Gaggioli Carlo Alberto, Belpassi Leonardo, Tarantelli Francesco, Harvey Jeremy N, Belanzoni Paola
Department of Chemistry, Chemical Theory Center and Supercomputing Institute, University of Minnesota, 207 Pleasant Street SE, 55455-0431, Minneapolis, Minnesota, USA.
Dalton Trans. 2017 Sep 12;46(35):11679-11690. doi: 10.1039/c7dt02170d.
The ligand effect on the recently uncovered feasible oxidative addition reaction of O on [LAuH] complexes has been investigated for a series of fifteen ligands. The activation barriers of this spin-forbidden reaction have been estimated at the minimum energy crossing points (MECP, relativistic scalar level) between the adiabatic triplet (reactants spin state) and singlet (product spin state) potential energy surfaces (PES) and calculated at the transition states by including Spin-Orbit Coupling (SOC) effects, as applied for the mechanistic study of this reaction in a previous study by us [Chem. Sci., 2016, 7, 7034-7039]. We find a sizeable effect of the ligand on the activation barriers, and some of the stronger electron donating phosphines are predicted to induce the highest catalyst efficiency. The inclusion of SOC effects lowers the activation barriers by about 3 kcal mol systematically with respect to the MECP values independently of the ligand type. We used the Charge-Displacement (CD) analysis to quantify the net electron charge donation from the ligand L towards the metallic fragment AuH in the [LAuH] series, and surprisingly only a poor correlation was found between the net electron donor character of L and the activation barriers. Application of the CD-NOCV (Natural Orbitals for Chemical Valence) approach, which allows the quantification of the Dewar-Chatt-Duncanson (DCD) L-AuH bond components, suggests that the ligand effect on the activation barriers is not easily predictable on the basis of solely the electronic properties of the ligand and depends significantly on the ligand nature or carbene or phosphine type. We show that for both phosphine and carbene ligand subsets, however, the σ donation component of the L-AuH bond quantitatively accounts for the ligand effect on the activation energy barriers (a larger σ-donor capability of L correlates with a smaller activation barrier), whereas the π back-donation, strongly affected by geometrical rearrangement, is a poor reactivity descriptor (π acceptor properties of the ligand L in the linear [LAuH] complexes are not transferable to the trigonal [LAuH(O)] transition state structures).
针对一系列15种配体，研究了配体对[LAuH]配合物上最近发现的氧可行氧化加成反应的影响。该自旋禁阻反应的活化能垒已在绝热三重态（反应物自旋态）和单重态（产物自旋态）势能面（PES）之间的最低能量交叉点（MECP，相对论标量水平）进行了估算，并通过纳入自旋轨道耦合（SOC）效应在过渡态进行了计算，正如我们之前的一项研究[《化学科学》，2016年，7卷，7034 - 7039页]中对该反应机理研究的应用那样。我们发现配体对活化能垒有显著影响，预计一些较强的给电子膦会诱导出最高的催化效率。纳入SOC效应后，相对于MECP值，活化能垒系统地降低了约3千卡/摩尔，且与配体类型无关。我们使用电荷位移（CD）分析来量化配体L向[LAuH]系列中金属片段AuH的净电子电荷转移，令人惊讶的是，在L的净电子给体特性与活化能垒之间仅发现了较弱的相关性。应用CD - NOCV（化学价自然轨道）方法，该方法能够量化Dewar - Chatt - Duncanson（DCD）L - AuH键成分，这表明仅基于配体的电子性质，配体对活化能垒的影响不易预测，并且很大程度上取决于配体性质或卡宾或膦的类型。然而，我们表明，对于膦和卡宾配体子集，L - AuH键的σ给体成分定量地解释了配体对活化能垒的影响（L的σ给体能力越大，活化能垒越小），而受几何重排强烈影响的π反馈，是一个较差的反应性描述符（线性[LAuH]配合物中配体L的π受体性质不能转移到三角[LAuH(O)]过渡态结构）。
