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

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

We report the synthesis, structures and magnetic properties of a series of chromium(III) metal-centered triangle (or “star”) clusters, [Cr-4{RC(CH2O)(3)} 2(4,4′-R'(2)-bipy)(3)Cl-6] [R = Et, R’ = H (2); R = HOCH2, R’ = H (3); R = Et, R’ = Bu-t (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)(2)Cl-2](2)[Cr2Cl8(MeCN)(2)] (1). In the absence of the diimine, a different family of tetrametallics is isolated: the butterfly complexes [Cr-4{EtC(CH2O)(3)}(2){NH(C(R)NH)(2)}(2)Cl-6] (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.