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On the Origin of Reversible Hydrogen Activation by Phosphine-Boranes

机译：膦-硼烷可逆氢活化的起源

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摘要

Mechanistic insights into the factors responsible for the reversible hydrogen-activation ability exhibited by an aryl phosphine-borane system ((CH3)(2)P-C6F4-B(CF3)(2)) are presented. A detailed evaluation of the energies of various intermediates, generated by the addition of molecular hydrogen, and their interconverting barriers have been carried out using ab initio and DFT methods. Several rearrangement possibilities of the H-2-phosphinoborane adduct have been investigated so as to unravel the lower energy pathways that convert the initial adduct to a series of other intermediates. The initial adduct formed by the heterolytic addition of a molecular hydrogen across the C-B bond is identified to undergo a series of rearrangement reactions until it terminates at the C-P end of the molecule. Among the possible 1,n-migrations (for which n = 1-5), 1,2-proton migrations are found to possess lower energy transition states, whereas 1,2-hydride (in a zwitterionic intermediate) and 1,4-proton-coupled electron transfers exhibited much higher energy transition states. The minimum energy pathway for the transfer of a proton and hydride from the C-B bond to the C-P bond is found to involve a cascade of 1,2-proton transfers followed by a 1,2-hydride migration and finally a 1,40 proton-coupled electron transfer. The higher energy pathways identified for the hydride transfer suggest the possibility of a cascade of reversible proton migrations from a thermodynamically stable intermediate (M-a). Possible uptake of two hydrogen molecules by the phosphine-borane system is additionally considered in the present study., in which relatively higher barriers than those with one molecule of hydrogen are observed. The computed thermodynamic parameters are found to be in accordance with the experimental observations, in which the uptake and storage of molecular hydrogen are carried out at lower temperatures whereas the liberation demands elevated temperatures.
机译：提出了对芳基膦-硼烷系统（（CH3）（2）P-C6F4-B（CF3）（2））表现出可逆氢激活能力的因素进行机理分析的机制。通过使用从头算和DFT方法，对通过添加分子氢产生的各种中间体的能量及其相互转换的势垒进行了详细的评估。已经研究了H-2-膦基硼烷加合物的几种重排可能性，以阐明将初始加合物转化为一系列其他中间体的较低能量途径。鉴定出通过氢原子跨C-B键的杂化加成反应形成的初始加合物经历了一系列重排反应，直到其终止于分子的C-P末端。在可能的1，n迁移（n = 1-5）中，发现1,2-质子迁移具有较低的能量跃迁状态，而1,2-氢化物（在两性离子中间体中）和1,4-质子耦合电子转移表现出更高的能量跃迁状态。发现质子和氢化物从CB键转移到CP键的最小能量路径涉及1，2-质子转移的级联，接着是1,2-氢化物的迁移，最后是1,40质子-耦合电子转移。为氢化物转移确定的较高能量途径表明，从热力学稳定的中间体（M-a）发生级联可逆质子迁移的可能性。在本研究中还考虑了膦-硼烷系统可能吸收两个氢分子的情况，其中观察到的势垒比具有一个氢分子的势垒要高。发现计算出的热力学参数与实验观察结果一致，其中分子氢的吸收和储存在较低的温度下进行，而释放需要升高的温度。

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