SYNTHESIS AND X-RAY STUDIES OF RUTHENIUM(II) COMPLEXES CONTAINING HYDRAZINE AND BENZYL ISOCYANIDE LIGANDS

The reaction of the polymeric species [{RuCl2(COD)}x] (1 ; x > 2; COD = cyclo-octa-1,5-diene) and hydrazine hydrate in methanol under reflux gave a pale pink solution from which the salt [Ru(COD)(N2H4)4][BPh4]2.CH3OH (2) was isolated on addition of NaBPh4. Treatment of 2 in refluxing acetone in the presence of the ligand benzyl isocyanide give a complex of stoichiometry [Ru(NH2N=CMe2)2(PhCH2NC)4][BPh4]2 (3) on the substitution of the labile COD ligand. The two compounds have been characterized by elemental analyses, IR and NMR measurements and single-crystal X-ray diffraction studies. The ruthenium in both compounds has a distorted octahedral coordination geometry.


INTRODUCTION
The coordination chemistry of η 4 -diene type ligands such as 1,5-cyclooctadiene (COD) and 2,5norbornadiene (NBD) relies on their ability to act as labile ligands [1].While the COD could bind to a metal via one (η 2 ) or two (η 4 ) of its double bonds it generally binds as an η 4 ligand.The polymeric compound [{RuCl 2 (COD) x }] has attracted particular attention due to the lability of the η 4 -coordinated COD ligands, which are able to undergo exchange reactions under very mild conditions [2].This unique property has been widely applied in numerous ligand exchange processes, making the polymer an ideal precursor for the synthesis of an extensive range of active ruthenium(II) compounds [3][4][5][6].
There is continued interest in the study of the chemistry of transition metal complexes containing hydrazine (NH 2 NH 2 ) or substituted hydrazine (RNHNH 2 ) as ligands.This is partly due to the interest in different coordination modes offered by hydrazine complexes [7][8][9][10] and also to the importance of hydrazine in the dinitrogen fixation process [11].The coordination of hydrazine to metal centers is usually through the nitrogen in a monodentate fashion but in a few cases, a bridging bidentate coordination mode can be adopted [11].To accurately determine the mode of coordination in these complexes, X-ray crystallographic analysis is required.

General procedures
The starting material [{RuCl 2 (COD)} x ] 1 was prepared as described previously [4].Hydrazine hydrate was obtained commercially from SAARCHEM (PTY) LTD, South Africa and was not further purified, while benzyl isocyanide was purchased from Sigma-Aldrich.The reactions involving benzyl isocyanide were carried out under a purified nitrogen atmosphere using standard Schlenk techniques while all other reactions were performed in air.Melting points were obtained on a Kofler hot-stage apparatus and were uncorrected.Infrared spectra of both compounds were recorded in the solid state (ATR) on a Bruker Tensor 27 FTIR spectrophotometer.The 1 H and 13 C{ 1 H}NMR NMR spectra of 2 were recorded on a Bruker DRX 250 spectrometer while that of 3 was recorded on a Bruker 300 MHz instrument at 298 K.The 1 H and 13 C{ 1 H}chemical shifts were calibrated to solvent peaks, which are reported relative to TMS.Microanalyses were carried out at the Center for Nanotechnology, Department of Chemistry, Rhodes University, South Africa.

Structure solution and refinement of compounds 2 and 3
The crystal structure determination of compounds 2 and 3 were carried out using a Bruker SMART APEX2 CCD-based X-ray diffractometer equipped with a low-temperature device and Mo-target X-ray tube (wavelength = 0.71073 Å).The X-ray data were collected at 100(2) K. Data collection, indexing, and initial cell refinements were carried out using APEX2 [13], with the frame integrations and final cell refinements carried out using SAINT [14].Absorption corrections were applied using SADABS [15], and all non-hydrogen atoms were refined anisotropically.Hydrogen atoms of the NH 2 moieties in compound 3 were located from the difference Fourier map and refined isotropically.The remaining hydrogen atoms were placed in idealized positions and were refined using a riding model.The structures were solved and refined using the SHELXTL program package software [16].Refinement details and structural parameters for compounds 2 and 3 are summarized in Table 1.One phenyl ring from a BPh 4 moiety in compound 2 was modelled disordered over two positions in a 68:32 ratio.The Ru atom was also modelled as disordered over two positions in a 89:11 ratio.

RESULTS AND DISCUSSION
The hydrazine complex  Tetraphenylborate was the only counter anion that effected precipitation and attempts to isolate the cations of 2 with other anions such as [BF 4 ] -and [PF 6 ] -were not successful, consistent with earlier reports [3].Complex 2 is stable in air and light for a period of six weeks after which slow decomposition starts to occur.It is soluble in solvents such as dimethyl sulphoxide (DMSO), acetone and dimethylformamide but not soluble in ethanol, methanol, dichloromethane and other common organic solvents.
Suitable crystals of 2 were grown from hydrazine hydrate/methanol mixture.The compound crystallizes in the monoclinic space group P2 1 /c with the unit cell dimensions a = 17.711, b = 11.518 and c = 24.886Å.A molecular diagram showing the numbering scheme of the compound 1, with selected bond lengths and angles in Table 2.The Ru(II) metal center is octahedrally coordinated to the cyclooctadiene ligand and four H 2 NNH 2 ligands.The angles around the Ru(II) atom are all close to 90°, in accordance with a distorted octahedral coordination sphere.The COD ligand adopts the usual boat or tub conformation with the four doubly bonded C atoms having an average bond distance of 2.208 Å which is slightly longer than bonding distance of Ru (2.14-2.19)[17].An average Ru(II)-NH 2 bond distance of 2.15 Å was observed, slightly longer than in compound 3.An average N-N distance of 1.45 Å was observed.The Ru(II)-N-N angles were found to be 114° on average.These values are all comparable to those of compound 3.No special features were observed for the tetraphenylborate anions.A molecule of methanol as well as a hydrate was observed in the asymmetric unit of 2.
Various attempts to substitute the ligands in these complexes with other groups such as CO, pyridine and CO 2 were not successful.Suitable crystals of compound 3 were grown from a methylene chloride/ethanol mixture in the ratio 1:4.The compound crystallizes in the monoclinic space group P2 1 /n with the unit cell dimension a = 13.4108,b = 19.156and c = 14.2231Å.The molecular diagram showing the numbering scheme for compound 3 is presented in Figure 2, with selected bond distances and angles in Table 3.The two hydrazone ligands are mutually trans to each other and the bonding to the ruthenium atom is through the expected amino N atoms (Figure 2).The isocyanide ligands are cis to one another and bonded to the ruthenium through the lone pair on the C atom.
The N-N distance of 1.45 Å in this Ru cation is shorter than the value of 1.51 Å reported in the literature for [OsCl(COD)(CN t Bu)(NH 2 N:CMe 2 ) 2 ] 2+ [6] but falls within the reported range of 1.45-1.48Å [18].The average Ru-NH 2 distance of 2.14 Å and Ru-N-N angles of 115 o in this complex are rather small when compared with the reported metal-NH 2 and metal-N-N angles of 2.17 Å and 119 o in [Ru(NH 2 N:CMe 2 ) 2 {P(OMe) 3 } 4 ] 2+ [19].In the hydrazone complex [W(η-C 5 H 5 )(CO) 3 (NH 2 N:CMe 2 ) 2 ] 2+ , the metal-NH 2 distance is 2.27 Å and the metal-N-N angle is 112 o .While the variation in the M-N distances in these complexes could be attributed to the differences in covalent radii of these metals, the wide variation in their bond angles remain difficult to explain but could be due to crystal packing effects.
Stirring a suspension of [{RuCl 2 (COD)} x ] (1) in boiling methanol under reflux for 5 min with hydrazine hydrate rapidly resulted in a pale red solution from which the salt [Ru(COD)(N 2 H 4 ) 4 ][(BPh 4 ) 2 ].CH 3 OH (Figure1) was isolated in high yield on addition of a methanolic solution of Na[BPh 4 ] (equation 1).Thus, it is not necessary to use anhydrous hydrazine to prepare complex 2, as described in the earlier reports.

Table 1 .
Crystal data and structure refinement for complexes 2 and 3.