The present contribution reports experimental and computational investigations of the interaction between [Cp*Fe(dppe)H] and different proton donors (HA). The focus is on the structure of the proton transfer intermediates and on the potential energy surface of the proton transfer leading to the dihydrogen complex [Cp*Fe(dppe)(H2)]+. With p-nitrophenol (PNP) a UV/Visible study provides evidence of the formation of the ion-pair stabilized by a hydrogen bond between the nonclassical cation [Cp*Fe(dppe)(H2)]+ and the homoconjugated anion ([AHA]-). With trifluoroacetic acid (TFA), the hydrogen-bonded ion pair containing the simple conjugate base (A-) in equilibrium with the free ions is observed by IR spectroscopy when using a deficit of the proton donor. An excess leads to the formation of the homoconjugated anion. The interaction with hexafluoroisopropanol (HFIP) was investigated quantitatively by IR spectroscopy and by 1H and 31P NMR spectroscopy at low temperatures (200-260 K) and by stopped-flow kinetics at about room temperature (288-308 K). The hydrogen bond formation to give [Cp*Fe(dppe)H]HA is characterized by DeltaH degrees =-6.5+/-0.4 kcal mol(-1) and DeltaS degrees = -18.6+/-1.7 cal mol(-1) K(-1). The activation barrier for the proton transfer step, which occurs only upon intervention of a second HFIP molecule, is DeltaH(not equal) = 2.6+/-0.3 kcal mol(-1) and DeltaS(not equal) = -44.5+/-1.1 cal mol(-1) K(-1). The computational investigation (at the DFT/B3 LYP level with inclusion of solvent effects by the polarizable continuum model) reproduces all the qualitative findings, provided the correct number of proton donor molecules are used in the model. The proton transfer process is, however, computed to be less exothermic than observed in the experiment.

The reaction between CpFe(dppe)H and a number of different proton donors (2-fluoroethanol, MFE; 2,2,2-trifluoroethanol, TFE; hexafluoro-2-propanol, HFIP; perfluoro-tert-butyl alcohol, PFTB; and trifluoroacetic acid, TFA) has been investigated spectroscopically by variable-temperature infrared, UV-visible, and NMR spectroscopy, and has been measured kinetically by the stopped-flow technique with UV-visible detection. The low-temperature IR study shows the establishment of hydrogen-bonding interactions which involve the hydride ligand as the proton accepting site. This investigation quantifies the thermodynamics of the hydrogen-bonding interaction and the basicity factor (E(j)) of the hydride complex. All techniques agree in indicating an equilibration process, after the immediate hydrogen-bond formation, between the hydride complex and an intermediate dihydrogen complex, [CpFe(dppe)(H(2))](+). The equilibrium is shifted toward the dihydrogen complex to a greater extent for the stronger alcohols and for higher alcohol/Fe ratios. The observed equilibration rate constant is linearly dependent on the alcohol concentration, in agreement with the involvement of two alcohol molecules and the formation of a homoconjugate pair. The rate constant increases with the acidity of the proton donor (TFE < HFIP < PFTB < TFA). The rate of the subsequent irreversible isomerization leading to the classical dihydride complex, [CpFe(dppe)H(2)](+), is first order, and the rate constant does not depend on the proton donor nature. The reaction continues, if conducted in CH(2)Cl(2), with a third, slower step leading to the paramagnetic [CpFe(dppe)Cl](+) product. The kinetic data are in accord with an isomerization mechanism consisting of an intramolecular reorganization, leading in one step from the dihydrogen complex to the classical dihydride species, and disagree with the occurrence of a proton-transfer process at the metal site.

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