Structure Elucidation of Dimethylformamide-Solvated Alkylzinc Cations in the Gas Phase

Organozinc iodides, useful for the synthesis of nonproteinogenic amino acids, are investigated in the gas phase by a combination of electrospray (ESI)-MS/MS, accurate ion mass measurements, and infrared multiphoton dissociation (IRMPD) spectroscopy employing a free electron laser. ESI allowed the full characterization of a set of dimethylformamide (DMF)-solvated alkylzinc cations formed by formal loss of I− in the gas phase. Gas phase ion structures of the organozinc cations were identified and optimized by computations at the B3LYP/6-311G** level of theory. The calculations indicate that the zinc cation in gas phase alkylzinc-DMF species preferentially adopts a tetrahedral coordination sphere with four ligands, namely the alkyl group, any internal coordinating group, and DMF (the number of which depends on the number of internal coordinating groups present). Besides the sequential loss of coordinated DMF, collision-induced dissociation (CID) patterns demonstrate that the zinc-DMF interaction in tetrahedral four-coordinate mono-DMF-zinc complex ions can be even stronger than covalent bonds. The IRMPD spectra of the alkylzinc-DMF species examined show a rich pattern of indicative bands in the range of 1000−1800 cm−1. All major features of the recorded IRMPD spectra are consistent with the computed IR spectra of the respective gas phase ion structures predicted by theory, allowing identification and assignment.