Building-block process for the synthesis of new chromium(iii) malonate complexes

We describe the crystal structures of two bimetallic compounds with the malonate and an exo-polydentate N-donor ligand {[Cu(tren)](4)[Cr-2(mal)(4)(OH)(2)]}(ClO4)(4)center dot 8H(2)O (3) and [Ni(Htren)(2)][Cr-2(mal)(4)(OH)(2)]center dot 8H(2)O (4) which are prepared from the dinuclear K-4[Cr-2(mal)(4)(OH)(2)]center dot 6H(2)O precursor (2) [tren = tris(2-aminoethyl) amine and H(2)mal malonic acid]. Their crystal packing and supramolecular structures are analyzed in the context of the influence of the dichromium(III) [Cr-2(mal)(4)(OH)(2)](4-) unit, which acts as a building-block. Different supramolecular motifs built up from hydrogen bonds are discussed, and their self-assembly to yield a 3D arrangement is described. The magnetic properties of the compounds 2-4 have been investigated as a function of the temperature. Weak ferro-(J = +2.41 cm(-1)) and antiferromagnetic (J = -0.21 cm(-1)) interactions within the di-mu-hydroxodichromium(III) unit occur in 2 and 4, respectively, their different nature being mainly dictated by small structural changes in the centrosymmetric di-mu-hydroxodichromium(II) core [Cr(1)-O- Cr(1a) = 99.48(7) (2) and 100.55(13)degrees (4) and Cr(1)...Cr(1a) = 3.0137(6) (2) and 3.0061(6) angstrom (4)]. An overall antiferromagnetic behaviour is observed for 3 with a maximum of the magnetic susceptibility at ca. 10.0 K, the intramolecular Cr(III)...Cr(III) (though the double hydroxo bridge) and Cu(II)...Cr(III) (across the carboxylate-malonate in the anti-syn conformation) magnetic couplings being J = -5.53 and 2.78 cm(-1), respectively. These magnetic parameters are discussed in terms of the structure of the compounds and compared with previous magneto-structural data on di-mu-hydroxodichromium(III) complexes.