Tuning the electronic communication and rates of intramolecular electron transfer of dimers of trinuclear ruthenium clusters: bridging and ancillary ligand effects

Ten new bridged dimers of oxo-centered triruthenium clusters with CO and 4-(dimethylamino)pyridine (dmap), pyridine (py), or 4-cyanopyridine (cpy) as terminal ligands and pyrazine-d(4) (d(4)-pz), 2,5-dimethylpyrazine (dmpz), 2-methylpyrazine (mpz), and 2-chloropyrazine (clpz) as bridging ligands were prepared. The carbonyl stretching frequency, nu(CO), was used as a probe for infrared spectroelectrochemical measurements. In the neutral and doubly reduced states, a single band was observed for each of the dimers, with a shift in frequency due to the oxidation state of the triruthenium clusters. In the singly reduced state, a range of nu(CO) line shapes was observed, depending on the nature of the ligands, from two bands centered at the frequencies of the bands of the neutral and doubly reduced species to one broad band at the average of these two frequencies. By synthesizing new combinations of bridging and ancillary ligands, electronic communication between two bridged triruthenium clusters was effectively tuned, and electron-transfer rates were estimated by IR spectral line-shape analysis. In dimers bridged by the asymmetric ligand mpz, it was possible through selective isotope labeling of one CO ligand to observe "mixed-valence isomers," the two alternate charge distributions of a mixed-valence complex.     

Substituent effects in five oxo-centered trinuclear rhodium(III) clusters

We here report the rates of water substitution by methanol-d(4) for four new oxo-centered trinuclear rhodium(III) clusters with different carboxylate-bridging ligands, [Rh3(mu3-O)(mu-O2CR)6(OH2)3]+ (R = CH2CH3, CH2CH2Cl, CH2Cl, and CHCl2), and [Rh3(mu3-O)(mu-O2CCH3)6(OH2)3]+. By varying the R group alkyl chain, water substitution rates were found to span almost 3 orders of magnitude (k(298K) = 1.2 x 10(-2)-2.3 x 10(-5) s(-1)) and reflect the following trend R = CH2CH3 > CH3 > CH2CH2Cl > CH(2)Cl > CHCl2. Activation parameters for substitution point toward a dissociative activation pathway (DeltaH = 99-115 kJ mol(-1); DeltaS = 48-52 J mol(-1) K(-1)), indicating that there is little association with the incoming methanol molecule during the formation of the transition-state complex. Because the mechanism for substitution in all five trimers has a considerable dissociative character, substitution rates are likely very similar to water exchange rates. These data suggest that the kinetic reactivity of the ligated waters is heavily influenced by the inductive ability of the aliphatic substituents, but yet the mechanism of substitution remains virtually unchanged. Structural data are also reported for the four new rhodium(III) trimer salts as well as 103Rh NMR spectra. We find that 103Rh NMR chemical shifts span more than 200 ppm and mirror the same reactivity trend found for the rates of water substitution (103Rh delta (9406-9620 ppm): R = CH2CH3 < CH3 < CH2CH2Cl < CH2Cl < CHCl2). Taken together, these data suggest a means for estimating water exchange rates for other oxo-centered rhodium(III) trimers from chemical shift data alone.     

Distinct water-exchange mechanisms for trinuclear transition-metal clusters

Mechanisms for water exchange from the bioxo-capped M-M-bonded trinuclear clusters, [M3(mu3-O)2(mu-O2CCH3)6(OH2)3]2+ [M = Mo(IV) and W(IV)], were investigated using high-pressure 17O NMR and compared to our previous work on a similar Rh(III) trimer. Reaction rates decrease by more than a factor of 2 when pressure is increased from 6 to 250 MPa for the Mo(IV) trimer, while exchange rates increase by less than a factor of 1.2 (10-229 MPa) for the W(IV) trimer. From the pressure dependence of the reaction rate, activation volumes (DeltaV()) were calculated to be DeltaV() = +8.0 (+/-0.4) cm(3) mol(-1) and DeltaV = -1.9 (+/-0.2) cm(3) mol(-1) for the Mo(IV) cluster and W(IV) cluster, respectively, which is the largest difference ( approximately 10 cm(3) mol(-1)) in activation volumes for any pair of 4d-5d (and 3d-4d) transition metal species located within the same group of the periodic table. If we interpret these activation volumes in terms of Swaddle's semiempirical model, which he established for simple octahedral monomers (Associative (A) = DeltaV approximately -13; Interchange (I) = DeltaV approximately 0; or Dissociative (D) = DeltaV approximately +13), our results suggest that water exchange follows a dissociative-interchange (Id) mechanism for the Mo(IV) cluster and an associative-interchange (Ia) activation mode for the W(IV) trimer. These volumes exhibit a unique changeover in the water-exchange mechanism despite considerable similarities in molecular structure and reactivity. This changeover could provide a standard for computational simulations of ligand-exchange pathways in molecules that are more complicated than monomers.     

Identifying and localizing intracellular nanoparticles using Raman spectroscopy

Raman microscopy is employed to spectroscopically image biological cells previously exposed to fluorescently labelled polystyrene nanoparticles and, in combination with K-means clustering and principal component analysis (PCA), is demonstrated to be capable of localising the nanoparticles and identifying the subcellular environment based on the molecular spectroscopic signatures. The neutral nanoparticles of 50 nm or 100 nm, as characterised by dynamic light scattering, are shown to be non-toxic to a human lung adenocarcinoma cell-line (A549), according to a range of cytotoxicity assays including Neutral Red, Alamar Blue, Coomassie Blue and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT). Confocal fluorescence microscopy identifies intracellular fluorescence due to the nanoparticle exposure, but the fluorescence distribution is spatially diffuse, potentially due to detachment of the dye from the nanoparticles, and the technique fails to unambiguously identify the distribution of the nanoparticles within the cells. Raman spectroscopic mapping of the cells in combination with K-means cluster analysis is used to clearly identify and localise the polystyrene nanoparticles in exposed cells, based on their characteristic spectroscopic signatures. PCA identifies the local environment as rich in lipidic signatures which are associated with localisation of the nanoparticles in the endoplasmic reticulum. The importance of optimised cell growth conditions and fixation processes is highlighted. The preliminary study demonstrates the potential of the technique to unambiguously identify and locate nonfluorescent nanoparticles in cells and to probe not only the local environment but also changes in the cell metabolism which may be associated with cytotoxic responses.     

Nanoprobes for intracellular and single cell surface‐enhanced Raman spectroscopy (SERS)

Surface‐enhanced Raman spectroscopy (SERS) is a promising and powerful label free technique for high resolution analysis of single cells. For intracellular analysis, there is a need for SERS‐active nanoprobes that are minimally invasive to cells, do not affect cell viability, and provide reproducible signals. This work reviews the state‐of‐the‐art tools currently available for intracellular SERS. Various types of SERS probes are considered, including colloidal gold and silver nanoparticles, metallized optical fibers, and tip‐enhanced Raman probes. We also discuss recently developed SERS‐active nanopipettes implemented on the basis of pulled glass microcapillaries. Finally, the critical aspects of selecting an optimal SERS nanoprobe for single‐cell analysis depending on a particular application are summarized. Copyright © 2012 John Wiley & Sons, Ltd.     

A definable failure of the singular cardinal hypothesis

We show first that it is consistent that κ is a measurable cardinal where the GCH fails, while there is a lightface definable wellorder of H(κ ⁺). Then with further forcing we show that it is consistent that GCH fails at ? _ω , ? _ω strong limit, while there is a lightface definable wellorder of H(? _ω+1) (“definable failure” of the singular cardinal hypothesis at ? _ω ). The large cardinal hypothesis used is the existence of a κ ⁺⁺-strong cardinal, where κ is κ ⁺⁺-strong if there is an embedding j: V → M with critical point κ such that H(κ ⁺⁺) ? M. By work of M. Gitik and W. J. Mitchell [12], [20], our large cardinal assumption is almost optimal. The techniques of proof include the “tuning-fork” method of [10] and [3], a generalisation to large cardinals of the stationary-coding of [4] and a new “definable-collapse” coding based on mutual stationarity. The fine structure of the canonical inner model L[E] for a κ ⁺⁺-strong cardinal is used throughout.     

Potent oxidative DNA cleavage by the di-copper cytotoxin: [Cu2(μ-terephthalate)(1,10-phen)4]2+

The di-copper(II) cation, [Cu(2)(μ-terephthalate)(1,10-phen)(4)](2+), is a powerful, non-sequence-specific, minor-groove oxidizer of duplex DNA which, unlike copper(II) bis-1,10-phenanthroline chloride, operates independently of exogenous reagents. The agent displays excellent in vitro cytoxicity towards cisplatin-resistant ovarian cancer cells, producing intracellular reactive oxygen species upon nano-molar exposure.     

Synthesis of 1,2,3-triazole-fused heterocycles via Pd-catalyzed cyclization of 5-iodotriazoles

A convenient approach toward polycyclic frameworks containing fused 1,2,3-triazoles is described. The synthesis consists of a Cu-catalyzed cycloaddition and an intramolecular Pd-catalyzed direct arylation or Heck reaction, and affords the products in good to excellent yields.