Nickel(II) tri-tert-butoxysilanethiolates with N-heterocyclic bases as additional ligands: Synthesis, molecular structure and spectral studies

Three heteroleptic, neutral nickel(II) tri-tert-butoxysilanethiolates with monodentate heterocyclic bases (pyridine, 2-methylpyridine and 3,5-dimethylpyridine) serving as additional ligands have been prepared following the same synthetic procedure. The complexes were characterized by single crystal X-ray structure determination and elemental analysis. For complexes 1 and 2, FT-IR and UV-Vis spectroscopy have been additionally recorded.Three different coordination motifs have been observed in these complexes. Molecules building tetragonal crystals of [Ni{SSi(O^tBu)₃}₂(C₅H₅N)] (1) feature Ni(II) coordinated by two S,O-chelating tri-tert-butoxysilanethiolato residues and one N atom of pyridine in a strongly distorted trigonal bipyramidal environment. The complex [Ni{SSi(O^tBu)₃}₂(C₆H₇N)₂] (2) forms triclinic crystals and its core atoms adopt a planar geometry with Ni(II) in the middle of the N₂S₂ plane. Molecules of complex [Ni{SSi(O^tBu)₃}₂(C₇H₉N)₂(H₂O)] (3) form orthorhombic crystals with penta-coordinated Ni(II) in a distorted tetragonal pyramidal NiN₂OS₂ environment. Complex 2 roughly mimics one of the two metal centers in the active site of the ACS/CODH enzyme.     

A DFT study of H-isomerisation in alkoxy-, alkylperoxy- and alkyl radicals: Some implications for radical chain reactions in polymer systems

Intramolecular 1-n H-shift (n = 2, 3… 7) reactions in alkoxy, alkyl and peroxy radicals were studied by density functional theory (DFT) at the B3LYP/6-311+G∗∗ level and compared with respective intermolecular H-transfers. It was found that starting from 1 to 3 H-shift the barrier heights stepwise decrease with increasing n reaching minimum for 1-5 and 1-6 H-shifts. This dependence can be ascribed to the decrease of the strain with increasing transition state (TS) ring size, which is minimal in six- and seven-member rings. The barrier heights of H-shifts in alkyl radicals are systematically larger than those in alkoxy radicals: the respective activation energies (E_a) of 1-5 and 1-6 H-shifts are about 59-67 kJ/mol for alkyl radical and 21-34 kJ/mol for alkoxy radicals. Further increase of the TS ring size in 1-7 H-shifts leads to the increase of the barrier to 44 kJ/mol in the hexyloxy radical and 84 kJ/mol for n-heptyl radical. We have also found that intermolecular H-transfer reactions in all three types of free radicals have smaller barriers than respective intramolecular 1-5 or 1-6 H-shifts by 4-25 kJ/mol. The mentioned difference can be explained in terms of enhanced nonbonding repulsion interaction in the cyclic TS structures compared to respective intermolecular TS. B3LYP/6-311+G∗∗ geometric parameters and imaginary frequencies for 1-n H-shifts TS are consistent with respective calculated barrier heights. Reactivity of some other radicals compared to alkoxy, peroxy and alkyl radicals as well as other factors influencing their reactivity (π-conjugation, steric effect and ring strain in cyclic TS, etc.) are also briefly discussed in relation to free radical reactions in polymer systems.     

A computational study of the reactivities of four-membered heavy carbene systems

The potential energy surfaces for the chemical reactions of four-membered N-heterocyclic group 14 heavy carbene species have been studied using density functional theory (B3LYP/LANL2DZ). Five four-membered group 14 heavy carbene species, (i-Pr)(2) NP(NR)(2) E:, in which E = C, Si, Ge, Sn, and Pb, were chosen as the model reactants in this work. Also, four kinds of chemical reactions, C-H bond insertion, water addition, alkene cycloaddition, and dimerization, have been used to study the chemical reactivities of these group 14 four-membered N-heterocyclic carbene species. Basically, our present theoretical work predicts that the larger the ∠NEN bond angle of the four-membered group 14 heavy carbene species, the smaller the singlet-triplet splitting, the lower the activation barrier, and, in turn, the more rapid, its chemical reactions to various chemical species. Moreover, our theoretical investigations suggest that the relative carbenic reactivity decreases in the order: C > Si > Ge > Sn > Pb. That is, the heavier the group 14 atom (E), the more stable is its four-membered carbene toward chemical reactions. As a result, our results predict that the four-membered group 14 heavy carbene species (E = Si, Ge, Sn, and Pb) should be more kinetically stable than the observed carbene species and, thus, can be also readily synthesized and isolated at room temperature. Furthermore, the singlet-triplet energy splitting of the four-membered group 14 carbene species, as described in the configuration mixing model attributed to the work of Pross and Shaik, can be used as a diagnostic tool to predict their reactivities. The results obtained allow a number of predictions to be made.     

Relativistic effects in HgHe and HgXe CCSD(T) ground state potential curves. Low‐density viscosity simulations of Hg:Xe mixture

The comparison of coupled cluster with single and double excitations and with perturbative correction of triple excitations [CCSD(T)] ground state potential curves of mercury with rare gases (RG): HgHe and HgXe, at several levels of theory is presented. The scalar relativistic (REL) effects and spin‐orbit coupling effects in the ground state potential curves of these weakly bounded dimers are considered. The CCSD(T) ground state potential curves at the level of the Dirac‐Coulomb Hamiltonian (DCH) are compared with CCSD(T) curves at the level of 4‐component spin‐free modified DCH, the scalar 2nd order Douglas‐Kroll‐Hess (DKH2) and the nonrelativistic (NR‐LL) (Lévy‐Leblond) Hamiltonian. In addition, London‐Drude formula and SCF interaction energy curves are employed in the analysis of different contributions of REL effects in dissociation energies of HgRG and Hg₂ dimers. Moreover, the large anharmonicity of the HgHe ground state potential curve is highlighted. The computationally less demanding scalar DKH2 Hamiltonian is employed to calculate the HgXe, Hg₂, and Xe₂ all electron CCSD(T) ground state potential curves in highly augmented quadruple zeta basis sets. These potential curves are used to simulate the shear viscosity of mercury, xenon, and mercury‐xenon (Hg:Xe) mixture. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2011     

Theoretical study on contribution of charge transfer effect to surface-enhanced Raman scattering spectra of pyridine adsorbed on Ag(n) (n = 2-8) clusters

We investigate surface-enhanced Raman scattering (SERS) spectra of pyridine–Ag_n (n = 2–8) complexes by density functional theory (DFT) and time-dependent DFT (TDDFT) methods. In simulated normal Raman scattering (NRS) spectra, profiles of pyridine–Ag_n (n = 2–8) complexes are analogical with that of isolated pyridine. Nevertheless, calculated pre-SERS spectra are strongly dependent on electronic transition states of new complexes. Wavelengths at 335 nm, 394.8 nm, 316.9 nm and 342.6 nm, which are nearly resonant with pure charge transfer excitation states, are adopted as incident light when simulating pre-SERS spectra for pyridine–Ag_n (n = 2–8) complexes, respectively. We obtain enhancement factors from 10³ to 10⁵ in pre-SERS spectra compared with corresponding NRS spectra. The obvious increase in Raman intensities mainly result from charge transfer resonance Raman enhancement. A charge difference densities (CDDs) methodology is adopted in describing chemical enhancement mechanism. This methodology aims at visualizing charge transfer from Ag_n (n = 2–8) clusters to pyridine on resonant electronic transition, which is one of the most direct evidences for chemical enhancement mechanism.     

PVA and BSA stabilized silver nanoparticles based surface-enhanced plasmon resonance probes for protein detection

To perform biosensing using nanoparticles in solution, silver particles were coated with bovine serum albumin (BSA) and polyvinyl alcohol (PVA) as control stabilizer. The plasmon resonance (420 nm) of the silver nanoparticles in solution was shifted slightly to longer wavelength (443 nm) when they were coated with BSA. The biointeractions of these engineered nanoparticles were studied using a mouse model. No significant changes in behavior or toxicity were observed. The nanoparticles were detected in all tissues including the brain. Antibody recognition was monitored via the change in light absorption which accompanied binding, indicating that the particles can be used as a biosensor to gain more insight into cellular mechanisms governing the function of organs in general, and the blood brain barrier (BBB) and brain in particular.     

Physico-chemical transformations in swift heavy ion modified poly(ethyleneterephthalate)

Thin films of poly(ethyleneterephthalate) (PET) were exposed to different radiation dose brought about by 80 MeV carbon and 98 MeV silicon ion beam. The UV-vis absorption studies reveal that there is decrease in optical band gap energy to the extent of ∼29.3 and 42.1%. The X-ray diffraction analyses have shown that crystallite size decreased by ∼18.6 and 52.6%, indicating amorphization of PET. The colour of PET films change from colourless to light yellowish followed by light brown as radiation dose is increased. The colour formation has been ascribed to an increase in conjugation in the carbon chain. In the case of PET irradiated with carbon ion, the electrical conductivity increased with frequency beyond a threshold value of 1 kHz. The increase in conductivity of PET films on irradiation is due to formation of defects and carbon clusters as a result of polymer chain scission. The thermal study further confirmed the increase in amorphous nature with increase in radiation dose. The results indicate that radiation dose brings about significant physicochemical transformations in PET.     

Fragmentation of oxime and silyl oxime ether odd‐electron positive ions by the McLafferty rearrangement: new insights on structural factors that promote α,β fragmentation

The McLafferty rearrangement is an extensively studied fragmentation reaction for the odd‐electron positive ions from a diverse range of functional groups and molecules. Here, we present experimental and theoretical results of 12 model compounds that were synthesized and investigated by GC‐TOF MS and density functional theory calculations. These compounds consisted of three main groups: carbonyls, oximes and silyl oxime ethers. In all electron ionization mass spectra, the fragment ions that could be attributed to the occurrence of a McLafferty rearrangement were observed. For t‐butyldimethylsilyl oxime ethers with oxygen in a β‐position, the McLafferty rearrangement was accompanied by loss of the t‐butyl radical. The various mass spectra showed that the McLafferty rearrangement is relatively enhanced compared with other primary fragmentation reactions by the following factors: oxime versus carbonyl, oxygen versus methylene at the β‐position and ketone versus aldehyde. Calculations predict that the stepwise mechanism is favored over the concerted mechanism for all but one compound. For carbonyl compounds, C–C bond breaking was the rate‐determining step. However, for both the oximes and t‐butyldimethylsilyl oxime ethers with oxygen at the β‐position, the hydrogen transfer step was rate limiting, whereas with a CH₂ group at the β‐position, the C–C bond breaking was again rate determining. n‐Propoxy‐acetaldehyde, bearing an oxygen atom at the β‐position, is the only case that was predicted to proceed through a concerted mechanism. The synthesized oximes exist as both the (E)‐ and (Z)‐isomers, and these were separable by GC. In the mass spectra of the two isomers, fragment ions that were generated by the McLafferty rearrangement were observed. Finally, fragment ions corresponding to the McLafferty reverse charge rearrangement were observed for all compounds at varying relative ion intensities compared with the conventional McLafferty rearrangement. Copyright © 2012 John Wiley & Sons, Ltd.     

Nickel(II) and copper(II) complexes of aza macrocycles derived from trichloromethane

Syntheses of nickel(II) complexes of the tetraaza macrocycles 2,7-dichloro-1,3,6,8-tetraazacyclodecane (DCCD) and 2,8-dichloro-1,3,7,9-tetraazacyclododecane (DICD) and a copper(II) complex of 2,6,8,12,13,17-hexaazabicyclo[5.5.5]heptadecane (HBCH) are reported in the template condensation of trichloromethane with 1,2-diaminoethane or 1,3-diaminopropane. Formulation of the synthesized products [Ni(DCCD)(H₂O)₂]Cl₂, [Ni(DICD)(H₂O)₂]Cl₂?·?H₂O, and [Cu₃(HBCH)(H₂O)₆]Cl₆, and the metal-free ligand hydrochloride HBCH?·?6HCl has been confirmed by elemental analyses, conductivity measurements, and spectral studies. Potentiometric studies of nickel(II) and copper(II) complexes of HBCH and structurally similar 2,5,8,10,13,16,17,20,23-nonaazabicyclo[7.7.7]tricosane (NACT, earlier derived from trichloromethane and diethylenetriamine) have also been performed in the structural support of HBCH. In 1?:?1, metal?:?HBCH solution, copper(II) is coordinated to four N-donors of two-HN(CH₂)₃NH– groups of the ligand in a non-planar tetraaza cavity. The equilibrium constant value (log?K?=?15.41) for the reaction Cu^2+?+?A???CuA²⁺ (A?=?HBCH) is in favor of the cyclic structure of the ligand. A high value (log?K?=?23.27) for corresponding reaction in the NACT system is due to conformational change in the ligand, where copper(II) organizes the macrocycle to form a nearly planar cavity in which the cation fits well.     

Synthesis,characterization, electrochemical study,and antimicrobial activity of bis(benzoylacetonato) vanadium(IV) complexes of biphenylphenols

New non-oxovanadium(IV) complexes of biphenylphenols, [VCl_{2? n} (bzac)₂(OAr^1,2) _n ], have been synthesized in quantitative yields from the reaction of bis(benzoylacetonato)dichlorovanadium(IV) with the trimethylsilyl derivative of 2- and 4-phenylphenols in carbon tetrachloride. The complexes have been characterized by physicochemical, magnetic moment measurements, IR, mass spectra, and electrochemical studies. The thermal behavior of the complexes has been studied by TGA–DTA. The complexes have been screened for their antimicrobial activity against some pathogenic bacteria, Escherichia coli and Staphylococcus aureus and fungi, Candida albicans, Aspergillus niger, and Fusarium oxysporum, by two-fold serial dilution.