Indian Journal of Chemistry Vol. ~9A April2000, pp. 400-406

Template synthesis of 1-(8 '-amino-a-naphthyl)-4-(8 '-amino-a-naphthylanline)-1-azabuta-1 ,3-diene, a tetraaza half-cyclised ligand, and

characterisation of its copper(II) complexes

Nita A Lewis

Department of Chemistry, University of Miami , Coral Gables, Florida 33 124, USA

and

Derek A Tocher

Department of Chemistry, University College London, 20 Gordon Street, London WCI H OAJ, UK

and

Goutam K Patra, Jnan P Naskar & Dipankar Datta*

Department of Inorganic Chemistry, Indian Association for the Culti:vation of Science, Calcutta 700 032, India

Received 20 July 1999

Reaction of I, I ,3,3-tetramethoxypropane with I ,8-diaminonaphthalene in presence of a nickel(ll ) template in aqueous medium generates the free acycli c ligand 1-(8 '-amino-a-naphthyl )-4-(8 '-amino-a-naphthylamine)-1-azabuta-1 ,3-diene (LH ; H: dissociable proton) as a 1.5 hydrate. The ligand undergoes acid dissociation in non-aqueous solvents which is reflected in its nc NMR spectrum in deuteriated dimethyl su lphcxide (DMSO). Its pK, value determined in DMSO from conductivity data is 5. 18. With copper( II ) perchlorate, LH forms air sensitive complexes of type CuL(Cl04).3 H20 and CuL(Cl04).DMF (DMF: dimethylformamide). In both the complexes, the perchlorate anion is weakly bound to the metal as revealed by their conductivity in DM F suggesting a CuN40 2+ chromophore. Their EPR spectra in DMF-toluene gl a~s at 77 K are almost identical and have rhombic features (<g3>, 2.40; <g2>, 2.08; <g 1>, 2.01; <A 3>, l37xl0-4 cm- 1

; <A 2>, 18x l0-4 cm- 1)

indicating that the coordination sphere around the copper ion is of di storted square-pyramidal type. ZINDO calculations show that the CuL+ fragment has a butterfly like shape with the naphthalene fragments spanning like two wings and t'le metal in the N4 plane. In cyclic voltamrnetric experiments in DMF at a platinum electrode, the complexes show a quasi~ reversible two-electron ligand oxidation with an E 112 {)f 0.12 Y vs saturated calomel electrode (SCE) and a irreversible metal oxidation process at a low potential ( < 0.5 Y vs SCE).

Jager's macrocycle, reported in 1964, is a 2 + 2

condensate of I ,2-phenylenediamine and acetyl­

acetone1. Later many derivat ives of it have been

prepared. They have an extensive conjugati on, like

that in naturally occurring porphyrins. This class of

tetraaza macrocycles (1) can be said to be true

synthetic analog of the porphyrins . They have given

rise to a very rich and versatile coordination

chemistry. For some selected aspects, see refs 2-7 .

Similar interesting chemistry is likely to emerge from

thei r 1 ,8-diaminonaphthalene ( 1 ,8-DAN) counterpart (2) which is not yet known. As such, no tetraaza macrocycle containing fused naphthalene ring(s) has been reported so far. Recently we have initiated a study in this direction. The results obtained so far are reported here.

lf1 OCN HNY')

NH N~ V'

Materials and Methods I ,8-DAN (97% ), I, I ,3,3-tetramethoxypropane (99+

%) and NaCI04.H20 were purchased from Lancaster, England, and deuteriated dimethyl sulphoxide (DMSO-d6; 99.9 atom % D) was purchased from Aldrich, USA. Other chemicals used were of AR grade. Fresh analytical grade DMF, procured from S. D. Fine-Chem Ltd., India, was used directly for electrochemistry without further purification. C, H and N analyses were performed by using a Perkin-

LEWIS eta!.: Cu(II) COMPLEXES OF A TETRAAZA HALF-CYCLISED LIGAND 401

Elmer 24001! analyzer. Copper was estimated gravimetrically as CuSCN. The melting point of LH was determined by a melting point apparatus procured from CBC Power Corporati<''l (Calcutta, India) and is uncorrected. IR spectra (KBr disc; 4000-400 cm-1

) were recorded on a Perkin-Elmer 783 spectrophotometer, UV -vis spectra on a Shimadzu UV-l60A spectrophotometer, 1H and ''c NMR on a Brucker DPX300 spectr9meter, X-band EPR spectra on a JEOL RE IX spectrometer and mass spectra on a VG-ZAB-SE instrument. Solution conductivity was measured by a Systronics (India) t(irect reading conductivity meter (model 304). Magnetic susceptibility was determined at room temperature by a PAR 155 vibrating sample magnetometer. The magnetometer was calibrated with HgCo(SCN)4 and the susceptibility data were corrected for diamagnetism using Pascal's constants. Cyclic voltammetry and coulometry were performed using EG&G PARC electrochemical analysis system (Model 250/5/0) under dry nitrogen atmosphere in conventional three electrode configurations with tetraethylammonium perchlorate as the supporting electrolyte. A planar EG&G PARC G0228 platinum milli electrode was used as the working electrode in cyclic voltammetry. The potentials are reported here with respect to SCE (saturated calomel electrode) and are uncorrected for liquid junction potentials. Under the experimental conditions employed here, the ferrocene-ferrocenium couple appears at 0.406 V vs SCE. Constant-potential coulometry was performed using a platinum wire gauge working electrode and a PAR 377 A cell system.

Syntheses 1-(8 '-amino-a-naphthyl)-4- (8 '-amino-a-naphthyl­

amine)-1-azabuta-1,3-diene 1.5 hydrate (LH.l.5H20) -1.582 g (10 mmol) of pulverised 1,8-DAN and 1.19 g (5 mmol) of NiCh.6H20 were taken in 500 cm3 of water. To this suspension was added 1.6 cm3 ( 10 mmo1) of 1, l ,3,3-tetramethoxypropane and the reaction mixture was stirred for 2 h. The resulting bluish green mixture was then refluxed for 9 h to obtain a light green solution which was decanted in hot condition and kept in the refrigerator overnight. A shinning fluffy compound precipitated (the colour of the compound varied from greenish to beige). It was filtered, washed thoroughly with water and dried in vacuo over fused CaCh It was . stored under N2 atmosphere; yield, 0.73 g (83%); m.p. 186-190°C. It

analysed as LH.l.5H20 [Found: C, 72.92; H, 6.01; N, 14.83. Calc. for C23H23N401.5 : C, 72.79; H, 6.11; N, 14.77%]. FAB mass spectrum: m/z 389 (LH + 2H20 + W). EI mass spectrum: m/z 387 (L + + 2H20), 370 (LW + H20), 352 (LW). Calcd for LH (C23H2oN4) 352.16. IR data (em-'): 3440 (vb), 3190 (w), 3110 (w), 3040 (s), 2860 (w), 2735 (w), 2710 (w), 2370 (w), 1905 (w), 1630 (s), 1610 (s), 1590 (vs), 1480 (s), 1440 (s), 1415 (s), 1375 (vs), 1335 (s), 1295 (s), 1205 (vs), 1170 (m), 1155 (m), 1110 (m), 1080 (s), 1030 (s, split), 960 (w), 875 (s), 820 (vs), 790 (w), 755 (vs), 660 (s), 640 (m, split), 590 (m), 525 (m), 490 (m), 450 (m) .

CuLCl04.3H20 (3a)--0.44 g (1.16 mmol) of LH and 0.1 g ( 1.22 mmol) of sodium acetate were dissolved in 20 cm3 of methanol and filtered . To the filtrate, 5 cm3 methanolic solution of 0.4 g ( 1.08 mmol) of Cu(Cl04)2.6H20 was added dropwise with stirring. Immediately a green flocculent precipitate appeared. It was stirred for another 2 min and was left in the air for 15 min . The green complex was filtered and washed with 2 cm3 of cold methanol. It was dried by keeping in the air overnight and then stored under nitrogen; yield, 0.39 g (68%). It analysed as a trihydrate of CuLCl04 [Found: C, 48.72; H, 4.31; N, 9.76; Cu, 11.10. Calc. for C23H25N4CuCl07: C, 48.57; H, 4.43; N, 9.85; Cu, 11.18%]. IR data (cm-1

): 3440 (vb), 3050 (w), 2460 (w), 1630 (s), 1610 (s), 1550 (w), 1480 (s), 1420 (w), 1375 (m), 1340 (w), 1310 (w), 1280 (w), 1210 (w)1 1150 (vs), 1120 (vs), 1080 (vs), 950 (w), 835 (s), 810 .(w), 775 (m), 680 (w), 630 (s, split), 500 (b).

CuLCl04.DMF (3bf-To a 20 cm3 DMF solution of 0.3 g (0.53 mmol) of CuLC104.3H20 was added 10 cm3 aqueous solution of 0.2 g NaC104.H20 dropwise with stirring. Immediately after the addition was complete, a green compound started precipitating. The reaction mixture was left in the air for 1 h, after which the green complex was filtered, washed thoroughly with diethylether and dried in vacuo over fused CaCh. It was stored under nitrogen; yield, 0.22 g (70%). It analysed as CuLCI04.DMF [Found: C, 53.00; H, 4.42; N, 11.98; Cu, 10.73. Calc. for C26H26NsCuCIOs : C, 53.12; H, 4.46; N, 11.92; Cu, 10.82%]. IR data (cm- 1

): 3445 (vb), 1650 (s), 1625 (s), 1600 (s), 1550 (s), 1485 (s), 1360 (b), 1305 (w), 1210 (w), 1175 (w), 1145 (s), 1120 (vs, split), 1090 (s), 850 (w), 830 (m) , 770 (b), 700 (s, split), 650 (s, split).

402 INDIAN J CHEM, SEC. A, APRIL 2000

Caution-Though we have not met with any incident during our studies, care should be taken in handling these compounds as perchlorate salts are potentially explosive. These should not be prepared and stored in larger amounts.

Results and Discussion Derivatives of 1 can be prepared by various thod 189 A · · h · · me s. · · convement way IS to synt es1se Its

nickel(m complex lb by Scheme 18 and subsequent stripping of the metal (by anhydrous HCl). In Scheme 1, the half-cyclised intermediate la can be isolated in the solid state. Here we have followed Scheme 1 for our purpose. Use of l ,8-diaminonaphthalene (I ,8-DAN) and NiCb.6H20 in Scheme I leads to the isolation.of the free ligand LH (H: dissociable proton) straight away as a I.5 hydrate; the tetraaza macrocycle 2 or its nickel(ll) complex is not formed . The reaction is carried out in aqueous medium.

LH

LH.l.5H20 is moderately air-stable in solid state as well as in solution. But prolonged exposure to air should be avoided. It undergoes acid dissociation in non-aqueous solvents [reaction (I)]. We have tried to

estimate its pKa value in dimethylsulphoxide (DMSO) from its conductivity in DMSO (Table 1). The molar conductance of LH.1.5H20 increases with dilution (Table I). The range of molar conductance for I: I electrolytes in DMSO as specified by Geary 10 is 50-70 n-1 cm2 mor1

• Assuming an average molar conductance of 60 n- 1 cm2 mor1 for a I : l electrolyte in DMSO, our estimation of pKa of LH. l.5Hz0 in DMSO yields a value of 5.18 ( cf. pKa of acetic acid in water, 4.74). This acid dissociation of LH.I.5Hz0 is reflected in its NMR spectra. Not much can be inferred from the 1H-NMR spectrum of LH.l .5H20 in

1b

Scheme 1

DMSO-d6 as it consists of three sharp signals in the region 6.95-7.29 ppm, an ill resolved and very broad signal (half-width: 0.58 ppm) around 6.38 ppm and a relatively less broad signal (half-width: 0.25 ppm) at 10.56 ppm. However, the 13C NMR spectrum is quite revealing (Fig. 1); the naphthalene C atoms resonate in the region 1I5.89-I47.24 ppm (thi s assignment is done by comparing the 13C NMR spectra of I ,8-DAN in DMSO-d6), three distinct signals due to the carbon atoms marked as I, 2 and 3 in LH in reaction (1) appear at 101.82, 104.82 and 105.26 ppm, and two distinct signals due to the carbon atoms marked as 1, 2 and I ' (I and 1' are magnetically equ ivalent) in L­in reaction (I) appear at 53.52 and 61 .63 ppm. From our pKa value, the population of L- in the solution of LH. l.5H20 used for 13C NMR is estimated as - I%.

Reaction of Cu(CI04)z.6H20 with LH.I .5Hz0 in equimolar proportion in methanol gives CuLCIOdH20 (Ja) in poor yield(- 10%). But when the reaction is carried out in presence of sodium acetate, 3a is obtained in 70% yield (see Experimental Section). Attempt to recrystallise 3a from dimethylformamide (DMF) yields CuLCI04.DMF (3b). The presence of DMF in 3b is confirmed by the appearance of a strong peak at I650 cm- 1 in its IR spectra assignable to the carbonyl func tion of DMF (this peak is absent in the IR spectra of 3a). The copper(ll) complexes 3a and 3b are air sensitive in solid state as well as in solution. Freshly prepared samples of 3a and 3b are completely soluble in DMF but sparingly soluble in other polar solvents like methanol, acetonitrile etc. Their solubility in DMF decreases gradually if kept in air. However, their solubility properties are retained if stored under N2. The freshly prepared solutions of 3a and 3b are light greenish brown in colour; however, after several

LEWIS eta!.: Cu(II) COMPLEXES OF A TETRAAZA HALF-CYCLISED LIGAND 403

Table I- Conductivity data for LH.l .5H20 in DMSO at various concentrationsh

Solute Solution Molar [LHr [LT [W]e pK/ concentration conductancec conductanced

11.920 16 1.34 11.653 0 .267 0 .267 5 .21

5.960 12 2.01 5.670 0.200 0.200 5. 16

2.980 8 2.68 2.847 0 .133 0.133 5.21

1.490 6 4.03 1.390 0 . 100 0 .100 5. 14

0 .745 4 5.37 0 .678 0.067 0.067 5.18

h Various concentrations are given in M. c In mho. d In mho cm2 mol- 1

.

e Equilibrium concentration. r Avearge value is 5.1 8.

150 130 110 90 6 (ppm)

Fig. I- 300 MHz 13C NMR spectra ofLH.I .5H20 in OM SO-d~.

hours of standing in air these become intense blue­green. Both 3a and 3b show molar conductance (in degassed DMF; Table 2) less than that stipulated for a I: 1 electrolyte 10 indicating that the perchlorate anion is weakly bound to the copper atom. This is al so reflected in the IR spectra of 3a and 3b where the perchlorate anion shows very well resolved three to four vibrations in the region I 080-1150 em_, characteristic of a C3v perturbation of the anion 11

•

Macrocycles of type 1 give rise to saddle shaped complexes. This property has been utilised very

recently in a very ingenious manner to trap globular molecules like c61.t So far, we have not been able to grow single crystals of 3a or 3b. In order to find what sort of geometry is produced by our ligand LH, we have performed ZINDO (Zemer-derived Intermediate Neglect of Differential Overlap) calculations 12

, with the CAChe suite of programs available from Oxford Molecular Group Inc .13

, on the CuL + moiety, The minimum energy structure obtained is displayed in Fig. 2. The CuL+ moiety is predicted to have a very beautiful butterfly like conformation; the two

404 INDIAN J CHEM, SEC. A, APRIL 2000

naphthalene fragments span like two wings. The copper atom is predicted to lie in the N4 plane. Since all the three water molecules of 3a are replaced by a single DMF molecule upon recrystallisation of 3a from DMF to yield 3b, it is evident that no solvent molecule is coordinated to the copper atom in 3a or 3b. Considering the results of our conductivity measurements on 3a and 3b together with Fig. 2, we can say that the copper atom in 3a and 3b possibly has a square-pyramidal N40 coordination with the oxygen atom of the CI04 anion occupying the apex.

The ligand LH.l.5H20 displays only one band at 334 nm (E = 30,500 dm3 mor1 cm-1

) in its electronic spectra in DMF. The electronic spectra of 3a and 3b in degassed DMF are essentially similar with rrunor

variations in the intensities (Table 2). The spectra comprise two closely spaced humps - one at :- 623 nm and another at 550 nm, and a very intense band around 340 nm. All the electronic transitions are of charge transfer origin. The high energy band seems to be the intra-ligand -charge transfer modulated by the metal. The electronic spectra of 3a and 3b in nujol mull (Table 2) indicate,· though the high intensity band could not be observed clearly, that 3a and 3b have more or less same strucmre in the solid and solution states.

The room temperature magnetic moments of 3a and 3b correspond to one unpaired electron (Table 2) as expected. Their X-band EPR spectra in solid state at room temperature as well as at 77 K are identical -

Fig. 2- Minimum energy structure of CuV fragment obtai ned by ZINDO calcu lations. (Colour code: red , Cu; violet , N; grey, C; white, H. )

Table 2- Some physical properties of CuLCI04.3H20 (3a) and CuLC104.DMF (3b)

Property

Conductivity"

Magnetic momenth

Electronic spectrac nujol

DMF

EPR spectra solid stated

DMF-toluene glassc

"In DMF; in mho cm2 mol- 1

hAt room temperature; in flB · cAmaxlnm (Eidm3 mol- 1 cm- 1

).

d At room temperature and 77 K.

3a

44

1.71

625,540

624 (600), 550 (870),

336 (22,000)

g.l = 2.01

gJ, 2.39; g2, 2.08; gJ . 2.01;:

A3,137.9x l0-4; A2,17.4x l0-4

cAt 77 K. The A values are given in cm- 1•

3b

29

1.79

615,530

622 ( 1,080), 550 ( I ,350) ,

344 ( 18,770)

g.l = 2.01

gJ, 2.41; g2, 2.07 ; gl> 2.01

A3, 134.9x i0-4; A2, 19.3xl0-4

LEWIS et al.: Cu(II) COMPLEXES OF A TETRAAZA HALF-CYCLISED LIGAND 405

-H

Fig. 3 - X-band EPR spectra of CuLCI04.3H20 (3a). Upper trace, in solid state at 7'7 K; lower trace, in DMF-toluene glass at 77K.

60

< 20 ~

-20

-6QL---~----~-----L----~

l.O 0.6 0.2 -0.2 -0.6

E (V) vs SCE

Fig. 4 - Cyclic voltammogram of LH.l .5H20 in DMF at a platinum electrode under N2 atmosphere; solute concentration, 0.812 mmol dm-3; supporting electrolyte, 0.1 mol dm-3

tetraethylammonium perchlorate; scan rate v = I V s- 1•

sort of axial type with the gJ. component appearing at 2.01 (Table 2; Fig. 3). These resolve into rhombic spectra in DMF-toluene glass at 77 K showing clear nuclear hyperfine splitting for the two higher g factors (Table 2; Fig. 3). The ratio (g2 - g1)/(g3 - g2), which can be considered as a measure of rhombiciti 4

, is quite small for the two complexes; it is 0.187 for 3a and 0.176 for 3b. This ratio indicates that the pentacoordinate copper(II) ion in these two complexes has a ground state which is quite close to a dx2-/ type rather than a dz2 type . The fact that A3 > A2

r:-. 2 also supports a dx2 _ / ground state. For a dz ground

state [resulting from a trigonal bipyramidal geometry for a 5-coordinate copper(II)], largest A value is associated with the lowest g factor 14

. Thus, our EPR

O.lt

4- O.lt 3

-1.2

- 2,01...-----J----L----L----' 1.0 0.6 0.2 -0.2 -0.6

E IV l vs S CE

ItO {b)

< 0 3

-40

-.so 1.0 0.6 0.2 -0.2 -0·6

E :vl vs SCE

Fig. 5- Cyclic voltammograms of CuLCI04.DMF (3b) in DMF at a platinum electrode under N2 atmosphere; solute concentration, 0.507 mmol dm-3

; supporting electrolyte, 0.1 mol dm-3 tetraethylammonium perchlorate. (a) scan rate v = 0.050 Y s-1; (b) v =IV s-1.

spectra suggest that the N40 coordination sphere around the copper(II)-ion in 3a and 3b is of somewhat distorted square-pyramidal type.

We have examined the electrochemical behaviour of the ligand LH.l.5H20 and its copper(II) complexes by cyclic voltamrnetry and coulometry in DMF using platinum working electrodes under N2 atmosphere. The ligand shows quasi-reversible voltamrnograms with half-wave potential £ 112 = 0.10 V vs SCE (saturated calomel electrode). Faster scan rates yield voltamrnograms of relatively better quality (Fig. 4) . Our coulometry at 0.4 V vs SCE establishes that the electrode process is oxidative and involves two electrons. The oxidation probably arise from the amino ends as I ,8-DAN itself is known to undergo an irreversible oxidation around 0.4 V vs SCE15

• The complexes 3a and 3b . give identical voltammograms in DMF. At slow scan rates, these show two waves on the anodic side but only one wave on the cathodic side; at faster scan rates the two anodic waves coalesce (Fig. 5). At the scan rate v = 10 mV s- 1

, the two anodic peaks appear at 0.15 and 0.35 V vs SCE and the cathodic peak appears at 0.08 V vs SCE

406 INDIAN J CHEM, SEC. A, APRIL 2000

[Fig. 5(a)]. Our coulometry at 0 .6 V vs SCE shows that the observed electrochemical process is oxidative and involves three electron. Our conclusion is that in Fig. 5(a) the peaks at 0.15 and 0.08 V vs SCE cmTespond to the quasi-reversible two-electron ligand oxidation (£112 = 0.12 V vs SCE) and the peak at 0 .35 V vs SCE with no cathodic counterpart corresponds to irreversible oxidation of Cu(II) to Cu(III) . Since the metal oxidation process is irreversible, it is evident that the ligand framework cannot stabilise the copper(III) state. Nevertheless, the potential for the oxidation of Cu(II) to Cu(III) is qu ite low and deserves some comments. While to the best of our knowledge there is no data on the redox potential for the Cu(II)/Cu(III) couple in an N40 coordination sphere, thi s couple in N4 and N20 2

coordination spheres is known to occur at very low potentials ( < 0.5 V vs SCE) in some copper complexes of ligands containing anionic amide N-donors 16 and phenolate 0-donors 17

.

Concluding remarks Here we have demonstrated that like I ,2-

phenylenediamine, I ,8-diamjno-naphthalene does not yield a tetraaza macrocycle in its reaction with I , I ,3,3-tetramethoxypropane in presence of nickel(II) chloride. The reaction product is a free acyclic tetraaza ligand, LH. Thi s we call half-cyclisation . The ligand birrds copper(II) in its monoanionic form giving rise to butterfly shaped air sensitive copper(II) complexes, Their sensitivity towards air can be attributed to their low oxidation potentials . The ligand framework is incapable of stabilising copper(III). This is possibly because the ligand itself gets oxidised before the metal oxidation takes place. In the perchlorate salts of our synthesised copper(II) complexes of LH, the anion is found to be weakly bound to the metal giving rise to penta coordinate complexes. This is in line with the general characteristics of the saddle shaped complexes which show a marked tendency to yield penta coordinate

. 9 spec1es .

So far, all our attempts. to synthes ise Ni(II) complexes of LH have failed. Since LH does not form any complex with Ni(II), the reaction of I ,8-DAN with I , I ,3,3-tetramethoxypropane in presence of a nickei(II) template does not proceed a ll the way (see Scheme I) to yield our desired macrocycle 2. It is noted that use of a copper(II) template instead of a nickel(II) one does not bring about even the half­cyclisation.

Acknowledgement DO wishes to thank the Depa1tment of Science and

Technology, New Delhi for financial support.

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and refs therein . 3 Eilmes J, Polyhedron, II ( 1992) 581. 4 Ricciardi G, Bavaso A, Rosa A, Lelj F & Cizov Y, 1 chern

Soc Dalton Trans , ( 1995) 2385. 5 Schumann H, lnorg Chern, 35 (1996) 1808. 6 Martin A, Uhrammer R, Gardner T G, Jo rdan R F & Rogers

R D, Organomelallics, 17 ( 1998) 382. 7 Andrews P C. Atwood L J, Barbour L 1, Nichols P J &

Raston C L, Chern Eur 1, 4 ( 1998) 1384. 8 Cutler A R & Dolphin D, 1 coo rei Chern , 6 ( 1976) 59 and

refs therein. 9 Goedken Y L & Weiss M C, lnorg Synlh , 20 ( 1980) 115.

10 Geary W J, Coord Chern Rev, 7 (1971 ) 81. II Parkovic S F & Meek D W, lnorg Chem, 4 ( 1965) I 09 1;

Wickenden, A E & Krause R A, lnorg Chem, 4( 1965) 404; Hathaway B 1 & Underhill A E, 1 chem Soc, (1961) 3091 ; Baldwin D A & Leigh G ), 1 Chem Soc (A), (1968) 1431 .

12 Anderson W P, Cundari T R, Drago R S & Zerner M C, lnorg Chem, 29 ( 1990) I and refs therein .

13 Oxford Molecular Group Inc, P 0 Box 4003, Beaverton, Oregon 97076, USA.

14 Ray N & Hathaway B J, 1 chem Sr>c Dalton Trans , ( 1980) I I 05 and refs therein.

15 Bagheri A & Na1eghi M R, Indian J Chetn, 37 A ( 1998) 606 and refs therein .

I 6 Levason W & Spicer M D, Coord Chem Rev, 76 ( 1987) 45 . 17 Abu-El-Wafa S M, lssa R M & McAul iffe C A, lnorg chim

Acta, 99 ( I 985) I 03; Anson F C, Collins T 1, Richmond T G, Santarsiero B D, Toth J E & Trcco B G R T , JA m chem Soc, I 09 ( 1987) 2974.