All-carbon materials based on sp2 hybrid atoms, such as fullerenes, carbon nanotubes, and graphene, have been extensively studied due to their significant physicochemical properties and application potential. Another unusual family of allotropic carbons is the ring[n]carbon (CN) composed of two coordination sp hybrid atoms.
They have been studied in the gas phase since the 20th century, but their high reactivity means that condensed phase synthesis and real space characterization have been challenging, making their exact molecular structure controversial. It wasn't until 2019 that an isolated C18 was generated on the surface and its polycrystalline structure was revealed by bond-resolved atomic force microscopy, followed by another report on C16. The work on C18 has led to theoretical studies clarifying the structure of cyclo[n]carbon below C100, although the synthesis and characterization of smaller CN allotropes remains difficult.
Here, fromTongji UniversityofXu Weiand other researchersModificationsBeforeSurface synthesis method, passedTip inductionofPerchlorinated naphthalene(c10cl8) andAnthracene(c14cl10)moleculesofDehalogenationwithAnti-Bergman open loop, respectivelyPreparationnowRingsCarbon(c10) andRingsCarbon(c14)。Related** titled "On-Surface Synthesis of Aromatic Cyclo[10]Carbon and Cyclo[14]Carbon" was published on November 29, 2023natureAbove.
Early theories **C10 is the watershed between annular (for n 10) and linear (for n < 10) CN and is the largest cumulative alkene aromatic cyclic carbon, while C14 is thought to be the cumulative alkene structure of C10, which is transformed by Pehrs distortion into the polyalkyne structure seen in C18. Therefore, it is of particular interest to study the structure of C10 and C14.
Advanced scanning tunneling microscopy (STM) and atomic force microscopy (AFM), especially when performed with CO-modified tips, enable both analytical bond characterization of molecular structures and triggering chemical reactions by manipulating individual atoms. Importantly, in a single-molecule, high-resolution AFM image, the polyalkyne groups can be clearly distinguished from the cumulative alkene groups (i.e., the characteristic bright feature of the triple bond and the uniform linearity feature of successive double bonds, respectively).
A critical first step is the precise synthesis of C10 and C14 in condensations, for which the researchers followed the previous surface synthesis method, in which the molecules are stabilized on the surface at extremely low temperatures. Be inspired by Bergman's reactions and draw on the previous onesA method for generating carbon cluster ions, InvestigatorThe reaction protocol shown in Figure 1 was used。As shown in Figure 1a, the Bergman and inverse Bergman reactions in solution involve one cyclization and one ring-opening reaction;The inverse Bergman ring-opening reaction has also been shown to be performed on surfaces (Figure 1B). The researchers used fully halogenated naphthalene (octachloronaphthalene, C10Cl8) and anthracene (Decachloroanthracene, C14Cl10) as molecular precursors with the aim of generating C10 and C14 on the surface through tip-induced dehalogenation and inverse Bergman reactions (Fig. 1C, D).
Figure 1Reaction scheme for the formation of cyclo[10]carbon and cyclo[14]carbon
Figure 2Four possible structures of ring[10] carbon
Figure 2 and its subplots illustrate the theoretically possible product structure of four cyclo[10]carbon. Of the four structures, two polyalne structures have D5H and C5H symmetry, showing non-zero bond length alternation (BLA ≠ 0) (Fig. 2A, B), while two accumulating alkene structures have D10H and D5H symmetry, showing zero bond length alternation (BLA = 0) (Fig. 2C, D). Compared to the polyalkyne structure of C18, calculations at different theoretical levels (e.g., density functional theory (DFT) and coupled cluster methods)**The ground state of C10 has the D5H cumulative alkene geometry shown in Figure 2d, with BLA = 0 and bond angle alternation (BAA) ≠ 0.
Figure 3Precursors, intermediates, and products (C10) are generated on the surface
In the Atomic Force Microscopy (AFM) image of Figure 3a, the Cl atoms of the C10Cl8 molecule show different brightness contrasts, suggesting a difference in adsorption height due to the spatial obstruction of the Cl atoms in the molecule. This was further confirmed by AFM simulations (Fig. 3a, (iv) and (v)) and AFM images with Laplace filtering (Fig. 3a, (vi)).
When performing dehalogenation experiments, positioning the tip on a single C10Cl8 molecule and gradually increasing the bias often results in the loss of two or three Cl atoms, forming C10Cl6 or C10Cl5 intermediates. The structures of these intermediates were characterized by AFM imaging (Fig. 3b, (ii)-(vi) and Fig. 3c, (ii)-(vi)) and confirmed by AFM simulation.
Eventually, further voltage sweeps induced complete dehalogenation, resulting in the final C10 product. The STM and AFM images (Figure 3E, (II)-(III)) clearly show the final product containing a carbon ring that can be definitively attributed to a single C10. The C10 molecule in the AFM image exhibits homogeneous characteristics similar to the cumulative alkene structure, indicating that it has a cumulative alkene structure. Further analysis of the AFM images (Fig. 3E, (vi)) revealed that the C10 structure deviated slightly from the perfect circle, possibly related to the D5H symmetry of the theoretical** C10.
Figure 4Surface-generated intermediates, products (C14), and AFM simulations of C14 under different BLAs
Figure 4 and its subplots show the possible structure and synthesis of cyclo[14]carbon.
Figure 4a illustrates six possible C14 structures. Based on the calculations of the present theory, the ground state of C14 is a Percee transition intermediate between the cumulative alkene structure of C10 and the polyalne structure of C18, with bond angle alternation (BAA)253° and small bond length alternate (BLA) 005å。This structure corresponds to the C7H intermediate.
In the experiment, C14Cl10 precursor is used to generate C14 on a bilayer NaCl surface. C14Cl10 shows non-planar configurations in STM and AFM images. Complete dehalogenation is induced by atomic manipulation, accompanied by a two-step inverse Bergmann ring-opening reaction to form a C14 product on the surface with a yield of about 24%. AFM imaging ((I)-(III)) of Figure 4B shows that the final product contains a carbon ring of a single C14.
In all AFM images, C14 exhibits a C10-like accumulator signature, despite the calculated BLA of 005å。As a result, C14 is thought to have a cumulative alkene-like structure that is distinctly different from the characteristic bright characteristics of polyalnes C18 and C16. The AFM image simulation in Figure 4E further explores the differences between the structures of different BLAs.
The formation of ring[14] carbon and the rearrangement of the skeleton were explored by studying intermediates (Fig. 4f-h). The dehalogenation of the C14Cl6 intermediate leads to the formation of C14Cl4, suggesting that the first step of the reverse Bergman reaction has occurred. Further dehalogenation yields C14Cl1, indicating that the second step of the inverse Bergman reaction has occurred. Most of the C-C bonds in C14Cl1 have reduced BLA compared to C14Cl4 intermediates.
In summary, the researchers in 4The successful formation of aromatic ring [10] carbon and ring [14] carbon on the surface of the bilayer NaCl Au(111) by atomic manipulation at 7 K enabled the confirmation of the Cumulus structure of C10 by bond-resolved AFM imaging, as theoretical. More interestingly, experimental AFM images of the Perce transition intermediate C14 also show the characteristics of cumulonimbus clouds. Although the AFM imaging resolution is not sufficient to detect the molecule, the 005 alternates between small bond lengths, but it does identify C14 as an intermediate structure between cumulative C10 and multimeric C18. The researchers expect that complements the researchers and previous surface synthesis strategies will enable the formation of other cyclo[n]carbons that may exhibit interesting properties.
References
sun, l., zheng, w., gao, w. et al. on-surface synthesis of aromatic cyclo[10]carbon and cyclo[14]carbon. nature 623, 972–976 (2023).
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