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

Abstract

We describe the crystal structures of two bimetallic compounds with the malonate and an exo-polydentate N-donor ligand {[Cu(tren)]₄[Cr₂(mal)₄(OH)₂]}(ClO₄)₄·8H₂O (3) and [Ni(Htren)₂][Cr₂(mal)₄(OH)₂]·8H₂O (4) which are prepared from the dinuclear K₄[Cr₂(mal)₄(OH)₂]·6H₂O precursor (2) [tren = tris(2-aminoethyl)amine and H₂mal = malonic acid]. Their crystal packing and supramolecular structures are analyzed in the context of the influence of the dichromium(III) [Cr₂(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-μ-hydroxodichromium(III) unit occur in 2 and 4, respectively, their different nature being mainly dictated by small structural changes in the centrosymmetric di-μ-hydroxodichromium(II) core [Cr(1)–O–Cr(1a) = 99.48(7) (2) and 100.55(13)° (4) and Cr(1)⋯Cr(1a) = 3.0137(6) (2) and 3.0061(6) Å (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-μ-hydroxodichromium(III) complexes