Phosphine-free Ru(ii)–CNC pincer complexes with mixed protic- and classical-NHCs in the same molecule for hydrogen production via oxidant-free benzyl alcohol dehydrogenation to benzoic acids

Abstract

Following our recent strategy of using suitably substituted ligand precursors for the synthesis of Ru(II)–CNC pincer complexes [Ru(C^HNC^Me)(PPh₃)₂Cl]Cl (1 and 2) with mixed protic- and classical-NHCs in the same molecule, synthesis of new phosphine-free complexes [Ru(C^HNC^Me)(CN^Me)I]PF₆ (3), [Ru(C^HNC^Me)(CN^i-Pr)I]PF₆ (4), [Ru(C^HNC^Ad)(CN^Me)I]PF₆ (5), and [Ru(C^HNC^Ad)(CN^i-Pr)I]PF₆ (6) has been achieved starting from our in-house made Ru(III)–NHC complexes [Ru(CN^R′)(H₂O)(Cl)₃] (R′ = Me (P1), i-Pr (P2)) as precursors. This is the first report of phosphine-free Ru-complexes having mixed, protic- and classical-NHC within the same pincer ligand platform. We also note that this synthetic strategy gives racemic mixtures of chiral-at-the-metal complexes from nonchiral precursors. All new complexes have been characterized as racemic mixtures using multinuclear NMR (¹H, ³¹P{¹H}, ¹³C{¹H} NMR) and high-resolution mass spectrometry (HRMS). The molecular structure of the racemic complex 6 has been determined by single-crystal X-ray diffraction and was found to contain both enantiomers in the same lattice. The catalytic activity of these complexes for oxidant-free, acceptorless dehydrogenation of benzyl alcohols to benzoic acids has been explored, revealing superior activity of complex 4 among the new complexes. Catalyst 4 was then screened for a wide range of substrates, including aliphatic, aromatic, and heteroaromatic benzyl alcohols, to give their corresponding carboxylic acids. Mechanistic investigations help identify some crucial intermediates during the catalytic reaction, and a plausible mechanism has been proposed. A maximum TON of 20 000 has been observed, greater than several previously reported ruthenium catalysts.