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