Protonation of Cp*M(dppe)H Hydrides: Peculiarities of the Osmium Congener
Résumé
The interaction between Cp*OsH(dppe) (1) and a series of proton donors (HA) of increasing strength [indole, CFH2CH2OH (MFE), CF3CH2OH (TFE), (CF3)2CHOH (HFIP), p‐nitrophenol, and HBF4·Et2O] has been investigated experimentally by variable temperature IR and NMR spectroscopy in solvents with different coordinating abilities (alkanes, dichloromethane and their mixtures) and computationally at the DFT/B3PW91 level using different models. Both the IR and NMR spectroscopic data for the interaction with weak proton donors conform to the criteria of M–H···H‐A bond formation. Theoretical calculations, however, indicate an asymmetric bifurcated interaction with a significant contribution from the metal atom, which is greater than that previously found for the corresponding Fe system. The basicity factor of 1 (Ej = 1.47) is greater than those of the Ru (1.39) and Fe (1.35) congeners, in agreement with previous studies on other compound families. The kinetic product of proton transfer, cis‐[Cp*Os(H)2(dppe)]+X–, which is selectively obtained at low temperatures, irreversibly rearranges to trans‐[Cp*Os(H)2(dppe)]+X– upon warming to >230 K. The latter compound with X = BF4 has been crystallographically characterized. The activation enthalpy of the isomerization process (ΔH≠ = 21.5 ± 1.0 kcal mol–1), obtained from kinetics investigation by 1H NMR in the 240–260 K range, is identical within experimental error to those previously reported for the M(η2‐H2)+BF4– → trans‐M(H)2+BF4– isomerization process of the iron and ruthenium analogues. The activation entropy (ΔS≠ = 12 ± 4 e.u.), on the other hand, is greater than for the lighter metals, following the order Fe < Ru < Os. The mechanism of proton transfer and trends in the energetics of the various steps upon changing the nature of the metal atom are discussed.
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