Nitrate and Nitrite Reductions at Copper(II) Sites: Role of Noncovalent Interactions from Second-Coordination-Sphere

Reductions of nitrate and nitrite (NO x –) are of prime importance in combatting water pollution arising from the excessive use of N-rich fertilizers. While examples of NO x – reductions are known, this report illustrates hydrazine (N2H4)-mediated transformations of NO x – to nitric oxide (NO)/nitrous oxide (N2O). For nitrate reduction to NO, initial coordination of the weakly coordinating NO3 – anion at [(mC)­CuII]2+ cryptate has been demonstrated to play a crucial role. A set of complementary analyses (X-ray diffraction and Fourier-transform infrared spectroscopy (FTIR), UV–vis, and NMR spectroscopies) on NO3 –-bound metal-cryptates [(mC)­MII(NO3)]­(ClO4) (1-M, M = Cu/Zn) demonstrates the binding of NO3 – through noncovalent (NH···O, CH···O, and anion···π) and metal–ligand coordinate interactions. Subsequently, reactions of [(mC)­CuII(14/15NO3)]­(ClO4) (1-Cu or 1-Cu/ 15 N) with N2H4·H2O have been illustrated to reduce 14/15NO3 – to 14/15NO. Intriguingly, in the absence of the second-coordination-sphere interactions, a closely related coordination motif [(Bz 3 Tren)­CuII]2+ (in 3-Cu) does not bind NO3 – and is unable to assist in N2H4·H2O-mediated NO3 – reduction. In contrast, nitrite coordinates at the tripodal CuII sites in both [(mC)­CuII]2+ and [(Bz 3 Tren)­CuII]2+ irrespective of the additional noncovalent interactions, and hence, the N2H4 reactions of the copper­(II)-nitrite complexes [(mC)­CuII(O14/15NO)]+ and [(Bz 3 Tren)­CuII(O14/15NO)]+ (in 2-Cu/4-Cu) result in a mixture of 14/15NO and N14/15NO.