The assembly line: self-assembling nanocars

Certain molecular structures that have controlled rolling properties on surfaces have been termed as nanocars. The syntheses of nanocars are generally multi-stepped. We report here the synthesis of two nanocars by a process resembling an assembly line where front and rear portions are attached using hydrogen bonding and metal complexation. Nanocars 1a and 2a,b were designed using 2-pyridone moieties for hydrogen bonding and terpyridyl ligands for metal complexation, respectively.     

Design and Manipulation of a Minimalistic Hydrocarbon Nanocar on Au(111)

Nanocars are carbon-based single-molecules with a precise design that facilitates their atomic-scale control on a surface. The rational design of these molecules is important in atomic and molecular-scale manipulation to advance the development of molecular machines, as well as for a better understanding of self-assembly, diffusion and desorption processes. Here, we introduce the molecular design and construction of a collection of minimalistic nanocars. They feature an anthracene chassis and four benzene derivatives as wheels. After sublimation and adsorption on an Au(111) surface, we show controlled and fast manipulation of the nanocars along the surface using the tip of a scanning tunneling microscope (STM). The mechanism behind the successful displacement is the induced dipole created over the nanocar by the STM tip. We utilized carbon monoxide functionalized tips both to avoid decomposition and accidentally picking the nanocars up during the manipulation. This strategy allowed thousands of maneuvers to successfully win the Nanocar Race II championship.     

Recent progress on nanovehicles

Nanovehicles are a new class of molecular machines consisting of a molecular scale chassis, axles, and wheels, that can roll across solid surfaces with structurally defined directions making them of interest to synthetic chemists, surface scientists, chemical engineers, and the general car enthusiast. In this tutorial review, following a brief introduction to the development of nanomachines, our recent progress on the nanovehicle project is presented including the design, synthesis, and testing of a series of nanocars, nanotrucks, and motorized nanocars.     

Synthesis of a dipolar nanocar

The synthesis of a nanocar with a built-in strong net dipole is disclosed. The synthesis was executed to explore a more effective handle on electric-field-induced rolling of nanocars.     

Diffusion of fullerene-based nanocars on the surface of a gold crystal

Based on molecular dynamics simulations of nanocars with fullerene wheels on the surface of an Au (110) gold crystal, estimates of diffusion coefficients are reported.     

Transition to organic materials science. Passive, active, and hybrid nanotechnologies

This article covers the author's transition from small molecule organic synthesis into polymeric materials and nanotechnology which led to receipt of the Arthur C. Cope Scholar Award in 2007. This includes his start in organometallic reaction development, synthesis of precisely controlled oligomers, conjugated polymers, planar conjugated polymers, and his work on fullerenes. Also mentioned are the people of influence in his life during that formative period. The meaning of nanotechnology is explained in light of bottom-up vs top-down construction and then more specifically related to the passive, active, and hybrid sides of nanotechnology research. These three areas are explained using examples from the author's laboratory: from the passive side, functionalization of carbon nanotubes and their use in composites; from the active side, molecular electronics and nanocars; and finally, the hybrid side, complementing silicon with molecules.     

Synthesis of a nanocar with an angled chassis. Toward circling movement

Nanocars with an angled chassis have been synthesized and imaged using scanning tunneling microscopy. These angled chassis nanocars were designed to further our understanding of the directional control and surface-rolling capabilities of this class of nanomachines. The alkylated carbazole inner core might enable the molecular scaffold to produce circular rolling motions of the nanovehicles on surfaces.     

Surface-rolling molecules

Design, syntheses, and testing of new, fullerene-wheeled single molecular nanomachines, namely, nanocars and nanotrucks, are presented. These nanovehicles are composed of three basic components that include spherical fullerene wheels, freely rotating alkynyl axles, and a molecular chassis. The use of spherical wheels based on C60 and freely rotating axles based on alkynes permits directed nanoscale rolling of the molecular structure on gold surfaces. The rolling motion observed by STM resembles the same motion performed by macroscopic entities in which rolling occurs perpendicular to the axles. A new synthesis methodology, in situ ethynylation of fullerenes, was developed for the realization of the fullerene-wheeled molecular machines. Four generations of the fullerene-wheeled structures were developed, and the latest fourth generation nanocar, 3b, along with three-wheeled triangular compounds, 4a and 4b, provided definitive evidence for fullerene-based wheel-like rolling motion, not stick-slip or sliding translation. The studies here underscore the ability to control directionality of motion in molecular-sized nanostructures through precise molecular design and synthesis.     

Graphs to chemical structures. 4: Combinatorial enumeration of planted three-dimensional trees as stereochemical models of monosubstituted alkanes

Planted three-dimensional (3D) trees, which are defined as a 3D version of planted trees, are enumerated by means of Fujita’s proligand method formulated in Parts 1–3 of this series [Fujita in Theor Chem Acc 113:73–79, 80–86, 2005; Fujita in Theor Chem Acc 115:37–53, 2006]. By starting from the concepts of proligand and promolecule introduced previously [Fujita in Tetrahedron 47:31–46, 1991], a planted promolecule is defined as a 3D object in which the substitution positions of a given 3D skeleton are occupied by a root and proligands. Then, such planted promolecules are introduced as models of planted 3D-trees. Because each of the proligands in a given planted promolecule is regarded as another intermediate planted promolecule in a nested fashion, the given planted promolecule is recursively constructed by a set of such intermediates planted promolecules. The recursive nature of such intermediate planted promolecules is used to derive generating functions for enumerating planted promolecules or planted 3D-trees. The generating functions are based on cycle indices with chirality fittingness (CI-CFs), which are composed of three kinds of sphericity indices (SIs), i.e., a _d for homospheric cycles, c _d for enantiospheric cycles, and b _d for hemispheric cycles. For the purpose of evaluating c _d recursively, the concept of diploid is proposed, where the nested nature of c _d is demonstrated clearly. The SIs are applied to derive functional equations for recursive calculations, i.e., a(x), c(x ²), and b(x). Thereby, planted 3D-trees or equivalently monosubstituted alkanes as stereoisomers are enumerated recursively by counting planted promolecules. The resulting values are collected up to 20 carbon content in a tabular form. Now, the enumeration problem initiated by mathematician Cayley [Philos Mag 47(4):444–446, 1874] has been solved in such a systematic and integrated manner as satisfying both mathematical and chemical requirements.     

Burnside Rings: Application to Molecules of Icosahedral Symmetry

The Burnside ring, B(G), of a group G is the set of isomorphism classes of orbits of G together with the operations of addition and product. The addition is defined as the disjoint union, and the product as the Cartesian product. This paper describes basic facts about this algebraic structure and develops some applications in chemistry, as the labelling of atoms in molecules of high symmetry and the construction of symmetry-adapted functions. For illustrating such applications, the concept of Burnside ring is applied to the icosahedral symmetry. Sets of points which are isomorphic to the orbits of the I group are described and the multiplication table of B({I}) is obtained from the table of marks. This multiplication table allows us to obtain an elegant labelling of the atoms of the buckminsterfullerene which is consistent with the icosahedral symmetry. Also, we obtain complete sets of symmetry-adapted functions for the buckminsterfullerene which span the Boyle and Parker's icosahedral representations.     

Theoretical analysis of K-region and bay-region indicators of carcinogenicity

The correlation of the K-region carcinogenicity indices of Pullman and of Mainster and Memory with bay-region carbocation delocalization energy (BCDE ) is displayed and shown to be maintained at the perturbational molecular orbital (PMO ) level. PMO algebraic relations are examined to reveal the underlying relationships. Some correlation between BCDE and K-region indices exists simply because carbocation formation occurs preferentially on carbons which are bonded to positions α (adjacent) to ring fusion centers. Sampling a K region is synonymous with sampling such α carbons. However PMO analysis shows that a very strong correlation exists between BCDE and indices for a certain type of K region (called “primitive”). It is further shown that high BCDE is favored by a straight-line chain of benzene rings as one moves away from the bay region. Kinks in the chain will lower the BCDE , and the closer the kink is to the bay region, the greater is its effect. Effects of methylation upon BCDE are shown to correlate with K-region indices if the methylation occurs at a carbon which is in the opposite class as the carbon to which the bay-region carbocation is attached.     

Synthesis and optical characterization of new ketocyanine dyes with extended polymethine chaines

New symmetrical and unsymmetrical near-infrared absorbing ketocyanine-type dyes are synthesized and their optical characterization is done. The relationships between their structure and optical properties are discussed. The synthesized ketocyanine dyes show a positive solvatochromism and are poorly fluorescent. A new procedure for the preparation of N,N,N′,N′-tetramethylvinamidinium perchlorate, which was a very useful reagent for the synthesis of various heterocyclic compounds as well as for the preparation of cyanine dyes is developed.     

Crystal Structure Of Betulin Ethanol Solvate

The crystal structure of the 1:1 ethanol betulin solvate, a widely spread natural compound, is determined. The betulin solvate crystallizes in orthorhombic symmetry in the P2₁2₁2₁ space group, Z= 4. The unit cell parameters are as follows: a = 7.0159(1) ?, b = 12.4425(2) ?, c = 33.7500(5) ?; V = 2946.22(8) ?³. Betulin molecules are hydrogen bonded to each other and ethanol molecules, as a result of which the layers are distinguished in the structure, which are perpendicular to the crystallographic direction. Inside the layer, all molecules are hydrogen bonded, while the layers are linked by van der Waals interactions.     

Synthesis of 1,2-Dihydropyridines, 2,3-Dihydro-4(1H)-pyridinone,and 1,2,3,4-Tetrahydropyridines via N-Acyl N,O-Hemiacetal Formation

New procedures are described for the synthesis of α,β-ethylenic and acetylenic aldehydes from 2-butene- and 2-butyne-1,4-diol, respectively (see Scheme 1). These are applied to the preparation of a particular δ-acetylamino-α,β-ethylenic aldehyde ((E)- 5 ) as well as of its acetylenic analogue 15 . On heating in the presence of a silyl enol ether, the former undergoes a complete dehydrative cyclization affording the N-acetyl-1,2-dihydropyridine 19 . The addition of HCl to aldehyde (E)- 5 results in the production of the 4-chloro-1,2,3,4-tetrahydropyridine 22 which is hydrolyzed to the corresponding alcohol 23 on silica gel. Similarly, the addition of HCl or HBr to the δ-acetyl-amino-α,β-acetylenic aldehyde 15 leads to the previously unknown 4-halo-1,2-dihydropyridines 26 ; these are easily hydrolyzed to the 2,3-dihydro-4(1H)-pyridinone 27 . The ring-forming process involves a N-acyl N,O-hemiacetal as intermediate which is eventually dehydrated.     

Microbial photodegradation of aminoarenes metabolism of 2-amino-4-nitrophenol byRhodobacter capsulatus

The phototrophic bacteriumRhodobacter capsulatus photoreduces 2,4-dinitrophenol to 2-amino-4-nitrophenol, which is further metabolized by an aerobic pathway that is also light-dependent. The catabolism of 2-amino-4-nitrophenol requires O₂ and the presence of alternative carbon (C) and nitrogen (N) sources, preferably acetate and ammonium.Rhodobacter capsulatus B10, a bacterium unable to assimilate nitrate, releases negligible amounts of nitrite when growing with 2-amino4-nitrophenol, thus suggesting that an oxygenase, nitrite-producing activity is not involved in the metabolization of the compound. The diazotrophic growth ofR. capsulatus increases in the presence of 2-amino4-nitrophenol, but growth with ammonium is clearly inhibited by the compound. Mutant strains ofR. capsulatus B10, which are affected innifHDK, nifR1, ornifR4 genes, unable to fix dinitrogen, do not grow with 2-amino-4-nitrophenol as the sole N source. This indicates that the compound cannot be used as a N source. Thenif mutants degrade 2-amino4-nitrophenol to the same extent as the wild-type in the presence of ammonium. The compound is not used as a C source by the bacterium, either. Aromatic stable intermediates, such as 2,4-diaminophenol or 4-nitrocatechol, are not detectable in microaerobic cultures ofR. capsulatus growing with 2,4-dinitrophenol or 2-amino-4-nitrophenol.     

Anionic syntheses of terminally functionalized ethylene oligomers

Synthetic procedures for preparation of terminally functionalized linear ethylene oligomers are described. The preferred synthetic method is anionic oligomerization of ethylene with n-butyllithium–tetramethylethylenediamine and electrophilic substitution of the living oligomer so-formed. Conditions and procedures for subsequent chemistry to elaborate the end groups of these oligomers are described. These procedures afford strictly linear ethylene oligomers which contain a wide variety of end groups and which range in molecular weight from 1000 to 4500 (M_n). The product oligomers were characterized spectroscopically as toluene-d₈ solutions at 110°C using multinuclear NMR, FT-IR, fluorescence, and UV-visible spectroscopies as appropriate. Alternative stepwise approaches to such oligomers are also discussed.     

Process Development of the Copper(II)-Catalyzed Dehydration of a Chiral Aldoxime and Rational Selection of the Co-Substrate

The access towards chiral nitriles remains crucial in the synthesis of several pharmaceuticals. One approach is based on metal-catalyzed dehydration of chiral aldoximes, which are generated from chiral pool-derived aldehydes as substrates, and the use of a cheap and readily available nitrile as co-substrate and water acceptor. Dehydration of N-acyl α-amino aldoximes such as N-Boc-l -prolinal oxime catalyzed by copper(II) acetate provides access to the corresponding N-acyl α-amino nitriles, which are substructures of the pharmaceuticals Vildagliptin and Saxagliptin. In this work, a detailed investigation of the formation of the amide as a by-product at higher substrate loadings is performed. The amide formation depends on the electronic properties of the nitrile co-substrate. We could identify an acceptor nitrile which completely suppressed amide formation at high substrate loadings of 0.5 m even when being used with only 2 equivalents. In detail, utilization of trichloroacetonitrile as such an acceptor nitrile enabled the synthesis of N-Boc-cyanopyrrolidine in a high yield of 92 % and with full retention of the absolute configuration.     

Complexes of Carbon Monoxide and Its Relatives: An Organometallic Family Celebrates Its Birthday

Nickel tetracarbonyl was discovered one hundred years ago. Its hundredth birthday deserves to be celebrated not only by metal carbonyl chemists. The pioneering work of Mond, Langer, and Quincke has led to developments which, particularly in the last 30 years, have had consequences in many areas outside that of metal carbonyl chemistry. The parent ligand of the family, CO, has been joined by a series of relatives which are isoelectronic with it and which in some cases are even more effective as π acceptors. These are in the main extremely reactive molecules such as CS, CNH, and C= CH₂, which, though very short-lived in the free state, form very stable complexes with transition metals. This article documents the family relationships by structure and reactivity comparisons; attention is also drawn to the synthetic potential of the metal carbonyl analogues, which is still virtually untapped. The final section ventures a look towards the next 100 years, which promise to be just as exciting as the past century.     

Synthesis and Aggregation Behavior of meso‐Sulfinylporphyrins: Evaluation of S‐Chirality Effects on the Self‐Organization to S–Oxo‐Tethered Cofacial Porphyrin Dimers

The synthesis and aggregation behavior of meso‐sulfinylporphyrins are described. The copper‐catalyzed C–S cross‐coupling reaction of a meso‐iodoporphyrin with benzenethiol and n‐octanethiol has proved to be an efficient method for the synthesis of meso‐sulfanylporphyrins, which are oxygenated by m‐chloroperbenzoic acid to produce the corresponding meso‐sulfinylporphyrins. Optically active zinc meso‐sulfinylporphyrins were successfully isolated by means of optical resolution of the racemates on a chiral HPLC column. Zinc sulfinylporphyrins readily undergo self‐organization through S–oxo–zinc coordination to form cofacial porphyrin dimers in solution, in which the hetero‐ and homodimers are present as a diastereomeric mixture. The aggregation modes of the S–oxo‐tethered porphyrin dimers were fully characterized by ¹H NMR, IR, and UV/Vis spectroscopy as well as DFT calculations on their model compounds, thus revealing that the self‐aggregation behavior depends on the combination of S chirality. The absolute configurations at the sulfur center can be determined by the exciton‐coupled CD method. The observed self‐association constant for the S–oxo‐tethered dimerization of (S)‐phenylsulfinylporphyrin in toluene is larger than that in dichloromethane, which reflects the difference in dipole moments between the homodimer and the monomer. In cyclic and differential pulse voltammetry, the first oxidation process of the cofacial dimers is split into two reversible steps, which indicates that the initially produced π radical cations are delocalized efficiently between the two porphyrin rings. The present findings demonstrate the potential utility of meso‐sulfinyl groups as promising ligands for investigating the effects of peripheral chirality on the structures and optical and electrochemical properties of metal‐assisted porphyrin self‐assemblies.