Metallosupramolecular Architectures Obtained from Poly-N

Article pubs.acs.org/accounts

Metallosupramolecular Architectures Obtained from Poly-Nheterocyclic Carbene Ligands Narayan Sinha and F. Ekkehardt Hahn* Institut für Anorganische und Analytische Chemie and NRW Graduate School of Chemistry, Westfälische Wilhelms-Universität Münster, Corrensstraße 30, D-48149 Münster, Germany CONSPECTUS: Over the past two decades, self-assembly of supramolecular architectures has become a field of intensive research due to the wide range of applications for the resulting assemblies in various fields such as molecular encapsulation, supramolecular catalysis, drug delivery, metallopharmaceuticals, chemical and photochemical sensing, and light-emitting materials. For these purposes, a large number of coordination-driven metallacycles and metallacages featuring different sizes and shapes have been prepared and investigated. Almost all of these are Werner-type coordination compounds where metal centers are coordinated by nitrogen and/or oxygen donors of polydentate ligands. With the evolving interest in the coordination chemistry of N-heterocyclic carbenes (NHCs), discrete supramolecular complexes held together by M−CNHC bonds have recently become of interest. The construction of such metallosupramolecular assemblies requires the synthesis of suitable poly-NHC ligands where the NHC donors form labile bonds with metal centers thus enabling the formation of the thermodynamically most stable reaction product. In organometallic chemistry, these conditions are uniquely met by the combination of poly-NHCs and silver(I) ions where the resulting assemblies also offer the possibility to generate new structures by transmetalation of the poly-NHC ligands to additional metal centers forming more stable CNHC−M bonds. Stable metallosupramolecular assemblies obtained from poly-NHC ligands feature special properties such as good solubility in many less polar organic solvents and the presence of the often catalyticlly active {M(NHC)n} moiety as building block. In this Account, we review recent developments in organometallic supramolecular architectures derived from poly-NHC ligands. We describe dinuclear (M = AgI, AuI, CuI) tetracarbene complexes obtained from bis-NHC ligands with an internal olefin or two external coumarin pendants and their postsynthetic modification via a photochemically induced single or double [2 + 2] cycloaddition to form dinuclear tetracarbene complexes featuring cyclobutane units. Even three-dimensional cage-like structures can be prepared by this postsynthetic strategy. Cylinder-like trinuclear, tetranuclear, and hexanuclear (M = AgI, AuI, CuI, HgII, PdII) complexes have been obtained from benzene-bridged tris-, tetrakis-, or hexakis-NHC ligands. These complexes resemble polynuclear assemblies obtained from related polydentate Werner-type ligands. Contrary to the Werner-type complexes, cylinder-like assemblies with three, four, or six silver(I) ions sandwiched in between two tris-, tetrakis-, or hexakis-NHC ligands undergo a facile transmetalation reaction to give the complexes featuring more stable M−CNHC bonds, normally with retention of the metallosupramolecular structure. This unique behavior of NHC-Ag+ complexes allows the prepration of assemblies containing various metals from the poly-NHC silver(I) assemblies. Narcissistic self-sorting phenomena have also been observed for mixtures of selected poly-NHC ligands and silver(I) ions. Even a very early type of metallosupramolecular assembly, the tetranuclear molecular square, can be prepared from four bridging dicarbene ligands and four transition metal ions either by a stepwise assembly or by a single-step protocol. At this point, it appears that procedures for the synthesis of metallosupramolecular assemblies using polydentate Werner-type ligands and metal ions can be transferred to organometallic chemistry by using suitable poly-NHC ligands. The resulting structures feature stable M−CNHC bonds (with the exception of the labile CNHC−Ag+ bond) when compared to M−N/M−O bonds in classical Werner-type complexes. The generally good solubility of the compounds and the presence of the often catalytically active {M(NHC)n} moiety make organometallic supramolecular complexes a promising new class of molecular hosts for catalytic transformations and encapsulation of selected substrates.

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INTRODUCTION Starting with the pioneering work by J.-M. Lehn on the synthesis of di- and trinuclear helicates from oligobipyridines and copper(I) ions, the self-assembly of metallosupramolecular architectures has become a field of intensive research.1 The coordination driven self-assembly of a large number of © 2017 American Chemical Society

supramolecular complexes of various sizes and shapes has independently been developed and explored by several research groups.2−5 This research has partly been driven by the search Received: March 31, 2017 Published: August 25, 2017 2167

DOI: 10.1021/acs.accounts.7b00158 Acc. Chem. Res. 2017, 50, 2167−2184

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Accounts of Chemical Research Scheme 1. Synthesis of Dinuclear Tetracarbene Complexes

for host molecules for the encapsulation of small molecular guests.6 In addition, reactive intermediates have been stabilized in the cavities of such metallosupramolecular assemblies.7 Selected molecular hosts have also been used as nanoreactors,8 where unusual catalytic reactions have been performed, and as drug delivery and release systems.9 Recently, some of the supramolecular complexes have been used as optical materials with light-harvesting and emissive properties.10 Most of the metallosupramolecular architectures described above are Werner-type coordination complexes where the metal centers are coordinated by nitrogen and/or oxygen donors of polydentate ligands. Over the last 10 years Nheterocyclic carbenes (NHCs)11,12 have emerged as a new class of potential donors for the synthesis of metallosupramolecular architectures featuring M−CNHC bonds. This Account will summarize recent efforts regarding the synthesis of metallosupramolecular assemblies obtained from poly-NHC ligands.

Figure 1. Molecular structure of dication [Au2(1)2]2+.

organized approach for the functionalization and modification of various supramolecular complexes in solution as well as in the solid state.17 The formation of a cyclobutane ring by a [2 + 2] cycloaddition requires a parallel arrangement of two CC bonds with a separation of less than 4.2 Å. Although PSM through photochemically induced [2 + 2] cycloaddition has been widely applied to polyhedra featuring polydentate Werner-type ligands, the photochemically induced [2 + 2] cycloaddition reaction for the PSM of organometallic complexes featuring poly-NHC ligands had not been studied until recently. For the PSM by [2 + 2] cycloaddition to be extended to the functionalization of organometallic molecular rectangles, the olefin-bridged dicarbene ligand precursors H22(PF6)2 and H23(PF6)3 have been prepared.18 The reaction of H22(PF6)2 or H23(PF6)3 with Ag2O gave the dinuclear AgI tetracarbene complexes [Ag2(2)2](PF6)2 and [Ag2(3)2](PF6)2 in good yields (Scheme 2). UV irradiation (Hg lamp, λ = 365 nm) of the AgI complexes [Ag2(2)2](PF6)2 and [Ag2(3)2](PF6)2 for 2.5 h in DMSO-d6 or CD3CN resulted in the formation of the rcttcyclobutane-silver(I) tetracarbene complexes [Ag2(4)](PF6)2 and [Au2(5)](PF6)2. The conversion was quantitative and stereospecific as judged by NMR spectroscopy. An X-ray diffraction structure analysis (Figure 2) confirmed the formation of complex cation [Ag2(5)]2+. The cyclobutane ring features typical C−C single bond distances but distorted intraring C−C−C bond angles of ∼90°.18 The olefin complexes [Ag2(2)2](PF6)2 and [Ag2(3)2](PF6)2 were found to be stable in solution. However, in the solid state they are light sensitive, and therefore, the potential singlecrystal to single-crystal [2 + 2] cycloaddition could not be studied in the solid state. Therefore, the more stable dinuclear AuI tetracarbene molecular rectangles [Au2(2)2](PF6)2 and [Au2(3)2](PF6)2 were synthesized via transmetalation from the corresponding AgI complexes (Scheme 2).

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MOLECULAR RECTANGLES AND THEIR POSTSYNTHETIC MODIFICATION Molecular rectangles obtained from bis-NHC ligands constitute the simplest derivatives in supramolecular NHC chemistry. Most of these were synthesized from two bis-NHC ligands and two transition metal ions using the Ag2O method13 followed by a transmetalation reaction. This transmetalation reaction also proved useful for the preparation of various other supramolecular complexes that will be discussed later. The benzene-bridged bisimidazolium salt H21(PF6)2 has been prepared and used for the preparation of organometallic molecular rectangles. Reaction of equimolar amounts of H21(PF6)2 and Ag2O under exclusion of light in acetonitrile yielded the dinuclear AgI tetracarbene complex [Ag2(1)2](PF6)2 (Scheme 1).14 Subsequently, the dicarbene ligands in [Ag2(1)2](PF6)2 were transferred to AuI ions via transmetalation to yield the digold(I) tetracarbene complex [Au2(1)2](PF6)2. The molecular structure analysis of [Au 2 (1) 2 ](PF 6 ) 2 confirmed the formation of a dinuclear complex from two AuI ions bridged by two dicarbene ligands (Figure 1). The gold atoms are coordinated by two NHC donors from different ligands and feature an almost linear coordination geometry with a Au···Au separation of 7.2 Å. Postsynthetic modifications (PSMs) of supramolecular architectures have become a powerful tool for the construction of complex structures with fascinating properties.15,16 PSMs have been used for the introduction of various functional groups into discrete supramolecular structures15 and metal− organic frameworks (MOFs).16 PSM through photochemically induced [2 + 2] cycloaddition reactions has furnished a well2168


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Accounts of Chemical Research Scheme 2. Synthesis and PSMs of Dicarbene-Bridged Molecular Rectangles

rather slowly in the [2 + 2] cycloaddition, and complex [Au2(5)](PF6)2 was obtained in only 25% yield together with unreacted [Au2(3)2](PF6)2 even after 72 h of irradiation. It has been assumed that this slow and low conversion might be due to an improper alignment of the CC double bonds in the solid state of [Au2(3)2](PF6)2. This assumption was confirmed by molecular structure analyses (Figure 3). In the solid state, because of interactions between the n-butyl substituents, the {Au(NHC)2} units in [Au2(3)2]2+ are rotated relative to each other, thereby forcing the two CC double bonds into an orthogonal arrangement and thus preventing the [2 + 2] cycloaddition. Contrary to this situation, the CC double bonds in [Au2(2)2]2+ are aligned in a parallel fashion enabling the solid-state to solid-state conversion. The cyclobutanebridged tetrakisimidazolium salts H44(PF6)4 and H45(PF6)4 can be liberated from the labile silver(I) complexes [Ag2(4)](PF6)2 and [Ag2(5)](PF6)2 by reaction with NH4Cl in methanol followed by anion exchange with NH4PF6.18 Benzene- or anthracene-bridged bisimidazolium salts H26(PF6)2 and H27(PF6)2 with two terminal coumarin pendants were used for the preparation of complex cations of

Figure 2. Molecular structure of dication [Ag2(5)]2+.

The PSM of [Au2(2)2](PF6)2 and [Au2(3)2](PF6)2 by [2 + 2] cycloaddition gave different results in solution and in the solid state. In solution, the [2 + 2] cycloaddition proceeded for both complexes to give complexes [Au2(4)](PF6)2 and [Au2(5)](PF6)2, respectively. UV irradiation (Hg lamp, λ = 365 nm) of a powdered crystalline sample of [Au2(2)2](PF6)2 for 12 h also resulted in the formation of the cyclobutane derivative [Au2(4)](PF6)2 in quantitative yield. Contrary to this, complex [Au2(3)2](PF6)2 bearing an n-butyl instead of an ethyl-substituted dicarbene ligand reacted upon irradiation 2169


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Accounts of Chemical Research

Figure 3. Molecular structures of cations [Au2(3)2]2+ (top) and [Au2(2)2]2+ (bottom) showing the alignment of the CC double bonds.

type [M2(bis-NHC)2]2+ (M = Ag, Cu, and Au) featuring terminal double bonds at the dicarbene donors (Scheme 3).19 The dicarbene ligands from the silver complexes [Ag2(6)2](PF6)2 and [Ag2(7)2](PF6)2 were transferred to AuI via transmetalation to yield the dinuclear gold(I) tetracarbene complexes [Au2(6)2](PF6)2 and [Au2(7)2](PF6)2, respectively. Gold complex [Au2(7)2](PF6)2 was characterized by an X-ray diffraction study confirming the formation of the dinuclear tetracarbene cation [Au2(7)2]2+ with four pendant coumarin groups arranged in a suitable orientation for a subsequent [2 + 2] cycloaddition (Figure 4). In principle, the photodimerization of coumarin can lead to four isomeric photodimers featuring cyclobutane rings composed of syn-head-to-head (syn-HH), anti-head-to-head (anti-HH), syn-head-to-tail (syn-HT), and anti-head-to-tail (anti-HT) coumarin building blocks. Because of the preorganization of the coumarin groups in [Au2(7)2]2+, only head-tohead (syn-HH and anti-HH) [2 + 2] cyclization products can form. The molecular structure of [Au2(7)2]2+ (Figure 4, bottom) shows two adjacent coumarin units arranged in a

Figure 4. Molecular structure of dication [Au2(7)2]2+ (top) and its schematic representation showing the orientation of the coumarin pendants (bottom).

nearly coplanar fashion due to π-stacking interactions with a separation of the midpoints of the C8=C9 double bonds of ∼4.2 Å. This alignment would favor the formation of the synHH [2 + 2] cycloaddition product. UV irradiation (Hg lamp, λ = 365 nm) of complexes [M2(6)2](PF6)2 (M = Ag, Cu, Au) and [M2(7)2](PF6)2 (M = Ag, Au) in DMSO-d6 or CD3CN at ambient temperature indeed yielded the syn-HH cycloaddition products [M2(8)](PF6)2 (M = Ag, Cu, Au) and [M2(9)](PF6)2 (M = Ag, Au), respectively (Scheme 4). These complexes bear macrocyclic tetracarbene ligands with the syn-HH stereochemistry of the cyclobutane rings established by 1H NMR

Scheme 3. Synthesis of Dinuclear AgI, CuI, and AuI Tetracarbene Complexes Featuring Four Coumarin Pendants

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Accounts of Chemical Research Scheme 4. Double [2 + 2] Cycloaddition in Coumarin-Tethered Tetracarbene Complexes

spectroscopy.19 In addition, photocleavage of the cyclobutane rings of [Au2(9)](PF6)2 upon irradiation at λ = 254 nm was performed and confirmed by the characteristic absorption for free coumarin groups at λ = 322 nm in the UV/vis spectrum. Hence, the photocleavage of the cyclobutane reconstituted the original complex [Au2(7)2](PF6)2 (Scheme 4). In a different approach, a tetranuclear molecular rectangle built from two bridging benzobiscarbene ligands20 and two bridging diphosphine ligands was prepared. Reaction of the benzobisimidazolium salt H210(Br)2 with Ag2O in dicholoromethane followed by addition of [AuCl(THT)] led to the dinuclear gold(I) complex [Au2(10)(Cl)2]. The subsequent reaction of 2 equiv of [Au2(10)(Cl)2] with 2 equiv of 1,2bis(diphenylphosphino)ethane (dppe) in the presence of 4 equiv of silver tetrafluoroborate gave the molecular rectangle [Au4(10)2(dppe)2](BF4)4 (Scheme 5).21 Only one resonance for the phosphorus atoms in [Au4(10)2(dppe)2](BF4)4 was observed in the 31P NMR spectrum, and the 13C{1H} NMR spectrum revealed the carbene carbon resonance at δ = 197.6 ppm as a doublet of doublets with coupling constants of 2JC,P = 124.5 Hz and 5JC,P = 1.6 Hz. The X-ray diffraction analysis confirmed the formation of the tetracation [Au4(10)2(dppe)2]4+, which is best described as a molecular rectangle with the opposing edges seized by two diphosphine and two dicarbene ligands, respectively (Figure 5). The two dicarbene ligands are aligned in an almost coplanar fashion, and their transannular separation measures ∼3.3 Å. This was taken as an indication for π···π interactions between the phenylene rings. The Au1···Au2 separation was calculated at 3.3108(2) Å, which is too large for an aurophilic interaction.

Scheme 5. Synthesis of the Tetragold Molecular Rectangle [Au4(10)2(dppe)2](BF4)4

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CYLINDER-LIKE ASSEMBLIES AND THEIR POSTSYNTHETIC MODIFICATION The first three-dimensional organometallic assembly reported by Meyer et al.22,23 was built up from three silver(I) ions and

Figure 5. Molecular structure of tetracation [Au4(10)2(dppe)2]4+.

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Accounts of Chemical Research Scheme 6. Synthesis of Cylinder-like Trinuclear Hexacarbene Complexes

two tripodal tricarbene ligands of type CH3−C(CH2−NHC)3. Later, a hexasilver dodecacarbene cylinder was obtained serendipitously from two equiv of a macrocyclic hexakisimidazolium salt and Ag2O.24 We became interested in the synthesis of organometallic cylinder-like structure from tricarbene ligands such as H311(PF6)3. With this tricarbene ligand precursor, trinuclear AgI, AuI, CuI, HgII, and PdII hexacarbene cylinder-like assemblies were prepared (Scheme 6).14,25−27 The reaction of the trisimidazolium salt H311(PF6)3 with Ag2O under exclusion of light in acetonitrile afforded the trisilver complex [Ag3(11)2](PF6)3. Transmetalation of [Ag3(11)2](PF6)3 with 3 equiv of [AuCl(SMe2)] or CuBr in acetonitrile gave the trinuclear AuI or CuI complexes [Au3(11)2](PF6)3 and [Cu3(11)2](PF6)3, respectively, with retention of the three-dimensional structures.25,26 The reaction of the trisimidazolium salt with Hg2O or [Pd(allyl)Cl]2 in the presence of Cs2CO3 in acetonitrile led to the formation of trinuclear HgII and PdII hexacarbene cylinders [Hg3(11) 2](PF6) 6 and [Pd3(11) 2(allyl)3 ](PF 6) 3 (Scheme 6).14,27

The cylinder-like trinuclear (M = AgI, AuI, CuI, HgII, PdII) hexacarbene complexes are reasonably soluble in polar solvents and could therefore be characterized by NMR spectroscopy in addition to X-ray diffraction analysis. The structure analyses revealed nonparallel almost linear CNHC−M−CNHC bonds except for the tripalladium cation [Pd3(11)2(allyl)3]3+, where a cis arrangement of the NHC donors at the metal centers (angles CNHC−Pd−CNHC 91.2(4)°−95.5(4)°) was found.27 Selected trimetallic hexacarbene molecular cylinders are depicted in Figure 6. Benzene-bridged tetrakis-NHC ligands also led to cylinderlike complexes (Scheme 7). The reaction of the tetrakisimidazolium salt H412(PF6)4 with Ag2O under exclusion of light gave the tetrasilver complex [Ag4(12)2](PF6)4. The tetrakis-NHC ligands in this silver complex can be transmetalated to AuI to give the tetragold complex [Au4(12)2](PF6)4 with retention of the three-dimensional structure (Scheme 7).26 In addition to the tetrapodal ligand precursor H412(PF6)4, a macrocyclic tetrakisimidazolium salt has also been reported to react with 2172


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Figure 6. Molecular structures of cations [M3(11)2]3+ (M = Ag, Au), [Hg3(11)2]6+, and [Pd3(11)2(allyl)3]3+. Only the first atom of each of the Nethyl substituents is shown.

Scheme 7. Synthesis of Cylinder-like Tetranuclear Octacarbene Complexes

Figure 7. Molecular structures of [Ag4(12)2]4+ and [Au4(12)2]4+. Only the first atom of each of the N-nBu substituents is shown. 2173


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Accounts of Chemical Research Scheme 8. Synthesis of Nanometer-Sized Cylinder-like Assemblies

Figure 8. Molecular structures of cations [Au3(13)2]3+ (left) and [Au3(14)2]3+ (right). Only the first atom of each of the N-nBu substituents (for [Au3(13)2]3+) and of each of the N-Et substituents (for [Au3(14)2]3+) is shown.

Ag2O to give a cylinder-like structure composed of four AgI ions sandwiched between two macrocyclic tetracarbene ligands.28 Even a hexakisimidazolium-substituted benzene has been described and reacted with Ag2O to give a cylinder-like hexanuclear dodecacarbene molecular assembly.29

Both [Ag4(12)2](PF6)4 and [Au4(12)2](PF6)4 were characterized by NMR spectroscopically and crystallographically. The octacarbene complex cations [Ag4(12)2]4+ and [Au4(12)2]4+ (Figure 7) feature an essentially planar M4 rectangle. 2174


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Accounts of Chemical Research Scheme 9. Synthesis of Cylinder-like Assemblies Featuring Six Coumarin Pendants

Au···Au separation found in the trigold cation [Au3(11)2]3+ derived from benzene-bridged tris-NHC ligands (∼6.0 Å). Similarly, cation [Au3(14)2]3+ features three AuI ions forming a distorted triangle where the Au···Au separations measure ∼1.4 nm. As was observed for cation [Au3(13)2]3+, the two tris-NHC ligands in [Au3(14)2]3+ also adopt a concave conformation with the midpoints of the ligands pointing toward each other and featuring a separation of the centroids of the central rings of 3.644 Å. This short separation can be considered as an indication for the presence of π···π interactions. The trinuclear cylinder-like hexacarbene complexes of type [M3(11)2]3+ (M = Ag, Au, Cu; Scheme 6) feature much larger centroid···centroid separations of the central phenyl rings (M = Ag, 4.914 Å; M = Au, 4.762 Å; M = Cu, 4.239 Å; Figure 6). By assuming a pseudotrigonal-prismatic structure for trication [Au3(14)2]3+, the estimated volume of the internal cavity measures 360 Å3, which is much larger than the internal volume estimated for the previously described trinuclear gold(I) hexacarbene cation [Au3(11)2]3+ (72 Å3; Figure 6). The larger internal cavity is much more suitable for the encapsulation of small guest molecules. It should be mentioned that recently a report has appeared describing a complex with the same topology as

The cylinder-like assemblies obtained from benzene-bridged poly-NHC ligands feature rather small cavities. These are normally not suitable for the encapsulation of even small guest molecules. As a consequence, cylinder-like assemblies from trisNHC ligands featuring large π-conjugated aromatic backbone groups have been prepared. Two trisimidazolium salts H313(PF6)3 and H314(PF6)3 featuring a triphenylamine or a 1,3,5-triphenylbenzene backbone, respectively, were reacted with Ag2O in acetonitrile under exclusion of light to give the trinuclear AgI hexacarbene complexes [Ag3(13)2](PF6)3 and [Ag3(14)2](PF6)3 (Scheme 8).30,31 By a transmetalation reaction, the silver-carbene complexes were converted into the trigold derivatives [Au3(13)2](PF6)3 and [Au3(14)2](PF6)3, and the three-dimensional structures remain intact. The copper(I) complex [Cu3(14)2](PF6)3 was also prepared via transmetalation from [Ag3(14)2](PF6)3 to CuI.31 X-ray diffraction analyses shed light on the metric parameters of the trinuclear AuI hexacarbene complex cations [Au3(13)2]3+ and [Au3(14)2]3+ (Figure 8). In trication [Au3(13)2]3+, the triphenylamine backbones are not planar but are bent in a concave distortion relative to each other. The three gold(I) ions form a triangle with Au···Au separations of ∼1.1 nm. This separation is much larger than the 2175


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Accounts of Chemical Research [Au3(14)2](PF6)3 but featuring mesoinic carbene (MIC) donor groups.32 Complex cations of type [M3L2]3+ (L = 11, 13, 14) are sandwich-shaped trinuclear assemblies composed of two disktype ligands that bear three NHC donor groups as the coordination sites. The planar donor groups are forced out of the plane of the central planar building block. Rotation of all three donors of one disk-shaped ligand in the same direction results in the formation of helical structures with P and M geometries as was confirmed by X-ray diffraction studies showing racemic mixtures of the P and M forms in the solid state. This feature has been noted previously for trinuclear silver(I) complexes of disk-shaped ligands bearing three thiazolyl or pyridyl groups where the interconversion of the P and M forms in solution has been studied.33 It appears that poly-NHC coordination compounds behave in this regard very similar to the classical Werner-type complexes, although so far only the formation of P and M isomers and not their interconversion has been demonstrated. Host−guest chemistry has been the driving force in the development of supramolecular interactions. In this regard, the metallospramolecular assemblies obtained from tris- or tetrakisNHC ligands have so far been disappointing as only irregular solvent incorporation and no encapsulation of substrates has been observed. Apparently, the cavity in these assemblies is still too small. However, the cavitand constructed from two macrocyclic tetrakis-NHC/dipyrazolato ligands and 8 Ag+ ions has been prepared. Its geometry is best described as sandwich-like with a tubular structure due to the macrocyclic nature of the ligand resembling the structure of the wellestablished class of organic pillar[n]-arenes.34 The analogous octagold complex is not only emissive as a solid in the blue region of the visible spectrum but also capable of selectively encapsulating linear molecules such as 1,8-diaminooctane. To study PSM on cylinder-like assemblies, we have synthesized trisimidazolium salts H 3 15(PF 6 ) 3 and H316(PF6)3, each featuring three pendant coumarin groups. These coumarin groups, when properly oriented, allow the generation of a hexacarbene cage upon a triple [2 + 2] cycloaddition. NHC precursors H315(PF6)3 or H316(PF6)3 react with Ag2O in acetonitrile under exclusion of light to give the trisilver hexacarbene complexes [Ag3(15)2](PF6)3 and [Ag3(16)2](PF6)3, respectively (Scheme 9).35 Transmetalation of the NHC ligands to give the trigold complexes [Au3(15)2](PF6)3 and [Au3(16)3](PF6)3 was also performed with [AuCl(THT)]. An X-ray diffraction analysis confirmed the formation of complex cation [Ag3(15)2]3+ with three silver ions sandwiched in between two tris-NHC ligands, each featuring three coumarin pendants (Figure 9).35 The three AgI ions in [Ag3(15)2]3+ form a triangle with Ag···Ag separations of ∼1.1 nm. Interestingly, the six coumarin pendants in [Ag3(15)2]3+ form three pairs with the distances between the midpoints of the C=C double bonds of the coumarin groups falling in the range of 3.662−3.881 Å. This might be due to π-stacking interactions (Figure 9) and will be advantageous for a subsequent [2 + 2] cycloaddition (Scheme 10). Irradiation (Hg lamp, λ = 365 nm) of [Ag3(15)2](PF6)3 in DMSO-d6 for 5 h at ambient temperature resulted in the formation of the trinuclear silver(I) hexacarbene complex [Ag3(17)](PF6)3 featuring a three-dimensional hexakis-NHC ligand cage (Scheme 10).35 The conversion from [Ag3(15)2](PF6)3 to [Ag3(17)](PF6)3 was found to be quantitative as

Figure 9. Molecular structure of [Ag3(15)2]3+ showing the orientation of the coumarin groups.

determined by 1H NMR spectroscopy. Complex [Ag3(17)](PF6)3 features three cyclobutane rings, two of which possess the syn-HH stereochemistry and the third featuring an anti-HH stereochemistry as could be expected from the orientations of the coumarin groups in the solid-state structure of [Ag3(15)2]3+ (Figure 9). From the HR-ESI mass spectra, it was concluded that only intramolecular photodimerization occurred. A similar result was observed with the trigold complex [Au3(15)2](PF6)3 to give via triple [2 + 2] cycloaddition complex [Au3(17)](PF6)3. Analogously, complexes [M3(16)2](PF6)3 (M = Ag or Au) underwent the photochemical [2 + 2] cycloaddition upon UV irradiation to give complexes [M3(18)](PF6)3 (M = Ag or Au) (Scheme 10), each featuring three cyclobutane rings. Interestingly, the cyclobutane rings in [M3(18)](PF6)3 (M = Ag or Au) all possess the syn-HH stereochemistry as was confirmed by 1H NMR spectroscopy. Complexes [M3(17,18)](PF6)3 are the first examples for organometallic complexes bearing cage-like three-dimensional hexacarbene ligands. Removal of metal ions from the silver complexes [Ag3(17,18)](PF6)3 leads to three-dimensional hexakisimidazolium salts. Such cyclophanes are promising anion receptors36 and might also find applications as chemosensors or molecular nanoreactors for selected catalytic reactions. Cage-like hexakisimidazolium salts or macrocyclic polyimidazolium salts are normally rather difficult to prepare by classical organic synthesis, particularly if derivatives with large internal cavities are desired. The template approach presented in Schemes 4 and 10 provides easy access to such compounds and allows the synthesis of rather large derivatives. It therefore should find various applications.

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NARCISSISTIC SELF-SORTING BEHAVIOR OF CYLINDER-LIKE ASSEMBLIES The known narcissistic self-sorting systems are based on Werner-type coordination compounds featuring metal centers coordinated by nitrogen and/or oxygen donor atoms of polydentate ligands.37 Related narcissistic self-sorting behavior of metallosupramolecular architectures obtained from polyNHC ligands has not been described until recently. The advantage of Werner-type bonding interactions is their reversibility, which allows rearrangements in a complex mixture to form the thermodynamically most stable assembly. Because of the stability of metal−carbon bonds and their normally irreversible formation, narcissistic self-sorting in classical organometallic compounds has not been studied. However, it is known that Ag−CNHC bonds are labile and that the carbene ligand can be transferred from silver complexes to other metal 2176


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Accounts of Chemical Research Scheme 10. Triple [2 + 2] Cycloaddition in Trinuclear Cylinder-like Complexes

good carbene transfer agents, transmetalation was also studied. A mixture of the three silver complexes [Ag3(11)2](PF6)3, [Ag3(19)2](PF6)3, and [Ag3(14)2](PF6)3 reacted with [AuCl(SMe2)] in a one-pot reaction to give exclusively the gold complexes [Au 3 (11) 2 ](PF 6 ) 3 , [Au 3 (19) 2 ](PF 6 ) 3 , and [Au3(14)2](PF6)3 with retention of metallosupramolecular architectures. No mixed-ligand or oligomeric products were observed, confirming the consecutive narcissistic self-sorting phenomenon.30 In an extension of the self-sorting study, another mixture of polyimidazolium salts composed of benzene-bridged tris-NHC ligand precursor H311(PF6)3 and benzene-bridged tetrakisNHC ligand precursor H420(PF6)4 was studied (Scheme 12). In this case, the ligand precursors feature the same backbone but a different number and orientation of the donor groups. Reaction of H311(PF6)3 and H420(PF6)4 with Ag2O in CH3CN at 60 °C over 2 d under exclusion of light produced only two organometallic molecular cylinders, [Ag3(11)2](PF6)3 and [Ag4(20)2](PF6)4, by narcissistic self-sorting.30 Apparently, the different number of donor groups of the ligands functioned as a molecular code in the control of the narcissistic self-sorting. Transmetalation of the tris- and tetrakis-NHC ligands from AgI to AuI with formation of complexes [Au3(11)2](PF6)3 and [Au4(20)2](PF6)4 was subsequently achieved in a one-pot procedure using [AuCl(SMe2)] (Scheme 12). Again, the

ions. Thus, metal−CNHC complexes may form a suitable combination to study self-sorting phenomena in organometallic metallosupramolecular assemblies. For such a self-sorting study, the trisimidazolium salts H311(PF6)3, H319(PF6)3, and H314(PF6)3 (Scheme 11) with different spacers but an identical number and type of donor groups were prepared. The one-pot reaction of 2 equiv each of H311(PF6)3, H319(PF6)3, and H314(PF6)3 with 9 equiv of Ag2O in acetonitrile at 60 °C for 2 d under exclusion of light resulted in the formation of three trinuclear AgI hexacarbene molecular cylinders, [Ag3(11)2](PF6)3, [Ag3(19)2](PF6)3, and [Ag3(14)2](PF6)3, respectively, each containing only one type of ligand.30 The three ligand precursors have exactly the same predonor groups (three ethyl-substituted imidazolium groups), and consequently, there exists a high probability for the formation of mixed ligand products or oligomeric complexes. However, no such crossover products were observed. The three ligands possess a backbone systematically increasing in size leading to an increase in the metal−metal distances in their trinuclear organometallic assemblies. We therefore assume that self-recognition was achieved by controlling the size of the cylinders formed.30 The one-pot formation of only three trisilver hexacarbene complexes was confirmed by 1H NMR spectroscopy and HRESI mass spectroscopy. Because silver carbene complexes are 2177


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Accounts of Chemical Research Scheme 11. Synthesis of Trinuclear AgI or AuI Hexacarbene Cylinder-like Assemblies in a One-Pot Reaction through Narcissistic Self-Sorting

Scheme 12. Narcissistic Self-Sorting to Give Organometallic Cylinder-like Assemblies from Poly-NHC Ligands with an Identical Backbone but Different Number of Donors

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Scheme 13. Rigidification of a Tetrakis-NHC Ligand by Complex Formation Leading to Highly Emissive TPE Derivatives

Scheme 14. Synthesis of a CpNiII-Capped Tetranuclear Molecular Assembly

other factors than aggregation. Exactly this situation is found in complexes [M221](PF6)2 (M = Ag+, Au+), where complex formation prevents the phenyl group rotation. In fact, the almost nonemissive imidazolium salts of type H411(PF6)4 show large fluorescence enhancement upon formation of the tetracarbene complexes [M221](PF6)2 (M = Ag+, Au+) in dilute solution. The complex formation can thus be considered an alternative to AIF. The related tetra-(4-pyridylphenyl)ethylene L forms together with a dicarboxylate coligand L′ and PtII octanuclear tetragonal-prismatic 2 + 4 + 8 supramolecular assemblies of type [Pt8L2L′4]8+, which are also highly emissive.10

metallosupramolecular structures were retained in the transmetalation, and no crossover products were observed. It should be noted that the type of metal determines what coordination compound is formed from tetrakis-NHC 20. Whereas linearly coordinated metals ions lead to complexes of type [M4(20)2](PF6)4 (M = Ag+, Au+),30 square-planar coordinated metal ions such as Pt2+ featuring two vacant cis positions at the metal center yield dinuclear complexes [Pd2(20)Br4], where two donors in 1,2-positions at the benzene ring coordinate the metal center in chelating fashion.26 A similar geometric situation exists in the tetraphenylethylenederived tetrakisimidazolium salt H421(PF6)4. With silver(I), tetracarbene 21 does not form the anticipated tetranuclear cylinder-like assembly of type [Ag4(21)2](PF6)4 but instead forms the dinuclear complexes [Ag221](PF6)2. Apparently, the enlarged distance between the NHC donors in 21, when compared to 20, can accommodate the linearly coordinated coinage metal ions and thus prevents the formation of the tetranuclear cylinder-like assembly (Scheme 13).38 Tetraphenylethylene (TPE) is a widely used chromophore that in the solid state exhibits strong aggregation-induced fluorescence (AIF). This fluorescence is a consequence of the restricted rotation of the phenyl rings and a twist of the C=C bond in the solid state. Consequently, fluorescence may be turned on by

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ORGANOMETALLIC MOLECULAR SQUARES Among the coordination-driven supramolecular architectures, M4L4-type molecular squares form the first and foremost studied class of compounds starting with the early work by Fujita et al. on an “inorganic” molecular square via self-assembly of end-capped PdII and 4,4′-bipyridine.39 Subsequently, related molecular squares were investigated by Stang et al.40 and others. Most of these molecular squares are built from four endcapped metals coordinated by four bidentate spacers having nitrogen or oxygen donor atoms. We set out to prepare a purely 2179


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Accounts of Chemical Research “organometallic” molecular square featuring only internal M− CNHC bonds. First, progress in this direction was made in 2008 with the preparation a molecular square featuring two dicarbene and two 4,4′-bipyridine bridging ligands.41 The reaction of benzobisimidazolium salt H210(Br)2 (Scheme 5) with nickelocene yielded the dinuclear NiII complex [Ni2(10)(Br)2]. Abstraction of the bromido ligands from [Ni2(10)(Br)2] with AgBF4 in dicholomethane followed by addition of 4,4′-bipyridine gave the tetranickel molecular square [Ni4(10)2(bpy)2](BF4)4 (Scheme 14). Molecular assembly [Ni4(10)2(bpy)2]4+ (Figure 10) features Ni···Ni separations of 10.910(2) (bpy) and 10.380(2) Å (bis-

NHC). These values are almost equidistant. However, cation [Ni4(10)2(bpy)2]4+ does not exactly represent a molecular square due to different bridging groups and different Ni···Ni separations, and it is therefore best described as a molecular rectangle. With the aim to prepare a molecular square held together exclusively by M−CNHC bonds, a different strategy was followed. It is known that β-functionalized isocyanides coordinated to electron-poor metal centers can cyclize in a template-controlled reaction to give NHC ligands.42 A β,β′difunctionalized phenyl-1,4-diisocyanide ligand would therefore make an interesting building block for the construction of a molecular square linked by four dicarbene ligands. The diisocyanide could first be used to construct a molecular square from four bridging diisocyanide ligands, and these diisocyanides could then be converted into bridging dicarbene ligands. Compound β,β′-bis(triisopropylsiloxy) phenyl-1,4diisocyanide 22 (Scheme 15) reacts with [Ir(Cp*)Cl2]2 in dichloromethane to afford the dinuclear complex [Ir2(22)(Cp*)2(Cl)4]. A subsequent reaction of equimolar amount of [Ir2(22)(Cp*)2(Cl)4] and diisocyanide 22 in the presence of AgBF4 in acetonitrile led to the formation of the tetranuclear iridium(III) complex [Ir4(22)4(Cp*)4(Cl)4](BF4)4. Finally, the reaction of [Ir4(22)4(Cp*)4(Cl)4](BF4)4 with isopropanolic HCl led to cleavage of all Si−O bonds followed by intramolecular attack of the liberated hydroxyl groups at adjacent isocyanide functions with formation of the molecular square [Ir4(23)4(Cp*)4(Cl)4](Cl)4 featuring four bis(NH,ONHC) dicarbene ligands (Scheme 15). 43 Complex [Ir4(23)4(Cp*)4(Cl)4](Cl)4 represents the first organometallic molecular square featuring four bridging dicarbene ligands and four metal ions. Compound [Ir4(23)4(Cp*)4(Cl)4](Cl)4 was characterized by an X-ray diffraction study (Figure 11). This study revealed an

Figure 10. Molecular structure of the tetracation [Ni4(10)2(bpy)2]4+. Only the first atom of each of the N-nBu substituents is shown for clarity.

Scheme 15. Stepwise Synthesis of an Organometallic Molecular Square

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This methodology was extended to the preparation of molecular assemblies composed of a classical benzobiscarbene bridge and a second linker generated from a β,β′-difunctionalized phenyl-1,4-diisocyanide. Reaction of benzobisimidazolium salt H 2 24(I) 2 with [PtCl 2 (dmpe)] (dmpe = bisdimethylphosphino)ethane) in the presence of sodium acetate yielded the dinuclear complex [Pt2(24)(dmpe)2(I)2](I)2. This complex was subsequently reacted with AgPF6 in acetonitrile to give complex [Pt2(24)(dmpe)2(NCCH3)2](PF6)4. Treatment of [Pt2(24)(dmpe)2(NCCH3)2](PF6)4 with 2 equiv of diisocyanide ligand 22 followed by removal of the four SiiPr3 protection groups with HBF4·Et2O in the presence of tetrabutylammonium fluoride resulted in the clean formation of molecular square [Pt4(23)2(24)2(dmpe)4](BF4)8 (Scheme 16).44 The X-ray diffraction analysis with crystals of [Pt4(23)2(24)2(dmpe)4](BF4)8 confirmed the unique presence of two different bridging dicarbene ligands located on opposite sides of the molecular assembly (Figure 12). The Pt···Pt separations measured 10.534 for the bis(NH,O-NHC) and 10.671 for the bis(NR,NR-NHC) linkers, respectively, making the complex cation almost a perfect molecular square. Although the two molecular squares [Ir4(23)4(Cp*)4(Cl)4](Cl)4 and [Pt4(23)2(24)2(dmpe)4](BF4)8 feature exclusively bridging dicarbene ligands, their synthesis is rather cumbersome, and the template-controlled cyclization of the bridging diisocyanides to the dicarbene ligands is only possible at electron-poor metal centers, thereby limiting the general

Figure 11. Molecular structure of complex cation [Ir4(23)4(Cp*)4(Cl)4]4+.

Ir···Ir separation of 10.5000(9) Å. This value is much shorter than the Ir···Ir separation measured in the dinuclear diidocyanide-bridged complex [Ir 2 (22)(Cp*) 2 (Cl) 4 ] (11.5787(8) Å). Shuttling between the dicarbene and the diisocyanide bridge, which can be initiated by a reduction of the metal centers to stabilize the isocyanide via M → C backbonding, might therefore initiate a unique change in the size of the molecule upon an electrical impulse.

Scheme 16. Stepwise Synthesis of a Molecular Square with Two Different Bridging Dicarbene Ligands

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Reaction of 4 equiv of benzobisimidazolium salt H224(PF6)2 with 2 equiv of [Pd(allyl)2Cl]2 or [Ir(COD)Cl]2 in the presence of Cs2CO3 in acetonitrile resulted in the formation of the tetranuclear octacarbene molecular squares [Pd4(24)4(allyl)4](PF6)4 and [Ir4(24)4(COD)4](PF6)4, respectively (Scheme 17).45 Both complexes feature exclusively M−C bonds, making them the first purely organometallic molecular squares. An X-ray diffraction analysis with crystals of [Ir4(24)4(COD)4](PF6)4 revealed the formation of the highly symmetrical tetracation [Ir4(24)4(COD)4]4+ (Figure 13). The

Figure 12. Molecular structure of cation [Pt4(22)2(23)2(dmpe)4]8+.

applicability of the protocol. In addition, the metal centers in these assemblies feature additional Ir−Cl or Pt−P bonds that make them not purely organometallic in nature. In searching for purely organometallic molecular squares (featuring metal atoms exclusively bound to carbon donors), a straightforward one-pot procedure was recently developed. This generally applicable and facile method resembles the procedure employed for the preparation of “inorganic” molecular squares from 4,4′-bipyridines and end-capped PdII complexes.39

Figure 13. Molecular structure of tetracation [Ir4(24)4(COD)4]4+.

Scheme 17. Single-Step Synthesis of Organometallic Molecular Squares

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Accounts of Chemical Research Notes

Ir···Ir separations measure 10.6204(6) and 10.6510(6) Å. Two dimethylformamide solvent molecules are encapsulated within the tetracation. The bridging dicarbene ligands are not exactly planar. Two of them (top and bottom) are distorted in a convex fashion relative to the Ir···Ir vector they span, and the other two are bent in a concave fashion relative to the relevant Ir···Ir connection line. Molecular squares obtained from Werner-type ligands are well-known,39,40 and even partially organometallic molecular squares (featuring half-sandwich rhodium and iridium building blocks at the vertices) have been described in detail.46,47 The facile preparation of molecular squares [Pd4(24)4(allyl)4](PF6)4 and [Ir4(24)4(COD)4](PF6)4 resembles the chemistry employed for the preparation of Werner-type molecular assemblies and would indicate that poly-NHC ligands might ultimately exhibit a similar rich coordination chemistry as those of selected Werner-type ligands.

The authors declare no competing financial interest. Biographies Narayan Sinha received his M.Sc. from the Indian Institute of Technology (IIT) in Guwahati, India in 2012 and his Ph.D. from Westfälische Wilhelms-Universität Münster, Germany (Supervisor: Prof. Dr. F. Ekkehardt Hahn) in 2016. His current research interest is the design of new poly-NHC ligands for the preparation of organometallic supramolecular complexes and advanced materials with attractive photophysical properties. F. Ekkehardt Hahn studied chemistry at the Technische Universität Berlin and the University of Oklahoma (M.S. 1982). He graduated with a Dr. rer. nat. from the Technische Universität Berlin in 1985. After a postdoctoral stay with Prof. Raymond at UC Berkeley (1985− 1988), he completed the Habilitation in 1990 and became Associate Professor at the Freie Universität Berlin (1992−1998) before moving in 1998 to a position as Chair of Inorganic Chemistry at the University of Münster. His research is centered on the chemistry of Nheterocyclic carbenes and isocyanide ligands. He was appointed Senior Editor of Chemistry Letters in 2014. Since 2004, he has acted as Permanent Secretary of the International Conference of Organometallic Chemistry (ICOMC).

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CONCLUDING REMARKS AND PROSPECTS This Account demonstrates that by rational ligand design various organometallic metallosupramolecular architectures can be obtained from poly-NHC ligands and transition metal ions. These comprise, among others, polynuclear cylinder-like structures and molecular squares with bridging dicarbene ligands. Postsynthetic modifications via [2 + 2] cycloadditions on assemblies bearing olefin-substituted NHC ligands are also possible. Organometallic supramolecular assemblies are made possible by the development of poly-NHC ligands of various types. The unique properties of the NHC groups allow the preparation of stable polydentate ligands with carbon donors. Selected poly-NHCs can mimick the coordination chemistry of related Werner-type ligands as was demonstrated with the selfsorting observed in the synthesis of cyclinder-like assemblies from poly-NHC ligands and silver ions. In addition, the CNHC− Ag+ interaction is labile in nature, allowing the formation of the thermodynamically most stable reaction product in coordination-driven assemblies. The lability of the CNHC−Ag+ bond also allows the transmetalation of the poly-NHC from silver to additional metal centers. We hope that our findings will advance the field of organometallic assemblies from poly-NHC ligands. For example, the synthesis of large two- and threedimensional polyimidazolium structures via the templatecontrolled [2 + 2] cycloaddition from olefin-substituted polyNHCs promises to become a facile method for the synthesis of new anion receptors, particularly because such compounds are difficult to prepare by standard organic synthesis. Cylinder-like assemblies, if they feature a suitable size of the internal cavity, could develop into new host molecules for selected substrates. The presence of the often catalytically active {M(NHC)n} moiety in the assemblies can also lead to interesting applications in catalysis if the metal centers are capable of interacting with encapsulated substrates. It can therefore be expected that metallosupramolecular assemblies from polyNHC ligands will form a promising area of study in the future.

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ACKNOWLEDGMENTS The authors thank the Deutsche Forschungsgemeinschaft (SFB 858 and IRTG 2027) for financial support. N.S. thanks the NRW Graduate School of Chemistry, Mü n ster for a predoctoral grant.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

F. Ekkehardt Hahn: 0000-0002-2807-7232 2183


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Metallosupramolecular Architectures Obtained from Poly-N

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