Electrospray ionization tandem mass spectrometry and DFT survey of copper(I) ate complexes containing coordinated borohydride anions

Copper(I) borohydride ate complexes of the type Cat⁺[XCu(BH₄)]⁻ have been previously postulated as intermediates in the reactions of copper salts with borohydride. Negative ion electrospray ionization of an acetonitrile solution of copper(I) phenylacetylide with a 10-fold excess of sodium borohydride (NaBH₄) revealed the formation of a diverse range of mononuclear, dinuclear and trinuclear cuprates with different numbers of BH₄^–, H^– and CN^– ligands, the latter likely being formed by abstraction of CN^– from the acetonitrile solvent. Collision-induced dissociation was used to examine the fragmentation reactions of the following borohydride containing cuprates: [Cu(H)(BH₄)]⁻, [Cu(BH₄)₂]⁻, [Cu(BH₄)(CN)]⁻, [Cu₂(H)(BH₄)₂]⁻, [Cu₂(H)₂(BH₄)]⁻, [Cu₂(BH₄)₂(CN)]⁻, [Cu₂(H)(BH₄)(CN)]⁻, [Cu₃(H)(BH₄)₃]⁻, [Cu₃(H)₂(BH₄)₂]⁻, [Cu₃(H)₃(BH₄)]⁻, [Cu₃(BH₄)₂(CN)₂]⁻, and [Cu₃(H)(BH₄)₂(CN)]⁻. In all cases, BH₃ loss is observed. For many of the dinuclear and trinuclear complexes cluster fragmentation by loss of CuH was also observed. In the case of [Cu₂(H)₂(BH₄)]⁻ and [Cu₃(H)₃(BH₄)]⁻, loss of H₂ was also observed. DFT calculations were used to explore potential structures of the various borohydride-containing cuprates and to predict the overall reaction energetics for the various fragmentation channels.