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, κN²-coordination of MeCN ₄^- was observed when the ancillary ligand was 2,2′-bipyridine (bpy): [Cp*M(bpy)(MeCN₄-κN ²)]PF₆ (Cp* = η⁵-C₅Me ₅; M = Ir: 1 and Rh: 2), while the corresponding complexes with 2-(2′-pyridyl)phenyl (ppy^-) were confirmed to have the κN¹-coordination of MeCN₄^-: [Cp*M(ppy)(MeCN₄-κN¹)] (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 κN¹- and κN²-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-κN¹(M′-ppy): κN³(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 η²:η²-intermediate without dissociation of Cp*M(bpy or ppy) fragments.