Zimmerman Paul M, Paul Ankan, Musgrave Charles B
Department of Chemical Engineering, Stanford University, Stanford, California 94305, USA.
Inorg Chem. 2009 Jun 15;48(12):5418-33. doi: 10.1021/ic900417z.
The development of ammonia borane (AB) as a promising hydrogen storage medium depends upon the ability to reversibly release H(2) from the system. We use density functional theory to investigate the mechanism of the catalytic dehydrogenation of AB by Ni N-heterocyclic carbene (NHC) complexes, which we show proceeds through Ni monocarbene and dicarbene species. Although Ni(NHC)(2) dehydrogenates AB, it competitively decomposes into a monocarbene species because AB readily displaces NHC from Ni(NHC)(2) and reaction of displaced NHC with abundant AB makes Ni monocarbene formation thermodynamically favored over the dicarbene catalyst. Prediction of NHC displacement by AB is consistent with the experimental observation of NHC-BH(3). The Ni monocarbene species Ni(NHC)(NH(2)BH(2)) competitively dehydrogenates AB with barriers consistent with the experimental temperature required to obtain reasonable reaction rates. The Ni monocarbene pathway also involves rate-limiting steps that exhibit both N-H and B-H kinetic isotope effects (KIEs), as observed experimentally. The predicted N-H and B-H KIEs are also in quantitative agreement with experiment. In contrast, AB dehydrogenation by Ni(NHC)(2) does not exhibit a B-H KIE. Activation of AB at both mono- and dicarbene catalysts proceeds through cis-carbene proton acceptance and involves transition states with significant electron delocalization over the pi-system of the carbene and its phenyl rings. NHC Ni catalysts involving carbenes with substituent groups containing steric factors that preclude planarity of the phenyl rings to the carbene aromatic system, such as the Imes and Idipp ligands, are predicted to have lower reactivity, in agreement with experiment. The addition of electron donating and withdrawing groups to the phenyl rings demonstrate the importance of pi-system electron delocalization by their influence on the barrier to cis-carbene proton acceptance.
氨硼烷（AB）作为一种很有前景的储氢介质，其发展取决于能否从该体系中可逆地释放出H₂。我们运用密度泛函理论研究了镍氮杂环卡宾（NHC）配合物催化AB脱氢的机理，结果表明该过程通过镍单卡宾和双卡宾物种进行。尽管Ni(NHC)₂能使AB脱氢，但它会竞争性地分解为单卡宾物种，因为AB很容易从Ni(NHC)₂中取代NHC，且被取代的NHC与大量AB的反应使得镍单卡宾的形成在热力学上比双卡宾催化剂更有利。AB对NHC的取代预测与NHC - BH₃的实验观察结果一致。镍单卡宾物种Ni(NHC)(NH₂BH₂)能竞争性地使AB脱氢，其能垒与获得合理反应速率所需的实验温度相符。镍单卡宾途径还涉及限速步骤，这些步骤表现出N - H和B - H动力学同位素效应（KIEs），这与实验观察结果一致。预测的N - H和B - H KIEs也与实验在定量上相符。相比之下，Ni(NHC)₂催化AB脱氢未表现出B - H KIE。单卡宾和双卡宾催化剂对AB的活化均通过顺式卡宾质子接受进行，且涉及的过渡态在卡宾及其苯环的π体系上有显著的电子离域。预测涉及带有取代基的卡宾的NHC镍催化剂，如Imes和Idipp配体，其取代基中的空间因素会阻止苯环与卡宾芳香体系共平面，这类催化剂的反应活性较低，这与实验结果一致。在苯环上添加供电子和吸电子基团，通过它们对顺式卡宾质子接受能垒的影响，证明了π体系电子离域的重要性。
