Electrospray ionization studies of transition-metal complexes of 2-acetylbenzimidazolethiosemicarbazone using collision-induced dissociation and ion-molecule reactions

The complexes of transition-metal ions (M2+, where M = Fe, Co, Ni, Cu, Zn, Cd, and Hg) with 2-acetylbenzimidazolethiosemicarbazone (L) are studied under electrospray ionization (ESI) conditions. The ESI mass spectra of Fe and Co complexes showed the complex ions corresponding to [M+2L-2H]+, and those of Ni and Zn complexes showed [M+2L-H]+ ions, wherein the metal/ligand ratio is 1:2 and the oxidation state of the central metal ion is +3 in the case of Fe and Co and +2 in the case of Ni and Zn. The Cd and Cu complexes showed preferentially 1:1 complex ions, i.e., [M+L-H]+ or [M+L+Cl]+, whereas Hg formed both 1:1 and 1:2 complex ions. During formation of the above complex ions one or two ligands are deprotonated after keto-enol tautomerism, depending on the nature and oxidation state of central metal ion. The structures and coordination numbers of the metal ions in the complex ions were studied by their collision-induced dissociation spectra and ion-molecule reactions with acetonitrile or propylamine in the collision cell. Based on these results it is concluded that Fe, Co, Ni and Zn form stable octahedral complexes, whereas tetrahedral or square planar complexes are formed preferentially for other metals. In addition, the Cu complex showed a [2L+2Cu-3H]+ ion with a Cu-Cu bond.     

Reactivity of molecules with nitrogen-containing functional groups toward H+ and SiMe3 + ions

The equilibrium constants of trimethylsilyl cation transfer reactions differ from those of proton transfer reactions by many orders of magnitude. The basicity of MeCN (1), MeNO₂ (2), and Et₂NH (3) in the gas phase decreases in the series3≫1>2, whereas the affinity of the same compounds for trimethylsilyl cation decreases in the series1≫3≈2. Semiempirical quantum-chemical MNDO calculations indicate that the formation of MeCN·SiMe₃ ⁺ ions is thermodynamically more favorable than that of MeNH₂·SiMe₃ ⁺ ions. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 6, pp. 1260–1261, June, 1998.     

A spectral study of transition-metal complexes on a chelating ion-exchange resin containing aminophosphonic acid groups

The coordination mode of a commercial chelating ion-exchange resin, ES 467, containing aminomethylphosphonic acid groups anchored on a macroporous styrenedivinylbenzene copolymer, with several metal ions, such as chromium(III), cobalt(II), nickel(II), copper(II) and zinc(II) has been studied with the aid of IR, ligand field and ESR spectra. The chelating group of ES 467, aminomethylphosphonic acid, appears to bind different metal ions mainly through oxygen atoms of the phosphonic acid group, though, in the case of those species containing low-loading of metal ions, the secondary amine nitrogen atom also appears to take part in coordination. Most of the metal ions studied seem to be present as six-coordinated species in the polymeric matrix, whereas cobalt(II) species show electronic spectra which are consistent with the presence of both four-coordinated (tetrahedral) and six-coordinated (octahedral) structures of these species. The distribution coefficients for cobalt(II), nickel(II), copper(II) and zinc(II) have been determined and it appears that the resin ES 467 binds copper(II) ions selectively. An attempt has been made to find a possible relationship between the selectivity of the chelating resin and the stereochemistry of the adsorbed metal ions.     

Stereochemistry of retro-diel-salder fragmentation in gas-phase ions formed by chemical ionization

Retro Diels Alder fragmentation is highly stereospecific in the diones 1 under chemical ionization conditions, both with methane and isobutane as the reagent gases. Only the cis-isomers yield abundant protonated diene and quinone ions. The isotope effect indicates preferential protonation on a CO oxygen, and a subsequent H-migration prior to the formation of the protonated diene cations in the cis isomers.     

Mass spectrometric synthesis. V. The gas-phase ion-molecule dimerization of some sulfur containing heterocycles

The gas-phase, ion-molecule reactions occurring in the ion source of a mass spectrometer between differently substituted thiazoles, employed as starting compounds for synthesis of polymers exhibiting semiconducting properties, have been studied in detail. The structural characterization of the dimeric moieties so obtained has been carried out by exact mass measurements, linked scans for metastable analysis and collisional spectroscopy.     

Formation of iminium ions by fragmentation of a2 ions

Tandem mass spectrometric experiments have been carried out on the protonated amides H-Gly-Ala-NH2, H-Ala-Gly-NH2, H-Ala-Val-NH2, H-Val-Ala-pNA, H-Leu-Phe-NH2, H-Phe-Leu-NH2, H-Phe-Tyr-NH2 and H-Tyr-Phe-NH2 with particular emphasis on the fragmentation of the isomeric a2 ions derived therefrom. Primary fragmentation reactions of the protonated amides involve formation of the y1" and b2 ions with further fragmentation of the b2 ion to form the a2 ion which fragments to form iminium ions. Collision-induced dissociation studies of the mass-selected a2 ions were carried out. For the Gly-Ala, Ala-Gly and Val-Ala a2 ions, weak signals were observed corresponding to loss of CO from the a2 ion. With the exception of the Gly-Ala, Ala-Gly and Val-Ala a2 ions, both possible iminium ions (a1 and the internal iminium ion) are observed with the most abundant being that formed by proton attachment to the imine of higher proton affinity. The results provide strong support for the recently proposed (El Aribi et al. J. Am. Chem. Soc. 2003; 125: 9229) mechanism of fragmentation of a2 ions which involves elimination of CO from the a2 ion to form a proton-bound complex of two imines. Based on this mechanism ab initio calculations of the total energies of the a2 ions and the transition states for fragmentation have been carried out giving the energy barrier for fragmentation of each a2 ion. The experimental results are interpreted in terms of these energetics data, unimolecular rate constants calculated by using the RRKM theory, and the imine proton affinities.     

Fragmentation and charge transfer in gas-phase complexes of divalent metal ions with acetonitrile

The development of electrospray has enabled generation of gas-phase multiply charged metal ion complexes with various solvent molecules. These species exhibit rich fragmentation chemistry, involving competition among neutral ligand loss, ligand cleavage, and dissociative electron and proton transfer. Acetonitrile is a common aprotic solvent. Here we present a comprehensive MS/MS study on acetonitrile complexes of divalent metal cations. We measured the critical sizes below which dissociation channels other than the trivial neutral evaporation become operative and minimum sizes at which dications remain stable against charge reduction. For all sizes between the two, low-energy fragmentation patterns have been elucidated in detail.     

Unimolecular gas-phase reactions of the methyl acetoacetate radical cation. Oxygen atom permutation via ion-molecule complexes

The reactions of metastable decomposing methyl acetoacetate (a mixture of keto a ad enol tautomers) are reported and discussed. The unimolecular fragmentations of the tautomers are different. The metastable decomposing radical cation of the keto form displays four specific ions: [M –CO]⁺˙, [M – CH₂O]⁺˙, [M – CH₂CO]⁺˙ and m/z 43. The results derived from D-, ¹³C- and ¹⁸O-labelled precursors together with thermochemical data have been used to study the mechanisms. Experimental results indicate that an unexpected isomerization occurs before dissociation. It formally corresponds to oxygen atom permutation of the two carbonyl groups without participation of the carbon atoms. This remarkable process is interpreted in terms of a mechanism involving ion-molecule complexes.     

Density functional study of hydrogen bond formation between methanol and organic molecules containing Cl, F, NH2, OH, and COOH functional groups

Various hydrogen-bonded complexes of methanol with different proton accepting and proton donating molecules containing Cl, F, NH(2), OH, OR, and COOH functional groups have been modeled using DFT with hybrid B3LYP and M05-2X functionals. The latter functional was found to provide more accurate estimates of the structural and thermodynamic parameters of the complexes of halides, amines, and alcohols. The characteristics of these complexes are influenced not only by the principle hydrogen bond of the methanol OH with the proton acceptor heteroatom, but also by additional hydrogen bonds of a C-H moiety with methanol oxygen as a proton acceptor. The contribution of the former hydrogen bond in the total binding enthalpy increases in the order chlorides < fluorides < alcohols < amines, while the contribution of the second type of hydrogen bond increases in the reverse order. A general correlation was found between the binding enthalpy of the complex and the electrostatic potential at the hydrogen center participating in the formation of the hydrogen bond. The calculated binding enthalpies of different complexes were used to clarify which functional groups can potentially form a hydrogen bond to the 2'-OH hydroxyl group in ribose, which is strong enough to block it from participation in the intramolecular catalytic activation of the peptide bond synthesis. Such blocking could result in inhibition of the protein biosynthesis in the living cell if the corresponding group is delivered as a part of a drug molecule in the vicinity of the active site in the ribosome. According to our results, such activity can be accomplished by secondary or tertiary amines, alkoxy groups, deprotonated carboxyl groups, and aliphatic fluorides, but not by the other modeled functional groups.     

Macromolecular metal complexes with transition-metal ions: Structure of a polymer and efficiency of complexation

A quantitative method of estimating the efficiency of formation of macromolecular metal complexes of carboxyl-containing (co)polymers with transition-metal ions in dilute solutions was developed. This method is based on the luminescence quenching of luminescently labeled macromolecules by transition-metal ions. With the use of this method, the effect of the chemical (including isomeric) structure of carboxyl-containing (co)polymers and external conditions on the stability of macromolecular metal complexes based on copper, nickel, and silver ions was assessed. With consideration for the data obtained, a targeted variation in the functional characteristics of macromolecular metal complexes may be accomplished.     

Substituent effects on the gas-phase fragmentation reactions of sulfonium ion containing peptides

The multistage mass spectrometric (MS/MS and MS3) gas-phase fragmentation reactions of methionine side-chain sulfonium ion containing peptides formed by reaction with a series of para-substituted phenacyl bromide (XBr where X=CH2COC6H4R, and R=--COOH, --COOCH3, --H, --CH3 and --CH2CH3) alkylating reagents have been examined in a linear quadrupole ion trap mass spectrometer. MS/MS of the singly (M+) and multiply ([M++nH](n+1)+) charged precursor ions results in exclusive dissociation at the fixed charge containing side chain, independently of the amino acid composition and precursor ion charge state (i.e., proton mobility). However, loss of the methylphenacyl sulfide side-chain fragment as a neutral versus charged (protonated) species was observed to be highly dependent on the proton mobility of the precursor ion, and the identity of the phenacyl group para-substituent. Molecular orbital calculations were performed at the B3LYP/6-31+G** level of theory to calculate the theoretical proton affinities of the neutral side-chain fragments. The log of the ratio of neutral versus protonated side-chain fragment losses from the derivatized side chain were found to exhibit a linear dependence on the proton affinity of the side-chain fragmentation product, as well as the proton affinities of the peptide product ions. Finally, MS3 dissociation of the nominally identical neutral and protonated loss product ions formed by MS/MS of the [M++H]2+ and [M++2H]3+ precursor ions, respectively, from the peptide GAILM(X)GAILK revealed significant differences in the abundances of the resultant product ions. These results suggest that the protonated peptide product ions formed by gas-phase fragmentation of sulfonium ion containing precursors in an ion trap mass spectrometer do not necessarily undergo intramolecular proton 'scrambling' prior to their further dissociation, in contrast to that previously demonstrated for peptide ions introduced by external ionization sources.     

Dissociation of gas-phase dimeric complexes of lactic acid and transition-metal ions formed under electrospray ionization conditions; the role of reduction of the metal ion

Dimeric complex ions of the type [M(A-H)A]+, where M=metal ion (Co, Ni, Cu, and Zn) and A=ligand (lactic acid, methyl lactate or ethyl lactate), were generated in the gas phase under electrospray ionization conditions. The collision-induced dissociation spectra of [M(A-H)A]+ ions were recorded to study the behaviour of ligand and metal ions in decomposition of these dimeric complex ions. Based on the fragmentation pathways observed for complex ions of lactic acid, it is found that both the carboxylic and hydroxyl groups of lactic acid are involved in the complex formation following displacement of a proton by the metal ion. The dimeric complex ions of Co, Ni, and Zn dissociated to yield similar types of ions, whereas that of Cu behaved differently. The dissociations of Co-, Ni-, and Zn-bound dimeric complexes involved losses of neutral molecules while keeping the oxidation state of the metal ion unchanged. However, elimination of radicals is found in the dissociation of dimeric complex ions of Cu, and the oxidation state of copper is reduced from Cu(II) to Cu(I) in the resulting fragment ions. The deprotonated ligand is involved in the fragmentation pathway of Cu complexes, whereas it is intact in other complexes. The oxidation state of the metal ion, nature of the ligand, and site of attachment to the metal ion are found to control the dissociation of these dimeric complex ions.     

Complexes in polymers IV. FTIR microscopic characterization of thin organic polymer films containing embedded transition-metal carbonyl complexes

Thin films of polystyrene (PS), poly(methyl methacrylate) (PMMA) and polystyrene-polyacrylonitrile copolymer (PS-AN), containing various embedded transition-metal complexes, have been studied by FTIR microscopy. The spatial distributions of the transition-metal carbonyl complexes throughout the thin organic polymer films have been determined by a two-dimensional IR mapping procedure. The spectral variations observed in the distribution of the metal carbonyls throughout the different polymer films are discussed. The IR data show that the technique used to prepare the organometallic-embedded thin films (viz. freeze-drying of solutions followed by hot mechanical pressing of the residues) does in general lead to homogeneous films which may eventually find industrial application, e.g. as membrane sensors for small molecules.     

Formation of charge-transfer complexes in systems containing substituted aromatic hydrocarbons and electron-donating organic compounds

Binary systems of monosubstituted benzene derivatives of type C₆H₅X (X=CN, NO₂, COMe, F, Cl, Br, I) and furan, pyrrole, thiophene, pyridine, tetrahydrofuran, and piperidine are analyzed physicochemically by a refractometric method.From deviations from additivity of n (V) isotherms for systems containing furan plus C₆H₅CN, C₆H₅NO₂, or C₆H₅COMe, conclusions can be drawn about interactions of components leading to formation of charge-transfer complexes (CTC) of 21 composition.With pyrrole possible intermolecular interaction must be confirmed by other methods of physicochemical analysis.Almost all of the systems containing tetrahydrofuran investigated, differ by showing more marked interaction between the components, giving CTC of 11 composition.n (V) isotherms for systems containing pyridine and C₆H₅NO₂ are slightly concave to the composition axis. At the same time piperidine clearly reacts with C₆H₅CN and C₆H₅NO₂, giving a 21 complex with the former, and a colored mixture with the latter, (formation of a complex in this case requires confirmation by other methods).All the thiophene binary systems investigated are characterized by linear refractive index isotherms, i.e., they are refractometrically ideal.The electron-donor properties of the heterocyclic compounds investigated vis a vis the aromatic ring with a lowered-electron density lie in the order tetrahydrofuran > furan > piperidine > pyrrole > thiophene.For Part III see [6].     

Density functional theory study on two D-π-A-type organic dyes containing different anchoring groups for dye-sensitized solar cells

The electronic structure and absorption spectra of two D-π-A-type organic dyes with different anchoring groups have been investigated using density functional theory (DFT) and time-dependent density functional theory (TD-DFT). The effect of anchoring groups on the electronic absorption of the free dyes on (TiO₂)₉ has been studied for the two carbazole dyes (MK1 and MK2). Results from DFT calculations indicate that hydroxamic acid anchoring group in MK2 lead to much stronger intermolecular charge transfer and adsorption energies on (TiO₂)₉ cluster. The effect of four different XC functionals (B3LYP, ωB97xD, M06-2X, and CAM-B3LYP) on the transition energies has been tested in order to explore the valid functional for the studied system. The wavelength values from the ωB97xD/6-31+G** level of theory are in excellent agreement with experimental data so this functional was considered to calculate the electronic absorption of the two studied dyes. The highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO), and the gap energy (H–L) of the studied dyes are slightly influenced by change of anchoring group. Results reveal that the LUMO energy levels of all studied dyes are higher than the conduction band (CB) of TiO₂ (??4.00 eV). Deprotonation process enhances the efficiency of dye-sensitized solar cells during decreasing adsorption energy of dyes with (TiO₂)₉ cluster.

Graphical abstract

     

The thermal decomposition of transition-metal complexes containing heterocyclic ligands: I. Benzothiazole complexes

Enthalpies of the decomposition reactions MX₂L₂(c)→MX₂(c) + 2L (g), where M is Mn, Co, Ni, Cu, or Cd, X is Cl and/or Br, and L is benzothiazole or 2-methyl-benzothiazole have been measured by use of a differential scanning calorimeter. Specific heats and enthalpies of sublimation of some of the complexes have been obtained.     

Reaction of lithium tris(tert-butylthiolato)hydridoaluminate with selected organic compounds containing representative functional groups

The approximate rates and stoichiometry of the reaction of lithium tri(2-methyl-2-propanethioxy)aluminum hydride (LTBSA) with selected organic compounds containing representative functional groups under the standardized conditions (tetrahydrofuran, 0 °C) were studied in order to define the reducing ability of LTBSA also compared with those of the parent lithium aluminum hydride (LAH) and lithium t-butoxyaluminum hydride (LTBA).