Mono- and binuclear bipyridyl derivatives of the Mo(η³-C ₃H₅)(CO)₂ fragment: Structural studies and fluxionality in solution

New mono- and binuclear complexes of the Mo(η³-C ₃H₅)(CO)₂ fragment, containing bipyridyl ligands (2,2′-bpy, 4,4′-Me₂-2, 2′-bpy) as chelates, and mono- (4-CNpy, 4-Mepy, NCMe, Br) or bidentate nitrogen ligands (4,4′-bpy, bipyridylethylene, pyrazine) as terminal or bridging ligands, respectively, were prepared. The binuclear complex [{Mo(η³-C₃H₅) (CO)₂(2,2′-bpy)}₂(μ-4,4′-bpy)] [PF₆]₂ (2) was shown by X-ray diffraction to assemble in the crystal forming large channels with a rectangular section. A longer bridge, such as bipyridylethylene, led to a different structure (3). 4-CNpy behaved as monodentate ligand (4), coordinating through the pyridine nitrogen as a terminal ligand. NMR spectroscopy studies showed that the complexes exhibited a fluxional behavior in solution, the endo and exo forms of the more symmetrical equatorial isomers being usually present and interconverting in solution. The solid state structures of the complexes revealed a preference for the more symmetrical equatorial isomer, with the two chelate ligands in trans positions in the binuclear species. The rings tended to become parallel in the organized crystal.