A triangular copper(I) complex displaying allosteric cooperativity in its electrochemical behavior and a mixed-valence Cu(I)-Cu(I)-Cu(II) state with unusual temperature-dependent behavior

Reaction of the tris-chelating hexadentate podand ligand tris[3-(2-pyridyl)pyrazol-1-yl]hydroborate (Tp(Py)) With [Cu(MeCN)(4)][PF6] affords [Cu-3(I)(Tp(Py))(2)][PF6] (1), which was crystallographically characterized. 1.(MeCN)(2): C52H44B2Cu3F6N20P, orthorhombic, Pna2(1); a = 24.592(7), b = 16.392(5), c = 13.365(5) Angstrom; Z = 4. Each Cu(I) ion is four coordinated by one N,N'-bidentate arm from each ligand; each ligand therefore donates each bidentate arm to a different Cu(I) ion. The isosceles triangular arrangement of Cu(I) ions with N-donor ligands is reminiscent of the tricopper(I) site of ascorbate oxidase. One-electron oxidation of 1 affords the (Cu2CuII)-Cu-I complex [Cu-3(Tp(Py))(2)][PF6](2) (2). The potentials of the Cu(I)/Cu(II) redox couples are affected by the ease with which the accompanying geometric rearrangement can occur. Thus, the first oxidation of 1 is facile (-0.52 V vs the ferrocene/ferrocenium couple, Fc/Fc(+)), but as a result of the concomitant structural rearrangement the second oxidation is rendered much more difficult (+0.12 V vs Fc/Fc(+)) and results in slow decomposition of the product. A third oxidation does not occur at accessible potentials. This complex therefore exhibits negative cooperative behavior, in which the geometric change accompanying one metal-based redox change hinders further redox changes at other sites via an allosteric effect. EPR studies on the mixed-valence complex 2 show that in frozen glasses below 120 K the unpaired electron is delocalized over two metal centers (7-line spectrum), but above 160 K the electron becomes localized and gives a simple axial spectrum. The electronic spectrum of 2 in solution shows an intense band at 910 nm (epsilon 2100 dm(3) mol(-1) cm(-1)) which we believe to be an IVCT band. The combination of EPR and electronic spectral studies show that 2 is class III (fully delocalized over 2 centers) below 120 K but class II (localized but strongly interacting) at higher temperatures.