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Reduction of Nitrous Acid with a Macrocyclic Rhodium Complex That Acts As a Functional Model of Nitrite Reductase

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journal contribution
posted on 21.02.2016, 14:36 by Kathleen E. Kristian, Andreja Bakac
Nitrous acid reacts with L2(H2O)­Rh2+ (L2 = meso-hexamethylcyclam) in acidic aqueous solutions to generate a strongly absorbing intermediate Int-1max 400 nm, ε = 1200 M–1 cm–1). The reaction follows a mixed second order rate law with k = (6.9 ± 0.3) × 104 M–1 s–1, independent of [H+]. The lack of acid dependence shows that Int-1 is a rhodium­(II) complex of HNO2, most reasonably assigned as L2(H2O)­Rh­(HNO2)2+. This species is analogous to the early iron and copper intermediates in the reduction of nitrite by nitrite reductases and by deoxyhemoglobin. In the presence of excess L2(H2O)­Rh2+, the lifetime of Int-1 is about 1 min. It decays to a 1:1 mixture of L2(H2O)­RhNO2+ and L2Rh­(H2O)23+ with kinetics that are largely independent of the concentration of excess L2(H2O)­Rh2+ and of [H+] at [H+] < 0.03 M. At [H+] > 0.03 M, an acid-catalyzed pathway becomes effective, suggesting protonation and dehydration of Int-1 to generate L2(H2O)­RhNO3+ (Int-2) followed by rapid reduction of Int-2 by excess L2(H2O)­Rh2+. Int-2, which was generated and characterized independently, is an analog of the electrophilic intermediates in the mechanism of biological reduction of nitrite to NO. Excess nitrite greatly reduces the lifetime of Int-1, which under such conditions decomposes on a millisecond time scale by nitrite-catalyzed disproportionation to yield L2(H2O)­RhNO2+ and L2Rh­(III). This reaction provides additional support for the designation of Int-1 as a Rh­(II) species. The complex reaction mechanism and the detection of Int-1 demonstrate the ability of inorganic complexes to perform the fundamental chemistry believed to take place in the biological reduction of HNO2 to NO catalyzed by nitrite reductases or deoxyhemoglobin.