Hydrogen Bonding and Isomerism Arising from the Coordination Modes of Bridging Benzimidazole-2-thiolate Ligands in Tetranuclear Rhodium Complexes.

Reaction of the dinuclear complex [Rh(2)(&mgr;-HBzimt)(2)(cod)(2)] with [Rh(2)(&mgr;-Cl)(2)(cod)(2)] (cod = 1,5-cyclooctadiene) gives the neutral tetranuclear complex [Rh(4)(&mgr;-HBzimt)(2)Cl(2)(cod)(4)] (2) in dichloromethane and the trinuclear cationic complex [Rh(3)(&mgr;-HBzimt)(2)(cod)(3)]Cl (3) in methanol, respectively. The ionization ability of the solvent seems to be the driving force to give 3, while the ability to coordinate a further RhCl(cod) fragment leads to 2 in poorer ionizing media. The complexes [M(4)(&mgr;-HBzimt)(2)Cl(2)(diolefin)(4)] (M = Rh, diolefin = tetrafluorobenzobarrelene (tfbb) (5); M = Ir, diolefin = cod (6)), formally analogous to 2, were isolated from the reactions of the appropriate complexes [MCl(H(2)Bzimt)(diolefin)] and [M(acac)(diolefin)] in acetone. A X-ray diffraction study on 2 shows the HBzimt(-) ligands to bridge two rhodium atoms through the sulfurs, forming a basic four-membered Rh(2)(&mgr;-(1:2kappaS)-HBzimt)(2) ring along with two RhCl(cod) moieties bonded to the nitrogen atoms. Two intramolecular hydrogen bonds between the chloro ligands and the acidic NH protons should stabilize the syn-endo disposition of the thiolate type bridging ligands. Replacement of the olefin in 2 by carbon monoxide gives [Rh(4)(&mgr;-HBzimt)(2)Cl(2)(cod)(CO)(6)] and [Rh(4)(&mgr;-HBzimt)(2)Cl(2)(CO)(8)] (7) depending on the reaction conditions. The X-ray structure of 7 shows the HBzimt(-) ligands in a HT-Rh(2)(&mgr;-(1kappaN,2kappaS)-HBzimt)(2) disposition with two RhCl(CO)(2) fragments coordinated to the sulfur atoms. In addition, two tetranuclear units 7 are associated in a dimer through four intermolecular 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 ln K(eq) versus 1/T, which gives DeltaH = 43.3 kJ mol(-)(1) and DeltaS = 114.7 J K(-)(1) mol(-)(1). Deprotonation of 7 with [Rh(2)(&mgr;-OMe)(2)(cod)(2)] gives the hexanuclear complex [Rh(6)(&mgr;-Bzimt)(2)(&mgr;-Cl)(2)(cod)(2)(CO)(8)] (10). Complexes 7 and 10 show identical conformations of the eight-membered HT-Rh(2)(&mgr;-(1kappaN,2kappaS)-Bzimt)(2) metallacycle and identical configurations of the sulfur atoms.