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 (L1) has been investigated. Reaction of L1 with equimolar copper(II) ions results in the formation of
the single-stranded mononuclear complex [Cu(L1)(ClO4)2] (1), whereas reaction with copper(I) ions results in the
double-stranded dinuclear helicate [Cu2(L1)2][PF6]2 (2). Both complexes were characterized by X-ray crystallography,
UV−vis spectroscopy, and electrospray ionization mass spectroscopy (as well as 1H NMR spectroscopy for
diamagnetic 2). Complex 2 is redox-active and, upon one-electron oxidation, forms the stable tricationic mixed-valence helicate [Cu2(L1)2]3+ (3). This species can also be prepared in situ by combining [Cu(MeCN)4][BF4], [Cu(H2O)6][BF4]2, and L1 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 [Cu2]3+ core varying from localized
to delocalized depending on the solvent polarity.