Hydrogen Bonding and Proton Transfer to the Trihydride Complex [Cp*MoH₃(dppe)]: IR, NMR, and Theoretical Investigations

Abstract

Keywords: Dihydrogen bonding / Hydride ligands / Molybdenum / Proton-transfer mechanism / DFT calculationsThe interaction between [Cp*MoH 3 (dppe)] (dppe = Ph 2 PCH 2 CH 2 PPh 2 ) and a variety of proton donors has been investigated by a combination of experiments and DFT calculations. Weak proton donors [2-monofluoroethanol (MFE) and trifluoroethanol (TFE)] allow the determination of basicity factor (E j = 1.42 ± 0.02) and thermodynamic parameters for the hydrogen bond formation (∆H HB = -4.9 ± 0.2 and -6.1 ± 0.3 kcal mol -1 ; ∆S HB = -15.7 ± 0.7 and -20.4 ± 1 cal mol -1 K -1 for MFE and TFE, respectively). For TFE, a stable low-temperature proton-transfer equilibrium (220-240 K) with the cationic classical tetrahydrido derivative [Cp*MoH 4 (dppe)] + could be investigated independently by UV/Vis (∆H°P T = -2.8 ± 0.4 kcal mol -1 and ∆S°P T = -15 ± 2 cal mol -1 K -1 ) and 1 H NMR (∆H°P T = -2.7 ± 0.5 kcal mol -1 and ∆S°P T = -11 ± 2 cal mol -1 K -1 ) spectroscopy. Upon warming, however, the tetrahydride evolves by dihydrogen loss and formation of a hydride-free diamagnetic product. Stronger proton donors [hexafluoroisopropanol (HFIP), p-nitrophenol (PNP), perfluoro-tert-butyl alcohol (PFTB), and HBF 4 ·OEt 2 ] lead to more extensive proton transfer at lower donor/Mo ratios. A 1:1 proton-transfer stoichiometry is indicated independently by a titration experiment with UV/Vis monitoring for the [Cp*MoH 3 (dppe)]-PNP reaction, and by a stopped-flow kinetics investigation for the

Solvent Control in the Protonation of [Cp*Mo(dppe)H3] by CF3COOH

“…Since the results of previous investigations into the protonation of [Cp*Mo(dppe)H 3 ] [13][14][15] serve as a basis for the new findings described in this paper, we summarize here the salient points that emerge from those studies. (i) The trihydride complex has a high basicity factor (E j = 1.42 Ϯ 0.02), placing it in the category of the most hydridic complexes known.…”

“…The formation of the 2:1 Hbonded adduct AЈ was also evidenced by low-temperature IR spectroscopy, but one molecule of TFA is sufficient for proton transfer to occur, as also indicated by previous ki-netics studies. [13] The proton transfer probably proceeds via a nonclassical intermediate (species B in Scheme 5), although we have not found any direct experimental evidence to support this proposition. Our recently reported theoretical calculations [13] indicate that the nonclassical complex lies in a very shallow minimum with a low barrier for conversion to the classical tetrahydride product, with the isomerization taking place without counteranion dissociation.…”

“…[13] The proton transfer probably proceeds via a nonclassical intermediate (species B in Scheme 5), although we have not found any direct experimental evidence to support this proposition. Our recently reported theoretical calculations [13] indicate that the nonclassical complex lies in a very shallow minimum with a low barrier for conversion to the classical tetrahydride product, with the isomerization taking place without counteranion dissociation. Figure 9), whereas it was completely consumed under the same solvent, temperature, and stoichiometry conditions in the presence of [Cp*Fe(dppe)-H].…”

“…This distortion is caused by steric repulsion between the Cp* and dppe ligands and by the small size of the two hydride ligands, which force the pseudoaxial (trans to the Cp* ligand) phosphorus donor to move up into the wedge of the two Mo-H bonds. The CNT-Mo1-P1 angle [122.648 (13) 2 ]. [18] This combined evidence indicates a significant ionic contribution to the bond between the Mo center and the monodentate anion, which then continues to promote some electronic delocalization between the two C-O bonds.…”

“…Theoretical calculations, however, located the latter complex on the proton-transfer potential energy surface, with a very low energy barrier for the intramolecular rearrangement. [13] Facile access to a nonclassical isomer is evidenced by the evolution of H 2 from [Cp*Mo(dppe)H 4 ] + at ambiet temperature. We now report new results on the protonation of [Cp*Mo(dppe)H 3 ] using trifluoroacetic acid (TFA) in various solvents of different coordinating (in par- Scheme 2. ticular H-bonding) ability and polarity: benzene, toluene, THF, CH 2 Cl 2 , and MeCN.…”