Dihydrogen Bond Intermediated Alcoholysis of Dimethylamine–Borane in Nonaqueous Media

Dimethylamine–borane (DMAB) acid/base properties, its dihydrogen-bonded (DHB) complexes and proton transfer reaction in nonaqueous media were investigated both experimentally (IR, UV/vis, NMR, and X-ray) and theoretically (DFT, NBO, QTAIM, and NCI). The effects of DMAB concentration, solvents polarity and temperature on the degree of DMAB self-association are shown and the enthalpy of association is determined experimentally for the first time (−Δ<i>H</i>°_assoc = 1.5–2.3 kcal/mol). The first case of “improper” (blue-shifting) NH···F hydrogen bonds was observed in fluorobenzene and perfluorobenzene solutions. It was shown that hydrogen-bonded complexes are the intermediates of proton transfer from alcohols and phenols to DMAB. The reaction mechanism was examined computationally taking into account the coordinating properties of the reaction media. The values of the rate constants of proton transfer from HFIP to DMAB in acetone were determined experimentally [(7.9 ± 0.1) × 10^–4 to (1.6 ± 0.1) × 10^–3 mol^–1·s^–1] at 270–310 K. Computed activation barrier of this reaction Δ<i>G</i>^‡theor_298 K(acetone) = 23.8 kcal/mol is in good agreement with the experimental value of the activation free energy Δ<i>G</i>^‡exp_270 K = 21.1 kcal/mol

Dimerization Mechanism of Bis(triphenylphosphine)copper(I) Tetrahydroborate: Proton Transfer via a Dihydrogen Bond

The mechanism of transition-metal tetrahydroborate dimerization was established for the first time on the example of (Ph₃P)₂Cu­(η²-BH₄) interaction with different proton donors [MeOH, CH₂FCH₂OH, CF₃CH₂OH, (CF₃)₂CHOH, (CF₃)₃CHOH, <i>p</i>-NO₂C₆H₄OH, <i>p</i>-NO₂C₆H₄NNC₆H₄OH, <i>p</i>-NO₂C₆H₄NH₂] using the combination of experimental (IR, 190–300 K) and quantum-chemical (DFT/M06) methods. The formation of dihydrogen-bonded complexes as the first reaction step was established experimentally. Their structural, electronic, energetic, and spectroscopic features were thoroughly analyzed by means of quantum-chemical calculations. Bifurcate complexes involving both bridging and terminal hydride hydrogen atoms become thermodynamically preferred for strong proton donors. Their formation was found to be a prerequisite for the subsequent proton transfer and dimerization to occur. Reaction kinetics was studied at variable temperature, showing that proton transfer is the rate-determining step. This result is in agreement with the computed potential energy profile of (Ph₃P)₂Cu­(η²-BH₄) dimerization, yielding [{(Ph₃P)₂Cu}₂(μ,η⁴-BH₄)]⁺

Dihydrogen Bonding in Complex (PP₃)RuH(η¹‑BH₄) Featuring Two Proton-Accepting Hydride Sites: Experimental and Theoretical Studies

Combining variable-temperature infrared and NMR spectroscopic studies with quantum-chemical calculations (density functional theory (DFT) and natural bond orbital) allowed us to address the problem of competition between MH (M = transition metal) and BH hydrogens as proton-accepting sites in dihydrogen bond (DHB) and to unravel the mechanism of proton transfer to complex (PP₃)­RuH­(η¹-BH₄) (<b>1</b>, PP₃ = κ⁴-P­(CH₂CH₂PPh₂)₃). Interaction of complex <b>1</b> with CH₃OH, fluorinated alcohols of variable acid strength [CH₂FCH₂OH, CF₃CH₂OH, (CF₃)₂CHOH (HFIP), (CF₃)₃COH], and CF₃COOH leads to the medium-strength DHB complexes involving BH bonds (3–5 kcal/mol), whereas DHB complexes with RuH were not observed experimentally. The two proton-transfer pathways were considered in DFT/M06 calculations. The first one goes via more favorable bifurcate complexes to BH_term and high activation barriers (38.2 and 28.4 kcal/mol in case of HFIP) and leads directly to the thermodynamic product [(PP₃)­RuH_eq(H₂)]⁺[OR]⁻. The second pathway starts from the less-favorable complex with RuH ligand but shows a lower activation barrier (23.5 kcal/mol for HFIP) and eventually leads to the final product via the isomerization of intermediate [(PP₃)­RuH_ax(H₂)]⁺[OR]⁻. The B–H_br bond breaking is the common key step of all pathways investigated