ELECTROCHEMISTRY OF NITRITE REDUCTASE MODEL COMPOUNDS (ELECTROCHEMISTRY, IRON-PORPHYRIN, HYDROXYLAMINE).

摘要

The electrochemistry of several iron tetraphenylporphyrin and tetraphenylchlorin complexes was investigated in non-aqueous solvents in order to model assimilatory and dissimilatory nitrite reductases. Iron tetraphenylchlorin, which was originally proposed to be a model of heme d(,1), was studied and compared with iron tetraphenylporphyrin which was a model of heme c. The results show these two complexes have very similar chemistry and electrochemistry. The studies of formation constants of substituted pyridines and amines with ferrous and ferric chlorin indicated that (pi)-interactions (back-bonding) were significant in the ferrous complexes, but were vary much less important for the ferric species. Iron nitrosyl complexes are readily formed by both assimilatory and dissimilatory nitrite reductases. The product of the reduction of this nitrosyl complex was investigated in detail. NMR results of reduced Fe(TPP)(NO) shows the added electron was placed in a MO which was a combination MO of (pi)*-MO in NO and d(,z)2 in iron center (22). IR, NMR and UV/visible studies indicated that Fe(TPP)(NO)('-) was very unstable at room temperature, but relatively stable at low temperature. It was also found that at room temperature, reduction of Fe(TPP)(NO) often leads to form Fe(TPP)(OH)('-). Cyclic voltammetry data of Fe(TPP)(NO) in the presence of weak acid, such as o-chlorophenol, indicated the possibility of second electron reduction followed by protonation to form NHOH or NH(,2)OH coordinated iron porphyrin complexes, which may then disproportion or experience a ECE process to Fe(TPP)(NO) and other nitrogen compounds. Iron tetraphenylporphyrin hydroxylamine complex was synthesized and characterized. It was stable at low temperature, but easy to disproportion to Fe(TPP)(NO) at room temperature and other nitrogen compounds. This complex was assigned as a six-coordinated diamagnetic iron(II) complex by NMR and visible studies at low temperature. It was proposed by this dissertation that this new complex could also be a very important intermediate in nitrite reductases processes.