Reduction of Copper(II) Complexes of Tripodal Ligands by Nitric Oxide and Trinitrosation of the Ligands

Abstract

The copper(II) centers in two copper(II) complexes of tripodal amine ligands, in acetonitrile solvent, upon exposure to nitric oxide have been found to produce a thermally unstable [Cu(II)-NO] intermediate followed by the reduction of copper(II) to copper(I). This reduction resulted in the concomitant trinitrosation of the ligands at their terminal amine centers.

Mechanisms of Nitric Oxide Reactions Mediated by Biologically Relevant Metal Centers

“…In this case, however, the ligand was not nitrosated, so a mechanism along the lines of Scheme 7 is likely, although there was no direct evidence for the formation of a Cu II NO intermediate. Previous studies by this research group with other ligands did observe ligand nitrosation occurring concomitant with NO reductions of Cu(II) complexes [149]. In another interesting study [150], these researchers prepared the cupric complexes Cu(2-aminomethylpyridine) 2 2+ and Cu(tren)(AN) 2+ (tren ¼ bis-(2-aminoethyl)amine, AN ¼ acetonitrile).…”

“…A different example has recently been reported where a {MNO} 7 complex displays a tendency to decompose slowly in aqueous solution presumably via protonation of the nitrosyl followed by dissociation of HNO. In this case, the complex was the water-soluble heme model Fe II (TPPS)(NO) that had been prepared in slightly acidic (pH 5.8) aqueous buffer [149]. Although, such ferrous porphyrinato nitrosyls are often considered to be quite unreactive, this solution slowly underwent spontaneous decay to give the ferric species Fe III (TPPS) (23).…”

Nitric oxide reactivity of a manganese(II) complex leading to nitrosation of the ligand

“…The reduction of Cu II centres in some proteins, such as cytochrome c oxidase and laccase, by NO is known for a long time [18,19]. In recent years this has been exemplified by a number of model copper(II) complexes [20][21][22][23][24][25][26][27][28].…”

“…In our laboratory, we have been studying the reactivity of NO with copper(II) complexes and found the reduction of metal ion by NO leads to the N-nitrosation and diazotization at the secondary and primary amine centers, respectively, of ligand frameworks [24][25][26][27][28]. Interestingly, the N-nitrosation, in cases of Cu(II) complexes, may not necessarily always proceed through Cu(II)-nitrosyl formation.…”

“…Depending upon the ligand denticity and geometry of the complex, it may proceed through a deprotonation mechanism in presence of base. For example, in case of tetradentate tripodal ligand, the N-nitrosation takes place through a Cu(II)-nitrosyl intermediate [25]. In case of tetradentate macrocyclic ligand, it proceeds through a deprotonation of the N-H group [36].…”