Zirconium and hafnium (1-pyridinio)imido complexes: functionalized terminal hydrazinediido analogues

Reaction of the diamidozirconium complex [Zr(N2(TBS)Npy)(NMe2)2] (1) (N2(TBS)Npy = CH3C(C5H4N)(CH2NSiMe2tBu)2) or the diamidohafnium complex [Hf(N2(TBS)Npy)(NMe2)2] (2) with one molar equiv. of 1-aminopyridinium triflate in the presence of one equiv. of pyridine gave the corresponding (1-pyridinio)imido complexes [Zr(N2(TBS)Npy)(=N-NC5H5)(OTf)(py)] (3) and [Hf(N2(TBS)Npy)(=N-NC5H5)(OTf)(py)] (4). These were converted to the acetylide complexes [Zr(N2(TBS)Npy)(=N-NC5H5)(CCPh)(py)] (5) and [Hf(N2(TBS)Npy)(=N-NC5H5)(CCPh)(py)] (6) by reaction with lithium phenylacetylide and substitution of the triflato ligand. Upon reaction of 3 and 4 with one molar equivalent of R-NC (R = tBu, Cy, 2,6-xyl), N-N bond cleavage in the (1-pyridinio)imido unit took place and the respective carbodiimido complexes [M(N2(TBS)Npy](N=C=NR)(OTf)(py)] (7-12) were formed instantaneously. A similar type of reaction with CO gave the isocyanato complex [Zr(N2(TBS)Npy](NCO)(OTf)(py)] (13). Finally, the abstraction of the pyridine ligand in compounds 3 and 4 with B(C6F5)3 led to the formation of the triflato-bridged dinuclear complexes [Zr(N2(TBS)Npy)(=N-NC5H5)(OTf)]2 (14) and [Hf(N2(TBS)Npy)(=N-NC5H5)(OTf)]2 (15).     

Crystal structure and spectroscopic properties of ammonium hydrogensquarate squaric acid monohydrate

The structure of ammonium hydrogensquarate squaric acid monohydrate has been determined by single crystal X-ray diffraction. The compound crystallizes in the monoclinic space group C2/c and exhibits a 3D network with molecules linked by intermolecular interactions with participation of the H₂Sq, HSq^?, NH₄ ⁺, and H₂O species. The HSq^? anion and the neutral H₂Sq form a strong head-to-tail dimer through O–H···O hydrogen bonding with lengths of 2.587 and 2.494 Å (protected space between numeral and unit). The layers are connected by ammonium cations and water molecules in a plane through the O···N (2.950, 2.978, 3.036 Å) and O···O (2.953, 2.781 Å) bonds. Another such layer is connected to the NH₄ ⁺ cation in the adjacent plane through bifurcated N–H···O hydrogen-bonding to form a double layer (NH···O bond lengths are 3.036, 2.978, 2.857, 2.909, 2.958, and 2.742 Å, respectively). The IR-band assignment of the compound was achieved using the polarized IR-spectroscopy of oriented colloids in a nematic host. Theoretical ab initio calculations were performed and achieved with a view to explain the IR-bands of the H₂Sq.HSq^? motif.     

A potential high-throughput method for the determination of lipase activity by potentiometric flow injection titrations

Potentiometric FIA titrations were performed to determine enzyme activities of lipase type B from Candida antarctica, CAL-B. Two substrates, triacetin and tributyrin were hydrolyzed in phosphate buffer solutions, and the concentration change of the base component of the buffer was titrated in a carrier solution containing hydrochloric acid and potassium chloride. The system was calibrated with butyric acid and acetic acid, respectively. FIA titration peaks were evaluated with respect to peak height and peak area. Butyric acid and acetic acid could be titrated in the buffer solution from 3 × 10⁻³ mol L⁻¹ to 0.1 mol L⁻¹. The detection limit of enzyme activity was determined to be 0.07 U mL⁻¹ (15 min reaction time) and the minimum activity was calculated to be 0.035 units corresponding to 35 nmol min⁻¹. The specific activities of lipase B for the hydrolysis of tributyrin and triacetin were determined as 16 ± 2 U mg⁻¹ and 2 ± 0.2 U mg⁻¹ (per mg commercial lipase preparation), respectively.     

2-amino-4-nitroaniline, a known compound with unexpected properties

2-amino-4-nitroaniline crystallizes in a noncentrosymmetric space group, which gives it significant potential for second-order nonlinear optical properties (NLO) in the bulk. Crystallographic and linear-polarized IR-spectrocopic data in the solid state unambiguously confirm a quinoide-like structure in the ground state in contrast to UV and theoretical data, which indicate an aromatic one for the excited state. UV-vis spectral elucidations in solutions with different polarities indicate a significant charge-transfer band with shifts of up to 100 nm, corresponding to a large value for the molecular first hyperpolarizability. Calculations of the UV- and IR-spectroscopic properties confirm the stabilization of the quinoide-like structure in the ground state, as well as the theoretically predicted NLO properties at the molecular level and provide a value of beta(tot), which is some 3.6 times higher than the analogous parameter for p-nitroaniline, a classical compound with experimentally confirmed NLO properties.     

Visualizing the structure of triglyceride nanoparticles in different crystal modifications

Colloidal suspensions of triglycerides are under investigation as potential drug carrier systems. The properties of the matrix lipids are altered in the nanoparticles compared to those of the bulk material. For instance, the metastable alpha-modification of the triglycerides usually transforms into the stable beta-polymorph quite rapidly in the colloidal particles. Recently, it was observed that the alpha-modification can be preserved for a considerable period of time in tristearin nanoparticles when the particles are stabilized with a blend of saturated long-chain phospholipids and the bile salt sodium glycocholate [Bunjes, H.; Koch, M. H. J. J. Controlled Release 2005, 107, 229-243]. As triglyceride nanoparticles in the alpha-modification may offer some advantages over those in the beta-form with regard to drug delivery applications, the structure of the corresponding dispersions was investigated in more detail with differential scanning calorimetry, X-ray diffraction, and electron microscopy. The electron microscopic investigations confirmed a platelet-like, layered structure for particles in the beta-modification and revealed a spheroidal shape with concentric layers for larger particles in the alpha-form. For the first time, not only was information on the internal structure of solid triglyceride nanoparticles obtained from freeze-fracture electron micrographs but also details were observed by cryoelectron microscopy.     

No Borel Connections for the Unsplitting Relations

We prove that there is no Borel connection for non‐trivial pairs of unsplitting relations. This was conjectured in [3].     

“Living” free radical copolymerization of styrene and N-vinylcarbazole

Poly[styrene-co-(N-vinylcarbazole)] copolymers with controlled molecular weights and narrow polydispersities were synthesized by nitroxide-mediated “living” free radical copolymerization using an initiator/capping agent system consisting of benzoyl peroxide (BPO) and the stable nitroxyl radical 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO). The copolymerization behaves in a “living” fashion and allows the synthesis of poly[styrene-co-(N-vinylcarbazole)]/polystyrene block copolymers via a controlled chain-extension reaction of the prepared copolymers with styrene.