Dinuclear Copper(II) Complexes with Carboxylate-Rich Coordination Environments. Models for Substituted Copper(II) Aminopeptidases
The dinucleating ligand N,N'-(2-hydroxy-5-methyl1-, 3-xylylene)bis(N-carboxymethylglycine) (CH3HXTA) has been used to synthesize the dinuclear Cu(II) diaqua complex [Cu2(CH3HXTA) (H2O)2]H•4H2O (1): monoclinic space group P21In (a = 15.092(3) Å, b = 10.842(3) Å, c = 15.219(4) Å, and β = 104.70(2)°). The structure shows two distinct square pyramidal Cu(II) centers with each Cu(II) ion bound to two carboxylate oxygen atoms, one amine nitrogen atom, the phenolate oxygen atom, and one water oxygen atom. The Cu- -Cu separation is 3.726 Å and Cu1-O1-Cu2 angle is 127.9°. The phenyl ring of the CH3HXTA ligand is twisted relative to the Cu1-O1-Cu2 plane and the resulting dihedral angle is 46.7°. The electronic absorption and EPR spectra of 1 in aqueous solution at pH 3 suggest a shift toward trigonal bipyramidal Cu(II) coordination in solution. Variable temperature magnetic susceptibility data for 1 indicates that it behaves as a simple Curie-Weiss paramagnet and hence the Cu(II) ions show little coupling (J ∼ 0). Titration of 1 with sodium hydroxide is accompanied by a reversible shift in the phenoxide-to-copper LMCT band at 420 nm and a concomitant decrease in the molar absorptivity. These data suggest that 1, is converted to a second dicopper(II) complex (2) with an apparent ka of 8. The structure of 2, derived from electronic absorption, EPR, NMR, and Evans susceptibility measurements, indicate it is the (μ-phenoxo)( μ-hydroxo)dicopper(II) complex [CU2 (CH3HXTA) μ-OH)]2-.