PREPARATION, STRUCTURE AND REACTIVITY OF THE BINUCLEAR DI-MU-OXO-BIS[(ETHYLENEDIAMINE-N,N,N',N'-TETRAACETATO)-RHENATE(IV)] COMPLEX

The dark brown dirhenium(IV) complex Ba2[Re2(mu-O)2(edta)2].4.5H2O(H4edta = ethylenediamine-N,N,N',N'-tetraacetic acid), peak at 450 nm (epsilon = 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) angstrom, beta = 123.40(1)degrees 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) angstrom is consistent with multiple metal-metal bonding. The structure is that of the meso DELTALAMBDA form. The H-1 and C-13 NMR spectra of the uncrystallised sodium salt in aqueous solution (pH 7.5) provide evidence for both meso DELTALAMBDA and racemic DELTADELTA or LAMBDALAMBDA forms. The UV-VIS and CD spectra of the (R)-pdta analogue, where pdta4- is propylenediamine-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(IV)2 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(IV)2 complex are reported.