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Preparation, structure and reactivity of the binuclear di-µ-oxo-bis[(ethylenediamine-N,N,N′,N′-tetraacetato)-rhenate(IV)] complex

The dark brown dirhenium(IV) complex Ba2[Re2(µ-O)2(edta)2]·4.5H2O(H4edta = ethylenediamine-N,N,N′,N′-tetraacetic acid), peak at 450 nm (Iµ= 7880 M–1 cm–1 per dimer), has been prepared. From X-ray crystallography, the compound crystallizes in a monoclinic space group C2/c with a= 15.867(3), b= 12.914(2), c= 9.686(2)A, β= 123.40(1)° and Z= 4. The structure is centrosymmetric, with the Re atoms having distorted octahedral co-ordination, and each tetradentate edta4– ligand having two uncoordinated –CH2CO2– groups. The Re2O2 ring is planar and the short Re–Re distance of 2.362(2)A is consistent with multiple metal–metal bonding. The structure is that of the mesoΔΛ form. The 1H and 13C NMR spectra of the uncrystallised sodium salt in aqueous solution (pH 7.5) provide evidence for both mesoΔΛ and racemic ΔΔ or ΛΛ forms. The UV–VIS and CD spectra of the (R)-pdta analogue, where pdta4– is proplenediamine-N,N,N′,N′-tetraacetate, are also reported. The edta complex exhibits irreversible electrochemical behaviour with an oxidation wave at +1.23 V vs. the normal hydrogen electrode at [H+]= 0.10 M. Kinetic studies on the oxidation of the ReIV2 complex to ReVII with the one-equivalent reagent cis-[VO2(H2O)4]+ in 1 M H+ indicate a rate-determining first stage with no evidence for stable intermediate states. Effects of the product [VO(H2O)5]2+ and H+ on the kinetics, and an unexpected difference in reactivity of isomeric forms of the ReIV2 complex are reported.