Gas-Phase Ion–Molecule Reactions of Copper Hydride Anions [CuH₂]⁻ and [Cu₂H₃]⁻

Abstract

Gas-phase reactivity of the copper hydride anions [CuH₂]^- and [Cu₂H₃]^- toward a range of neutral reagents has been examined via multistage mass spectrometry experiments in a linear ion trap mass spectrometer in conjunction with isotope labeling studies and Density Functional Theory (DFT) calculations. [CuH₂]^- is more reactive than [Cu₂H₃]^-, consistent with DFT calculations, which show it has a higher energy HOMO. Experimentally, [CuH₂]^- was found to react with CS₂ via hydride transfer to give thioformate (HCS₂^-) in competition with the formation of the organometallic [CuCS₂]^- ion via liberation of hydrogen; CO₂ via insertion to produce [HCuO₂CH]^-; methyl iodide and allyl iodide to give I^- and [CuHI]^-; and 2,2,2-trifluoroethanol and 1-butanethiol via protonation to give hydrogen and the product anions [CuH(OCH₂CF₃)]^- and [CuH(SBu)]^-. In contrast, the weaker acid methanol was found to be unreactive. DFT calculations reveal that the differences in reactivity between CS₂ and CO₂ are due to the lower lying π* orbital of the former, which allows it to accept electron density from the Cu center to form the initial three-membered ring complex intermediate, [H₂Cu(η²-CS₂)]^-. In contrast, CO₂ undergoes the barrierless side-on hydride transfer promoted by the high electronegativity of the oxygen atoms. Side-on S_N2 mechanisms for reactions of [CuH₂]^- with methyl iodide and allyl iodide are favored on the basis of DFT calculations. Finally, the DFT calculated barriers for protonation of [CuH₂]^- by methanol, 2,2,2-trifluoroethanol, and 1-butanethiol correlate with their gas-phase acidities, suggesting that reactivity is mainly controlled by the acidity of the substrate.