REDOX CHARACTERISTICS OF MOLYBDENUM(0), MOLYBDENUM(II), AND DINUCLEAR RHENIUM COMPLEXES: ISOMERIZATIONS AND ELECTRON TRANSFER RATES

KAY ANN CONNER, Purdue University

Abstract

Standard heterogeneous electron transfer rate constants, k(,s), have been measured at Pt electrodes by cyclic and ac voltammetry for the two sequential one-electron oxidations of triply-bounded dirhenium-(II) complexes Re(,2)X(,4)(PR(,3))(,4) (X = Cl or Br; PR(,3) + PMe(,2)Ph or PMe(,3)) in CH(,2)Cl(,2), CH(,3)CN , and DMF. Variable temperature studies were done to determine the temperature dependence of k(,s). Rate constants for Re(,2)Cl(,5)(PMe(,2)Ph)(,3), Re(,2)Cl(,6)(PMe(,2)Ph)(,2), Re(,2)Cl(,4)(dppe)(,2), and Re(,2)Br(,4)(dppm)(,2) were also measured. For Re(,2)Cl(,4)(PMe(,2)Ph)(,4) ('2+,+,0) whose x-ray crystal structures are known, the objective was to explore the dependence of k(,s) on structural changes accompanying electron transfer and to compare the results with predictions of current electron transfer theory. Although significant variations in k(,s) were observed for related redox couples, the trends appear to be a consequence of electrostatic work term differences rather than structural differences. Electrochemical oxidation of ((eta)('3)-RC(,3)H(,4))MoX(CO)(,2)L(,2) and ((eta)('3)-RC(,3)-H(,4))Mo(CO)(,2)L(,3) X' (R = H or CH(,3); L(,2) = bpy, Me(,2)bpy, phen, dppm, dppe, or dpae; L(,3) = dien or dpma; X = Cl or O(,2)CCF(,3): X' = PF(,6) or BF(,4)) in CH(,2)Cl(,2)-0.2 M (n-Bu)(,4)NPF(,6)(TBAH) solutions generates the ESR-active 17-electron cations ((eta)('3)-RC(,3)H(,4))MoX(CO)(,2)L(,2) ('+) and ((eta)('3)-RC(,3)H(,4))Mo(CO)(,2)L(,3) ('2+). Differences between the spectra of the compounds containing nitrogen and phosphine/arsine ligands can be attributed to structural differences between the neutral and cationic species containing N vs P/As donors. The ionization of ((eta)('3)-C(,3)H(,5))MoCl(CO)(,2)(CH(,3)CN)(,2) in CH(,2)Cl(,2)-0.2 M TBAH, viz. 3((eta)('3)-C(,3)H(,5))MoCl(CO)(,2)(CH(,3)CN)(,2) (DBLHARR) ((eta)('3)-C(,3)H(,5))Mo(CO)(,2)(CH(,3)CN)(,3) ('+) + ((eta)('3)-C(,3)H(,5))(,2)Mo(,2)Cl(,3)(CO)(,4) ('-) + 3 CH(,3)CN, can be followed by cyclic voltammetry (CV). Whereas ionization in this solvent system is essentially complete, little ionization occurs in CH(,3)CN-0.2 M TBAH solutions. The complexes Mo(CO)(,2)(CNR)(,2)(PR(,3)')(,2) (R = methyl, i-propyl, t-butyl, cyclohexyl, p-tolyl, xylyl, or mesityl; PR(,3)' = PEt(,3), P-n-Pr(,3), PEt(,2)Ph, PMePh(,2), or PPh(,3)) have been prepared and shown to have a cis-disposition of their CO and CNR ligands. Electrochemical oxidation generates the 17-electron cations Mo(CO)(,2)(CNR)(,2)(PR(,3)')(,2) ('+) which have an all-trans geometry. The intramolecular isomerizations have been investigated using CV and coulometry, and electrogenerated products have been characterized by ESR, IR, and UV-vis spectroscopies. Some cations have been produced chemically using (Cp(,2)Fe)PF(,6) as oxidant and have identical properties to the electrochemical products. The rates of isomerization have been estimated using CV and a mechanism for the isomerization has been suggested.

Degree

Ph.D.

Subject Area

Chemistry

COinS