Gas-phase ion-molecule reactions of copper hydride anions [CuH2]- and [Cu2H3]-

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.