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.