Polynuclear chromium(III) carboxylates. 1. Synthesis, structure, and magnetic properties of an octanuclear complex with a ring structure

A novel cyclic octanuclear chromium(III) complex with hydroxo and acetato bridging ligands was isolated and its structure determined by X-ray crystallography. The complex [Cr-8(ON)(12)(OAc)(12)] (1) (OAc- = CH3CO2-), as found in crystals of 1.34H2(O), is obtained by refluxing an aqueous solution of the trinuclear "basic" chromium acetate. 1.34H(2)O crystallizes in the tetragonal space group I (4) over bar 2d with the following unit cell dimensions: a 16.592(2) Angstrom, c = 31.557(4) Angstrom, V = 8687(1) Angstrom(3), and Z = 2. A total of 2000 unique data with I > 3 sigma(I) were used to solve and refine the structure to R(F-0) = 0.066 and R-w(F-0) = 0.085. The structure consists of eight Cr(III) ions that form a ring structure and are bridged by hydroxo and acetato ligands. Each of the two neighboring metal atoms in 1 is bridged either by two OH- ligands and one OAc- ligand, with a Cr ... Cr distance of 2.949(2) Angstrom, or by two OAc- ligands and one OH- ligand, with a Cr ... Cr distance of 3.383(2) Angstrom in an alternating fashion. The complex resides on a crystallographic 4 center, and the overall symmetry of 1 is S-4. The magnetic susceptibility of 1.34H(2)O was measured in the temperature range of 5-240 K. Our theoretical modeling of the susceptibility data indicates alternating antiferromagnetic exchange interactions between adjacent spin 3/2 Cr3+ ions around the ring, of magnitude J/k(B) = 13.7 and 8.9 K, respectively.