Copper(II) complexes with unusual axial phenolate coordination as structural models for the active site in galactose oxidase: ^x-ray crystal structures and spectral and redox properties of [Cu(bpnp)X] complexes

Abstract

The crystal structures of [Cu(bpnp)(SCN)]·NH₄SCN (1), [Cu(bpnp)(CH₃COO)]·CH₃OH·C₈H₁₀ (2), and [Cu(bpnp)ClO₄] (3) [Hbpnp = 2-(bis(pyrid-2-ylmethyl)aminomethyl)-4-nitrophenol] reveal a distorted square pyramidal geometry around Cu(II) with an unusual axial coordination of phenolate. The mononuclear complex [Cu(bpnp)(SCN)]·NH₄SCN crystallizes in the triclinic space group P ↑ with a = 10.796(2) Å, b = 10.804(2) Å, c = 12.559(2) Å, α = 71.38(1) ° , β = 72.68(1) ° , γ = 61.69(1) ° , and Z = 2. The mononuclear acetate [Cu(bpnp)(CH₃COO)]·CH₃OH·C₈H₁₀ crystallizes in the triclinic space group P↑ with a = 10.480(6) Å, b = 12.116(4) Å, c = 12.547(3) Å, α = 98.77(3) ° , β = 113.37(3) ° , γ = 100.78(3) ° , and Z = 2. The binuclear perchlorate complex crystallizes in the monoclinic space group C2/c with a = 13.417(3) Å, b = 20.095(2) Å, c = 16.401(2) Å, α = 102.21(2) ° , and Z = 8. The coordination plane in all these complexes is comprised of the tertiary amine and two pyridine nitrogens. The fourth equatorial position is occupied by SCN^-/CH₃COO^- in the mononuclear complexes but by the coordinated phenolate ion from the adjacent molecule in the perchlorate complex, resulting in its dimerization. The unusual occupation of phenolate ion in the axial site is possibly due to the steric constraint at copper imposed by the 5,5,6-chelate ring sequence. The thiocyanate/acetate coordination geometry is reminiscent of the active site of the radical copper enzyme galactose oxidase (GOase) with an axial phenolate and equatorial SCN^-/CH₃COO^- ligands. Further, the present complexes exhibit several spectral features also similar to this enzyme. The addition of chloride or thiocyanate or acetate ions dissociates the dimeric structure of the perchlorate complex to produce the corresponding monomeric derivatives. The study of the interaction of the acetate complex with N₃^- and CN^- ions provide insight into the anion binding properties of the enzyme. The sensitivity of the acetate complex to protons suggests the facile dissociation of the axial phenolate which then acts as a base to bind to protons. The implication of this reaction to the GOase mechanism is discussed.