Structure and Dynamics of Individual Diastereomeric Complexes on Platinum: Surface Studies Related to Heterogeneous Enantioselective Catalysis

Substrate Tumbling in a Chemisorbed Diastereomeric α-Ketoester/1-(1-Naphthyl)ethylamine Complex

Scanning tunneling microscopy (STM) data for α-ketoester/1-(1-naphthyl)ethylamine complexes on Pt(111) reveal a tumbling motion that couples two neighboring binding states. The interconversion, resulting in prochiral inversion of the α-ketoester, occurs in single complexes without breaking them apart. This is a surprising observation because the overall motion requires rotation of the α-ketoester away from the surface without branching exclusively into diffusion away from the complex or desorption. The multi-step interconversion is rationalized in terms of sequences of bound states that combine transient H-bond interactions with the chiral molecule and weakened adsorption interactions with the metal. The observation of tumbling in single long-lived complexes is of relevance to self-assembly and directed molecular motion on surfaces, to ligand-controlled surface reactions, and most directly to stereocontrol in asymmetric heterogeneous catalysis.

Designing Sites in Heterogeneous Catalysis: Are We Reaching Selectivities Competitive With Those of Homogeneous Catalysts?

A critical review of different prominent nanotechnologies adapted to catalysis is provided, with focus on how they contribute to the improvement of selectivity in heterogeneous catalysis. Ways to modify catalytic sites range from the use of the reversible or irreversible adsorption of molecular modifiers to the immobilization or tethering of homogeneous catalysts and the development of well-defined catalytic sites on solid surfaces. The latter covers methods for the dispersion of single-atom sites within solid supports as well as the use of complex nanostructures, and it includes the post-modification of materials via processes such as silylation and atomic layer deposition. All these methodologies exhibit both advantages and limitations, but all offer new avenues for the design of catalysts for specific applications. Because of the high cost of most nanotechnologies and the fact that the resulting materials may exhibit limited thermal or chemical stability, they may be best aimed at improving the selective synthesis of high value-added chemicals, to be incorporated in organic synthesis schemes, but other applications are being explored as well to address problems in energy production, for instance, and to design greener chemical processes. The details of each of these approaches are discussed, and representative examples are provided. We conclude with some general remarks on the future of this field.

Highly-active platinum nanoparticle-encapsulated alumina-doped resorcinol–formaldehyde carbon composites for asymmetric hydrogenation

Alumina-doped resorcinol–formaldehyde carbon supported Pt nanoparticles were synthesized and employed in asymmetric hydrogenation with high enantioselectivity, good reusability, and unprecedentedly high TOF.

Adsorption geometry and self-assembling of chiral modifier (R)-(+)-1-(1-naphthylethylamine) on Pt(111)

A mechanistic study on interaction of a chiral modifier - (R)-(+)-1-(1-naphthylethylamine) (R-NEA) - with a single crystalline Pt(111) surface is reported. The details of the adsorption geometry of individual R-NEA molecules and their intermolecular interactions are addressed by combination of infrared reflection absorption spectroscopy (IRAS) and scanning tunneling microscopy (STM). The spectroscopic observations suggest that the molecules are tilted with respect to the underlying metal substrate with the long axis of the naphthyl ring being parallel and the short axis tilted with respect to the surface. In the medium coverage range, formation of directed 3-5 membered chains was observed by STM for the first time, which points to intermolecular bonding between individual molecules and might account for an unusual tilted adsorption geometry deduced from the IR spectra. Based on the STM images revealing the atomic structure of the Pt grid close to the R-NEA chains, we propose the adsorption configuration of NEA fitting both the IRAS and STM data. The obtained results suggest that this strong intermolecular interaction energetically stabilizes the tilted adsorption geometry of the naphthyl ring, which otherwise would be expected to lie flat on the metal to maximize the dispersive interactions. At the coverage close to saturation, R-NEA builds a self-assembled overlayer with hexagonal symmetry, exhibiting intermolecular distances larger than in the directed chains.

Barrier to internal rotation, symmetry and carbonyl reactivity in methyl 3,3,3-trifluoropyruvate

High-resolution rotational spectroscopy was used to investigate the conformational landscape of methyl-3,3,3-trifluoropyruvate, a small, partially-fluorinated molecule, which is of interest because of its chemical properties and reactivity in contrast to the unfluorinated species. Methyl 3,3,3-trifluoropyruvate is also subject to two possible large amplitude motions of the methyl and trifluoromethyl group. However, only the methyl rotor gives rise to the tunneling splitting specific to individual conformers. In the rotational spectrum measured in the frequency region from 6 to 27 GHz, the identified conformers, s-cis and s-trans, were fitted to experimental accuracy, resulting in the accurate determination of the vibrational ground state rotational constants A 0 = 2185.05827 36  MHz ${A}_{0}=2185.05827\left(36\right)\text{\hspace{0.17em}MHz}$ , B 0 = 1023.300   31 17  MHz ${B}_{0}=1023.30031\left(17\right)\text{\hspace{0.17em}MHz}$ , and C 0 = 803.520287 95  MHz ${C}_{0}=803.520287\left(95\right)\text{\hspace{0.17em}MHz}$ for the s-cis conformer, and A 0 = 2706.9024 ( 49 )  MHz ${A}_{0}=2706.9024\left(49\right)\text{\hspace{0.17em}MHz}$ , B 0 = 864.889   539 ( 81 )  MHz ${B}_{0}=864.889539\left(81\right)\text{\hspace{0.17em}MHz}$ , and C 0 = 746.532   896 ( 71 )  MHz ${C}_{0}=746.532896\left(71\right)\text{\hspace{0.17em}MHz}$ for the s-trans conformer. Additionally the barrier heights of the methyl rotor V 3 CH 3 = 363 . 116 94  cm − 1 ${V}_{3}\left({\text{CH}}_{3}\right)=363.116\left(94\right){\text{\hspace{0.17em}cm}}^{-1}$ and V 3 ( CH 3 ) = 389.290 ( 80 )  cm − 1 ${V}_{3}\left({\text{CH}}_{3}\right)=389.290\left(80\right){\text{\hspace{0.17em}cm}}^{-1}$ were obtained for the s-cis and s-trans conformer, respectively.

Supramolecular Assemblies on Surfaces: Nanopatterning, Functionality, and Reactivity

Understanding how molecules interact to form large-scale hierarchical structures on surfaces holds promise for building designer nanoscale constructs with defined chemical and physical properties. Here, we describe early advances in this field and highlight upcoming opportunities and challenges. Both direct intermolecular interactions and those that are mediated by coordinated metal centers or substrates are discussed. These interactions can be additive, but they can also interfere with each other, leading to new assemblies in which electrical potentials vary at distances much larger than those of typical chemical interactions. Earlier spectroscopic and surface measurements have provided partial information on such interfacial effects. In the interim, scanning probe microscopies have assumed defining roles in the field of molecular organization on surfaces, delivering deeper understanding of interactions, structures, and local potentials. Self-assembly is a key strategy to form extended structures on surfaces, advancing nanolithography into the chemical dimension and providing simultaneous control at multiple scales. In parallel, the emergence of graphene and the resulting impetus to explore 2D materials have broadened the field, as surface-confined reactions of molecular building blocks provide access to such materials as 2D polymers and graphene nanoribbons. In this Review, we describe recent advances and point out promising directions that will lead to even greater and more robust capabilities to exploit designer surfaces.

Helical nanoparticle-induced enantiospecific adsorption of N3 dyes

N3 dyes are enantiospecifically adsorbed on silver helical nanoparticles, and the adsorption enantiospecificity is primarily determined by the helical handedness and maximized at a nominal helical pitch of ∼15 nm.