Microsolvated and chelated butylzinc cations: formation, relative stability, and unimolecular gas-phase chemistry

Solutions of butylzinc iodide in tetrahydrofuran, acetonitrile, and N,N-dimethylformamide were analyzed by electrospray ionization mass spectrometry. In all cases, microsolvated butylzinc cations [ZnBu(solvent)(n)](+), n=1-3, were detected. The parallel observation of the butylzincate anion [ZnBuI(2)](-) suggests that these ions result from disproportionation of neutral butylzinc iodide in solution. In the presence of simple bidentate ligands (1,2-dimethoxyethane, N,N-dimethyl-2-methoxyethylamine, and N,N,N',N'-tetramethylethylenediamine), chelate complexes of the type [ZnBu(ligand)](+) form quite readily. The relative stabilities of these complexes were probed by competition experiments and analysis of their unimolecular gas-phase reactivity. Fragmentation of mass-selected [ZnBu(ligand)](+) leads to the elimination of butene and formation of [ZnH(ligand)](+). In marked contrast, the microsolvated cations [ZnBu(solvent)(n)](+) lose the attached solvent molecules upon gas-phase fragmentation to produce bare [ZnBu](+), which subsequently dissociates into [C(4)H(9)](+) and Zn. This difference in reactivity resembles the situation in organozinc solution chemistry, in which chelating ligands are needed to activate dialkylzinc compounds for the nucleophilic addition to aldehydes.