Synthesis and Study of 5,5′-Bibenzimidazolylidenes and Their Bimetallic Complexes

A new, high-yielding synthesis of 3,3',4,4'-tetrachlorobiphenyl was developed, facilitating access to a new family of fluorescent 5,5'-bibenzimidazolium salts bearing phenyl or bulky tertiary alkyl N-substituents. Deprotonation of these salts afforded the respective 5,5'-bibenzimidazolylidenes in excellent isolated yields (>= 86 %), which were characterized in the solid state and solution. Treatment of these ditopic ligands with [Rh(COD)Cl](2). (COD = 1,5-cyclooctadiene) afforded bimetallic complexes composed of two Rh atoms connected through a bis(carbene) linker. Electrochemical analyses of these complexes revealed single, quasi-reversible oxidations at E-1/2 = +0.54 to +0.59 V (vs. SCE) arising from the Rh-I/II redox couples. A bimetallic Rh carbonyl complex bearing a 5,5'-bibenzimidazolylidene was also synthesized and found to exhibit v(CO) signals similar to a monotopic analogue. Collectively, these results suggested that the two metal centers in the aforementioned bimetallic complexes were electronically decoupled. These observations were supported by the crystal structures of one of the 5,5'-bibenzimidazolium salts and its bis(carbene) derivative, which revealed relatively large dihedral angles of 51.2 degrees and 47.8 degrees, respectively, between the two aryl systems. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)