Alkoxo-bridged dinuclear copper(II) compounds with 2-amino-picolines as ligands - Synthesis, spectroscopy, magnetism and X-ray crystal structures

The synthesis, spectroscopic, magnetic and structural characterisation of four new alkoxo-bridged dinuclear copper(II) compounds are described. All four compounds have the general formula [Cu(mu-OR-)(L)(2)](2)(A(-))(2), in which mu-OR=CH3O- or CH3CH2O-, L=2-amino-3-picoline (abbreviated as 3pic) and 2-amino-5-picoline (abbreviated as 5pic) and A=NO3- or ClO4-. The title compounds all consist of dinuclear units with bridging methoxo groups for [Cu(3pic)(2)(CH3O)(2)(NO3)(2)](CH3OH)(2) (1), [Cu(3pic)(2)(CH3O)(2)(ClO4)(2)](CH3OH)(2) (2), Cu(5pic)(2)(CH3O)(2)(ClO4)(2) (3) and a bridging methoxo group for Cu(5pic)(2)(C2H5O)(2)(ClO4)(2) (4) with two picoline ligands linked to each copper via the pyridine N atom, providing a square planar CuN2O2 unit. The nitrate anions in compound (1) are bridging at semi-coordination distance (Cu-O-NO3 2.735-2.793 Angstrom), the perchlorate anions in compounds (2) and (3) are at semi-coordination distance (Cu-O-ClO4 2.701-3.042 Angstrom); on the other hand the perchlorate anion in compound (4) has only one semi-coordination bond (Cu-O-ClO4 2.826 Angstrom). The Cu-Cu distances within the dinuclear units are 2.9957(12), 2.9872(15), 2.996(5), and 3.0123(18) Angstrom for compounds (1), (2), (3) and (4), respectively, with Cu-O-Cu angles of 102.3(8), 101.33(13), 103.8(5), and 103.38(16)degrees for compounds (1), (2), (3) and (4), respectively. The structures are stabilized by intramolecular H-bonds between the amino hydrogens and O-methoxo and/or O-anion atoms (shortest contacts N-H ... O-methoxo 3.108 Angstrom; N-H ... O-anion 2.914 Angstrom). The magnetic susceptibility measurements of the alkoxo-bridged compounds display a diamagnetic behaviour below room temperature with an estimated exchange parameter 2J of < - 600 cm(-1). All compounds have LF absorption maxima in the range 16.9-17.6 x 10(3) cm(-1), in agreement with tetragonal Cu(II) and one or two axial ligands at relatively large distances. (C) 1999 Elsevier Science Ltd. All rights reserved.