Intercalation polymerisation has been extensively employed for preparing layered nanocomposites.19,20 However, to the best of our knowledge, the use of this protocol to synthesise 2D polymers has not been reported. It is envisaged that a pathway similar to that shown in Figure 1c might occur, when biscyclopropene BCP-0 with a linker is employed (Figure 3a). Thus, the ROMP of BCP-0 will initially generate stereospecifically a linear oligomer/polymer (e.g., Tetra-0) that is structurally similar to PMEV. As shown in Figure 1c, the pendant of the incoming cyclopropene unit and the pendant directly attached to the ruthenium carbene intermediate as in Ru-carb-1 will be in opposite directions. Cross metathesis between these two neighbouring cyclopropene rings on the same side of the polymeric backbone in Tetra-0 is unlikely. Instead, cross metathesis of one of the cyclopropene groups in BCP-0 from the bulk with two cyclopropene rings on the same side of Tetra-0 would lead to the formation of 2D-0. The continuation of these procedures might eventually produce two-dimensional stromaphane SP-0 (Figure 3a). The overall process can be considered to be the self-intercalation polymerisation of BCP-0.
2D-COFs need rigid frameworks.7,8 The incorporation of flexible linkers into the framework may cause defects in the crystal structure, even though highly crystalline, 2D-COFs containing somewhat flexible motifs have recently been explored.39-4 As shown in Figure 2, irrespective of the conformation for alkenyl motifs in stromaphane, the separation between linkers should be in the subnanometre range. Room for the movement of linkers would therefore be fairly limited. It is envisioned that the presence of slightly flexible linker might facilitate the intercalation polymerisation.
Synthesis of BCP-1
Biscyclopropene BCP-1 was obtained in 77% yield via the SN2 reaction of carboxylate CP-138 and a,a’-dibromo-p-xylene (Figure 3b). The benzene triad (three benzene rings) in the linker of BCP-1 has limited flexibility to adopt the right conformation for the chain growth process.
Self-intercalation polymerisation of BCP-1
Treatment of BCP-1 with 10 mol % of G-I in DCM at ambient temperature for 2 h afforded the corresponding crude polymer (hereafter abbreviated as CDP) in 95% yield (Figure 3b). Immediately after workup, CDP was initially sparingly soluble in chloroform, but it gradually formed aggregates that were insoluble in any solvent. Accordingly, immediately after CDP was obtained, characterisation and microscopic examinations were performed Gel permeation chromatography (GPC) analysis of CDP shows a bimodal distribution with Mn values around 6.0 and 12.2 kD (Figure S1).
The 1H NMR spectrum of CDP (Figure S2) exhibits two broad peaks centred at d 5.80 and 6.10 ppm that were assigned to the absorptions of olefinic protons attributed to PMEV backbone and cyclohexadiene fragments, respectively. The ratio was about 4.3 : 1 after deconvolution analysis of these peaks. It is worth noting that this ratio may not reflect the actual composition because of the solubility problem. These results suggest that CDP may constitute two polymeric components with one containing PMEV motifs and the other cyclohexadiene moieties. Another possibility would be a single species containing both PMEV and cyclohexadiene functionalities.
As will be discussed in the next two sections, the flexibility of the benzene triad linker is an impediment for the resolution of the powder XRD and STM images. Nevertheless, the long range ordered structural pattern offers a strong evidence for crystallinity feature and two-dimensional scaffold.
XRD pattern and the structure of stromaphane SP-1
Under optical microscopy, a sample of CDP was found to consist of crystalline SP-1 (Figure S3) and amorphous powder AMP-1 (Figure S4) which were separated manually. A sample of SP-1 in a 0.3 mm capillary tube was subjected to the XRD experiments XRD measurements were performed using beamlines TPS09A and TLS01C2 with wavelengths of 0.082656 nm and 0.103321 nm, respectively. Figure 4a shows the one-dimensional XRD patterns obtained with 0.103321 nm, and the corresponding two-dimensional image pattern is displayed in Figure S5. The results confirm the high crystallinity for SP-1. Further processes (see Methods and Supplementary Information) resulted in having cell constants: a = 6.0455(3) nm, b = 0.92292(4) nm, c = 1.15838(6) nm, and b = 90.649(3)o with monoclinic Pa space group and the agreement factors were Rwp = 1.17% and Rp = 0.72% (Figure S5).
As shown in Figure 4b, the two nitrogen atoms on the linker flanked by the benzene triad are separated by about 2.3 nm and the distance between two benzene rings in the triads is 0.9 nm. The orientation of benzene rings in benzene triads are not homogeneous. The fluxional behaviour of these aromatic motifs reflects the presence of a somewhat disordered crystal structure of SP-1. The centre-to-centre distance between the benzene ring of a triad to the corresponding benzene ring of the adjacent triad is in the range of 0.44-0.56 nm. This spacing represents the span of each monomeric unit and is also the width of the slits in SP-1. In comparison with the cartoons in Figure 2, the two geminal olefinic moieties at the C3-position of each azetidine ring in PMEV skeletons may likely adopt anti-anti conformation. The spacing of 0.44-0.56 nm allows fairly little room for the limited movement of the triad linker. Such a minor structural disorder may not significantly affect the long-range order of crystallinity in stromaphane SP-1. In order to avoid steric congestion, the dihedral angle between the two geminal olefinic groups at the C3-position of azetidine ring ranges from 30 to 42o. In addition, both the simulated XRD structure and the STM image shown in the next section revealed that the triad linker in a ladderphane stripe and the linkers in the two immediate next two ladderphane stripes are in a staggered relationship.
The length of the slit, that is the distance between two quaternary carbon atoms at both ends of the slit, is about 3.3 nm. This spacing would approximately be the separation between two PMEV skeletons in SP-1.
Figure 5 shows the STM images of SP-1 on highly ordered pyrolytic graphite (HOPG). The long-range ordered and striped features shown in Figure 5a confirm the simulation results. Figure 5b further confirms the XRD analysis results that indicate that SP-1 is somewhat rigid and 2D-patch-like polymer. The 0.45 nm separation between adjacent triad linkers further suggests that olefinic moieties in the PMEV skeleton may adopt the E-configuration and anti-anti conformation.
The stripe directions are attributed to the PMEV backbones, and the angles of approximately 120° between the stripes of different patches (Figure 5a) are consistent with the three-fold symmetry of the underlying HOPG. These results suggest fairly strong interactions between the SP-1 and the substrate. Figures 5c and d reveal that, in between stripes, there are other fine structures that are benzene triad linkers. As mentioned above, these linkers are somewhat fluxional, and a deviation from the alignment may occasionally occur. In addition, the bright features that may be attributed to the images of benzene triads, are significantly smaller than what was expected for the benzene rings. As shown in Figure 4b, some benzene rings are tilted from the plane of the 2D scaffold in the simulated structure for SP-1. Indeed, about 25% variation has been found in the distances between two benzene rings in adjacent linkers in this calculated structure. Figure 5d is superimposed with a model calculated from the XRD results. The interactions between SP-1 and HOPG may also account for the structural discrepancy. Regardless of these minor deficiencies, the structural features shown in Figures 4 and 5 are consistent with a 2D-sheet-like structure for stromaphane SP-1.
As shown in Figure 5a and b, the crystalline morphology of SP-1 can spread over 40 × 40 nm2 in a highly regular manner on the HOPG surface. The length of each monomeric unit is about 3.0 nm, and the distance between two adjacent linkers is 0.45 nm. The total number of monomeric units on this area (40 × 40 nm2) would be around 1,200 or 75 monomeric units per 100 nm2.
How can cyclohexadiene be excluded from SP-1?
In the previous sections, both the STM image (Figure 5) and XRD simulation results (Figure 4) suggest the homogeneous structural feature for SP-1. It is immensely striking to see the selective formation of SP-1 with all double bonds in the trans configuration and anti-anti conformation. As shown in Figure 1c, the initial product such as Ru-carb-2 in the chain growth process adopts the trans-configuration and anti-conformation for the newly formed double bond. When the product is a single stranded polymer like PCP, equilibrium may exist between Ru-carb-2anti and Ru-carb-2syn or the like and the latter with the syn conformation may be thermodynamically preferred. However, this kind of equilibrium might not be particularly favourable for the growth of more rigid 2D-framework of SP-1. In other words, the initially formed anti conformation would continuously generate the corresponding anti conformation throughout the polymerisaton process.
During the course of polymerisation, the intermediate species having a Z-double bond might be theoretically generated along with those with the E-double bond. However, the homogeneous nature of SP-1 suggests that the Z-double bond might not be incorporated in the polymeric scaffold. If the Z-double bond were formed and the fragment containing such double bond were linked to the 2D-framework (Figure 6a), the ruthenium carbene moiety (2D-Z-Ru-1) would be trapped into the sub-nanometric gap between the two adjacent pendants. It is known that tricyclohexylphosphine ligand remains ligating to ruthenium in the ruthenium-carbene-catalysed metathesis process.26 The spacing allowed for ruthenium carbene in 2D-Z-Ru-1 might not be sufficient to contain this bulky ligand.
In a similar manner, the tricyclohexylphosphine ligand would not fit into the aperture between the two adjacent pendants in the possible cyclohexadiene-comprising intermediate 2D-CHD-Ru-1. It seems unlikely that the ruthenium carbene species in intermediates similar to 2D-Z-Ru-2 would continue catalysing the chain growth process because of steric congestions. If the positions of Ru-carbene and the Z-double bond are swapped such that the Ru moiety is at the exo position as in 2D-Z-Ru-exo-1 (Figure 6c), severe strain will exist around the Z-double bond.
When ROMP takes place between 2D-E-Ru- 2 and a biscyclopropene (such as BCP-1), the initially formed 2D-Z-Ru-2 with the Z-double bond would have an unreacted cyclopropene group at the end of the linker. This fragment would be movable to generate a space to house the tricyclohexylphosphine ligand as in 2D-Z-Ru-2. After ring closure metathesis, the cyclohexadiene-containing fragment CHD-2 would be liberated from the framework to regenerate 2D-E-Ru-2 that would continue the chain growth process leading to SP-1 (Figure 6b). These hypotheses reasonably explain why the cyclohexadiene moiety was absent in SP-1.