Theoretical, thermodynamic, spectroscopic, and structural studies of the consequences of one-electron oxidation on the Fe-X bonds in 17-and 18-electron Cp*Fe(dppe)X complexes (X = F, Cl, Br, I, H, CH3)

The compounds Cp*Fe(dppe)X ([Fe*]X) and the corresponding cation radicals [ Fe*]X-circle+ are available for the series X = F, Cl, Br I, H, CH3. This ha s allowed for a detailed investigation of the dependence of the nature of F e-X bonding on the identity of X and the oxidation state (charge) of the co mplex. Cyclic voltammetry demonstrates that the electrode potentials for th e [Fe*]X0/+ couples decrease in the order I > Br > Cl > H > F > CH3. An "in verse halide order" is seen, in which the most electronegative X leads to t he most easily oxidized complex. This suggests that F is the best donor amo ng the halides. The halide trend is also reflected in NMR spectroscopic dat a. Mossbauer spectroscopy data also suggest that the F ligand is a strong d onor (relative to H and CH3) in [Fe*]Xcircle+. DFT calculations on CpFe(dpe )X ([Fe]X) model complexes nicely reproduce the trend in the electrode pote ntials for the [Fe*]X0/+ couples. Analysis of the theoretical data within t he halogen series indicates that the energy of the [Fe]X HOMO does not corr elate with the extent of its Fe(d(pi))-X(p(pi)) antibonding character, whic h varies in the order I > Br > Cl > F, but rather depends on the destabiliz ing electrostatic effect caused by X. This effect varies in the order F > C 1 > Br > L A thermochemical cycle that incorporates the [Fe*]X0/+ and [Fe*] (0/+) electrode potentials was used to investigate the effect of the oxidat ion state of the complex on the homolytic bond dissociation energy (BDEhom) , defined for the processes Fe-X --> Fe-circle + X-circle and Fe-Xcircle+ - -> Fecircle+ + X-circle. For all X, it was found that a one-electron oxidat ion leads to a weakening of the Fe-X bond. This trend was reproduced by the DFT calculations. On the other hand, IR nu (Fe-X) spectroscopy data showed an increase in the stretching frequencies for X = H and Cl upon oxidation. X-ray crystallographic data showed a shortening of the Fe-Cl bond upon oxi dation. The trends in IR and Fe-Cl bond distances were reproduced in the DF T calculations. The combined data therefore suggest that oxidation leads to weaker, but shorter, Fe-X bonds. A second thermochemical cycle was applied to investigate the effect of the one-electron oxidation on the heterolytic bond dissociation energies (BDEhet), defined for the processes Fe-X --> Fe + + X- and Fe-Xcircle+ --> Fe2+ + X-. In this case, the oxidation led to bo nd strengthening in all cases. The computed BDE values have been analyzed w ithin Ziegler's transition state methodology and decomposed into two compon ents, one electrostatic and one covalent, describing the interaction betwee n the unrelaxed fragments. In all the computed BDEhom and BDEhet values of the [Fe]X models the electrostatic component is important. This helps to un derstand their respective variations upon oxidation.