van Leest Nicolaas P, de Bruin Bas
Homogeneous, Supramolecular and Bio-Inspired Catalysis Group, Van 't Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands.
Inorg Chem. 2021 Jun 21;60(12):8380-8387. doi: 10.1021/acs.inorgchem.1c00910. Epub 2021 Jun 5.
Cobalt porphyrin complexes are established catalysts for carbene and nitrene radical group-transfer reactions. The key carbene and mono- and bisnitrene radical complexes coordinated to [Co(TPP)] (TPP = tetraphenylporphyrin) have previously been investigated with a variety of experimental techniques and supporting (single-reference) density functional theory (DFT) calculations that indicated doublet ( = /) ground states for all three species. In this contribution, we revisit their electronic structures with multireference N-electron valence state perturbation theory (NEVPT2)-complete-active-space self-consistent-field (CASSCF) calculations to investigate possible multireference contributions to the ground-state wave functions. The carbene () and mononitrene () radical complexes were confirmed to have uncomplicated doublet ground states, although a higher carbene or nitrene radical character and a lower Co-/ bond order was found in the NEVPT2-CASSCF calculations. Supported by electron paramagnetic resonance analysis and spin counting, paramagnetic molar susceptibility determination, and NEVPT2-CASSCF calculations, we report that the cobalt porphyrin bisnitrene complex () has a quartet ( = /) spin ground state, with a thermally accesible multireference and multideterminant "broken-symmetry" doublet spin excited state. A spin flip on the porphyrin-centered unpaired electron allows for interconversion between the quartet and broken-symmetry doublet spin states, with an approximate 10-fold higher Boltzmann population of the quartet at room temperature.
钴卟啉配合物是卡宾和氮宾自由基基团转移反应的既定催化剂。此前，已利用多种实验技术和辅助（单参考）密度泛函理论（DFT）计算对与[Co(TPP)]（TPP = 四苯基卟啉）配位的关键卡宾以及单氮宾和双氮宾自由基配合物进行了研究，结果表明这三种物质均具有二重态（(S = 1/2)）基态。在本论文中，我们运用多参考N电子价态微扰理论（NEVPT2）-完全活性空间自洽场（CASSCF）计算重新审视了它们的电子结构，以研究基态波函数中可能存在的多参考贡献。卡宾（(S = 1/2)）和单氮宾（(S = 1/2)）自由基配合物被证实具有简单的二重态基态，不过在NEVPT2-CASSCF计算中发现卡宾或氮宾自由基特征更强，且Co-卟啉键级更低。在电子顺磁共振分析、自旋计数、顺磁摩尔磁化率测定以及NEVPT2-CASSCF计算的支持下，我们报告钴卟啉双氮宾配合物（(S = 3/2)）具有四重态自旋基态，以及一个热可及的多参考和多行列式“破缺对称性”二重态自旋激发态。卟啉中心未成对电子上的自旋翻转使得四重态和破缺对称性二重态自旋态之间能够相互转换，在室温下四重态的玻尔兹曼布居数大约高出10倍。
