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 a lso obtained, albeit in lower yield, from BrRe(CO)(5) (1) and bma in r efluxing l,2-dichloroethane. fac-ReBr(CO)(3)(bma) has been isolated an d characterized in solution by IR and NMR ((31)p and C-13) spectroscop ies. The oxidation/reduction chemistry of fac-ReBr(CO)(3)(bma) was exp lored by cyclic and rotating disk electrode voltammetric techniques. T wo 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 reducti ve 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, a nd the product of one-electron reduction, [fac-ReBr(CO)(3)(bma)].(-)(4 ), has been characterized by IR and UV-vis spectroscopies. Large infra red frequency shifts in the carbonyl stretching bands of the bma ligan d in [fac-ReBr(CO)(3)(bma)].(-) are observed, as expected for a reduct ion process confined to the electron-accepting bma ligand. The unequiv ocal identity of [fac-ReBr(CO)(3)(bma)].(-) was independently establis hed by the isolation and structural characterization of the product fo rmed from the cobaltocene reduction of fac-ReBr(CO)(3)(bma). The molec ular structures of compounds 3 and 4 have been determined by single-cr ystal 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)de grees, 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 monocl inic 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 compar ison 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 in volving electron addition to the bma ligand.