Comparison of ancillary ligand effects between 2,2'-bipyridine and 2-(2'-pyridyl)phenyl in the linkage and bridging isomerism of 5-methyltetrazolato iridium(III) and/or rhodium(III) complexes.

Several new iridium(III) and rhodium(III) complexes bearing 5-methyltetrazolate (MeCN4(-)) have been prepared, and their structures in the crystals and in solution have been determined by X-ray analysis and by NMR spectroscopy, respectively. In the crystals of the mononuclear complexes, κN(2)-coordination of MeCN4(-) was observed when the ancillary ligand was 2,2'-bipyridine (bpy): [Cp*M(bpy)(MeCN4-κN(2))]PF6 (Cp* = η(5)-C5Me5; M = Ir: 1 and Rh: 2), while the corresponding complexes with 2-(2'-pyridyl)phenyl (ppy(-)) were confirmed to have the κN(1)-coordination of MeCN4(-): [Cp*M(ppy)(MeCN4-κN(1))] (M = Ir: 3 and Rh: 4). In solution, the Ir(III) complexes (1 and 3) were robust enough to maintain their molecular structures, but the Rh(III) complexes (2 and 4) existed as an equilibrium mixture of the κN(1)- and κN(2)-isomers. In addition to the Ir(III)-Ir(III) and Rh(III)-Rh(III) homodinuclear complexes bridged by MeCN4(-) (5-8), the corresponding heterodinuclear Ir(III)-Rh(III) complexes (9-12) were prepared using the mononuclear Ir(III) complexes (1 and 3) as precursors. The molecular structures of these dinuclear complexes were also characterised. Interestingly, both of the heterodinuclear complexes comprised of Cp*M(bpy)(2+) and Cp*M'(ppy)(+) fragments, [Cp*M(bpy)(μ-MeCN4)M'(ppy)Cp*](PF6)2 (M = Ir, M' = Rh: 10 and M = Rh, M' = Ir: 11), exhibited selective crystallisation of a specific μ-κN(1)(M'-ppy):κN(3)(M-bpy) isomer. In solution, the dinuclear complexes with a Rh-N(MeCN4) bond and more than one positive charge (6 and 9-11) showed a dissociation equilibrium, while monocationic MeCN4-bridged complexes (7, 8 and 12) were inactive for dissociation. Furthermore, the heterodinuclear complexes of 9-12, as well as the Rh(III)-Rh(III) complex of 8, exhibited a bridging isomerisation, which would proceed via a η(2):η(2)-intermediate without dissociation of Cp*M(bpy or ppy) fragments.