(1 ,lO-phenanthroline)iron(ll) complexes take place through the following equilibria:

Fe 2++ LL [Fe(LL)] 2÷

[Fe(LL)] 2+ + LL- ~[Fe(LL)2] 2+

[Fe(LL)2]2" + LL- " [Fe(LL)3] :+

° Values can be found in "Fable 5 (v.i.).

Transition Met. Chem. 3, 162-164 (19781 Substitution at Tris(5-bromo-l, lO-phcnanthroline)-Iron(ll)and-lron(ll l) 163

It is the add i t i on or toss of the th i rd l igand molecule which is ra te -de te rmin ing , since this reac t ion involves a high- s p i n ~ low-spin change (81. The fo rma t ion -d i s soc ia t ion equi- libria can be forced in the d i rec t ion of d issocia t ion by adding % v:v

• , 2 + . a reagent whmh scavenges e i ther the Fe or the lllzand. The organic ligand may be scavenged by' acids (8), cer ta in m e t a ( i o n s (9), or cosolvent

bv hydrogen pe rox ide ( l ° ) ; the iron may be scaveniged by' 10 such s t rong complex ing agents as c i t ra te or edta 0 'J. The 2o results of such k inet ic expe r i m en t s for the [Fe (5 -Brphen)3 ] 2÷ 3o ca t ion are shown in Table 1. As should be the case, the ra te

T a b l e 1 . Rate constants for dissociation and aquation of the I Fe(5-Brphen)3] 2+ cation

Reagent Conch. 103k o bs/s- 1

(mol dm -3) 298.2 K 308.2 K

Sulphuric acid 0.54 0.20 0.81

Sodium citrate 0.03 0.95 0.30 1.o

Zinc(ll) sulphate 0.O1 0.83 o.1o 0.85

Nickel(ll) sulphate 0.01 0.84 0.10 0,86

Disodium edta O.O1 0.70 o. 10 0.89

Hydrogen peroxide 0.60 0.20 3.00 0.16

cons t an t for aqua t i on (dissocia t ion) at a given t e m p e r a t u r e is a lmos t i n d e p e n d e n t of the na ture and c o n c e n t r a t i o n of the scavenging agent (we have avoided low c o n c e n t r a t i o n s of weak scavenging agents O t, 12)). The kinet ic results in Table 1 indica te an a p p r o x i m a t e ac t iva t ion energy of 115 kJ tool -1 and an A fac tor of 1016's for aqua t ion of this 5 -b romo- i ron( l I ) complex ; values for the 5-chloro complex

- 1 18 .1 - 2 + are E a = 124 kJ mo and A = 10 , and for [be(phen)3] itself E a = 125 kJ mo1-1 a n d A = 1017s (13)

Rate cons t an t s for aqua t i on of the [Fe(5-Brphen)3] 2÷ cat ion in some binary aqueous solvent mix tures are repor ted in Table 2; the mix tu res here include "typically, a q u e o u s " (e thanol , t -bu ty l a lcohol , d ioxan) , " typ ica l ly n o n a q u e o u s posi t ive" (acetoni t r i le ) and " typ ica l ly n o n a q u e o u s negat ive" (d ime thy l su lphoxide) (141. The pa t te rn of react ivi ty in b inary

Table 2. First-order rate constants for aquation of the IFe(5-Brphen)3] 2÷ cation in binary aqueous mixtures at 298.2 K, 0.54 mol dm -3 H2NO 4

103kobs/S -1 EtOH t-BuOH Dioxan MeCN DMSO

0.22 0.2(I 0.25 0.21 0.29 0.25 0.25 0.27 0.22 0.36 O.33 0.31 0.28 0.25 0.46

aqueous mix tures for the 5-bromo complex is compared wi th those for the 5-nitro- and 4 ,7 -d ime thy l complexes in the Figure. The behav iour of the 5 -bromo complex is in termedi- ate, i. e. nearer to t ha t of the 5-nitro c o m p o u n d ; the varia- t ion of react ivi ty of the fo rmer with solvent compos i t i on is marked ly tess t han t h a t of the lat ter .

tron(ll): Base hydrolysis

Observed f i rs t -order rate cons tants , kob s, ( hyd rox ide present in large excess) for base hydrolys is of [Fe(5-Brphen)3] 2+ in aqueous so lu t ion are repor ted in Table 3. Kinetics have been invest igated at two d i f fe rent ionic s t rengths , at I = 0 .243 mol d m -3 (NaOH: NaCI) for compari-

son wi th o the r p h e n a n t h r o l i n e i r o n ( l l ) complexes (vide infra) and at I = 0 .50 mol dm -3 (KOH: KCI) for compar i son with cyanide a t t ack rates (again 'vide infra). The var ia t ion of kobs with hyd rox ide c o n c e n t r a t i o n can be a c c o m m o d a t e d by the usual rate law for t r i s -1 ,10 -phenan th ro l ine i ron ( l l ) co m- plexes(l s, t61.

- d [ c o m p l e x ] / d t = {kl + k2 [ O H - ] ) [ c o m p l e x ]

In the present case, k2 [ O H - ] >> kl ( aqua t ion rate con- s tant , vide supra) and the ra te law is apparen t ly s imple second-order with a negligible intercept , k l . The lower value of k2 in the med ium of higher ionic s t rength is as expected.

1ron(ll).. Cyanide attack

Observed f i rs t -order rate cons tants , kob s, for reac t ion of [Fe(5-Brphen)3] 2. with cyanide (present in large excess), to

0.4

1

o

~ 0.2 0

/ ./

/ /0

0

/ .

X 2 ~

0.2

/ / -o2 o • / /

. ~ - 04

0'.1 X2~

0.1 x2~

N o

O

Figure. ]'he variation ofaquation rate constant with solvent composition for substitutcd tris-l,lO-phenanthrolinciron(ll) complexes in binary aqueous solvent mixtures. Cosolvents: • EtOH; ,.' t-BuOH; + MeCN. Rate constants at 298.2 K except for the 4,7-Me2phen complex in aqueous alcohols (308.2 K).

164 J. Burgess and F.M. Mekhail Transition Met. Chem. 3, 162-165 (19781

give Fe(5-Brphen)2(CN)2, in aqueous solut ion are repor ted in Table 3. As for base hydrolysis, the results conform to the usual rate law (Is ' 17).

- d [ c , , m p l e x l / d t = {k, + k : l C N - l } [ c o m p l e x ]

establ ished for analogous i ron( l l ) complexes . Thus, for example, act ivat ion enthalpies of 120, 124, and 125 kJ mol -I have been repor ted for the aquat ion of [Fe(5-NO2phen)3] 2+, [Fe(5_Clphen)3]2 . . . . 113) , a n d [ F e ( p h e n ) 2 ] " respectwely .

with the k I term insignificant in the range of cyanide concent ra t ions we have used. For most phenan thro l ine iron(l I) complexes , second-order rate constants for a t tack by hydrox ide and by cyanide under comparable condi t ions are similar. For the [Fe(5-Brphen)3] 2÷ cation, those rate constants are equal at I = 0.5 mol dm -3 and 298.2 K. The rate cons tant for cyanide at tack at this complex at 308.2 K is compared with rates for o ther complexes in the general discussion section below. From the kz valucs de te rmined at 298.2 K and at 308.2 K, an activation energy for cyanide attack [at 1 = 0.5 tool dm -3 (KOH: KCl)] at [Fe(5-Brphen)3] 2*

can be es t imated as 85 kJ mol - l . This is, as usual, markedly less than the activation energy for aquat ion, vide supra.

The k2 terms for hydroxide and for cyanide at tack at [Fe(5-Brphen)3] z* presumably cor respond to a bimolecular mechanism, but whe ther a t tack takes place at the metal (15' 161

or at the coordina ted ligand 1181 cannot be decided on the

basis of our kinetic results.

I r o n ( I l l ) : A q u a t i o n

Conclusions

The general pat tern of reactivity of the t r is-5-bromo tris-5-bromo-1 , lO-phenanthro l ine- i ron( l I ) and -iron(Il l) complexes , in relation to that of o ther related complexes and to t h e r m o d y n a m i c proper t ies of the ligands and their iron complexes , is shown in Table 5. This Table shows the expec ted closeness of behaviour of the 5-bromo- to the cor responding 5-chloro-compounds . These 5-bromo-com- plexes can there fore take their place alongside their 5-chloro- analogues as useful examples of c o m p o u n d s containing modera te ly e lec t ron-wi thdrawing ligand subst i tuents .

Acknowledgemen t s

We are grateful to the Royal Society for the award of a Grant-in-aid for the purchase of the Unicam SPSOOA s p e c t r o p h o t o m e t e r and to Mr. J. A. Brivati for the design and cons t ruc t ion of the t h e rmo s t a t t ed syringe mixing device and o ther technical assistance.

In contrast to the iron(l l) complexes , t r i s -1 ,10-phenanthrol ine i ron( l l l ) complcxcs undergo aquat ion at rates which arc strongly acid concen t ra t ion depen- dent(6, 9, i9, 20). We have de te rmined rates for the

[Fc(5-Brphcn)3] 3÷ cation in sulphuric acid of concent ra t ion 1.09 mol dm -3 (Table 41, for ease of compar ison with published results for related complexes 161. The tempera ture variation of these rate constants indicates an activation energy (enthalpy) of 96 + 5 (93 -+ 5) kJ mol and an A factor of 10141 for aquat ion of this 5-bromo complex in 1.09 mol dm -3 sulphuric acid; the activation en tha lpy for [Fc(5-NO2phen)3] 3÷ under analogous condi t ions is 84 +- 4 kJ mol -/ (161. A lower activation enthalpy for a s tronger e lec t ron-wi thdrawing group parallels the behaviour

References

(1) J. Burgess, Inorg. React. Mechanisms, 1, 177 (1971); 2, 168 119721;3, 215 119741; P. Moore, ibid., 4, 174, 178 11976); and refs. therein.

(2) E. Bielli, R. D. Gillard and D. W. James, J. Chem. Soc. Dalton Thins., 1837 11976);R. D. Gillard, C.T. Hughes, andP. A. Williams, Transition Met. Chem., 1, 51 11976); R. D. Gillard, C. T. Hughes, L. A. P. Kane-Maguire, and P. A. Williams, ibid., 1, 226 119761; R. D. Gillard, L. A. P. Kane-Maguire, and P. A. Williams, ibid., 2, 12 119771; K. H. AI-Obaidi, R. D. Gillard, L. A. P. Kane-Maguire, and P. A. Williams, ibid., 2, 64 119771; J. Burgess and R. H. Prince, J. Cbem. Soc., 4697 (19651.

(3) G. F. Smith and F. P. Richter, Pbenantbroline and Subst i tuted Pbenanthroline Indicators, G. F. Smith Chemical Co., Columbus, Ohio, 1944, Sect. 2.

Table 3. Observed first-orderrateconstltntstkob s) and derived second-order ratc constants (k 2) for reactions of the [Fe(5-Brphen)3] 2+ cation with h.vdroxide and with cyanide in aqueous solution

Reagent Conditions Concns. k 2 and kob s values (mol dm -3)

Hydroxide 1 = 0.5 M (KCI) 298.2 K [KOll]/mol dm -3 0.065 O. 108 O. 152 11.105 0.238 11.036 103kobs/S-! 2.2 4.3 5.3 7.9 9.4

I = 0.243 M (NaCI) 298.2 K [NaOlll/mol dm -3 0.049 0.097 O.145 O.057 103kobs/S -I 2.3 5.0 6.5 /

1 = 0.5 bl (KCI) 298.2 K [KCNI/mol dm -3 0.025 1/.1/51/ 0.075 O. 10 O.15 } 0.036 103kobs/S -1 1.6 2.2 3.(1 3.5 6.2

1 =11.5 MiKCI) 308.2 K IKCNI/mol dm -3 (I.50 O.11 10 3kobs/s-I 55 /

Cyanide

Table 4. First-order rate constants for aquation of thelFe(5-Brphen) 3 ]3* cation in 1.09M H2SO4

T(K) 3o2.3 3113.5 306.2 309.0 311.2 314.4 315.4 318.0 103k/s -I 3.17 3.88 5.1 8.4 9.6 16.O 17.0 18.7

Transit ion Met. Chem. 3, 1 6 5 - 1 6 9 (1978) Complexes of 2- (Di- t -butylphosphinomethyl) - l -methoxy-4-methylbenzene 165 ~)Verlag Chemie, GmbH, D-6940 Weinheim, 1978

Table 5. Comparison of some kinetic and thermodynamic parameters for lFe(5-Brphen)3] 2÷,{Fc(5-Brphcl,)3] 3÷and 5-Brphen with those for other subst i tu ted 1,10-phenanthrolines and their iron(ll) and iron(liD tris-complexes

Thermodynamic Kinetic lron(ll) Iron(Ill)

Ligand Ligand Complex IFe(LL)3] 2+/3÷ 104kaq (308.2)g) k o H (298 K) h) kCN- (308 .2 ) i) 104kaq (307.5 K) jl pK a log/33 e °" 0.54 M H2SO4 l = 0.243 M I = 1).5 M (KCN/KCI) 1.09 M 1-12504

(NaOH/NaCI)

5~N02 phen 3.57 a) 17.8 a) 1.25e) 23 0.094 0.51 200 5-Clphen 4.26 a) 19.7 a) 1.12e) 12 0.052 0.10 22 5-Brphen 4.20 k) 1.12 e) 8.6 0.057 O. 11 6.4 phen 4.96 a) 21.2 c) 1.O6 e) 3.8 O.011 0.034 1.2 4,7-Me2phen 5.94 b) 23.1 d) 0.87 f) 1.1 (I.007 ca. 0.006 0.13

a) Ref. 21' b) ref. 22; c) ref. 23; d) ref. 24; e) ref. 25; f) ref. 26; g) sec -1 , rcf. 13; h) mol d m - 3 s -I , ref. 16; i) mol d m - 3 s -1 , rcf. 17; j) s -1 ,

ref. 6; k') ref. 27.

(4) See pp. 51 and 78 of ref. 3. (5) F. P. Dwyer and E. C. Gyarfas, J. Am. Chem. Soc., 74, 4699

(1952); J. E. Dickens, F. Basolo and H. M. Neumann, ibid., 79, 1286 (1957).

(6) J. Burgess and R. I. Haines, J. lnorg. NucL Chem., 39, 1705 (1977).

(7) E. R. Gardner, F. M. Mekhail, and J. Burgess, hlternat. J. Cbem. Kinetics, 6, 133 (1974).

(8) T. S. Lee, I. M. Ko[thoff and D. L. Leussing, J. Am. Cbem. Soc., 70, 2348, 3596 (1948).

(9) L. Seiden, F. Basolo and H. M. Neumann, J. Am. Cbem. Soc., 81, 3809 (1959).

(10) J. Burgess and R. H. Prince, J. Cbern. Sot., 6061 (1965). (11) J. Burgess and M. V. Twigg, Transition Met. Chem., 3, 88

(1978). (12) V. V. S. E. Dutt and H.A. Mottola, Ana/yt. Cbem., 49. 319

(1977). (13) J. Burgess and R .H . Prince, J. Cbem, Soc., 5752 (1963). (14) M. J. Blandamer andJ . Burgess, Cbem. Soc. Rev., 4, 55 (1975). (15) D. W. Margerum and L. P. Morgenthaler, j . Am. Chem. Soc., 84,

706 (1962). (16) J. Burgess and R. H. Prince, J. Chem. Soc., 4697 (1965).

(17) J. Burgess, Inorg. Cbtm. Acta, 5, 133 (1971). (18) R. D. Gillard, lnorg. Cbim. Acta, 11, 1.21 (1974); Coord. Chem.

Rev., 16, 67 (1975). (19) J. E. Dickens, F. Basolo and H. M. Neumann, J. Am. Cbem. Soc.,

79, 1286 (1957). (20) R. D. Gillard, L . A . P . Kane-Magui rcandP. A. Williams,

J. Cbem. Soc. Dalton Trans.. 1792 (1977). (21) C. J. Hawkins, H. Duewell and W. F. Pickering, Ana(vt. Cbirn.

Acta, 25, 257(1961) . (22) M. Yasada, K. Sone, and K. Yam~Lsaki, J. Phys. Cbem., 60, 1667

(1956). (23) H. l rv ingand D. H. Mellor, J. Cbem. Sac., 1962, 5222. (24) D. A. Brisbin andW. A. E. McBryde, Gzn. J Cbem., 41, 1132

(1963). (25) In 1F H2SO 4, see p. 42 of ref. 3. (26) A. I. Vogel, Quantitative Inorganic Analysis, 3rd Edit.,

l .ongmans, London, 1961, p. 101. (27J C. V. Banks and R. 1. Bystroff, J. Am. Chem. Sot., 81, 6153

(1959).

T M C 7 7 / 1 3 8

Some Complexes of Rhodium and Iridium with 2- (Di-t-butylphosphinomethyl)- 1-methoxy-4-methylbenzene*

H. David Empsall, Peter N. Heys, a n d B e r n a r d L. Shaw**

D e p a r t m e n t o f I n o r g a n i c and S t r u c t u r a l C h e m i s t r y , T h e U n i v e r s i t y , Leeds L S 2 9 J T , U.K.

( R e c e i v e d J a n u a r y 5 th , 1978 )

Summary

T h e n e w t e r t i a r y p h o s p h i n e t -Bu2 ( 2 - M e O - 5 - M e C r H 3 C H z ) P , L, was p r e p a r e d b y t he base

t r e a t m e n t o f t h e p h o s p h o n i u m sal t , LH+I - , w h i c h was in t u r n

* No reprints are available. ** To whom all correspondence should be addrcsscd.

p r e p a r e d f r o m t -Bu2P[ 1, 2 - M e O - 5 - M e C r t t 3 C I t 2 C [ and s o d i u m iodide . T h i s p h o s p h i n e , L, r eac t s w i th va r i ous c h l o r o - r h o d i u m

and - i r i d ium spec i e s w i t h o u t O - d e m e t h y l a t i o n . T h i s c o n t r a s t s

wi th t h e b e h a v i o u r o f t - B u 2 P C o t t 4 O M e - 2 w h i c h d e m e t h y l a t e s

read i ly on s imi l a r t r e a t m e n t to give c h e l a t e c o m p l e x e s . T h e

c o m p l e t e l y d i f f e r e n t b e h a v i o u r b e t w e e n th i s new 2 - m e t h o x y b e n z y l p h o s p h i n e , L, and t he 2 - a n i s y l p h o s p h i n e , t - B u 2 P C o H 4 O M e - 2 , is e x p l a i n e d in t e r m s o f s te r ic e f f ec t s .