Revisiting the Electronic Structure of Cobalt Porphyrin Nitrene and Carbene Radicals with NEVPT2-CASSCF Calculations: Doublet versus Quartet Ground States

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 (S = 1/2) ground states for all three species. In this contribution, we revisit their electronic structures with multi-reference 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 ([Co-III(TPP)((CHCO2Et)-C-center dot)]) and mononitrene ([Co-III(TPP)((center dot)NNs)]) radical complexes were confirmed to have uncomplicated doublet ground states, although a higher carbene or nitrene radical character and a lower Co-C/N 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 ([Co-III(TPP center dot)((center dot)NNs)(2)]) has a quartet (S = 3/2) 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.

Automated summary

In this study, the electronic structures of cobalt porphyrin complexes were re-investigated using NEVPT2-CASSCF calculations, revealing higher carbene or nitrene radical character and lower Co-C/N bond order. Specifically, the cobalt porphyrin bisnitrene complex was found to have a quartet spin ground state, along with a multireference and multideterminant broken-symmetry doublet spin excited state. Additionally, a spin flip mechanism was identified that allows for interconversion between the quartet and doublet spin states, with a predominance of the quartet state at room temperature.