Hetero-Bimetallic Complexes Involving Vanadium(V) and Rhenium(VII) Centers, Connected by Unsupported μ-Oxido Bridge: Synthesis, Characterization, and Redox Study

Heterobimetallic complexes of a vanadium(V) and rhenium(VII) combination connected by a μ-oxido bridge [LVO(μ-O)ReO3]·H2O [H2L = N, N′-ethylene bis(salicylideneimine) (H2salen) and its methoxy derivative] (1, 2) are reported. The compounds have been prepared by a single-pot synthesis in which the precursor [VIVOL] complexes are allowed to be oxidized aerially in the presence of added perrhenate. The oxidized [VVOL]+ species accommodate the ReO4− anion in their vacant coordination site, trans to the terminal oxido group, providing the complexes 1 and 2. The later generates a binuclear oxovanadium(V) compound [H2en][(TBC)VO(μ-TBC)2OV(TBC)]·5H2O (3) when treated with tetrabromocatechol. Single crystal X-ray diffraction analysis and 1H NMR spectroscopy have been used to establish their identities. In compound 2, the Re(1)−O(11)−V(1) bridge angle is barely linear [170.2(3)°] with a Re···V separation of 3.9647(9) Å. The redox behavior of 1 and 2 are quite interesting, each undergoing two reductions both in the positive potential range at E1/2 = 0.59 (process I) and E1/2 = 0.16 V (process II) versus Ag/AgCl reference (corresponding potentials are 0.59 and 0.18 V for 2). Process I has a single-electron stoichiometry involving the [VO(salen)] part of the complexes as established by combined coulometry-Electron Paramagnetic Resonance (EPR) experiments which provide an eight-line isotropic EPR pattern at room temperature (⟨g⟩ = 1.967; ⟨A⟩ = 87 × 10−4 cm−1), characteristic of an unpaired electron being coupled to a vanadium nuclear spin (51V, I = 7/2). The almost linear V−O−Re bridge in 1 and 2 allows this unpaired electron to interact effectively with the neighboring Re nuclear spin, leading to familiar “two-line pattern” superhyperfine coupling (A (185,187Re) = 20.7 × 10−4 cm−1). Process II, on the other hand, is based on a Re(VII/VI) electron transfer as confirmed by differential pulse and normal pulse voltammetric experiments.