title: Metalloporphyrin-Nitroxyl Interactions: The Low-Energy States of Reduced Manganese, Iron, and Cobalt Porphyrin Nitrosyls.
abstract: DFT calculations employing the OLYP and B3LYP functionals have been used to map out the low energy states of the metalloporphyrin-nitroxyl adducts "M(Por) + NO(-)" and "M(Por) + HNO", where M = Fe, Co, and Mn and Por(2-) is the dianion of unsubstituted porphyrin. For [Fe(Por)(NO)](-), the calculations yield two low-energy solutions, with MS = 0 and 1. The MS = 0 solution is thought to represent the experimentally observed diamagnetic ground states of {FeNO}(8) porphyrins, and both functionals yield FeNO geometrical parameters in excellent agreement with a recent crystal structure. For [Co(Por)(NO)](-), the lowest-energy solution for both OLYP and B3LYP is a true {CoNO}(9) state that appears to be best described as a high-spin Co(II) center with a dxy(2)dxz(1)dyz(1)dz2(2)dx2-y2(1) configuration antiferromagnetically coupled to a NO(-) diradical. Such an electronic configuration is expected to lead to diagnostic structural features, including long equatorial Co-N distances (∼2.1 Å), a strong displacement (∼0.4 Å) of the metal from the mean plane of the equatorial nitrogens, and a relatively short Co-N(O) distance (1.8 Å), which should all be experimentally observable. The dx2-y2(1) electronic configuration should also lead to characteristic EPR hyperfine parameters. The calculations also indicate a number of other low-energy states for [Co(Por)(NO)](-), including multiple {CoNO}(8) porphyrin anion radical states. For [Mn(Por)(NO)](-), both functionals indicate a rather complex electronic state landscape, including multiple {MnNO}(6) porphyrin anion radical states as well as a high-spin S = 3/2 {MnNO}(7) state. Both functionals clearly indicate a low-spin Fe(II) state for [Fe(Por)(HNO)]. On the other hand, two comparably low-energy states are predicted for both [Co(Por)(HNO)] and [Mn(Por)(HNO)]. For [Co(Por)(HNO)], the two states are a low-spin Co(II) state with a dxy(2)dxz(2)dyz(2)dz2(1) configuration and a low-spin Co(III)(HNO)(•-) state. For [Mn(Por)(HNO)], the two states may be described as low- (S = 1/2) and intermediate-spin (S = 3/2) Mn(II). The latter state has a relatively long Mn-N(O) distance of about 2.07 Å, which may be indicative of facile HNO dissociation.
authors.affiliation: Department of Chemistry and Center for Theoretical and Computational Chemistry, UiT - The Arctic University of Norway , 9037 Tromsø, Norway.|Department of Chemistry, University of the Free State , 9300 Bloemfontein, Republic of South Africa.
authors.forename: Jeanet
authors.initials: J
authors.lastname: Conradie
authors.affiliation: Department of Chemistry and Center for Theoretical and Computational Chemistry, UiT - The Arctic University of Norway , 9037 Tromsø, Norway.
authors.forename: Abhik
authors.initials: A
authors.lastname: Ghosh