CpRu(PH3)2SH与HNCS 的模型化反应机理

应用密度泛函理论(DFT), 通过CpRu(PH3)2SH(Cp=环戊二烯基)与HNCS的模型化反应, 探讨了CpRu-(PPh3)2SH与RNCS(R=Ph, 1-naphthyl)反应生成CpRu(PPh3)S2CNHR的两种可能的反应机理. 一种可能的机理是, 一个PH3配体先从反应物CpRu(PH3)2SH解离出来, 得到一个16e中间体, 然后经过一个氢转移反应, 得到产物; 另一种可能的机理是, 先经过一个氢转移反应, 然后一个PH3配体再从金属中心解离出来, 得到产物. 通过分析两种机理的势能曲线发现, 反应的决速步骤为从硫原子到氮原子的氢迁移过程. 第一种反应机理中反应的最高活化能明显比第二种反应机理的最高活化能高. 因此, 我们预测反应倾向于先发生氢迁移, 然后配体PH3再从金属中心上解离出来. 在该反应机理中, 尽管和产物相连的中间体稳定性稍高于产物, 由于熵效应致使最终产物仍然是实验中所得到的产物.

Mechanism of the Model Reaction between CpRu(PH3)2SH and HNCS

Reaction mechanisms for the reaction between CpRu(PPh3)2SH (Cp=cyclopentadienyl) and RNCS (R = Ph, 1-naphthyl) were investigated by density functional theory using the model reaction between CpRu(PH3)2SH and HNCS. Two possible mechanisms are proposed. First, one PH3 ligand dissociates from CpRu(PH3)2SH to give a 16e intermediate upon which hydrogen migration occurs giving the product. Second, hydrogen migration occurs before the dissociation of a PH3 ligand, giving the product. Based on our calculations, the second mechanism is more favorable. From the potential energy curves for the two possible mechanisms, the rate-determining step for the reaction is hydrogen migration. The overall reaction activation energy for the first mechanism is markedly higher than that for the second mechanism. Therefore, we predict that this reaction tends to experience hydrogen migration before the dissociation of PPh3 from the metal center. In the second mechanism, the product is eventually obtained because of an increase in entropy but the product is thermally less stable than the intermediate that directly connects to the product.