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Tris-triazolium Salts as Anion Receptors and as Precursors for the Preparation of Cylinder-like Coordination Cages Fadi AL-Shnani,† Gregorio Guisado-Barrios,*,‡ Daniel Sainz,† and Eduardo Peris*,‡ †

Departament de Química Inorgànica i Orgànica, Universitat de Barcelona, Marti i Franquès 1, Barcelona, E-08028, Spain Institute of Advanced Materials (INAM), Centro de Innovación en Química Avanzada (ORFEO−CINQA), Universitat Jaume I, Av. Vicente Sos Baynat s/n, Castellón, E-12071, Spain

‡

Organometallics Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/25/19. For personal use only.

S Supporting Information *

ABSTRACT: The synthesis and characterization of a “click chemistry” derived tripodal tris-1,2,3-triazolium bearing different counterions are described. The trifluoromethanesulfonate trisazolium salt is used as a receptor for halides and oxoanions, reflecting the formation of 1:1 complexes with binding affinities ranging between 200 and 1200 M−1 in CD3CN. The same tris-1,2,3-triazolium cation was used as precursor of a 1,2,3-triazolylidene ligand for the preparation of trinuclear prismatic cages of Ag and Au, via metal-controlled selfassembly.

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INTRODUCTION Cationic imidazolium salts have been widely used as anion receptors due to their strong affinity with anions through (C− H)+···X− interactions, which combine hydrogen bonding with favorable electrostatic interactions.1 The relatively easy synthesis of azolium motifs has facilitated their integration in a large number of host structural networks designed to target specific anion guests.2 However, while cationic imidazolium receptors have been widely used for anion recognition, 1,2,3triazolium motifs with cationic CH hydrogen bond donor groups have been rarely studied.3 1,2,3-Triazoliums can also serve as precursors of mesoionic 1,2,3-triazoly-5-ylidenes (MICs),4 which have been extensively used as ligands for transition metal complexes that found a variety of applications in materials sciences and in homogeneous catalysis.5 The synthetic availability of the ligand precursors, through either CuAAC-reaction “click chemistry” or cycloaddition between alkynes and 1,3-diaza-2-azoniaallene salts,6 provides access to a large variety of functional groups for incorporation onto the triazolylidene scaffold.7 In our current interest for designing scaffolds for the construction of supramolecular assemblies,8 we recently reported the formation of the first MIC-based homoleptic supramolecular cylinder-like trinuclear Ag(I) and Au(I) complexes via selfassembly.9 By mixing tripodal trisazolium and tris-triazolium salts of the same topology, we were also able to prepare the heteroleptic MIC-NHC coordination cages,9 illustrating an atypical case of a social self-sorting phenomenon.10 These complexes constituted very rare examples of organometallicbased supramolecular assemblies, a field fully dominated by poly-imidazolylidene-based ligands.11 Aiming to widen the scope of triazolium salts, both in the preparation of effective anion receptors and as precursors for © XXXX American Chemical Society

the preparation of molecular cages, we herein report the synthesis of a “click-chemistry” derived tris-triazolium salt, which we have used as a receptor for anions, and for the preparation of Ag(I) and Au(I) trigonal prismatic coordination cages.

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RESULTS AND DISCUSSION 1,3,5-Tris(1-mesityl-1H-1,2,3-triazol-4-yl)benzene (1) was obtained in 75% yield via copper catalyzed [3 + 2] cycloaddition between the 2,4,6-trimethylphenyl azide and 1,3,5-triethynylbenzene (Scheme 1). Further alkylation to produce the corresponding tris-triazolium salt was accomplished by using either methyl trifluoromethanesulfonate or trimethyloxonium tetrafluoroborate, affording [H3(2)](X)3 (with X = OTf or BF4) in 87% and 88% yield, respectively. The chloride salt, [H3(2)](Cl)3, was obtained from [H3(2)](OTf)3 by anion exchange using a Cl-containing resin.12 The salts [H3(2)](X)3 (with X = OTf, BF4, or Cl) were characterized by means of NMR spectroscopy and mass spectrometry. Both the 1H and 13C NMR are consistent with the three-fold symmetry of the compounds. The molecular structure of [H3(2)](OTf)3 was confirmed by X-ray diffraction studies (Figure 1). An interesting feature of the 1H NMR spectra of [H3(2)](OTf)3 and [H3(2)](BF4)3 in CD2Cl2 is that the resonances of the acidic protons of the triazolium salts appear at 8.93 and 8.72 ppm, respectively, while, for [H3(2)](Cl)3, this resonance is observed at 10.29 ppm. This significant downfield shift of the resonance of the NCHN proton of the chloride salt suggests that a strong hydrogen bonding Received: December 3, 2018

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DOI: 10.1021/acs.organomet.8b00874 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

these two signals is also accompanied by a smaller change in the chemical shift of the signal due to the protons at the Nmethyl group (Δδmax = 0.3, for the addition of [NBu4]Cl). These observations suggest that the interaction of the anion is mainly located at the interior of the tripod-shaped host. In the case of the titration with [NBu4](H2PO4), the formation of an insoluble product (most likely [H3(2)](H2PO4)3) precluded the determination of the association constant with this tetrahedral-shaped oxoanion. As an illustrative example, Figure 2 shows the series of spectra recorded for the titration of

Scheme 1. Synthesis of [H3(2)](X)3 (with X = OTf, BF4, and Cl)

Figure 2. 1H NMR spectra of [H3(2)](OTf)3 upon titration with tetrabutylammonium chloride [NBu4]+Cl− solution in acetonitrile-d3 at 298 K. The inset plot represents δ against [Cl−]/[H3-2(OTf)3]. Blue and orange dots represent the experimental values, and solid lines represent the output fitting curve according to the 1:1 model used.

[H3(2)](OTf)3 with [NBu4]+Cl−. The analyses of the binding isotherms resulting from the 1H NMR titrations allowed us to conclude that the curves were best fitted to a 1:1 stoichiometry model. Job plots also supported this stoichiometry,13 but their limited applicability prompted us to also analyze the residual distribution of the titration data fitting,14 and in all cases, the 1:1 stoichiometry gave the lowest residuals compared to potential 1:2 or 1:3 stoichiometries. The 1:1 association constants were calculated by global nonlinear regression analysis, by including all three protons showing the chemical shift variations.15 As can be seen in the data shown in Table 1, K11 values follow the trend Cl− > Br− > I−, in agreement with the basicity trend of the anions, with affinity constants ranging from 209 (for I−) to 1211 (for Cl−) M−1. The affinities for the

Figure 1. X-ray molecular structure of [H3(2)](OTf)3 (50% displacement ellipsoids). Hydrogen atoms (except the ones at the triazolium rings) and solvent molecules have been omitted for clarity.

interaction is occurring between the acidic protons of the cation and the chloride anions, and therefore, we have a good indication that these salts may serve as effective receptors for the recognition of anions. The anion recognition properties of the tripod receptor [H3(2)](OTf)3 were studied by 1H NMR titration experiments in CD3CN. The anions chosen for the study (X− = Cl−, Br−, I−, C7H10SO3−, C6H5CO2−, and H2PO4−) are of contrasting geometries and charge density, and they all were added as tetrabutylammonium salts. All titrations were performed at room temperature, at a constant concentration of host (2 mM). In general, the addition of the anion produced important downfield shifts of the resonances due to the proton at the triazolium and at the central benzene ring, with maximum shifts of Δδ = 1.6 and Δδ = 0.81, respectively, for the case of the titration with chloride. The perturbation of

Table 1. Association Constants (Ka)a for 1:1 Complexes of Host [H3(2)](OTf)3 with [NBu4]+X− (with X− = Cl−, Br−, I−, C7H10SO3−, C6H5CO2−, and H2PO4−), in Acetonitrile, 25 °C entry

anion

host K11 [M−1]a

1 2 3 4 5

Cl− Br− I− tosylate benzoate

1211 (6%) 418 (4%) 209 (3%) 315 (4%) 903 (6%)

a

K11 values calculated by global nonlinear regression analysis.15 Titrations were carried out at 298 K, using constant concentrations of host 2 mM in CD3CN. Errors are given in parentheses. B

DOI: 10.1021/acs.organomet.8b00874 Organometallics XXXX, XXX, XXX−XXX

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Organometallics aryl-substituted anions, p-tolylsulfonate and benzoate, are 315 and 903 M−1, respectively, again indicating that a higher affinity is observed for the more basic oxoanion. The affinities obtained are similar to those shown by previously reported tripodal-shaped imidazolium and triazolium-based receptors, in which the azoliums were bound to a central 1,3,5-mesityl connector.3d,16 In principle, the presence of the methylene groups separating the azoliums from the central benzene ring was supposed to be useful for defining a better cavity for trapping the anions, but in view of our new results, the presence of this cavity seems to not enhance the binding affinities of these anion hosts. Next, we decided to carry out coordination studies with the 1,2,3-tris-triazolium salts [H3(2)](X)3 (with X = OTf, BF4, and Cl) and Ag2O in order to prepare homoleptic coordination cages by self-assembly. Treatment of [H3(2)](X)3 (with X = BF4 and Cl) with 1.5 equiv of Ag2O in acetonitrile or methanol at 60 °C under the exclusion of light produced the tris-Ag(I) hexacarbene cages [Ag3(2)2](X)3 (with X = BF4 and Cl) in 92% and 93% yield, respectively (Scheme 2). Attempts to

of the supramolecular structure. The three-fold symmetry for this trimetallic complex was confirmed by the 1H NMR spectrum of the complex. The 13C NMR spectrum displays the signals due to the equivalent carbene carbons at 174.6 ppm. The molecular structures of [Ag3(2)2](Cl)2(AgCl2) and [Au3(2)2](Cl)3 were confirmed by X-ray diffraction studies. In general, both structures resemble the previously reported triAg and tri-Au prismatic structures reported by Hahn and coworkers, with the difference that, in their case, the cage is based on a tris-imidazolylidene ligand.17 Figure 3 displays the

Scheme 2. Synthesis of [Ag3(2)2](X)3 (with X = Cl and BF4)

Figure 3. Two perspectives of the molecular structure for [Ag3(2)2](Cl)2(AgCl2). Hydrogen atoms and counterions (2 Cl− and 1 AgCl2−) omitted for clarity. Bond distances (Å) and angles (deg) (only related to Ag and Ccarbene atoms): Ag(1)−C(5) 2.080(10), Ag(1)−C(7) 2.057(12); Ag(2)−C(11) 2.056(9), Ag(2)−C(9) 2.04(1), Ag(3)−C(15) 2.05(1), Ag(3)−C(13) 2.05(1), C(7)− Ag(1)−C(5) 176.9(5), C(11)−Ag(2)−C(9) 177.7(4), C(15)− Ag(3)−C(13) 178.6(4).

obtain [Ag3(2)2](OTf)3 from the related salt [H3(2)](OTf)3 did not produce the desired compound. These two Ag(I) [Ag3(2)2](X)3 cages contain three silver atoms sandwiched between two tris-(1,2,3-triazolylidene)benzene ligands. The 1H NMR spectra of [Ag3(2)2](X)3 (with X = BF4 and Cl) are consistent with the three-fold symmetry of the cage. This is exemplified by the four singlets due to the protons of the four inequivalent methyl groups, one bound at the nitrogen of the 1,2,3-triazolylidene, and three from the mesityl group. The 13C NMR spectra display the resonance due to the equivalent metalated carbene-carbon atoms at 170.16 for [Ag3(2)2](BF4)3, and 169.5 ppm for [Ag3(2)2](Cl)3. The two doublets due to the coupling of the carbene carbons with the two silver isotopes (1JC‑Ag107 = 169.5 Hz, 1JC‑Ag109 = 168.8 Hz) were clearly observed for the case of [Ag3(2)2](Cl)3. The HR-MS spectra display the most intense peaks at m/z 1849.4 (assigned to [M + 2BF4]+) and m/z 855.3 (assigned to [M + Cl]2+). Transmetalation of [Ag3(2)2](Cl)3 with 3 equiv of [AuCl(SMe2)] in dichloromethane at room temperature produced exclusively [Au3(2)2](Cl)3, in 92% yield, with the full retention

molecular structure of [Ag3(2)2](Cl)3. The structure contains three silver atoms sandwiched between two tris-(1,2,3triazolylidene)benzene ligands. The tricationic nature of the supramolecular cage is balanced with two Cl− and one AgCl− counteranions. The average Ag−Ccarbene bond distance is 2.069 Å, and the average Ccarbene−Ag−Ccarbene angle is 176.9°. The separation between the three silver atoms is quasi-identical, and averages 6.06 Å. The cage may be viewed as a twisted triangular prism, so the symmetry is pseudo-D3d. The distance between the planes established by the two central benzene rings of the molecule is 5.73 Å. The structure of the tri-Au cage [Au3(2)2](Cl)3 (see molecular diagram in the Supporting Information) is similar to that of its tri-Ag analogue, with an average Au···Au separation of 6.22 Å, and a distance between the two central benzene rings of 5.38 Å, thus slightly shorter than that observed for its silver analogue. C

DOI: 10.1021/acs.organomet.8b00874 Organometallics XXXX, XXX, XXX−XXX

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2014, 2014 (20), 4201−4223. (b) Cai, J. J.; Sessler, J. L. Neutral CH and cationic CH donor groups as anion receptors. Chem. Soc. Rev. 2014, 43 (17), 6198−6213. (c) Sato, K.; Arai, S.; Yamagishi, T. A New Tripodal Anion Receptor with C-H···X- Hydrogen Bonding. Tetrahedron Lett. 1999, 40, 5219−5222. (d) Alcalde, E.; Alvarez-Rua, C.; Garcia-Granda, S.; Garcia-Rodriguez, E.; Mesquida, N.; PerezGarcia, L. Hydrogen bonded driven anion binding by dicationic 1(4) imidazoliophanes. Chem. Commun. 1999, No. 3, 295−296. (e) Ihm, H.; Yun, S.; Kim, H. G.; Kim, J. K.; Kim, K. S. Tripodal nitroimidazolium receptor for anion binding driven by (C-H)(+)-Xhydrogen bonds. Org. Lett. 2002, 4 (17), 2897−2900. (f) Amendola, V.; Boiocchi, M.; Colasson, B.; Fabbrizzi, L.; Douton, M.-J. R.; Ugozzoli, F. A metal-based trisimidazolium cage that provides six C-H hydrogen-bond-donor fragments and includes anions. Angew. Chem., Int. Ed. 2006, 45 (41), 6920−6924. (g) Wong, W. W. H.; Vickers, M. S.; Cowley, A. R.; Paul, R. L.; Beer, P. D. Tetrakis(imidazolium) macrocyclic receptors for anion binding. Org. Biomol. Chem. 2005, 3 (23), 4201−4208. (h) Molina, P.; Zapata, F.; Caballero, A. Anion Recognition Strategies Based on Combined Noncovalent Interactions. Chem. Rev. 2017, 117 (15), 9907−9972. (2) (a) Xu, Z.; Kim, S. K.; Yoon, J. Revisit to imidazolium receptors for the recognition of anions: highlighted research during 2006−2009. Chem. Soc. Rev. 2010, 39 (5), 1457−1466. (b) Baker, M. V.; Brown, D. H. Azolium cyclophanes. Mini-Rev. Org. Chem. 2006, 3 (4), 333− 354. (3) (a) Cai, J. J.; Hay, B. P.; Young, N. J.; Yang, X. P.; Sessler, J. L. A pyrrole-based triazolium-phane with NH and cationic CH donor groups as a receptor for tetrahedral oxyanions that functions in polar media. Chem. Sci. 2013, 4 (4), 1560−1567. (b) Mercurio, J. M.; Knighton, R. C.; Cookson, J.; Beer, P. D. Halotriazolium Axle Functionalised 2 Rotaxanes for Anion Recognition: Investigating the Effects of Halogen-Bond Donor and Preorganisation. Chem. - Eur. J. 2014, 20 (37), 11740−11749. (c) Nepal, B.; Scheiner, S. Substituent Effects on the Binding of Halides by Neutral and Dicationic Bis(triazolium) Receptors. J. Phys. Chem. A 2015, 119 (52), 13064−13073. (d) Ruiz-Botella, S.; Vidossich, P.; Ujaque, G.; Peris, E.; Beer, P. D. Tripodal halogen bonding iodo-azolium receptors for anion recognition. RSC Adv. 2017, 7 (19), 11253−11258. (e) Gilday, L. C.; White, N. G.; Beer, P. D. Halogen- and hydrogen-bonding triazole-functionalised porphyrin-based receptors for anion recognition. Dalton Trans. 2013, 42 (44), 15766−15773. (f) Gilday, L. C.; White, N. G.; Beer, P. D. Triazole- and triazolium-containing porphyrin-cages for optical anion sensing. Dalton Trans. 2012, 41 (23), 7092−7097. (g) Mullen, K. M.; Mercurio, J.; Serpell, C. J.; Beer, P. D. Exploiting the 1,2,3-Triazolium Motif in Anion-Templated Formation of a Bromide-Selective Rotaxane Host Assembly. Angew. Chem., Int. Ed. 2009, 48 (26), 4781−4784. (4) (a) Guisado-Barrios, G.; Bouffard, J.; Donnadieu, B.; Bertrand, G. Crystalline 1H-1,2,3-Triazol-5-ylidenes: New Stable Mesoionic Carbenes (MICs). Angew. Chem., Int. Ed. 2010, 49 (28), 4759−4762. (b) Mathew, P.; Neels, A.; Albrecht, M. 1,2,3-triazolylidenes as versatile abnormal carbene ligands for late transition metals. J. Am. Chem. Soc. 2008, 130 (41), 13534−13535. (5) (a) Vivancos, A.; Segarra, C.; Albrecht, M. Mesoionic and Related Less Heteroatom-Stabilized N-Heterocyclic Carbene Complexes: Synthesis, Catalysis, and Other Applications. Chem. Rev. 2018, 118 (19), 9493−9586. (b) Donnelly, K. F.; Petronilho, A.; Albrecht, M. Application of 1,2,3-triazolylidenes as versatile NHC-type ligands: synthesis, properties, and application in catalysis and beyond. Chem. Commun. 2013, 49 (12), 1145−1159. (c) Kruger, A.; Albrecht, M. Abnormal N-heterocyclic Carbenes: More than Just Exceptionally Strong Donor Ligands. Aust. J. Chem. 2011, 64 (8), 1113−1117. (d) Crabtree, R. H. Abnormal, mesoionic and remote N-heterocyclic carbene complexes. Coord. Chem. Rev. 2013, 257 (3−4), 755−766. (e) Marichev, K. O.; Patil, S. A.; Bugarin, A. Recent advances in the synthesis, structural diversity, and applications of mesoionic 1,2,3triazol-5-ylidene metal complexes. Tetrahedron 2018, 74 (21), 2523− 2546. (f) Guisado-Barrios, G.; Soleilhavoup, M.; Bertrand, G. 1H-

CONCLUSIONS In summary, we reported the synthesis of a tripodal tris-1,2,3triazolium bearing different counterions. The trifluoromethanesulfonate trisazolium salt was used as a receptor of several anions, reflecting the formation of 1:1 complexes with good binding affinities. The same tris-1,2,3-triazolium salts were used for the preparation of trinuclear coordination cages of silver and gold, yielding trinuclear prismatic cages via metalcontrolled self-assembly. Although several studies may be found in the literature in which poly-azolium salts are used for the recognition of anions and for the preparation of selfassembly molecular cages, to the best of our knowledge, this is the first work merging both studies in a single contribution. We believe that our work reflects how poly-azolium salts may be designed with suitable topologies for being used in multiple applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00874. All synthetic procedures, characterization methods, NMR and mass spectra. Titrations experiments and crystallographic details (PDF) Accession Codes

CCDC 1881940−1881942 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.G.-B.). *E-mail: [email protected] (E.P.). ORCID

Fadi AL-Shnani: 0000-0002-7961-806X Gregorio Guisado-Barrios: 0000-0002-0154-9682 Daniel Sainz: 0000-0002-1079-7020 Eduardo Peris: 0000-0001-9022-2392 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the Universitat Jaume I (UJI-B2017-07). We are grateful to the ́ Serveis Centrals d’Instrumentació Cientifica (SCIC-UJI) for the spectroscopic and X-ray diffraction facilities. G.G.-B thanks MINECO for a “Juan de la Cierva Fellowship” (GGB, IJCI2015-23407). F.A-S. thanks the European Commission for the scholarship funded within the Erasmus+ KA1 Programme (2016-1920/001-001). We would like to dedicate this article to Professor Pablo Espinet, one of the internationally renowned Spanish organometallic chemistry researchers, on the occasion of his 70th birthday.

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DOI: 10.1021/acs.organomet.8b00874 Organometallics XXXX, XXX, XXX−XXX