"Switching on" the properties of single-molecule magnetism in triangular manganese(III) complexes

The reaction between oxide-centered, triangular [MnIII3O(O2CR)6(py)3](ClO4) (R = Me (1), Et (2), Ph (3)) compounds and methyl 2-pyridyl ketone oxime (mpkoH) affords a new family of Mn/carboxylato/oximato complexes, [MnIII3O(O2CR)3(mpko)3](ClO4) [R = Me (4), Et (5), and Ph (6)]. As in 1-3, the cations of 4-6 contain an [MnIII3(mu3-O)]7+ triangular core, but with each Mn2 edge now bridged by an eta1:eta1:mu-RCO2- and an eta1:eta1:eta1:mu-mpko- group. The tridentate binding mode of the latter causes a buckling of the formerly planar [MnIII3(mu3-O)]7+ core, resulting in a relative twisting of the three MnIII octahedra and the central O2- ion now lying approximately 0.3 A above the Mn3 plane. This structural distortion leads to ferromagnetic exchange interactions within the molecule and a resulting S = 6 ground state. Fits of dc magnetization data for 4-6 collected in the 1.8-10.0 K and 10-70 kG ranges confirmed S = 6 ground states, and gave the following D and g values: -0.34 cm(-1) and 1.92 for 4, -0.34 cm(-1) and 1.93 for 5, and -0.35 cm(-1) and 1.99 for 6, where D is the axial zero-field splitting (anisotropy) parameter. Complexes 4-6 all exhibit frequency-dependent out-of-phase (chi" M) ac susceptibility signals suggesting them possibly to be single-molecule magnets (SMMs). Relaxation rate vs T data for complex 4 down to 1.8 K obtained from the chi" M vs T studies were supplemented with rate vs T data measured to 0.04 K via magnetization vs time decay studies, and these were used to construct Arrhenius plots from which was obtained the effective barrier to relaxation (Ueff) of 10.9 K. Magnetization vs dc field sweeps on single-crystals of 4.3CH2Cl2 displayed hysteresis loops exhibiting steps due to quantum tunneling of magnetization (QTM). The loops were essentially temperature-independent below approximately 0.3 K, indicating only ground-state QTM between the lowest-lying Ms = +/-6 levels. Complexes 4-6 are thus confirmed as the first triangular SMMs. High-frequency EPR spectra of single crystals of 4.3CH2Cl2 have provided precise spin Hamiltonian parameters, giving D = -0.3 cm(-1), B40 = -3 x 10(-5) cm(-1), and g = 2.00. The spectra also suggest a significant transverse anisotropy of E > or = 0.015 cm(-1). The combined work demonstrates the feasibility that structural distortions of a magnetic core imposed by peripheral ligands can "switch on" the properties of an SMM.     

Separation of molecular tracers sorbed onto atmospheric particulate matter by flash chromatography

In to order increase sensitivity and to reduce the background induced by matrix effects, a method was developed that uses flash chromatography to separate various compounds present in atmospheric aerosol samples prior to their analysis with different analytical techniques (GC–MS, GC–FID, HPLC). For this purpose, flash chromatography using a 4 g silica gel column crossed by eluent at a flow rate of 20 mL min⁻¹ was used. An eluent with enhanced polarity is needed to separate nonpolar (linear and branched alkanes), semipolar (PAH, nitro-PAH and cholesterol) and polar (methoxyphenols, alkanoic acids, and levoglucosan) compounds. Three combinations of solvents were used: hexane for the nonpolar fraction (F1), toluene/hexane for the semipolar fraction (F2) and dimethylformamide for the polar fraction (F3). The use of different eluents for each fraction allows separation of the sample to be accomplished with good repeatability and satisfying yields [85 ± 5% for F1, 81 ± 8% (PAHs), 89 ± 6% (nitro-PAHs) and 74 ± 7% (cholesterol) for F2 and 79 ± 7% (n-alkanoic acids), 40 ± 11% (methoxyphenols) and 77 ± 6% (levoglucosan) for F3]. The methoxyphenol yields were low due to losses during the concentration/evaporation step. This method was then applied to analyse the organic composition of particles collected at an urban site in Strasbourg (France).     

Sizing large proteins and protein complexes by electrospray ionization mass spectrometry and ion mobility

Mass spectrometry (MS) and ion mobility with electrospray ionization (ESI) have the capability to measure and detect large noncovalent protein-ligand and protein-protein complexes. Using an ion mobility method of gas-phase electrophoretic mobility molecular analysis (GEMMA), protein particles representing a range of sizes can be separated by their electrophoretic mobility in air. Highly charged particles produced from a protein complex solution using electrospray can be manipulated to produce singly charged ions, which can be separated and quantified by their electrophoretic mobility. Results from ESI-GEMMA analysis from our laboratory and others were compared with other experimental and theoretically determined parameters, such as molecular mass and cryoelectron microscopy and X-ray crystal structure dimensions. There is a strong correlation between the electrophoretic mobility diameter determined from GEMMA analysis and the molecular mass for protein complexes up to 12 MDa, including the 93 kDa enolase dimer, the 480 kDa ferritin 24-mer complex, the 4.6 MDa cowpea chlorotic mottle virus (CCMV), and the 9 MDa MVP-vault assembly. ESI-GEMMA is used to differentiate a number of similarly sized vault complexes that are composed of different N-terminal protein tags on the MVP subunit. The average effective density of the proteins and protein complexes studied was 0.6 g/cm(3). Moreover, there is evidence that proteins and protein complexes collapse or become more compact in the gas phase in the absence of water.     

An electrochemiluminescence-based competitive displacement immunoassay for the type-2 brevetoxins in oyster extracts

A new competitive electrochemiluminescence-based immunoassay for the type-2 brevetoxins in oyster extracts was developed. The assay was verified by spiking known amounts of PbTx-3 into 80% methanol extracts of Gulf Coast oysters. We also provide preliminary data demonstrating that 100% acetone extracts, aqueous homogenates, and the clinical matrixes urine and serum can also be analyzed without significant matrix interferences. The assay offers the advantages of speed ( 2 h analysis time); simplicity (only 2 additions, one incubation period, and no wash steps before analysis); low limit of quantitation (conservatively, 50 pg/mL = 1 ng/g tissue equivalents); and a stable, nonradioactive label. Due to the variety of brevetoxin metabolites present and the lack of certified reference standards for liquid chromatography-mass spectrometry confirmation, a true validation of brevetoxins in shellfish extracts is not possible at this time. However, our assay correlated well with another brevetoxin immunoassay currently in use in the United States. We believe this assay could be useful as a regulatory screening tool and could support pharmacokinetic studies in animals and clinical evaluation of neurotoxic shellfish poisoning victims.     

Supported ionic liquid phase catalysis with supercritical flow

Rapid hydroformylation of 1-octene (rates up to 800 h(-1)) with the catalyst remaining stable for at least 40 h and with very low rhodium leaching levels (0.5 ppm) is demonstrated when using a system involving flowing the substrate, reacting gases and products dissolved in supercritical CO(2) (scCO(2)) over a fixed bed supported ionic liquid phase catalyst.     

Synthesis and structural characterization of cross-linked histidine-phenol Cu(ii) complexes as cytochrome c oxidase active site models

Tridentate cross-linked histidine-phenol Cu(ii) ether and ester complexes, chemical analogs of the active site of several heme-copper oxidases, have been synthesized and crystallized.     

New ligand platforms for developing the chemistry of the Ti=N-NR2 functional group and the insertion of alkynes into the N-N bond of a Ti=N-NPh2 ligand

Two broadly applicable strategies for extending the available ligand platforms of the virtually unexplored terminal Ti=N-NR2 functional group are described, along with the highly selective room temperature insertion of alkynes into the N-N bond of Ti{MeN(CH2CH2NSiMe3)2}(NNPh2)(py) and the catalytic cis-diamination of PhC[triple bond]CMe by diphenylhydrazine.     

Oligomeric cadmium-phthalocyanine complexes: novel supramolecular free radical structures

Certain cadmium-metallated phthalocyanines give rise to EPR active triple-decker sandwich complexes containing two Cd ions and three phthalocyanine (Pc) ligands. These have been shown to form when the ligands bear either eight non- peripheral alkyl or alkenyl substituents or eight peripheral 2-ethylhexyl groups. They can be derived either from three equivalents of a cadmium phthalocyanine precursor or from a 2:1 mixture of a cadmium phthalocyanine (CdPc) and a metal-free phthalocyanine (H(2)Pc). The mode of their formation has been investigated by a series of "cross" experiments. The results indicate that the triple-decker structures are formed by a self-assembly process. This is deduced from results that show that they can disassemble and reassemble with incorporation of differently substituted ligands derived from either an H(2)Pc or CdPc. The reassembled structures in these cross experiments can contain more than one ligand that originated from either the added CdPc or, and more surprisingly, the H(2)Pc compound. Mass spectrometry has also established that higher order oligomers can be formed when steric requirements between the alkyl substituents on adjacent rings in the stack are reduced. Thus an isotopic cluster for a Cd(5)Pc(6) complex has been observed when the eight peripheral substituents are hexyl chains and tetrameric complexes are formed when two different ligands are incorporated within a stack, with one carrying substituents at the peripheral sites and the other bearing substituents at the non-peripheral sites.