Synthesis, Structural Characterization, and Gas-Phase Unimolecular Reactivity of Bis(diphenylphosphino)amino Copper Hydride Nanoclusters [Cu3(X)(μ3‑H)((PPh2)2NH)3](BF4), Where X = μ2‑Cl and μ3‑BH4

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)4(BF4)], bis­(diphenylphosphino)­amine (dppa = (Ph2P)2NH = L), and NaBH4 revealed a major peak, which based on both the accurate masses and isotope distribution was assigned as [Cu3(BH4)­(H)­(L)3]+. This prompted synthetic efforts resulting in isolation of the dppa ligated trinuclear copper hydride nanoclusters, [Cu3(μ2-Cl)­(μ3-H)­(L)3]­(BF4) and [Cu3(μ3-BH4)­(μ3-H)­(L)3]­(BF4), 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 Cu3 geometry coordinated on the bottom face by a μ3-hydride and surrounded by three dppa ligands. ESI-MS of [Cu3(μ2-Cl)­(μ3-H)­(L)3]­(BF4) and [Cu3(μ3-BH4)­(μ3-H)­(L)3]­(BF4) produces [Cu3(μ2-Cl)­(μ3-H)­(L)3]+ and [Cu3(μ3-BH4)­(μ3-H)­(L)3]+. 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 [Cu3(μ3-H)­(X)­(L)2]+, which is in competition with BH3 loss in the case of X = BH4. DFT calculations on the fragmentation of [Cu3(μ3-BH4)­(μ3-H)­(LMe)3]+ suggest that BH3 loss produces the hitherto elusive [Cu3(μ3-H)­(μ2-H)­(LMe)3]+, which undergoes further fragmentation via H2 loss. CID of the deuterium labeled cluster [Cu3(μ3-D)­(μ3-BD4)­(L)3]+ reveals that the competing losses of ligand and BD3 yield [Cu3(μ3-BD4)­(μ3-D)­(L)2]+ and [Cu3(D)2(L)3]+ as primary products, which subsequently fragment via further losses of BD3 or a ligand to give [Cu3(D)2(L)2]+. The coordinated hydrides in the latter ion are activated toward elimination of D2 to give [Cu3(L)2]+. Loss of HD and 2HD are minor channels, consistent with higher DFT predicted endothermicities to form [Cu3(D)­(L)­(L-H)]+ and [Cu3(L-H)2]+.