Ruthenium p ‐Cymene Iminophosphonamide Complexes: Activation under Basic Conditions and Transfer Hydrogenation Catalysis

Complex [(η 6 ‐Cym)RuCl( NPN )] {Cym = p ‐cymene; NPN = ( p TolN) 2 PPh 2 } ( 1 ) yields a thermally sensitive hydride derivative [(η 6 ‐Cym)RuH( NPN )] ( 2 ) by reaction with i PrOH in the presence of a strong base, via an observable isopropoxide intermediate [(η 6 ‐Cym)Ru(O i Pr)( NPN )] ( 3 ), or with NaBHEt 3 in toluene. Partial conversion also occurs in i PrOH in the absence of base. 2 is stabilized by dihydrogen bonding with isopropyl alcohol, but attempts to isolate it induce isomerization by hydride migration to a ring CH position to yield a 16‐electron cyclohexadienyl derivative [{η 5 ‐ p ‐C 6 H 5 (Me)( i Pr)}Ru( NPN )], which has been crystallographically characterized as a disordered mixture of two regioisomers ( 4 / 4′ ). Complex 2 is able to release H 2 upon treatment with medium strength proton donors (fluorinated alcohols), but also slowly with i PrOH. 2 is an active catalyst for the transfer hydrogenation of acetophenone to phenylethanol in isopropyl alcohol. The catalytic transformation is first order in acetophenone and first order in catalyst, with k = 117 ± 10 m –1  h –1 at 40 °C. The temperature dependence of the rate constant (25–80 °C) gave the activation parameters Δ H ‡ = 9.6 ± 1.3 kcal mol –1 and Δ S ‡ = –31 ± 4 cal mol –1  K –1 . DFT calculations have validated the slow isomerization of 2 to 4 / 4′ (high energy TS), the preference of the cyclohexadienyl system for 4 / 4′ relative to the other isomers 4 Me and 4 i Pr , where the hydride has migrated to the CMe or C i Pr position, and suggest that the hydrogen transfer mechanism involves outer sphere hydride transfer to the ketone substrate with H‐bonding assistance of isopropyl alcohol to yield a σ complex intermediate [(η 6 ‐Cym)Ru + ( NPN ){H‐C(Me)(Ph)O – }].