Gas-Phase Solvation Behavior of Ni(II) in Water/N,N-Dimethylformamide Mixtures

Abstract

Electrospray ionization−mass spectrometry (ESI-MS) is used to investigate the complexation of nickel(II) by N,N-dimethylformamide (DMF) in different water/DMF mixtures. The types of solvated cations observed are independent of the solvent composition and are part of the series [Ni(DMF)n]2+ with n = 2−6 and [Ni(DMF)nX]+ with n = 1−4 (where X = Cl, Br, NO3). The number of ligands, n, and thus the extent of solvation depends on the cone voltage (UC) of the ESI source. At low UC multiply ligated ions prevail, whereas ligation drops with rising UC and further increase of UC causes dissociation of the solvent molecules as well as reduction of Ni(II) to Ni(I). The collision-induced dissociation (CID) spectra of the multiply ligated ions [Ni(DMF)n]2+ with n ≥ 3 and [Ni(DMF)nX]+ with n ≥ 2 show only losses of neutral DMF ligands. Quite different is the behavior of the bisligated ions [Ni(DMF)2]2+ and [Ni(DMF)X]+. In the CID spectrum of [Ni(DMF)2]2+, electron transfer from DMF to the metal leads to the reduced complex [Ni(DMF)]+ concomitant with ionized DMF+. Charge-stripping spectra of mass-selected monocations [Ni(DMF)n]+ (n = 1, 2) confirm that the dication [Ni(DMF)2]2+ is stable, whereas monoligated [Ni(DMF)]2+ appears to be intrinsically unstable toward Coulomb explosion. In the CID spectra of [Ni(DMF)Cl]+ and [Ni(DMF)Br]+, bond activation of the solvent takes place, and in the case of [Ni(DMF)NO3]+ the formation of a cationic species is observed which formally corresponds to the solvated metal-oxide cation [(DMF)NiO]+.