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

• Published in: Inorganic chemistry Vol. 60; no. 12; pp. 8380 - 8387
• Main Authors: van Leest, Nicolaas P, de Bruin, Bas
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 21.06.2021
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.1c00910

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 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 ([Co III (TPP)­( • CHCO 2 Et)]) and mononitrene ([Co III (TPP)­( • 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 • )­( • 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.