Relative Binding Energies of Gas-Phase Pyridyl Ligand/Metal Complexes by Energy-Variable Collisionally Activated Dissociation in a Quadrupole Ion Trap

Abstract

The relative binding energies of a series of pyridyl ligand/metal complexes of the type [M(I)L(2)](+) and [M(II)L(3)](2+) are investigated by using energy-variable collisionally activated dissociation in a quadrupole ion trap mass spectrometer. The pyridyl ligands include 1,10-phenanthroline and various alkylated analogues, 2,2'-bipyridine, 4,4'-dimethyl-2,2'-bipyridine, and 2,2':6',2' '-terpyridine, and the metal ions include cobalt, nickel, copper, zinc, cadmium, calcium, magnesium, lithium, sodium, potassium, rubidium, and cesium. The effect of the ionic size and electronic nature of the metal ion and the polarizability and degree of preorganization of the pyridyl ligands on the threshold activation voltages, and thus the relative binding energies of the complexes, are evaluated. Correlations are found between the binding constants of [M(II)L(3)](2+) complexes in aqueous solution and the threshold activation voltages of the analogous gas-phase complexes determined by collisionally activated dissociation.

A simple method for estimating activation energies using the fragmentation yield: Collision-induced dissociation of iron(II)-phenanthroline complexes in an electrospray ionization mass spectrometer

“…k o ϭ Aexp͑ϪsE a ⁄ E int, o͒ and k ϭ Aexp͓ϪsE a ⁄ ͑Eint, o ϩ E int͔͒ (6) Assuming that E int Ͼ ϾE int,o and substituting eq 6 into eq 4 the fragmentation yield according to the Arrhenius equation ( ARR ) is given by eq 7.…”

“…The gasphase ions formed in such a way are then mass analyzed and/or mass-selected, followed by collision with the collision gas molecules (CID) in the collision cell or surface (SID) to induce fragmentation and, thus, structural information and bond dissociation energies for the analyte ions can be obtained. Several methods have been applied for determination of relative gas-phase stabilities and critical energy for fragmentation including threshold dissociation voltage [6], critical slope method [7], sigmoid-type evaluation [9], as well as a highly sophisticated computer program based on the Rice-Ramsperger-Kassel-Marcus (RRKM) approximation is also available [10].In the most commercial ESI-MS instruments with no MS/MS capabilities, fragmentation can only be enhanced upon increasing the potential difference between the orifice (capillary) and the skimmer. The ions entering from the atmospheric pressure region into the first stage of the vacuum system are accelerated by the orifice-skimmer potential difference, meanwhile several low and/or high-energy collisions occur between the ions and the background gas molecules.…”

“…2 omparison of the stabilities of the metal-containing complexes in the gas-phase to those in the solution phase may provide a deeper insight into the processes affecting the complex formation. The electrospray ionization (ESI) technique [1] combined with tandem mass spectrometry offers a unique way for the investigation of the metal-containing complexes by generating gas-phase ions from solutions under the influence of a constant electric field [2][3][4][5][6][7][8]. The gasphase ions formed in such a way are then mass analyzed and/or mass-selected, followed by collision with the collision gas molecules (CID) in the collision cell or surface (SID) to induce fragmentation and, thus, structural information and bond dissociation energies for the analyte ions can be obtained.…”

“…The gasphase ions formed in such a way are then mass analyzed and/or mass-selected, followed by collision with the collision gas molecules (CID) in the collision cell or surface (SID) to induce fragmentation and, thus, structural information and bond dissociation energies for the analyte ions can be obtained. Several methods have been applied for determination of relative gas-phase stabilities and critical energy for fragmentation including threshold dissociation voltage [6], critical slope method [7], sigmoid-type evaluation [9], as well as a highly sophisticated computer program based on the Rice-Ramsperger-Kassel-Marcus (RRKM) approximation is also available [10].…”

Tunable transition metal-ligand complexation for enhanced elucidation of flavonoid diglycosides by electrospray ionization mass spectrometry

“…Electron-releasing substituents increase the metal binding energies of pyridyl ligands and therefore in some cases destabilize the desired [M(II) (flavonoid -H) pyridyl ligand] ϩ complexes. Pyridyl ligands have intrinsically high metal binding energies, both in solution [39 -43] and in the gas-phase [44,45], arising from their ability to chelate metal ions by their nitrogen atoms. Addition of electron-withdrawing substituents should temper the metal binding energies and thus expand the options for creating tunable auxiliary ligands for the differentiation of isomers, such as flavonoids, by metal complexation strategies.…”