HNO is an important species in nitric oxide (NO) biology and the global N-cycle. Heme-HNO species are proposed key intermediates in several important biotransformations such as NO detoxification by fungal cyt P450 NO reductases (via hydride attack at the ferric heme-NO) en route to N2O formation, and nitrite reduction by cytochrome c nitrite reductase (via proton attack on a heme-bound NO). Although, hemeprotein-HNO complexes are known and have been implicated in some important biological processes, it is surprising that there are currently only a few well-characterized heme-Fe HNO models that could be used to correlate their chemical properties with their observed reactivities and biological effects. As part of our on-going investigation into heme-mediated chemistry of NOx and its derivatives, we examined the factors that lead to successful hydride attack at the coordinated NO of ferric hemes to form Fe-HNO products. Following up on our earlier report, we show that hydride attack at the bound NO in six-coordinate ferric nitrosyl porphyrins at low temperature can be quite general, yielding the corresponding (por)Fe(HNO)(L) (por = OEP, PPDME, TTP; L = ImH, 1-MeIm, 5-MeIm) derivatives. We find, however, that hydride attack at the five-coordinate [(OEP)Fe(NO)]+ complex results in direct H-attack at the metal to give the hydride (OEP)Fe(NO)H. Both the N-path for hydride attack to form Fe-HNO, and the H-path to form Fe-H have been probed by DFT calculations. Interestingly, the six-coordinate (por)Fe(HNO)(L) compounds react with external NO to yield N2O, modeling the critical N−N bond formation step carried out by the fungal cyt P450 NO reductase enzyme. The preparation, low temperature IR and 1H NMR characterization, DFT calculations, and reactivity of these species will be presented.

Fe−HNO vs. (NO)Fe−H formation from hydride attack at ferric nitrosyl porphyrins

254th National Meeting (2017)

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