Gas-phase reactions of divalent Ni complex ions with acetonitrile: Chelate ring size, inductive, and steric effects

Abstract

Ni(II) complexes of a series of pentadentate polyamine ligands have been reacted with CH 3 CN in the gas phase using a modified quadrupole ion trap mass spectrometer. The ligands have structural features such that upon complexation, chelate ring size, sterics, and inductive effects can be evaluated in the gas phase. Rate and equilibrium constants for CH 3 CN addition to the metal complexes show that there is a general decrease in the gas-phase reactivity as the chelate ring size is increased. Density functional theory calculations at the B3LYP/LANL2DZ level of theory have been used to obtain minimum energy structures and Mulliken charges for the complexes. The decreased reactivity observed as the chelate ring size is increased correlates with a decrease in the atomic charge on the metal. A larger chelate ring size enhances ligand flexibility and improves the overlap of the ligand's donor atoms with the metal center. Adding methyl groups adjacent to or on the nitrogen donor groups of a ligand also decreases the rate and equilibrium constants for the reactions of a given complex with CH 3 CN. Analysis of Mulliken charges for these complexes indicates that both inductive and steric effects are responsible for lower complex reactivity. These results suggest that while the gas-phase reactivity of a metal complex with CH 3 CN is very dependent on the functional groups directly bound to the metal, in some cases steric effects can conceal the correlation between reactivity and coordination structure. T he chemistry of transition metal ions is quite diverse mainly because they can be tuned by the type, number, and orientation of the ligands coordinated to them. Knowledge of this coordination sphere provides insight into metal complex reactivity, and a variety of techniques including X-ray spectroscopies, NMR, and electron paramagnetic resonance (EPR) have been used to gather coordination structure information. If metal complexes are present at low concentrations or in complicated mixtures, though, these techniques can have difficulty providing coordination structure. Consequently, we have begun to investigate the advantage of using mass spectrometry (MS) to gather coordination structure information.For several reasons we have chosen not to rely on the typical dissociation methods (e.g., collision-induced dissociation [CID]) for acquiring metal complex structural information by MS. One reason is that, while the dissociation patterns and dynamics of metal complexes have not been exhaustively studied, most results indicate that rearrangement reactions involving the loss of small neutrals from large ligands or the loss of intact ligands from multi-ligated metal ions are typical [1][2][3][4][5][6][7][8][9][10][11]. Unfortunately, neither provides much insight into the coordination sphere, as they do not necessarily reveal whether the groups lost were coordinated to the metal or not. Furthermore, during collisional activation, even at low energies, scrambling of the metal's coordination sphere can occur because of relatively w...

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