Butera Valeria, Russo Nino, Sicilia Emilia
Department of Chemistry, Università della Calabria, 87036 Arcavacata di Rende (Italy), Fax: (+39) 0984-492044.
Chemistry. 2014 May 12;20(20):5967-76. doi: 10.1002/chem.201304329. Epub 2014 Apr 2.
Mg and Ca β-diketiminato silylamides [HC{(Me)CN(2,6-iPr2C6H3)}2M(THF)n{N(SiMe3)2}] (M = Mg, n = 0; M = Ca, n = 1) were studied as precatalysts for the dehydrogenation/dehydrocoupling of secondary amine-boranes R2HNBH3 . By reaction with equimolar quantities of amine-boranes, the corresponding amidoborane derivatives are formed, which further react to yield dehydrogenation products such as the cyclic dimer [BH2-NMe2]2. DFT was used here to explore the mechanistic alternatives proposed on the basis of the experimental findings for both Mg and Ca amidoboranes. The influence of the steric demand of amine-boranes on the course of the reaction was examined by performing calculations on the dehydrogenation of dimethylamine-borane (DMAB), pyrrolidine-borane (PB), and diisopropylamine-borane. In spite of the analogies in the catalytic activity of Mg- and Ca-based complexes in the dehydrocoupling of amine-boranes, our theoretical analysis confirmed the experimentally observed lower reactivity of Ca complexes. Differences in catalytic activity of Mg- and Ca-based complexes were examined and rationalized. As a consequence of the increase in ionic radius on going from Mg(2+) to Ca(2+), the dehydrogenation mechanism changes and formation of a key metal hydride intermediate becomes inaccessible. Dimerization is likely to occur off-metal in solution for DMAB and PB, whereas steric hindrance of iPr2NHBH3 hampers formation of the cyclic dimer. The reported results are of particular interest because, although amine-borane dehydrogenation is now well established, mechanistic insight is still lacking for many systems.
镁和钙的β-二酮亚胺基硅酰胺[HC{(Me)CN(2,6-iPr2C6H3)}2M(THF)n{N(SiMe3)2}]（M = Mg，n = 0；M = Ca，n = 1）被研究作为仲胺硼烷R2HNBH3脱氢/脱氢偶联反应的预催化剂。通过与等摩尔量的胺硼烷反应，形成相应的氨基硼烷衍生物，其进一步反应生成脱氢产物，如环状二聚体[BH2-NMe2]2。这里使用密度泛函理论（DFT）来探索基于镁和钙氨基硼烷的实验结果所提出的机理选择。通过对二甲胺硼烷（DMAB）、吡咯烷硼烷（PB）和二异丙胺硼烷的脱氢进行计算，研究了胺硼烷的空间需求对反应过程的影响。尽管基于镁和钙的配合物在胺硼烷脱氢偶联反应中的催化活性存在相似性，但我们的理论分析证实了实验观察到的钙配合物较低的反应活性。研究并合理解释了基于镁和钙的配合物催化活性的差异。由于从Mg(2+)到Ca(2+)离子半径的增加，脱氢机理发生变化，关键金属氢化物中间体的形成变得难以实现。对于DMAB和PB，二聚化可能在溶液中于金属外发生，而iPr2NHBH3的空间位阻阻碍了环状二聚体的形成。所报道的结果特别令人感兴趣，因为尽管胺硼烷脱氢现在已经得到充分证实，但许多体系仍缺乏机理方面的深入了解。
