The effect of ligand substituents on spectroscopic and catalytic properties of water-compatible Cp*Ir-(pyridinylmethyl)sulfonamide-based transfer hydrogenation catalysts

Transition metal-based hydrogenation catalysts have applications ranging from high-value chemical synthesis to medicinal chemistry. A series of (pyridinylmethyl)sulfonamide ligands substituted with electron-withdrawing and -donating groups was synthesized to study the influence of the electronic contribution of the bidentate ligand in Cp*Ir piano-stool complexes. A variable temperature NMR investigation revealed a strong correlation between the electron-donating ability of the substituent and the rate of stereo-inversion of the complexes. This correlation was partially reflected in the catalytic activity of the corresponding catalysts. Complexes with electron-withdrawing substituents followed the trend observed in the variable temperature NMR study, thereby confirming the rate-determining step to be donation of the hydride ligand. Strongly electron-donating groups, on the other hand, caused a change in the rate-determining step to the formation of the iridium-hydride species. These results demonstrate that the activity of these catalysts can be tuned systematically via changes in the electronic contribution of the bidentate (pyridinylmethyl)sulfonamide ligands.