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

Reductions of nitrate and nitrite (NO^-) are of prime importance in combatting water pollution arising from the excessive use of N-rich fertilizers. While examples of NO^- reductions are known, this report illustrates hydrazine (N₂H₄)-mediated transformations of NO^- to nitric oxide (NO)/nitrous oxide (N₂O). For nitrate reduction to NO, initial coordination of the weakly coordinating NO₃^- anion at [(mC)Cu^II]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)M^II(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)Cu^II(^14/15NO₃)](ClO₄) (1-Cu or 1-Cu/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 [(BzTren)Cu^II]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 Cu^II sites in both [(mC)Cu^II]²⁺ and [(BzTren)Cu^II]²⁺ irrespective of the additional noncovalent interactions, and hence, the N₂H₄ reactions of the copper(II)-nitrite complexes [(mC)Cu^II(O^14/15NO)]⁺ and [(BzTren)Cu^II(O^14/15NO)]⁺ (in 2-Cu/4-Cu) result in a mixture of ^14/15NO and N^14/15NO.