Multiple nitrene insertions into metal-sulfur bonds of dithiocarbamate complexes: synthesis of sulfido-amido and zwitterionic tetraamido complexes

The iodine(III) reagent, PhI[double bond, length as m-dash]NTs, acts as a source of the nitrene fragment NTs, which undergoes facile insertion into the metal-sulfur bonds of a range of dithiocarbamate complexes. Addition of two equivalents of PhI=NTs to [M(S(2)CNR2)2] affords sulfido-amido complexes [M{SC(NR2)SNTs}2](M=Ni, Cu), which insert two further nitrene fragments to afford zwitterionic tetraamido complexes [M{TsNSC(NR2)SNTs}2](M=Co, Ni, Cu). Crystallographic studies have been carried out on both types of complex allowing possible resonance hydrids of the new ligand types to be assessed.     

Selective cleavage of P-N bonds and the conversion of rhodium N-pyrrolyl phosphine complexes into diphosphoxane-bridged dimers

Rhodium(I) complexes trans-[RhCl(CO)(PR(2)[NC(4)H(3)C(O)Me-2])(2)] (R = Ph, NC(4)H(4)) react with water to give the diphosphoxane-bridged dimers [Rh(2)Cl(2)(CO)(2)(mu-PR(2)OPR(2))(2)] following cleavage of the P-N bonds to the 2-acetyl-N-pyrrolyl groups. The two dimers have been crystallographically characterized and show a number of structural differences, with the PPh(2)OPPh(2) compound possessing semibridging chloride and carbonyl ligands whereas the P(NC(4)H(4))(2)OP(NC(4)H(4))(2) compound contains only terminal chlorides and carbonyls. No evidence for cleavage of the P-N bonds involving the unfunctionalized N-pyrrolyl groups in trans-[RhCl(CO)(P[NC(4)H(4)](2)[NC(4)H(3)C(O)Me-2])(2)] was observed.     

The structural characteristics of organozinc complexes incorporating N,N'-bidentate ligands

Dimethylzinc reacts with an excess of N-2-pyridylaniline 6 to give the homoleptic species, Zn[PhN(2-C(5)H(4)N)](2) 8. Single crystal X-ray diffraction reveals a solid-state dimer based on an 8-membered (NCNZn)(2) core motif. Zn[CyN(2-C(5)H(4)N)]Me (Cy =c-C(6)H(11)) 10, prepared by the combination of ZnMe(2) with the corresponding cyclohexyl-substituted pyridylamine, is also dimeric in the solid state but reveals a central (ZnN)(2) metallacycle. Employment of (p-Tol)NH(2-C(5)H(4)N)(p-Tol = 4-MeC(6)H(4)) 11 yielded the tris(zinc) adduct Zn(3)[(p-Tol)N(2-C(5)H(4)N)](4)Me(2) 12, which incorporates a central chiral molecule of 'Zn[(p-Tol)N(2-C(5)H(4)N)](2)' 12a, that bridges two 'Zn[(p-Tol)N(2-C(5)H(4)N)]Me' 12b units. A similar trimetallic structure is noted when the pyridylaniline substrate 11 is replaced with the bicyclic guanidine 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine (hppH), affording Zn(3)(hpp)(4)Me(2) 13. Spectroscopic studies point to retention of the solid-state structure of in hydrocarbon solution. Reaction of 13 with dimesityl borinic acid, Mes(2)BOH (Mes = mesityl), affords Zn(3)(hpp)(4)(OBMes(2))(2) 14 in which the trimetallic core is retained. This reactivity is in contrast to the closely related reaction of dimeric Zn[Me(2)NC[N(i)Pr](2)]Me 15 with Mes(2)BOH, which yielded Zn[Me(2)NC[N(i)Pr](2)][OBMes(2)].Me(2)NC[N(i)Pr][NH(i)Pr] 16 as a result of protonation at the guanidine ligand in addition to the Zn-Me bond.     

A study of [Co2(alkyne)(binap)(CO)4] complexes (BINAP=(1,1'-binaphthalene)-2,2'-diylbis(diphenylphosphine))

Understanding the interaction of chiral ligands, alkynes, and alkenes with cobaltcarbonyl sources is critical to learning more about the mechanism of the catalytic, asymmetric Pauson-Khand reaction. We have successfully characterized complexes of the type [Co2(alkyne)(binap)(CO)4] (BINAP=(1,1'-binaphthalene)-2,2'-diylbis(diphenylphosphine)) and shown that diastereomer interconversion occurs under Pauson-Khand reaction conditions when alkyne=HC[triple bond]CCO2Me. Attempts to isolate [Co2(alkyne)(binap)(CO)x] complexes with coordinated alkenes led to the formation of cobaltacyclopentadiene species.     

Two dimensional reversed-phase-reversed-phase separations isomeric separations incorporating C18 and carbon clad zirconia stationary phases

Informational theory and a geometric approach to factor analysis were employed to evaluate the degree of orthogonality of a two-dimensional reversed-phase-reversed-phase chromatographic system. The system incorporated a C18 column as one dimension and a carbon clad zirconia column as the second dimension. In order to study the resolving power of this system, the separation of a sample matrix containing an artificial mix of 32 isomers (structural and diastereoisomers) was evaluated. Using this system, between 25 and 28 of the 32 isomers could be separated, depending on the mobile phase combinations--with resolution that could not possibly be achieved in a single one dimensional separation. The results from this study indicate that in order to fully evaluate the resolving power of a 2D system multiple methods of analysis are most appropriate. This becomes increasingly important when the sample contains components that are very closely related and the retention of solutes is clustered in one quadrant of the 2D space. Ultimately, the usefulness of the 2D separation is determined by the goals of analyst.     

Intramolecular fluorescence quenching in luminescent copolymers containing fluorenone and fluorene units: a direct measurement of intrachain exciton hopping rate

The quenching process of fluorescence emission in polyfluorene (PF) due to the presence of intramolecular 9-fluorenone (9 FL) moieties is studied in dilute toluene solution as a function of 9 FL content in eight copolymers containing both fluorene and fluorenone units (PF/FL(x)). The absorption spectrum of PF/FL(x) copolymers clearly shows a new absorption band, redshifted relatively to the PF and 9-fluorenone absorption, which increases in intensity when the fluorenone fraction increases and also decreases with solvent polarity. Fluorescence emission spectra of PF/FL(x) show that this redshifted and unstructured emission does not coincide with the 9-fluorenone emission and, with increasing solvent polarity, it further redshifts and decreases in intensity. An isoemissive point is clearly observed on the fluorescence emission spectra of PF/FL(x) as a function of fluorenone content, showing that the new emission band is formed at the expense of PF. We propose the formation of an intramolecular charge transfer complex (ICTC) between PF units and 9-fluorenone to explain the appearance of the new emission band. Global analysis of time resolved fluorescence decays collected at 415 nm (PF emission) and 580 nm (the ICTC emission) show that three exponentials are generally needed to achieve excellent fits. Two of the components (420 ps and 6.5 ns) are independent of 9-fluorenone fraction. A further fast component is strongly dependent on fluorenone fraction and ranges between 280 and 70 ps. This component appears as a decay time at 415 nm and as a rise time at 580 nm and is ascribed to the migration of exciton to quenching sites (formation of intramolecular CT complex or exciton ionization at CT complex). A kinetic mechanism involving three different kinetic species, quenched PF units kinetically coupled with the ICTC complex, and unquenched PF units is proposed to explain the experimental data and the quenching rate constant is obtained, k(1) congruent with 10(11) s(-1). This is an experimental measurement of the intrachain exciton hopping rate.     

Dynamic light scattering studies of poly(ethylene oxide) adsorbed on Laponite: layer conformation and its effect on particle stability

Dynamic light scattering has been used to determine the hydrodynamic thickness of poly(ethylene oxide) (PEO) adsorbed on synthetic anisotropic clay particles (Laponite) as a function of molecular weight. The layer thicknesses, and their increase with molecular weight, indicate that the conformation of the adsorbed layer is very compact and is much smaller than those normally observed for polymer adsorption on flat interfaces. The aggregation kinetics of the polymer coated particles in 5 mM NaCl was analyzed in a quantitative manner, revealing that the potential barrier to aggregation is strongly enhanced when polymer is present.     

Rearrangement of 5-trimethylsilylthebaine on treatment with L-selectride: an efficient synthesis of (+)-bractazonine

Treatment of 5-trimethylsilylthebaine with L-Selectride gave rise to a rearrangement to 10-trimethylsilylbractazonine through migration of the phenyl group, whereas treatment of thebaine with strong Lewis acids is known to lead to a similar rearrangement through migration of the alkyl bridge to give, after reduction, (+)-neodihydrothebaine. It is suggested that the rearrangement of the alkyl group of thebaine is favored due to the formation of a tertiary benzylic cation. However, for 5-trimethylsilylthebaine, the lithium ion of L-Selectride acts as the Lewis acid and the beta-silyl effect dominates in the stabilization of any positive charge. This rearrangement provides a clear example of the greater relative migratory aptitude of phenyl groups over alkyl groups, and provides an efficient synthesis of (+)-bractazonine from thebaine.     

Organic nanoparticles whose size and rigidity are finely tuned by cross-linking the end groups of dendrimers

Dendrimers with molecular weights ranging from ca. 2700 to 11 000 and from 16 to 64 homoallyl ether end groups were cross-linked using the Grubbs ring-closing metathesis reaction. A combination of SEC, MALDI-TOF-MS, and AFM were used to characterize the cross-linked nanoparticles. The data suggest a significant decrease in volume with cross-linking and a concomitant increase in rigidity, both of which can be controlled independently with a fair degree of precision.     

Transport and kinetics at microheterogeneous electrodes: Part V. The methylviologen/platinum system

Results are reported for the reaction of methylviologen radical cation, MV⁺ with platinum colloidal particles, studied by stopped flow spectrophotometry. The rate of the reaction depends on the gaseous pretreatment of the particles. For particles reduced by hydrogen, the kinetics are usually first order with respect to MV⁺. The reaction is also first order in the concentration of platinum, and is inhibited in a first order manner by the product MV²⁺. This inhibition suggests that MV²⁺ is adsorbed on the particle surfaces, and this has been confirmed by ac, ring—disc electrode studies on macroscopic platinum electrodes. At high concentrations of MV⁺ some deviation from first order kinetics is observed. These results are all explained by a kinetic model in which either the desorption of MV²⁺ or the adsorption of MV⁺ is the rate limiting process. The rate of consumption of MV⁺ on an oxidised surface is an order of magnitude faster than that on the reduced surface. Ring—disc studies show that this is because the MV⁺ is not producing H₂ but is reducing the surface oxide. The results are shown to fit a simple model which takes into account this titration of the oxide layer. The model also explains why the rate on partially oxidised surfaces will appear to have an order greater than one in [Pt].