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 [Fe-II(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 d(z)(2) orbital, where the Fe-N-nitro 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 d(yz) orbital, whereas the SOMO of the Fe((III))-nitro complexes may be identified as d(xz). 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-N-nitro distance in this molecule.