Dihydrogen Bonding in Complex (PP<sub>3</sub>)RuH(η<sup>1</sup>‑BH<sub>4</sub>) Featuring Two Proton-Accepting Hydride Sites: Experimental and Theoretical Studies
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Abstract
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 (PP<sub>3</sub>)RuH(η<sup>1</sup>-BH<sub>4</sub>) (<b>1</b>, PP<sub>3</sub> = κ<sup>4</sup>-P(CH<sub>2</sub>CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub>). Interaction of complex <b>1</b> with CH<sub>3</sub>OH, fluorinated alcohols of variable acid strength [CH<sub>2</sub>FCH<sub>2</sub>OH, CF<sub>3</sub>CH<sub>2</sub>OH, (CF<sub>3</sub>)<sub>2</sub>CHOH (HFIP), (CF<sub>3</sub>)<sub>3</sub>COH], and CF<sub>3</sub>COOH 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<sub>term</sub> and high activation barriers (38.2 and 28.4 kcal/mol in case of HFIP) and leads directly to the thermodynamic product [(PP<sub>3</sub>)RuH<sub>eq</sub>(H<sub>2</sub>)]<sup>+</sup>[OR]<sup>−</sup>. 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 [(PP<sub>3</sub>)RuH<sub>ax</sub>(H<sub>2</sub>)]<sup>+</sup>[OR]<sup>−</sup>. The B–H<sub>br</sub> bond breaking is the common key step of all pathways investigated
