Encouraging chromium(III) ions to form larger clusters: Syntheses, structures, magnetic properties and theoretical studies of di- and octametallic Cr clusters

Reaction of anhydrous CrCl2 with the tripodal ligands 1,1,1-tris(hydroxymethyl)ethane (H(3)thme), 1,1,1-tris(hydroxymethyl)propane (H(3)tmp) and pentaerythritol (H(4)peol) produces three new Crib clusters that have been structurally and magnetically characterised. The dimeric complex [Cr-2(H(2)tmp)(2)-Cl-4]center dot 2MeOH (1.2MeOH) is generated under reflux. An analogous reaction, but under solvothermal conditions produces the octametallic species [Cr8O2(thme)(2)(Hthme)(4)Cl-6]center dot 2MeOH (2.2MeOH) and [Cr8O2(Hpeol)(2)(H(2)peol)(4)Cl-6]center dot 3MeOH (3.3MeOH). Complex 1 is a simple dimeric species whereas the structures of 2 and 3 are based on the decametallate {M10O28}(26-) ion. Variable-temperature direct-current (dc) magnetic susceptibility data were collected for complexes 1 and 2 in the 1.8-300 K temperature range in fields up to 5.0 T. Complex 1 has a ground state of S = 0 with the best-fit parameters J = -12.30 +/- 0.04 cm(-1) and g = 1.990 +/- 0.003. Electronic structure calculations based on density functional theory (DFT) on 1 and 2 have been carried out and good agreement with the experimental data was found for 1. For 2, the theoretical results have been used as starting point to fit the J values to the experimental data, revealing the presence of competing antiferromagnetic (J(4,7) = -39.0 cm(-1); J(3,7) = J(4,6) = +1.1 cm(-1), J(4,5) = J(3,4) = -20.0 cm(-1), J(3,6) = -16.0 cm(-1) and J(1,3) = J(1,6) = J(1,4) = J(1,7) = -4.8 cm(-1)) exchange interactions between the Cr-III centres that suggests a singlet ground state with very close S = 1, 2, 3 and 4 excited states. Magneto-structural correlations developed on a model complex based on 1 show a strong dependence of J with Cr-O-Cr angle. A similar magneto-structural correlation is found for the J values obtained by theoretical calculations on 2. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)