Enhancing the Magnetic Coupling of Oxalato-Bridged Re(2)(IV)M(II) (M = Mn, Co, Ni, and Cu) Trinuclear Complexes via Peripheral Halide Ligand Effects

Four heterotrinuclear ReIV 2MII compounds of general formula (NBu4)2[{ReIVBr4(μ-ox)}2MII(Him)2] [NBu4+ = tetra-n-butylammonium cation, ox = oxalate, Him = imidazole; M= Mn (1), Co (2), Ni (3), and Cu (4)] have been synthesized by using the novel mononuclear complex [ReIVBr4(ox)]2- as a ligand toward divalent first-row transition metal ions in the presence of imidazole. Compounds 1-4 are isostructural complexes whose structure contains discrete trinuclear [{ReIVBr4(μ-ox)}2MII(Him)2]2- anions and bulky NBu4+ cations. The Re and M atoms are six-coordinated: four peripheral bromo and two oxalate-oxygens (at Re), and two cis-coordinated imidazole molecules and four oxygen atoms from two oxalate ligands (at M), build distorted octahedral surroundings. Two peripheral [ReBr4(ox)]2- units act as bidentate ligands through the oxalate group toward the central [MII(Him)2] fragment affording the trinuclear entities. The values of the intramolecular Re...M separation are 5.62(1) (1), 5.51(1) (2), 5.46(1) (3), and 5.55(1) Å (4). Magnetic susceptibility measurements on polycrystalline samples of 1-4 in the temperature range of 1.9-300 K show the occurrence of intramolecular antiferro- [J = -1.1 cm1 (1)] and ferromagnetic interactions [J = +3.9 (2), +19.7 (3), and +14.4 cm1 (4)], the Hamiltonian being defined as H^ =-J [S^M(S^Re1+S^Re2)]. The larger spin delocalization on the oxalato bridge in 1-4 when compared to the trinuclear ReIV 2MII complexes with chloro instead of bromo as peripheral ligands (1'-4') accounts for the strengthening of the magnetic interactions in 1-4 [J =-0.35 (1'),+14.2 (3'), and +7.7 cm1 (4')]. An incipient frequency dependence of the out-of-phase ac signals of 3 at very low temperatures is reminiscent of a system with slow relaxation of the magnetization, a phenomenon characteristic of single-molecule magnet behavior.