Instituto de Química, Universidade Federal Fluminense, Niterói-RJ, Brazil.
J Phys Chem A. 2011 Nov 10;115(44):12259-70. doi: 10.1021/jp202762p. Epub 2011 Oct 13.
A thorough analysis of the electronic structure and thermodynamic aspects of Grignard reagents and its associated equilibrium composition in ethereal solutions is performed. Considering methylmagnesium halides containing fluorine, chlorine, and bromine, we studied the neutral, charged, and radical species associated with their chemical equilibrium in solution. The ethereal solvents considered, tetrahydrofuran (THF) and ethyl ether (Et(2)O), were modeled using the polarizable continuum model (PCM) and also by explicit coordination to the Mg atoms in a cluster. The chemical bonding of the species that constitute the Grignard reagent is analyzed in detail with generalized valence bond (GVB) wave functions. Equilibrium constants were calculated with the DFT/M06 functional and GVB wave functions, yielding similar results. According to our calculations and existing kinetic and electrochemical evidence, the species R(•), R(-), (•)MgX, and RMgX(2)(-) must be present in low concentration in the equilibrium. We conclude that depending on the halogen, a different route must be followed to produce the relevant equilibrium species in each case. Chloride and bromide must preferably follow a "radical-based" pathway, and fluoride must follow a "carbanionic-based" pathway. These different mechanisms are contrasted against the available experimental results and are proven to be consistent with the existing thermodynamic data on the Grignard reagent equilibria.
对格氏试剂及其在醚溶液中的平衡组成的电子结构和热力学方面进行了深入分析。考虑含氟、氯和溴的甲基卤化镁，我们研究了与其在溶液中的化学平衡有关的中性、带电和自由基物种。所考虑的醚溶剂四氢呋喃（THF）和乙醚（Et（2）O）使用极化连续体模型（PCM）进行建模，并且还通过与 Mg 原子在簇中的配位进行建模。用广义价键（GVB）波函数详细分析了构成格氏试剂的物种的化学键合。用 DFT/M06 函数和 GVB 波函数计算平衡常数，得到了相似的结果。根据我们的计算和现有的动力学和电化学证据，在平衡中必须以低浓度存在物种 R（•）、R（-）、（•）MgX 和 RMgX（2）（-）。我们得出结论，取决于卤素，必须采用不同的途径来在每种情况下产生相关的平衡物种。氯化物和溴化物必须优选遵循“基于自由基的”途径，而氟化物必须遵循“基于碳负离子的”途径。这些不同的机制与可用的实验结果形成对比，并被证明与格氏试剂平衡的现有热力学数据一致。
