Heterotetranuclear Oxalato-Bridged ReM(M = Mn, Fe, Co, Ni, Cu) Complexes: A New Example of a Single-Molecule Magnet (M = Ni)

The use of the mononuclear species (NBu4)2[ReIVCl4(ox)] (NBu4+ = tetra-n-butylammonium cation; ox = oxalate dianion) as a ligand toward fully solvated divalent first-row transition metal ions affords the tetranuclear complexes (NBu4)4[{ReIVCl4(µ-ox)}3MII] with M = Mn (1), Fe (2), Co (3), Ni (4) and Cu (5). Their structure is made up of discrete [{ReCl4(µ-ox)}3M]4- anions and bulky NBu4+ cations. The complexes 2-5 crystallize in the triclinic system with space group P-1; 2 and 5 are isostructural as well as 3 and 4. The Re and M atoms exhibit somewhat distorted ReCl4O2 and MO6 octahedral surroundings, the oxalate groups adopting the bis-bidentate bridging mode. Magnetic susceptibility measurements on polycrystalline samples of 1-5 in the temperature range 1.9-300 K show the occurrence of intramolecular antiferro- [J = -1.30(1) cm-1 (1)] and ferromagnetic couplings [J = + 1.62 (2), +3.0 (3), +16.3 (4) and +4.64 cm-1 (5)] the Hamiltonian being defined as H = -J [SM . (SRe1 + SRe2 + SRe3)]. Compound 4 is the first example of an oxalato-bridged heterometallic species that behaves as a single-molecule magnet (SMM) with a ground spin state S = 11/2 and D = -0.8(1) cm-1, as shown by the study of its static and dynamic magnetic properties and a high-frequency EPR study on polycrystalline samples together with detailed Micro-SQUID measurements on single crystals.