Confinement of a Au–N-heterocyclic carbene in a Pd6L12 metal–organic cage

A Au(i)–N-heterocyclic-carbene (NHC)-edged Pd6L12 molecular metal–organic cage is assembled from a Au(i)–NHC-based bipyridyl bent ligand and Pd2+. The octahedral cage structure is unambiguously established by NMR, electrospray ionization-mass spectrometry and single crystal X-ray crystallography. The electrochemical behaviour was analyzed by cyclic voltammetry. The octahedral cage has a central cavity for guest binding, and is capable of encapsulating PF6− and BF4− anions within the cavity.


Introduction
The self-assembly of metal-organic cages (MOCs) 1-3 has been investigated for diverse potential applications, such as catalysis, 4 sensing, 5,6 molecular storage and sequestration, 7 drug delivery, 8 and so on. For MOCs the shape, cavity size, and ligand can inuence their guest binding and transformation capability. 9-11 N-heterocyclic-carbenes (NHCs) are a class of electrondonating ligands, 12 and have been applied in catalyzing various organic transformations. 13,14 Discrete assemblies based on NHC ligands including molecular metallocycles and cages have received increasing attention [15][16][17][18] because they may induce reactivity change, selectivity and product distribution variation. 19,20 However, few molecular cages with well-dened cavities are reported in the literature. [21][22][23] Remarkably, Nitschke et al. reported M 4 L 6 (M ¼ Zn(II), Cd(II)) cages with Au(I)-NHCbased ligand. 23 In our effort to build NHC-based cages with large cavity, we wish to report herein that a Pd 6 L 12 metalorganic cage containing twelve Au(I)-NHC centres is assembled from a rigid, bent N-heterocyclic-carbene-based bispyridyl ligand and palladium(II) ions.

Results and discussion
For the self-assembly of Pd 6 L 12 cage, bis(pyridyl)-functionalised Au(I)-NHC ligand L was designed and synthesized (Fig. 1a). First diiodo-functionalised imidazolium chloride (b) was synthesized from bis-Schiff's compound a 24 and paraformaldehyde via a ring-closing step in the presence of chlorotrimethylsilane. Then Au(I)-NHC compound c 25 was synthesized by the reaction of imidazolium salt b with HAuCl 4 $4H 2 O and 3-chloropyridine in the presence of base (Na 2 CO 3 ). Subsequently, L was synthesized by Suzuki-Miyaura cross-coupling reaction between c and 3-pyridylboronic acid ( Fig. S1-S10 †). 1 H NMR shows that the imidazolium C-H resonance appeared at d 10.34 for b and disappeared for L. 13 C NMR spectra show the resonance of the imidazolium C-H carbon at 139.60 ppm for b and the resonance of the metallated carbenecarbon atoms at 185.55 ppm. The coordination cage Pd 6 L 12 was successfully assembled by heating a 2 : 1 mixture of L and Pd(NO 3 ) 2 $2H 2 O in DMSO at 70 C for 6 h. 1 H NMR indicates that the quantitative formation of Pd 6 L 12 (12 NO 3 À anions are omitted for clarity, the same hereinaer) ( Fig. 2 and S11 †). The cage is highly symmetric. Compared with the free ligand L the original sharp signals of pyridine moieties turn into broad peaks and shi downeld, for example the signals at 9.05, 8.64 and 8.26 ppm for the H i , H h and H c pyridyl protons are downeld-shied to 9.74, 9.09 and 8.62 ppm, respectively, which is ascribed to the coordination of palladium(II) ions to the ligand. Gratifyingly, single crystals of Pd 6 L 12 (NO 3 À salt) suitable for X-ray diffraction analysis were obtained over one month by slow diffusion of ethyl acetate vapour into a solution of Pd 6 L 12 in DMSO (Table S1 †). The crystals crystallize in the trigonal space group R 3. The cubic symmetric unit is composed of six Pd(II) metal centres linked by twelve L ligands, forming the octahedron edges (Fig. 1b). Nevertheless, nitrate ions and solvent molecules could not be reasonably located in this highly disordered structure. The bend angle between pyridine rings and central Au-NHC ring is 174.1 . Two pyridyl donors on the same ligands adopt syn-conformation. The bond angles C carbene -Au-I trans range from 176 to 179 are in fact close to linearity (C18-Au1-I1 176.1 (11) ) and the Au1-C18 bond length of 1.96(3) A, which is close to those of the reported (NHC)gold(I) complexes. 26 Each Pd(II) has a square planar geometry with Pd-N bond distances of 1.98(2)-2.03(2) A and the angle between the two 3-pyridyl coordinating motifs in L is 88-93 , within the normal range for those reported analogous pyridine-based ligand assembled Pd 6 L 12 complexes. 27,28 The cavity size is approximately 20.2 Â 20.4 Â 28.7 A 3 , which is dened by the six Pd(II) ions. The opposing Pd(II)-Pd(II) distance is 28.7 A and adjacent Pd-Pd distances are 20.2-20.4 A. One of the two diisopropyl groups on each ligand points to the cavity, and the C-C distance of diisopropyl groups on opposite ligands is 16.3 A.
Further evidence for Pd 6 L 12 was provided by 1 H-1 H homonuclear correlation spectroscopy (COSY) and 1 H-1 H nuclear overhauser effect spectroscopy (NOESY), which both reveal important cross peaks between the two observed sets of NMR signals (e.g. H c -H b and H c -H h ) ( Fig. S12 and S13 †). In addition, diffusion-ordered NMR spectroscopy (DOSY) shows the selective formation of a single species (Fig. S14 †). The same diffusion coefficient at D ¼ 5.75 Â 10 À11 m 2 s À1 corresponds to the dynamic radius of 19.0 A according to the Stokes-Einstein equation. 28 Further structural evidence was given by electrospray ionization-mass spectrometry (ESI-MS) (Fig. 3). Aer anion exchange of NO 3 À for PF 6 À ions in DMSO solution, a series of prominent peaks with continuous charge states of were detected for Pd 6 L 12 . The isotopic distribution patterns of each peak agreed well with the simulated patterns.
The chemical composition of L and Pd 6 L 12 were characterized by Fourier transform infrared spectroscopy (FT-IR) (Fig. S15 †). The absorption of Pd 6 L 12 at 1632 cm À1 corresponding to the C]N in-ring stretching in the pyridine rings shis to lower energy compare with L at 1638 cm À1 , which is attributed to the coordination of L to Pd 2+ via the nitrogen atom. Both L and Pd 6 L 12 exhibit characteristic C-N carbene bands typically at 1467 and 1443 cm À1 . 29 The strong absorption band at approximately 1384 cm À1 is attributed to the NO 3 À ions for To investigate the electrochemical behaviour, cyclic voltammetry analyses were carried out in the potential range from À2.0  to +2.0 V at a scan rate of 50 mV s À1 in a solution of tetrabutylammonium hexauorophosphate (TBAPF 6 ) in dry DMSO (0.10 mol L À1 ) as a supporting electrolyte on glassy carbon electrode (3 mm in diameter) (Fig. 4). L gave a reduction wave at a potential of À1.068 V, indicating the reduction of Au(I) to metallic Au(0) corresponding to previously reported Au(I)-NHC analogues. 30,31 The analogous process was signicantly shied to a more anodic potential at À0.890 V for Pd 6 L 12 . In addition to the reduction processes, a single irreversible oxidation peak was also observed at 0.812 V for Pd 6 L 12 , positively shied by 0.287 V from L (+0.525 V), which is consistent with the oxidation processes of I À to I 2 on clean glassy carbon surface. 32,33 It is worth noting that a new irreversible cathodic potential appeared at À0.732 V, attributed to the reduction process of NO 3 À to NO 2 À within the cage, 34 perhaps owing to the change of ionic status in the cavity aer coordination. 35 The reduction current for Pd 6 L 12 were acquired at scan rates of 50-200 mV s À1 , showing that two reduction waves are located below the scan rates of 100 mV s À1 (Fig. S16 †). With the increase in the sweep rate, the two waves merge into one broad peak, suggesting that the redox process takes place under simple diffusion-control for the cage. 36 The resulting cage structure of Pd 6 L 12 was further investigated to explore its encapsulation of anions. The original nitrate anions are difficult to detect by NMR, so anion exchange of nitrate with other anions were performed. Various anions (20 equiv., excess) were added to a DMSO solution of Pd 6 L 12 (0.75 mmol L À1 ) and the mixture was allowed to react for 4 h at room temperature, then diethyl ether was added to precipitate out pale yellow solid prior to the acquisition of NMR spectroscopy. Aer the introduction of NaPF  . 5b and S18 †). Additionally for 1 H NMR H i proton resonance inside the central cavity is upeld shied by 0.70 ppm (Fig. S19 †), while other proton resonances are essentially unaffected. These results demonstrate that two types of PF 6 À anions are present in the solution, some are  This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 39323-39327 | 39325 encapsulated within the cavity, and others are free in the solution.
BF 4 À anions can also be encapsulated inside the cavity. With the addition of KBF 4 to Pd 6 L 12 , 19 F NMR shows two signals, one strong signal at À148.27 ppm corresponding well with the peak of free BF 4 À in DMSO-d 6 (À148.3 ppm), and the other new signal at À144.1 ppm (Fig. S20 and S21 †). 11 B NMR show that a new signal at 2.79 ppm is also assigned to bound BF 4 À in the cavity ( Fig. S22 and S23 †). 38,39 Simultaneously 1 H NMR reveals that the proton resonance of H i inside the central cavity is shied upeld (Dd ¼ 0.30 ppm) compared to Pd 6 L 12 (Fig. S19 †). These results suggest that some BF 4 À anions are bound within the cavity of Pd 6 L 12 . However, addition of OTf À to Pd 6 L 12 causes neither new signals nor notable shis, and 19 F NMR gives only one sharp signal corresponding to free OTf À , suggesting that the nitrate ions in the cage was unaffected by OTf À (Fig. S19 and S24 †).

Conclusions
In summary, we have successfully synthesized Au(I)-NHC bispyridine ligand (L) and Au(I)-NHC-edged Pd 6 L 12 coordination cage has been assembled from L and Pd 2+ . The cage Pd 6 L 12 has been fully characterized by NMR, diffusion-ordered NMR spectroscopy, FT-IR, mass spectrometry and cyclic voltammogram. The single crystal X-ray diffraction conrms unequivocally the octahedral structure of Pd 6 L 12 . The octahedral cage has central cavity for guest binding, and has been shown to be capable of encapsulating PF 6 À and BF 4 À anions within the cavity. It paves a way to build NHC-based metal-organic container molecules with large cavity for subsequent guest binding and transformation. to give colourless crystals of Pd 6 L 12 over one month. A crystal was picked (0.2 Â 0.2 Â 0.2 mm 3 ) and coated in paratone oil, attached to a glass silk which was inserted in a stainless steel stick, then quickly transferred to the Agilent Technologies SuperNova X-ray diffractometer with the Enhance X-ray Source of Cu Ka radiation (l ¼ 1.54184 A) using the u-f scan technique. Data collection were measured at 150.00 (10) K. The unit cell parameters were solved by direct methods and the unit cell parameters rened against all data by anisotropic full-matrix least-squares methods on F 2 with the SHELXL program. 40 Hydrogen atoms were calculated in ideal positions (riding model). All nitrate ions and solvent molecules could not be reasonably located because of highly disordered structures, and were removed by PLATON/SQUEEZE routine. 41 Crystallographic data for Pd 6 L 12 : C 74 H 84 Au 2 I 2 N 8 Pd FW ¼ 1839.62, trigonal, R 3, a ¼ 29.8768 (16) A, b ¼ 29.8768 (16) A, c ¼ 74.341 (3) A, a ¼ 90 , b ¼ 90 , g ¼ 120 , V ¼ 57 468 (7) A 3 , Z ¼ 18, T ¼ 150.00 (10) K, l ¼ 1.54184 A, r calcd ¼ 0.957 mg m À3 , m ¼ 9.349 mm À1 , 13 205 reections were collected (7693 were unique) for 6.856 < 2q < 79.938, R(int) ¼ 0.0372, R 1 ¼ 0.0888, wR 2 ¼ 0.2668 [I $ 2s(I)], R 1 ¼ 0.1033, wR 2 ¼ 0.2821 (all data) for 725 parameters and 825 restraints, GOF ¼ 1.059. Selected bond lengths and bond angles are presented in Table S1. † The crystal structure was submitted to the Cambridge Structural Database under the CCDC number 2013514.

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