Indian Journal of Chemistry Vol. 42A, March 2003, pp. 478-483

Stabilisation of two Cu(l)-amino N bonds in presence of imino N's

Michael G B Drew

Department of Chemistry, University of Reading, Whiteknights, Reading RG6 6AD, UK

and

Shubhamoy Chowdhury, Goutam K Patra & DilPankar Datta*

Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Calcutta 700 032, India

Received 9 July 2002; revised 12 December 2002

Two structurally characterised examples of air stable Cu'(amino Nh( irnino Nh chromophores having a Cu(I1/1 ) potential of 0.01-0.19 V vs SCE in CH2C1 2 are provided using two tetradentate N-donor li gands.

Examples of copper(l) complexes containing a discrete bond between "soft" CuO) and "hard" H20 , two species apparently incompatible with each other according to Pearson 's Hard-Soft Acid-Base (HSAB) principle ' , are very few

2•4. Amines are less "hard"

than water. For example, chemical hardness 11 of H20 is 9.5 eV and that of NH3 i.s 8.2 eV I . Consequently, examples of copper(l) complexes containing Cu(l)-amino N (Nam) bond(s) are not few

5. '3 . Curiously, to

stabilise multiple (2-3) Cu(l)-Nam bonds, bonding of the metal with a strong 1t-acid like CO, olefins, nitriles or phosphines, which are "soft", has been necessary. For an idea of the relative 1t-acidity of CO, olefins, nitriles and phosphines, see ref. 14. Cu l N3 chromophores having one Cu(I)-Nam bond in presence of two bonded pyridine N (N py) atoms (reported by Karlin et al)IO.1I or benzimidazole N atoms l2 are not stable in air. A copper(I) complex with three Cu(l)-Naill bonds and a Cu(J)-Npy bond reported by Halfen et al. IJ has been found to be extremely air sensitive. Only very recently, two examples of copper(l) complexes containing only Cu(J)-Nam bonds have been reported together with their X-ray crystal structures-in one case, it is [Cu(NH3ht encapsulated in a zeolite '5 and in the other case, it is a CuN4 + chromophore isolated by Hubin et al. 16 with a tetradentate macrocycle containing tertiary N donors. While the cation [Cu(NH3ht seems to be quite stable in air within the zeolitic cavity, nothing is reported abou t the air stabi lity/sensitivity of the complex of Ilubin et af. Herein we describe some structurally characteri sed air stable Cu lN4 chromophores contain ing two Cu(I)-Nam bonds and two Cu(I)-imino N (N im) bonds isolated by using the tetradentate N-donor ligands shown below.

Materials and Methods N,N' -bis(2-aminoethyl)propane-l ,3· diamine (2.3.2-

tet) and [Cu(MeCN)4]CI04 were prepared by literature methods ' 7. '8 . Tetrabutylammonium perchlorate (TBAP) was prepared by the action of perchloric acid on tetrabutylammonium bromide [procured from Spectrochem (India)]. Triethylene-tetramine (trien) was purchased from Aldrich (USA) and AR grade benzaldehyde from SO Fine-Chern Ltd . (India). Purified dichloromethane was used for electrochemistry. All other chemicals and so l vents used for other purposes were of analytical grade. C, H and N analyses were performed by using a Perkin-Elmer 2400II analyzer. Copper was estimated gravimetrically as CuSCN. IR spectra (KBr di sc ; 4000-400 em· l ) were recorded on a Perkin-Elmer 783 spectrophotometer and UV -VIS spectra on a Shimadzu UV -160A spectrophotometer. Solution conductivity was measured by a Systronics (India) direct reading conductivity meter (model 304) . Cyclic voltammetry and coulometry were performed using an EG&G PARC electrochemical analysis system (Model 250/5/0) under dry nitrogen atmosphere in conventional three electrode configurations with TBAP as the supporting electrolyte. A planar EG&G PARC G0228 platinum milli electrode o r a planar EG&G PARC G0229 glassy carbon milli electrode was used as the working electrode in cyc li c

r (CH2)n,,\ P~ /\ ,;---\ Ph )=N N N N==<

R H H R

HL: R = H, n = 2 Mel: R = Me, n = 2 Hl': R = H, n =3

DREW el al .. STABILISATION OF TWO Cu(I)-AM INO N BONDS 479

vo ltammetry . Constant-potential coulometry was pe rformed using a platinum wire gauge working e lectrode and a PAR 377A cell sys te m. Under the experimental conditi ons employed here, the ferrocene-ferrocenium coupl e appears in dichloro-methane at OA7 V vs SCE (saturated calomel e lec trode) with a peak-to-peak separati on of 0.09 V at a scan rate (v) of 0.050 V s-' at a g lassy carbon e lectrode.

Syntheses MeL- 7.74 cm' (66A mmo l) of freshl y di still ed

benzaldehyde and 5 cm' (33.2 mmol) of freshly di still ed tri en were retlu xed in 50 cm' of anhydrous methano l for 6 h. The yellow reacti o n mixture was evaporated at room te mperature under reduced pressure to - 20 cm' . Then it was poured into 200 cm' of ice-cold water dropwise and stirred for 30 min. The resulting white suspension was kept in the refrigerato r overnight to obtain a ye llow ish semisolid . The solvent was decanted o ff and the semi solid was dried ill vacuo over fu sed CaCI2; y ie ld , 9.7 g (80%).

HC- OAI cm} (4 mmol ) of freshly di still ed benzaldehyde and 0.32 g (2 mmo l) of freshly prepared 2.3.2- tet were re flu xed in 20 cm} of anhydrous methano l for 6 h. Then the solvent was removed complete ly from the red reacti on mixture at room temperature under reduced pressure to o btain a redd ish viscous liquid . It was washed with 10 cm' of hexane (65-70°C fraction from petrol eum) and kept ill vacuo over fused CaCl:!; yield, 0 .5 g (73 %).

{Cu(MeL)}C10,,-0.35 g ( I mmo l) of MeL was dissolved in 20 cm3 of anhydrous, degassed methano l to which 0 .33 g ( I mmol) of freshly prepared [Cu(MeCN)4]CI04 was added under dry N2 atmosphere. The reac ti o n mixture was stirred for 15 min. The reddi sh yellow co mpound precipitated was filtered, washed with 3 cm' of methanol and dri ed ill vacuo over fused CaCl 2. It was recrystalli sed from di chl oromethane-hexane (65-70°C fraction fro m petroleum) mi xture as redd ish ye ll ow needles. Yield, 0.8 g (83 %). Elementa l analyses were consistent with the stoichi ometry Cn H,oClCuN40 4 [Fo und : C, 51.37 ; H, 5.8 1; N, 11 .01 ; Cu , 12 .33. Ca lc : C , 51A3 ; H, 5.89; N, 10.9 1; Cu, 12.29%]. AM/mho c m" mol" (C H, OH ): 99 ( I : I e lec trolyte). IR data (cm-') : 1605vs (C=N );

1085vs, b, 6 15m (Cl04 ) . UV -vis (C H}OH ): Alnm

(£/dm' mo l" cm-'): 235 (22,000), 292 sh (3,900), 382 sh (2,350).

{Cu(HC)}CI04- 1t was synthes ised by s tarting wit h 0.34 g (1 mmol) of HL' dissolved in 20 em} of

anhydrous, degassed methano l and 0 .33 g (I mmo l) of solid [Cu (MeCN)4]CI04 in a manner exactl y simi lar to that for [Cu (MeL)]CI04 as deep red needles. Yie ld, 0.35 g (72 %). Elemental ana lyses were consisten t with the stoichiometry C2 , H2SClCuN40 4 [Found : C, 50.55 ; H, 5.59; N, 11.26; Cu , 12.7 1. Calc: C, 50.50;

H, 5.65; N, 11.22; Cu, 12.72%]. AM/mho cm2 mol" (C H30H): 95 ( 1:1 e lectrolyte). IR data (cm-'): 16 10vs (C=N); lllOvs, b, 630m, split (CI04). UV-vis

(C H30H): A/nm (£/dm' mol" cm-'): 250 (24,400), 300 sh (3,400), 390 sh (3,600).

Cautioll-Though we have not met with any inc ident while handling [Cu(MeL)]CI04 and [Cu(HL ')]CI04, care should be taken as perchl orate sa lts are potentially explosive.

X-ray crystallography Single c rystals of [Cu (MeL)]CI04 and

[Cu(HL')]CI04 were grown by direct diffusio n of hexane (65 -70°C fraction from petro leum) into dilute

diehloromethane solutions of the complexes. Data collections were performed with Mo- Ka radi atio n

(0.71073 A) using the MARresearch Image Pl ate Sys te m at 293(2) K. The crystals were pos itioned at

70 mm from the Image Pl ate. 95 frames were measured at 2° inte rva ls with a counting time of 2

min . Data analyses were carried o ut with the XDS program' 9. The structures were solved using direct

methods with the SHELXS-86 program20. The perchl orate oxygen atoms in [Cu(MeL)]Cl04 were

di sordered over 2 sets of tetrahedral positions. In both

the structures the no n-hydrogen ato ms apart from the

di sordered atoms were refined with ani so tropic thermal paramete rs. The hydrogen atoms were

inc luded in geometric positions and g iven thermal parameters equi va lent to 1.2 times those of the ato m

to which they were attached. The structures were refined on F2 using SHELXL-93 (ref. 21). Significant crystal data for [Cu(Me L)]Cl04 and lC u(HL')]CI04 are g iven in Table I . The fractional atom ic coordinates of the no n-hydrogen atoms In [C u(MeL)]CI04 and [C u(HL')]CI04 are li sted in

T ables 2 and 3 respectivel y. The c rys tal structures have been deposited at the Cambri dge

Crystallographic Data Centre and a llocated the depos itio n numbers CC DC 199846 & 199847. Cop ies of the data can be obtained free o f charge o n

application to CCDC, 12 Unio n Road, Cambri dge CB2 I EZ, UK; fax: (+44) 1223 -336-03 3; e- mail : deposit @ccdc .cam.ac.uk .

480 INDIAN J CHEM, SEC A, MARCH 2003

Table I-Crystal Data fo r [Cu(MeL)]CI0 4 and [Cu(HL')]CI0 4

[Cu(MeL)]CI04 [Cu(HL')]CI04 Empirical formula C22H30ClCuN404 C21 H28C1CuN40 4 Formula weight 513.99 499.46 Crystal system, space group monoclinic, P2/a orthorhombic, Pbca Unit cell dimensions (A,O) a 16.288( 17) 13.639(14) b 8.812( 10) 16.800(17) c 17.601 (21) 20.506(23)

13 107.110( 10) 90 Volume/A3 24 14(5) 4699(9) Z, Calculated density/g cm·3 4,1.413 8, 1.412 Absorption coefficient/mm·1 1.050 1.077 Reflections collected I unique (Rin,) 8456 I 4429 (0.0386) 12711 / 4062(0.0417) Data I restraints I parameters 442910 I 291 4062 10 I 281 Final R indices [1>20(1)] : R I, wR2 0.0712, 0.1849 0.0674,0.1749 R indices (a ll data): R I, wR2 0.1075, 0.2068 0.1 193, 0.1965 Largest diff. peak and hole/e k 3 0.756, -0.549 1.067,0.725

Table 2-Fractional atomic coordinates (A) and equivalent Table 3-Fractional atomic coordinates (A) and equiva lent isotropic displacement parameters (A 2) of the non-hydrogen isotropic displacement parameters (A 2) of the non-hydrogen atoms in [Cu(MeL)]CI04 with their standard deviations 111 atoms in [Cu(HL')]CI04 with their standard devia ti ons in parentheses" parentheses"

Atom x y z Ucq Atom x y Z Ueq

Cui 0.0 1390(4) 0.78466(7) 0.26289(4) 0.049(1) Cui 0.82746( 4) 0.09841(4) 0.32449(3 ) 0.054(1) NI 0.0302(2) 0.6183(5) 0.3422(2) 0.042(1) NI 0.7173(2) 0.0275(2) 0.29545( 19) 0.049(1 )

4 -0.0028(3) 0.9315(5) 0.3570(3) 0.054(2) N7 -0.1014(3) 0.8970(6) 0.1947(3) 0.062(2) N4 0.8193(3) 0.0393(2) 0.4160(2) 0.057(2)

NIO 0.0615(3) 0.8360(5) 0.1741(2) 0.051 (2) N8 0.8228(3) 0.2 178(3) 0.3578(2) 0.058(2 )

C2 0.0319(4) 0.6847(6) 0.4198(3) 0.051 (2) Nil 0.9405(2) 0.1327(2) 0.2684(2) 0.049( I) C3 -0.0268(4) 0.8199(7) 0.4080(3) 0.061 (2) C2 0.7037(4) -0.0328(3) 0.3472(3) 0.065(2) C5 -0.0712(4) 1.0433(7) 0.3203(4) 0.075(2) C3 0.7196(4) 0.0040(3) 0.4123(3) 0.066(2 ) C6 -0.1389(4) 0.9764(8) 0.2506(4) 0.080(3) C5 0.8350(4) 0.0865(4) 0.4761 (3) 0077(2 ) C8 -0.0655(4) 0.9976(8) 0.1460(4) 0.077(2) C6 0.7988(4) 0.1716(3) 0.4723(3) 0077(2) C9 -0.0062(4) 0.9119(8) 0.1101(3) 0.071 (2) C7 0.8552(4) 0.2254(4) 0.4261(3) 0.076(2 ) CII 0.0533(3) 0.4801 (6) 0.3383(3) 0.043(2)

C9 0.8871 (4) 0.2607(3) 0.3 122(3) 0.067(2) CI2 0.0841(5) 0.3790(7) 0.4095(3) 0.078(3) CI3 0.0524(3) 0.4184(6) 0.2591 (3) 0.044(2) CIO 0.9746(3) 0.2108(3) 0.2934(3) 0.057(2)

CI4 0.1144(4) 0.3124(6) 0.2534(4) 0.057(2) CI2 0.9714(3) 0.1116(3) 0.2130(3) 0.055(2)

CI5 0. 1 19 1 (4) 0.2659(7) 0.1801(4) 0.069(3) CI3 0.9440(3) 0.0368(3) 0. 1797(2) 0.051(2) CI6 0.0610(5) 0.3221(8) 0.1117(4) 0.074(3) CI4 0.9479(3) 0.0342(4) 0. 1125(3) 0.069(2 ) CI7 -0.0027(4) 0.4189(7) 0.1 160(3) 0.063(2) CI5 0.9235(4) -0.0351 (5) 0.0798(3) 0.087(3) C I8 -0.0076(4) 0.4681 (6) 0.1901(3) 0.05 1(2) CI6 0.8950(5) -0.1021(4) 0. 1132(4) 0.086(3 ) C21 0.1315(4) 0.7890(7) 0. 1614(3) 0.058(2)

CI7 0.89 12(4) -0.0992(4) 0.1808(3) 0.075(3 ) C22 0.1495(5) 0.8048(10) 0.0823(4) 0.09 1 (3) C23 0. 1975(3) 0.7136(7) 0.2270(3) 0.052(2) CI8 0.9153(3) -0.0310(3) 0.2 139(3) 0.056(2)

C24 0.2108(3) 0.7596(7) 0.3057(3) 0.055(2) C21 0.6717(3) 0.0160(3) 0.2414(3) 0.056(2)

C25 0.2680(4) 0.6858(9) 0.3676(4) 0.074(3) C22 0.6751(3 ) 0.0671(3) 0.1843(2) 0.049(2 ) C26 0.3140(4) 0.5642(10) 0.3531 (5) 0.087(3) C23 0.6246(4) 0.0433(3) 0.1284(3) 0.066(2 ) f;27 0.3049(4) 0.5199(9) 0.2769(5) 0.087(3) C24 0.6257(4) 0.0902(4) 0.0731(3) 0.074(3 ) C28 0.2473(4) 0.5936(8) 0.2136(4) 0.072(3)

C25 0.6759(4) 0.1605(4) 0.0720(3) 0.078(3) CI3 114 0.2823(3) 0 0.080(1 ) *031 0.2 109(7) 0. 1567(13) 0.0360(6) 0.098(3) C26 0.7260(4) 0.1865(4) 0.1272(3) 0.073(2 )

*032 0.2738( 13) 0.227(2) -0.0594(9) 0.178(8) C27 0.7242(4) 0.1393(3) 0.1828(3) 0.061(2 )

*033 0.3070(8) 0.345 1(16) 0.064 1(7) 0. 125(5) CI2 0.53794( I 0) 0.31532(9) 0.38506(7 ) 0.069(1)

*034 0.1741 (8) 0.3721(15) -0.0304(8) 0.124(5) 021 0.5504(4) 0.3884(3) 0.4176(3) 0.120(2) CI2 1/4 0.0679(2) 1/2 0.065(1) 022 0.6138(3) 0.2986(4) 0.3414(3) 0.127(3) 021 0.1766(3) -0.0183(7) 0.4689(4) 0.127(3) 023 0.5314(6) 0.2535(4) 0.4306(3) 0.180(4) 022 0.2644(5) 0.1621( 10) 0.4402(4) 0.149(4)

024 0.4496(3) 0.3180(4) 0.3508(3) 0.143(3) "Ueq is defined as one thi rd of the trace of the orthogonalised Uij tensor. Starred atom sites have a S.O.F. of 0.5. For the atom ·Ucq is defined as one third of the trace of the orthogonalised U" labell i ng scheme, see Fig. I. tensor. For the atom labelling scheme, see Fig. 2.

DREW et at.: STABILISATION OF TWO Cu(l)-AMINO N BONDS 48 1

Results and Discussion The ligand MeL has been synthesised by refluxing

one mol of trien with two mol of acetophenone, and HL'by refluxing one mol of 2.3.2-tet with two mol of benzaldehyde in anhydrous methanol. As isolated, these ligands are not analytically pure. Attempts to purify them by chromatography (over silica gel and alumina) or recrystallisation have not been successful. However, their copper(I) complexes, [Cu(MeL)]C104 (reddish yellow) and [Cu(HL')]C104 (deep red), obtained by reacting the respective ligands with [Cu(MeCN)4]C104 in equimolar proportion in anhydrous methanol under an N2 atmosphere, are analytically pure. The complexes are quite stable towards aerial oxidation. [Cu(MeL)]CI04 is stable for more than two weeks in the solid state while [Cu(HL')]C104 is stable for more than two months in the solid state. Their solution stability towards aerial oxidation depends on the solvent. For example, in CH2Clz these are stable for more than 24 h but in methanol [Cu(MeL)]C104 is stable for about 2 hand [Cu(HL')]CI04 for about 12 h. On standing in air, orange-red methanol solutions of [Cu(MeL)]C104 and [Cu(HL ' )]C104 gradually become violet through green. The violet colour arises due to the generation of the tetramine complexes of copper(IJ) via slow hydrolysis of the ImInO fragments. The 2: 1 condensate of acetophenone and 2.3.2-tet yields gummy material when reacted with [Cu(MeCN)4]C104. Syntheses of HL and its air stable copper(l) complex [Cu(HL)]C104 have been reported by us elsewhere in connection with the photolumine-scence of [Cu(HL)]Cl04 (ref. 22).

The structures of the cations in [Cu(MeL)]C104 and [Cu(HL ' )]CI04 as determined by X-ray crystallography are shown in Figs 1 and 2 respectively. Selected bond lengths and angles are given in Tables 4 and 5. In both the complexes, the N4 coordination sphere of the metal is significantly di storted from tetrahedral in similar fashion presumably due to the steric constraints of the ligands. The Cu-Nim bonds vary from 1.988(4)-2.008(4) and Cu-Nam from 2.121 (5)-2.182(5) A in length. Shorter Cu-Nil11 bonds clearly indicate the preference of copper(l) for an imino N over an amino one. The phenyl rings in both the complexes adopt EE configuration.

The electrochemical behaviour of the two complexes has been examined by cyclic voltammetry and coulometry in dichloromethane under an N2 atmosphere at platinum and glassy carbon electrodes.

Fig. I-The structure of the cation in [Cu(MeL)]Cl04 with ellipsoids at 30% probability.

Fig. 2-The structure of the cation in [Cu(HL')]ClO-l with ellipsoids at 30% probability.

They display a quasireversible Cu(IlII) couple on the positive side of SCE. The couple is more reversible at a glassy carbon electrode (Fig. 3) . The Cu(ll/l ) potential in [Cu(HL')]C104 (0.19 V vs SCE) is considerably higher than that in [Cu(MeL)]ClO.J (0.0 I V vs SCE). For comparison, we mention that the Cu(II1I) potential in [Cu(HL)]CI04 is 0. 15 V vs SCE22. In a Cu l N4 chromophore, the Cu(II1I ) potent ial increases with the 1t-acidity of the ligand and the extent of tetrahedral distortion in the correspondi ng Cu"N4 moiety23 . Effect of 1t-acidity can be readily assessed here. When we compare the Cu(lI/l ) potential of [Cu(HL')]CI04 with that of [Cu(MeL)]CI04, we realise that presence of elec tron donating groups like methyl drags the potential or a

482 INDIAN J CHEM, SEC A, MARCH 2003

Table 4- Selected bond lengths (A) and angles (deg) in [Cu(MeL)]CI04 a

Cul-NI 1.988(4) Cul-N7 2. 150(5) Cu l -N4 2.1 82(5) Cul-NIO 1.991(5) N I-CI I 1.283(6) NIO-C21 1.292(7) NI-Cu l -N4 85.66( 19) NI-Cul-N7 130.2 1 ( 18) NI-Cul-N IO 135.82( 18) N7-Cul-NIO 86.3(2) N4-Cu l-N7 82.38( 19) N4-Cu i-N 10 128.75(19) Cul-NI-C II 130.0(3) Cul-NlO-C21 129.0(4) C2-N I-CI I 11 9.9(4) C9-N I0-C21 120.7(5)

a For the atom labelling scheme, see Fig. I .

Table 5- Selected bond lengths (A) and angles (deg) in [Cu(HL ')]CIO/

Cul -N I 2.008(4) Cui -Nil 2.008(4) Cul-N4 2.127(4) Cul -N8 2.121 (5) NI-C2 1 1.286(7) NII -CI2 1.262(6) N I-Cul ·N8 129.38( 15) N4-Cul-N8 98.96( 17) N I-C ul -NII 125 .1 0(17) N4-Cul-NII 132.85(14) NI-Cul -N4 86.87(16) N8-Cul-NII 86.34( 17) Cul -N I-C2 1 135.0(3) Cul-NIl-CI2 133.7(3) C2-N I-C21 11 7. 1(4) CIO-NII-CI2 117.0(4)

" For the atom labelling scheme, see Fig. 2.

Cu(IIIl) couple towards negative. Recently we have demonstrated that an imino N has the ability to stabi li se copper(l) to a great extent23 . For example, the Cu(IIII) potential in 1, where the ligand is closely related to HL, MeL and HL', is 0.81 V vs SCE in dichloro-methane23 . According to Pearson's HSAB principle, bonding of copper to a strong cr-donor like amino N leads to destabilisation of the metal 's oxidation state I. Consequently a combination of Naill and Nilll is expected to lower the potential of the Cu(IIIl) couple in a CulN4 chromophore compared to that in a Cul(N;1ll)4 core. This is what is observed here. So far, the Cu(IlII) potential for only two copper(I) complexes containing Cu-Nalll bonds are known; these are the copper(l) complex of Curtis' macrocycle (2) which has been generated in situ24 and the one reported by Karlin el al lO . While 2 may have a CulN4 moiety., the complex of Karlin et al has a CulN3 chromophore. The Cu(IlIl) potential in 2 is -0.9 V vs SCE and that in the complex of Karlin et al. 10 -0.08 V vs SCE in dimethylformamide. No electrochemical data are available for the complex of Halfen et al. 13; however, considering its high sensitivity towards O2, the Cu(IlIf) potential is likely to be very much negative. Thus the import of our present electrochemical results is that here we have been able to stabilise two (multiple) CuI-Naill bonds to a very significant extent without using any classical IT-acid as a co- ligand.

T x

J

j

0.9

-- '-.

0.1 E (V) vs SeE

/ '" I , "-"- '-.

-------

-0.3

Fig. 3-Cyclic vohammograms of ICu(MeL)ICI04 (broken line: concentration c = 1.03 mmol dm·3, v = 0.05 V S· I. currenl scale x = 2 ).tA) and [Cu(HL')]C104 (full line; c = 1.07 mmol dm ·.1 . v = I V S· I , X = 10 ).tA) in dichloromethane (0.1 mol dnf3 in TBAP) al a glassy carbon electrode. At v < I V S· I, the cathod ic peak in [Cu(HL')]CI04 appears with a shoulder.

2

Here we wish to point out that the IT-acidity of an imino N can at the most be comparable to that of pyridine N25 . When we consider the Cu(I1/1) potential of 0.06 V vs SCE in [CU(PY)]4 + in water26, it is apparent that IT-acidity alone cannot be respons ible for the observed magnitudes of the Cu(IIII) potential s of our Cul(Nalllh(Nimh chromophores. A closer look at the crystal structures of the cations in [Cu(MeL)]CIO~ and [Cu(HL')]C104 reveal that the phenyl rings in these two complexes are stacked approx imately on top of each other with the shortest C ... C di stance of 3.46 A in [Cu(MeL)]CI04 and 3.49 A in [Cu(HL')]CI04; the angle between the two phenyl rings is 10.6 and 24.5° in [Cu(MeL)]Cl04 and [Cu(HL')]C!0 4 respectively. Some degree of IT-IT interactions between the phenyl ri ngs in the two complexes cannot be ruled out. This may restrict the inherent tendency of the lahn-Teller sensiti ve copper(II) center generated upon oxidation to undergo a flattening distortion raising the Cu(II1I) potentials in [Cu(MeL)]CI04 and [Cu(HL')]CI04 higher than what would be expected only on the basis of the IT-acid strengths of the ligand frameworks.

DREW et af.: STABILISATION OF TWO CU(l)-AMINO N BONDS 483

Acknowledgment M G B D thanks EPSRC and the University of

Reading for funds for the Image Plate System and D D the Department of Science and Technology, New Delhi, India for financial support.

References I Hati S, Datta 0 , Proc Indian Acad Sci, Chem Sci, 108 (1996)

143 and refs therein . 2 Naskar J p, Hati S, Datta 0 & Tocher 0 A J chem Soc, Chem

COmIlIUI1 , ( 1997) 1319. 3 Filinchuk Y E, Davydov V N & Mys' kin M G, Russ J coord

Chem, 27 (2001) 505; Melnyk 0 P, Schollmeyer 0, Olijnk V V & Filinchuk Y E, Acta Crystallogr, C57 (2001) 151.

4 Naskar J P, Chowdhury S, Drew M G B & Datta 0, New J Chem, 26 (2002) 170 and refs therein.

5 Hathaway B 1, Comprehensive Coordination Chemistry, edited by G Wilkinson, R 0 Gillard & J A McCleverty (Pergamon Press, Oxford) vol 5, 1987, P 533.

6 Munakata M, Kitagawa S & Maekawa M, J Inorg Biochem, 43 ( 1991) 198.

7 Halfen 1 A & Tolman W B, J Am chem Soc, 116 (1994) 5475.

8 Mahadevan V, Hou Z, Code A P, Root 0 E, Lal T K, Solomon E I & Stack TOP, J AIR chem Soc. 119 (1997) 11996.

9 Tolman W B, Acc Chem Res, 30 ( 1997) 227 . 10 Karlin K 0 , Gultneh Y, Hutchinson 1 P & Zubieta J, JAm

chem Soc, 104 ( 1982) 5240.

II Karlin K 0 , Gan Q & Tyeklar Z, J chem Soc. Chl'lII Commun, ( 1999) 2295.

12 Monzani E G, Battaini G A, Peretti A, Casella L, Gullotti M. Santagostini L, Nardin G, Randaccio L, Geremia S. Zane lla P & Opromolla G, Inorg Chem, 38 ( 1999) 5359.

13 Halfen 1 A, Young V G, 1r. & Tolman W B, J Am chelll Soc. 118 (1996) 10920.

14 Naskar 1 P, Hati S & Datta 0 , Proc Indian Acad Sci. Chelll Sci , 108 (1996) 10 1.

15 Gomez-Lor B, Iglesias M, Cascales C, Guiterez-Puebla E & Monge M A, Chem Mater, 13 (2001 ) 1364.

16 Hubin T J, Alcock N W & Busch 0 H, Acta Crystal/og /'. C56 (2000) 37.

17 Brubaker G R & Schaefer 0 P, Ill org Chem. 10 ( 197 1) 968. 18 Hemmerich P & Sigwart C, Experientia, 19 ( 1963) 488. 19 Kabsch W, J App/ Crystallogr, 21 (1 988) 916. 20 Sheldrick G M, SHELXS-86: Program for c rystal structure

solution, Acta Cystallogr, A46 ( 1990) 467. 21 Sheldrick G M, SHELXL-93, Program for crystal structure

solution and refinement (University of Gottingen , Germany) 1993.

22 Panja S, Chowdhury S, Drew M G B & Datta D. Illorg Chl'lll Commun, 5 (2002) 304.

23 Chowdhury S, Patra G K, Drew M G B, Chattopadhyay N & Datta 0 , J chem Soc, Dalton Trans, (2000) 235.

24 Gagne R R, Allison J L & Ing le 0 M, Inorg Chem, 18 ( 1979) 2767.

25 Datta 0, J chem Soc, Dalton Trans, (1986) 1907. 26 James B R & Williams R 1 P, J chem Soc, (1961) 2007.