School of Chemical Sciences, Indian Institute of Technology Mandi, Mandi, Himachal Pradesh 175075, India.
Inorg Chem. 2023 Apr 10;62(14):5810-5821. doi: 10.1021/acs.inorgchem.3c00463. Epub 2023 Mar 28.
Metal-bound nitrene species are the crucial intermediate in catalytic nitrene transfer reactions exhibited by engineered enzymes and molecular catalysts. The electronic structure of such species and its correlation with nitrene transfer reactivity have not been fully understood yet. This work presents an in-depth electronic structure analysis and nitrene transfer reactivity of two prototypical metal-nitrene species derived from Co(TPP) and Fe(TPP) (TPP = -tetraphenylporphyrin) complexes and tosyl azide nitrene precursor. Parallel to the well-known "cobalt(III)-imidyl" electronic structure of the Co-porphyrin-nitrene species, the formation mechanism and electronic structure of the elusive Fe-porphyrin-nitrene have been established using density functional theory (DFT) and multiconfigurational complete active-space self-consistent field (CASSCF) calculations. Electronic structure evolution analysis for the metal-nitrene formation step and CASSCF-derived natural orbitals advocates that the electronic nature of the metal-nitrene (M-N) core of Fe(TPP) is strikingly different from that of the Co(TPP). Specifically, the "imidyl" nature of the Co-porphyrin-nitrene [(TPP)Co-NTos] (Tos = tosyl) () is contrasted by the "imido-like" character of the Fe-porphyrin-nitrene [(TPP)Fe[Formula: see text]NTos] (). This difference between Co- and Fe-nitrene has been attributed to the additional interactions between Fe-d and N-p orbitals in Fe-nitrene, which is further complemented by the shortened Fe-N bond length of 1.71 Å. This stronger M-N bond in Fe-nitrene compared to the Co-nitrene is also reflected in the higher exothermicity (ΔΔ = 16 kcal/mol) of the Fe-nitrene formation step. The "imido-like" character renders a relatively lower spin population on the nitrene nitrogen (+0.42) in the Fe-nitrene complex , which undergoes the nitrene transfer to the C═C bond of styrene with a considerably higher enthalpy barrier (Δ = 10.0 kcal/mol) compared to the Co congener (Δ = 5.6 kcal/mol) possessing a higher nitrogen spin population (+0.88) and a relatively weaker M-N bond (Co-N = 1.80 Å).
金属结合的氮宾物种是工程酶和分子催化剂中催化氮宾转移反应的关键中间体。然而，这类物种的电子结构及其与氮宾转移反应性的关系尚未得到充分理解。本工作对源自 Co(TPP)和 Fe(TPP)（TPP = -四苯基卟啉）配合物和对甲苯磺酰叠氮氮宾前体的两种典型金属-氮宾物种进行了深入的电子结构分析和氮宾转移反应性研究。与众所周知的 Co-卟啉-氮宾的“钴(III)-亚氨基”电子结构平行，使用密度泛函理论（DFT）和多组态完全活性空间自洽场（CASSCF）计算建立了难以捉摸的 Fe-卟啉-氮宾的形成机制和电子结构。金属-氮宾形成步骤的电子结构演化分析和 CASSCF 衍生的自然轨道表明，Fe(TPP)金属-氮宾（M-N）核的电子性质与 Co(TPP)明显不同。具体来说，Co-卟啉-氮宾 [(TPP)Co-NTos]（Tos = 对甲苯磺酰基）（）的“亚氨基”性质与 Fe-卟啉-氮宾 [(TPP)Fe[Formula: see text]NTos]（）的“亚硝酰基样”特征形成对比。这种 Co-和 Fe-氮宾之间的差异归因于 Fe-氮宾中 Fe-d 和 N-p 轨道之间的附加相互作用，这进一步由 1.71 Å 的缩短 Fe-N 键长补充。与 Co-氮宾相比，Fe-氮宾中更强的 M-N 键也反映在 Fe-氮宾形成步骤的更高放热性（ΔΔ= 16 kcal/mol）中。Fe-氮宾复合物中氮宾氮上相对较低的自旋密度（+0.42）赋予了其“亚硝酰基样”特征[4]，这使得 Fe-氮宾更容易与苯乙烯的 C═C 键发生氮宾转移，其焓垒（Δ= 10.0 kcal/mol）明显高于具有较高氮自旋密度（+0.88）和相对较弱的 M-N 键（Co-N = 1.80 Å）的 Co 同系物[3]（Δ= 5.6 kcal/mol）。
