Highly efficient large bite angle diphosphine substituted molybdenum catalyst for hydrosilylation

Abstract

Treatment of the complex Mo(NO)Cl3(NCMe)2 with the large bite angle diphosphine, 2,2′-bis(diphenylphosphino)diphenylether (DPEphos) afforded the dinuclear species [Mo(NO)(P∩P)Cl2]2[μCl]2 (P∩P = DPEphos = (Ph2PC6H4)2O (1). 1 could be reduced in the presence of Zn and MeCN to the cationic complex [Mo(NO)(P∩P)(NCMe)3]+[Zn2Cl6]2–1/2 (2). In a metathetical reaction the [Zn2Cl6]2–1/2 counteranion was replaced with NaBArF4 (BArF4 = [B{3,5-(CF3)2C6H3}4]) to obtain the [BArF4]− salt [Mo(NO)(P∩P)(NCMe)3]+[BArF4]− (3). 3 was found to catalyze hydrosilylations of various para substituted benzaldehydes, cyclohexanecarboxaldehyde, 2-thiophenecarboxaldehyde, and 2-furfural at 120 °C. A screening of silanes revealed primary and secondary aromatic silanes to be most effective in the catalytic hydrosilylation with 3. Also ketones could be hydrosilylated at room temperature using 3 and PhMeSiH2. A maximum turnover frequency (TOF) of 3.2 × 104 h–1 at 120 °C and a TOF of 4400 h–1 was obtained at room temperature for the hydrosilylation of 4-methoxyacetophenone using PhMeSiH2 in the presence of 3. Kinetic studies revealed the reaction rate to be first order with respect to the catalyst and silane concentrations and zero order with respect to the substrate concentrations. A Hammett study for various para substituted acetophenones showed linear correlations with negative ρ values of −1.14 at 120 °C and −3.18 at room temperature.