Communication pubs.acs.org/Organometallics
Hexanuclear Cylinder-Shaped Assemblies of Silver and Gold from Benzene−Hexa-N-heterocyclic Carbenes Candela Segarra,† Gregorio Guisado-Barrios,† F. Ekkehardt Hahn,*,‡ and Eduardo Peris*,† †
Dpto. de Quı ́mica Inorgánica y Orgánica, Universitat Jaume I, Avda. Vicente Sos Baynat, E-12071 Castellón, Spain Institut für Anorganische und Analytische Chemie, Westfälische Wilhelms-Universität Münster, Corrensstraße 30, D-48149 Münster, Germany
‡
S Supporting Information *
ABSTRACT: In this work we describe the preparation of a new benzene− hexaimidazolylidene ligand, which we have coordinated to silver and gold. The new complexes constitute unique examples of cylinder-like macromolecular arrangements, in which six metal atoms are sandwiched between two hexacarbene ligands.
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preparation of cylinder-type structures, disk-shaped NHCfunctionalized benzenes allowed the preparation of tri- or tetranuclear sandwich-like complexes such as those depicted in Scheme 1.4c,d The metal-controlled self-assembly methodology
uring the last two decades, there has been an increasing interest in the design of macromolecular self-assembled structures with nanocavities.1 The rapid development in the preparation of multicomponent assemblies has grown parallel to the development of new ligands that serve as scaffolds for sophisticated metallo-organic macromolecules, and the complexity of some new structures artificially obtained is starting to rival that of biological systems.1a,2 Scientists became interested in this type of chemistry not only because of the fascinating structures that these supramolecules possess but also because of their unusual sensing, magnetic, optical, and catalytic properties, which make this type of research a highly interdisciplinary field. While all of the early examples for metal−organic assemblies relate to molecular architectures based on Werner-type complexes featuring oxygen or nitrogen donor ligands, in the last 5 years a new generation of organometallic molecular assemblies has been developed as a consequence of the rapid development of N-heterocylic carbene (NHC) chemistry. Among NHC-based metallo-organic assemblies, those containing Ag+ ions have played an important role because of their facile preparation via the Ag2O route. In addition, the coordination chemistry of Ag(I), featuring a linear arrangement that can hold two transoid NHC ligands, has allowed the preparation of a number of macromolecular arrangements, which today include molecular squares and triangles,3 cylinderlike structures,4 and organometallic polymers.5 While a large number of poly-NHC ligands are known,6 those featuring the topologic requirements to serve as suitable bridging ligands for the construction of metallo-organic frameworks are still limited to a few well-defined examples. For example, for the © 2014 American Chemical Society
Scheme 1. Previously Related Cylinder-like NHC-Based Assemblies
used for the preparation of these cylinders constituted elegant examples for the high-yielding syntheses of three-dimensional organometallic molecules with cavities, whose size can be controlled by the thoughtful design of the ligand. In principle, such structures may be used for the development of new chemical entities, in which the reactivity of selected compounds may be confined to the interior of the macromolecular cage or even used for the selective recognition of small neutral or ionic molecules. Received: July 16, 2014 Published: September 11, 2014 5077
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Communication
sandwiched between two benzene−hexaimidazolylidenes. The good solubility of the complexes formed in organic solvents suggested that discrete molecular structures were formed, rather than polymeric structures that should have provided materials with low solubility. At this point it may be worth noting that probably the formation of 3 and 4 may be under thermodynamic control. It may not be discarded that in the formation process the [AgCl(NHC)] complexes are formed first and then reorganized to form the final [Ag(NHC)2](AgCl2) species, which in the presence of an excess of the azolium salt yields the final [Ag(NHC)2]+ complex. Such types of NHC-M species (M = Ag, Au) have recently been described.8 Unfortunately, we could not obtain consistent microanalytical data of either of these two complexes, probably due to a combination of their highly hygroscopic nature and to their slow decomposition in air. The 1H NMR spectrum of the hexaAg-dodecaimidazolylidene complex 3 indicates a highly symmetrical structure (for example, it shows only one resonance due to the 12 methyl protons, at 3.88 ppm). The 13C NMR spectrum of 3 shows the most representative signal due to the magnetically equivalent metalated carbene carbon atoms at 181.4 ppm. Interestingly, this resonance exhibits the rarely observed coupling to both silver isotopes (3: dd, 1JC−107Ag = 182.0 Hz, 1JC−109Ag = 210.1 Hz), which fall in the range described for similar cylinder-type complexes, such as I and II (Scheme 1),4c,d and other related complexes.4a,9 The NMR spectra of 4 reveal a loss of symmetry of the complex, with respect to the highly symmetrical pattern shown by 3. The 1H NMR spectrum shows two resonances due to the protons of the methylene groups. In addition, the signals due to the protons at the imidazolylidene backbones indicate a situation with two different coordination environments in a 2:1 (or 4:2 in our case for six silver centers) ratio. This interpretation is supported by the 13C NMR spectrum, which displays two signals due to the metalated carbene carbons at 182.8 and 181.3 ppm. Slow diffusion of diethyl ether into a concentrated solution of 4 in acetonitrile provided single crystals suitable for X-ray diffraction analysis. The molecular structure of the complex cation (Figure 1) is built from six silver(I) ions sandwiched between two hexacarbene ligands related to each other by a pseudoinversion center. For the formation of 4, the planes of the NHC donors must rotate out of the plane of the central phenyl ring. For steric reasons, the six imidazol-2-ylidene rings of both ligands rotate from an imaginary perpendicular orientation relative to the central phenyl ring in an anticlockwise direction. The relative orientation of the two benzene−hexaimidazolylidene ligands is staggered, as can be seen from a top view of the molecule. The six silver ions form a slightly distorted hexagon, with four Ag−Ag distances being 3.569(7) Å and two being 3.416(6) Å. The shortest distance is slightly shorter than the sum of the van der Waals radii of two silver atoms10 and is very similar to the shortest Ag−Ag distance established in the related tetrasilver complex derived from complex II (Scheme 1).4d This situation implies that the molecule in the solid state has two symmetry environments for the silver centers and may be related to the symmetry of the molecule in solution, as revealed by the symmetry pattern of the NMR spectra. Each Ag(I) ion is coordinated by two NHC donors from two different hexacarbene ligands in an almost linear fashion (C carbene −Ag−C carbene range: 173.4(6)− 171.1(4)°). The Ag−Ccarbene bond lengths range from
Inspired by these unique NHC-based polyhedral coordination cages with hollow central cavities, we now report the preparation of a new disk-shaped hexaazolium benzene, which we have used for the preparation of related Ag(I) and Au(I) coordination cages, in which six metal ions (Ag(I) or Au(I)) are sandwiched between two hexadentate NHC ligands. The p r epar a tion of t h e 1 ,2 ,3 ,4 ,5 , 6-he xa ki s( Nalkylimidazolium)benzene salts (1 and 2; alkyl = Me, Et) was performed as depicted in Scheme 2. First, hexa(imidazol-1Scheme 2. Preparation of the Hexaazolium Salts 1 and 2
yl)benzene was prepared according to a known literature method,7 by deprotonation of imidazole and subsequent reaction with hexafluorobenzene. Subsequently, the hexa(imidazol-1-yl)benzene was reacted with either methyl trifluoromethanesulfonate to afford 1 (93% yield) or triethyloxonium tetrafluoroborate to afford 2 (78% yield). Both 1 and 2 were characterized by NMR spectroscopy and mass spectrometry. The 1H and 13C NMR spectra of the salts reveal a highly symmetric geometry in solution, which supports the possibility to prepare sandwich-like assemblies resembling those shown for I and II in Scheme 1. The reaction of 1 (or 2) with Ag2O in MeOH at 50 °C afforded the cylinder-type complexes 3 and 4 (Scheme 3), consisting of six Ag(I) centers Scheme 3. Synthesis of Cylinder-Shaped Hexa-NHC-Based Assemblies
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
Communication
S Supporting Information *
Text, figures, a table, and a CIF file giving details of the synthesis and characterization data for all new complexes and crystallographic details of complex 4. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Authors
*E-mail for F.E.H.:
[email protected]. *E-mail for E.P.:
[email protected]. Author Contributions
All authors contributed equally. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge financial support from Ministerio de Economia y Competitividad of Spain (CTQ2011-24055/ BQU). C.S. thanks the Ministerio de Ciencia e Innovación for a fellowship. The authors are grateful to the Serveis Centrals d’Instrumentació Cientı ́fica (SCIC) of the Universitat Jaume I for providing all characterization data.
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Figure 1. Two perspectives of the molecular structure of the complex cation in 4. Thermal ellipsoids are at the 50% probability level. Hydrogen atoms and counteranions (6BF4−) are omitted for clarity. Selected bond distances (Å) and angles (deg): Ag2a−C22a 2.060(11), Ag1a−C12a 2.078(10), Ag1a−C32a 2.054(7), Ccentroid(1)−Ccentroid(2) 5.387(7); C22a−Ag2a−C22a 173.4(6), C12a−Ag1a−C32a 171.1(4).
REFERENCES
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2.054(7) to 2.078(10) Å and fall within the limits for related silver carbenes in cylinder-like structures.4c,d The separation between the two phenyl ring centroids measures 5.387(7) Å; therefore, any type of noncovalent interaction can be discarded. In order to obtain a related hexagold complex, we performed the reaction of 3 and 4 with AuCl(DMS) (DMS = dimethyl sulfide). Both silver-NHC complexes proved to be excellent NHC-transfer agents, and the corresponding hexagold− dodecaimidazolylidenes 5 and 6 were obtained in 80 and 84% yields, respectively. Both complexes were characterized by means of NMR spectroscopy and mass spectrometry. The NMR spectra of 5 and 6 reveal highly symmetric structures. The 13C NMR spectra display signals due to the magnetically equivalent carbene carbons at 183.9 and 182.2 ppm for 5 and 6, respectively. ESI-MS shows monoisotopic peaks at m/z 649.4 (5, [M − 4(OTf)]4+) and at m/z 660.1340 (6, [M − 2(BF4)]2+). In summary, we have developed the preparation and coordination of a new benzene−hexaimidazolylidene, which we have used for the self-assembly of a series of cylinder-like complexes of silver and gold. The new complexes constitute unique examples of discrete molecular arrangements in which six metal atoms are sandwiched between two hexacarbene ligands, thus affording a very singular type of molecule, which can only be related to other existing cylinder-shaped NHCbased complex of lower nuclearity.4c,d The formation of these types of supramolecular complexes may open the door to the preparation of high-nuclearity complexes with well-defined cavities, which in turn may be modulated by the choice of the ligand. We are currently developing methods to prepare polyNHC-based ligands that can give rise to larger cavities, aiming to explore their capabilities in the selective recognition of ions and neutral small molecules. 5079
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NOTE ADDED AFTER ASAP PUBLICATION Figure 1 caption was corrected on September 12, 2014.
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