Intermolecular insertion of the carbene ligand of pentacarbonyl tungsten benzylidenes into the M–H bond of transition metal hydrido complexes

Abstract

The strongly electrophilic carbene complexes [(CO)5WC(H)C6H4R-p] (2) [R=H (a), Me (b), OMe (c)] react with the terminal transition metal hydrides [(CO)5MnH] (3) and [Cp(CO)3WH] (5) by a formal transfer and insertion of the carbene ligand into the M–H bond (M=Mn, W) to give the benzyl complexes [(CO)5MnCH2C6H4R-p] (4a–c) and [Cp(CO)3WCH2C6H4R-p] (6a–c). Similarly, 6a is formed when solutions of [(CO)5CrC(H)Ph] (7a) or [Cp(CO)2MnC(H)Ph] (8a) are treated with 5. The analogous reaction of 2c with [Cp*(CO)3WH] (9) (Cp*=C5Me5) affords [Cp*(CO)3WCH2C6H4OMe-p] (10). In contrast, the reactions of 2b with [Cp2WH2] (11) and PNP[(CO)4FeH] (13) afford binuclear complexes, [Cp2W(H)(CH2C6H4Me-p)W(CO)5] (12) and PNP[(CO)4Fe(CH2C6H4Me-p)W(CO)5] (14). Spectroscopic data confirm the existence of a Fe–W bond in the structure of 14 while in complex 12 the W(CO)5 fragment is coordinated to the W–H bond of the second metal center in a three center two electron mode. In contrast to 3, 5, 9, 11, and 13, the μ-hydrido complexes [H3Mn3(CO)12], [HW2(CO)9(NO)], and NEt4[HW2(CO)10] show no comparable reaction with the benzylidene complexes 2. Kinetic measurements of the reaction of 2c with 5 reveal second-order kinetics, first-order in the concentrations of the benzylidene complex 2c and the metal hydride 5. The activation enthalpy ΔH≠ is small (ΔH≠=19±3 kJ mol−1), the activation entropy ΔS≠ strongly negative (ΔS≠=−120±10 J mol−1 K−1). The kinetic isotope effect for the reaction of 2c with [Cp(CO)3WX] (X=H, D) is kH/kD=2.6±0.4. The reaction rate increases (a) with increasing electrophilicity of the carbene carbon atom and (b) with decreasing steric demand of the ligands at the metal hydride. The reaction mechanism is discussed on the basis of an associative rate-determining step with an early transition state.