Synthesis and characterization of completely delocalized mixed-valent dicopper complexes

The multidentate ligands tris[(N'-tert-butylureayl)-N-ethyl)]amine (H(6)1) and 1-(tert-butylaminocarbonyl)-2,2-dimethylaminoethane (H(2)2) have been used to investigate the assembly and properties of complexes with (CuCu1.5)-Cu-1.5 units. The complexes [Cu(H(5)1)](2)(+) and [Cu(H2)](2)(+) have been isolated and structurally characterized by X-ray diffraction methods. [Cu(H(5)1)](2)(+) has a (CuCu1.5)-Cu-1.5 core, with each copper ion having square planar coordination geometry. The copper ions are linked through two mono-deprotonated urea ligands, which coordinate as mu-1,3-(kappaN:kappaO) ureate bridges to produce a Cu-Cu distance of 2.39 Angstrom. The remaining two urea arms of [H(5)1](-) form intramolecular hydrogen bonds, the result of which is to confine the (CuCu1.5)-Cu-1.5 unit within a pseudomacrocycle. The structure of [Cu(H2)12+ lacks intramolecular hydrogen bonds and thus does not have a pseudomacrocyclic structure. However, the structural properties of the (CuCu1.5)-Cu-1.5 core in [Cu(H2)](2)(+) are nearly identical to those of [Cu(H(5)1)](2)(+). Both complexes exhibit rhombic EPR spectra at 77 K, which do not change upon cooling to 4 K. The optical spectra of [Cu(H(5)1)](2)(+) and (Cu(H2)](2)(+) are dominated by an intense band at similar to700 nm. These spectral characteristics are consistent with [Cu(H(5)1)](2)(+) and [Cu(H2)](2)(+) being classified as fully delocalized (type III) mixed-valent species.