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Showcasing research from Professor Sarkar’s laboratory, Institut für Anorganische Chemie, Universität Stuttgart, As featured in: Stuttgart, Germany. The transformations of a methylene-bridged bis-triazolium salt: a mesoionic carbene based metallocage and analogues of TCNE and NacNac A methylene-bridged bis-triazolium salt transforms via H+/e- transfers to an electron-poor tetratriazolium substituted alkene. A silver(I) metallocage based on mesoionic carbene donors derived from the aforementioned tetratriazolium salts is presented. Additionally, the mono-deprotonated form of the methylene-bridged bis-triazolium salt, which is a NacNac analogue, is also isolated. The mechanisms of these unusual transformations were probed via a combination See Biprajit Sarkar et al., of electrochemistry, spectroelectrochemistry, EPR Chem. Sci., 2021, 12, 3170. spectroscopy, NMR spectroscopy, X-ray crystallography and theoretical calculations.

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The transformations of a methylene-bridged bis- triazolium salt: a mesoionic carbene based Cite this: Chem. Sci.,2021,12,3170 †‡ All publication charges for this article metallocage and analogues of TCNE and NacNac have been paid for by the Royal Society a b b a ab of Chemistry Jessica Stubbe, Simon Suhr, Julia Beerhues, Maite Noßler¨ and Biprajit Sarkar *

Unusual and unexpected chemical transformations often provide access to completely new types of functional molecules. We report here the synthesis of a methylene-bridged bis-triazolium salt designed as a precursor for a new bis-mesoionic carbene (MIC) ligand. The direct metalation with silver oxide led to the isolation and crystallographic characterization of a cationic tetranuclear octacarbene–silver(I) complex. During metalation the formal bis-MIC precursor undergoes signiﬁcant structural changes and chemical transformations. A combined synthetic, crystallographic and (spectro-)electrochemical approach is used to elucidate the mechanistic pathway: starting from the methylene-bridged bis- Received 21st December 2020 triazolium salt a single deprotonation leads to a NacNac analogue, which is followed by a redox-induced Accepted 2nd February 2021

Creative Commons Attribution 3.0 Unported Licence. radical dimerization reaction, generating a new tetra-MIC ligand coordinated to silver(I) central atoms. DOI: 10.1039/d0sc06957d Decomplexation led to the isolation of the corresponding tetratriazoliumethylene, a profoundly rsc.li/chemical-science electron-poor alkene, which is an analogue of TCNE.

Introduction generating metal complexes of 1,2,3-triazol-5-ylidenes. Exam- ples of multinuclear silver complexes include the dinuclear Ag(I) 7 Due to their eﬃcacy in improving catalyst activities, N- complexes reported by Bielawski, Sessler and co-workers and 8 heterocyclic carbenes (NHCs) are among the most important by Crudden and co-workers (see Fig. 1). In this context, it 1 This article is licensed under a ligand classes in organometallic chemistry. The popularity of should be mentioned that NHCs have been extensively used in these carbenes predominantly arose because of their strong the last years in developing metal-based supramolecular 9 donor abilities and high covalent bonding to the metal centers.2 assemblies. Apart from displaying remarkable structures, such The majority of publications around NHCs is based on supramolecular assemblies have also been used in host–guest Open Access Article. Published on 11 February 2021. Downloaded 10/3/2021 1:09:42 PM. Arduengo-type imidazole-2-ylidenes and 1,2,4-triazol-5-yli- recognition chemistry, and for reactions inside conned spaces. 3 denes, due to an easy handling of the free carbenes, which are The aforementioned silver(I)–triazolylidene complexes consti- stabilized by heteroatoms adjacent to the carbene.3a Alterna- tute structurally characterized intermediates of the popular tively, abnormal NHCs,4 especially mesoionic carbenes (MIC) transmetalation route towards 1,2,3-triazol-5-ylidene- 5 based on 1,2,3-triazol-5-ylidenes, have been postulated as even containing complexes. Usually, silver(I)–triazolylidene 2a better sigma donors.4e The corresponding triazoles can be easily complexes are unstable and challenging to isolate. Therefore, constructed in a modular fashion using the copper-catalyzed the silver oen only serves as an auxiliary agent for further azide–alkyne cycloaddition, arguably the most famous “click complexation, in which the in situ generation of the silver–car- reaction”.4d,6 The exibility of the substitution in 1- and 4- bene complex is commonly monitored via multinuclear NMR position at the triazole is nearly limitless due to the accessibility spectroscopy. of a large variety of azides and acetylenes. Either deprotonation Here, we report the synthesis of a methylene-bridged, 2 of the triazolium salt with a base, or its reaction with Ag2O methyl-substituted bis-triazolium salt H2 (BF4)2, originally followed by (trans)metalation are typical synthetic routes for designed as a precursor for the formation of a bis-MIC ligand. In the following, we show the various chemical transformations

aInstitut Fur¨ Chemie und Biochemie, Anorganische Chemie, Freie Universitat¨ Berlin, that we observed while attempting the metalation, or the Fabeckstraße 34–36, 14195 Berlin, Germany deprotonation of this salt, and we provide a consistent mech- bLehrstuhl fur¨ Anorganische Koordinationschemie, Institut fur¨ Anorganische Chemie, anistic picture for the observations. Aer metalation with silver ﬀ Universitat¨ Stuttgart, Pfa enwaldring 55, 70569 Stuttgart, Germany. E-mail: oxide, a cationic tetranuclear octacarbene–silver(I) complex [email protected] could be isolated and crystallographically characterized, which † Dedicated to Prof. Dr W. Kaim on the occasion of his 70th birthday. revealed signicant structural changes and chemical trans- ‡ Electronic supplementary information (ESI) available. CCDC 2025018, 2025037, 2025039 and 2025043. For ESI and crystallographic data in CIF or other electronic formations in the ligand under the chosen conditions. The format see DOI: 10.1039/d0sc06957d formal bis-triazolium salt undergoes a single deprotonation,

Fig. 1 First isolated and crystallographically characterized Ag(I)–MIC complexes by (left) Bielawski and Sessler,7 and (right) Crudden.8

Scheme 1 Synthesis of H22(BF4)2.

followed by a redox-induced radical dimerization under formal With the bis-triazolium precursor in hand, we turned our dihydrogen cleavage, leading to the formation of a tetra-MIC- attention to the coordination chemistry of this ligand. First 4 4 alkene . Decomplexation led to the isolation of H4 (BF4)4 as complexation reactions were accomplished by the direct met- tetratriazoliumethylene, which shows the potential of bearing alation of H 2(BF ) with an excess of Ag O in the presence of

Creative Commons Attribution 3.0 Unported Licence. 2 4 2 2 a comparable rich electron-transfer chemistry as seen for tet- Cs2CO3. This procedure is based on standard transmetalation racyanoethylene (TCNE). TCNE and related molecules have techniques without further treatment of additional metal functioned either as pure organic electron acceptors, or as precursors,12 and led to the formation of the air stable tetra-

radical ligands in multinuclear metal complexes. Both metal- nuclear silver complex 3(BF4)4 (Scheme 2). The molecular

free reduced TCNE, and the corresponding TNCE-radical structure of 3(BF4)4 in the crystal could be unambiguously metal complexes display very interesting magnetic and also determined through single crystal X-ray diﬀraction analysis optical properties.10 In the following, a combined synthetic, (Fig. 3, le). 3 crystallographic, spectroscopic, (spectro-)electrochemical and The molecular structure of (BF4)4 draws attention to

This article is licensed under a theoretical approach is used to describe the products, inter- signicant structural changes and chemical transformations in

mediates and reaction pathways. the native ligand H22(BF4)2 under the metalation conditions: dimerization via the formal methylene bridge takes place and leads to the formation of the ethylene-bridged tetra-MIC ligand Open Access Article. Published on 11 February 2021. Downloaded 10/3/2021 1:09:42 PM. Results and discussion 4, in which the newly formed C–C bond (1.336(8) A) is best Our studies began with the synthesis of the bidentate triazole described as a double bond. This bond length is shorter precursor 1, which was achieved by cycloaddition of methyl compared to C–C bond lengths of tetra-substituted-ethylenes 13 azide to the singly TMS-protected 1,4-pentadiyne in the pres- (ca. 1.37 A) which have neutral heteroaromatic substituents.

ence of base under standard click conditions (CuSO4 and The reactive nature of the protons in the methylene bridge is sodium ascorbate) (Scheme 1). The yield is 27%, which is due to safety issues in handling the explosive methyl azide, a reagent which is generated in situ and carefully dealt with under diluted conditions, following a synthetic procedure earlier reported from our group.11 In proton NMR, the reaction product shows three singlets at 8.23, 4.50 and 4.24 ppm, corresponding to the triazole-H, methylene-bridge and methyl groups, respectively

(Fig. S1‡). Methylation with Me3OBF4 results in the formation of

H22(BF4)2 in excellent yields (Scheme 1). A clean bis- methylation was conrmed by the preservation of the protons' chemical equivalency and by the presence of a sharp singlet at 4.24 ppm in proton NMR, corresponding to the methyl groups in the generated triazolium salt (Fig. S3‡). 2 The solid-state molecular structure of H2 (BF4)2 displays bond lengths in accordance with values previously reported in Fig. 2 ORTEP representation of H 2(BF ) : ellipsoids drawn at 50% 5a 2 4 2 the literature (Fig. 2). The triazole moieties are tilted by probability. Solvent molecules and H-atoms omitted for clarity (for 73.2(1) to each other. selected bond length and angles see ESI Table S3‡).

Scheme 2 Synthesis of 3(BF4)4 and H44(BF4)4.

also known from methylene-bridged bis-NHC ligands.14 our knowledge, this is the rst structurally characterized However, in many of those cases, the activation of the methy- example of a cationic tetranuclear octa-MIC–silver(I) complex. lene protons opens up a pathway for decomposition of the Additionally, the four MICs within the tetra-MIC ligand 4 are

compound. The ratio of silver to MIC donors in 3(BF4)4 is 1 : 2. connected via aC]C double bond, thus delivering a completely Each silver ion is bonded to two MICs and each MIC, four units new type of a multi-donor MIC ligand. of which constitute one molecule of ligand 4, coordinates to one As 4 was generated in situ during the complexation reaction, silver(I) center. The overall structure is best described as we were next interested in the decomplexation reaction to a sandwich-type metallocage, in which the four silver ions lie in generate the corresponding tetratriazolium salt. This was

between two ligands of type 4. However, the structure is not accomplished by the addition of NH4Cl to abstract silver as symmetric, as the sandwiching ligands are twisted by 24.7(5) to AgCl, and the addition of NH4BF4 to generate the ligand as the each other in order to obtain the nearly linear geometry around BF4 containing tetratriazolium salt H44(BF4)4 (Fig. 3 right, Creative Commons Attribution 3.0 Unported Licence. 18 the coordinated silver ions. The CMIC–Ag–CMIC bond angles Scheme 2). In the molecular structure in the crystal, H44(BF4)4 range from 172.1(2) to 176.2(2). The average Ag–Ag distance is displays a local center of inversion. The central C]C bond 3.458(2) A, and in comparison to the sum of the van der Waals length is 1.343(5) A and ts nicely with a C]C double bond. The radii for Ag(O) (3.440 A),15 it is consistent with weak metal– C–C and C–N bond lengths within the triazolium units are all in 16 5a metal interactions. The average Ag–CMIC bond length is the expected range (Table S5‡). H44(BF4)4 can thus be 2.086(2) A and is comparable with previously reported values.7,8 described as a tetratriazolium substituted ethylene.

In proton NMR, the formation of 3(BF4)4 was accompanied In H44(BF4)4 the positively charged triazolium units, which

by the disappearance of the triazolium-H and the methylene-H2 are strongly electron withdrawing groups, induce an electron- ‡  This article is licensed under a signals (see ESI, S5 and S6 ). A set of four singlets is obtained, poor ole n-moiety, comparable with the famous tetracyano- corresponding to four inequivalent methyl groups caused by the ethylene (TCNE), rst reported in 1957 by Middleton and 3 13 19  twisted geometry of (BF4)4.In C NMR characteristic reso- coworkers. TCNE is a valuable organic compound and nds nances at 172.3, 171.4, 170.0 and 168.9 ppm, corresponding to applications in several diﬀerent reaction: it is an excellent Open Access Article. Published on 11 February 2021. Downloaded 10/3/2021 1:09:42 PM. the triazolylidene-C–Ag moieties, are observed.17 To the best of dienophile, carries good leaving groups, is easy to both oxidize

Fig. 3 ORTEP representation of (left) 3(BF4)4 (for selected bond length and angles see ESI Table S5‡) and (right) H44(BF4)4 (for selected bond length and angles see ESI Table S6‡). Ellipsoids drawn at 50% probability. Solvent molecules and H-atoms omitted for clarity.

Fig. 4 (Left) Cyclic voltammogram and diﬀerential pulse voltammogram of a 0.1 mM solution of H44(BF4)4 with 0.1 M NBu4PF6 in acetonitrile. 1 1 (Top) 100 mV s and (bottom) 20 mV s and (right) combined cyclic voltammograms of a 0.1 mM solution of 2(BF4)2 and 4(BF4)4 with 0.1 M 1 NBu4PF6 in acetonitrile at 100 mV s .

and reduce and is available for coordination to metal centers.20 not as electron donors like TDAE or the electron-rich olens. As ﬃ 3 Due to its high electron a nity and reversible reductions, TCNE expected, the tetrasilver(I) complex (BF4)4 is not very robust has been termed the “E. coli” of electron-transfer chemistry.21 towards redox processes. However, in the cyclic voltammogram 4+ Encouraged by the rich electrochemistry of TCNE, ligand H44 of 3(BF4)4 two distinct reduction steps can be seen at 1.62 and Creative Commons Attribution 3.0 Unported Licence. has been investigated by (spectro-)electrochemical methods 1.74 V (see ESI, Fig. S16‡). We tentatively assign these redox (Fig. 4, 5 and 8). waves to two two-electron waves that arise from the simulta-

The cyclic voltammogram of H44(BF4)4 in CH3CN/0.1 M neous reduction of both the ligands. This would be expected as

NBu4PF6 displays two reversible reduction processes at E1/2 ¼ the two ligands are far apart and do not display either through- ¼ 0.65 V and E1/2 0.88 V versus the ferrocene/ferrocenium space or through-bond electronic coupling. Additionally, the couple, with peak-to-peak separations of 95 mV and 69 mV, large negative shi of reduction potentials on moving from respectively. The diﬀerence in the half-wave potentials is triazolium salts to the corresponding 1,2,3-triazolylidene-based

230 mV, which translates to a comproportionation constant, Kc, MICs has been observed earlier. value of ca. 8 103, displaying a reasonable thermodynamic In solution, the native form of H 44+ displays absorption

This article is licensed under a 4 stability of the one-electron reduced form. Further irreversible bands in the UV region at 219, 234 and 293 nm and in the visible reductions occur at signicantly lower potentials (see ESI, region at 405 nm. Upon one-electron reduction, a broad band Fig. S10‡). The aforementioned redox-potentials also have appears at 505 nm, while the band at 293 nm is shied to 22 Open Access Article. Published on 11 February 2021. Downloaded 10/3/2021 1:09:42 PM. similarities to tetrakis(dimethylamino)ethylene (TDAE) as well slightly lower energies. Such low energy bands in the visible or as to certain electron-rich olens derived from carbenes.23 in the NIR region are oen observed for organic radicals. The 4  However, we believe that a better comparison of H4 (BF4)4 is second reduction further shi s this absorption band to 339 nm, with TCNE (E1/2(1.Red) ¼0.23 V and E1/2(2.Red) ¼0.95 V vs. while the bands at 405 nm and 505 nm coalesce into one broad HFc/HFc+ in MeCN),20 as the tetracationic compound reported band centered at 495 nm. Reversing the potential to the starting here is expected to act as an electron acceptor like TCNE, and potential led to the quantitative regeneration of the spectrum

4+ Fig. 5 Changes in UV/vis spectrum of H44 in MeCN with 0.1 M NBu4PF6 during the ﬁrst reversible reduction (left) and second reversible reduction (right).

Fig. 6 (Left) Optimized structure of native tetracationic species. (Right) Side view along central C–C bond of native, singly and doubly reduced species. H atoms omitted for clarity.

Table 1 Selected calculated bond distances and dihedral angles which decreases upon second reduction for TCNE, but not for 4+ H44 . Native Singly reduced Doubly reduced The next question arises regarding the site of reduction in 44+ r(C1–C4) [A] 1.382 1.438 1.479 H4 . 1,2,3-Triazolium-centered reductions are usually irre- 25 r(C1–C2) [A]˚ 1.478 1.449 1.422 versible or in the best case quasi-reversible, thus it seems d(C2–C1–C4–C6) [ ] 167 152 132 unlikely that the reversible reductions observed in the present case are triazolium-centered. This can be elucidated by the

Creative Commons Attribution 3.0 Unported Licence. 44+ direct comparison of the electrochemical behavior of H4 to 44+ 2+ corresponding to the native form H4 , thus proving the the native bis-triazolium salt H22 (Fig. 4, right).  2+ reversible nature of both the rst and second reduction in the The reductions observed for H22 are triazolium-centered, UV/vis-NIR spectroelectrochemical time scale. which are at signicantly more cathodic potentials and are 2+ 4+ These observations, including both the electrochemical and irreversible. The structural similarities of H22 and H44 allow spectroelectrochemical behavior, can be compared with those a careful comparison of the electrochemical behavior, which 24  44+ 44+ reported for TCNE: The rst reduction potential of H4 is does not support a triazolium-based reduction in H4 .In cathodically shied in comparison to TCNE. Upon one-electron comparison to TCNE, it seems likely that the electron-poor  44+

This article is licensed under a reduction, both compounds display the appearance of a broad ole n-moiety of H4 undergoes reversible reductions. 44+ band in the visible region (TCNE: 420 nm, H4 : 505 nm), Open Access Article. Published on 11 February 2021. Downloaded 10/3/2021 1:09:42 PM.

Fig. 7 (Left) Spin density of the singly reduced species (contour value 0.005 A 1). (Right) Side and top view of the SOMO of the singly reduced species (contour value 0.05 A1).

population of p*-orbitals and thus a predominant reduction of the alkene moiety. This interpretation is substantiated by an inspection of the spin population in the singly reduced open-shell species, calculated at the TPSSh/def2-TZVP level of theory. According to the Loewdin population analysis, approximately 40% of the spin population are located on the central carbon atoms C1 and C4, while each triazolium-moiety carries about 15% (Fig. 7, le). Furthermore, the SOMO of the one-electron reduced species shows a distinct p*-character with respect to the central C–C bond (Fig. 7, right). The location of the radical's spin in the singly reduced species was monitored via EPR spectroscopy (Fig. 8). Here, the electrochemically generated radical shows a signal centered at g ¼ 2.003 without resolved hyperne coupling to neighboring Fig. 8 EPR-spectroelectrochemical measurement of H 44+ with 4 nuclei, additionally supporting a reduction at the alkene-moiety 0.1 M NBu PF in acetonitrile, electrolysis at 0.2 V versus silver wire at 4 6  0 C. in signi cant distance to the nitrogen atoms in the triazolium- moiety. The expected hyperne couplings are likely not resolved due to unfavorable line-width to hyperne coupling ratios. A ﬀ Indeed, DFT studies on the singly reduced species support series of simulations using di erent line widths supports this ‡ a predominantly olen-centered reduction. hypothesis (Fig. S25 ). The molecular structures of the native, the singly and the According to the combined (spectro-)electrochemical, spec- 4+ Creative Commons Attribution 3.0 Unported Licence. doubly reduced species of [H 4] were optimized at the BP86/ troscopic and theoretical data the reductions can be considered 4  def2-TZVP level of theory starting from the crystallographically as predominantly ole n-based. The presence of four highly 44+ determined structure H 4(BF ) . The optimized structure of the electron withdrawing groups in H4 decreases the electron 4 4 4  native species and side views of all calculated species are shown density around the ole nic group and thus makes it a strong (Fig. 6). electron acceptor. With suitable electron donors, complexes Selected bond distances and dihedral angles are given (Table with intermolecular charge-transfer interactions are expected.  1). Two trends are apparent upon reducing the olenic species: In a rst attempt at gauging the electron acceptor properties of 44+ the central C1–C4 bond is elongated while the dihedral angle H4 we reacted it with TDAE (oxidation potentials of 1.01 + C2–C1–C4–C6 decreases. Both the increased bond length and and 1.18 V vs. FcH/FcH in CH3CN). As can be seen from This article is licensed under a ‡ the more twisted geometry indicate a decrease in bond order Fig. S21, this reaction leads to a large increase in the signal c+ between C1 and C4 upon reduction, which implies the corresponding to a (TDAE) radical cation, thus displaying 44+ successful electron transfer from TDAE to H4 . The signal 43+c corresponding to H4 , which is expected to be featureless Open Access Article. Published on 11 February 2021. Downloaded 10/3/2021 1:09:42 PM.

2+ + Fig. 9 ORTEP representation of [H22 –H ](BF4): ellipsoids drawn at 50% probability. H-atoms (except those at C3, C5 and C13) are omitted for clarity. (Left) Single molecule, (right) unit cell (for selected bond length and angles see ESI Table S4‡).

2+ + + Scheme 3 Synthesis of [H22 –H ] .

Fig. 11 Combined cyclic voltammograms of a 0.1 mM solution of 2+ + + 4+ [H22 –H ] and H44 with 0.1 M NBu4PF6 in acetonitrile at 100 mV s1. 4+ 2+ + + Scheme 4 Formation of H44 from [H22 –H ] .

2+ + The conversion of H22(BF4)2 to [H22 –H ](BF4) is supported (Fig. 8), is likely “hidden” below the intense and well-resolved by NMR spectroscopic and single crystal X-ray diﬀraction signal of (TDAE)c+. Further detailed studies will be necessary measurements (see ESI, Fig. S3 and S9,‡ and above). The rear-

Creative Commons Attribution 3.0 Unported Licence. to determine the exact nature of the compound that is formed. rangement is then followed by a redox-induced radical dimer- 2 Eventually, the structural changes from H2 (BF4)2 to ization under formal dihydrogen cleavage (Scheme 4): oxidation 4 2+ + + H4 (BF4)4 within the metalation is still an open question. At of [H22 –H ] at the methylene-bridging carbon leads to the rst, we focused on the deprotonation of H22(BF4)2, in which we formation of an open-shell species, which we were able to 2+ were able to isolate the singly deprotonated species [H22 – generate electrochemically aer the in situ deprotonation of + H ](BF4) (Fig. 9). H22(BF4)2. The open-shell species was monitored via EPR In the solid state, the molecule is perfectly planar and nearly spectroscopy. The resulting spectrum does not show any symmetric. Slight discrepancies in symmetry are caused by hyperne coupling to neighboring nuclei and is centered at g ¼ diﬀerent bond lengths between the bridging C–H moiety and 2.004 (Fig. 10). This article is licensed under a the triazolium units with 1.404(5) A and 1.388(6) A. However, The actual formation of H44(BF4)4 through radical dimer- both bond lengths are shorter in comparison to those observed ization (Scheme 4) was conrmed via cyclic voltammetry

in H22(BF4)2, which give them considerable double bond char- (Fig. 11). 2+ + + Open Access Article. Published on 11 February 2021. Downloaded 10/3/2021 1:09:42 PM. acter. In order to maintain the symmetry and planarity, the Here, the in situ generated [H22 –H ] initially displays an charges in this molecule appear to be separated, yet delocalized oxidation step at ca. 0.05 V (Fig. 11). On reversing the potential (Scheme 3). at 0.3 V versus the ferrocene/ferrocenium couple, dimerization takes place within the CV timescale, thus leading to the appearance of the characteristic two olen-centered reduction 4+ waves of H44 at E1/2 ¼0.65 V and E1/2 ¼0.88 V, with peak- to-peak separations of 95 mV and 69 mV, respectively. These 4+ new reduction peaks corresponding to H44 appear in response to the aforementioned oxidation process. The combination of all these control experiments thus provide insights into the 4+ formation of H44 . With these observations in hand, we set out to decipher the 4 conditions under which H4 (BF4)4 can be directly synthesized starting from H22(BF4)2 (Scheme 5). We investigated the deprotonation reaction with several bases, in which KHMDS proved to be an appropriate base for a clean single deprotonation (see ESI, Fig. S9‡). The oxidation requires a mild oxidizing agent such as ferrocenium tetrauoroborate. The reaction leads to a mixture of starting material and product, in which the 4  desired tetratriazoliumethylene H4 (BF4)4 can be isolated a er 2+ + + Fig. 10 EPR-spectroelectrochemical measurement of [H22 –H ] recrystallization, and the starting material regained. Attempts versus with 0.1 M NBu4PF6 in acetonitrile, electrolysis at 0.8 V silver for the optimization of this reaction are currently being pursued wire at 0 C.

Scheme 5 Direct synthesis of H44(BF4)4.

4 in our group. The formation of H4 (BF4)4 through the activation References of the methylene protons of H22(BF4)2 as described here is reminiscent of a few recent investigations on the trans- 1(a) S. C. Sau, P. K. Hota, S. K. Mandal, M. Soleilhavoup and formations of bis-heteroaromatic substituted methanide G. Bertrand, Chem. Soc. Rev., 2020, 49, 1233–1252; (b) ligands.26 M. H. Wang and K. A. Scheidt, Angew. Chem., Int. Ed., 2016, In conclusion, we have presented the synthesis of a new bis- 55, 14912–14922; (c) J. D. Egbert, C. S. J. Cazin and triazolium salt, which undergoes signicant structural changes S. P. Nolan, Catal. Sci. Technol., 2013, 3, 912–926; (d) and chemical transformations under metalation conditions C. C. Loh and D. Enders, Chem.–Eur. J., 2012, 18, 10212– with silver oxide. We were able to isolate and crystallographi- 10225; (e) M. Melaimi, M. Soleilhavoup and G. Bertrand, cally characterize the cationic tetranuclear octacarbene–silver(I) Angew. Chem., Int. Ed., 2010, 49, 8810–8849; (f)P.de metallocage and the corresponding newly formed tetra- Fr´emont, N. Marion and S. P. Nolan, Coord. Chem. Rev.,

Creative Commons Attribution 3.0 Unported Licence. triazoliumethylene through decomplexation. An independent 2009, 253, 862–892; (g) V. Charra, P. de Frémont and synthesis of the tetratriazoliumehtylene has been presented as P. Braunstein, Coord. Chem. Rev., 2017, 341,53–176; (h) well which does not require the prior formation of the silver- H. V. Huynh, Chem. Rev., 2018, 118, 9457–9492; (i) based metallocage. Here, we could reveal important aspects of S. Kuwata and F. E. Hahn, Chem. Rev., 2018, 118, 9642– the mechanistic pathways: a single deprotonation, followed by 9677; (j) E. Peris, Chem. Rev., 2018, 118, 9988–10031. a redox-induced radical dimerization causes the structural 2(a) K. F. Donnelly, A. Petronilho and M. Albrecht, Chem. changes and chemical transformations from the formal bis- Commun., 2013, 49, 1145–1159; (b) F. E. Hahn and triazolium salt to the tetratriazoliumethylene. The newly M. C. Jahnke, Angew. Chem., Int. Ed., 2008, 47, 3122–3172; formed tetratriazolium alkene is an analogue of TCNE, which (c) D. Bourissou, O. Guerret, F. P. Gabbai and G. Bertrand, This article is licensed under a could be illustrated by the rst (spectro-)electrochemical Chem. Rev., 2000, 100,39–91. investigations on that compound. Furthermore, the electron 3(a) A. J. Arduengo, R. L. Harlow and M. Kline, J. Am. Chem. acceptor ability of the tetratriazoliumethylene was proven by its Soc., 1991, 113, 361–363; (b) W. A. Herrmann, Angew. 41 – Open Access Article. Published on 11 February 2021. Downloaded 10/3/2021 1:09:42 PM. reduction with the electron donor TDAE. Additionally, the Chem., Int. Ed., 2002, , 1290 1309; (c) D. Enders and singly deprotonated compound (a NacNac analogue) and the T. Balensiefer, Acc. Chem. Res., 2004, 37, 534–541; (d) tetratriazolium salt are expected to be exciting ligands for H. Braband, O. Blatt and U. Abram, Z. Anorg. Allg. Chem., generating a host of functional metal complexes. The 2006, 632, 2251–2255; (e) D. Enders, O. Niemeier and compounds reported here are thus likely to be relevant for A. Henseler, Chem. Rev., 2007, 107, 5606–5655; (f)M.He diverse elds of chemistry such as organic materials, redox- and J. W. Bode, J. Am. Chem. Soc., 2008, 130, 418–419; (g) active supramolecular cages, and redox-switchable catalysis. A. Zanardi, R. Corberán, J. A. Mata and E. Peris, Organometallics, 2008, 27, 3570–3576; (h) M. C. Jahnke and F. E. Hahn, Top. Organomet. Chem., 2010, 30,95–129. fl 4(a) J. M. Aizpurua, R. M. Fratila, Z. Monasterio, N. Pérez- Con icts of interest Esnaola, E. Andreieff, A. Irastorza and M. Sagartzazu- 38 – There are no conicts to declare. Aizpurua, New J. Chem., 2014, , 474 480; (b) R. H. Crabtree, Coord. Chem. Rev., 2013, 257, 755–766; (c) J. D. Crowley, A.-L. Lee and K. J. Kilpin, Aust. J. Chem., 2011, 64, 1118–1132; (d) D. Schweinfurth, N. Deibel, Acknowledgements F. Weisser and B. Sarkar, Nachr. Chem., 2011, 59, 937–941; (e) M. Heckenroth, E. Kluser, A. Neels and M. Albrecht, J. The high-performance computing facilities at ZEDAT of the FU Am. Chem. Soc., 2008, 6242–6249; (f) S. Klenk, L. Suntrup in Berlin, Germany, are acknowledged for access to computing and B. Sarkar, Nachr. Chem., 2018, 66, 717–721; (g) resources. The core facility (BioSupraMol) is gratefully G. Guisado-Barrios, J. Bouffard, B. Donnadieu and acknowledged. The authors are grateful to the DFG Priority G. Bertrand, Angew. Chem., Int. Ed., 2010, 49, 4759–4762. Program SPP 2102, “Light-controlled reactivity of metal complexes” (SA 1840/7-1) for nancial support.

5(a) D. Schweinfurth, L. Hettmanczyk, L. Suntrup and 11 R. Maity, A. Mekic, M. van der Meer, A. Verma and B. Sarkar, B. Sarkar, Z. Anorg. Allg. Chem., 2017, 643, 554–584; (b) Chem. Commun., 2015, 51, 15106–15109. G. Guisado-Barrios, M. Soleilhavoup and G. Bertrand, Acc. 12 L. Hettmanczyk, S. Manck, C. Hoyer, S. Hohloch and Chem. Res., 2018, 51, 3236–3244; (c) A. Vivancos, C. Segarra B. Sarkar, Chem. Commun., 2015, 51, 10949–10952. and M. Albrecht, Chem. Rev., 2018, 118, 9493–9586; (d) 13 (a) L. Viglianti, N. Xie, H. H. Y. Sung, A. A. Voityuk, G. Kleinhans, A. K. Chan, M. Y. Leung, D. C. Liles, N. L. C. Leung, Y. Tu, C. Baldoli, I. D. Williams, M. A. Fernandes, V. W. Yam, I. Fernandez and R. T. K. Kwok, J. W. Y. Lam, E. Licandro, L. Blancafort and D. I. Bezuidenhout, Chem.–Eur. J., 2020, 26, 6993–6998; (e) B. Z. Tang, Angew. Chem., Int. Ed., 2020, 59, 8552–8559; (b) R. E. Cordero-Rivera, D. Rendon-Nava,´ C. Angel-Jij´ on,´ R. G. Khoury, L. Jaquinod and K. M. Smith, Chem. O. R. Suárez-Castillo and D. Mendoza-Espinosa, Commun., 1997, 1057–1058. Organometallics, 2020, 39, 1887–1895; (f) M. Flores-Jarillo, 14 A. S. McCall, H. Wang, J. M. Desper and S. Kra, J. Am. Chem. D. Mendoza-Espinosa, V. Salazar-Pereda and S. González- Soc., 2011, 133, 1832–1848. Montiel, Organometallics, 2017, 36, 4305–4312. 15 A. Bondi, J. Phys. Chem., 1964, 68, 441–451. 6(a) H. C. Kolb, M. G. Finn and K. B. Sharpless, Angew. Chem., 16 J. C. Garrison and W. J. Youngs, Chem. Rev., 2005, 105, 3978– Int. Ed., 2001, 40, 2004–2021; (b) V. V. Rostovtsev, L. G. Green, 4008. V. V. Fokin and K. B. Sharpless, Angew. Chem., Int. Ed., 2002, 17 The Ag–MIC peak was detected indirectly by 1H–13C HMBC 41, 2596–2599; (c) C. W. Tornøe, C. Christensen and NMR spectroscopy. M. Meldal, J. Org. Chem., 2002, 67, 3057–3064. 18 L. Zhang, R. Das, C. T. Li, Y. Y. Wang, F. E. Hahn, K. Hua, 7 J. Cai, X. Yang, K. Arumugam, C. W. Bielawski and L. Y. Sun and Y. F. Han, Angew. Chem., Int. Ed., 2019, 58, J. L. Sessler, Organometallics, 2011, 30, 5033–5037. 13360–13364. 8 E. C. Keske, O. V. Zenkina, R. Wang and C. M. Crudden, 19 T. L. Cairns, R. A. Carboni, D. D. Caffman, V. A. Engelhardt, Organometallics, 2012, 31, 456–461. R. E. Heckert, E. L. Little, E. G. McGeer, B. C. McKusick and 79 –

Creative Commons Attribution 3.0 Unported Licence. 9(a) Y. Li, Y. Y. An, J. Z. Fan, X. X. Liu, X. Li, F. E. Hahn, W. J. Middleton, J. Am. Chem. Soc., 1957, , 2340 2341. Y. Y. Wang and Y. F. Han, Angew. Chem., Int. Ed., 2020, 59, 20 J. S. Miller, Angew. Chem., Int. Ed., 2006, 45, 2508–2525. 10073–10080; (b) N. Sinha and F. E. Hahn, Acc. Chem. Res., 21 B. Olbrich-Deussner, W. Kaim and R. Gross-Lannert, Inorg. 2017, 50, 2167–2184; (c) M. M. Gan, J. Q. Liu, L. Zhang, Chem., 1989, 28, 3113–3127. Y. Y. Wang, F. E. Hahn and Y. F. Han, Chem. Rev., 2018, 22 J. Broggi, T. Terme and P. Vanelle, Angew. Chem., Int. Ed., 118, 9587–9641; (d) V. Martinez-Agramunt, T. Eder, 2014, 53, 384–413. H. Darmandeh, G. Guisado-Barrios and E. Peris, Angew. 23 J. Messelberger, A. Grunwald,¨ S. J. Goodner, F. Zeilinger, Chem., Int. Ed., 2019, 58, 5682–5686; (e) S. Ibanez and P. Pinter, M. E. Miehlich, F. W. Heinemann, E. Peris, Angew. Chem., Int. Ed., 2019, 58, 6693–6697; (f) M. M. Hansmann and D. Munz, Chem. Sci., 2020, 11,

This article is licensed under a L. Y. Sun, N. Sinha, T. Yan, Y. S. Wang, T. T. Y. Tan, L. Yu, 4138–4149. Y. F. Han and F. E. Hahn, Angew. Chem., Int. Ed., 2018, 57, 24 N. Okumura, M. Goto and B. Uno, Chem. Pharm. Bull., 2000, 5161–5165; (g) Y. S. Wang, T. Feng, Y. Y. Wang, F. E. Hahn 48, 537–541. and Y. F. Han, Angew. Chem., Int. Ed., 2018, 57, 15767– 25 L. Hettmanczyk, L. Suntrup, S. Klenk, C. Hoyer and Open Access Article. Published on 11 February 2021. Downloaded 10/3/2021 1:09:42 PM. 15771; (h) C. Mejuto, G. Guisado-Barrios, D. Gusev and B. Sarkar, Chem.–Eur. J., 2017, 23, 576–585. E. Peris, Chem. Commun., 2015, 51, 13914–13917; (i) 26 (a) P. Ghosh, R. Naastepad, C. F. Riemersma, M. Lutz, S. Ib´anez,˜ D. G. Gusev and E. Peris, Organometallics, 2020, M. E. Moret and R. J. M. Klein Gebbink, Chem.–Eur. J., 39, 4078–4084. 2017, 23, 10732–10737; (b) S. Panda, S. K. Bera, P. Goel, 10 (a) A. J. Fatiadi, Synthesis, 1986, 249–284; (b) A. J. Fatiadi, A. K. Dutta and G. K. Lahiri, Inorg. Chem., 2019, 58, 11458– Synthesis, 1987, 749–789; (c) A. J. Fatiadi, Synthesis, 1987, 11469; (c) J. Kretsch, A. Kreyenschmidt, T. Schillmoller, 959–978; (d) J. S. Millera and A. J. Epstein, Chem. Commun., R. Herbst-Irmer and D. Stalke, Inorg. Chem., 2020, 59, 1998, 1319–1325; (e) J. S. Miller and A. J. Epstein, Coord. 13690–13699. Chem. Rev., 2000, 206–207, 651–660.