Synthesis and Characterization of Poly-NHC-Derived Silver (I) Assemblies and Their Transformation into Poly-Imidazolium Macrocycles

Three metallosupramolecular assemblies composed of two bis-NHCs and two silver atoms, [ 4 ](PF 6 ) 2 , two tetra-NHCs and four silver atoms, [ 7 ](PF 6 ) 4 , and two tri-NHCs and three silver atoms [ 8 ](BF 4 ) 3 , have been prepared. Assemblies [ 4 ](PF 6 ) 2 and [ 7 ](PF 6 ) 4 feature NHC ligands decorated with terminal olefin groups. Irradiation of [ 4 ](PF 6 ) 2 yielded complex [ 5 ](PF 6 ) 2 with two terminal cyclobutane rings linking the two bis-NHC ligands. Liberation of the macrocyclic tetrakisimidazolium salt H 4 - 6 (PF 6 ) 4 was achieved by reaction of [ 5 ](PF 6 ) 2 with NH 4 Cl/NH 4 PF 6 . No [2 + 2] cycloaddition was observed upon irradiation of [ 7 ](PF 6 ) 4 , apparently due to an unfavorable orientation of the olefin groups. Irradiation of complex [ 8 ](BF 4 ) 3 with three internal pairs of olefin groups leads to [ 9 ](BF 4 ) 3 as a mixture of two isomers that differ on the relative orientation of the internal cyclobutane rings. Reaction of [ 9 ](BF 4 ) 3 with NH 4 Cl/NH 4 BF 4 yields an isomer mixture of the novel cage-line hexakisimidazolium salt H 6 - 10 (BF 4 ) 6 .


Introduction
Over the last three decades, research on supramolecular coordination complexes (SCCs) [1] has emerged as an important sub-discipline of coordination chemistry.Particularly, the wide range of applications has stimulated interest in these compounds.Among others, SCCs have found applications in catalysis, [2] molecular recognition, [3] the stabilization of highly reactive species, [4] and as drug delivery/release vectors. [5]oordination-driven self-assembly [1e,6] arguably constitutes the most widely used method for the construction of discrete SCCs.From the ligand perspective, metallosupramolecular assemblies have been mainly constructed from O-, N-, and P-donor Werner-type polydentate ligands.More recently, poly N-heterocyclic carbene ligands (poly-NHCs) [7] have emerged as an alternative for the generation of organometallic metallosupramolecular assemblies. [8]The rapid development of organometallic supramolecular assemblies is illustrated by the coinage of the term supramolecular organometallic complexes (SOCs), [9] referring to all metal-containing assemblies in which the linkermetal connection is established by MÀ C bonds.
Over the last 10 years, the number of metallosupramolecular assemblies featuring MÀ C NHC bonds has grown steadily.Among them, a large number of NHC-derived SOCs is based on group 11 metals.These metals in oxidation state of + 1 offer certain advantages for the preparation of metal assemblies such as the trend to form linear C NHC À MÀ C NHC moieties and therefore facilitate the construction of assemblies in which the metal is sandwiched between two polydentate NHCs. [10]In addition, the AgÀ C NHC bond is usually labile allowing rearrangements to the thermodynamically most stable assembly.Known poly-NHC derived assemblies with group 11 metals, include diverse architectures such as rectangles, triangles [11] and cylinders. [12]8a,12h,13] This constitutes a very interesting feature, because PAM reactions facilitate the generation of architectures with tailored functionalities normally not directly accessible from metal ions and ligands.8a,14] Cationic imidazolium salts have been widely used as anion receptors because they combine potential hydrogen bonding with favorable electrostatic interactions in their interactions with selected substrates. [15]However, finding effective ways for preparing imidazolium-based multivalent anion receptors still constitutes a challenge.14a] The oktakisimidazolium salt turned out to be an efficient multivalent anion receptor with a binding affinity significantly larger than that shown by the original tetrakisimidazolium salt.
Based on these previous findings, we describe here the preparation and characterization of a series of metallosupramolecular assemblies from poly-imidazolium salts featuring terminal or internal olefins and Ag I ions.The resulting poly-NHC-Ag I assemblies were then subjected to photochemically induced [2 + 2] cycloaddition reactions, by activation of the pendant cinnamic ester groups or the internal olefins.This procedure linked the individual poly-NHC ligands to larger macrocyclic or cage-like poly-NHCs.Demetallation of the poly-NHC complexes yielded novel poly-imidazolium salts not accessible by conventional organic synthesis.
Reaction of H 2 -1(PF 6 ) 2 with a slight excess of Ag 2 O yielded the metallorectangle [4](PF 6 ) 2 in 83 % yield (Scheme 2).The formation of the rectangular assembly was confirmed by NMR spectroscopy and mass spectrometry.The 1 H NMR spectrum of [4](PF 6 ) 2 is consistent with the pseudo-D 2h symmetry of the molecule.This is exemplified by the appearance of two resonances assigned to the protons of the imidazolylidene rings (δ = 7.98 and 7.78 ppm), the three signals due to the protons of the terphenylene linker (δ = 7.88, 7.80 and 7.64 ppm) and the two doublets assigned to the olefin protons (δ = 7.52 and 6.48 ppm).The metallation of the NHC donors with Ag I has been confirmed by the resonance at δ = 179.4ppm observed in the 13 C{ 1 H} NMR spectrum for the C NHC carbon atom.The dimetallic nature of the complex is evident from the electrospray mass spectrum, which shows the base peak at m/z = 818.4,assigned to [4] 2 + .
tetranuclear Rh I and Ir I complexes obtained from a related pyrene-bridged tetrakisimidazolylidene ligand displaying interesting structural and catalytic features. [16]The reaction of H 4 -2(PF 6 ) 4 with Ag 2 O, affords the tetranuclear cylinder-like assembly [7](PF 6 ) 4 in 74 % yield.Compound [7](PF 6 ) 4 was characterized by NMR spectroscopy and mass spectrometry.The 1 H NMR spectrum displays a set of signals in accord with a pseudo-D 2h symmetric molecule.For example, two resonances due to the protons of the pyrene bridge were observed at δ = 8.18 and 7.96 ppm.In addition, two signals due to the protons of the imidazolylidene rings (δ = 7.56 and 7.45 ppm) and two doublets assigned to the olefin protons (δ = 7.52 and 6.39 ppm) were observed.The 13 C{ 1 H}NMR spectrum shows a signal at δ = 182.8ppm assigned to the metallated carbene carbon atom.The tetrametallic nature of the complex was unambiguously confirmed by electrospray mass spectrometry, which reveals peaks at m/z = 689.6 and 967.8 assigned to [7] 4 + and [7 + PF 6 ] 3 + , respectively.
Attempts to perform a post synthetic modification on [7](PF 6 ) 4 via a photochemically induced [2 + 2] cycloaddition reaction were next investigated.A solution of [7](PF 6 ) 4 in acetonitrile was irradiated with a high pressure mercury lamp at ambient temperature for 3 h.Under these reaction conditions, the expected cycloaddition product was not formed and only a mixture of unidentified reaction products was obtained.For the [2 + 2] cycloaddition to proceed, the olefins must be oriented in a parallel fashion with a distance between the midpoints measuring about 3.6 Å. [17] While these geometric features were observed for a related cylinder-like assembly obtained from two tetra-NHC ligand and four silver atoms, [14a] we believe that the orientation of the olefin groups in [7](PF 6 ) 4 does not meet the geometric requirements.Unfortunately, we were not able to obtain crystals suitable for an X-ray diffraction study with [7](PF 6 ) 4 to confirm this assumption.
In order to shed more light on the geometric requirements for the [2 + 2] cycloaddition and the possible outcome of this reaction, we decided to extend our studies to tri-NHC ligands featuring internal olefin groups.In this regard, the reaction of The three silver atoms form a rather symmetrical triangle with an average non-bonding Ag•••Ag separation of 17.6 Å.The average AgÀ C NHC distance measures 2.08 Å and falls in the typical range for this type of bond. [14]The central benzene rings are arranged parallel but twisted by about 4°relative to each other.Interestingly, the three pairs of olefin groups are not oriented in a parallel fashion.
While the separation between the olefins in the three pair measures between 3.35 to 3.83 Å and thus fall in the range required for a subsequent [2 + 2] cycloaddition, [17] their nonparallel orientation might prevent this reaction.14f] Nevertheless, complex [8](BF 4 ) 3 was irradiated with a highpressure lamp in order to prove our hypothesis.We observed changes in the 1 H NMR spectra, thus indicating that the cycloaddition reaction might be taking place.After 16 h of reaction, no more changes were observed in the 1 H NMR and it was assumed that the formation of the triple cyclobutane was complete (Scheme 4).
This assumption was corroborated by the 1 H NMR spectrum of the reaction product [9](BF 4 ) 3 , showing no resonances for the olefin protons.However, the 1 H NMR spectrum features a total of 12 new resonances for the cyclobutane protons (Figure 3) instead of the two resonances expected and observed after cyclobutane formation in [5](PF 6 ) 2 and related compounds. [14]his observation is consistent with the formation of two isomeric complexes (Figure 4) in the cycloaddition, featuring either three different (isomer A) or three identical cyclobutane rings (isomer B).
Assuming the formation of the isomers A and B of [9](BF 4 ) 3 , all resonances in the 1 H NMR spectrum (Figure 3   The formation of the isomer mixture should lead to two separate spin systems.These two spin systems were indeed observed by 1 H, 1 H-ROESY NMR spectroscopy (Figure 5).The ROESY spectrum clearly shows the correlation between protons H13 and H10/H11 for the two isomers.Resonances for the isomer mixture of [9](BF 4 ) 3 were also observed by 13 C{ 1 H} NMR spectroscopy (see the Supporting Information).

Conclusions
In summary, new bis-and tetrakisimidazolium salts with terminal olefins and a trisimidazolium salt with internal olefins have been prepared and used for the synthesis of di-, tetra-and trinuclear silver-NHC assemblies.The di-silver tetra-NHC metallorectangle [4](PF 6 ) 2 underwent effective photochemically induced [2 + 2] cycloaddition to form a new macrocycle in which the two di-NHC ligands are linked by two terminal cyclobutane units.Removal of the Ag I ions with NH 4 Cl/NH 4 PF 6 afforded a novel macrocyclic tetrakisimidazolium salt H 4 -6(PF 6 ) 4 .The pyrene-bridged tetrakismidazolium salt H 4 -2(PF 6 ) 4 reacted with Ag 2 O to give the cylinder-like tetrasilver assembly [7](PF 6 ) 4 featuring four pairs of terminal olefins.Irradiation of this assembly did not yield the derivative with bridging cyclobutanes, most likely due to an unfavorable orientation of the olefin groups.Finally, trisimidazolium salt H 3 -3(Br) 3 reacted with Ag 2 O to give the trinuclear assembly [8](BF 4 ) 3 featuring three internal pairs of olefins.Irradiation of this assembly proceeded via triple cyclobutane formation leading to a mixture of two isomeric complexes [9](BF 4 ) 3 with either three different or three identical cyclobutane linkers.Liberation of two isomeric hexakisimidazolium cages from [9](BF 4 ) 3 proved possible.We present an efficient method for the synthesis of macrocyclic or cage-like poly-imidazolium salts via photochemical [2 + 2] cycloaddition at a silver-carbene template.We are confident that this methodology can be extended to the synthesis of novel cyclic poly-azolium salts not accessible by conventional organic synthesis.

Experimental Section
General considerations.1,3,6,8-Tetraimidazolylpyrene was prepared according to a reported method. [16]All other reagents were used as received from commercial suppliers.Anhydrous solvents were dried using a solvent purification system (SPS M BRAUN) or were distilled by standard procedures prior to use.NMR spectra were recorded on Bruker AVANCE III 400 or 300 MHz spectrometers.NMR spectra were obtained at room temperature in acetonitrile-d 3 or DMSO-d 6 .
Electrospray mass spectra were recorded on a Micromass Quatro LC instrument, MeOH or CH 3 CN were used as mobile phase, and nitrogen was employed as drying and nebulizing gas.Satisfactory microanalytical data for most ligands and metal complexes could not be obtained due to the large fluorine content in the hexafluorophosphate counterions.A complete set of NMR spectra is provided instead.

Synthesis of bisimidazolyl-p-terphenylene
4,4'-Dibromoterphenyl (500 mg, 1.29 mmol), imidazole (173 mg, 2.54 mmol), K 2 CO 3 (710 mg, 5.14 mmol) and CuI (49 mg, 0.26 mmol) were placed together in a high pressure Schlenk tube fitted with a Teflon cap.The tube was evacuated and filled with nitrogen three times.The solids were suspended in anhydrous DMF (20 mL) and the resulting mixture was heated under reflux for 72 h.The reaction mixture was then allowed to reach ambient temperature.Distilled water (75 mL) was added, and the resulting suspension was stirred for 2 h.The solid was collected by filtration and washed with water.Compound bisimidazolyl-p-terphenylene was isolated as a white solid.Yield: 440 mg (1.21 mmol, 94 %).

Synthesis of H 2 -1(PF 6 ) 2
Bisimidazolyl-p-terphenylene (80 mg, 0.22 mmol) and methyl 3-(4bromomethyl)cinnamate (113 mg, 0.44 mmol) were placed together in a thick-walled Schlenk tube fitted with a Teflon cap.The tube was evacuated and filled with nitrogen three times.The solids were suspended in anhydrous DMF (16 mL) and the resulting mixture was heated to 145 °C for 24 h.Once at ambient temperature, the solvent was distilled under vacuum.The resulting white solid was washed several times with diethyl ether and dried under vacuum.The obtained bromide salt H 2 -1(Br) 2 (101 mg, 0.12 mmol) was suspended in MeOH (20 mL), heated to 40 °C and treated with NH 4 PF 6 (56 mg, 0.34 mmol).The resulting suspension was stirred at 40 °C for 16 h.Filtration of the suspension yielded compound H 2 -1(PF 6 ) 2 as a light brown solid.Yield: 154 mg (0.15 mmol, 68 %).

Synthesis of H 6 -10(BF 4 ) 6
A solution of [9](BF 4 ) 3 (120 mg, 0.058 mol) and NH 4 Cl (19 mg, 0.36 mmol) in MeOH (20 mL) was stirred for 2 h at 25 °C.A white solid (AgCl) immediately precipitated.The suspension was filtered through Celite and NH 4 BF 4 (38 mg, 0.36 mmol) was added.The resulting solution was stirred at 25 °C for 2 h.After this time, a white solid precipitated, which was isolated by filtration.The solid was brought to dryness under vacuum to give compound H 6 -10(BF 4 ) 6 as a white powder.Yield: 70 mg (35 mmol, 60 %).Due the low solubility of H 6 -10(BF 4 ) 6 , not all the carbon atom resonances were detected in the 13 C{ 1 H} NMR spectra.NMR spectra showed resonances for two isomeric reaction products, one with three identical and one with three different cyclobutane-units.The two isomers could not be separated, but the two different spin systems were resolved by 2D 1 H NMR spectroscopy.Due of the overlap between the resonances of the two isomers, the relative amounts of the isomers could not be determined exactly.
, bottom) can be assigned.For the unsymmetric isomer A, three resonances are observed for each of the protons H10, H11 and H13.The symmetric isomer B gives only one set of resonances for H10, H11 and H13.The resonances for the other protons of the isomers fall together in additional multiplets, which could not be resolved.

Figure 3 .
Figure 3.Sections of the 1 H NMR spectra of [8](BF 4 ) 3 (top) and [9](BF 4 ) 3 (bottom).The resonances for isomers A and B are labelled in blue and red, respectively.Overlapping resonances for both isomers are depicted in black.

Figure 5 .
Figure 5. Section of the 1 H, 1 H ROESY NMR spectrum for the isomers of [9](BF 4 ) 3 showing the two independent spin systems (resonances for the unsymmetrical isomer A in blue and for the symmetrical isomer B in red).