Synthesis and study of 5,5'-bibenzimidazolylidenes and their bimetallic complexes

Abstract

A new, high-yielding synthesis of 3, 3-,4,4-tetrachlorobi-phenyl was developed, facilitating access to a new family offluorescent 5,5-bibenzimidazolium salts bearing phenyl or bulky tertiary alkyl N-substituents. Deprotonation of thesesalts afforded the respective 5, 5-bibenzimidazolylidenes inexcellent isolated yields (86%), which were characterizedin the solid state and solution. Treatment of these ditopic li-gands with [Rh(COD)Cl] 2 (COD = 1,5-cyclooctadiene) af-forded bimetallic complexes composed of two Rh atoms con-nected through a bis(carbene) linker. Electrochemical analy-ses of these complexes revealed single, quasi-reversible oxi-dations at E1/2 = +0.54 to +0.59 V (vs. SCE) arising from theRhI/IIredox couples. A bimetallic Rh carbonyl complex bear-ing a 5, 5-bibenzimidazolylidene was also synthesized andfound to exhibit ?CO signals similar to a monotopic analogue. Collectively, these results suggested that the two metal cen-ters in the aforementioned bimetallic complexes were elec-tronically decoupled. These observations were supported bythe crystal structures of one of the 5, 5-bibenzimidazoliumsalts and its bis (carbene) derivative, which revealed rela-tively large dihedral angles of 51.2° and 47.8°, respectively,between the two aryl systems. © Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009.