Division of Chemistry and Chemical Engineering, Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, California 91125, United States.
J. Heyrovský Institute of Physical Chemistry, Czech Academy of Sciences, Prague 8, 18223 Czech Republic.
Inorg Chem. 2020 Jul 6;59(13):8707-8715. doi: 10.1021/acs.inorgchem.0c00249. Epub 2020 Jun 8.
Iron porphyrin carbenes (IPCs) are important reaction intermediates in engineered carbene transferase enzymes and homogeneous catalysis. However, discrepancies between theory and experiment complicate the understanding of IPC electronic structure. In the literature, this has been framed as whether the ground state is an open- vs closed-shell singlet (OSS vs CSS). Here we investigate the structurally dependent ground and excited spin-state energetics of a free carbene and its IPC analogs with variable trans axial ligands. In particular, for IPCs, multireference ab initio wave function methods are more consistent with experiment and predict a mixed singlet ground state that is dominated by the CSS (Fe(II) ← {:C(X)Y}) configuration (i.e., electrophilic carbene) but that also has a small, non-negligible contribution from an Fe(III)-{C(X)Y} configuration (hole in d(), i.e., radical carbene). In the multireference approach, the "OSS-like" excited states are metal-to-ligand charge transfer (MLCT) in nature and are energetically well above the CSS-dominated ground state. The first, lowest energy of these "OSS-like" excited states is predicted to be heavily weighted toward the Fe(III)-{C(X)Y} (hole in d()) configuration. As expected from exchange considerations, this state falls energetically above a triplet of the same configuration. Furthermore, potential energy surfaces (PESs) along the IPC Fe-C(carbene) bond elongation exhibit increasingly strong mixings between CSS/OSS characters, with the Fe(III)-{C(X)Y} configuration (hole in d()) growing in weight in the ground state during bond elongation. The relative degree of electrophilic/radical carbene character along this structurally relevant PES can potentially play a role in reactivity and selectivity patterns in catalysis. Future studies on IPC reaction coordinates should evaluate contributions from ground and excited state multireference character.
铁卟啉碳烯（IPCs）是工程化碳烯转移酶和均相催化中的重要反应中间体。然而，理论和实验之间的差异使得 IPC 电子结构的理解变得复杂。在文献中，这被表述为基态是开壳（OSS）还是闭壳（CSS）单重态。在这里，我们研究了自由碳烯及其具有可变轴向配体的 IPC 类似物的结构依赖性基态和激发自旋态能。特别是对于 IPC，多参考从头算波函数方法与实验更一致，并预测混合单重态基态，其由 CSS（Fe(II)←{:C(X)Y}}构型（即亲电碳烯）主导，但也有来自 Fe(III)-{C(X)Y}构型（d 轨道中的空穴，即自由基碳烯）的小而不可忽略的贡献。在多参考方法中，“OSS 样”激发态本质上是金属-配体电荷转移（MLCT），能量远高于 CSS 主导的基态。这些“OSS 样”激发态中能量最低的第一激发态预计主要由 Fe(III)-{C(X)Y}（d 轨道中的空穴）构型组成。根据交换考虑，预计该态的能量高于相同构型的三重态。此外，IPCFe-C（碳烯）键伸长的势能面（PES）表现出 CSS/OSS 特征之间越来越强的混合，随着键伸长，基态中 d 轨道中的空穴（Fe(III)-{C(X)Y}）构型的权重增加。在这种结构相关的 PES 上，亲电/自由基碳烯特征的相对程度可能在催化反应的活性和选择性模式中发挥作用。未来对 IPC 反应坐标的研究应评估基态和激发态多参考特征的贡献。
