Ru(ii)–CNC pincer complexes with bis(protic-NHC): comparative analysis with bis(classical-NHC) and mixed protic- & classical-NHC analogues

Abstract

This report concludes our efforts on the synthesis of a series of Ru(II)–CNC pincer complexes with stepwise replacement of classical-NHCs by protic-NHCs. Building upon our work on Ru(II)–CNC pincer complexes with classical-NHCs (1a–d) and mixed protic- & classical-NHCs (2a–d), a new set of air- and moisture-stable ruthenium(II)–CNC pincer complexes (3a–d), bearing two protic-NHC ligands, has been synthesized via high-temperature reactions of bis(imidazolyl)- and bis(benzimidazolyl)pyridine ligands with Ru(III)–NHC precursors. All new complexes have been characterized by multinuclear NMR spectroscopy, HRMS, and the molecular structure of 3a has been determined by single-crystal X-ray diffraction. Comparative UV-vis and electrochemical analyses indicate that bis(protic-NHC) complexes display red-shifted absorption bands and lower oxidation potentials relative to their mono-protic- and classical-NHC counterparts, despite possessing less electron-rich metal centres. DFT calculations attribute this trend to greater solvation stabilization of the oxidized bis(protic-NHC) species. This study presents a direct comparison between mixed- and bis(protic-NHC)-Ru pincer complexes, providing valuable insights into the change in the electronic structure and redox behaviour upon the stepwise replacement of classical-NHC with protic-NHC.