Abstract

The compounds [L1Cu(μ-OH)2CuL1] (ClO4)2 (1), [L2Cu(μ-OH)2CuL 2](ClO4)2 (2), [L3Cu(μ-OH) 2CuL3](ClO4)2 (3), [L 3Cu-(μ-OMe)2CuL3](ClO4) 2 (4), [L3Cu(μ-F)2CuL3](BF 4)2 (5), [L4Cu-(μ-Cl)2CuL 4](ClO4)2 (6), and [Cu(μ-L5) 2Cu](ClO4)2 (7), where L1 = bis(pyridin-2-ylmethyl)benzylamine, L2 = (6-methyl-pyridin-2- ylmethyl)(pyridin-2-ylmethyl)benzylamine, L3 = bis(6-methylpyridin-2- ylmethyl)benzylamine, L4 = (pyridin-2-ylethyl)(pyridin-2-ylmethyl) benzylamine, and L5 = (6-oxidomethylpyridin-2-ylmethyl)(6- methylpyridin-2-yl-methyl)benzylamine, were structurally characterized and studied by low-temperature EPR spectroscopy. The formation of compound 5 involved fluoride abstraction from BF4 - and that of 7 involved O2-assisted C-H (methyl) activation/monooxygenation of L3. Whereas the dications of 1-4 and 7 possess a square-pyramidal configuration at the copper(II) centers with hydroxide or alkoxide bridges in equatorial positions, the dications of 5 and 6 exhibit equatorial/axial arrangements for the bridging halide ions. In the dication of 5, the tridentate ligand binds in a meridional fashion and includes two adjacent five-membered chelate rings. Such a situation has been observed previously only with chloride, alkoxide, or aqua bridges. In contrast to the syn configuration of the dication of 2, the dications of complexes 1, 3, 4, and 7 adopt an anti configuration with respect to the binding of Ln. The dications of 1, 2, and 7 have the pyridinyl or 6-methyl-pyridinyl groups in the axial position, whereas those of 3 and 4 feature the tertiary amine N atom at that site. DFT calculations of various configurations of the dications of 1-6 reproduce the experimentally observed structural alternatives in complexes 1-4, but they do not predict the axial/equatorial structure of 5 as the lowest-energy configuration. The systematic overestimation of repulsive forces and thus of distances by DFT may result in an underestimation of the π-π interaction between the largely coplanar mer-L3 ligands separated by about 3.6 Å in 5, which probably affects its structure. By incorporating these and previous results, the syn versus anti, equatorial/equatorial versus equatorial/axial, and axial pyridine versus axial amine structural alternatives are discussed. © Wiley-VCH Verlag GmbH & Co. KGaA, 2007.