Copper(I) Nitro Complex with an Anionic [HB(3,5-Me2Pz)3]− Ligand: A Synthetic Model for the Copper Nitrite Reductase Active Site

The new copper­(I) nitro complex [(Ph3P)2N]­[Cu­(HB­(3,5-Me2Pz)3)­(NO2)] (2), containing the anionic hydrotris­(3,5-dimethylpyrazolyl)­borate ligand, was synthesized, and its structural features were probed using X-ray crystallography. Complex 2 was found to cocrystallize with a water molecule, and X-ray crystallographic analysis showed that the resulting molecule had the structure [(Ph3P)2N]­[Cu­(HB­(3,5-Me2Pz)3)­(NO2)]·H2O (3), containing a water hydrogen bonded to an oxygen of the nitrite moiety. This complex represents the first example in the solid state of an analogue of the nitrous acid intermediate (CuNO2H). A comparison of the nitrite reduction reactivity of the electron-rich ligand containing the CuNO2 complex 2 with that of the known neutral ligand containing the CuNO2 complex [Cu­(HC­(3,5-Me2Pz)3)­(NO2)] (1) shows that reactivity is significantly influenced by the electron density around the copper and nitrite centers. The detailed mechanisms of nitrite reduction reactions of 1 and 2 with acetic acid were explored by using density functional theory calculations. Overall, the results of this effort show that synthetic models, based on neutral HC­(3,5-Me2Pz)3 and anionic [HB­(3,5-Me2Pz)3]− ligands, mimic the electronic influence of (His)3 ligands in the environment of the type II copper center of copper nitrite reductases (Cu-NIRs).