Chromium(iii) stars and butterflies: synthesis, structural and magnetic studies of tetrametallic clusters

Abstract

We report the synthesis, structures and magnetic properties of a series of chromium(III) metal-centered triangle (or “star”) clusters, [Cr₄{RC(CH₂O)₃}₂(4,4′-R′₂-bipy)₃Cl₆] [R = Et, R′ = H (2); R = HOCH₂, R′ = H (3); R = Et, R′ = ^tBu (4)], prepared by two-step solvothermal reactions starting from [CrCl₃(thf)₃]. The product of the first stage of this reaction is the salt [Cr(bipy)₂Cl₂]₂[Cr₂Cl₈(MeCN)₂] (1). In the absence of the diimine, a different family of tetrametallics is isolated: the butterfly complexes [Cr₄{EtC(CH₂O)₃}₂{NH(C(R)NH)₂}₂Cl₆] (R = Me (5), Et (6), Ph (7)] where the chelating N-acetimidoylacetamidine NH(C(R)NH)₂ ligands are formed in situ via condensation of the nitrile solvents (RCN) under solvothermal conditions. Magnetic measurements show the chromium stars to have an isolated S = 3 ground state, arising from antiferromagnetic coupling between the central and peripheral metal ions, analogous to the well-known Fe(III) stars. Bulk antiferromagnetic ordering is observed at 0.6 K. The butterfly complexes have a singlet ground state, with a low-lying S = 1 first excited state, due to dominant wing-body antiferromagnetic coupling.