Ferromagnetic trinuclear carbonato-bridged and tetranuclear hydroxo-bridged Cu(II) compounds with 4,4′-dimethyl-2,2′-bipyridine as ligand. X-ray structure, spectroscopy and magnetism

A controlled synthesis, characterisation and single-crystal X-ray analysis of two novel copper(II) compounds with the ligand 4,4′-dimethyl-2,2′-bipyridine (abbreviated dmbipy) is described. In a CO2 atmosphere, with sodium hydroxide added, the carbonato-bridged triangular trinuclear compound [Cu3(dmbipy)6(μ3-CO3)](BF4)4(C2H5OH)(H2O) (1) is obtained. Compound 1 crystallises in the monoclinic space group P21/n with a=16.169(6), b=23.351(11), c=21.312(7) Å, β=91.26(3), Z=4. The three copper ions are connected via the oxygen atoms from the symmetrically bridging carbonato group, resulting in a triangular array of copper atoms. Each copper has a distorted square-pyramidal environment with a basal plane formed by three nitrogen atoms of the two chelating bipyridine groups and the oxygen atom of the bridging carbonato group (Cu–N/O distances about 2.0 Å). The apical position at each copper is occupied by the fourth nitrogen atom of the bipyridines with distances varying from 2.100(11) to 2.146(11) Å. In all other experimental conditions the tetranuclear hydroxo-bridged compound [Cu4(dmbipy)4(μ3-OH)2(μ-OH)2(H2O)2](BF4)4(H2O)4 (2) is obtained. Compound 2 crystallises in the monoclinic space group P21/c with a=13.274(8), b=21.685(7), c=11.266(7) Å, β=107.71(4), Z=2. The structure consists of two bis(hydroxo)-bridged dinuclear planar units which are connected with long Cu–O bonds to form a tetranuclear unit. Each Cu ion has a similar square-pyramidal coordination geometry: the equatorial plane of each Cu ion consists of two nitrogen atoms of the dmbipy ligand (Cu–N distances 1.945–2.003 Å), and two bridging hydroxo oxygen atoms (Cu–O distances 1.945–1.973 Å). The apical position of Cu1 is occupied by an oxygen atom of a water molecule with a distance of 2.262 Å. The second copper atom, Cu2, has the apical position occupied by an oxygen atom of a bridging hydroxo group at a distance of 2.349 Å; this bond is responsible for the formation of the tetranuclear unity. Compound 1 exhibits a ferromagnetic interaction between the copper ions with a J=29.3 cm−1 and a very weak ferromagnetic intercluster interaction with zj′=2.4 cm−1. Compound 2 also exhibits a ferromagnetic interaction between the copper ions with a J=31.1 cm−1 and an overall magnetic interaction between the two dimeric units J′=8.76 cm−1