Heterometallic Co^II-Co^III-M^II alkoxido-bridged heptanuclear motifs (M=Cu, Zn). Syntheses, crystal structures and magnetic properties

Two new alkoxido-bridged heterometallic complexes of formula [Co^IICo^III ₃Cu^II ₃(dea)₆(CH₃COO)₃](ClO₄)_0.75(CH₃COO)_1.25 (1) and [Co^II ₂Co^III ₂Zn^II ₃(tea)₂(piv)₆(CH₃O)₂(OH)₂(CH₃OH)₂]·H₂O 2 (H₂dea=diethanolamine, H₃tea=triethanolamine and Hpiv=pivalic acid) have been assembled using aminoalcohol ligands. The cationic core in 1 possesses a threefold crystallographic axis, and it exhibits a set of three copper(II), one cobalt(II) and three cobalt(III) ions arranged as a hexagon of alternating copper(II) and cobalt(III) ions around the central cobalt(II) ion. Each edge of the hexagon is defined by a double alkoxido bridge, the outer one being bis-monodentate with copper(II)-cobalt(III) pair whereas the inner one adopts a tris-monodentate coordination mode linking the Cu(II)-Co(III) pair with the central cobalt(II) ion. The acetate groups act as bidentate ligands towards the copper(II) ions. The intramolecular cobalt(II)-copper(II) separation is 3.157Å. A hexagon of two cobalt(II), two cobalt(II) and two zinc(II) ions distributed along three types of edges with a central zinc(II) ion occurs in 2. A tris-modentate alkoxido bridge is involved in all edges together with another bis-monodentate alkoxido bridge (Co^III-Co^II and Co^II-Zn^II pairs) and a carboxylate-pivalate bridge in the syn-syn conformation. The five coordination at each peripheral zinc(II) ion is completed by a monodentate carboxylate-pivalate ligand whereas a methoxo group allows to achieve the six-coordination at each cobalt(III) ion. The intramolecular cobalt(II)-cobalt(II) separation through the inner zinc(II) ion is 6.309Å. The magnetic properties of 1 and 2 have been investigated in the temperature range 1.9-300K. The exchange interaction between the six-coordinate cobalt(II) and copper(II) ions through the double alkoxido bridges in 1 and that between the six-coordinate cobalt(II) ions across the diamagnetic zinc(II) ion in 2 are found to be ferromagnetic [J =+5.39(5) (1) and +1.05(2)cm^-1 (2)], the accidental orthogonality between the interacting magnetic orbitals accounting for this behaviour in the two cases.