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(mu-HBzimt)(2)(cod)(2)] with [Rh-2(mu-Cl)(2)(cod)(2)] (cod = 1,5-cyclooctadiene) gives the neutral tetranuclear complex [Rh-4(mu-HBzimt)(2)Cl-2(cod)(4)] (2) in dichloromethane and the trinuclear cationic complex [Rh-3(mu-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(mu-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(1:2 kappa S)-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(mu-HBzimt)(2)Cl-2(cod)(CO)(6)] and [Rh-4(mu-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(mu-(1 kappa N,2 kappa S)-HBzimt)(2) disposition with tyro 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 In K-eq versus 1/T, which gives Delta H = 43.3 kJ mol(-1) and Delta S = 114.7 J K-1 mol(-1). Deprotonation 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 show identical conformations of the eight-membered HT-Rh-2(mu-(1 kappa N,2 kappa S)-Bzimt)(2) metallacycle and identical configurations of the sulfur atoms.