Synthesis, molecular structure, and reactivity of dinuclear copper(II) complexes with carboxylate-rich coordination environments

The dinucleating ligand N,N'-(2-hydroxy-5-methyl-1,3-xylylene)bis(N-(carboxymethyl)glycine) (CH(3)HXTA) has been used to synthesize the dinuclear Cu(II) bis(pyridine) complex Na[Cu-2(CH(3)HXTA)(Py)2]. 1.5(1,4-dioxane) (Na(1)): triclinic space group P (1) over bar (a = 12.550(3) Angstrom, b = 13.413(3) Angstrom, c = 13.540(4) Angstrom, alpha = 117.12(2)degrees, beta = 104.70(2)degrees, and gamma = 92.13(2)degrees). The structure shows two distinct distorted square pyramidal Cu(IT) centers with each Cu(II) ion bound by two carboxylate oxygen atoms, one amine nitrogen atom, a phenolate oxygen atom, and one pyridine nitrogen atom. The Cu--Cu separation is 3.531 Angstrom, and the Cu1-O1-Cu2 angle is 123.7 degrees. The phenyl ring of the CH(3)HXTA ligand is twisted relative to the Cu1-O1-Cu2 plane, and the resulting dihedral angle is 44.2 degrees. The electronic absorption spectrum of 1 in aqueous solution at pH 3 suggests a shift toward trigonal bipyramidal Cu(II) coordination in solution. Spectral titration of Na[Cu-2(CH(3)HXTA)(H2O)(2)] with L (where L = pyridine or sodium cyanide) results in complexes with terminal L groups. These exogenous ligands appear to bind in a positive cooperative stepwise fashion. Variable-temperature magnetic susceptibility data for 1 indicate that the Cu(II) ions are antiferromagnetically coupled (-2J = 168 cm(-1)). X-band EPR spectra of an aqueous solution of 1 shows isotropic signals with g = 2.14, while a powdered sample of 1 provides no EPR spectrum. A Delta M-s= 2 transition at g = similar to 4.5, expected for weakly magnetically coupled Cu(IT) ions, is not observed for powdered samples but is observed for a methanolic solution sample of 1. On the basis of these data, the two Cu(II) ions are antiferromagnetically coupled in the solid state but due to a coordination geometry change become weakly ferromagnetically or antiferromagnetically coupled in solution. H-1 NMR studies on a methanol solution of 1 are consistent with weak spin-coupling in solution.