Catalytic Activity of Diphenyl‐ <i>N</i> ‐carbazolyl Phosphine‐Based Ruthenium (II) <i>p</i> ‐Cymene Complex Toward Hydrogen Borrowing Reactions

Abstract The use of phosphine‐based transition metal complexes in hydrogen borrowing (HB) reactions at low catalyst loading and milder conditions is challenging due to their air‐sensitive nature. In order to address this issue, an air‐stable Ru(II) complex, [( p ‐cymene)RuCl 2 (PPh 2 Cbz)] , (Ru‐PCbz) (Cbz = N ‐carbazolyl), has been synthesized from diphenyl‐ N ‐carbazolyl phosphine (PPh 2 Cbz) and [Ru( p ‐cymene)( μ ‐Cl)Cl] 2 , and characterized using various spectroscopic and analytical techniques. The molecular structure of the complex has been determined through single crystal X‐ray diffraction analysis. Catalytic investigations reveal that Ru‐PCbz is both versatile and highly efficient in performing transfer hydrogenation of ketones, dehydrogenation of secondary alcohols, and N‐methylation of anilines. In order to unravel the role of a carbazolyl substituent in the observed catalytic activity of Ru‐PCbz , catalytic studies were also conducted with triphenyl phosphine (PPh 3 ) bearing Ru(II) complex [( p ‐cymene)RuCl 2 (PPh 3 )] (Ru‐PPh) as the control. The results reveal that Ru‐PCbz significantly outperforms Ru‐PPh for all the three catalytic transformations investigated. By performing other appropriate control experiments, most plausible mechanistic pathways for these reactions using Ru‐PCbz have been proposed, often by trapping the intermediates using various spectroscopic techniques.