Efficient Catalyst for Acceptorless Alcohol Dehydrogenation: Interplay of Theoretical and Experimental Studies

Abstract

The promoterless AAD (acceptorless alcohol dehydrogenation) reaction mediated by an iridium catalyst Cp*Ir(bpyO) 1–Ir (Cp* = penta­methyl­cyclo­pentadienyl, bpyO = α,α′-bipyridonate) has been theoretically investigated with the density functional theory. The reaction occurs through three steps, including alcohol dehydrogenation, formation of dihydrogen complex, and H2 elimination from the iridium center. In the first two steps, the metal center and the bpyO ligand work cooperatively via the aromatization/dearomatization process of the bpyO ligand. The second step is rate-determining, where the ΔG 0≠ and ΔG 0 values are 23.9 and 13.9 kcal/mol, respectively. Our calculations demonstrate that the aromatization of the bpyO ligand as well as the charge transfer (CT) from the Cp* ligand to the iridium center plays important roles in stabilizing the transition state of the rate-determining step. We have theoretically and experimentally examined the 4d rhodium analogue Cp*Rh(bpyO) 1–Rh and found that it exhibits similar activity to that of 1–Ir. On the basis of those results, a new catalyst (HMB)Ru(bpyO) 1–Ru (HMB = hexamethyl­benzene) is designed both theoretically and experimentally, where a cheaper and more abundant 4d ruthenium element is employed with the HMB and bpyO ligands. Theoretical calculations certainly show that 1–Ru is active for the promoterless AAD reaction via the same reaction mechanism as that of the reaction by 1–Ir. The experiments also demonstrate that 1–Ru is as efficient as 1–Ir for the AAD reaction.

Mechanism of N,N,N-Amide Ruthenium(II) Hydride Mediated Acceptorless Alcohol Dehydrogenation: Inner-Sphere β-H Elimination versus Outer-Sphere Bifunctional Metal–Ligand Cooperativity

“…Ru(HbMepi)(PPh 3 )Cl[PF 6 ]. THF (3 mL) was added to a vial charged with HbMepi (51.6 mg, 0.158 mmol), RuCl 2 (PPh 3 ) 3 (137.4 mg, 0.143 mmol), TlPF 6 (50.1 mg, 0.143 mmol), and a stir bar.…”

“…Yield: 94 mg (75%). 6 ]. THF (5 mL) was added to a 20 mL vial charged with Ru(HbMepi)(PPh 3 )Cl[PF 6 ] (10 mg, 0.0115 mmol), NaO t Bu (1.1 mg, 0.0115 mmol), and a stir bar.…”

“…Synthesis and crystal structure (thermal ellipsoids of 5 depicted at 50% probability) of Ru(HbMepi)(PPh 3 )Cl[PF6 ]. The outer-sphere PF 6 anion, PPh 3 phenyl groups, and hydrogen atoms, except the NH, are omitted for clarity.…”

Reversible catalytic dehydrogenation of alcohols for energy storage

“…Homogeneous catalysts for the acceptorless dehydrogenation of primary and secondary alcohols for the most part contain precious metals, such as Ru (19), Rh (20), and Ir (21). By comparison,…”

Metall‐Ligand‐Kooperation

“…Der Mechanismus der akzeptorfreien Alkohol‐Dehydrierung mit 166 b ′ als Katalysator wurde durch DFT‐Rechnungen untersucht, die ergaben, dass der konzertierte Außensphärenmechanismus günstiger ist als ein Innensphärenmechanismus mit β‐Hydrid‐Eliminierung 133…”