Gediga Micheal, Feil Christoph M, Schlindwein Simon H, Bender Johannes, Nieger Martin, Gudat Dietrich
Institut für Anorganische Chemie, University of Stuttgart, Pfaffenwaldring 55, 70550, Stuttgart, Germany.
Department of Chemistry, University of Helsinki, P.O Box 55, A.I. Virtasen aukio 1), 00014, Helsinki, Finland.
Chemistry. 2017 Aug 25;23(48):11560-11569. doi: 10.1002/chem.201701442. Epub 2017 Jul 21.
A neutral N-heterocyclic phosphenium complex of manganese was synthesised by a metathesis approach and characterised by IR, NMR, and XRD studies. The short P-Mn distance suggests a substantial metal-ligand double bond character. Reaction with a hydride produced an anionic phosphine complex, which was also characterised by IR and NMR spectroscopy and, after anion exchange, a single-crystal XRD study. Protonation of the anion occurs at the metal to yield a neutral phosphine metal carbonyl hydride, which releases dihydrogen upon irradiation with UV light. These reactions confirm the electrophilic nature of the phosphenium ligand and suggest that the title complex might undergo reactions displaying metal-ligand cooperativity. Surprisingly, reaction with ammonia borane (AB) did not proceed under transfer hydrogenation of the Mn=P double bond but through the catalytic dehydrogenation of AB. The phosphenium complex behaves here as a class II catalyst, which dehydrogenates AB to NH BH that was trapped with cyclohexene. Computational model studies led to the identification of two possible catalytic cycles, which differ in the regioselectivity of the initial AB activation step. In one case, the activation proceeds as cooperative transfer hydrogenation of the Mn=P bond, whereas in the other case a H /H pair is transferred to the phosphorus atom and a nitrogen atom of the phosphenium unit, resulting in a ligand-centred reaction in which the metal fragment acts merely as stabilising substituent. Unexpectedly, this pathway, which constitutes a new reaction mode for phosphenium complexes, seems to be better in accord with experimental findings on the course of the catalysis.
通过复分解方法合成了一种中性的锰氮杂环磷鎓配合物，并通过红外光谱、核磁共振和X射线衍射研究对其进行了表征。较短的P-Mn距离表明存在显著的金属-配体双键特征。与氢化物反应生成了一种阴离子膦配合物，也通过红外光谱和核磁共振光谱对其进行了表征，并且在阴离子交换后进行了单晶X射线衍射研究。阴离子在金属上发生质子化，生成中性的膦金属羰基氢化物，该氢化物在紫外光照射下会释放出氢气。这些反应证实了磷鎓配体的亲电性质，并表明标题配合物可能会发生显示金属-配体协同作用的反应。令人惊讶的是，与氨硼烷（AB）的反应不是通过Mn=P双键的转移氢化进行的，而是通过AB的催化脱氢进行的。在这里，磷鎓配合物表现为II类催化剂，它将AB脱氢生成NH BH，后者被环己烯捕获。计算模型研究确定了两个可能的催化循环，它们在初始AB活化步骤的区域选择性上有所不同。在一种情况下，活化以Mn=P键的协同转移氢化方式进行，而在另一种情况下，一对H /H转移到磷鎓单元的磷原子和氮原子上，导致以配体为中心的反应，其中金属片段仅作为稳定取代基起作用。出乎意料的是，这条构成磷鎓配合物新反应模式的途径似乎更符合催化过程的实验结果。
