Enhancing the Magnetic Coupling of Oxalato-Bridged ReIV2MII (M = Mn, Co, Ni, and Cu) Trinuclear Complexes via Peripheral Halide Ligand Effects

Four heterotrinuclear ReIV2MII 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 14 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 14 in the temperature range of 1.9–300 K show the occurrence of intramolecular antiferro- [J = −1.1 cm–1 (1)] and ferromagnetic interactions [J = +3.9 (2), +19.7 (3), and +14.4 cm–1 (4)], the Hamiltonian being defined as Ĥ = –J [ŜM(ŜRe1 + ŜRe2)]. The larger spin delocalization on the oxalato bridge in 14 when compared to the trinuclear ReIV2MII complexes with chloro instead of bromo as peripheral ligands (1′–4′) accounts for the strengthening of the magnetic interactions in 14 [J = −0.35 (1′), +14.2 (3′), and +7.7 cm–1 (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.