Activation of Carbon Monoxide by (Me₃tpa)Rh and (Me₃tpa)Ir

Cationic iridium and rhodium carbonyl complexes supported with the tetradentate N-donor ligand Me3tpa (Me3tpa = N,N,N-tri(6-methyl-2-pyridylmethyl)amine) were synthesized. Their structures were investigated using X-ray diffraction, NMR, and IR, and their redox properties were probed using cyclic voltammetry and EPR spectroscopy. Iridium forms a trigonal bipyramidal 18 VE biscarbonyl complex, while rhodium forms a square-planar 16 VE monocarbonyl species. In both compounds the Me3tpa ligand coordinates in a κ3 rather than the expected κ4 mode and is fluxional on the NMR time scale. Complex [Ir(κ3-Me3tpa)(CO)2]PF6 (2+) reacts at room temperature with methanol and water to form the methoxycarbonyl iridium compound [Ir(κ3-Me3tpa)(CO)(H)(COOMe)]PF6 (4+) and the hydroxycarbonyl iridium compound [Ir(κ3-Me3tpa)(CO)(H)(COOH)]PF6 (5+), respectively. The hydroxycarbonyl iridium complex 5+ is not stable and decomposes to bis-hydride iridium complex [Ir(κ3-Me3tpa)(CO)(H)2]PF6 (6+), thus illustrating subsequent steps of the water gas shift reaction. Complex 2+ is converted to [Ir(κ4-Me3tpa)(CO)(H)](PF6)2 (7+) upon one-electron oxidation with AgPF6. Rhodium complex [Rh(κ3-Me3tpa)(CO)]PF6 (3+) is selectively oxygenated by O2 in the presence of water to form the carbonato rhodium complex [Rh(κ3-Me3tpa)(CO3)]PF6 (8+). This represents a rare example of formation of a stable rhodium carbonate complex via oxygenation of a coordinated CO molecule.