Localization and delocalization in a mixed-valence dicopper helicate

The coordination chemistry of the tetradentate pyridyl-thiazole (py-tz) N-donor ligand 6,6'-bis(4-phenylthiazol-2-yl)-2,2'-bipyridine (L-1) has been investigated. Reaction of L-1 with equimolar copper(II) ions results in the formation of the single-stranded mononuclear complex [Cu(L-1)(ClO4)(2)] (1), whereas reaction with copper(I) ions results in the double-stranded dinuclear helicate [Cu-2(L-1)(2)][PF6](2) (2). Both complexes were characterized by X-ray crystallography, UV-vis spectroscopy, and electrospray ionization mass spectroscopy (as well as H-1 NMR spectroscopy for diamagnetic 2). Complex 2 is redox-active and, upon one-electron oxidation, forms the stable tricationic mixed-valence helicate [Cu-2(L-1)(2)](3+) (3). This species can also be prepared in situ by combining [Cu(MeCN)(4)][BF4], [Cu(H2O)(6)][BF4](2), and L-1 in a 1:1:2 ratio in nitromethane. X-ray crystallographic analysis of 3 provides structural evidence for the presence of an internuclear Cu-Cu bond, with an even distribution of spin density across the two Cu centers. Room-temperature UV-vis spectroscopy is consistent with this finding; however, frozen-glass EPR spectroscopic investigations suggest solvatochromic behavior at 110 K, with the [Cu-2](3+) core varying from localized to delocalized depending on the solvent polarity.