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

Abstract

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 (ε= 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)Å, β= 123.40(1)° and Z= 4. The structure is centrosymmetric, with the Re atoms having distorted octahedral co-ordination, and each tetradentate edta^4– ligand having two uncoordinated –CH₂CO₂^– groups. The Re₂O₂ ring is planar and the short Re–Re distance of 2.362(2)Å 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 pdta^4– 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 Re^IV₂ complex to Re^VII 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 Re^IV₂ complex are reported.