Metalloporphyrin–Nitroxyl Interactions: The Low-Energy States of Reduced Manganese, Iron, and Cobalt Porphyrin Nitrosyls

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.