Terphenyl Complexes of Molybdenum and Tungsten with Quadruple Metal-Metal Bonds and Bridging Carboxylate Ligands

Mono- and bis-terphenyl complexes of molybdenum and tungsten with general composition M-2(Ar'(O2CR)(3) and M-2(Ar')(2)(O2CR)(2), respectively (Ar' = terphenyl ligand), that contain carboxylate groups bridging the quadruply bonded metal atoms, have been prepared and structurally characterized. The new compounds stem from the reactions of the dimetal tetracarboxylates, M-2(O2CR)(4) (M = Mo, R = H, Me, CF3; M = W, R = CF3) with the lithium salts of the appropriate terphenyl groups (Ar' = Ar-xy12, Ar-Mes2, Ar-Dipp2, and Ar-Trip2). Substitution of one bidentate carboxylate by a monodentate terphenyl forms a M-C sigma bond and creates a coordination unsaturation at the other metal atom. Hence in M-2(Ar')(2)(O2CR)(2) complexes the two metal atoms have formally a low coordination number and an also low electron count. However, the unsaturation seems to be compensated by a weak M-C-arene bonding interaction that implicates one of the aryl substituents of the terphenyl central aryl ring, as revealed by X-ray studies performed with some of these complexes and by theoretical calculations.