Nitrate and Nitrite Reductions at Copper(II) Sites: Role of Noncovalent Interactions from Second-Coordination-Sphere
Aditesh Mondal, Kiran P. Reddy, Shubham Som, Deepak Chopra, Subrata Kundu
Abstract
Reductions of nitrate and nitrite (NOx–) are of prime importance in combatting water pollution arising from the excessive use of N-rich fertilizers. While examples of NOx– reductions are known, this report illustrates hydrazine (N2H4)-mediated transformations of NOx– 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/15N) 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 [(Bz3Tren)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 [(Bz3Tren)CuII]2+ irrespective of the additional noncovalent interactions, and hence, the N2H4 reactions of the copper(II)-nitrite complexes [(mC)CuII(O14/15NO)]+ and [(Bz3Tren)CuII(O14/15NO)]+ (in 2-Cu/4-Cu) result in a mixture of 14/15NO and N14/15NO.