Highly Efficient Large Bite Angle Diphosphine Substituted Molybdenum Catalyst for Hydrosilylation

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.