Abstract
The copper-containing nitrite reductases (CuNIRs) are a class of enzymes that mediate the reduction of nitrite to nitric oxide in biological systems. Metal–ligand complexes that reproduce the salient features of the active site of CuNIRs are therefore of fundamental interest, both for elucidating the possible mode of action of the enzymes and for developing biomimetic catalysts for nitrite reduction. Herein, we describe the synthesis and characterization of a new tris(2-pyridyl) copper complex ([Cu1(NO2)2]) that binds two molecules of nitrite, and displays all three of the common binding modes for , with one nitrite bound in an asymmetric quasi-bidentate κ2-ONO manner and the other bound in a monodentate fashion with a linkage isomerism between the κ1-ONO and κ1-NO2 binding modes. We use density functional theory to help rationalize the presence of all three of these linkage isomers in one compound, before assessing the redox activity of [Cu1(NO2)2]. These latter studies show that the complex is not a competent nitrite reduction electrocatalyst in non-aqueous solvent, even in the presence of additional proton donors, a finding which may have implications for the design of biomimetic catalysts for nitrite reduction.
Highlights
The oxides of nitrogen are key constituents of the natural nitrogen cycle
We report the synthesis and characterization of a new copper complex which contains a tripodal N-donor ligand and two molecules of nitrite. This complex ([Cu1(NO2)2]) forms immediately upon addition of nitrite to a solution of the precursor copper complex [Cu(1)2]2+, the synthesis and characterization of which we report. [Cu(1)2]2+ shares a common geometry with a number of similar complexes reported in the 1990s by Jonas & Stack [28] the effects of adding nitrite to these previously reported complexes have hitherto not been documented. [Cu1(NO2)2] displays all three common binding modes for NO2−, with one nitrite bound in an asymmetric quasi-bidentate κ2-ONO manner and the other bound in a monodentate fashion with linkage isomerism between κ1-ONO and κ1-NO2 binding modes
A general route to the synthesis of ligand 1 and copper complex [Cu(1)2](PF6)2 is given in scheme 1
Summary
The oxides of nitrogen are key constituents of the natural nitrogen cycle. Of these nitrogen oxides, nitrite (NO2−) occupies arguably the most prominent position, being an intermediate2017 The Authors. The oxides of nitrogen are key constituents of the natural nitrogen cycle. Of these nitrogen oxides, nitrite (NO2−) occupies arguably the most prominent position, being an intermediate. In a range of transformations between the various oxidation states of nitrogen [1]. The reduction of nitrite to nitric oxide (NO) is one such transformation, which is a critical step during the process of denitrification performed by a range of bacteria. The overall equation for this conversion is
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