Comparative Coordination Chemistry of PNP and SNS Pincer Ruthenium Complexes

Ruthenium carbonyl complexes supported by PNP pincer ligands are prominent catalysts for a range of hydrogenation and dehydrogenation reactions. Recently, Ru complexes with cheaper, more air stable SNS pincer ligands have emerged as attractive alternatives for the development of improved catalysts. However, there is currently a paucity of information on how the replacement of the phosphine donors in PNP ligands with the sulfur donors in SNS ligands influences the synthesis, structure, and electronic properties of the resulting metal complexes. Herein, the coordination chemistry of a series of Ru carbonyl complexes with SNS pincer ligands has been systematically compared with related PNP-ligated species. Three different SNS pincer ligands were explored including a pyridyl based NC5H3{CH2(StBu)}2 ligand and two aliphatic ligands, HN­{CH2CH2(StBu)}2 and NCH3{CH2CH2(StBu)}2, along with different combinations of monodentate ancillary ligands. The geometric structures of the SNS and PNP Ru complexes were studied using NMR spectroscopy and X-ray crystallography. Additionally, the redox properties and electronic structures of these complexes were probed through a combination of cyclic voltammetry and DFT calculations. Overall, differences between SNS and PNP complexes extend well beyond simply modulating inductive donation to the metal and include changes in synthetic outcomes, as well as variations in geometry that impact redox behavior. Our study reveals fundamental information about the coordination chemistry of the SNS ligand, which may aid in interpreting catalytic results.