Substitution reactions of a water-soluble metalloporphyrin with azide and 1,1,3,3-tetramethyl-2-thiourea

The substitution reactions of tetrakis-(4-N-methylpyridyl)porphinecobalt (III) (Co^IIITMpyP) with azide and with 1,1,3,3-tetramethyl-2-thiourea (TMTU) have been studied as a function of pH at 25° and an ionic strength of 0.5 M. The mechanistic pathway proposed for thiocyanate [1] and pyridine [2] is applicable to these ligands as well once allowance is made for two attacking forms of azide, N₃^— and HN₃. A TMTU axial substituent has about the same influence on the rate of further ligand substitution as does SCN^— and a much larger influence than does azide. Similar behavior between bound SCN^— and bound TMTU is also shown in electron-transfer reactions with Ru(NH₃)₆²⁺ - Whereas both sulfur-containing ligands enhance the rate relative to the diaquo complex, the azide complex undergoes reduction an order of magnitude more slowly than does the diaquo complex.