Intramolecular CH/OH Bond Cleavage with Water and Alcohol Using a Phosphine‐Free Ruthenium Carbene NCN Pincer Complex

Abstract

Transition metal complexes that exhibit metal-ligand cooperative reactivity could be suitable candidates for applications in water splitting. Ideally, the ligands around the metal should not contain oxidizable donor atoms, such as phosphines. With this goal in mind, we report new phosphine-free ruthenium NCN pincer complexes with a central N-heterocyclic carbene donor and methylpyridyl N-donors. Reaction with base generates a neutral, dearomatized alkoxo-amido complex, which has been structurally and spectroscopically characterized. The tert-butoxide ligand facilitates regioselective, intramolecular proton transfer through a CH/OH bond cleavage process occurring at room temperature. Kinetic and thermodynamic data have been obtained by VT NMR experiments; DFT calculations support the observed behavior. Isolation and structural characterization of a doubly dearomatized phosphine complex also strongly supports our mechanistic proposal. The alkoxo-amido complex reacts with water to form a dearomatized ruthenium hydroxide complex, a first step towards phosphine-free metal-ligand cooperative water splitting.

Metal–Ligand Cooperation

“…96 Activation of water by an NCN pincer complex containing a central NHC ligand and two pyridine arms was also reported by Morris et al. Dearomatization of one or both Py arms by a CH 2 Py group deprotonation was observed in these systems 97…”

Optimum bifunctionality in a 2-(2-pyridyl-2-ol)-1,10-phenanthroline based ruthenium complex for transfer hydrogenation of ketones and nitriles: impact of the number of 2-hydroxypyridine fragments

“…7,8 Remarkably, bio-inspired complexes have emerged as a privileged class of compounds in organometallic chemistry. Inspired by the pioneering work in this field of Noyori, [11][12][13][14] Shvo,15,16 Morris, [17][18][19][20][21] and Milstein, [22][23][24][25] new generations of bifunctional catalysts have been developed. Inspired by the pioneering work in this field of Noyori, [11][12][13][14] Shvo,15,16 Morris, [17][18][19][20][21] and Milstein, [22][23][24][25] new generations of bifunctional catalysts have been developed.…”

Ruthenium(II) Complexes Having a Pincer‐Type Ligand with Two N‐Heterocyclic Carbene Moieties

“…These values are longer than those of Ru–C bonds (1.98–2.00 Å) observed in other Ru II –NHC complexes. [3d], This indicates the strong trans influences provided by strong σ donating NHC ligands that are positioned mutually at the trans position …”