Rhodium(I) complexes bearing N-donor ligands: catalytic activity towards intramolecular cyclization of alkynoic acids and ligand lability

The structures of rhodium(I) complexes bearing tris(pyrazol-1-yl) toluidine (tpt) and tris(N-methylimidazol-2-yl) methanol (tim) ligands were examined in the solid state using single crystal X-ray diffraction, and in the solution state using variable temperature NMR spectroscopy. The solid state structures of the rhodium(I) tpt and rhodium(I) tim complexes showed that the ligands are bound to the rhodium(I) centre in the kappa(2) binding mode, rather than the kappa(3) binding mode. In the solution state, rhodium(I) complexes bearing the tpt ligand undergo fluxional behaviour at room temperature, which was attributed to rotation of the toluidine substituent about the C-C bond or the equilibrium between kappa(2) and kappa(3) binding modes. At low temperatures, rhodium(I) complexes bearing the tpt ligand adopted the kappa(2) binding mode, consistent with the coordination mode in the solid state structures. The efficiency of the complexes as catalysts for the intramolecular hydroalkoxylation of 4-pentynoic acid and 5-hexaynoic acid to form the corresponding lactone was established. The presence of the third unbound N-donor was shown to reduce the catalytic efficiency of the complexes with tridentate ligands when compared to their counterparts bearing bidentate ligands, due to either the steric hindrance or competitive binding of the third N-donor with the substrate during the catalytic cycle.