p-Benzyne derivatives that have exceptionally small singlet-triplet gaps and even a triplet ground state

In an effort to find a p-benzyne (1,4-didehydrobenzene) derivative with a triplet ground state, we have investigated tetrasubstitution by -F, -NH(2), -CH(3), and -NO(2) groups. These were predicted to reduce the singlet-triplet gap, but none led to a triplet ground state because of unexpected destabilization of one of the radical orbitals. This effect is likely the result of rehybridization of the substituted C atom, which has been observed for substituted benzene and perturbs the side sigma and sigma* orbital energies of the phenyl ring. The role of substituent rotation on the energy difference between the two nominally singly occupied orbitals (S and A) was then investigated. The energy of the A radical orbital was found to be much more sensitive to perturbations within the sigma C[bond]C framework than the S MO. Consequently, we believe that rehybridization of the ring carbons destabilizes the A radical orbital and can lead to large singlet-triplet splittings. To test this hypothesis, calculations on a p-benzyne with 2,6 substitution by oxygen were performed. Interestingly, a triplet ground state was predicted. Yet, examination of the geometry and wave function showed that 2,6-quinone p-benzyne is a very twisted molecule with a C3-C4-C5 allene linkage and a C1 triplet carbene center.     

Osmium diboride, an ultra-incompressible, hard material

The need for wear- and scratch-resistant materials drives the quest for new superhard materials. In this work, we apply two design parameters to identify ultra-incompressible, superhard materials-high valence electron density and high bond covalency. Our first example of such a material is OsB2. The bulk modulus of OsB2 was measured using in situ high-pressure X-ray diffraction and was determined to be in the range of 365-395 GPa. While this value is slightly less than that of the bulk modulus of diamond, due to the anisotropic crystal structure of OsB2, the axis compressibility in the orthorhombic c-direction is less than the axis compressibility found in diamond. OsB2 also scratches the surface of a sapphire window, indicating that the hardness of OsB2 exceeds 2000 kg/mm2.     

Organoplatinum(IV) compounds : II. Preparation and characterization of trimethylplatinum(IV) compounds with chelating nitrogen donor ligands. The crystal and molecular structure of iodotrimethyl[bis(3,5-dimethyl-1-pyrazolyl)methane]platinum(IV)

Compounds of the types Me₃PtX(NN) (where X = Cl, I, OAc, NO₃; NN = bis(1-pyrazolyl)methane (pz₂CH₂), bis(3,5-dimethyl-1-pyrazolyl)methane ((Me₂pz)₂CH₂), or bis(2-pyridyl)methane (py₂CH₂)), [Me₃Pt(NNN)][PF₆] (where NNN=tris(1-pyrazolyl)methane (pz₃CH), or tris(2-pyridyl)methane (py₃CH)), and [Me₃Pt((Me₂pz)₂CH₂(py)][PF₆] have been prepared and characterized by elemental analyses and ¹H and ¹³C NMR spectroscopy; the structure of Me₆PtI[(Me₂pz)₂CH₂] (1) has also been determined by X-ray crystallography. Crystals of 1 are orthorhombic, space group Pcmn with four molecules in a unit cell of dimensions a 11.936(5), b 14.462(4), c 10.138(5) Å. The structure was solved by the heavy-atom method and refined by full-matrix least-squares calculations to R = 0.022 for 1719 observed data. The molecule has crystallographic mirror symmetry. The Pt atom has octahedral geometry with one methyl group trans to iodine and two methyl groups trans to the N atoms of the bidentate ligand (Pt---I 2.843(1), Pt---N, 2.236(4), Pt---C 2.077(6) (trans to I) and 2.032(5) Å (trans to N)).     

Resonance Raman Spectroscopy,and Its Application to Inorganic Chemistry. New Analytical Methods (27)

Resonance Raman spectra are obtained when the wave number of the exciting radiation is close to, or coincident with, that of an electronic transition of the scattering species. Such spectra are usually characterized by a very large enhancement of the intensities of particular Raman bands, sometimes with the appearance of intense overtone and combination tone progressions. The technique provides detailed information about excited electronic states because it is only the vibrational modes associated with the chromophore that are resonance-Raman active. Additionally, the high sensitivity is such that compounds at concentrations as low as 10^?6 mol/L may be detected, enabling resonance Raman spectroscopy to be used as an analytical tool and for the study of chromophores in molecules of biological interest.     

Atom transfer radical cyclisation reactions mediated by copper complexes

This article describes recent advances in the use of copper complexes in mediating atom transfer radical cyclisation reactions (ATRC). Recent developments have included the design of activated complexes which mediate the cyclisation of tri-, di-, and mono-halo derived substrates at ambient temperatures. Using this methodology, cyclisation to give a variety of ring sizes (4-18 membered rings) has been demonstrated. In addition tandem and radical-polar crossover reactions have also been developed. The design of solid supported and perfluorous complexes that mediate cyclisations may make this approach to the synthesis of rings more attractive towards industrial applications.     

A theoretical investigation of the ground and core hole states of linear and bent NiCO; a prototype for CO adsorbed on nickel

Non-empirical LCAO MO SCF calculations within the Hartree-Fock formalism have been performed on linear and bent conformations of NiCO, for both the states arising from the 3d⁹4s¹ and 3d¹⁰ electronic configurations of Ni, for ground and core ionized species. Binding and relaxation energies are computed for the nickel atom, the free ligand (CO) and the absorbed species (NiCO). Comparison with available experimental data reveals good agreement for the shifts in core binding energies for the bent, 3d⁹4s¹ NiCO system. The main contribution to these binding energy shifts is found to be due to relaxation effects, which may be rationalized in terms of atomic population and bond overlap changes accompanying core ionization.     

Poisoning of heterogeneous, late transition metal dehydrocoupling catalysts by boranes and other group 13 hydrides

Borane reagents are widely used as reductants for the generation of colloidal metals. When treated with a variety of heterogeneous catalysts such as colloidal Rh, Rh/Al2O3, and Rh(0) black, BH3.THF (THF = tetrahydrofuran) was found to generate H2 gas with the concomitant formation of a passivating boron layer on the surface of the Rh metal, thereby acting as a poison and rendering the catalyst inactive toward the dehydrocoupling of Me2NH.BH3. Analogous poisoning effects were also detected for (i) colloidal Rh treated with other species containing B-H bonds such as [HB-NH]3, or Ga-H bonds such as those present in GaH3.OEt2, (ii) colloidal Rh that was generated from Rh(I) and Rh(III) salts using borane or borohydrides as reductants, and (iii) for other metals such as Ru and Pd. In contrast, analogous poisoning effects were not detected for the catalytic hydrogenation of cyclohexene using Rh/Al2O3 or the Pd-catalyzed Suzuki cross-coupling of PhB(OH)2 and PhI. These results suggest that although this poisoning behavior is not a universal phenomenon, the observation that such boron layers are formed and surface passivation may exist needs to be carefully considered when borane reagents are used for the generation of metal colloids for catalytic or materials science applications.     

Physical properties of poly(ethylene glycol) (PEG)-based resins for combinatorial solid phase organic chemistry: a comparison of PEG-cross-linked and PEG-grafted resins

Three series of poly(ethylene glycol) (PEG)-based polymers were synthesized and characterized with respect to their physical properties. Polyoxyethylene-polyoxypropylene (POEPOP), polyoxyethylene-polyoxetane (SPOCC), and polyoxyethylene-polystyrene (POEPS-3) were synthesized respectively by anion polymerization, cation polymerization, and radical polymerization. Both bulk and suspension modes were used to synthesize the polymers from derivatized PEG monomers (PEG 400, PEG 900, and PEG 1500). The three supports were compared with two commercially available PEG-grafted supports (TentaGel S OH, ArgoGel-OH) and two polystyrene supports (aminomethylated polystyrene [PS-NH2] and macroporous aminomethylated polystyrene [PLAMS]) with respect to their swelling properties, loading, NMR spectral quality, as well as solvent and reagent accessibility. Loadings of 0.3-0.7 mmol/g were obtained for the PEG-based resins. Swelling of the PEG-based resins was determined to be higher than that of the PEG-grafted resins and polystyrene supports. The PEG-based resins gave better resolved high-resolution NMR spectra than the PEG-grafted resins when examined by magic angle spinning nanoprobe (MAS) NMR spectroscopy. Moreover, fluorescence quenching of polymer bound 2-amino-benzoate by protonation with p-toluenesulfonic acid showed moderate to fast diffusion through the polymer depending on the solvent and the polymer matrix.     

Determination of the bioavailability of biotin conjugated onto shell cross-linked (SCK) nanoparticles

Shell cross-linked nanoparticles (SCKs) presenting surface- and bioavailable biotin functional groups were synthesized via a mixed micelle methodology, whereby co-micellization of chain terminal biotinylated poly(acrylic acid)-b-poly(methyl acrylate) (PAA-b-PMA) and nonbiotinylated PAA-b-PMA were cross-linked in an intramicellar fashion within the shell layer of the mixed micelles, between the carboxylic acid groups of PAA and the amine functionalities of 2,2'-(ethylenedioxy)diethylamine. The hydrodynamic diameters (D(h)) of the micelles and the SCKs with different biotinylated block copolymer contents were determined by dynamic light scattering (DLS), and the dimensions of the SCKs were characterized with tapping-mode atomic force microscopy (AFM) and transmission electron microscopy (TEM). The amount of surface-available biotin was tuned by varying the stoichiometric ratio of the biotinylated PAA-b-PMA versus the nonbiotinylated PAA-b-PMA, as demonstrated with solution-state, binding interaction analyses, an avidin/HABA (avidin/4'-hydroxyazobenzene-2-carboxylic acid) competitive binding assay, and fluorescence correlation spectroscopy (FCS). The avidin/HABA assay found the amount of available biotin at the surface of the biotinylated SCK nanoparticles to increase with increasing biotin-terminated block copolymer incorporation, but to be less than 25% of the theoretical value. FCS measurements showed the same trend.