COPPER(I) TETRAHYDROBORATE DERIVATIVES CONTAINING PHOSPHINE AND PHENANTHROLINE LIGANDS - AN ELECTROSPRAY MASS-SPECTROMETRIC STUDY OF SPECIES IN SOLUTION

Electrospray mass spectra have been obtained from dichloromethane/methanol for cationic species existing in solutions of copper(I) tetrahydroborate derivatives containing phosphine- and phenanthroline-type ligands. Complexes of the type (L-L)Cu(PPh3)(BH4) (L-L = phenanthroline ligand) exist in solution as cationic [(L-L)Cu(PPh3)]+ species which are easily observed by electrospray mass spectrometry (ESMS). The triphenylpbosphine ligand is labile and readily exchanges with excess triphenylphosphine, or another monodentate phosphine, at rates which are moderately fast on the P-31 NMR time scale at room temperature. However, under these conditions the [(L-L)-Cu(PPh3)2]+ cation can be observed by ESMS. When two different phosphines are present, two[(L-L)Cu(PR3)]+ ions and all three possible [(L-L)Cu(PR3)2]+ cations are observed. Thus ESMS can observe individual cations from a solution in which the species are rapidly exchanging. Tandem MS/MS spectra show that the [(L-L)Cu(PR3)]+ cations readily lose phosphine and the [(L-L)Cu(PR3)2]+ cations are very unstable in the gas phase toward loss of phosphine. Reaction of (L-L)Cu(PR3)(BH4) With 1 mol of L-L gives [(L-L)2Cu]+ ions which are observed in the ES mass spectrum. Interaction of[(L-L)2Cu]+ and a different phenanthroline complex [(L'-L')2Cu]+, or interaction of [(L-L)2Cu]+ With free L'-L', gives all three possible bis(phenanthroline)copper cations, thus demonstrating that the phenanthroline ligands are also labile. The compound (PPh3)2Cu(BH4) exists in solution as cationic [Cu-(PPh3)2]+, which is also observable by ESMS. Addition of excess PPh3 gives fast ligand exchange (NMR), but ESMS shows the presence of both [Cu(PPh3)2]+ and [Cu(PPh3)3]+ ions. Addition of 2 mol of another phosphine (PR3) to [Cu(PPh3)2]+ also gives sharp averaged P-31 NMR signals for each phosphine at room temperature, but the ES mass spectrum shows peaks corresponding to all the bis(phosphine) and tris(phosphine) cations. When 2 mol of each of two different phosphines are added to 1 mol of [Cu(PPh3)2]+, the ES mass spectrum gives peaks for every possible combination of bis(phosphine)- and tris(phosphine)copper cations.