Hydrogen elimination reactivity of ruthenium pincer hydride complexes: a DFT study

Abstract

The tridentate rigid coordination of a pincer ligand in meridional configuration is expected to enhance the catalytic reactivity of a pincer transition metal complex compared to a non-pincer complex composed of a similar ligand environment. Here a DFT study is conducted on a Ru(II) PNN type Milstein catalyst and a large variety of trans-hydrido carbonyl Ru(II) pincer complexes having XZY-type pincer ligands (‘pincer atoms’ X and Y are selected from N, O, S, and P and Z is either C or N) to quantify the reactivity of the water splitting hydrogen elimination reaction. A homodesmotic reaction scheme in conjunction with molecular electrostatic potential (MESP) analysis is used to quantify and characterize the stabilizing/destabilizing effect of the pincer coordination in comparison with non-pincer coordination. The pincer type coordination always led to a more electron-rich ruthenium centre than the non-pincer coordination. The H₂ elimination mechanism revealed for the Ru(II) complexes showed a decreasing activation energy (ΔE^#) trend with an increase in the electron rich character of the metal centre which is quantified in terms of the MESP at Ru(II), V_Ru. Pincer complexes having pincer atom ZN showed a ΔE^# of 17.5–21.1 kcal mol⁻¹ (NNN is an exception) for the rate determining H₂ elimination step while those having ZC showed a ΔE^# of 6.9–13.0 kcal mol⁻¹. These results suggest that benzene based XCY type pincer ligands can be more effective than pyridine based XNY type ligands for designing water splitting catalysts.