Highly Efficient Large Bite Angle Diphosphine Substituted Molybdenum Catalyst for Hydrosilylation
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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.
将配合物Mo(NO)Cl₃(乙腈)₂（NCMe为乙腈）与大咬角双膦配体2,2′-双(二苯基膦基)二苯醚（2,2′-bis(diphenylphosphino)diphenylether, DPEphos）反应，得到双核物种[Mo(NO)(P∩P)Cl₂]₂[μ-Cl]₂（其中P∩P为DPEphos，即(Ph₂PC₆H₄)₂O），记为化合物1。在锌（Zn）与乙腈存在下，化合物1可被还原为阳离子配合物[Mo(NO)(P∩P)(NCMe)₃]⁺与[Zn₂Cl₆]²⁻按2:1计量比成盐的产物，记为化合物2。在复分解反应中，将原抗衡阴离子[Zn₂Cl₆]²⁻（按2个阳离子对应1个阴离子的计量比）替换为NaBArF₄（BArF₄ = [B{3,5-(CF₃)₂C₆H₃}₄]⁻，即四(3,5-二(三氟甲基)苯基)硼酸根），得到[BArF₄]⁻盐[Mo(NO)(P∩P)(NCMe)₃]⁺[BArF₄]⁻，记为化合物3。实验发现，化合物3可在120℃下催化多种对位取代苯甲醛、环己基甲醛、2-噻吩甲醛以及2-糠醛的硅氢化反应。硅烷类试剂筛选实验表明，一级与二级芳基硅烷在化合物3催化的硅氢化反应中效果最佳。此外，在室温下使用化合物3与甲基苯基硅烷（PhMeSiH₂）即可实现酮类的硅氢化反应。在以PhMeSiH₂为硅源、化合物3为催化剂的体系中，对甲氧基苯乙酮的硅氢化反应在120℃下的最大转化频率（TOF）可达3.2×10⁴ h⁻¹，室温下的TOF为4400 h⁻¹。动力学研究表明，该反应的速率对催化剂与硅烷浓度为一级反应，对底物浓度为零级反应。针对多种对位取代苯乙酮开展的哈米特研究显示，反应速率与取代基常数呈线性相关，120℃下的哈米特ρ值为-1.14，室温下为-3.18。
