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Multireference Ground and Excited State Electronic Structures of Free- versus Iron Porphyrin-Carbenes.
Inorganic Chemistry ( IF 4.3 ) Pub Date : 2020-06-08 , DOI: 10.1021/acs.inorgchem.0c00249 Gautam D Stroscio 1 , Martin Srnec 2 , Ryan G Hadt 1
Inorganic Chemistry ( IF 4.3 ) Pub Date : 2020-06-08 , DOI: 10.1021/acs.inorgchem.0c00249 Gautam D Stroscio 1 , Martin Srnec 2 , Ryan G Hadt 1
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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}0) 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(xz), 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(yz)) 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(xz)) 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.
中文翻译：
游离和铁卟啉-卡宾酸酯的多参考基态和激发态电子结构。
卟啉铁碳酸钙（IPC）是工程化卡宾转移酶和均相催化中的重要反应中间体。但是，理论和实验之间的差异使对IPC电子结构的理解更加复杂。在文献中，这被定义为基态是开壳还是闭壳单峰态（OSS和CSS）。在这里，我们研究了自由卡宾及其具有可变反式轴向配体的IPC类似物的结构相关的基态和激发自旋态能量。特别是，对于IPC，多引用从头算函数函数与实验更加一致，并且可以预测由CSS（Fe（II）←{：C（X）Y} 0主导的混合单重态基态。）构型（即亲电卡宾），但也具有Fe（III）– {C（X）Y} -•构型（d（xz）中的孔，即自由基卡宾）的微小贡献，不可忽略。在多参考方法中，“类OSS”激发态本质上是金属到配体的电荷转移（MLCT），并且在能量上远高于CSS主导的基态。预计这些“类OSS”激发态的第一，最低能量将偏重于Fe（III）– {C（X）Y} -•（d（yz中的孔））配置。如从交换考虑所期望的那样，该状态在能量上落在相同配置的三元组之上。此外，沿IPC Fe–C（卡宾）键延伸的势能面（PESs）在CSS / OSS特征之间表现出越来越强的混合，具有Fe（III）– {C（X）Y} -•构型（d孔（xz））在键伸长期间在基态下重量增加。沿该结构上相关的PES的亲电/自由基卡宾特征的相对程度可能在催化反应性和选择性模式中发挥作用。IPC反应坐标的未来研究应评估基态和激发态多参考特征的贡献。
更新日期：2020-07-06
中文翻译：
游离和铁卟啉-卡宾酸酯的多参考基态和激发态电子结构。
卟啉铁碳酸钙（IPC）是工程化卡宾转移酶和均相催化中的重要反应中间体。但是，理论和实验之间的差异使对IPC电子结构的理解更加复杂。在文献中，这被定义为基态是开壳还是闭壳单峰态（OSS和CSS）。在这里，我们研究了自由卡宾及其具有可变反式轴向配体的IPC类似物的结构相关的基态和激发自旋态能量。特别是，对于IPC，多引用从头算函数函数与实验更加一致，并且可以预测由CSS（Fe（II）←{：C（X）Y} 0主导的混合单重态基态。）构型（即亲电卡宾），但也具有Fe（III）– {C（X）Y} -•构型（d（xz）中的孔，即自由基卡宾）的微小贡献，不可忽略。在多参考方法中，“类OSS”激发态本质上是金属到配体的电荷转移（MLCT），并且在能量上远高于CSS主导的基态。预计这些“类OSS”激发态的第一，最低能量将偏重于Fe（III）– {C（X）Y} -•（d（yz中的孔））配置。如从交换考虑所期望的那样，该状态在能量上落在相同配置的三元组之上。此外，沿IPC Fe–C（卡宾）键延伸的势能面（PESs）在CSS / OSS特征之间表现出越来越强的混合，具有Fe（III）– {C（X）Y} -•构型（d孔（xz））在键伸长期间在基态下重量增加。沿该结构上相关的PES的亲电/自由基卡宾特征的相对程度可能在催化反应性和选择性模式中发挥作用。IPC反应坐标的未来研究应评估基态和激发态多参考特征的贡献。
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