Enhancing the Magnetic Coupling of Oxalato-Bridged Re^IV₂M^II (M = Mn, Co, Ni, and Cu) Trinuclear Complexes via Peripheral Halide Ligand Effects

Four heterotrinuclear Re^IV₂M^II compounds of general formula (NBu₄)₂[{Re^IVBr₄(μ-ox)}₂M^II(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 [Re^IVBr₄(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 [{Re^IVBr₄(μ-ox)}₂M^II(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 [M^II(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 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 [Ŝ_M(Ŝ_Re1 + Ŝ_Re2)]. The larger spin delocalization on the oxalato bridge in 1–4 when compared to the trinuclear Re^IV₂M^II 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 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.