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.

以大咬角双膦配体2,2'-双(二苯基膦基)二苯醚(DPEphos, (Ph₂PC₆H₄)₂O)处理配合物Mo(NO)Cl₃(NCMe)₂，可制得双核配合物[Mo(NO)(P∩P)Cl₂]₂(μ-Cl)₂（其中P∩P代表DPEphos，对应结构为(Ph₂PC₆H₄)₂O，记为化合物1）。
化合物1可在锌粉与乙腈(MeCN)存在下被还原，得到阳离子配合物[Mo(NO)(P∩P)(NCMe)₃]^+[Zn₂Cl₆]^(2-)_(1/2)（记为化合物2）。
通过复分解反应，使用四(3,5-双(三氟甲基)苯基)硼酸钠(NaBArF4，其中BArF4 = [B{3,5-(CF₃)₂C₆H₃}₄])替换化合物2中的[Zn₂Cl₆]^(2-)抗衡阴离子，可得到[BArF4]^−盐[Mo(NO)(P∩P)(NCMe)₃]^+[BArF4]^−（记为化合物3）。
实验证实，化合物3可在120 ℃条件下催化多种对位取代苯甲醛、环己烷甲醛、2-噻吩甲醛以及2-糠醛的硅氢化反应。
硅烷筛选实验显示，一级与二级芳香族硅烷在以化合物3为催化剂的硅氢化体系中催化效果最佳。此外，以化合物3与PhMeSiH₂为试剂，可在室温下实现酮类化合物的硅氢化反应。
以4-甲氧基苯乙酮为底物、PhMeSiH₂为硅烷时，在化合物3催化下，120 ℃下测得最大周转频率(TOF)为3.2 × 10⁴ h⁻¹，室温下的TOF为4400 h⁻¹。
动力学研究结果表明，反应速率对催化剂与硅烷浓度呈一级反应，对底物浓度呈零级反应。
针对多种对位取代苯乙酮的哈米特(Hammett)研究显示，反应速率与取代基常数呈线性相关，120 ℃下的哈米特常数ρ为-1.14，室温下为-3.18。