Nitrite Reduction with H₂S/Thiol Mediated by Cobalt and Manganese Porphyrins in the Solid State

The endogenous reduction of nitrite to nitrosyl is drawing increasing attention as a protective mechanism against hypoxic injury in mammalian physiology and as an alternative source of NO, which is involved in a wide variety of biological activities. Thus, chemical mechanisms for this transformation, which are mediated by metallo proteins, are of considerable interest. The study described here examines the reactions of the biomimetic models Co(TTP)(NO₂) (TTP = meso-tetratolylporphyrinato dianion) and Mn(TPP)(ONO) (TPP = meso-tetraphenyl-porphyrinato dianion) in sublimated solid films with hydrogen sulfide (H₂S) and with ethanethiol (EtSH) at various temperatures from 77 K to room temperature using in situ infrared and optical spectroscopy. In both cases, the coordinated nitrite complex is eventually converted to the respective nitrosyl Co(TTP)(NO) and Mn(TPP)(NO); however, reaction at low temperature first gave a novel six-coordinate complex M(Por)(RSH)(nitrite). Warming these films in the presence of excess thiol resulted in the formation of the two nitrosyl complexes. Mass spectrometric analysis of volatile products and DFT computations of possible intermediates are reported, and potential mechanisms for reduction of the coordinated nitrite ions are discussed.