HYDROGEN-BONDING AND ISOMERISM ARISING FROM THE COORDINATION MODES OFBRIDGING BENZIMIDAZOLE-2-THIOLATE LIGANDS IN TETRANUCLEAR RHODIUM COMPLEXES

Reaction of the dinuclear complex [Rh-2(mu-HBzimt)(2)(cod)(2)] with [R h-2(mu-Cl)(2)(cod)(2)] (cod = 1,5-cyclooctadiene) gives the neutral te tranuclear complex [Rh-4(mu-HBzimt)(2)Cl-2(cod)(4)] (2) in dichloromet hane and the trinuclear cationic complex [Rh-3(mu-HBzimt)(2)(cod)(3)]C l (3) in methanol, respectively. The ionization ability of the solvent seems to be the driving force to give 3, while the ability to coordin ate a further RhCl(cod) fragment leads to 2 in poorer ionizing media. The complexes [M-4(mu-HBzimt)(2)Cl-2(diolefin)(4)] (M = Rh, diolefin = tetrafluorobenzobarrelene (tfbb) (5); M = Ir, diolefin = cod (6)), fo rmally analogous to 2, were isolated from the reactions of the appropr iate complexes [MCl(H(2)Bzimt)(diolefin)] and [M(acac)(diolefin)] in a cetone. A X-ray diffraction study on 2 shows the HBzimt(-) ligands to bridge two rhodium atoms through the sulfurs, forming a basic four-mem bered Rh-2(1:2 kappa S)-HBzimt)(2) ring along with two RhCl(cod) moiet ies bonded to the nitrogen atoms. Two intramolecular hydrogen bonds be tween the chloro ligands and the acidic NH protons should stabilize th e syn-endo disposition of the thiolate type bridging Ligands. Replacem ent of the olefin in 2 by carbon monoxide gives [Rh-4(mu-HBzimt)(2)Cl- 2(cod)(CO)(6)] and [Rh-4(mu-HBzimt)(2)Cl-2(CO)(8)] (7) depending on th e reaction conditions. The X-ray structure of 7 shows the HBzimt(-) li gands in a HT-Rh-2(mu-(1 kappa N,2 kappa S)-HBzimt)(2) disposition wit h tyro RhCl(CO)(2) fragments coordinated to the sulfur atoms. In addit ion, two tetranuclear units 7 are associated in a dimer through four i ntermolecular hydrogen bonds. This association occurs even in solution , where the two species are observed. The equilibrium constant for the dissociation fits a linear plot of In K-eq versus 1/T, which gives De lta H = 43.3 kJ mol(-1) and Delta S = 114.7 J K-1 mol(-1). Deprotonati on of 7 with [Rh-2(mu-OMe)(2)(cod)(2)] gives the hexanuclear complex [ Rh-6(mu-Bzimt)(2)(mu-Cl)(2)(cod)(2)(CO)(8)] (10). Complexes 7 and 10 s how identical conformations of the eight-membered HT-Rh-2(mu-(1 kappa N,2 kappa S)-Bzimt)(2) metallacycle and identical configurations of th e sulfur atoms.