Jian-guo Wang

Nitrogen-Doped sp2-Hybridized Carbon as a Superior Catalyst for Selective Oxidation

Current approaches for efficient C H bond activation are usually mediated by heterogeneous or homogeneous catalysts. The basis is the employment of transition metals or organometallic centers, which is pivotal for the successful attack on the targeted C H bonds. However, we have reported that it is feasible to use carbon-based nanomaterials to activate short-chain alkanes in catalytic dehydrogenation reactions although relatively high reaction temperatures are required. It is of particular interest to know whether it is possible to activate C H bonds to get high value-added products at a moderate reaction temperatures by using cheap metal-free catalysts. To this end, an elegant approach using metalor boron-doped carbon nitrides as catalysts has been developed for the selective oxidation of allylic and benzylic hydrocarbons in organic solvents with moderate conversion. Attempts to achieve higher activity also include the application of N-alkoxysulfonyloxaziridines for the activation of C(sp) H bonds, although a complicated catalytic system for efficient reaction circulation was required. Layered carbon, that is, highly exfoliated graphitic structures containing one or a few graphene layers, has an unconventional electronic structure, which was speculated to have a high chemical reactivity. Indeed, researchers observed that layered carbon can catalyze hydrogenation, ring-opening polymerization, and C H oxidation reaction, and that it could serve as a support for metal oxide catalysts. Herein we describe nitrogen-doped graphene materials that can activate the benzylic C H bond with exceptionally high activity. The nitrogen atoms introduced are preferentially bound at graphitic sites in the carbon framework. This induces high charge and spin density at the adjacent ortho carbon, which promotes the formation of reactive oxygen species and the materials display exceptional catalytic activity even at room temperature. Firstly, we examined the oxidation of ethylbenzene in aqueous phase with tert-butyl hydroperoxide (TBHP) as the oxidant and without using catalyst. However, no obvious activity was observed by GC after a reaction time of 24 h (Table 1, entry 1). Then we used a graphene sample prepared by the arc-discharge method (referred to as Arc-C) as the catalyst for this reaction. Surprisingly, Arc-C activated ethylbenzene at 353 K to generate acetophenone in 20.7% yield (Table 1, entry 2). As Arc-C had been prepared by a directcurrent arc-discharge method with a pure graphite rod as the electrode in an NH3/He atmosphere, besides trace nitrogen (0.7%), no element other than carbon was detected by elemental analysis (EA) (oxygen cannot be detected by this method). The full X-ray photoelectron spectrum showed a C content of 97.9% and low amounts of nitrogen and oxygen of 0.9% and 1.1%, respectively. This promising observation suggests that it is layered carbon material itself that catalyzed the oxyfunctionalization of the hydrocarbon. As Arc-C has a relatively low surface area (61.3 mg ) which is an obvious drawback when it is used as catalyst, we decided to use the highly exfoliated layered carbon (multilayer graphene) with much higher surface area (423.6 mg ) in order to obtain a better catalytic performance. The layered carbon (referred to as LC) was prepared by the pyrolytic method with graphene oxide as the precursor. However, this material showed similar acetophenone yield to that of Arc-C (Table 1, entry 4) although its surface area was around seven times higher than that of Arc-C. This result suggests that the catalytic activity is not directly related to the surface area of the catalysts. It is worth noting that this material also contains trace amount of nitrogen (0.4%). In the next phase, we investigated the relationship between the nitrogen content of the catalyst and its catalytic activity. We prepared nitrogen-modified layered carbon with different nitrogen loadings by a gas-phase CVDmethod using acetonitrile as the nitrogen source. The nitrogen introduced [*] Y. Gao, Dr. G. Hu, Prof. Dr. Z. Shi, Prof. Dr. D. Ma Beijing National Laboratory for Molecular Sciences College of Chemistry and Molecular Engineering Peking University, Beijing 100871 (China) E-mail: dma@pku.edu.cn

Guanine- and Potassium-Based Two-Dimensional Coordination Network Self-Assembled on Au(111)

In this study, through the choice of the well-known G-K biological coordination system, bioligand-alkali metal coordination has for the first time been brought onto an inert Au(111) surface. Using the interplay between high-resolution scanning tunneling microscopy and density functional theory calculations, we show that the mobile G molecules on Au(111) can effectively coordinate with the K atoms, resulting in a metallosupramolecular porous network that is stabilized by a delicate balance between hydrogen bonding and metal-organic coordination.

Supramolecular Porous Network Formed by Molecular Recognition between Chemically Modified Nucleobases Guanine and Cytosine

The involvement of surfaces in the origin of the first genetic molecules on Earth has long been suggested. Prior to the emergence of nucleic acid polymerases in the prebiotic soup, the self-assembly of primitive nucleobase building blocks may have relied on surface-mediated recognition events which catalyzed the formation of a covalent backbone in prototype oligonucleotides that subsequently may have functioned as templates in a primitive copying mechanism. This initial replication process may have been catalyzed by surfaces or chemical substances in solution—including RNA itself, as postulated in the RNA world hypothesis. Today, the role and the relative importance of the basic, fundamental driving forces for nucleic acid replication such as base pairing, base stacking, and steric effects are still under intense debate. Watson–Crick hydrogen bonding has traditionally been thought to be a prerequisite for high-fidelity DNA replication. However, recent studies on nucleobase analogues with the same size and shape as the natural ones but without relevant hydrogen-bonding groups have revealed that these analogues can recognize each other with high fidelity when incorporated into DNA sequences in vivo. Watson–Crick hydrogen bonding thus seems not to be a requisite for the selectivity of base pairing in DNA replication. However, in the absence of polymerases in the prebiotic soup, Watson– Crick hydrogen bonding may have played a more crucial role in the molecular recognition between the nucleobase building blocks at surfaces and for further polymerization. In support of this postulation, molecular recognition between complementary bases, most likely driven by hydrogen bonding alone, has already been observed both at the liquid/solid (HOPG) interface and on the noble Au(111) surface under extreme ultrahigh vacuum (UHV) conditions. These previous experiments were, however, conducted with nucleobases alone, and hence did not take the presence of deoxyribose into account. It is therefore of utmost importance to explore the role that Watson–Crick hydrogen bonding plays at surfaces in chemical structures that mimic nucleotides so as to address the fundamental question of how the polymerization of nucleotides may have started in the prebiotic soup in the absence of enzymes. The development of the scanning tunneling microscopy (STM) technique has advanced our understanding of supramolecular self-assembly systems on surfaces and has allowed intermolecular interactions to be explored at the submolecular scale. Herein we show by using a combination of high-resolution STM imaging and density functional theory (DFT) that sequential co-deposition of N-aryl-modified nucleobases cytosine (C) and guanine (G) onto the Au(111) surface under UHV conditions results in the formation of highly ordered supramolecular porous networks, where Watson–Crick hydrogen bonding between chemically modified C and G molecules plays the primary role in their stabilization. As the N-arylation of the nucleobases has been performed on the nitrogen atom normally attached to the sugar moiety in DNA or RNA (Scheme 1), these N-aryl-modified nucleobases thus represent two-dimensional (2D) structural mimics of naturally occurring nucleotides. The current results outline a new route for directing the self-assembly of nucleobase-derived nanostructures at the surface. Furthermore, the observed [*] Prof. W. Xu, Dr. M. F. Jacobsen, Dr. M. Yu, Prof. E. Laegsgaard, Prof. I. Stensgaard, Prof. T. R. Linderoth, Prof. J. Kjems, Prof. K. V. Gothelf, Prof. F. Besenbacher Interdisciplinary Nanoscience Center (iNANO) and Center for DNA Nanotechnology (CDNA), Department of Physics and Astronomy, Department of Chemistry, and Department of Molecular Biology, Aarhus University 8000 Aarhus C (Denmark) E-mail: fbe@inano.au.dk