Copper(II) complexes with {N,N′,N,N′-bis[(2-hydroxybenzyl) (2-pyridylmethyl)]-1,3-propanediamine}—H2bbppn: their suitability as models for the inactive form of galactose oxidase

This paper summarizes the results of our studies on the structural, spectral and redox properties of new mononuclear copper(II) complexes which can mimic some structural and spectroscopic features of the enzyme galactose oxidase (GAO) in its inactive form. The crystal structure of the [CuII(H2bbppn)](ClO4)2 complex 1 (where H2bbppn={N,N′,N,N′-bis[(2-hydroxybenzyl) (2-pyridylmethyl)]-1,3-propanediamine}) has been determined by X-ray crystallography. This structure reveals a distorted octahedral geometry around the Cu(II) atom, where the equatorial plane is occupied by alkylamine and pyridine nitrogen atoms (in a cis fashion), and also reveals unusual coordination by two protonated oxygen atoms (from phenol groups) in the axial positions. The long Cu(II)–Ophenol distances of 2.474(5) and 2.501(5) Å in complex 1, are significantly similar to the Cu(II)–O(Tyr495) distance (≈2.6 Å) in the GAO. The equatorial Cu–N bonds showing mean values of 2.03 Å, are also in good agreement with the values of ≈2.10 Å for GAO. The addition of base to complex 1 results in the formation of a green complex 2, for which a square-pyramidal geometry is suggested, with one oxygen atom in an equatorial position while the other oxygen atom is not coordinated as indicated by potentiometric titration. Furthermore, the spectroscopy (UV–Vis and EPR) properties of 2 are essentially similar to those observed in the inactive form of GAO.