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)
M. Tilset et al., 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), J AM CHEM S, 123(41), 2001, pp. 9984-10000
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.