In Situ Formation of Heterobimetallic Salen Complexes Containing Titanium and/or Vanadium Ions

Tuning ligand electronics and peripheral substitution on cobalt salen complexes: structure and polymerisation activity

A series of cobalt salen complexes, where salen represents an N2O2 bis-Schiff-base bis-phenolate framework, are prepared, characterised and investigated for reversible-termination organometallic mediated radical polymerisation (RT-OMRP). The salen ligands contain a cyclohexane diimine bridge and systematically altered para-substituted phenoxide moieties as a method to examine the electronic impact of the ligand on complex structure and reactivity. The complexes are characterised by single crystal X-ray diffraction, cyclic voltammetry, X-ray photoelectron spectroscopy, electron paramagnetic resonance spectroscopy and computational methods. Structural studies all support a tailorable metal centre reactivity altered by the electron-donating ability of the salen ligand. RT-OMRP of styrene, methyl methacrylate and vinyl acetate is reported and suggests that cobalt-carbon bond strength varies with the ligand substitution. Competing β-hydrogen abstraction affords long-chain olefin-terminated polymer chains and well controlled vinyl acetate polymerisations, contrasting with the lower temperature associative exchange mechanism of degenerative transfer OMRP.

Assessment of the various ionization methods in the analysis of metal salen complexes by mass spectrometry

Metal salen complexes are one of the most frequently used catalysts in enantioselective organic synthesis. In the present work, we compare a series of ionization methods that can be used for the mass spectral analysis of two types of metalosalens: ionic complexes (abbreviated as Com(+)X(-)) and neutral complexes (NCom). These methods include electron ionization and field desorption (FD) which can be applied to pure samples and atmospheric pressure ionization techniques: electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI) which are suitable for solutions. We found that FD is a method of choice for recording molecular ions of the complexes containing even loosely bonded ligands. The results obtained using atmospheric pressure ionization methods show that the results depend mainly on the structure of metal salen complex and the ionization method. In ESI spectra, Com(+) ions were observed, while in APCI and APPI spectra both Com(+) and [Com + H](+) ions are observed in the ratio depending on the structure of the metal salen complex and the solvent used in the analysis. For complexes with tetrafluoroborate counterion, an elimination of BF3 took place, and ions corresponding to complexes with fluoride counterion were observed. Experiments comparing the relative sensitivity of ESI, APCI and APPI (with and without a dopant) methods showed that for the majority of the studied complexes ESI is the most sensitive one; however, the sensitivity of APCI is usually less than two times lower and for some compounds is even higher than the sensitivity of ESI. Both methods show very high linearity of the calibration curve in a range of about 3 orders of magnitude of the sample concentration.

Fluorescent, Theoretical and Structural Investigations on [Me4bim][CdCl4] (Me4bim = 1,1′,3,3′-tetramethyl-2,2′-bi-1H-imidazolium)

The compound [Me4bim][CdCl4] (Me4bim = 1,1′,3,3′-tetramethyl-2,2′-bi-1 H-imidazolium), has been prepared via a solvothermal reaction and has an isolated (0-D) structure. It displays an emission in the blue region that is ascribed to ligand-to-ligand charge transfer on the basis of a theoretical investigation.

Kinetics and mechanism of the racemic addition of trimethylsilyl cyanide to aldehydes catalysed by Lewis bases

The mechanism by which four Lewis bases, triethylamine, tetrabutylammonium thiocyanate, tetrabutylammonium azide and tetrabutylammonium cyanide, catalyse the addition of trimethylsilyl cyanide to aldehydes is studied by a combination of kinetic and spectroscopic methods. The reactions can exhibit first or second order kinetics corresponding to three different reaction mechanisms. Spectroscopic evidence for the formation of hypervalent silicon species is obtained for reaction between all of the tetrabutylammonium salts and trimethylsilyl cyanide. The reactions are accelerated by the presence of water in the reaction mixture, an effect which is due to a change in the reaction mechanism from Lewis to Brønsted base catalysis. Tetrabutylammonium thiocyanate is shown to be an excellent catalyst for the synthesis of cyanohydrin trimethylsilyl ethers on a preparative scale.

In Situ Synthesis, X-Ray Structure, Fluorescence and Theoretical Investigations of [Cu2Cl4][(C2H5)2-4,4'-Bipyridine]

A novel N,N'-diethyl-4,4'-bipyridine copper complex, [Cu2Cl4][(C2H5)2-4,4'-bipyridine] (1), has been obtained in situ through a solvothermal reaction and structurally characterised by X-ray diffraction. Complex 1 comprises discrete Cu2Cl4 moieties and (C2H5)2-4,4'-bipyridine molecules interlinked via hydrogen bonding interactions to give a three-dimensional supramolecular open framework. Photoluminescent study shows that the complex exhibits an emission in the green region. Theoretical study reveals that the emission is attributed to the π → π* charge-transfer interaction of the (C2H5)2-4,4'-bipyridine moieties.

In Situ Synthesis, Structure, Photoluminescence and Theoretical Study of [Cu(2-PyCOOH)2(H2O)2]Cl2

A new pyridine-2-carboxylic acid complex, [Cu(2-PyCOOH)2(H2O)2]Cl2, has been prepared in situ through a hydrothermal reaction and its structure comprises discrete [Cu(2-PyCOOH)2(H2O)2]2+ cations and chloride anions which are interlinked via hydrogen bonding interactions to give a 3-D supramolecular motif. Photoluminescent study shows that the complex exhibits an emission in the blue region. Theoretical studies reveal that the emission can be ascribed to metal-to-ligand charge transfer.

Kinetics and mechanism of vanadium catalysed asymmetric cyanohydrin synthesis in propylene carbonate

Propylene carbonate can be used as a green solvent for the asymmetric synthesis of cyanohydrin trimethylsilyl ethers from aldehydes and trimethylsilyl cyanide catalysed by VO(salen)NCS, though reactions are slower in this solvent than the corresponding reactions carried out in dichloromethane. A mechanistic study has been undertaken, comparing the catalytic activity of VO(salen)NCS in propylene carbonate and dichloromethane. Reactions in both solvents obey overall second-order kinetics, the rate of reaction being dependent on the concentration of both the aldehyde and trimethylsilyl cyanide. The order with respect to VO(salen)NCS was determined and found to decrease from 1.2 in dichloromethane to 1.0 in propylene carbonate, indicating that in propylene carbonate, VO(salen)NCS is present only as a mononuclear species, whereas in dichloromethane dinuclear species are present which have previously been shown to be responsible for most of the catalytic activity. Evidence from ⁵¹V NMR spectroscopy suggested that propylene carbonate coordinates to VO(salen)NCS, blocking the free coordination site, thus inhibiting its Lewis acidity and accounting for the reduction in catalytic activity. This explanation was further supported by a Hammett analysis study, which indicated that Lewis base catalysis made a much greater contribution to the overall catalytic activity of VO(salen)NCS in propylene carbonate than in dichloromethane.

Synthesis, Structure and Photoluminescence of [N-ethyl-4,4′-bipyridinium][CdBr4] with N-ethyl-4,4′-bipyridinium Generated in situ

A new viologen(4,4′-bipyridinium)-based complex [ N-ethyl-4,4′-bipyridinium][CdBr4], in which N-ethyl-4,4′-bipyri-dinium ( EQ2+) has been generated in situ, has been synthesised via hydrothermal reaction and structurally characterised by single-crystal X-ray diffraction. [ N-ethyl-4,4′-bipyridinium][CdBr4] features an isolated structure, based on an N-ethyl-4,4′-bipyridinium moiety and a cadmium atom terminally bound by four bromine atoms. Photoluminescence investigation reveals a broad and intense emission in blue region, which may originated from π → π* charge-transfer interaction of the N-ethyl-4,4′-bipyridinium moiety.