SYNTHESIS, REDOX REACTIVITY, AND X-RAY-DIFFRACTION STRUCTURES OF THE RHENIUM CARBONYL-COMPLEXES FAC-REBR(CO)(3)(BMA) AND [FAC-REBR(CO)(3)(BMA)] [CP(2)CO] - STRUCTURAL CONSEQUENCES OF ELECTRON ACCESSION IN FAC-REBR(CO)(3)(BMA)

Solvent displacement in the rhenium complex ReBr(CO)(3)(THF)(2) (2) by the redox-active diphosphine ligand 2,3-bis(diphenylphosphino)maleic anhydride (bma) affords the corresponding bma-substituted complex fac-ReBr(CO)(3)(bma) (3) in near quantitative yield. The same product is also obtained, albeit in lower yield, from BrRe(CO)(5) (1) and bma in refluxing l,2-dichloroethane. fac-ReBr(CO)(3)(bma) has been isolated and characterized in solution by IR and NMR ((31)p and C-13) spectroscopies. The oxidation/reduction chemistry of fac-ReBr(CO)(3)(bma) was explored by cyclic and rotating disk electrode voltammetric techniques. Two diffusion-controlled one-electron reduction processes at E(1/2) = -0.33 V and E(1/2) = -1.23 V and an irreversible oxidation at E(pa) = 1.58 V are observed in CH2Cl2/TBAP at a platinum electrode. The reductive electrochemistry is discussed in the context of a scheme involving the formal reduction of the bma ligand, a property that is common with other known 18 + delta paramagnetic compounds containing an ancillary bma ligand. Reductive bulk electrolyses have been carried out on 3, and the product of one-electron reduction, [fac-ReBr(CO)(3)(bma)].(-)(4), has been characterized by IR and UV-vis spectroscopies. Large infrared frequency shifts in the carbonyl stretching bands of the bma ligand in [fac-ReBr(CO)(3)(bma)].(-) are observed, as expected for a reduction process confined to the electron-accepting bma ligand. The unequivocal identity of [fac-ReBr(CO)(3)(bma)].(-) was independently established by the isolation and structural characterization of the product formed from the cobaltocene reduction of fac-ReBr(CO)(3)(bma). The molecular structures of compounds 3 and 4 have been determined by single-crystal X-ray diffraction analysis. fac-ReBr(CO)(3)(bma) crystallizes in the triclinic space group P $($) over bar$$ 1: a = 9.855(1) Angstrom, b = 12.1153(4) Angstrom, c = 13.7751(8) Angstrom, alpha = 85.322(4)degrees, beta = 73.828(6)degrees, gamma = 75.812(6)degrees, V= 1531.3(2) Angstrom(3), Z = 2, d(calc) = 1.771 g . cm(-3); R = 0.0552, R(w) = 0.0771 for 3342 observed reflections. The radical anion [fac-ReBr(CO)(3)(bma)][Cp(2)Co], as the CH(2)CL(2) solvate, crystallizes in the monoclinic space group C2/c: a = 39.600(3)Angstrom b = 10.2625(2) Angstrom, c = 23.253(2) Angstrom,beta = 120.278(7)degrees, V = 8161(1) Angstrom(3), Z = 8, d(calc) = 1.793 g . cm(-3); R = 0.0511, R(w), = 0.0598 for 3811 observed reflections. These two structures permit a direct comparison regarding the consequences of electron accession in fac-ReBr(CO)(3)(bma). Extended Huckel calculations have been performed on the model compound fac-ReBr(CO)(3-)(H(4)bma), the results of which are used to support the electrochemical data and the proposed reduction pathway involving electron addition to the bma ligand.