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

Reaction of the dinuclear complex [Rh2(μ-HBzimt)2(cod)2] with [Rh2(μ-Cl)2(cod)2] (cod = 1,5-cyclooctadiene)
gives the neutral tetranuclear complex [Rh4(μ-HBzimt)2Cl2(cod)4] (2) in dichloromethane and the trinuclear cationic
complex [Rh3(μ-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 [M4(μ-HBzimt)2Cl2(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(H2Bzimt)(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 Rh2(μ-(1:2κS)-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 [Rh4(μ-HBzimt)2Cl2(cod)(CO)6] and [Rh4(μ-HBzimt)2Cl2(CO)8] (7) depending on the reaction conditions. The X-ray structure of
7 shows the HBzimt- ligands in a HT-Rh2(μ-(1κN,2κS)-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 Keq versus 1/T, which gives ΔH = 43.3 kJ
mol-1 and ΔS = 114.7 J K-1 mol-1. Deprotonation of 7 with [Rh2(μ-OMe)2(cod)2] gives the hexanuclear complex
[Rh6(μ-Bzimt)2(μ-Cl)2(cod)2(CO)8] (10). Complexes 7 and 10 show identical conformations of the eight-membered
HT-Rh2(μ-(1κN,2κS)-Bzimt)2 metallacycle and identical configurations of the sulfur atoms.