Burjor Captain

Helical chiral 2-aminopyridinium ions: A new class of hydrogen bond donor catalysts

Helical chiral 2-aminopyridinium ions were designed as a significantly more acidic (active) dual hydrogen-bonding catalyst than commonly used (thio)urea-based systems. The helicene framework was specifically utilized to position an inherently chiral barrier on the hydrogen-bonding side of the catalyst. The catalyst reactivity and enantioselectivity were successfully demonstrated in additions of 4,7-dihydroindoles to nitroalkenes (0.5-2 mol % catalyst loadings, up to 98:2 er).

Unusual Structures and Reactivity of Mixed Metal Cluster Complexes Containing the Palladium/Platinum Tri-t-butylphosphine Grouping

Polynuclear metal carbonyl complexes have a range of applications in chemical research: for example, they can serve as surface models to probe features of heterogeneous catalysis and can perform novel transformations of organic molecules in solutions. Mixed metal complexes can demonstrate bimetallic cooperativity and synergism and can also serve as precursors to multimetallic heterogeneous catalysts that have superior activities and selectivities. This Account describes the results of our recent comprehensive study of the chemistry of mixed metal cluster complexes containing the sterically encumbered M(PBu(t)(3)), M = Pd or Pt, group. This grouping readily adds to the metal-metal bonds of metal carbonyl cluster complexes and modifies their reactivity. We have prepared new, highly electronically unsaturated mixed metal complexes that exhibit unusually high reactivity toward hydrogen. The platinum atom of the Pt(PBu(t)(3)) grouping can bond to as many as five metal atoms, and it can interconvert, sometimes rapidly, between the different bonding modes. The large steric effects of the PBu(t)(3) ligand allowed us to prepare highly unsaturated, stable, mixed-metal complexes, and these complexes react with hydrogen, sometimes reversibly, under very mild conditions to yield polyhydride complexes. Strong evidence suggests that the Pt(PBu(t)(3)) group can also activate metal-hydrogen bonds in other complexes. In the future, we expect that researchers will prepare a greater variety of mixed metal complexes containing the Pd/Pt(PBu(t)(3)) group or other similar bulky groups, and that some of these complexes will exhibit even more unusual chemistry than what we have observed so far.

Synthesis, characterization, electronic structure and catalytic performance of bimetallic and trimetallic nanoparticles containing tin

When anchored on a high-area, siliceous supports, nanoparticle catalysts, consisting of two or three different metals, but totaling no more than twenty atoms in all, exhibit exceptional activities and selectivities in solvent-free, one-step hydrogenation reactions at low temperatures (< 420 K) and much lower pressures (e.g. 30 bar) than those required in current industrial manufacture. The two selective hydrogenations illustrated here are the conversion of (a) cyclododecatriene (CDT) to cyclododecene (CD) and (b) dimethyl terephthalate (DMT) to cyclohexane dimethanol (CHDM); each of these products is extensively used in the polymer industry. All our mixed-metal nanoparticles are derived from an appropriately chosen parent (precursor) mixed-metal carbonyl having phenyl-containing tin ligands, e.g. Ru4(mu4-SnPh)2(CO)12. Various techniques are used to characterize the denuded, anchored cluster catalysts; and it is expected that aberration-corrected high-resolution electron microscopy (and other techniques, which are outlined) will be invaluable in such characterization. Density functional theory has provided important insights into the structures and electronic properties of our catalysts and their precursors.

Photoactivatable BODIPYs Designed To Monitor the Dynamics of Supramolecular Nanocarriers

Self-assembling nanoparticles of amphiphilic polymers can transport hydrophobic molecules across hydrophilic media and, as a result, can be valuable delivery vehicles for a diversity of biomedical applications. Strategies to monitor their dynamics noninvasively and in real time are, therefore, essential to investigate their translocation within soft matrices and, possibly, rationalize the mechanisms responsible for their diffusion in biological media. In this context, we designed molecular guests with photoactivatable fluorescence for these supramolecular hosts and demonstrated that the activation of the fluorescent cargo, under optical control, permits the tracking of the nanocarrier translocation across hydrogel matrices with the sequential acquisition of fluorescence images. In addition, the mild illumination conditions sufficient to implement these operating principles permit fluorescence activation within developing Drosophila melanogaster embryos and enable the monitoring of the loaded nanocarriers for long periods of time with no cytotoxic effects and no noticeable influence on embryogenesis. These photoresponsive compounds combine a borondipyrromethene (BODIPY) chromophore and a photocleavable oxazine within their covalent skeleton. Under illumination at an appropriate activation wavelength, the oxazine ring cleaves irreversibly to bring the adjacent BODIPY fragment in conjugation with an indole heterocycle. This structural transformation shifts bathochromically the BODIPY absorption and permits the selective excitation of the photochemical product with concomitant fluorescence. In fact, these operating principles allow the photoactivation of BODIPY fluorescence with large brightness and infinite contrast. Thus, our innovative structural design translates into activatable fluorophores with excellent photochemical and photophysical properties as well as provides access to a general mechanism for the real-time tracking of supramolecular nanocarriers in hydrophilic matrices.

Self aggregation of supramolecules of nitroxides@cucurbit[8]uril revealed by EPR spectra.

Supramolecular complexation behavior of cucurbiturils with paramagnetic nitroxide spin probes was examined by (1)H NMR, X-ray diffraction studies of crystals, computation, and EPR. Both cucurbit[7]uril (CB7) and cucurbit[8]uril (CB8) form a 1:1 complex with 4-(N,N,N-trimethylammonium)-2,2,6,6-tetramethylpiperidinyl-N-oxy bromide (CAT1). The structure of the complex in the solid state was inferred by X-ray diffraction studies and in the gas phase by computation (B3LYP/6-31G(d)). Whereas ESI-MS data provided evidence for the existence of the complex in solution, indirect evidence was obtained through (1)H NMR studies with a structural diamagnetic analogue, 4-(N,N,N-trimethylammonium)-2,2,6,6-tetramethyl-N-methylpiperidine iodide (DCAT1). The EPR spectrum of the CAT1@CB7 complex consisting of three lines suggested that probe CAT1 is associated with host CB7 such that the nitroxide part is exposed to water. The spectral pattern was independent of the concentration of the complex and the presence of salt such as NaCl. The most interesting observation was made with CB8 as the host. In this case, in addition to the expected three-line spectrum, an additional spectrum consisting of seven lines was recorded. The contribution of the seven-line spectrum to the total spectrum was dependent on the concentration of the complex and added salt (NaCl) to the aqueous solution. The coupling constant for the seven-line spectrum for (14)N-substituted CAT1 is 5 G, and that for the four-line spectrum for (15)N-substituted CAT1 is 7.15 G. The only manner by which we could reproduce the observed spectra by simulation for both (14)N- and (15)N-substituted CAT1@CB8 was by assuming a spin exchange among three nitroxide radicals. To account for this observation, we hypothesize that three CAT1 molecules included within CB8 interact in such a way that there is an association of three supramolecules of CAT1@CB8 (i.e., [CAT1@CB8](3)) in a triangular geometry that leads to spin exchange between the three radical centers. We have established, with the help of 13 additional examples, that this is a general phenomenon. We are in the process of understanding this unusual phenomenon.

Facile introduction of bridging MPh2 groups (MGe, Sn, Pb) into platinum–pentaruthenium and hexaruthenium carbido carbonyl cluster complexes

Abstract The reactions of PtRu5(CO)16(μ6-C) (1) with Ph3GeH and Ph3SnH afforded the trimetallic cluster complexes PtRu5(CO)15(μ-GePh2)(μ6-C) (4) and PtRu5(CO)15(μ-SnPh2)(μ6-C) (5), respectively, in good yields. Both the compounds consist of an octahedral cluster of one platinum and five ruthenium atoms with an interstitial carbido ligand in the center. The bridging CO ligand in 1 was replaced by a bridging GePh2 group in 4 and a bridging SnPh2 group in 5. The lead homologue PtRu5(CO)15(μ-PbPh2)(μ6-C) (6) was obtained from the reaction of 1 with Pb2Ph6. The reaction of PtRu5(CO)15(PMe2Ph)(μ6-C) (2) with Ph3SnH yielded the phosphine derivative of 5, PtRu5(CO)14(μ-SnPh2)(PMe2Ph)(μ6-C) (7). Compound 7 was obtained in a higher yield from the reaction of 5 with PMe2Ph. The reaction of Ru6(CO)14(η6-C6H6)(μ6-C) (3) with Ph3SnH yielded the new hexaruthenium complex Ru6(CO)13(μ-SnPh2)(η6-C6H6)(μ6-C) (8) containing a bridging SnPh2 ligand. Evidence for benzene formation in the formation of compound 5 indicates the fate of the phenyl group that was cleaved from the tin atom in that reaction.

Luminescent charge-transfer complexes: tuning emission in binary fluorophore mixtures.

Charge-transfer (CT) complexes composed of a π-electron-poor naphthalene diimide (NDI) derivative combined with a series of π-electron-rich donors were investigated. Solutions of the CT complexes are nonemissive; however, solid-state complexes and aqueous suspensions display emission that is dependent on the energy of the HOMO of the electron donor. Crystallographic analysis of a pyrene-NDI complex reveals columnar packing and a high degree of frontier molecular orbital (FMO) overlap that likely contributes to the observed optical properties. The fluorescent CT particles are utilized as imaging agents; additional luminescent CT complexes may be realized by considering FMO energies and topologies.

Photodimerization of HCl salts of azastilbenes in the solid state

A simple strategy that is based on conversion of an electron-rich pyridyl to an electron-deficient pyridinium ion is able to orient thirteen of the sixteen olefins investigated towards a single dimer in quantitative yield in the crystalline state. The reagent used is HCl, the method involves grinding the olefin with a drop of HCl and the dimerization is achieved by exposure of the crystalline stilbazolium·HCl salts to light. The weak force involved in modifying the crystal packing is most likely charge-transfer interaction.

New alkyne bridged mixed-metal clusters and studies of their activity for catalytic hydrosilylation of alkynes

Abstract The homologous series of compounds MPt2(CO)5(PPh3)2(PhC2Ph), M=Fe (1), M=Ru (3) and M=Os (4) were obtained in the yields 55%, 24% and 30% from the reactions of Pt(PPh3)2(PhC2Ph) with Fe(CO)5, Ru3(CO)10, and Os(CO)5, respectively. Each of the products was characterized by IR, elemental analysis and a single crystal X-ray diffraction analysis. A second mixed-metal cluster product, Ru2Pt(CO)7(PPh3)2(PhC2Ph) (2) (45% yield) was also obtained from the reaction that yielded 3. In fact, it is obtained in a higher yield than that of 3. Compounds 1, 3, and 4 are isostructural and are comprised of a MPt2 (M=Fe, Ru or Os) triangular metal cluster containing a triply bridging diphenylacetylene ligand. The structure of compound 2 is similar to that of 1, 3, and 4, but contains two ruthenium and one platinum atom in the triangular cluster. One coproduct Os(CO)2(PPh3)2(PhC2Ph) (5) (25% yield) was obtained from the reaction of Os(CO)5 with Pt(PPh3)2(PhC2Ph). Compound 5 contains only one metal atom with an approximately trigonal bipyramidal coordination having the two phosphine ligands in the axial positions and the alkyne in an equatorial site. The reaction of 2 with H2 in refluxing hexane afforded the tetranuclear complex H2Ru2Pt2(CO)8(PPh3)2 (6) (11% yield) that contains an Ru2Pt2 tetrahedral shaped cluster with two bridging hydride ligands. The ability of the solutions of compounds 1–6 and Pt(PPh3)2(PhC2Ph) to produce catalytic hydrosilylation of diphenylacetylene by triethylsilane to yield (E)-[(1,2-diphenyl)ethenyl]triethylsilane at 30°C, and 1,4-bis(trimethylsilyl)butadiyne with triethylsilane to yield (E)-2-triethylsilyl-1,4-bis(trimethylsilyl)-1-buten-3-yne at 60°C was investigated. Pt(PPh3)2(PhC2Ph) was the best catalyst for these reactions. Compound 5 is completely inactive. A combination of evidence suggests that the catalytic activity exhibited by the mixed-metal cluster complexes is produced principally by fragmentation products which are almost certainly mononuclear platinum complexes.

The importance of methylation in the binding of (ferrocenylmethyl)tempammonium guests by cucurbit[n]uril (n = 7, 8) hosts

The extent of methylation of the central nitrogen in (ferrocenylmethyl)tempammonium guests determines the main binding site for complexation by the cucurbit[8]uril host.