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 (N 2 H 4 )-mediated transformations of NO x - to nitric oxide (NO)/nitrous oxide (N 2 O). For nitrate reduction to NO, initial coordination of the weakly coordinating NO 3 - 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 NO 3 - -bound metal-cryptates [( mC )M II (NO 3 )](ClO 4 ) ( 1-M , M = Cu/Zn) demonstrates the binding of NO 3 - through noncovalent (NH···O, CH···O, and anion···π) and metal-ligand coordinate interactions. Subsequently, reactions of [( mC )Cu II ( 14/15 NO 3 )](ClO 4 ) ( 1-Cu or 1-Cu/ 15 N) with N 2 H 4 ·H 2 O have been illustrated to reduce 14/15 NO 3 - to 14/15 NO. Intriguingly, in the absence of the second-coordination-sphere interactions, a closely related coordination motif [( Bz 3 Tren )Cu II ] 2+ (in 3-Cu ) does not bind NO 3 - and is unable to assist in N 2 H 4 ·H 2 O-mediated NO 3 - reduction. In contrast, nitrite coordinates at the tripodal Cu II sites in both [( mC )Cu II ] 2+ and [( Bz 3 Tren )Cu II ] 2+ irrespective of the additional noncovalent interactions, and hence, the N 2 H 4 reactions of the copper(II)-nitrite complexes [( mC )Cu II (O 14/15 NO)] + and [( Bz 3 Tren )Cu II (O 14/15 NO)] + (in 2-Cu / 4-Cu ) result in a mixture of 14/15 NO and N 14/15 NO.