DFT Functional Benchmarking on the Energy Splitting of Chromium Spin States and Mechanistic Study of Acetylene Cyclotrimerization over the Phillips Cr(II)/Silica Catalyst
DFT Functional Benchmarking on the Energy Splitting of Chromium Spin States and Mechanistic Study of Acetylene Cyclotrimerization over the Phillips Cr(II)/Silica Catalyst
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DOI：
10.1021/jp302529q
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发表时间：
2012-07-19
影响因子：
2.9
通讯作者：
Liu, Boping
中科院分区：
文献类型：
--
作者：
Liu, Zhen;Cheng, Ruihua;Liu, Boping
本文用密度泛函理论(DFT)对Phillips Cr(II)/SiO_2催化剂上的乙炔环三聚反应的两态反应机理进行了系统的研究。由于在催化循环中证实了自旋交叉现象，因此准确预测低自旋态和高自旋态之间的能隙对于描述涉及两态反应的反应至关重要。因此，以CASPT2能隙为参照，对集群模型进行了大规模的DFT功能基准测试。因此，选择了具有28%Hartree-Fock交换能的B3PW91*进行了以下的机理研究。探索了每一种可能的势能面，包括单重态、三重态和五重态。在五元组表面上，反应开始于铬中心上的乙炔配位生成pi配位的络合物。随后的氧化偶联通过与第二个乙炔的进一步配位被预测为一个两步反应，以生成一个色环戊二烯物种。在过渡态(TS)-T-5[C-E](双匕首)=31.1千卡/摩尔的存在下，这种转变被发现是能量受阻的。然而，在三重态表面上，乙炔的配位生成了一个色环丙烯物种，没有表现出任何活化势垒。通过铬中心配位，然后插入铬环丙烯的Cr-C sigma键的第二次乙炔加成反应被预测为一个简单的反应途径(双匕首)=10.2千卡/摩尔。第三个乙炔通过形成氢键被团簇模型捕获。三重态表面的后一步转变为分子间的[4+2]环加成反应，完成环化反应。由于L-3分子筛中苯环的芳香性不足，导致在单一三重态表面上的反应路径不完整。因此，描述了由两个表面的两个低位最小能量交叉点(MECP)连接的两态反应路径。值得注意的是，团簇模型仅通过形成氢键捕获了三(乙炔)铬络合物中的第三乙炔，排除了ZecChina等人提出的[2+2+2]协同一步反应途径。[物理。化学。化学。太棒了。2003、5、4414]。与在三重态和五重态表面上进行的反应相比，单重态反应的能量分布要高得多。
In this work, a two-state reaction mechanism for the acetylene cyclotrimerization over a cluster model for the Phillips Cr(II)/silica catalyst were systematically investigated using density functional theory (DFT). Since spin crossover phenomenon was confirmed in the catalytic cycle, an accurate prediction of the energy gap between low- and high-spin states is crucial for the description of a reaction involving a two-state reactivity. Therefore, a massive DFT functional benchmarking test has been conducted on the cluster model by taking a CASPT2 energy gap as a reference. Consequently, B3PW91* with 28% Hartree-Fock exchange energy was selected for the following mechanistic investigation. Each of the possible potential energy surface including singlet, triplet, and quintet surfaces was explored. On the quintet surface the reaction begins with a coordination of an acetylene on the chromium center to generate a pi-coordinated complex. The following oxidative coupling through further coordination with a second acetylene was predicted to be a two-step reaction to generate a chromacyclopentadiene species. This transformation was found to be energetically prohibitive by the presence of the transition state (TS)-T-5[C-E] (Delta G(double dagger) = 31.1 kcal/mol). On the triplet surface, however, the coordination of an acetylene generates a chromacyclopropene species without showing any activation barrier. The second acetylene incorporation proceeding via a coordination on the chromium center followed by an insertion into a Cr-C sigma-bond of the chromacyclopropene was predicted to be a facile reaction pathway (Delta G(double dagger) = 10.2 kcal/mol). The third acetylene was captured by the cluster model through the formation of a hydrogen bond. The later transformation on the triplet surface was found to be an intermolecular [4 + 2] cycloaddition to finish the cyclization. The lack of the aromaticity of the benzene ring in L-3 results in an uncompleted reaction pathway on a single triplet surface. Consequently, a two-state reaction pathway that is connected by two low-lying minimum-energy crossing points (MECPs) of the two surfaces is thus described. It is worthy of note that the third acetylene in the tri(acetylene)chromium complex captured by the cluster model only through the formation of a hydrogen bond rules out the [2 + 2 + 2] concerted one-step reaction pathway proposed by Zecchina et al. [Phys. Chem. Chem. Phys. 2003, 5, 4414]. The singlet reaction profile is far higher in energy compared with that proceeded on the triplet and quintet surfaces.