Syntheses,Structures, and Polymorphism of β‐Diketonato Complexes – Co(thd)3

Oxidation of Co(thd)₂ dissolved in different solvents has been investigated in air and oxygen atmosphere. In oxygen atmosphere and at the boiling point of the solvents this treatment leads to oxidation of Co^II to Co^III, but also to degradation of some of the thd ligands and formation of a new mixed‐ligand complex. Three pure‐cultivated crystalline Co(thd)₃ phases are reported: 1 (room‐temperature phase), 2 (low‐temperature phase), and 3 (metastable phase) and in addition there exists an amorphous Co(thd)₃ phase ( 4 ) with approximate composition Co(thd)₃·xH(thd); x = 0.06. Reaction of metal(II) oxides (MO, M = Mn, Fe, and Co) with H(thd) under air or O₂ atmosphere is an easy direct route to M(thd)₃ complexes. Structure determinations are reported for Co(thd)₃ ( 1 – 3 ) based on single‐crystal X‐ray diffraction data. Modification 1 crystallizes in space group with a = b = 18.8100(10), c = 18.815(2) Å at 295 K; R(wR2) = 0.180, modification 2 in space group C2/c with a = 28.007(12), b = 18.482(8), c = 21.356(9) Å, β = 97.999(5)° at 100 K; R(wR2) =0.211, and modification 3 in space group Pnma with a = 19.2394(15), b = 18.8795(15), c = 10.7808(8) Å at 100 K; R(wR2) = 0.193. The molecular structures of 1 – 3 all comprise a central Co atom octahedrally co‐ordinated by the ketonato O atoms of three thd ligands. The transformation between modifications 1 and 2 is of a fully reversible second‐order character. Modifications 1 and 3 are, on the other hand, related by a quasi‐reversible cycle. Heat treatment (specifically sublimation) of 1 leads to 3 whereas re‐crystallization or prolonged storage at room temperature is required to regenerate 1 . Co(thd)₃ has sufficient thermal stability to permit sublimation without degradation. The various forms of Co(thd)₃ are all diamagnetic, viz. a confirmation of the Co^III valence state.     

Configurationally stable longitudinally twisted polycyclic aromatic compounds

Two strategies for the synthesis of configurationally stable twisted polycyclic aromatic compounds (PACs) were pursued. The first approach employed dissymmetrically positioned 1-naphthyl substituents to bias the direction of twist in highly substituted PACs. 2,3-Bis(1-naphthyl)-1,4-diphenyltriphenylene (7) was prepared, and its meso cis-dinaphthyl and enantiomeric trans-dinaphthyl isomers were resolved by preparative supercritical fluid chromatography (SFC) on chiral supports. Similarly, several naphthyl-substituted derivatives of the more highly twisted 9,10,11,12,13,14-hexaphenylbenzo[b]triphenylene (2) were prepared. Of these, 10-(1-naphthyl)-9,11,12,14-tetraphenylbenzo[b]triphenylene (13) was resolved by SFC on a chiral support. The pure enantiomers of trans-7 showed moderately large specific rotations ([alpha]D(25) = -330 and +320 degrees), but the specific rotations for the enantiomers of 13 were unexpectedly small ([alpha]D(25) = -23 and +23 degrees). Computational studies suggest that the latter result is due to presence of a minor conformation of 13 possessing a larger rotation of opposite sign than the major conformation. Both 7 and 13 showed strong circular dichroism and moderately strong circularly polarized luminescence. A byproduct of these syntheses was 9,10,19,21-tetraphenyldiphenanthro[9,10-b:9,10-h]carbazole (15), a very crowded carbazole that exhibits an 81 degree end-to-end twist but is not resolvable. In the second approach, the large, twisted, polycyclic aromatic ligand 9,10,11,12,13,14-hexaphenylbenzo[h]naphtho[2,3-f]quinoline (21, an aza-2) was used to prepare the chiral, cyclometallated iridium(III) complex 4. The ligand 21 was prepared via an unusually stable benzannulated norbornadienone, for which the free energy of activation for decarbonylation was a remarkable 33.5 kcal/mol. The iridium complex 4 proved to be configurationally stable and resolvable by analytical HPLC on chiral supports, but the low solubility of 4 prevented its resolution on a preparative scale. A much more soluble dibutyl analogue of 4 (complex 28) was then prepared, but it was not resolvable on any of the available media.     

Role of exposed metal sites in hydrogen storage in MOFs

The role of exposed metal sites in increasing the H2 storage performances in metal-organic frameworks (MOFs) has been investigated by means of IR spectrometry. Three MOFs have been considered: MOF-5, with unexposed metal sites, and HKUST-1 and CPO-27-Ni, with exposed Cu(2+) and Ni(2+), respectively. The onset temperature of spectroscopic features associated with adsorbed H2 correlates with the adsorption enthalpy obtained by the VTIR method and with the shift experienced by the H-H stretching frequency. This relationship can be ascribed to the different nature and accessibility of the metal sites. On the basis of a pure energetic evaluation, it was observed that the best performance was shown by CPO-27-Ni that exhibits also an initial adsorption enthalpy of -13.5 kJ mol(-1), the highest yet observed for a MOF. Unfortunately, upon comparison of the hydrogen amounts stored at high pressure, the hydrogen capacities in these conditions are mostly dependent on the surface area and total pore volume of the material. This means that if control of MOF surface area can benefit the total stored amounts, only the presence of a great number of strong adsorption sites can make the (P, T) storage conditions more economically favorable. These observations lead to the prediction that efficient H2 storage by physisorption can be obtained by increasing the surface density of strong adsorption sites.     

Design and use of organic nanoparticles prepared from star-shaped polymers with reactive end groups

Star-shaped poly(isobornyl acrylate) (PiBA) was prepared by atom transfer radical polymerization (ATRP) using multifunctional initiators. The optimal ATRP conditions were determined to minimize star-star coupling and to preserve high end group functionality (>90%). Star-shaped PiBA with a narrow polydispersity index was synthesized with 4, 6, and 12 arms and of varying molecular weight (10,000 to 100,000 g x mol(-1)) using 4 equiv of a Cu(I)Br/PMDETA catalyst system in acetone. Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) analysis, NMR spectroscopy, and size exclusion chromatography (SEC) confirmed their controlled synthesis. The bromine end group of each arm was then transformed to a reactive end group by a nucleophilic substitution with methacrylic acid or cinnamic acid (conversion >90%). These reactive star polymers were used to prepare PiBA nanoparticles by intramolecular polymerization of the end groups. The successful preparation of this new type of organic nanoparticles on a multigram scale was proven by NMR spectroscopy and SEC. Subsequently, they have been used as additives for linear, rubbery poly(n-butyl acrylate). Rheology measurements indicated that the viscoelastic properties of the resulting materials can be fine-tuned by changing the amount of incorporated nanoparticles (1-20 wt %), as a result of the entanglements between the nanoparticles and the linear polymers.     

NMR studies of the dynamics of a bifunctional rhodamine probe attached to troponin C

Fluorescence polarization measurements of bifunctional rhodamine (BR) probes provide a powerful approach to determine the in situ orientation of proteins within ordered complexes such as muscle fibers. For accurate interpretation of fluorescence measurements, it is important to understand the probe dynamics relative to the protein to which it is attached. We previously determined the structure of the N-domain of chicken skeletal troponin C, BR-labeled on the C helix, in complex with the switch region of troponin I, and demonstrated that the probe does not perturb the structure or dynamics of the protein. In this study, the motion of the fluorescence label relative to the protein has been characterized using NMR relaxation measurements of 13C-labeled methyl groups on the BR probe and 15N-labeled backbone amides of the protein. Probe dynamics were monitored using off-resonance 13C-R(1rho), 13C-R(1) and {1H}-13C NOE at magnetic field strengths of 500, 600, and 800 MHz. Relaxation data were interpreted in terms of the overall rotational correlation time of the protein and a two-time scale model for internal motion of the BR methyl groups, using a numerical optimization with Monte Carlo parameter error estimation. The analysis yields a 1.5 +/- 0.4 ps correlation time for rotation around the three-fold methyl symmetry axis, and a 0.8 +/- 0.4 ns rotational correlation time for reorientation of the 13C-14N bond with an associated S2s of 0.79 +/- 0.03. Order parameters of the backbone NH vectors in the helix to which the probe is attached average S2 approximately 0.85, implying that the amplitude of independent reorientation of the BR probe is small in magnitude, consistent with results from fluorescence polarization measurements in reconstituted muscle fibers.     

Regioselective activation of oxo ligands in functionalized dawson polyoxotungstates

The organic side chain of tin-substituted Dawson polyoxotungstates alpha1- and alpha2-[P2W17O61{SnCH2CH2COOH}]7- can be used to direct regioselective acylations of oxo ligands in the inorganic backbone, which was examined both experimentally and computationally. Acylation of the oxo ligand gave exalted electrophilicity to the acyl moiety, and the compounds that were obtained led to direct ligation of POMs to complex organic molecules.     

Combined use of atomic force microscopy, X-ray photoelectron spectroscopy, and secondary ion mass spectrometry for cell surface analysis

Understanding the surface properties of microbial cells is a major challenge of current microbiological research and a key to efficiently exploit them in biotechnology. Here, we used three advanced surface analysis techniques with different sensitivity, probing depth, and lateral resolution, that is, in situ atomic force microscopy, X-ray photoelectron spectroscopy, and secondary ion mass spectrometry, to gain insight into the surface properties of the conidia of the human fungal pathogen Aspergillus fumigatus. We show that the native ultrastructure, surface protein and polysaccharide concentrations, and amino acid composition of three mutants affected in hydrophobin production are markedly different from those of the wild-type, thereby providing novel insight into the cell wall architecture of A. fumigatus. The results demonstrate the power of using multiple complementary techniques for probing microbial cell surfaces.     

Biomimetic dual templating of silica by polysaccharide/protein assemblies

The control of silica growth by living organisms such as diatoms is known to involve the templating effect of several biomolecules working concomitantly. However, until now, biomimetic studies involving model molecules have mainly been performed with single templates. We show here that the addition of two biopolymers, gelatin and alginic acid, to silicate solutions allows the formation of complex structures resulting from the combined templating effect of both components at different scales. Gelatin is able to activate silica formation resulting in hybrid aggregates at the nanoscale. Alginic acid does not interfere with silica condensation but is able to control silica morphology through the assembly of these gelatin-silica aggregates at the microscale. For all materials, calcination up to 700 degrees C degrades the polymer component of the hybrid material and opens macroporosity in the silica network. In parallel, the high thermal stability of gelatin allows a good preservation of initial silica nanoparticle size upon heating whereas a coarsening process is observed in the sole presence of alginate. These results correlate well with previous models of biosilicification and suggest that the use of multiple templates is a suitable approach to elaborate more complex silica architectures.     

Spectroscopic properties of orthometalated platinum(II) bipyridine complexes containing various ethynylaryl groups

Neutral orthometalated platinum(II) complexes of the deprotonated 6-phenyl-2,2'-bipyridine ligand (bearing a trialkoxygallate, tolyl, ethynyltrialkoxygallate, or ethynyltolyl substituent) and a sigma-bonded Cl, ethynyltolyl, or ethynyltrialkoxygallate coligand have been prepared by a stepwise procedure based on copper-promoted cross-coupling reactions. The X-ray structure of the [2-(p-tolyl)ethynyl][4-{2-(p-tolyl)ethynyl}-6-phenyl-2,2'-bipyridyl)]platinum(II) complex revealed a coplanar arrangement of all residues bound to platinum, although the tolylethynyl groups exhibit position-dependent bending in the solid state. The complexes exhibit charge-transfer absorption in the visible region. All except two of the complexes also exhibit charge-transfer emission, typically from an excited state that has a submicrosecond lifetime at room temperature in deoxygenated dichloromethane solution. In accordance with the presence of a carbometalated polypyridine ligand, the emitting state is assumed to have a mixture of metal-to-ligand charge-transfer (MLCT) and intra-ligand charge-transfer (ILCT) character. However, spectral comparisons and electrochemical data suggest that the emissive state also exhibits interligand charge-transfer (LLCT) character when an electron-rich ethynylaryl group is bound to platinum. In keeping with altered orbital parentage in the latter systems, the emission occurs at longer wavelength. The excited-state lifetime is also shorter, evidently due to vibronic interactions. The decay is so efficient when an ethynyltrialkoxygallate group binds to platinum that there is no detectable emission in fluid solution, although the complexes do emit in a frozen glass. The excited states are subject to associative (exciplex) quenching by Lewis bases, but the admixture of ILCT and/or LLCT character diminishes efficiency, except for relatively strong bases like dimethyl sulfoxide and dimethylformamide.