NMR Investigation of the Dihydrogen‐Bonding and Proton‐Transfer Equilibria between the Hydrido Carbonyl Anion [HRe 2 (CO) 9 ] − and Fluorinated Alcohols

The interaction of fluorinated alcohols with the anionic hydrido complex [HRe 2 (CO) 9 ] − ( 1 ) has been investigated by NMR spectroscopy. According to the acidic strength of the alcohols, the interaction may result not only in the formation of dihydrogen‐bonded ROH ⋅⋅⋅ [HRe 2 (CO) 9 ] − adducts 2 , but also in proton transfer to give the neutral species [H 2 Re 2 (CO) 9 ] ( 3 ). With the weaker acid trifluoroethanol (TFE) evidence for the occurrence of the dihydrogen‐bonding equilibrium was obtained by 2D 1 H NOESY. The dependence of the hydride chemical shift on TFE concentration at different temperatures provided values for the constants of this equilibrium, from which the thermodynamic parameters were evaluated as Δ H °=−2.6(2) kcal mol −1 , Δ S °=−9.3(2) cal mol −1  K −1 . This corresponds to a rather low basicity factor ( E j =0.64). Variable‐temperature T 1 measurements allowed the proton–hydride distance in adduct 2 a to be estimated (1.80 Å). In the presence of hexafluoroisopropyl alcohol (HFIP) simultaneous occurrence of both dihydrogen‐bonding and proton‐transfer equilibria was observed, and the equilibria shifted versus the protonated product 3 with increasing HFIP concentration and decreasing temperature. Reversible proton transfer between the alcohol and the hydrido complex occurs on the NMR timescale, as revealed by a 2D 1 H EXSY experiment at 240 K. For the more acidic perfluoro‐ tert ‐butyl alcohol (PFTB) the protonation equilibrium was further shifted to the right. Thermal instability of 3 prevented the acquisition of accurate thermodynamic data for these equilibria. The occurrence of the proton‐transfer processes (in spite of the unfavorable p K a values) can be explained by the formation of homoconjugated RO ⋅⋅⋅ HOR − pairs which stabilize the alcoholate anions.