Thermal stability and absorption spectroscopic behavior of (μ-Peroxo)dicopper complexes regulated with intramolecular hydrogen bonding interactions

In order to clarify the effect of hydrogen bonding on the stabilities of (mu-peroxo)dicopper complexes with a trans-mu-1,2-peroxo form, novel copper complexes with intramolecular hydrogen bonding interaction sites have been synthesized, and their spectroscopic properties and thermal stabilities studied. The selected tripodal tetradentate ligands were tris(2-pyridylmethyl)amine (TPA) derivatives bearing pivalamido and amino groups at the 6-position of the pyridine ring in TPA, {[6-(pivalamido)pyrid-2-yl]methyl}bis(pyrid-2-ylmethyl)amine (MPPA) and [(6-aminopyrid-2-yl)methyl]bis(pyrid-2-ylmethyl)amine (MAPA). The single-crystal X-ray structure of a monomeric Cu-II complex with N-3(-) namely [Cu(mppa)N-3]ClO4 (1a), revealed an interligand hydrogen bonding interaction between the substituent NH group of MPPA and the azide nitrogen atom in the axial position. The Cu-I complexes of MPPA and MAPA were immediately oxygenated with dioxygen in acetone solution at -78 degreesC to give the p-peroxo dinuclear copper(II) complexes [{Cu(mppa)}(2)(O-2)](2+) (1b) and [{Cu(mapa)}(2)(O-2)](2+) (2b). These complexes exhibited two kinds of characteristic absorption bands (pi*(sigma)-->d(sigma), pi*(v)-->d(sigma)) originating from the ligand-metal charge transfer (LMCT) Of O-2(2-) to Cu. Affected by the hydrogen bonding interaction, the pi*(sigma)-->d(sigma) CT band shifted significantly to a higher energy region and the pi*(v)-->d(sigma) CT absorbance decreased due to stabilization of the pi*(sigma) orbital and restriction of the Cu-O bond rotation. The thermal stabilities of the (mu-peroxo)dicopper(II) complexes were estimated from their decomposition rates which decreased in the order, 2b approximate to [{Cu(tpa)}(2)(O-2)](2+) (3b) > 1b much greater than [{Cu(6-metpa)}(2)(O-2)](2+) (4b) (6-MeTPA = [(6-methylpyrid-2-yl)methyl]bis(pyrid-2-ylmethyl)amine). The above findings indicate that the interligand hydrogen bonding interaction, although overcome to some extent by the adverse effect of the steric bulk of the NH group, is inclined to stabilize (mu-peroxo)dicopper(II) complexes. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003.