Synthesis, Structural Characterization, and Gas-Phase Unimolecular Reactivity of Bis(diphenylphosphino)amino Copper Hydride Nanoclusters [Cu(X)(μ-H)((PPh)NH)](BF), Where X = μ-Cl and μ-BH.

An electrospray ionization mass spectrometry (ESI-MS) survey of the types of cationic copper clusters formed from an acetonitrile solution containing a 1:1:20 mixture of tetrakis(acetonitrile)copper(I) tetrafluoroborate [Cu(MeCN)(BF)], bis(diphenylphosphino)amine (dppa = (PhP)NH = L), and NaBH revealed a major peak, which based on both the accurate masses and isotope distribution was assigned as [Cu(BH)(H)(L)]. This prompted synthetic efforts resulting in isolation of the dppa ligated trinuclear copper hydride nanoclusters, [Cu(μ-Cl)(μ-H)(L)](BF) and [Cu(μ-BH)(μ-H)(L)](BF), which were subsequently structurally characterized using high resolution ESI-MS, X-ray crystallography, NMR, and IR spectroscopy. The X-ray structures reveal a common structural feature of the cation, in which the three copper(I) ions adopt a planar trinuclear Cu geometry coordinated on the bottom face by a μ-hydride and surrounded by three dppa ligands. ESI-MS of [Cu(μ-Cl)(μ-H)(L)](BF) and [Cu(μ-BH)(μ-H)(L)](BF) produces [Cu(μ-Cl)(μ-H)(L)] and [Cu(μ-BH)(μ-H)(L)]. The unimolecular gas-phase ion chemistry of these cations was examined under multistage tandem mass spectrometry conditions using collision-induced dissociation (CID). CID of both cations proceeds via ligand loss to give [Cu(μ-H)(X)(L)], which is in competition with BH loss in the case of X = BH. DFT calculations on the fragmentation of [Cu(μ-BH)(μ-H)(L)] suggest that BH loss produces the hitherto elusive [Cu(μ-H)(μ-H)(L)], which undergoes further fragmentation via H loss. CID of the deuterium labeled cluster [Cu(μ-D)(μ-BD)(L)] reveals that the competing losses of ligand and BD yield [Cu(μ-BD)(μ-D)(L)] and [Cu(D)(L)] as primary products, which subsequently fragment via further losses of BD or a ligand to give [Cu(D)(L)]. The coordinated hydrides in the latter ion are activated toward elimination of D to give [Cu(L)]. Loss of HD and 2HD are minor channels, consistent with higher DFT predicted endothermicities to form [Cu(D)(L)(L-H)] and [Cu(L-H)].