Anion binding and luminescent sensing using cationic ruthenium(II) aminopyridine complexes

The synthesis of a series of ruthenium(II) based anion sensors of the type [Ru(eta(6)-C(6)H(4)MeCHMe(2))Cl(L)(2)][BF(4)] (2) is reported in which ligand L represents a series of substituted pyridinylmethyl-amine derivatives. The carbazole based ligand L(3) exhibits a fluorescent intraligand charge-transfer (ILCT) state that is quenched by ligand-to-metal charge transfer (LMCT) upon coordination to ruthenium in the 1:1 complex [Ru(eta(6)-C(6)H(4)MeCHMe(2))Cl(2)(L(3))] (1 c). The 1:2 complex 2 c is fluorescent, however, and acts as a fluorescent anion sensor because of the mixing of an anion-dependent charge-transfer component into the excited state. The 1:2 complexes of type 2 all exhibit interesting low symmetry (1)H NMR spectra that also are a useful handle on anion complexation. The electronic structures of L(3), 1 c and 2 c have been probed by time-dependent DFT calculations.     

Travelling wave ion mobility mass spectrometry studies of protein structure: biological significance and comparison with X-ray crystallography and nuclear magnetic resonance spectroscopy measurements

The three-dimensional conformation of a protein is central to its biological function. The characterisation of aspects of three-dimensional protein structure by mass spectrometry is an area of much interest as the gas-phase conformation, in many instances, can be related to that of the solution phase. Travelling wave ion mobility mass spectrometry (TWIMS) was used to investigate the biological significance of gas-phase protein structure. Protein standards were analysed by TWIMS under denaturing and near-physiological solvent conditions and cross-sections estimated for the charge states observed. Estimates of collision cross-sections were obtained with reference to known standards with published cross-sections. Estimated cross-sections were compared with values from published X-ray crystallography and nuclear magnetic resonance (NMR) spectroscopy structures. The cross-section measured by ion mobility mass spectrometry varies with charge state, allowing the unfolding transition of proteins in the gas phase to be studied. Cross-sections estimated experimentally for proteins studied, for charge states most indicative of native structure, are in good agreement with measurements calculated from published X-ray and NMR structures. The relative stability of gas-phase structures has been investigated, for the proteins studied, based on their change in cross-section with increase in charge. These results illustrate that the TWIMS approach can provide data on three-dimensional protein structures of biological relevance.     

One-pot catalytic enantioselective domino nitro-michael/michael synthesis of cyclopentanes with four stereocenters

A highly enantioselective organocatalytic one-pot synthesis of nitro-, formyl-, and ester-functionalized cyclopentanes with four stereocenters is presented. The cyclopentanes were formed as a predominant diasteroisomer and isolated in high yields with 97-99 % ee.     

Charge recombination in organic photovoltaic devices with high open-circuit voltages

A detailed charge recombination mechanism is presented for organic photovoltaic devices with a high open-circuit voltage. In a binary blend comprised of polyfluorene copolymers, the performance-limiting process is found to be the efficient recombination of tightly bound charge pairs into neutral triplet excitons. We arrive at this conclusion using optical transient absorption (TA) spectroscopy with visible and IR probes and over seven decades of time resolution. By resolving the polarization of the TA signal, we track the movement of polaronic states generated at the heterojunction not only in time but also in space. It is found that the photogenerated charge pairs are remarkably immobile at the heterojunction during their lifetime. The charge pairs are shown to be subject to efficient intersystem crossing and terminally recombine into F8BT triplet excitons within approximately 40 ns. Long-range charge separation competes rather unfavorably with intersystem crossing--75% of all charge pairs decay into triplet excitons. Triplet exciton states are thermodynamically accessible in polymer solar cells with high open circuit voltage, and we therefore suggest this loss mechanism to be general. We discuss guidelines for the design of the next generation of organic photovoltaic materials where separating the metastable interfacial charge pairs within approximately 40 ns is paramount.     

Synthesis of functionalized biodegradable polyesters

This tutorial review summarizes recent developments in the syntheses of functionalized aliphatic polyesters. These polymers are attracting attention as sustainable alternatives to petrochemicals and for applications in medicine. Two main syntheses are described: step polymerization using mild chemo/enzymatic catalysis and ring opening polymerization, which is usually initiated by metal complexes. The methods are compared and their utility illustrated with reference to interesting new materials.     

Evidence for a SN2-type pathway for phosphine exchange in phosphine-phosphenium cations, [R2P--PR'3]+

Abstraction of a Cl(-) ion from the P-chlorophospholes, R4C4PCl (R=Me, Et), produced the P--P bonded cations [R4C4P--P(Cl)C4R4]+, which reacted with PPh3 to afford X-ray crystallographically characterised phosphine-phosphenium cations [R4C4P(PPh3)]+ (R=Me, Et). Examination of the 31P-{1H} NMR spectrum of a solution (CH2Cl(2)) of [Et4C4P-(PPh3)]+ and PPh3 revealed broadening of the resonances due to both free and coordinated PPh3, and importantly it proved possible to measure the rate of exchange between PPh3 and [Et4C4P-(PPh3)]+ by line shape analysis (gNMR programmes). The results established second-order kinetics with DeltaS( not equal)=(-106.3+/-6.7) J mol(-1) K(-1), DeltaH( not equal)=(14.9+/-1.6) kJ mol(-1) and DeltaG( not equal) (298.15 K)=(46.6+/-2.6) kJ mol(-1), values consistent with a SN2-type pathway for the exchange process. This result contrasts with the dominant dissociative (S(N)1-type) pathway reported for the analogous exchange reactions of the [ArNCH2CH2N(Ar)P(PMe3)]+ ion, and to understand in more detail the factors controlling these two different reaction pathways, we have analysed the potential energy surfaces using density functional theory (DFT). The calculations reveal that, whilst phosphine exchange in [Et4C4P(PPh3)]+ and [ArNCH2CH2N(Ar)P(PMe3)](+) is superficially similar, the two cations differ significantly in both their electronic and steric requirements. The high electrophilicity of the phosphorus center in [Et4C4P]+, combined with strong pi-pi interactions between the ring and the incoming and outgoing phenyl groups of PPh3, favours the SN2-type over the SN1-type pathway in [Et4C4P(PPh3)]+. Effective pi-donation from the amide groups reduces the intrinsic electrophilicity of [ArNCH2CH2N(Ar)P]+, which, when combined with the steric bulk of the aryl groups, shifts the mechanism in favour of a dissociative SN1-type pathway.     

A mechanistic investigation of the asymmetric Meerwein-Schmidt-Ponndorf-Verley reduction catalyzed by BINOL/AlMe(3)-structure, kinetics, and enantioselectivity

The kinetics of the Al-catalyzed asymmetric Meerwein-Schmidt-Ponndorf-Verley (MSPV) reduction are presented. Structural identification of the catalytic precursor formed in situ between (S)-2,2'-dihydroxy-1,1'-binapthyl ((S)-BINOL), AlMe3, and 2-propanol was established through 1H and 27Al NMR spectroscopies, and APCIMS. All experimental evidence points toward the formation of a BINOL-chelated, pentacoordinate aluminum species in solution. Ligand-accelerated catalysis was confirmed for the phenolate/AlMe3/2-propanol system. The rate law for the catalytic reaction was determined to be nearly unimolecular dependent on aluminum, zero-order dependent on substrate, and inversely dependent on 2-propanol. At the low catalyst loading employed in the BINOL/AlMe3 system, the inherent reversibility of the MSPV reaction does not affect product yield or enantiomeric excess over time. Systematic ligand studies imply that while a tetrahedral geometry around the aluminum center may result in the most active MSPV reduction catalysts, the enantioselectivity of the reaction is enhanced when the aluminum center allows for a 2-point coordination of the substrate to achieve a pentacoordinate geometry with the fifth ligand weakly coordinated to the axial site of a pseudo square pyramid.