Indi an Journal of Chemistry

Vol. 40A, March 200 1, pp.22R-23I

Synthesis of a novel binuc1eating symmetrical ~-bis(tetradentate) Schiff base ligand based on pentan-2,4-dione, 2-hydroxybenzaldehyde and 1,4,7,10-

tetraazadecane: A new family of dicopper(II/II) complexes Uday Mukhopadhyay & Debashis Ray*

Departmcnt of Chcmistry, Indian Inst itutc o f Technology, Kh aragpur 72 1 302, India

E-mail : dray @chcm.iitkgp.ernel.in

Received 7 A llg lIst 2000

The new acycl ic binucleating ligand H,L has been synthesizcd from a different type of SchilT ~ase co~~en sati on reacti on of acetyl acetone, salicylaldehyde and tneth ylenetet ram1l1e. The li gand has been characteri zed by H and C NMR, IR and FAB mass spectra. A famil y o f binuclear Cu(I I)-Cu( lI ) complexes of thi s diimine li gand containing imidazolidine backbone and other supporting exogcnous bridges havc been synthesized and eharactcrized by elemental analysis, IR, UV­vis, EPR spectral , and magneti c studies. The complexes of gcncral formul a [Cu2(f,I -X )(f,I -L»).2H20 (X=OAc, OMe, OEt) arc obtained by stoichiometric reacti on of the ligand, CuCI2.4H20 or Cu(OAch .. H20 and NaX in aqueous-methanol. The ligand reacts with Cu(CH,C02)2.H20 in aqueous methanol in air at ambient temperaturc affording crystalline [Cu 2(f,I -OAc)(p­L )] .2H20 in excellent yiclds. The complexcs havc two CuN20 1 coordinati on spheres bridged by one exogenous li gand. They rep r'escnt a new famil y of imidazol idinc bridgcd dicopper(ll/ll) complexes in unsymmetri cal compartments o f a novel J.l ­bi s( tctradentate) liganci (H1L J.

There is considerable interest in li gand s which can coordinate two metal ions at a suitable di stance whereby a small molecule (or molecules) can bind between the metal centres. Such metal complexed systems may show interesting magnetic behaviour I , may activate the bound molecule (phosphate ester hj'drolysis) or may serve as models for metalloenzymes.- In nature there are many phosphoesterases that are activated by two or more metal ions ." Nature also creates high affinity and very selective receptors for proteins empl oy ing a large number of simultaneous and weak interactions. Strong and dir~cted metal-li§and interactions could b~ l!Se~ as bas Is of recogl11tlon . Design and syntheS IS ot fleXible and rigid binucleating ligands are important to position two metal ions at a suitable and predetermined distance to be used as receptors,) A novel recognition process is operative to bind protein molecules selectively using two simultaneous interacti ons from two metal centres. The donor atoms in a suitably des igned binucleating ligand should be able to stabili ze a particular coordination geometry for a metal ion that is not possible with a simple ligand . By this means attempts can be made to mimic the entatic state of an active site,(' Recently, we7 and others8 have found that phenol­containing polydentate ligands are useful to stabilize both homo and heterobimetallic complexes of di storted coordination geometries. Herein we report the synthes is of a hitherto unknown symmetrical binucleating !l -bi s­(tetradentate) ligand (H,L 1) hav ing unsymmetrica l compartments of l-methyl-3-oxobut- l-enyl end groups and its dinuc lear copper(lI) complexes. This new acyc lic binucleating ligand H, L gives triply bridged binuclear

Cu(II)-Cu(II) complexes (2) in reasonable yields. The unsymmetrical salenacac winged ligand could take-up one metal ion in each wing and folded along the spacer backbone with the help of triatomic(acetate) or monoatomic(alkoxide) exogenous spacer.

Materials and Methods Cu(CH3COOh,H20 was obtained from Sarabhai

M. Chemicals, Vadodara, India. CuCI2.2H20 was purchased from BDH, MiJmbai , India. Triethylenetetramine was obtained fro m SD Fine Chemicals, Mumbai , India and salicylaldehyde was purchased from SRL, Mumbai, India. All manipulations were carried out under aerobic conditions. Super dry alcohols were prepared by following a literature procedure,'> using magnesium turnings and iodine. Sodium alkox ides were obtained by a reaction of dry alcohols with small pieces of sodium metal and removing the alcohol in vacuo. The solvents were obtained from BDH, Mumbai, India and used wi th out further purification . The solution electrical conductivity and e lectronic spectra were obtained using a Unitech type U 131 C digital conductivity meter with a solute concentration of about 10.3 M and a Shimadzu UV 3 100 UV -vis-NIR spectrophotometer respecti ve ly. The room temperature magnetic susceptibilities in the solid state were measured using a home built Gouy balance fitted with a Polytronic d.c. power supply. The experimental magnetic susceptibilities were corrected for the diamagnetic response using Pasca l" s constants. IR spectra and thermal data were recorded on a Perk i n­Elmer 883 spectrophotometer and Shimadzu DT40 thermal analyzer respectively. The elemental analyses

MUKHOPADHYAY el 0 1. : SYNTHESIS OF A ~l - BIS (TETRADENTATE) SCHIFF BASE 229

(C, H, N) were performed by the microanalytical laboratory of the Indian A ssociation for the Cultivation of Science, Calcutta with a Perkin -Elmer model 240 °C elemental analyzer. X-band EPR spectra were recorded on a V arian E-109C spectrometer fitted with a quartz Dewar tl ask for measurements at 77 K (l iquid nitrogen). The spectra were ca librated w ith diphenylpicrylhydrazyl (dpph ) (g = 2.0037). The microwave power level was maintained at ca. 0.2 mW.

Synthesis (~l th e iiRand and complexes

In search of newer and interesting binucleating Schiff base ligands of higher tlex ibility, basicity and so lubility with higher aliphatic and lower aromatic parts around the same trien backbone, we have synthesi zed

ligand H,L. The synthes is of the ligand was achieved in steps as shown in Scheme I in n % yield . The product thus obtained was analyti ca lly pure. Elemental analys is, [R , I Hand I·' C NMR studies characteri ze the li gand. Infrared spectrum of the Schiff base li gand showed a peak around 1605 cm·l , which is characteri stic of imine C=N bonds. The strong band at 1546 cm· 1 indicates the Vc~o stretching frequency. It is noted that in thi s Schiff base, fi ve-membered imidazo lidine rings were formed at the backbone after the condensation and that the middle arm is, therefore, unique and different from the other two aliphatic arms.

In I H NMR spectrum the signal due to phenolic OH of the middle arm is observed at I D.n ppm lO That due to imidazo lidine hydrogen (Hi) is observed at 3.n ppm. It is also observed that there is no broad peak above 12 ppm indicating the absence of enol hydrogens. In LIC NMR a peak at 194.99 ppm indicates the presence of C=O group and absence of enol form of the ligand in so lution.

H3C\ /'.. / H3 Ii If o 0

H,C~ f\ f\ 1\ CH3

- N N N :z H H -0 -

H,C CH,

11 .• 1.

MeOH

DoC .R.;

OH

c5-c~ MeOH

OOc· RT

Scheme I

2-(21-Hydroxyphellyl)- I.3-hi.\· / i -a?(/ -3 -( Ili-lI1 ethyl-f ­oxo/)ut- I -ell vl)-prop-31-ell - f' -vI/ - I , 3 - illl id(/ '!.olidill e, fi lL

A solution of trien (7 .00 g, 47. 8 mmol) In methanol (50 mL) was added to an ice-cold so lution or acetyl acetone (9.57 g, 95 .6 mmol) in methanol ( 100 mL). The resulting pale yellow solution was stirred for ca. 15 min at - 0 0c. The hexadentate precursor diketone, H2L' is a ye llow oily product and was not iso lated for the synthesi s of H ,L. [n the nex t slep a solution of salicy laldehyde (5.84 g, 47 .8 mmol ) in methanol (50 mL) was added to the previous solution of diketone and the yellow solution was stirred initiall y at - 0 °c for 15 min and finall y at ambient temperature (30 °C) for 2 h. The so lution was rotary evaporated to a yellow crystalline compound. The product was iso lated by filtration , washed with waleI' and hexane and finall y dried ill vacuo over fu sed CaCI2; yield 15.46 g (7X%). mp 85-87 Dc. Anal. Calc . for C2.,H:\4N40 , : C, 66.64: H, 8.27; N, 13.52. Found : C. 66.52 ; H , 8. 16; N. 13.57. Mass spectrum (El) : ml z 4 14 (M + = H, L+). Infrared spectrulll (cm·l . KBr disk ): 1605 (vs, VC~N) , 1546. 1443 (s, vc~c) . An.,x (M eCN . E, I it mol· 1 cm·l) 32 1 ( 14%5), 285 (20670), and 244 ( 13420). I H NMR (200 MHz. CDCI, ) 81'I'm : 1.76 (6H, 5, h), 2.65(4H. m, a), 2.77(gH. m, b, c) , 3.78( IH, s, i), 4.90(4H, s, e), 6.95( IH, dd.6), 7.20( IH, tdA), 6.79(2H , m, 3,5), 10.73(H , b, phenolic OH). LIC NMR (50 MHz, CDCI, ) 81'1'''' : I g.55 (g) . 28.65 (h), 42. 17 (a), 50.95 (b), 52 .04 (c) , 89.33 (I), 95.62 (e), 11 6.87 (3), Il g.!Q (5), 120.34 ( I ), 130. 35 ((6). 13D.XO (4), 158.03 (2), 162.70 (d), 194.99 (t').

/\/\/\ eN Ny N 1\\ )~}<o~

R

CompoLind R

2a Oae

2b

2<.:

MeO

EtO

( i ) / CU2(l-l-L)(p-OAc) /.2h'20 (20)

An aqueous so lution of copper(II) acetate monohydrate (0.482 g, 2.4 1 mmol) was added to a methanolic (20 ml ) so lution of H, L (0.5 g, 1.2 1 mmol ). The reaction mixture was st irred magneticall y at rOO ll1 temperature in air 1'0 1' I h. On concentration in air a li ght green compound separated from the reaction mixture; it was filtered on a G 4 frit and washed thoroughly w ith water and methanol and finally dried in vacllo over p ~O 10 . The yield of the compound was 0.497 g (65%). Anal. Calc. for C2,H:1xN~07CU 2: C, 47.23; H, 6.34: N. X.84; Cu, 20.06%. Found: C, 47 .28; H , 6.3 1; N, X.79; Cu, 19.94%. M olar conductance AM, 5.9 ohm·lc l11 ~ m () I · I.

(ii) / Cll i p-L)(I-I -OMe) /. 2H 20 (2b )

A n aqueous solution of copper(Il) chloricie dihydrate (0.4 12 g, 2.42 I11mol) was added to a

230 I DIAN J CHEM , SEC. A, MARCH 200 1

methanolic (20 ml) solution of H,L ( 0.5 g. 1.2 1 mmol) . The green co lour reaction mixture was stirred magnetically for 10 min . Methanolic solution of CH,ONa (0.07 g, 1.296 mmol) was added into that mixture. Immediately, a green compound was separated out. The compound was fi Itered. washed w ith water and methanol and dried in vacuo over P4010. The second crop of the compound was obtained by concen trating the filtrate in air. The yield of the compound was 0.51 g (70%). Anal. Calc. for C24H,sN40 6CU2: C. 47.62: H . 6.28: N , 9.25; Cu, 20.98%. Found: C, 47 .55 ; H , 6.31 ; N , 9.2 1; Cu, 20.80%. M olar conductance A M, 7.5 ohm' I cm2mor I .

The other alkoxide bridging compound was prepared by the same procedure as above using acetone instead of methanol as so lvent.

(iii) {Cui p-L)().1-0Er) ]. 2H20 (2e i : Ana l. Calcd for C2sH40N40 6CU2: C. 48.48: H , 6.46: N . 9.04 : Cu. 20.5 1 %. Found : C. 48.40; H , 6.4 1; N . 9. 10: Cu. 20.40%. M olar conductance AM. 10.4 ohm·1 cm2 mor' .

Results and Discussion

The ligand reported in thi s note belongs to a new family of Il-b is(tetradentate) pentan-2-one-4-imine ended imidazo l idines and has easy synthetic accessibility. T wo bridging spacer groups in the form of a trisubstituted imidazo lidine ring and a pendent phenol ic group are introduced inside the hexadentate bis(pentan-2-one-4-i mine-eth y leneamine) I igand frame work. In a two-step synthetic route it was prepared by the condensation of trien with 2 equiva lents of pentan-2. 4-dione(acety lacetone) and I equivalent of sal icy laldehyde. It can form chelates w ith relatively ri gid five-membered and more fl ex ible six-membered rings respectively, at each metal site. The ligand used in the present work is abbreviated as H , L. where the three protons correspond to one phenolic and two enol functi ons. During complex formation s. the two types of protons undergo dissociation. The hex adentate ketoimineamine li gand. H2L/ is syn thes ized by mild heating of acetyl acetone with trieth ylenetctramine in 2: I mole ratio in absence of any so lvent. The present ligand. H IL is then synthes ized by imidazolidine ring formation with sa licylaldehyde. The reaction of H,L with Cu(OAc h.H 20 in a 1:2 mole ratio at room temperature leads to the formation of dinuclear penta-coordinated complexes. The format ion of a l CU2(Il-phenoxo )(p ­OAc)f+ core involves opening of the tetra-Il-acetato cage as shown in Eq . I. Another series of monoatomic alkox ide bridging complexes is synthesized by the reaction of CuCI2.2H20. H IL and different sod ium alkoxides in 2: I : I mole ratio (Eq. 2) . A nal ytical and spectral data and room temperature magnetic momen t val ues es tab li sh the dinuclear comp lexes of compos iti on rCu2(Il-L)(Il.-0Ac)1.2H20 and [Cu2(p-L)(p-X)].2H20 (X = MeO, and E tO). The crystals grown so far by

solven t diffusion (DCM-hexane) techniques were not of X-ray quality. The complexes may have distorted trigonal bipyramidal or square pyramidal geometr ies around each metal ions with deprotonated enolate and phenolate donors. The ligand provides four donor po ints around each copper(JI) centre. W e are so far unab le to synthes ize the mononuclear comp lexes of H , L. Those complexes would have been controls in comparing the change in spectral and magnetic behav iour through dinuclear complex formation . One exogenous 0.0/­bridging from acetate or 0 bridging from alkox ides fulfils the fifth coordi nati on site. The comp lexes are so luble in most common organic solven ts and stab le for several days in so lution in the presence of air and moisture. Solution electri cal conducti Vtty measurements

----+. ICu!(p·L )(p·O;\c)J.2H20 + .I MeC02H H20

McO I Vi\CChl l~

R.T. Stirring

X= MeO . EIO

• IC",Ht- L )(p·XlJ.211,() 111CI + NaCI + 211,()

I I I

show the neutral character of the comp lexes . Without the use of NaX in the reaction , the correspond ing

tetracoordinated complex of type rCu2(1l.-L ) ICI or pentacoordinated complex of type ICu2 (p-L)( ~t-C1 ) 1 could not be iso lated.

In CU2 comp lexes the metal s are bonded in the pentacoordinated CuN20 , polyhedron w ith a typica l three-point N (imidazolidine), O(phenol ic) , O(acetate/ alkox ide) bridging between the two metal cen tres. The imine and imidazo lidine nitrogen pairs are coordinated cis to each other, so also are the bridging phenol ic oxygen and non-bridgi ng enolic oxygen pai r. The three chelate rings around each copper(U) ions are either six­or fi ve-membered. All the compounds (2a-2c ) have similar geometry consisting of ~1l-acetato/alk(Jxo) dicopper(IVII) cores and no termi na l monodentate li gand. The second bridging group is provided by the pendent ary lox ide oxygen of the p -b is(tetradcntare) H, L , and an in-built imidazo lidine r ing present in the ligand backbone acts as a third bridging ligand. In all the three comp lexes, a dinuclear triply bridged structure is present. The Cu-Cu non-bonded separat ion in these molecules and the magneto-structural correlations thereof would be dependent on both the number and nature of the bridging ligands.

The IR spectra of the two types of comp lexes show strong C=N stretching frequem y of the terminal imine functions at around 1597 cm·1 (for free ligand it is at 1605 cm·I

). The complexes have charac ter istic P2-bridging phenolic CoO stretching freq uencies in the range of 15 J 3- 1526 clll· l(ref l I ). For [Cu2(p-L)(p­OAc)].2H20 (2a) the Vcoo- (asym ) and vcoo- (SYIII ) appear at 1587 and 1404 cm·1 respectively. The ,0.v= I X3

MUKHOPADHYAY el a l .: SYNTHESIS OF A p-BIS(TETRADENTATE) SCHIFF BASE 23 1

cm'l corresponds to the familiar bidentate 1{ 11 I, f..lr bridging mode I 2. 1 ~ . Al so a broad band around 34 10 cm' l suggests the presence of lattice coord inated water onl y. The alkoxide bridging C-O stretching frequencies lie around I 12 1-11 23 cm' l . The terminal C-O (enol form) frequencies are in the range of 1249-1 272 cm' I (refI 3). The complexes exhibit d-d transitions in the 65 3-664 nm region . The moderately intense band observed in the near-v isible region is due to overlap of the azomethine transition with the charge-tran sfer band from bridging phenolic oxygen to the vacant d-orbita l of the Cu ll (ref. 14 ).

The effec ti ve magnet ic moments of 2a-2c are 2.50. 2.52 and 2.54 f..lB respec ti ve ly at 300 K . A diamagnetic correction of 278.43 x 10,1) cgsu per complex. as calcul ated from Pascal's constants I." was used . The observed magnetic moments of the pentacoordinated triply-bridged comp lexes due to each copper centre at room temperature are sli ght ly higher than the spin-onl y value ( 1.73 11 13) ' This suggests the operation of a ferromagnetic spin-exchange interaction in these complexes. In these comp lexes an imidazolidine group. besides the acetato/alkoxo and phenoxy groups. bridges the copper(II) centres. The imidazo lidine group is not expected to contribute to the magnetic exchange in these molecules. The X-band EPR spectrum of [Cu2(I1-L)(11-0Ac)].2H20 (2a) at 77 K in M eCN-toluene ( I : I ) glass shows a rhombic signal (gl = 2.3 1 I . g2 = 2. 106 and g~ = 1.889) in the g = 2.000 reg ion. A very weak 'half-field ' signal in the g = 4 region (g = 4.29), characteristic of a triplet state and t:-..Ms = 2 transiti on, has also been observed. The nature of the spectrum and the g va lues are typica l of a variety of bridged dicopper(IVII) complexes Iii with a half-field signa l at or around g = 4. In the strong ly coordinating solvent acetonitrile. the bridged structure is retained and no typica l tetragonal EPR spectrum is observed due to the formation of a mononuclear complex. The EPR spectrum of a powdered samp le of IC u(I1-L)(I1-OMe)1.2H20 (2b) at 77 K shows an isotropic signal at g = 2. 10. The spec trum is quite broad with a peak-to-peak separati on of 169 G. The correspond ing frozen spec trum in M eCN-toluene ( I : I ) glass at 77 K shows a nearly axial pattern with gil = 2.3 15 and g.l = 2.089 (A ll = 125 G ). The nature of the spectra arc completely different from that of rCu 2(f..l-L) (f..l-OAc)].2H20 (2a). suggesting a different molecular geometry and copper(ll) environment in frozen solution . The thermal decomposition of 2a was studied using TG and DTA techniques. The elimination of lattice water molecule is

D clearly observable at SO - 125 C. The TG curve shows a single step, first mass loss between 50- 126 De, wh ich corresponds exact ly to the release of water content (Found: 5.44%; calc.: 5.66%). The comparati ve ly lower temperature of water loss shows that these are not coordinated to the copper centres. The anhydrous

complex decomposes in three steps in the ranges 170-309, 309-410 and in a slower process at 410-600 0e, The absence of any well-defined TG plateau during decomposition indicates that stable intermed iates are not formed . The final , thermally stable residue is CuO (Found : 25 .3 1 %; Calc.: 25 .03%). Similar thermal behaviour is observed for the other members of this class of complexes irrespect ive of the nature of the bridging groups.

Acknowledgement We thank the INSA, New Delhi , India for

financial support.

References I Kahn O. Molecula r II/{/glleli.nll. (VCH. New York and

Cambridge). 1993. 2 Beese L S. Steitz T A. EMBO ./ .. 10 ( 199 1) 25: Davies J F.

Hoslomsky Z. Jordon S R & Mathews D A. Sc iellce. 252 ( 19') I ) 88; Lahm A , Vol veda S & Suck D . ./ II/olec Bioi. 2 15 ( 1990) 207: Kim E E & Wyckoff H W . ./lIIolec Bio I. 2 18 ( 199 I ) 449

3 Bioil1ol'ga ll ic chell/islry of ClJI' l'er . ediled by K D Karlin & Z Tyektar .(Chapman and Hall. London) 1993: Spodine E. Al ria A W. Manzur Z. Gracia A M . Hocquel A . Sanhueza E. Baggio R. Pena 0 & Sai ll ard J Y . ./ chclII SoC, Daillm Tml/S. ( 1997) 3683: Reim J & Krebs B . ./ Chi'1I1 SoC, Dulloll Tmll s. ( 1997 ) 3793.

4 Mallick S. Johnson R D & Arnold F H . ./ Alii chell I SoC, 116 ( 1994) 8902.

5 Mallick I & Mallick S. S\'III(' /I . ( 1996). 734. 6 Howells P . Kenny J W. Nelson J H & Henry R A. I lIol'g

G elll. 15 ( 1976) 124. 7 (a) Mukhopadhyay S & Ray D. I lldiall./ Chelll. 34A ( 1995) 466:

(b) Mukhopadhyay S. Mukhopadhyay U & Ray D. IJm c I lIrlillll Acari Sci (Chelll Sci). 107 ( 1995) 273: (c) Mukhopadhyay U & Ray D. Pl'Oc l lldiaIlAcadSci(CheIll Sci.). II O(1998)5 17:( d )

Mukhopadhyay U. Govi ndaswamy L. Velmurugan D & Ray D. Illorg Chelll COIIIIIIl/II. I ( 1998) 152: (e) Fal ve llo L R. Urriolabeitia E P. Mukhopadhyay U & Ray D. M C/(/

Crys /(IIIog l'" C55 ( 1999) 170: (I') Singha Chowdhury P K. Mukhopadhyay U & Ray D. Ill dioll ./ ChC/l/. 38 A ( 1999) I 159: (g) Mukhopadhyay U & Ray D . ./. G elll . Rt's. (S). (2000) 5X .

H (a) Suzuki M . Uehara A . Oshio H . Endo K . Yanaga M. Kida S & Sainto K . Blfll chell/ Soc ./apoll. 60 ( 1987) 3547 : (11 ) Borov ic A S & Que Jr L. ./ Alii chelll Soc. 110 ( 1988) 19X6.

9 Voge l's lexliJook or I'mclical orgallic chelllis ll'v . edited hy G H Jeffery. J Bassell . J Mendham & R C Denney (ELBS) 5' 1' cdll . 1989.400.

10 Yang L -W. Shuang E. Wong E. Rellig S J & Orvig e. I I/(lrg ChclII . 34 ( 1995) 2 164.

II Das Sarma \3 & J.e. Bail ar .I,. J e. ./ Alii chell/ SoC, 77 ( 1955).5476.

12 Deacon G B & Phi llips R J. Coorrl Chelll Rev. 4 ( I Y(9 ) YJ I . 13 Nakamoto K. I I//i 'ored al/d ROlllal/ sl ,eum of il/orgll l/ic lIlIrI

cOO l'dill(l/ iOIl COIllPOtlllds, 3'" edn .. (Wiley. New York ). 197X. 14 Okawa H. Tadokoro M. Ara lake Y. Ohba M . Shindo K.

Mitsumi M. Koikawa M. TOlllono M & Fenlon D E . .I chell I Soc. Dalloll 7'rlll1.\·. ( 1993) 253.

15 Carlin R L. MoglI ('lochelllisl r \', (Springer' Verl ag. New York /. 1986.

16 Sanyal I , M ahroof-Tahir M , Nasir M S, Ghosh P, Cru sc R W. Farooq A. Karlin K D , Liu S & Zubicta J, I llo rg Chelll. 3 1 ( 1992) 4322.