Copper complexes relevant to the catalytic cycle of copper nitrite reductase: Electrochemical detection of NO(g) evolution and flipping of NO _2 binding mode upon Cu~(II) → Cu~I reduction

摘要

Copper complexes of the deprotonated tridentate ligand, N-2-methylthiophenyl-2′-pyridinecarboxamide (HL1), were synthesized and characterized as part of our investigation into the reduction of copper(II) o-nitrito complexes into the related copper nitric oxide complexes and subsequent evolution of NO(g) such as occurs in the enzyme copper nitrite reductase. Our studies afforded the complexes [(L1)Cu~(II)Cl] _n (1), [(L1)Cu~(II)(ONO)] (2), [(L1)Cu~(II)(H _2O)](ClO_4)·H_2O (3·H _2O), [(L1)Cu~(II)(CH_3OH)](ClO_4) (4), [(L1)Cu~(II)(CH_3CO_2)]·H_2O (5·H_2O), and [Co(Cp)_2][(L1)Cu~I(NO _2)(CH_3CN)] (6). X-ray crystal structure determinations revealed distorted square-pyramidal coordination geometry around Cu~(II) ion in 1-5. Substitution of the H_2O of 3 by nitrite quantitatively forms 2, featuring the κ~2-O,O binding mode of NO _2 ~- to Cu~(II). Reduction of 2 generates two Cu~I species, one with κ~1-O and other with the κ~1-N bonded NO_2 ~- group. The Cu ~I analogue of 2, compound 6, was synthesized. The FTIR spectrum of 6 reveals the presence of κ~1-N bonded NO_2 ~-. Constant potential electrolysis corresponding to Cu~(II) → Cu ~I reduction of a CH_3CN solution of 2 followed by reaction with acids, CH_3CO_2H or HClO_4 generates 5 or 3, and NO(g), identified electrochemically. The isolated Cu~I complex 6 independently evolves one equivalent of NO(g) upon reaction with acids. Production of NO(g) was confirmed by forming [Co(TPP)NO] in CH _2Cl_2 (λ_(max) in CH_2Cl _2: 414 and 536 nm, ν_(NO) = 1693 cm~(-1)).