Richard W. Vachet

Ion–molecule reactions in a quadrupole ion trap as a probe of the gas-phase structure of metal complexes

A method is described in which the coordination number in metal complexes can be determined using ion–molecule reactions in a quadrupole ion trap mass spectrometer. Complexes of first-row transition metals in the +2 oxidation state, including manganese through zinc, are electrosprayed, isolated in the ion trap and allowed to react with gases. The coordination number is ascertained by observing the reagent ligands that successfully react with the complex. It was generally observed that six-coordinate complexes are unreactive, five-coordinate complexes react with pyridine and ethylamine, four-coordinate complexes react with pyridine, ethylamine and ammonia and threecoordinate complexes react with all the reagent ligands studied, including water and methanol. The order of reactivity for a given complex reacting with the various reagent ligands is found to follow the order of the electron-donating ability of the reagent ligands. In addition, the effect of the metal center on the reactivity of the complexes in the gas phase is analogous to solution-phase trends; electronic structure strongly influences the gas-phase reactions. These results were then used to predict the complexation behavior of novel podand ligands for which condensed phase information is not available. The results indicate that ion–molecule chemistry in the gas phase may be useful in predicting the interactions between novel multidentate ligands and metals in solution.

Quadrupole ion trap studies of the structure and reactivity of transition metal ion pair complexes

Ion pairs are common species observed in the electrospray mass spectra of transition metal coordination complexes. To understand the nature of these ion pairs, a systematic study of the gas-phase chemistry of these species using ion-molecule reactions and collision-induced dissociation (CID) was carried out. Ion pair complexes of the type MLnX+ (where M is Mn(II), Fe(II), Co(II), Ni(II), Cu(II) or Zn(II), L is 1,10-phenanthroline, 2,2-bipyridine, ethylenediamine, diethylenetriamine or 1,4,8,11-tetraazacyclotetradecane and X is Cl-, NO3-, acetylacetonate, ClO4-, acetate or SCN-) were studied. Ion-molecule reactions can distinguish whether the counterion in an ion pair is an inner- or outer-sphere ligand and can determine the coordination mode of the counterion. In addition, CID and ion-molecule reactions reveal some interesting chemistry of these complexes and unique coordination modes for some of the anions studied here were inferred from the ion-molecule reactions. For example, the thiocyanate ion is found to coordinate in a bidentate fashion in Zn(II) and Ni(II) complexes, contrasting behavior typically observed in solution. Also, certain Co(II) and Fe(II) ion pair complexes undergo oxidation reactions in which species such as dioxygen and nitric oxide from the counterions ClO4- and NO3- are transferred to the Co(II) and Fe(II) complexes, showing the inherent affinity of these metals for these molecules. These complexes were also studied by ion-molecule reactions and CID. Dioxygen in complexes formed by CID is demonstrated to be bidentate, suggesting the formation of a peroxo ligand with concurrent oxidation of the metal.

A comparison of the gas, solution, and solid state coordination environments for the copper(II) complexes of a series of aminopyridine ligands of varying coordination number

Abstract The synthesis and characterization of the copper(II) complexes of a series of aminopyridine ligands that range from tri- to heptadentate is described. The ligands include the tripod ligand tris(2-((2-pyridylmethyl)amino)ethyl)amine, TREN-pyr, and the following linear ligands: (2-pyridylmethyl)(2-(2-(2-((2-pyridylmethyl)amino)ethyl)amino)ethyl)amine, TRIEN-pyr; bis(2-((2-pyridylmethyl)amino)ethyl)amine, DIEN-pyr; (2-pyridylmethyl)(2-((2-pyridylmethyl)amino)ethyl)amine, EN-pyr; bis(2-pyridylmethyl)amine, AM-pyr; and methyl(2-((-pyridylmethyl)amino)ethyl)amine, MeEN-pyr. The following methods were used to determine the binding geometries of the Cu(II) complexes in the solid, solution, and gas phases: magnetic susceptibility measurements, absorption spectroscopy, EPR spectroscopy, electrochemistry, and analyzing the gas phase ion-molecule reactions in a mass spectrometer. The solid state structures for the TRIEN-pyr and DIEN-pyr complexes were determined by X-ray crystallography. [Cu(TRIEN-pyr)](BF 4 ) 2 crystallized in the octahedral system, space group Pbca , with a =9.4107(7), b =17.536(13), c =30.132(6) A, and Z =8 ( R =0.064, R w =0.060). [Cu(DIEN-pyr)](BF 4 ) 2 crystallized in the octahedral system, space group Pbca , with a =29.137(5), b =14.829(3), c =9.850(1) A, and Z =8 ( R =0.071, R w =0.066). The coordination numbers of the Cu(II) complexes in this study are dependent on the denticity of the ligand. The complexes with AM-pyr, MeEN-pyr and TRIEN-pyr appear to be six-coordinate, DIEN-pyr to be five-coordinate and the EN-pyr to be four-coordinate in the solid state. The TREN-pyr complex was isolated as a dimer in the solid state but appears to be a tetragonally distorted monomeric complex in the solution and gas phases. For all of the complexes except DIEN-pyr, the coordination sphere appears to be independent of the phase of the complex. The Cu(II) DIEN-pyr complex appears to be five-coordinate in the solid phase, a mixture of four- and five-coordinate in the solution phase, and predominately four-coordinate in the gas phase.

Gas, solution, and solid state coordination environments for the nickel(II) complexes of a series of aminopyridine ligands of varying coordination number

Abstract The synthesis and characterization of the nickel(II) complexes from a series of aminopyridine ligands that range from tri- to heptadentate is described. The ligands include the tripod ligand tris(2-((2-pyridylmethyl)amino)ethyl)amine, TREN-pyr, and the following linear ligands: (2-pyridylmethyl)(2-(2-((2-((2-pyridylmethyl)amino)ethyl)amino)ethyl)amine, TRIEN-pyr; bis(2-((2-pyridylmethyl)amino)ethyl)amine, DIEN-pyr; (2-pyridylmethyl)(2-((2-pyridylmethyl)amino)ethyl)amine, EN-pyr; bis(2-pyridylmethyl)amine, AM-pyr; and methyl(2-((-pyridylmethyl)amino)ethyl)amine, MeEN-pyr. The following methods were used to determine the binding geometries of the nickel(II) complexes in the solid, solution, and gas phases: magnetic susceptibility measurements, absorption spectroscopy, electrochemistry, and analyzing the gas phase ion-molecule reactions in a mass spectrometer. The linked five-membered chelate character of the linear ligands appear to have imposed high-spin, octahedral geometry on the complexes in the condensed phases. The tripod ligand TREN-pyr and the two tridentate ligands, AM-pyr and MeEN-pyr, form six-coordinate complexes in the gas phase. In contrast, the rest of the complexes (TRIEN-pyr, DIEN-pyr, and EN-pyr) were found to have lower coordination numbers in the gas phase (five-, five-, and four-coordinate, respectively). The potentially heptadentate tripod ligand TREN-pyr does not appear to confer any unusual properties on the Ni(II) ion in the solution or solid phases, but does appear to be more effective than the hexadentate ligand TRIEN-pyr at maintaining the six-coordinate geometry in the gas phase.

The use of static pressures of heavy gases within a quadrupole ion trap.

The performance of quadrupole ion traps using argon or air as the buffer gas was evaluated and compared to the standard helium only operation. In all cases a pure buffer gas, not mixtures of gases, was investigated. Experiments were performed on a Bruker Esquire ion trap, a Finnigan LCQ, and a Finnigan ITMS for comparison. The heavier gases were found to have some advantages, particularly in the areas of sensitivity and collision-induced dissociation efficiency; however, there is a significant resolution loss due to dissociation and/or scattering of ions. Additionally, the heavier gases were found to affect ion activation and deactivation during MS/MS, influencing the product ion intensities observed. Finally, the specific quadrupole ion trap design and the ion ejection parameters were found to be crucial in the quality of the spectra obtained in the presence of heavy gases. Operation with static pressures of heavy gases can be beneficial under certain design and operating conditions of the quadrupole ion trap.

Characterization of Cu(II)-binding ligands from the Chesapeake Bay using high-performance size-exclusion chromatography and mass spectrometry

The speciation of Cu(II) in marine waters is dominated by organic ligands, which have resisted detailed chemical characterization. In this work we have used immobilized-metal affinity chromatography (IMAC) to isolate Cu(II)-binding ligands from the Chesapeake Bay. We have then used high-performance size-exclusion chromatography (HPSEC) and mass spectrometry (MS) to gather information about the size distributions, molecular weights, and chemical functionality of the ligands isolated by IMAC. Results show that weaker-binding ligands have molecular weights that range from about 230 up to >20,000 Da. A portion of these weaker-binding ligands has molecular weight distributions that are consistent with humic substances. The molecular weight distribution of stronger-binding ligands is significantly more narrow with molecular weight values that are less than 1600 Da. Both HPSEC and MS show that the most abundant stronger-binding ligands have molecular weights around 270 Da. Furthermore, mass spectral analysis allows some empirical molecular formulas to be postulated for several ligands. These empirical formulas show that the ligands are abundant in nitrogen, oxygen, and sulfur functionality. In addition, consistency between MS data and data from HPSEC when peptides are used for the calibration combined with the low molecular weights and prevalence of nitrogen, oxygen, and sulfur functionality suggest that the stronger-binding ligands may have been produced directly by organisms in the water. It is not clear at this point, however, the degree to which the molecular information we have gathered represents the majority of the Cu(II)-binding ligands at our sampling site. Nonetheless, combining IMAC, HPSEC, and MS seems to be a promising approach for characterizing Cu(II)-binding ligands in natural waters.

The utility of ion–molecule reactions in a quadrupole ion trap mass spectrometer for analyzing metal complex coordination structure

Gas-phase ion–molecule reactions of metal complex ions with acetonitrile in a quadrupole ion trap mass spectrometer are shown to have some potential for determining the number and types of functional groups bound to a metal. Metal complexes with varying coordination spheres show significant differences in their gas-phase reactivity with acetonitrile. The relative product ion intensities observed in the mass spectrum after a given reaction time provide the appropriate data to distinguish complexes with different coordination spheres. Experimental parameters suspected to have an effect on the reproducibility of these intensities are examined in order to understand better which factors need to be most closely controlled to maximize precision. Ni(II) and Cu(II) complex ions of the pentadentate ligands 1,9-bis(2-imidazolyl)-2,5,8-triazanonane (DIEN-(imi)2) and 1,9-bis(2-tetrahydrofuranyl)-2,5,8-triazanonane (DIEN-(THF)2) are reacted in the gas-phase at different temperatures, pressures of acetonitrile, and pressures of buffer gas (helium) in a modified quadrupole ion trap mass spectrometer. The effects of such experimental variations on the product ion intensities are measured and analyzed. Under conditions where the precision is maximized, the temperature and acetonitrile pressure seem to have the biggest impact on the reproducibility of the resulting product ion intensity with the acetonitrile level being slightly more important. The reproducibility of ion intensities for the reactions of the Ni(II) complexes are the lowest due to their higher reactivity, while the measurements for the Cu(II) complexes have a higher degree of precision. In general, with careful control of the temperature and reagent gas pressure, ion–molecule (I–M) reactions seem to be a promising method for providing a rapid and sensitive analysis of the functional groups bound to a metal in a given complex.

Application of external customized waveforms to a commercial quadrupole ion trap

The Finnigan LCQ quadrupole ion trap has recently become part of the repertoire of instruments for many analytical laboratories. The LCQ commercial design, while employing complex waveforms to manipulate ions, does not allow the application of many state-of-the-art user-defined waveforms that enable one to perform other complex ion manipulations. The work presented here describes the simple modifications made to the LCQ electronics to allow the application of external customized waveforms. Results show that externally generated waveforms can be applied to the endcap electrodes while still working within the context of the commercial software and hardware. Double resonance, multiple ion isolation, and multiple ion excitation experiments are demonstrated to reveal the effectiveness of these modifications.