Axial binding of 2-methylimidazole to the tetraarylcarboxylatodiruthenium (II, III) core: X-ray crystal structure of [Ru₂(O₂CC₆H₄-<em>p</em>-OMe)₄ (2-mimH)₂](CIO₄). 1.75CH<sub>2<

Axial binding of 2-methylimidazole to the tetraarylcarboxylatodiruthenium (II, III) core: X-ray crystal structure of [Ru2(02CC6H4-p-OMe)4

(2-mimH)2](CI04). 1.75CH2C12.H20

Chellamma Sudha &Akhil R Chakravarty Department of Inorganic and Physical Chemistry

Indian Institute of Science. Bangalore 560 012 Received 5 August 1997: revised 24 October 1997

Diruthenium (II. III) complexes of the type [RU2(02CAr)4 (2- mimH12)(CI04) (Ar =C6H4-P-X :X=OMe.l: X=Me.

2: 2-mimH=2- methylimidazole) have been isolated from the reaction of RU2CI(02CAr)4 with 2- mimH inCH2Ch followed by the addition of NaCI04. The crystal structure of 1. 1.75CH2Cb.H20 has been determined, The crystal belongs to the monoclinic space group p211c with the following unit cell dimensions for the C4oH40N4016CIRu2. J.75CH2Cb.H20 (M= 1237.0) :a= 12.347 (3)A. b= 17615(5)A,c = 26.148(2)A.p =9288(lt v = 5679(2)A'. Z=4.De= 1.45 g cm-'. ).(Mo- Ka) = 0.7\07 A. ~(Mo-Ka) = 8.1 em-I. T= 293 K. R = 0.0815 (wR- = 0.2118) for 5834 reflections with 1>2 0(1). The complex has atetracarboxylatodiruthenium (II. III) core and two axially bound 2-methylimidazole ligands. The Ru-Ru bond length is 2.290( I)A. The Ru-Ru bond order is 2.5 and the complex is three-electron paramagnetic. The complex shows an irreversible RU2(11,I1I)~ RU2(1I.II) reduction near -0.2 Vvs SCE inCH2C1i-0.1 MTBAP. The complexes exemplify the first adduct of the tetracarboxylatodiruthenium (11.111) core having N-donor ligands.

Diruthenium (11,111)tetracarboxylates with a spin- quarte ground state and a Ru-Ru bond order of 2.5 are known to form axial adducts with only weakly bound aqua or halide ligands!". The stability of the core structure depends primarily upon the nature of the terminal (axial) and bridging (equatorial) ligands.

Axial n-acceptor ligands weaken the ruthenium-ru- thenium multiple bond and the core becomes suscep- tible to conversion in presence of N- and P-donor ligands

8-

12. It has been observed

I I

that in presence of N-donor bases like imidazole or I-methylimidazole, RU2(~-02CRt4 core readily converts to the RU2(~- O)(~-02CRh2+ unit. However, a similar reaction with 2-methylimidazole leads to the formation of an unusual axial adduct. Herein we report the synthesis and structural characterization of [RU2(02CAr)4(2- mimHh](CI04) (Ar

=

C6H4-p-OMeJ;

^C6H4-p-Me, 2).

Materials and Methods

Solvents and reagents were used as obtained from commercial sources. Precursor complexes were pre- pared by literature procedure". 2-Methylimidazole was of Fluka make. Preparation of tetrabutylam-

monium perchlorate (TBAP) and purification of di-

ch loromethane were carried out by conventional methods. Elemental analyses were done on a Heraeus

CHN-O Rapid instrument. Conductivity measure- ments were made on a Century CC603 digital con- ductivity meter. Cyclic voltammetric measurements were made at 25°C on a EG&G PAR 253 Versastat PotentiostatiGalvanostat with a three-electrode set- up using platinum disc working, platinum wire aux- iliary and saturated calomel reference (SCE) electrodes. The electrochemical data were uncor- rected for junction potentials. Measurements were made in CH2Ch containing 0.1 MTBAP. Ferrocene was used as an internal standard to verify the poten- tials against the SCE and to obtain the electron trans- fer stoichiometries of the redox process by peak- current measurements. Constant potential electroly- sis was done to determine the nature of the redox process. Room temperature magnetic susceptibility measurements were carried out using George Asso- ciates model 3000 Lewis-coil force magnetometer.

Hg[Co(NCS)4] was used as the calibrant. The dia- magnetic corrections for complexes

1

and 2 were 496 x 10-

6

and 482 x 10-

6

cm

3

mol", respectively.

Synthesis oj[RU2(02CAr)4(2-mimH)2} (CI04) (Ar

=

C6H4-P-X X= OMe. 1; Me. 2)

RU2Cl (02CAr)4 (0.2 g, - 0.25 mmol) in CH2Ch

(20 em:') was reacted with 2-mimH (0.06, - 0.7

mmol) under stirring condition at 25°C for 30 min.

2

INDIAN J CHEM, SEC. A, JANUARY 1998

The brown solution thus obtained was stirred with NaCI04 (0.04g, - 0.33 mmol) for 10 min, filtered and the brown product was precipitated by the addition of petroleum ether (bp 60-80°C). The solid was iso- lated and dried in vacuo (yield - 70%). [Found: C, 45.1; H, 3.7; N, 5.3. Calc. for C4o!tJoN4016CIRu2: C, 44.9; H, 3.7; N, 5.2%; Found: C, 47.8; H, 4.1; N, 5.8.

Calc. for C4oH40N4012CIRu2: C, 47.7; H, 4.0; N, 5.6%].

Safety Note: Perchlorate salts of metal complexes containing organic ligands are potentially explosive and should be handled with great caution using only small amounts of the materials.

X-ray crystallographic procedures

Single crystals of 1 were obtained by diffusion technique using a dichloromethane solution of 1 and petroleum ether. The crystals were found to be unsta- ble on removal from the mother liquor. Hence a brown coloured rectangular crystal (approximate di- mension: 0.20 x 0.08 x 0.06 mm) was mounted inside a Lindemann capillary along with the mother liquor.

The unit cell parameters were obtained from least-squares treatment of25 reflections in the range 18 < 28 < 32°. Intensity data (hk±l) were collected for 0 ::; h s 14, 0 s k s 20, -31 ::; 1 s 31 with theta range 1.39 to 24.98° using co-scan on an Enraf- Nonius CAD-4 diffractometer equipped with a graphite monochromated Mo-K, radiation. The data were corrected for Lorentz, polarisation and absorp- tion effects 13.

Out of 10700 reflections collected, there were 9976 unique reflections of which 5834 reflections with I >20'(1) were used for structure solution using Patterson method which revealed the position of the metals in the crystallographic asymmetric unit [R(int) =0.0538]. The remaining atoms were located from the difference Fourier maps and refined by least-squares techniques. There were positionally disordered lattice solvent molecules that were best modelled as 1.75 CH2Ch and H20. The source of water could be the solvents used for crystallization of 1. All non-hydrogen atoms ofthe complex cation and the perchlorate anion except those of the positionally disordered solvent molecules were refined anisot- ropically. The hydrogen atoms attached to the atoms in the complex were generated and assigned isotropic thermal parameters, riding on their parent atoms and used fOI' structure factor (F2) calculation only. The full-matrix least-squares refinement on F2 converged to R = 0.0815 and WR2 =

[L[w(Fo2-

Fc2)/L w~02)2]]112

=

0.2118 with a weighting scheme w

=

[0' (F02) + (0.1385 p)2 + 45.83 P}

-I

where P

=

[max.

(Fo2,0) + 2Fc2]3and using 622 parameters [R indices

(all data): R

=

0.1498, wR2

=

0.2720; F(OOO) = 2498].

The structure did not show any chemically suspecting features in the final difference Fourier map which showed the largest peak and hole as 1.102 and -1.190 eA-3. The goodness-of-fit on F2 was 1.11. All calcu- lations were carried out using VAX88 and IBM com- puter systems at the Indian Institute of Science using SHELXS-86 and SHELXL-93 programs 14. Atomic scattering factors were taken from ref. 15. The posi- tional parameters along with equivalent isotropic thermal parameters are given in Table I. The hydro- gen atom coordinates, full lists of bond distances and angles, anisotropic thermal parameters, and the struc- ture factor tables are available with the authors on request.

Crystal data

The cell constants and crystallographic data are:

mol. formula C4oH40N4016CIRu2. 1.75CH2Ch.H20;

mol. wt. 1237.0; monoclinic

^{P2I1c; a}

= 12.347(3)A;

b = 17.612(5)A; c = 26. 148(2)A; [3= 92.88(1)0; V=

5679(2)A3; Z =4; D, = 1.45 g cm'; J..L(Mo-K

^a)

= 8.1 em"; A = 0.7107 A, T= 293 K; transmission coeff. : 0.99-1.16.

Results and Discussion

The precursor complex RU2CI(02CAr)4 is a poly- meric species. The RU2(02CAr)4 units are covalentli linked into an infinite chain by the chloride Iigands

^I..

The axial Ru-CI bond is ca. 2.6 A in length. The reaction of RU2CI(02CAr)4 with P- and N-donor Iigands in an alcoholic or acetonitrile medium leads to the cleavage of the RU2(02CAr)4 core. The reac- tion with PPh3 readily forms RU20(02CAr)4(PPh3h·

Similarly, the reaction with 1- methylimidazole or imidazole forms complexes with a [RU2(J..L-O)(J..L- 02CArhf+ core

lO•ll.

The RU2(02CAr)4+ to [RU2(J..L- O)(J..L- 02CArhf+ core conversion has been found to be facile and quantitative. However, from a similar reaction between RU2CI(02CAr)4 and 2-methylimi- dazole we have observed a selective substitution of the axial chloride to form the diaxial adduct RU2CI(02CAr)4(2-mimHht, isolated as a per- chlorate salt (Ar = C6H4-P-X; X = OMe, 1; X = Me, 2). The complexes, obtained in high yield are stable in the solid as well as in solution and are 1:1 electro- lytes giving

^AM

values of 145 for

¹

and 132 mho cm2 mol" for 2 in MeCN. The complexes are three-elec- tron paramagnetic giving

^/lefT

value of 4.2 for 1 and 3.9 BM for 2 at 25°C. This suggests a ~1t482(8·1t ·)3 electronic configuration for the ground state with a metal-metal bond order of2.5.

The complexes are redox active and undergo a

one-electron irreversible reduction RU2(1I, III) ~

Table I - Fractional atfmic coordinates and the isotropic Atom x y z Ueq~iSO)

(A )8 thermal parametes (x10 ) for the non-hydrogen atoms In

[Ru2( 02CC6H4 -p-OMe )4(2-mim)2]( CI04 ).1.75CH2Ch. H2O

C(25) 0.3896(9) 0.6577 (7) 0.1936(4) 48 (3) (I,I.75CH2Ch.H20)

0.41 09( I 0) 0.5978 (7) 0.1559 (5) 52 (3) with their standard deviations (e.s.d.s) in parenthesis C(26)

C(27) 0.3999(1l) 0.5218 (7) 0.1710 (4) 57 (3)

y z Ueq~iSO) C(28) 0.4155(11) 0.4635 (7) 0.1355 (5) 63 (3)

Atom x

(A )8

C(29) 0.4431(11) 0.4816 (8) 0.0846 (5) 62 (3) Ru(l) 0.33504(7) 0.71822 (5) 0.29032 (3) 40 (0.3)

C(30) 0.4506(12) 0.5568 (7) 0.0708 (5) 65 (3) Ru(2) 0.35987(7) 0.80943 (5) 0.22947 (3) 40 (0.3)

C(31) 0.4365( II) 0.6142 (7) 0.1049(4) 60 (3) 0(1) 0.1775(6) 0.7124 (5) 0.2657 (3) 50 (2)

0(12) 0.4591(9) 0.4285 (5) 0.0482 (3) 75 (3) 0(2) 0.2011(7) 0.8006 (5) 0.2066 (3) 55 (2)

C(32) 0.4453(15) 0.3517 (8) 0.0618 (5) 83 (3) 0(3) 0.4930(6) 0.7268 (5) 0.3125 (3) 51 (2)

N(I>, 0.3102(10) 0.6315 (6) 0.3513 (4) 79 (3) 0(4) 0.5169(6) 0.8171(5) 0.2545 (3) 53 (2)

C(33) 0.2764(18) 0.5669 (5) 0.3535 (12) '210 (3) 0(5) 0.3013(7) 0.8004 (5) 0.3403 (3) 53 (2)

N(2) 0.2997(11) 0.5375 (6) 0.4045 (5) 85 (3) 0(6) 0.3240(7) 0.8902 (5) 0.2813 (3) 57 (2)

C(34) 0.3745(20) 0.5958 (18) 0.4332 (8) 173 (4)

1

^{0(7) 0.3700 (6) 0.6377 (4) 0.2393 (3) 50 (2)} C(35) 0.3722(20) 0.6517(11) 0.4068 (8) 145 (3)

0(8) 0.3947(7) 0.7273 (5) 0.1794(3) 51 (2)

C(36) 0.2226(19) 0.5253 (15) 0.3189(14) 228 (4) C(I) 0.1395(9) 0.7541 (7) 0.2303 (4) 46 (3)

N(3) 0.3949(8) 0.9078 (6) 0.1767 (4) 62 (3) C(2) 0.0242(9) 0.7468 (6) 0.2126 (4) 44 (3)

C(37) 0.3770(11) 0.9186 (9) 0.1286(6) 80 (3) C(3) -0.0391(10) 0.6939 (7) 0.2343 (5) 56 (3)

N(4) 0.4118(11) 0.9914 (7) 0.1160(4) 86 (3)

j

C(4) -0.1456(11) 0.6896 (8) 0.2183 (5) 65 (3)

C(38) 0.4461(13) 1.0271 (9) 0.1599 (7) 91 (3)

I

C(5) -0.1944(10) 0.7386 (7) 0.1835 (3) 53 (3)

C(39) 0.4380(12) 0.9768 (9) 0.1967 (5) 75 (3)

,

I

C(6) -0.1284(10) 0.7925 (8) 0.1611(5) 64 (3)

C(40) 0.3375(14) 0.8637 (9) 0.0898 (6) 93 (3)

I

C(7) -0.0213(12) 0.7957 (8) 0.1745 (5) 62 (3)

CI(I) 0.3320(4) 0.3801 (2) 0.4995 (I) 98 (2)

I

I

^{0(9)C(8) 0.3021(7)0.3567(11) 0.7325 (5)0.7893 (9) 0.1741 (3)0.1424 (5) 67 (3)76 (3) 0(13)0(14) 0.2573(11)0.4125(16) 0.3859 (7)}0.3271 (12) ^{0.4565 (4)}0.4871 (8) 210 (4)^{117 (3)} C(9) 0.5527(9) 0.7735 (7) 0.2901 (4) 45 (3)

0(15) 0.3785(16) 0.4507 (7) 0.5090 (5) 175 (3) C(IO) 0.6708(9) 0.7819 (7) 0.3070 (4) 50 (2)

0(16) 0.2902(20) 0.3562 (13) 0.5426 (6) 271 (3) C(II) 0.7134(10) 0.7342 (8) 0.3460 (5) 65 (3)

CI(2) 0.9406(7) 0.0165 (5) 0.7552 (3) 182 (3) C(12) 0.8222(12) 0.7412 (9) 0.3622 (3) 73 (2)

CI(3)' 0.9239(15) -0.0022 (II) 0.8699 (7) 207 (5) C(13) 0.8845(11) 0.7942 (II) 0.3371 (6) 88 (3)

C(3')

•

_0.8701(15) -0.0450 (II) 0.8434 (7) 209 (5) C(14) 0.8417(12) 0.8426 (9) 0.2999 (6) 80 (3)

C(41) 0.8557(17) 0.0100 (14) 0.8028 (9) 139 (5) C(15) 0.7369(10) 0.8361 (7) 0.2854 (5) 54 (3)

CI(4)' 0.8858(11) 0.0569(8) 0.3380 (5) 158 (4) 0(10) 0.9946(9) 0.803

i

(8) 0.3500 (5) 112 (3)

CI(5)

•

_0.9102(15) -0.0282 (II) 0.4288 (7) 224 (5) C(16) 1.0487(16) 0.7480 (15) 0.3819 (8) 147 (4)

C(42)" 0.9581(29) 0.0126 (22) 0.3717 (15) 120 (6) C(17) 0.3037(10) 0.8698 (6) 0.3277 (5) 49 (3)

CI(6)# 0.9759(21) 0.2003 (16) 0.9674 (10) 157 (5) C(18) 0.2836(10) 0.9302 (7) 0.3650 (4) 51 (3)

CI(7)# 0.8969(32) 0.3191 (25) 0.9383 (16) 261 (6) C(19) 0.2656(13) 0.9107 (8) 0.4148 (6) 77 (3)

C(43)# 0.9364(40) 0.2620 (33) 0.9602 (23) 87 (6) C(20) 0.2463(15) 0.9639 (9) 0.4504 (6) 91 (3)

0(17)

"

0.9134(21) 0.0872 (16) 0.0194 (10) 135 (5) C(21) 0.2455(12) 1.0430 (8) 0.4348 (6) 75 (3)

0(18)

"

1.2323(21) -0.2765 (16) 1.0170 (10) 129 (5) C(22) 0.2663(12) 1.0625 (8) 0.3862 (6) 73 (3)

C(23) 0.2856(11) 1.0058 (8) 0.3537 (5) 63 (3) 8 The expression for the equivalent isotropic thermal parame-

0(11) 0.2264(12) 0.0919 (7) 0.4744 (5) 123 (3)

^{ter isUeq =[LiLij. ai.aj.ai.aj¥3.} " Atoms were refine~ with. a site occupancy factor of 0.5. Atoms were refined With a site C(24) 0.2292(12) 0.1.1640 (14) 0.4632 (9) 147 (4) occupancy factor of 0.25.

Contd

4 INDIAN J CHEM, SEC. A, JANUARY 1998

Table 2 - Selected bond distance (Aand angles (deg) for [RU2(02CC6H4-p-OMe)4 (2-mimH)2](CI04). 1.75CH2Cb.H2.

Ru( I) -Ru(2) Ru(1) -0(1) Ru( I) -0(3) Ru(I)-0(5) Ru(l) -0(7) Ru(I)-N(I) Ru(2) -0(2) Ru(2) -0(4) Ru(2) -0(6) Ru(2) -O(S) Ru(2) -N(3) 0(1) -C(l) 0(2) -C(I) 0(3) -C(9) 0(4) -C(9) 0(5) -C(17) 0(6) -C(17)

0(1) -C( I)

0(7) -C(2S) O(S) -C(2S)

Bond distancestA) N(I)-C(33) N( I)-C(33) N(2)-C(33) N(2)-C(34) C(34)-C(3S) C(33)-C(36) N(3)-C(37) N(3)-C(30 N(4)-C(3S) N(4)-C(37) C(3S)-C(39) C(37)-C( 40) C(1 )-C(2) C(9)-C(10) C( 17)-C( IS) C(25)-C(26) C I( I )-O(13) C(37)-C( 40) C I( I )-C( 14) CI(l)-C( IS) CI(I) - 0(16) N(3) - Ru(2) -0(6) N(3) - Ru(2) -0(8) 0(2) - Ru(2) -0(4) 0(2) - Ru(2) -0(6) 0(2) - Ru(2) -0(8) 0(4) - Ru(2) -0(6) 0(4) - Ru(2) -0(8) 0(6) - Ru(2) -0(8) N( I) - C(33) -N(2) N(I) - C(33) -C36(6) C(36) -C(33) -N(2) C(33) -N(2) -C(34) C(35) -C(34) -N(2) C(34) -C(35) -N(I) C(33) -N(I) -C(35) C(37) -N(3) -C(39) N(3) -C(37) -N(4) C(37) -N(4) -C(38) N(4) -C(38) -C(39) C(38) -C(39) -N(3) N(3) -C(37) -C(40) N(4)'-C(37) -C(40) 2.290( I)

2.020( I) 2.027(7) 2.022(7) 2.019(7) 2.248(9) 2.031(7) 2.030(7) 2.027(7) 2.028(7) 2.290(9) 1.265( 12) : .283( 12) 1.278(12) 1.260( 12) I. 284( 12) 1.287( 12) 1.265(12) 1.274(11) 1.266(12)

Ru( I) - Ru(2) -0(2) Ru( I) - Ru(2) -0(4) Ru(1) - Ru(2) -0(6) Ru(l) - Ru(2) -0(8) Ru( I) - Ru(2) -N(3) Ru(2) -Ru( I) -O( I) Ru(2) - Ru(1) -0(3) Ru(2) - Ru(l) -0(5) Ru(2) - Ru( I) -0(7) Ru(2) - Ru(l) -N(I) N(I) - Ru(l) -0(1) N(I) - Ru(l) -0(3) N(I) - Ru(\) -0(5) N(I) - Ru(l) -0(7) 0(1) - Ru( I) -0(3) 0(1) - Ru(I) -O(S) 0(1) - Ru( I) -0(7) 0(3) - Ru( I) -0(5) 0(3) - Ru(l) -0(7) 0(5) - Ru(l) -0(7) N(3) - Ru(2) -0(2) N(3) - Ru(2) -0(4)

89.6(2) 89.0(2) 89.2(2) 89.1(2) 173.3(3 ) 88.9(2) 89.2(2) 89.6(2) 89.3(2) 178.4(3) 92.1(4) 89.8(4) 89.1(3) 92.0(3) 177.7(3) 90.9(3) 89.3(3) 89.3(3) 90.4(3)

\78.8(3) 94.8(3) 86.6(3)

1.21(2) I.S8(3) 1.37(3) 1.57(3) 1.13(3) 1.38(3) 1.28(2) 1.40(2) 1.36(2) 1.31(2) 1.33(2) 1.47(2) 1.469(\4) 1.485( 14) 1.47(2) 1.47(2) 1.422( II) 1.47(2) 1.44(2) 1.393( 13) 1.31(2) 8S.7(3) 96.1(3) 178.2(3) 90.6(3) 89.3(3) 88.6(3) 91.5(3 )

178.6(3) 114(3) 124(2) 122(2) 103(2) 107(3) 111(3) 108(2) 105.3(10) 109.2(13) 108.2( 13) 105.9(14) 110.4(13) 128.1(4) 122.5( 13)

((15)

Fig. I - An ORTEP view of [RU2(02CC6H4-p-OMe)4(2- miml-lj-]" cation alongwith atom numbering scheme showing the thermal ellipsoids at the 50%probability level.

RU2(II,II) near -0.2V vs SCE in CH2Cb-0.1

M

TBAP at 20-200 m V s". Diruthenium (II, III) tetracarboxy- lates with axial halide ligands are known to exhibit a quasireversible (L\Ep - 250m V) reduction near 0.0 V vs SCE. Since addition of an electron takes place in the antibonding level, it is expected to reduce the metal-metal bond order and the reduced species may undergo chemical conversion. This is particularly true as the Ru-Ru and Ru-L(axial) distances in the diruthenium(II,III) core are considerably different from those observed in the diruthenium(I1,III) com-

plexes'.

The molecular structure of 1 has been established by an X-ray diffraction study. An ORTEpI6 view of the cationic complex is shown in Fig. I. Selected bond distances and angles are given in Table 2. In the cationic complex, the diruthenium unit is held by four

syn-syn

bridging carboxylates and 2-mimH ligands occupy the axial sites of the core. Each ruthenium atom has a near octahedral geometry. The N-Ru(I)-O angles lie between 88 and 92°. The variation is greater for the N-Ru(2)-O angles giving values between 85 and 96°. The

cis

and

trans

O-Ru-O angles range between 88 and 91°, and 178 and 179° respectively.

The Ru-Ru distance of 2.290( I)A is similar to that found in analogous complexes having a metal-metal bond order of 2.5 (ref. I). The axial Ru-N distances are 2.240( II ) and 2.270( II )A. The equatorial Ru-O distances are in the range of 2.007-2.032

A.

A notable structural feature is the

cis

orientation of the methyl group of 2-mimH with respect to the Ru-Ru bond. A close proximity of the NH group of an imidazole ring with an oxygen atom of the per- chlorate anion has been observed in the crystal pack- ing diagram. The N(2) ....O( 13) distance of 3.06

A

indicates the presence of a weak hydrogen bonding interaction involving the cationic species and the anion.

The reactivity of 1and 2 with l-rnethylimidazole (I-mim) has been studied. The reaction monitored by visible spectroscopy shows an isosbestic point at 478 nm. The yield of the product [RU20(02C Ar)2( 1- mim)6](CI04h is quantitative. The effect of the methyl substituent of the imidazole ring ⁰¹¹ the sta- bility of the tribridged core structure has been studied by molecular modelling using BIOSYM INSIGHT in Silicon Graphics workstations at the Il Sc Com- puter Centre. It has been observed that [RU20(02CAr)2(2- mimH)6]2+ should have severe steric crowding caused by the close proximity of the methyl groups of the terminal2-mimH ligands. Such a complex is thus expected to be unstable and the conversion of the [Ru2(1l-02Ar)4(2-mimHhr core to the [Ru2(1l-0)(1l-02CAr)2(2-mimH)6f+ unit may not be a favourable process. For analogous imidazole.

l-rnethyl imidazole or 4-methylimidazole bases. the tribridged species are stable and there is no apparent steric crowding observed at the fac ia I term inaI sites II. In the core conversion reaction. formation of the diaxial adduct is the first intermediate step. The adduct undergoes further reaction in presence of imi- dazole bases to form the tribridged species. The stcric bulk of the 2-mimH gives the unusual stabilitx ofthe diaxial adduct and complexes 1 and 2 exempt it\ the first adduct of the tetracarboxylatodiruthcn ium (II,III) core having N-donor ligands.

Acknowledgement

We thank Dr NY Vasanthacharya forthe magnetic data; the Alexander van Humboldt Foundation (Ger- many) for a donation of an

EG&G PAR electroana-

Iytical system; and the Department of Science and Technology, Government of India. for financial sup- port.

6 _INDIAN

_JCHEM, SEC. A, JANUARY 1998

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