Iron(III)-nitro porphyrins: theoretical exploration of a unique class of reactive molecules

DFT(PW91/TZP) calculations, including full geometry optimizations, have been carried on [FeII(P)(NO2)]-, Fe(III)(P)(NO2), [Fe(II)(P)(NO2)(py)]-, Fe(III)(P)(NO2)(py), [Fe(III)(P)(NO2)2]-, and Fe(III)(P)(NO2)(NO), where P is the unsubstituted porphine dianion, as well as on certain picket fence porphyrin (TPivPP) analogues. The bonding in [Fe(II)(P)(NO2)]- and Fe(III)(P)(NO2), as well as in their pyridine adducts, reveals a sigma-donor interaction of the nitrite HOMO and the Fe dz2 orbital, where the Fe-Nnitro axis is defined as the z direction and the nitrite plane is identified as xz. Both molecules also feature a pi-acceptor interaction of the nitrite LUMO and the Fe dyz orbital, whereas the SOMO of the Fe(III)-nitro complexes may be identified as dxz. The Fe(III)-nitro porphyrins studied all exhibit extremely high adiabatic electron affinities, ranging from about 2.5 eV for Fe(III)(P)(NO2) and Fe(III)(P)(NO2)(py) to about 3.4 eV for their TPivPP analogues. Transition-state optimizations for oxygen-atom transfer from Fe(III)(P)(NO2) and Fe(III)(P)(NO2)(py) to dimethyl sulfide yielded activation energies of 0.45 and 0.77 eV, respectively, which is qualitatively consistent with the observed far greater stability of Fe(III)(TPivPP)(NO2)(py) relative to Fe(III)(TPivPP)(NO2). Addition of NO to yield {FeNO}6 nitro-nitrosyl adducts such as Fe(P)(NO2)(NO) provides another mechanism whereby Fe(III)-nitro porphyrins can relieve their extreme electron affinities. In Fe(P)(NO2)(NO), the bonding involves substantial Fe-NO pi-bonding, but the nitrite acts essentially as a simple sigma-donor, which accounts for the relatively long Fe-Nnitro distance in this molecule.