Pasteurian Segregation on a Surface Imaged In Situ at the Molecular Level

Abstract

Dedicated to Professor Sir J. Fraser Stoddart on the occasion of his 70th birthdayThe separation of enantiomers, which is an important step in the production of optically active chemicals for use in a variety of applications, is frequently performed using classical diastereomeric resolution, or Pasteurian resolution. [1][2][3][4] This crystallization process relies on the different solubility and crystallization propensity of the diastereomers formed by the enantiomers of a target racemate with a complementary chiral compound (the resolving agent), which forms a complex or a salt with it. The identification of the resolving agents that work for a given racemic mixture in this process relies on an empirical screening, [5] although some rationalization based on phase behavior or arguments based on supramolecular chemistry and molecular recognition can be invoked largely with hindsight. [6] The study of Pasteurian resolution involving surfaces is interesting for a number of reasons, many of which arise from the reduced symmetry possibilities in this low-dimensional environment. [7] Unlike the situation in bulk crystals, spontaneous resolution is observed very frequently on crystalline surfaces wherein domains of opposite handedness arise. [8] Enantiomers often adsorb in mirror-image domains, though their separation and collection is a tremendous task. A demonstration of Pasteurs triage at the nanoscale was shown for mirror-image clusters using the tip of a scanning tunneling microscope (STM) as a nanoscale probe. [9] Diastereomeric interactions on surfaces could be more powerful for practical separations. For instance, transfer of chirality from chemisorbed monolayers to bulk crystals has been used for the selective growth of crystals of one enantiomer in a racemic mixture in solution, which presumably involves diastereomeric interactions between the surface and the crystal nucleus, [11] while for similar cases conglomerates are formed. [12] Visualization of diastereomeric interactions on surfaces is rare. While the observation of diastereomeric interactions for the complex formed between phenylglycine and adenine was seen under ultrahigh vacuum conditions on a metal surface, [10] this process originates from the formation of chiral strips of the prochiral base followed by adhesion of the amino acid to it. It is therefore akin to a spontaneous resolution. There is no information regarding the formation of two-dimensional (2D) crystals of diastereomers in a dynamic system in solution. This observation could be very relevant to separation procedures involving the interaction of chiral molecules with surfaces.Herein we demonstrate diastereoselective adsorption on an achiral surface by surface-mediated complex formation and the in situ visualization at the molecular level by employing STM at the liquid-solid interface (Scheme 1). As part of the process, a 2D crystal is formed, and as such it is a crystallization process, albeit more dynamic than the case of bulk crystals. Although only a small fraction of the molecules...

Chiral segregation driven by a dynamical response of the adsorption footprint to the local adsorption environment: bitartrate on Cu(110)

“…Fig. 4 shows the trimer assembly as the repeat distance of the unit cell is progressively elongated along the [1][2][3][4][5][6][7][8][9][10] direction, from 7 to 10 Cu atoms. We find that there is no significant change in the bitartrate geometry.…”

“…The nanoscale control of chiral molecular assembly at surfaces is of central importance in fields as diverse as chiral separations, 1,2 heterogeneous enantioselective catalysis, 3,4 plasmonics, 5 chiroptical switching 6,7 and biosensors. 8 In all cases, the surface function is profoundly influenced by how individual enantiomers organize at the local and the global level, therefore the outstanding problem in the field is to understand the factors that drive enantiomer assembly and segregation.…”

Exploitation of the Large‐Area Basal Plane of MoS2 and Preparation of Bifunctional Catalysts through On‐Surface Self‐Assembly

“…Considering that the inner HER activity is not high and water molecules are difficult to diffuse into the bulk, it can be concluded that HER mainly concentrates on the surface N sites of Ni(abt) 2 crystal. In this respect, it will be highly material‐saving if Ni(abt) 2 molecules are processed into ultrathin films or even monolayer, which can be potentially realized by bottom‐up on‐surface self‐assembly …”

“…Next, we will explore how to prepare self‐assembled Ni(abt) 2 monolayers on MoS 2 . Many studies have shown that ultrathin self‐assembled organic monolayers can be prepared through a facile liquid‐deposition method, i.e., organic molecules are first dissolved in solvent and self‐assembled organic monolayers can be naturally generated on the substrate on which the mixed solution is dropped. From the preparation process, two key points toward the preparation of self‐assembled Ni(abt) 2 monolayers on MoS 2 can be derived: (i) an appropriate solvent that can dissolve Ni(abt) 2 and (ii) dissolved Ni(abt) 2 molecules that can deposit onto MoS 2 .…”

Stereochemistry of 2D Molecular Crystallization

“…[14] In 2D systems diastereomeric selectivity has been observed for acid-base pairs at the solid-liquid interface. [15] 2D crystallization experiments with helicenes and ortho-annulated carbohelicenes which have outstanding material and chiroptical properties, [16] showed interesting behavior. [17,18] On Cu(111) heptahelicene ( [7]H, C 30 H 18 (Fig.…”