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

Abstract

The new copper(I) nitro complex [(Ph(3)P)(2)N][Cu(HB(3,5-Me(2)Pz)(3))(NO(2))] (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 [(Ph(3)P)(2)N][Cu(HB(3,5-Me(2)Pz)(3))(NO(2))]·H(2)O (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 (CuNO(2)H). A comparison of the nitrite reduction reactivity of the electron-rich ligand containing the CuNO(2) complex 2 with that of the known neutral ligand containing the CuNO(2) complex [Cu(HC(3,5-Me(2)Pz)(3))(NO(2))] (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-Me(2)Pz)(3) and anionic [HB(3,5-Me(2)Pz)(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).

Electrochemical reduction of Ttz copper(II) complexes in the presence and absence of protons: Processes relevant to enzymatic nitrite reduction (TtzR,R′= tris(3-R, 5-R′-1, 2, 4-triazolyl)borate)

“…[30] Comparing to other (L)Cu II NO 2 complexes in the literature, our Cu II/I redox potential is much higher than the values for L 1 and L 2 complexes described in the introduction (E 1/2 = -0.027 V and -0.375 V vs. SCE with L 1 and L 2 , respectively) which makes sense because these ligands are stronger donors than Ttz tBu,Me . [26][27][28] Notice that the reduction potential we observed for 2 at 0.18 V is much higher than the value we previously reported for E pc of -1.45 V (vs. a non-aqueous Ag/Ag(ClO 4 ), or -1.15 V vs. SCE reference electrode in CH 2 Cl 2 ). [8,65] We conclude that this reduction potential was in error due to the high resistivity of the solution (CH 2 Cl 2 ) which was previously not accounted for using the Ohmic drop (iR) method.…”

“…We postulated that the reduction potential from Cu II to Cu I of (Ttz tBu,Me )Cu II NO 2 (2) should similarly be sensitive to the addition of acid. The cyclic voltammogram of (Ttz tBu,Me )Cu(II)NO 2 (2) with varied amounts of HBF 4 and the resulting electrochemical data are shown in Figure 6 and Table 2 [27] The difference here is due to Ttz tBu,Me being a weaker electron donor vs. Tp Me,Me . [30] Comparing to other (L)Cu II NO 2 complexes in the literature, our Cu II/I redox potential is much higher than the values for L 1 and L 2 complexes described in the introduction (E 1/2 = -0.027 V and -0.375 V vs. SCE with L 1 and L 2 , respectively) which makes sense because these ligands are stronger donors than Ttz tBu,Me .…”

“…[21][22][23][24][25] For instance, Dilworth et al reported that LCu(II)NO 2 complexes supported by the weak donor ligand, L 1 = tris-[(benzimidazol-2-yl)methyl]amine, were more readily reduced than complexes of a stronger donor ligand, L 2 = tris-(2-aminoethyl)]amine (E 1/2 = -0.027 V and -0.375 V vs. SCE with L 1 and L 2 , respectively). [26][27][28] In contrast to altering the electronic properties by synthetic changes, our group has demonstrated that the electron donating properties of Ttz ligand can be tuned by protonation at the fourth position nitrogen atom of the triazole ring (Figure 2, right). [29] This point is described further below.…”

Nitric oxide generation study of unsymmetrical β-diketiminato copper(ii) nitrite complexes

“…Previous reports illustrating the copper(I)-nitrito complexes could be a good model system for bio-relevant nitrite reduction despite their instability issue. [29][30][31] On the other hand, Cu(II)metal scaffolds (Scheme 1) such as the N 2 S ligand 24,25 and the hydrotris(triazolyl)borate ligand have been used for the reduction of nitrite to nitric oxide in the presence of an acid source using a cyclic voltametric reduction process. 31 Apart from them, triphenylphosphine (PPh 3 ) has also been used but under higher temperature conditions (60 °C) (Scheme 1).…”

An investigation into the unusual linkage isomerization and nitrite reduction activity of a novel tris(2-pyridyl) copper complex

“…The apparently rather simple active-site structure of CuNIR (a tripodal N-donor ligand which supports a copper centre undergoing a one-electron redox reaction) has led numerous groups of coordination chemists to attempt the synthesis of functional analogues of this enzyme [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. Most of these complexes bind only one molecule of nitrite, but Woollard-Shore et al [20] have reported two examples where two nitrite molecules are bound to a single Cu centre.…”