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

• Published in: New journal of chemistry Vol. 43; no. 36; pp. 14634 - 14642
• Main Authors: Remya, Geetha S, Suresh, Cherumuttathu H
• Format: Journal Article
• Language: English
• Published: Cambridge Royal Society of Chemistry 2019
• Subjects: Atomic properties; Benzene; Carbonyls; Catalysis; Catalysts; Coordination compounds; Hydrogen storage; Ligands; Reactivity; Ruthenium compounds; Transition metal compounds; Water splitting
• ISSN: 1144-0546; 1369-9261
• DOI: 10.1039/c9nj03100f

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 H 2 elimination mechanism revealed for the Ru( ii ) complexes showed a decreasing activation energy (Δ E # ) 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 Z&z.dbd;N showed a Δ E # of 17.5-21.1 kcal mol −1 ( NNN is an exception) for the rate determining H 2 elimination step while those having Z&z.dbd;C showed a Δ E # 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. The pincer effect is explained for various pincer hydride complexes, differing in the donor atoms, using activation barriers, and MESP parameters.