Dissociation of doubly charged transition metal/polyether/pyridyl ligand complexes in a quadrupole ion trap mass spectrometer

The dissociation of a series of doubly charged pyridyl ligand/polyether/transition metal complexes is studied using electrospray ionization and collision activation methods. Both doubly charged mixed-ligand dimer and trimers are observed by electrospray ionization. The mixed-ligand trimer complexes always dissociate by cleavage of one entire ligand, whereas the mixed-ligand dimers show a more diverse array of fragmentation pathways, including charge reduction processes. The fragmentation pathways of these mixed-ligand dimers are influenced by the second ionization energy and electron configuration of the metal and relative coordination strength of the ligands.     

Investigation of uranyl nitrate ion pairs complexed with amide ligands using electrospray ionization ion trap mass spectrometry and density functional theory

Ion populations formed from electrospray of uranyl nitrate solutions containing different amides vary depending on ligand nucleophilicity and steric crowding at the metal center. The most abundant species were ion pair complexes having the general formula [UO(2)(NO(3))(amide)(n=2,3)](+); however, singly charged complexes containing the amide conjugate base and reduced uranyl UO(2)(+) were also formed as were several doubly charged species. The formamide experiment produced the greatest diversity of species resulting from weaker amide binding, leading to dissociation and subsequent solvent coordination or metal reduction. Experiments using methyl formamide, dimethyl formamide, acetamide, and methyl acetamide produced ion pair and doubly charged complexes that were more abundant and less abundant complexes containing solvent or reduced uranyl. This pattern is reversed in the dimethylacetamide experiment, which displayed lower abundance doubly charged complexes, but augmented reduced uranyl complexes. DFT investigations of the tris-amide ion pair complexes showed that interligand repulsion distorts the amide ligands out of the uranyl equatorial plane and that complex stabilities do not increase with increasing amide nucleophilicity. Elimination of an amide ligand largely relieves the interligand repulsion, and the remaining amide ligands become closely aligned with the equatorial plane in the structures of the bis-amide ligands. The studies show that the phenomenological distribution of coordination complexes in a metal-ligand electrospray experiment is a function of both ligand nucleophilicity and interligand repulsion and that the latter factor begins exerting influence even in the case of relatively small ligands like the substituted methyl-formamide and methyl-acetamide ligands.     

Study of diketone/metal ion complexes by electrospray ionization mass spectrometry: Influence of Keto-Enol tautomerism and chelation

The formation and collisionally activated dissociation (CAD) behavior of a series of complexes containing cyclic or linear diketone ligands and alkali, alkaline earth, or transition metal ions are investigated. Electrospray ionization (ESI) is utilized for introduction of the metal ion complexes into a quadrupole ion trap mass spectrometer. The proximity of the carbonyl groups is crucial for formation and detection of ion complexes by ESI. For example, no metal ion complexes are observed for 1,4-cyclohexanedione, but they are readily detected for the isomers, 1,2-and 1,3-cyclohexanedione. Although the diketones form stable doubly charged complexes, the formation of singly charged alkaline earth complexes of the type (nL + M²⁺ ? H⁺)⁺ where L = 1,3-cyclohexanedione or 2,4-pentanedione is the first evidence of charge reduction. CAD investigations provide further evidence of charge reduction processes occurring in the gas-phase complexes. The CAD studies indicate that an intramolecular proton transfer between two diketone ligands attached to a doubly charged metal ion, followed by elimination of the resulting protonated ligand, produces the charge reduced complex. For transition metal complexation, the preference for formation of doubly charged versus singly charged complexes correlates with the keto-enol distribution of the diketones in solution.     

Multidentate Acyclic Neutral Ligands and Their Complexation

The properties of cyclic crown ethers are approximated by acyclic neutral ligands (popdands), which are compared and contrasted with open-chain bioionophores and acidic chelating agents in this article. Variations of the endo-polarophilicity/exo-lipophilicity balance, complex stability, ion slectivity can often be accomplished more easily, with greater versatility, and at less expense with acyclic polyethers than with their cyclic counterparts; complexation and decomplexation are generally faster in acyclic systems; and the pseudocavity usually has greater conformational flexibility. Acyclic crown ethers and open-chain cryptands stiffened by rigid “terminal groups” containing donor atoms readily form crystalline complexes of alkali and alkaline earth metals. Some oppen-chain neutral ligands form helical conformations in their crycstalin complexes. The observed coordination numbers and geometries are of theoretical interst. Attractive terminal group interactions lead to pseudocyclic species occupying a position intermediate between cyclic and acyclic ligands. It has recently proved possible to isolate crystalline complexes of alkali and alkaline earth metal ions with weakly donating oligo(ethylene glycol ethers) and with glycols; such complexes have also been obtained with sugars. Acyclic neutral ligands can serve as simple models of nigericin-type bioionophores and be used analytically in microelectrodes. The recently discoverd crystalline stoichiometric complexes formed by some acyclic neutral ligands with guest molecules such as urea, thiourea, and water provide a fresh insight into weak interactions between neutral molecules and for the development of urea receptors.     

Metal ion complexation by acyclic polyethers with lipophilic amide, thioamide, and amine end groups

A series of acyclic polyethers with lipophilic amide, thioamide, and amine end groups was synthesized. Metal ion transport across bulk liquid membranes and measurement of thermodynamic parameters for ligand-metal ion complexation by titration calorimetry show strong selectivity for complexation of lead ion over other metal ion species for the diamide ligand. Lead ion complexation by the acyclic polyether diamide involves the amide oxygens and silver ion coordination by a dithioamide analog involves the thioamide sulfurs. With a proper length of the ethereal linkage, the ligand wraps around the metal ion in a pseudocyclic fashion.     

Threshold dissociation energies of protonated amine/polyether complexes in a quadrupole ion trap

Electrospray ionization mass spectrometry (ESI-MS) is increasingly used to probe the nature of noncovalent complexes; however, assessing the relevance of gas-phase results to structures of complexes in solution requires knowledge of the types of interactions that are maintained in a solventless environment and how these might compare to key interactions in solution. This study addresses the factors impacting the strength of hydrogen bonding noncovalent interactions in the gas phase. Hydrogen bonded complexes consisting of ammonium ions bound to polydentate ethers are transported to the gas phase with ESI, and energy-variable collisional activated dissociation (CAD) is used to map the relative dissociation energies. The measured relative dissociation energies are correlated with the gas-phase basicities and steric factors of the amine and polyether constituents. To develop correlations between hydrogen bonding strength and structural features of the donor and acceptor molecules, a variety of amines with different gas-phase basicities and structures were selected, including primary, secondary, and tertiary amines, as well as those that are bidentate to promote intramolecular hydrogen bonding. The acceptor molecules are polydentate ethers, such as 18-crown-6. Four primary factors influence the observed dissociation energies of the polyether/ammonium ion complexes: the gas-phase basicities of the polyether and amine, steric effects of the amines, conformational flexibility of the polyethers, and the inhibition of intramolecular hydrogen bonds of the guest ammonium ions in the resulting ammonium/polyether noncovalent complexes.     

Formation Constants of Complexes of Monovalent Cations with Benzocrown Ethers in Nitromethane

The complexation reactions between the macrocyclic polyethers dibenzo-18-crown-6, benzo-18-crown-6, benzo-15-crown-5 and polyethers bearing a stilbene unit with alkali metal and silver cations have been studied conductometrically in nitromethane. The formation constants of 1 : 1 and 1 : 2 (metal : ligand) complexes were determined and found to decrease with increasing cation diameter. The stability of the stilbene crown – metal cation complexes is lower than for complexes of other investigated crown ethers with analogous cations. There seem to be some effects of double bond-silver ion interactions.     

Comparison of the orders of gas-phase basicities and ammonium ion affinities of polyethers by the kinetic method and ligand exchange technique

The orders of relative gas-phase basicities and ammonium ion affinities of a series of polyethers obtained by application of the kinetic method and ligand exchange technique are compared to evaluate the discrepancies of results between the two techniques. The order of gas-phase basicities determined by the ligand exchange technique in a quadrupole ion trap agrees with the order established previously by Kebarle using equilibrium methods in a high-pressure mass spectrometer. The order obtained by the kinetic method in a triple quadrupole mass spectrometer varies for the ranking of one polyether (12-crown-4), and this discrepancy is attributed to a difference in the rates of the competing dissociation pathways from the triethylene glycol dimethyl ether/12-crown4 proton-bound adduct, owing to a substantial variation in the flexibilities of these two ethers. For the order of gas-phase ammonium ion affinities, the kinetic method results agree overall with the ligand exchange results; however, the order of ammonium ion affinities for tetraethylene glycol dimethyl ether and 15-crown-5 could not be differentiated by the ligand exchange method because of the rapidity of ammonium ion transfer between the two polyethers in both directions.     

High energy collision-induced dissociation of alkali-metal ion adducts of crown ethers and acyclic analogs.

High energy collision-induced dissociation (CID) techniques were applied for structural elucidation of alkali-metal ion adducts of crown ethers. The CID of alkali-metal adducts of tetraglyme and hexaethylene glycol were also evaluated to contrast the fragmentation pathways of the cyclic ethers with those of acyclic analogs. A common fragmentation channel for alkali-metal ion adducts of all the ethers, which results in distonic radical cations, is the homolytic cleavage of carbon-carbon bonds. Additionally, dissociation by carbon-oxygen bond cleavages occurs, and these processes are analogous to the fragmentation pathways observed for simple protonated ethers. The proposed fragmentation pathways for alkali-metal ion adducts of crown ethers result mostly in odd-electron, acyclic product ions. Dissociation of the alkali-metal ion adducts of the acyclic ethers is dominated by losses of various neutral species after an initial hydride or proton transfer. The CID processes for all ethers are independent of the alkali-metal ion sizes; however, the extent of dissociation of the complexes to bare alkali-metal ions increases with the size of the metal.     

The effect of magnetic field on the stability of (18-crown-6) complexes with potassium ion

Macrocyclic polyethers are ligands with selectivity for metal ions. In order to understand the interactions between ligand, ion and solvent we resorting to study of magnetic field effect on ion-macrocyclic complexes. Therefore, we studied the complexation between 18-crown-6 and potassium ion in water through the conductometry technique (in 25+0.05 °C) by a nonlinear least-square program (Genplot) under magnetic field.We observed that stability constants of complexes in the presence of the magnetic field, were decreased. Like-wise, we observed that, magnetic field influenced on ion, solvent and ligand one by one.     

Doubly charged porphyrin ion tandem mass spectrometry: Implications for structure elucidation

Under electron ionization (EI) conditions, porphyrins yield unusually high intensities of doubly charged molecular and fragment ions. These doubly charged ions offer unique opportunities for the structure elucidation of porphyrins by tandem mass spectrometry (MS/MS). First, they fragment to a greater extent than the corresponding singly charged ions under both EI/MS and EI/MS/MS conditions. Second, doubly and singly charged porphyrin ions often fragment via different pathways, and can therefore yield different structural information. This paper describes several ways in which analyses of doubly charged porphyrin ions with a triple quadrupole tandem mass spectrometer can be useful in structure elucidation of porphyrins. The effect of the metal atom on the fragmentation of metalloporphyrins in an EI source is demonstrated by correlating the extent of doubly charged fragment ion formation to a stability index. Doubly charged porphyrin ions are shown to yield predominantly doubly charged daughter ions upon collisionally activated dissociation (CAD), and are also shown to fragment to a greater extent than corresponding singly charged porphyrin ions. Advantages and disadvantages of doubly charged porphyrin ion MS/MS for structure elucidation are discussed.     

Retardation of dibenzo polyether-formaldehyde resin formation by alkali-metal cations

Macrocyclic polyethers are well-known complexing agents for alkali-metal and alkaline earth-metal cations. The influence of alkali-metal cations upon the polycondensation rate of acyclic and cyclic dibenzo polyethers with formaldehyde in formic acid and alkali-metal cation sorption by some of the resultant resins have been investigated. For certain dibenzo polyether and alkali-metal cation combinations, polymer formation is markedly reduced. The alkali-metal cation that provides the best fit for the macrocyclic polyether cavity produces the greatest retardation of polymer formation. It is proposed that metal ion complexation renders the dibenzo polyether monomer inert to polymerization under the reaction conditions. No template effect for alkali-metal cation sorption by dibenzo polyether carboxylic acid resins was observed. © 1995 John Wiley & Sons, Inc.     

Ion-selective electrode studies on novel organic molecule sensors.

Researches on prospects for novel ion-selective electrodes, based on organic molecule sensors, are described. The organic molecules are large crown ethers extending from bis(metaphenylene)-26-crown-8 to bis(metaphenylene)-38-crown-12, small crown ethers, bis-crown ethers, and acyclic polyethers consisting of diphenyl ethers of tetraethylene glycol and receptor molecules of planar and tetrahedral tripodal types.     

Determination of binding selectivities in host-guest complexation by electrospray/quadrupole ion trap mass spectrometry

The quantifiable relationship between the equilibrium solution composition and electrospray (ESI) mass spectral peak intensities of simple host-guest complexes was investigated. Specifically, host-guest complexes of simple crown ethers or glymes with alkali metals and ammonium ions were studied. Comparisons were made between the theoretical concentrations of host-guest complexes derived in solution from known stability constants and the peak intensities for the complexes observed by ESI mass spectrometry (ESI-MS). Two types of complexation experiments were undertaken. First, complexation of a single guest ion, such as an alkali metal, and two crown ethers was studied to evaluate the determination of binding selectivities. Second, complexation of two different guest ions by a single polyether host was also examined. In general, solvation was found to play an integral part in the ability to quantify binding selectivities by ESI-MS. The more similar the solvation energies of the two complexes in the mixture, the more quantifiable their binding selectivities by ESI-MS. In some cases, excellent correlation was obtained between the theoretically predicted selectivity ratios and the ESI mass spectral ratios, in particular when the ESI ratios were adjusted based on evaluation of ESI response factors for the various host-guest complexes.     

Temperature dependence of ion association constants in nonaqueous solutions of metal perchlorates

The ion association constants in nonaqueous solutions of singly and doubly charged metal perchlorates in organic solvents in a wide range of temperatures have been calculated by integrating the ligand distribution density function. The standard Gibbs energies, entropies, and enthalpies of formation of ion pairs have been calculated. Comparison with experimental data have shown that diluted solutions of doubly charged metal perchlorates are dominated by solvent-separated ion pairs, whereas such pairs coexist with contact ion pairs in solutions of alkali metal perchlorates. It has been demonstrated that the dielectric continuum approximation satisfactorily predicts the temperature dependence of ion association constants.     

Facilitated transport from ternary cation mixtures through water—chloroform—water membrane systems containing macrocyclic ligands

Cation fluxes were determined for various three-component, equimolar mixtures of alkali metal, alkaline earth, and Pb²⁺ cations in a H₂O---CHCl₃---H₂O liquid membrane system incorporating macrocyclic polyethers as carriers. Carrier ligands studied were 18-crown-6, dicyclohexano-18-crown-6, 1,10-diaza-18-crown-6, 21-crown-7, dibenzo-24-crown-8, and cryptand [2.2.2]. Correlations were found between transport and relative cation:polyether cavity radii, the type of substituents present on the polyether ring, and the type and number of donor atoms present. All the ligands studied transported Pb²⁺ at higher rates than the other Mⁿ⁺ in the mixtures. Transport behavior in these multi-cation systems can be predicted from Mⁿ⁺—polyether complex stability constant data in most cases.     

Separation of alkali and alkaline earth metals by polyethers using extraction chromatography. Effect of diluent

The separation of alkali and alkaline earth metals by means of an acyclic polyether, 1,13-bis(8-chinolinyl)-1, 4, 7, 10, 13-pentaoxatridecane (CPOD), and cyclic polyethers, benzo-15-crown-5 (BC), dibenzo-18-crown-6 (DBC) and dicyclohexyl-18-crown-6 (DCHC), using extraction chromatography has been studied. The alkali metals can be effectively separated using SCN⁻ ions. Different elution sequences for these groups were observed using chloroform and mesitylene and diluents for the polyethers.     

DFT calculation of benzoazacrown ethers and their complexes with calcium perchlorate

The complexation constants of several azacrown ethers with Ca(ClO₄)₂ were determined and turned out to be the higher, the large the macrocycle. The structures of free ligands and their complexes and the complexation energies were calculated by the DFT method. In the aza-12(15)-crown-4(5) ether complexes with Ca(ClO₄)₂, the metal cations lie outside the averaged plane of heteroatoms of the macrocycle, and the coordination of both counterions is V-like. In the complexes of aza-18-crown-6 ethers, the counterions are in the axial position relatively to the macrocycle in the center of which the Ca²⁺ ion is localized. The complexation energies increase with an increase in the size of the azacrown ether macrocycle. The involvement of the nitrogen atom in binding with the Ca²⁺ ion decreases with the expansion of the macrocycle. Two methods for quantitative estimation of the degree of pre-organization of ligands to complexation were considered: geometric and energetic methods. Benzoaza-15-crown-5 ether is a ligand which is more pre-organized to complexation than N-phenylaza-15-crown-5 ether.     

Promotion of Sandmeyer hydroxylation (homolytic hydroxydediazoniation) and hydrodediazoniation by chelation of the copper catalyst: bidentate ligands

Relative to the rate observed for the hexa-aqua ion, Cu(OH(2))(6)(2+), chelation of the copper catalyst by certain bidentate ligands enhances the rate of hydroxydediazoniation reaction (Sandmeyer hydroxylation); the ligands also provide a source of hydrogen in competitive hydrodediazoniation (H-transfer) reactions. By using the cyclisation of 2-benzoylphenyl radical as a radical clock, it has been possible to evaluate absolute rate constants for both processes effected by a variety of complexes involving one or two bidentate ligands (2-aminocarboxylate, 2-hydroxycarboxylate, 1,3-dicarboxylate, 1,2-diamine). The radical exhibits electrophilic character in both processes. The pattern of behaviour observed suggests the rate determining step in hydroxylation is reaction of the aryl radical at the metal centre to form an organocopper adduct which is rapidly converted into products. The relative reactivities of different complexes are explained qualitatively in terms of variations in the ligand field and Jahn-Teller distortion splittings of the copper d orbitals. Hydrodediazoniation is an S(H)2 H-abstraction process. Generally, coordination by Cu(2+) deactivates the first added ligand relative to its reactivity as a free species in the same state of protonation. For the majority of complexes studied, the relative reactivity as H-donors of 1 : 1 and 1 : 2 complexes is statistically determined but an additional electronic effect is discerned for doubly charged ions.     

Ion selectivity of crown ethers investigated by UV and IR spectroscopy in a cold ion trap

Electronic and vibrational spectra of benzo-15-crown-5 (B15C5) and benzo-18-crown-6 (B18C6) complexes with alkali metal ions, M(+)?B15C5 and M(+)?B18C6 (M = Li, Na, K, Rb, and Cs), are measured using UV photodissociation (UVPD) and IR-UV double resonance spectroscopy in a cold, 22-pole ion trap. We determine the structure of conformers with the aid of density functional theory calculations. In the Na(+)?B15C5 and K(+)?B18C6 complexes, the crown ethers open the most and hold the metal ions at the center of the ether ring, demonstrating an optimum matching in size between the cavity of the crown ethers and the metal ions. For smaller ions, the crown ethers deform the ether ring to decrease the distance and increase the interaction between the metal ions and oxygen atoms; the metal ions are completely surrounded by the ether ring. In the case of larger ions, the metal ions are too large to enter the crown cavity and are positioned on it, leaving one of its sides open for further solvation. Thermochemistry data calculated on the basis of the stable conformers of the complexes suggest that the ion selectivity of crown ethers is controlled primarily by the enthalpy change for the complex formation in solution, which depends strongly on the complex structure.