Experimental and computational studies of hydrogen bonding and proton transfer to [Cp*Fe(dppe)H]

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 ΔH° = -6.5 ± 0.4 kcal mol-1 and ΔS° = -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 ΔH‡ = 2.6 ± 0.3 kcal mol-1 and ΔS‡ = -44.5 ± 1.1 cal mol -1 K-1. The computational investigation (at the DFT/B3LYP 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.