Does a multiply bonded oxo ligand directly participate in B-H bond activation by a high-valent di-oxo-molybdenum(VI) complex? A density functional theory study

Huang Liangfang; Wang Jiandi; Wei XiaoqinWei Haiyan

摘要

The reduction of organic substrates using high-valent oxo-transition metal complexes represents a new catalytic activity. In this study, we theoretically investigated the mechanism of catalytic reduction of amides, amines, nitriles and sulfoxides with boranes by the high-valent di-oxo-molybdenum(VI) complex MoO2Cl2. Our computational results reveal that reduction of sulfoxides with boranes catalyzed by MoO2Cl2 proceeds via a [2 + 2] addition pathway involving the B-H bond of borane adding across the Mo=O bond to form a metal hydride intermediate, followed by the elimination of the new species HOBcat, accompanied by the loss of the sulfide. The activation free energy of the turnover-limiting step is calculated to be 24.0 kcal mol(-1). By contrast, borane additions to either amide, amine or nitrile proceed through an ionic outersphere mechanism, in which the substrates attack the boron center to prompt the heterolytic cleavage of the B-H bond, generating an anionic molybdenum(VI) hydride paired with a borylated amide/amine/nitrile cation. Then, the activated organic substrates abstract a hydride from the molybdenum(VI) center to complete the catalytic cycle. The activation free energies of the turnover-limiting step along the ionic outersphere pathway are calculated to be similar to 22.7, 19.7 and 30.6 kcal mol(-1) for benzamide, N-(iphenylmethylene) benzenamine, and benzonitrile, respectively. These values are energetically more favorable (similar to 3-8.0 kcal mol(-1)) than those via the [2 + 2] addition pathway. Along the ionic outer-sphere pathway, the multiply bonded oxo ligand does not participate in the activation of the B-H bond. The ionic outer-sphere mechanism suggests that the high-valent di-oxo-molybdenum(VI) complex MoO2Cl2 acts as a Lewis acid in catalyzing the reduction reaction and activation of B-H bonds.

机译：使用高价值氧转变金属配合物减少有机底物代表了一种新的催化活性。在这项研究中，我们从理论上研究了高价值的Di-oxo----------氧化物（VI）复合物MOO2CL2的酰胺，胺，硝酸盐和亚氧化物的催化减少机制。我们的计算结果表明，用MOO2CL2催化的硼烷的硫氧化物通过A [2 + 2]的加法途径进行进行，涉及硼烷B-H键在MO = O键上添加的B-H键，形成金属氢化物中间体，然后消除新的氢化物中间体物种霍布卡特（Hobcat），伴随着硫化物的损失。离职限制步骤的激活自由能计算为24.0 kcal mol（-1）。相比之下，在酰胺，胺或氮中添加硼烷通过离子Outersphere机制进行，其中底物攻击硼中心以促使B-H键的异质裂解，产生阴离子钼（VI）与氢化物配对的阴离子钼（VI） /胺/氮阳离子。然后，活化的有机底物从钼（VI）中心抽象氢化物以完成催化循环。对于苯甲酰胺，N-（iphenylmethylene）苯胺和苯甲酸酯，计算沿离子Outersphere途径的限制步骤的激活自由能与22.7、19.7和30.6 kcal mol（-1）相似。与通过[2 + 2]加法途径相比，这些值在能量上更有利（类似于3-8.0 kcal mol（-1））。沿着离子外球途径，多键氧配体不参与B-H键的激活。离子外球机制表明，高价值的Di-oxo-------氧化（VI）复合物MOO2CL2在催化B-H键的还原反应和激活时充当刘易斯酸。

Does a multiply bonded oxo ligand directly participate in B-H bond activation by a high-valent di-oxo-molybdenum(VI) complex? A density functional theory study

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