Structural elucidation of transition metal complex by single crystal EPR study

Single crystal EPR studies of Mn(II) doped hexaaquazincdiaquabis(malonato) zincate [Zn(H₂O)₆][Zn(mal)₂(H₂O)₂] have been carried out at room temperature using X-band spectrometer to identify the location of the dopant. Single crystal rotations along the three orthogonal axes show more than 30 line pattern EPR spectra indicating the presence of two types of dopant ions in the host lattice, with intensity ratio of 6:1. However, the latter could not be followed due to its low intensity during crystal rotations. The spin-Hamiltonian parameters, estimated from the three mutually orthogonal crystal rotations are: g_xx = 1.972, g_yy = 2.000, g_zz = 2.023, A_xx = 8.95, A_yy = 9.48, A_zz = 9.93 mT, D_xx = −34.49, D_yy = −3.26, D_zz = 37.74 mT and E = 15.6 mT. The direction cosines of one of the principal values of g match with that of Zn-O bond in the host lattice, suggesting that the Mn(II) ion entered the lattice substitutionally. The large value of E is indicative of low symmetry of the substitutional site, in accordance with the crystal structure of the isomorphous [Zn(H₂O)₆][Cu(mal)₂(H₂O)₂]. Covalency of Mn-O bond, estimated from Matamura’s plot, is 9%. Various admixture coefficients, bonding and optical parameters have also been calculated.     

Modification of glass capillary gas chromatographic columns by alkylation of the glass surface with pentafluorobenzyl bromide

Summary Glass surface alkylation with pentafluorobenzyl (PFB) bromide yields glass capillary gas chromatographic columns with modified retention characteristics. Glass-alkylated OV-225 columns have been tested in the analysis of PFB fluoroacetate, and the substantial increase in retention time of this highly volatile compound was found to improve the precision of analysis. PFB-alkylated columns should prove generally useful in gas chromatographic analysis of small relative molecular mass compounds.     

Graphene nucleation on transition metal surface: structure transformation and role of the metal step edge

The nucleation of graphene on a transition metal surface, either on a terrace or near a step edge, is systematically explored using density functional theory calculations and applying the two-dimensional (2D) crystal nucleation theory. Careful optimization of the supported carbon clusters, C(N) (with size N ranging from 1 to 24), on the Ni(111) surface indicates a ground state structure transformation from a one-dimensional C chain to a 2D sp(2) C network at N ≈ 10-12. Furthermore, the crucial parameters controlling graphene growth on the metal surface, nucleation barrier, nucleus size, and nucleation rate on a terrace or near a step edge are calculated. In agreement with numerous experimental observations, our analysis shows that graphene nucleation near a metal step edge is superior to that on a terrace. On the basis of our analysis, we propose the use of graphene seeds to synthesize high-quality graphene in large area.     

Potential of thermo-optical methods for the study of molecular layers bonded to a flat glass surface

Thermal-lens spectrometry and photothermal deflection spectroscopy (mirage-effect spectroscopy) are used to study active bright red 5SKh bonded to a glass surface. The theoretically calculated dependence of the thermal-lens signal on the absorbance of the surface-absorbing layers on a nonabsorbing substrate is used to evaluate the concentration of the reagent in the bonded layer. The concentration of the reagent varies in the range of (2–8) × 10^?11 mol/cm², depending on the method of surface modification. Using photothermal deflection spectroscopy, it is shown that the thickness of the layer on the glass surface does not exceed 700 nm. Data obtained by the thermo-optical methods are supplemented with the results of the analysis of the surface relief obtained using electron microscopy in the secondary electron emission mode, and the uniformity of the modification of the sample surface is characterized. Based on the obtained data, a comparative analysis is performed for the potential of the considered methods for the investigation of the molecular layers bonded to the flat glass surface.     

Structural isomer effects on the morphology of block copolymer/metal salts hybrids

The structural isomer effects on phase behavior of block copolymer/FeCl₃ hybrids were investigated by comparing structures of two series of blends based on polystyrene‐b‐poly(4‐vinylpyridine) (PS‐P4VP) and polystyrene‐b‐poly(2‐vinylpyridine) (PS‐P2VP), with the same molecular weight and the same composition. By conbining fourier transform infrared (FT‐IR) spectroscopy and differencial scaninng calorimetry, successful achievements of selective dispersion of FeCl₃ into poly(vinylpyridine) phase via coordination were verified. Complementary morphological observation by transmission electron microscopy and small‐angle X‐ray scattering (SAXS), it has been clarified that phase behavior for two isomer series is considerably different. That is, neat PS‐P4VP formed thicker cylindrical domains than that of neat PS‐P2VP due to much stronger Flory‐Huggins interaction parameter χ, χ_PS‐P4VP » χ_PS‐P2VP. As for PS‐P2VP/FeCl₃ hybrids, morphological transition can be taken place at the smaller amount of metal salt; furthermore, P2VP blend series form lamellar structures with evidently larger periodic length at the same amount of metal salt. This is probably caused by the event that excess metal salt also contributes to lamellar expansion by localizing at the center of P2VP lamellar phase. Moreover, the saturation limit of introduced metal salt in P2VP was smaller than that in P4VP due to the steric hindrance for a lone pair electrons on nitrogen atoms directed to the main chain of P2VP. These results can be explained by the structural isomer effects on the conformation of the P2VP chains at coordinated state with FeCl₃, that is, P2VP chains prefer to form the intramolecular coordination due to the short range interaction so as to make themselves stiffer, whereas P4VP chains tend to adopt the long range interaction including intra‐ and intermolecular coordinations. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 377–386     

Catalytic properties of transition metal salts immobilized on nanoporous silica polyamine composites II: hydrogenation

The transition metal compounds Pd(OAc)₂, RhCl₃·4H₂O and RuCl₃ · nH₂O were adsorbed onto the nanoporous silica polyamine composite (SPC) particles (150–250 µm), WP‐1 [poly(ethyleneimine) on amorphous silica], BP‐1 [poly(allylamine) on amorphous silica], WP‐2 (WP‐1 modified with chloroacetic acid) and BP‐2 (BP‐1 modified with chloroacetic acid). Inductively coupled plasma‐atomic emission spectrometry analysis of the dried samples after digestion indicated metal loadings of 0.4–1.2 mmol g^?1 except for RhCl₃·4H₂O on BP‐2 which showed a metal loading of only 0.1 mmol g^?1. The metal loaded composites were then screened as hydrogenation catalysts for the reduction of 1‐octene, 1‐decene, 1‐hexene and 1, 3‐cyclohexadiene at a hydrogen pressure of 5 atm in the temperature range of 50–90 °C. All 12 combinations of SPC and transition metal compound proved active for the reduction of the terminal olefins, but isomerization to internal alkenes was competitive in all cases. Under these conditions, selective hydrogenation of 1,3‐cyclohexadiene to cyclohexene was observed with some of the catalysts. Turnover frequencies were estimated for the hydrogenation reactions based on the metal loading and were in some cases comparable to more conventional heterogeneous hydrogenation catalysts. Examination of the catalysts before and after reaction with X‐ray photoelectron spectroscopy and transmission electron microscopy revealed that, in the cases of Pd(OAc)₂ on WP‐2, BP‐1 and BP‐2, conversion of the surface‐ligand bound metal ions to metal nano‐particles occurs. This was not the case for Pd(OAc)₂ on WP‐1 or for RuCl₃ · nH₂O and RhCl₃· 4H₂O on all four composites. The overall results are discussed in terms of differences in metal ion coordination modes for the composite transition‐metal combinations. Suggested ligand interactions are supported by solid state CPMAS ¹³C NMR analyses and by analogy with previous structural investigations of metal binding modes on these composite materials. Copyright © 2011 John Wiley & Sons, Ltd.     

Complexation of transition metal ions on the surface of carboxyethylated aminopolysiloxanes

The effect of the nature of the sorbent matrix and the state of ions of some transition metals in solution on their sorption by carboxyethylated alumino-, zircono-, and titano-aminopolysiloxanes was studied. Modification of the aminopolysiloxane matrix with zirconia, alumina, or titania results in displacement of the optimal sorption ranges for copper(II), nickel(II), and cobalt(II) to alkaline pH. Comparison of the results of structural studies of the complexes of N-aryl-3-aminopropionic acids in solutions and compositions of equilibrium solutions upon sorption of metal ions from ammonia-acetate buffer systems demonstrated that metal ions are simultaneously coordinated by the functional groups of iminodipropionic acid, which are rigidly attached to on the polysiloxane matrix, and by monodentate molecular ligands (ammonia molecules) present in the buffer solution. A competitive influence of copper(II), nickel(II), and cobalt(II) on the sorption from mixed solutions was established.     

Effects of metal salts on polymerization. Part II. Polymerization of vinylpyridine complexed with the group IIb metal salts

The following stoichiometric vinylpyridine complexes have been prepared: (4-VP)₂—Zn(SCN)₂, (2-VP)₂—Zn(SCN)₂, (MVP)₂—Zn(SCN)₂, (MVP)₂—ZnCl₂, (MVP)₂—ZnBr₂, (MVP)₂—ZnI₂, and (MVP)₂—HgCl₂, where 4-VP, 2-VP, and MVP denote 4-vinylpyridine, 2-vinylpyridine, and 2-methyl-5-vinylpyridine, respectively. Results of radical polymerization initiated by azobisisobutyronitrile indicate that the effect of complex formation between the monomers and the metal salts is to enhance the rate of polymerization with the exception of the 2-VP complex. The R_p for the solution polymerization in dimethylformamide increases in the following order: (1) (MVP)₂—Zn-(SCN)₂ > (MVP)₂-ZnCl₂ > (MVP)₂—ZnBr₂ > (MVP)₂—ZnI₂ > free MVP; (2) (4-VP)₂—Zn(SCN)₂ > (MVP)₂—Zn(SCN)₂ > free MVP > (2-VP)₂—Zn(SCN)₂; and (3) MVP + Zn(CH₃COO)₂ < MVP + Cd(CH₃COO)₂. When ethanol, acetone, or tetrahydrofuran is used as solvent, the change in R_p is more marked, partly due to insolubility of the PMVP complexed with the metal salts. The increase in R_p would be attributed to the change in k_p since the molecular weights of PMVP are nearly proportional to R_p when (MVP)₂—ZnX₂ where X is Cl^?, Br^?, I^?, or SCN^? is polymerized in DMF under fixed conditions. Copolymerizations of MVP—ZnX₂ complexes (where X is Cl^?, Br^?, I^?, or CH₃COO^?) with styrene indicate that the e values of complexed MVP are more positive than that of free vinylpyridine, and the amounts of the positive shift in e values increase with decreasing polarizability of the halide anions. These results are discussed in terms of the charge-transfer properties of anions, the nature of coordination bonds, and the structures of vinylpyridines. The complexed monomers are hardly polymerized by a cationic or an anionic mechanism. Radiation-induced solid-state polymerization gives polymers in low yields.     

Modification of polymethylhydrosiloxane by dehydrocoupling reactions catalyzed by transition metal complexes: Evidence for the preservation of linear siloxane structures

Polysiloxanes with pendant poly(ethylene oxide) side chains (4 were prepared by the dehydrocoupling reaction of poly(methylhydrosiloxane) (PMHS, 3 with 2-(2-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)ethoxy)ethanol (1 and poly(ethylene glycol) methyl ether (2 using a metal catalyst. Catalysts investigated were tin(II) 2-ethylhexanoate, Rh(Ph₃P)₃Cl, and Pd₂(dba)₃. The reaction of a cyclic siloxane, D₄^H, with 1 catalyzed by Pd₂(dba)₃ was also carried out to synthesize siloxane 6. The polysiloxanes were characterized by ¹H NMR, ²⁹Si NMR, FT-IR, and GPC. ²⁹Si NMR study of these comb-like polysiloxanes revealed that there is a significant difference in the structure of the siloxane polymers prepared depending upon the catalyst. M, D, and T units were observed when tin(II) was used as a catalyst, but only M and D units were detected when Rh(Ph₃P)₃Cl or Pd₂(dba)₃ was employed. Furthermore, M and T units are negligible for the cyclic siloxane 3 using Pd₂(dba)₃. A mechanism is proposed to account for these observations.     

Structural changes above the glass transition and crystallization in aluminophosphate glasses: an in situ high-temperature MAS NMR study

We present an in situ high-temperature nuclear magnetic resonance study on the structural changes in aluminophosphate glasses occurring in the temperature range between the glass transition temperature Tg and the crystallization temperature Tc, Tg < T < Tc. Decisive changes in the network organization between Tg and Tc in potassium aluminophosphate glasses in the compositional range 50K2O-xAl2O3-(50 - x)P2O5 with 2.5 < x < 20 could be monitored for the first time employing 1D 31P- and 27Al-MAS NMR. Accompanying ex situ NMR experiments (31P-RFDR NMR and 31P-{27Al} CP-HETCOR NMR) on devitrified samples were performed at room temperature to further characterize the phases formed during the crystallization process. The structural role of boron-which is known to inhibit the crystallization process in these aluminophosphate glasses-on short and intermediate length scales was analyzed employing 11B-MQMAS, 11B-{27Al} TRAPDOR and 11B-{31P} REDOR NMR spectroscopy.     

Chemistry of organosilicon compounds : LXXI. Ring closure of (4-pentenyl)hydrosilanes by intramolecular hydrosilylation with transition metal salts

The intramolecular hydrosilylation of (4-pentenyl)hydrosilanes with transition metal salt catalysts afforded five- and six-membered ring-closure products in high yields, with the former predominating. This has been rationalized in terms of a reaction scheme based on the Harrod—Chalk mechanism in which both Si-metal and C-metal bonds were involved. The seven-membered intermediate leading to the six-membered ring products is apparently less favorable than the six-membered intermediate. However, with dicobalt octacarbonyl, no ring closure occurred, and only izomerization was observed.     

Separation of cephalosporins on thin silica gel layers impregnated with transition metal ions and by reversed-phase TLC

Cephalosporin antibiotics were separated on thin layer plates impregnated with transition metal ions, Mn(2+), Fe(2+), Co(2+), Ni(2+) and Cu(2+), using different concentrations. Various solvent systems were developed for the study and were used for separation of these analytes. Impregnation was observed to have an effect on hR(F) values, removed tailing of analytes and improved the resolution. The results have been discussed for each metal ion and compared, and the best conditions of separation have been identified. Activation time of thin layer plates impregnated with 0.1% FeSO(4) was found to affect both hR(F) values and resolution of cephalosporins. New solvent systems are reported for both normal phase and reversed-phase TLC.     

Effects of metal salts on polymerization. VI. Photo and thermally catalyzed polymerization of N-vinylimidazole in the presence of metal salts

Polymerization of 2-methyl-1-vinylimidazole (MVI) and 2-ethyl-1-vinylimidazole (EVI) was found to be markedly photosensitized in the presence of oxidizing metal salts such as UO₂(NO₃)₂, Ce(NH₄)₂(NO₃)₆, Hg(CH₃COO)₂, AgNO₃; non-oxidizing metal salts such as Zn^II did not act as photosensitizers. The interaction of monomer with a metal salt is discussed on the basis of infrared and electronic spectroscopy. This photopolymerization is very specific with respect to the kind of monomer. The polymerization of noncomplexing monomer (styrene) is not photosensitized by these metal salts. Consequently, photosensitized electron transfer between monomer and metal salt via complex formation is considered to be the most probable initiation mechanism. Cupric acetate and sodium chlorolaurate, which have been reported as efficient initiators for the polymerization of vinylpyridine and N-vinylcarbazole, respectively, act as linear terminators of growing radicals. The radical polymerizability of the zinc complex of MVI was studied by means of copolymerization with styrene. The reduction of the reactivity of MVI on complexing was explained by correlating with the spectroscopic observations. Because the polymerization system is heterogeneous, a detailed discussion was not possible.     

Alkaline-earth overlayers on furrowed transition metal surfaces: An example of tailoring the surface properties

Alkaline-earth (AE) layers adsorbed on furrowed transition metal surfaces are remarkable for a variety of their atomic structures thus offering a good possibility for exploring the interplay between atomic and electronic structures as well as dynamic properties of surfaces. As the coverage increases, the commensurate-incommensurate transition in the AE layers is accompanied by the nonmetal-to-metal transition (NMT) in the films which leads to dramatic changes in characteristics of photoemission and surface diffusion thus enabling one to tailor the surface properties. The calculations for one-dimensionally compressing monolayers elucidate some important features of the NMT in such adsorbed films. These findings have been exploited to develop effective Mg-Ba alloy photocathodes.     

Effects of metal salts on polymerization. Part IV. Polymerization of N-vinylcarbazole initiated by oxidizing metal nitrates

The polymerization of N-vinylcarbazole (VCZ) in ethylene dichloride, acetone, benzene, and dioxane with cupric nitrate, ferric nitrate, and ceric ammonium nitrate as catalyst was studied. In all cases the polymerization seemed to be of a cationic nature, judged by copolymerization with styrene. Electron spin resonance (ESR) spectroscopy was made for the polymerization system and also for a system containing N-ethylcarbazole instead of VCZ. Singlet ESR spectra were observed for all systems containing ceric salt and for some systems containing ferric salt but not for systems containing cupric salt. The ESR spectra indicated the formation of an ion radical by electron transfer between the oxidizing metal salt and the carbazole derivatives. Mechanisms of initiation other than electron transfer were less likely, and it was concluded that the initiation process was most likely to be of the electron transfer type.     

Structural diversity of homobinuclear transition metal complexes of the phenazine ligand: theoretical investigation

DFT/BP86 calculations have been carried out on a series of hypothetical binuclear compounds of general formula (L₃M)₂(C₁₂N₂H₈) (M?=?Sc–Ni, L₃?=?(CO)₃, (PH₃)₃ and Cp^?, and C₁₂N₂H₈?=?phenazine ligand-denoted Phn). The various structures with syn and anti configurations have been investigated, in order to determine the phenazine’s coordination to first-row transition metals of various spin states with syn and anti conformations. The lowest energy structures depend on the nature of the metal, the spin state, and the molecular symmetry. This study has shown that the electronic communication between the metal centers depends on their oxidation state and the attached ligands. The tricarbonyl and the triphosphine ligands gave rise to comparable results in terms of stability order of isomers, metal-metal bond distances, and the coordination modes. Metal-metal multiple bonding has been evidenced for Sc, Ti, and V complexes to compensate the electronic deficiency. The Cr, Mn, Fe, Co, and Ni-rich metals prefer the anti conformation due to the enhancement of the metal valence electron count. The spin density values calculated for the triplet and quintet spin structures point out that the unpaired electrons are localized generally on the metal centers. The Wiberg bond indices are used to evaluate the metal-metal bonding. Furthermore, calculations using the BP86-D functional which take into account the attractive part of the van der Waals type interaction potential between atoms and molecules that are not directly connected to each other gave comparable results to those obtained by BP86 functional in terms of coordination modes, HOMO-LUMO gaps, metal-metal bond orders, and the stability order between isomers, but with slight deviation of M–C, M–N, and M–M bond distances not exceeding 3%.     

Electrospray mass spectrometry: a study on some aqueous solutions of metal salts

Aqueous metal salt solutions were used as models to probe the origin of the species observed in the electrospray mass spectrum. A qualitative or semiquantitative correlation among different species was observed between electrospray responses and calculated equilibrium aqueous solution concentrations. Quantitative correlations were obtained, however, when ions that were identical in charge and similar in type were selected for comparison. In these experiments the ions experienced very similar electrospray-related processes and their effects on the responses were canceled in a comparison of these ions. Consequently, the relative abundances of these ions in the electrospray mass spectrum closely matched the calculated relative abundances in aqueous solution. Our results suggest that the basic principle that determines ionic distribution in the electro spray mass spectrum in aqueous solution chemistry.