Adaptive coordination assemblies based on a flexible tetraazacyclododecane ligand for promoting carbon dioxide fixation

Coordination hosts based on flexible ligands have received increasing attention due to their inherent adaptive cavities that often show induced-fit guest binding and catalysis like enzymes. Herein, we report the controlled self-assembly of a series of homo/heterometallic coordination hosts (Me4enPd)2n(ML)n [n = 2/3; M = Zn(ii)/Co(ii)/Ni(ii)/Cu(ii)/Pd(ii)/Ag(i); Me4en: N,N,N′,N′-tetramethylethylenediamine] with different shapes (tube/cage) from a flexible tetraazacyclododecane-based pyridinyl ligand (L) and cis-blocking Me4enPd(ii) units. While the Ag(i)-metalated ligand (AgL) gave rise to the formation of a (Me4enPd)4(ML)2-type cage, all other M(ii) ions led to isostructural (Me4enPd)6(ML)3-type tubular complexes. Structural transformations between cages and tubes could be realized through transmetalation of the ligand. The buffering effect on the ML panels endows the coordination tubes with remarkable acid–base resistance, which makes the (Me4enPd)6(ZnL)3 host an effective catalyst for the CO2 to CO32− conversion. Control experiments suggested that the integration of multiple active Zn(ii) sites on the tubular host and the perfect geometry match between CO32− and the cavity synergistically promoted such a conversion. Our results provide an important strategy for the design of adaptive coordination hosts to achieve efficient carbon fixation.


Introduction
Carbon dioxide (CO 2 ) in the atmosphere has been rising due to humans' reliance on fossil fuels, causing a range of climate problems such as rising temperature and changing sea-levels.
To eliminate such problems, it is necessary to develop efficient and economic carbon xation technologies. 1,2 In order to transform CO 2 into valuable compounds, various carbon xation technologies such as metal-participated chemical conversions, 3 electrocatalysis, 4 and photocatalysis 5,6 have been developed. Ideally, the carbon xation process itself should avoid CO 2 emissions to achieve carbon neutralization. However, it remains a big challenge to develop such carbon xation processes.

Results and discussion
Controlled self-assembly and structural transformations between coordination tubes 1 and 3, and cages 2 The tetraazacyclododecane moiety located at the center of ligand L is prone to being metalated due to the macrocyclic effect. Indeed, treatment of L with Zn(NO 3 ) 2 led to the quantitative formation of the ZnL metalloligand, as conrmed by 1 HNMR (Fig. 1A and B, S1-S8 †) and single crystal X-ray diffraction (SCXRD) studies. In the 1 HNMR spectrum of ZnL, the signal of H d is not only down-eld shied but also splits into two peaks, suggesting that metallation of L by Zn(II) rigidied its conformation in solution. In the crystal structure, four pyridine groups on ZnL are in the same plane rather than the randomly distributed form seen on the free L ( Fig. 2A and B, S9 and S10 †). Reacting Me 4 enPd(NO 3 ) 2 with ZnL in D 2 O at 70 C for 3 h led to formation of one clean complex (1-Zn). 1 HNMR analyses show that both the signals of H a and H b on 1-Zn shi to the down-eld than that on ZnL, due to the coordination of pyridine groups to Pd(II) (Fig. 1C, S11-S13 †). Diffusion ordered NMR spectroscopy (DOSY) further indicates the purity and the size (about 2.1 nm) of 1-Zn (Fig. S14 †). The X-ray crystal structure of 1-Zn was also obtained, which clearly conrmed its tubular structure [(Me 4 enPd) 6 (ZnL) 3 ] 18+ (Fig. 2C, S15 †). Six Pd(II) capping units are located on the six vertices of the triangular prism, while three ZnL ligands dene the three faces of the triangular prism, with one NO 3 À encapsulated in the tubular cavity. The chelated Zn(II) ion displays a tetragonal pyramid geometry through coordinating with four N atoms from L and one O atom from water. Such coordination mode makes the new-formed metalloligand ZnL adopt the planar conformation, leading to the formation of a tubular structure. The triangular apertures on the tube are about 13.7Å Â 13.7Å Â 13.7Å. No anion effect was observed on the self-assembly of tube 1-Zn, as isostructural tubular complexes were obtained using other Me 4 enPd (BF 4 À , SO 4 2À , ClO 4 À , SbF 6 À salts) (Fig. S16-S20 †). Onepot self-assembly starting from L, Zn(NO 3 ) 2 and Me 4 enPd salt also leads to the coordination tube 1-Zn (Fig. S21 †). When other transition metal ions M(II) (M ¼ Co, Ni, Cu) that can adopt similar coordination geometries to Zn(II) ions were used, heterometallic coordination tubes [(Me 4 enPd) 6 Fig. 1D and E, S22-S27 †). SCXRD analyses conrm that complexes 1-Ni and 1-Cu are isostructural to tube 1-Zn, except that no NO 3 À can be found in the cavity of 1-Cu ( Fig. 2E and F, S29 and S30 †). Although the chelated Co(II) ion in 1-Co displays a distorted octahedral conguration, it has the same coordination form with the ligand L as the Zn(II), so the Co(II) ion also induces the formation of a tubular structure (Fig. 2D, S28 †). In short, Zn(II)/ Co(II)/Ni(II)/Cu(II) ions can be used as templates to induce the self-assembly of heterometallic tubular tubes (1-M) because they can drive the metalloligand MLs to adopt a planar conformation (Scheme 1A). To our surprise, cage complexes 2 were obtained by the in situ self-assembly of L, PdMe 4 en(Cl) 2 S41 †), further con-rming that Ag(I) ions can template the self-assembly of coordination cages (2) (Scheme 1B). It then turns out that the metallation of L was a prerequisite for the successful self-assembly. Trail on direct self-assembly of L with Me 4 enPd(NO 3 ) 2 gave a mixture of products ( Fig. S42 and S43 †). Interestingly, reacting ligand L with Me 4 enPd(ClO 4 ) 2 led to a new (Me 4 enPd) 6 (PdL) 3 -type tube complex (3), with the in situ metallation of L with Pd(II). The 1 HNMR spectrum of 3 is clearly different from those of both 1 and 2 ( Fig. 1G and S44-S46 †). DOSY shows that the size of 3(ClO 4 À ) is about 2.2 nm (Fig. S47 †), very close to that of tube 1-Zn. SCXRD nally conrms that 3(ClO 4 À ) is a Pd-chelated triangular coordination tube [(Me 4 -enPd) 6 (PdL) 3 ](ClO 4 ) 18 (Fig. 2G, S48 †). The crystal structure of 3(ClO 4 À ) contains two independent tubular molecules. Both of them have a triangular prism structure similar to that of 1-Zn, where six Pd nodes are located on six vertices of the triangular prism and three PdL metalloligands span three faces of the prism. The Pd(II) ions chelated in the PdL ligands adopt the square-planar coordination geometry. Like ZnL, PdL also adopts a planar conformation, so a similar tube complex is formed. A clean tubular complex 3 (SbF 6 À salt) could also be obtained when Me 4 enPd(SbF 6 ) 2 was used ( Fig. S49-S52 †). However, both Me 4 -enPd(BF 4 ) 2 and Me 4 enPdSO 4 lead to a mixture of products, demonstrating an obvious anionic effect (Fig. S53 †).
With the structure of hetero/homometallic coordination tubes 1 and 3, and cages 2 determined, we then wondered whether controllable structural transformation between cages and tubes can be achieved through transmetalation on the ML. Indeed, the addition of Zn(II) ions to the Ag-chelated cage 2 triggered its complete transformation into Zn-chelated tube 1-Zn (Fig. S54-S56 †). Due to the much stronger coordination ability of Zn(II) than Ag(I) with L, it is difficult for Ag(I) ions to drive the back transformation. Even with a large excess of Ag(I) ions, only a partial transformation from tube 1-Zn to cage 2 was observed ( Fig. S57-S59 †). Similarly, cage 2 can be completely transformed into tube 3 by the addition of Pd(II) salts, where use of Pd(SbF 6 ) 2 can greatly promote this transformation (Fig. S60-S62 †), while back transformation from tube 3 to cage 2 is successful by the addition of AgBF 4 or AgNO 3 , but not for AgSbF 6 (Fig. S63-S65 †). We infer that SbF 6 À may have a stronger interaction with tube 3 than cage 2, so it can promote the conversion of cage 2 to tube 3 but hamper the back transformation. In brief, controllable transformations between cages and tubes can be realized via transmetalation on the ligands with a certain degree of anion effects (Scheme 1D-G).

Fine-tuned shapes and cavities on the tubular complexes
It's worth noting that the utilization of the exible tetraazacyclododecane-based ligand not only facilitates the incorporation of various metal ions on the coordination tubes, but also provides a good opportunity to ne-tune the volumes of their central cavities. Based on the crystal data, deformations of geometry on L depending on its metallation with different metal ions are clearly observed, leading to gradual volume changes from 316 (1-Ni) to 409 [3(ClO 4 À )]Å 3 (Fig. 3B). For the isostructural heterometallic tubes 1-Zn/Ni/Cu, their central metals share the same coordination environment, and the volumes of the tubes are found to be proportional to the M-N bond lengths on MLs (N-Ni < N-Cu < N-Zn). We infer that shorter coordination bonds make the MLs panels more convex, while such conformational changes do not cause signicant changes to the triangular opening (Fig. 3A). 1-Co and 3(ClO 4 À ) with quite different coordination environments on MLs bear much larger volumes than the above three tubes, with the largest volume of 409Å 3 measured for tube 3(ClO 4 À ), which is ca. 1.3 times that for 1-Ni. We infer that the volume increases in these two tubes are due to their less curved ML panels.

Acid-base resistance of coordination tubes
Stability of the coordination assemblies is an important prerequisite for their applications. 21,23 Impressively, 1 H-NMR for tube 1-Zn in aqueous solutions conrmed that it maintained its structural integrity under both strongly acidic (pH ¼ 1) and basic conditions (pH ¼ 12) but would decompose in stronger acidic (pH ¼ 0) or basic (pH ¼ 13) solutions (Fig. S66 †). Tubes 1-Co and 1-Ni also remain stable at pH 12, but their acid resistance is poorer than that of 1-Zn ( Fig. S67 and S68 †). For comparison, a simple coordination complex (Me 4 enPd) Py 2 (NO 3 ) 2 (Py ¼ pyridine) was synthesized. 1 H-NMR tests indicate that it can only remain intact in a smaller pH range (pH 3-11; Fig. S69 †), suggesting that multi-component cooperativity on the tube enhances its structural stability. The buffering effect of ML, i.e., accepting H + on the N atoms of tetraazacyclododecane and OH À on the metal centers via axial coordination, should also contribute to the observed acid-base resistance of the tube complexes. While many acid or base resistant coordination hosts have been reported, few of them are stable toward both acidic and basic conditions. Coordination hosts built from inert coordination bonds, such as Pt-N, and Zr-O bonds, tend to show enhanced stability. 107,108 Compared with the strategy of using stronger coordination bonds, here we improve the acidbase stability of the host by creating a local buffer environment, which provides a useful strategy for the construction of stable coordination hosts.

CO 2 xation promoted by coordination tubes
Considering that the coordination tube 1-Zn contains three Zn active sites and is tolerable toward basic solutions, we then studied its applications in carbonic anhydrase mimicking. The CO 2 dissolution experiment indicates that CO 2 has better solubility in DMSO/H 2 O (1 : 2 v/v) mixed solvent than pure water (Fig. S70 †). So, a DMSO-d 6 /D 2 O mixed solvent containing 1-Zn (5 mM) and NaOH (0.01 M) was used for the CO 2 capture reaction. When CO 2 gas was continuously injected into this solution at room temperature for 3 h, a signal at 160 ppm assignable to CO 3 2À was clearly observed in 13 CNMR (Fig. 4B(c) and (d)), whereas no CO 3 2À was observed without adding 1-Zn, or with 1-Co, 1-Ni as the catalysts (Fig. 4B(a) and (b), S71 †). A control experiment with a mononuclear ZnL 1 (NO 3 ) 2 (L 1 ¼ 1,4,7,10-tetraazacyclododecane) complex as the catalyst, which contains a similar Zn(II) active site, also showed no CO 2 conversion (Fig. S72-S74 †). Taking together all these experiments, it is clear that both the existence of the Zn metal sites  Further studies indicate that adding more NaOH and prolonging the reaction time cannot enhance the yield of CO 3 2À in solution (Fig. S75 †). The IR spectra of tube 1-Zn aer the reaction show a new absorption peak at 1350 cm À1 , which is obviously red-shied compared with that in Na 2 CO 3 ($1440 cm À1 ) (Fig. 4C, S76 †). The crystal of the 1-Zn-CO 3 2À host-guest complex was obtained through slowly diffusing THF vapor into the reaction system. SCXRD conrmed that one CO 3 2À is tightly captured inside the tube cavity. Encapsulation of CO 3 2À also causes induced-t cavity deformations, with the size of the triangular window changing to 13.1Å Â 13.6Å Â 14.0Å and the volume increasing to 377Å 3 compared to that of 1-Zn (Fig. 3B, S77 †). Multiple hydrogen bonds and the existence of strong electrostatic interactions lead to a strong host-guest interaction between CO 3 2À and the 1-Zn host (Fig. 4D). We speculate that the formed 1-Zn-CO 3 2À host-guest complex is inactive and hinders the turnover of further CO 2 conversion. Considering that Ca(II) ions can react with CO 3 2À to produce water-insoluble CaCO 3 (K sp ¼ 2.80 Â 10 À9 ), they have been used to grab the CO 3 2À encapsulated inside cavity. Further experiments showed that aer removal of CO 3 2À encapsulated by the cavity, the tube 1-Zn can be regenerated and reused (Fig. 4B(e) and (f)). Although many coordination hosts that can x CO 2 by binding with carbonate have been reported, 95-102 the newly formed strong metal-oxygen interactions make these hosts unable to be reused. Thus, tube 1-Zn represents a rare coordination host that can catalyze CO 2 conversion like carbonic anhydrase. In short, efficient and mild CO 2 conversion like the carbonic anhydrase can be realized by using an acid-base resistant coordination tube 1-Zn (Fig. 4A). While the efficiency is weaker, our complex is much cheaper and can tolerate harsh conditions.
A possible cavity-assisted CO 2 conversion process was proposed as follows (Fig. S78 †): (1) Zn(II) ions in tube 1-Zn are coordination unsaturated, which can coordinate with OH À to form Zn-OH units under basic conditions; (2) The Zn-OH group attacks the CO 2 in solution to form HCO 3 À , which is the ratedetermining step of the conversion of CO 2 . Compared to the OH À coordinated to harder Co(II) and Ni(II), the enhanced nucleophilicity of Zn-OH makes it more efficient to attack CO 2 ; (3) HCO 3 À deprotonates into CO 3 2À due to the pK a -shiing effect inside the cavity of the highly charged cationic host. 109 In fact, DFT calculations show that the CO 3 2À 31-Zn host-guest complexation energy was as high as À1349 kJ mol À1 ; (4) the CO 3 2À sequestration by Ca(II) achieves the recycle of the catalyst and facilitates the reaction turnover.

Conclusions
A series of homo/heterometallic coordination assemblies have been synthesized from a tetraazacyclododecane-based ligand. Both self-assembly and structural transformations of coordination tubes/cages can be controlled in a cooperative manner using metal templates and auxiliary anions. The tubular complexes show adaptive cavities depending on the metallation of the ligand and the anions. The Zn-containing heteroleptic tubular complex not only shows impressive acid-base stability but also can catalyze the CO 2 to CO 3 2À conversion, standing for a rare example of an efficient and recyclable carbonic anhydrase-mimicking supramolecular system. Mechanistic studies conrm that the integration of multiple active metal sites and an open CO 3 2À -binding pocket is important for the CO 2 xation. This work not only offers a new ligand candidate for the self-assembly of multi-variant coordination hosts, but also provides an important principle for the design of supramolecular catalysts toward CO 2 xation.

Data availability
All experimental and computational data associated with this work are available in the ESI. †

Conflicts of interest
There are no conicts to declare.