This is the accepted version of the journal article: Vellé, Alba; Rodríguez Santiago, Luis; Sodupe Roure, Mariona; [et al.]. «En- hanced metallophilicity in metal-carbene systems : stronger character of au- rophilic interactions in solution». Chemistry, Vol. 26, issue 5 (Jan. 2022), p. 997-1002. DOI 10.1002/chem.201904507 This version is available at https://ddd.uab.cat/record/279252 under the terms of the license COMMUNICATION Enhanced Metallophilicity in Metal–Carbene Systems: Stronger Character of Aurophilic Interactions in Solution Alba Vellé,[a] Luis Rodríguez-Santiago,[b] Mariona Sodupe,[b] and Pablo J. Sanz Miguel*[a] Abstract: Metallophilicity is an essential concept that builds upon of the Ag2 host concerns its dynamic behavior in solution. Such the attraction between closed shell metal ions. We report on the species coexists in two conformers, namely U and Z (Figure S1), [M2(bisNHC)2]2+ (M = AuI, AgI; NHC = N-Heterocyclic Carbene) or syn/anti,[32] and exhibit different aggregation properties. systems, which display almost identical features in solid state. Switch between both conformers occurs even at moderate However, in solution the Au2 cation exhibits a significant higher temperatures, and the conversion rate depends on external degree of rigidity owed to the stronger character of the aurophilic factors as concentration, temperature and the presence of Ag+ interactions. Both Au2 and Ag2 cationic constructs are able to ions in the media. accommodate Ag+ ions via M-M interactions, despite their inherent In order to compare metallophilic interactions, we report on Coulombic repulsion. When electrostatic repulsion between host and the study of an analogous system: [Au2(bisNHC)2]2+. Treatment guest is partially diminished, M–M distances are substantially of [Ag2(bisMeOEtIm)2]2+ (1)[30] (bisMeOEtIm = bis(2- shortened. Quantum chemical calculations estimate intermetallic methoxyethyl-1-imidazole)methane) under mild conditions with bond orders up to 0.2. Although at the limit of (or beyond) the van [Au(tht)Cl] (tht = tetrahydrothiophene) results in the der Waals radii, metallophilic interactions are responsible for their transmetallation product [Au2(bisMeOEtIm)2](BF4)2·CH3CN (2). behavior in solution. Molecular arrangement of 2 (Figure 1) is similar to that of 1, bearing in mind the expected differences regarding M–C bonds (Supporting Information).[33] In solid state, cation 2 is folded in U- The term aurophilicity was established by Schmidbaur to conformation (open-book), with a dihedral angle of 81.4(1)°. The describe attractive interactions between AuI centers, relying on intramolecular Au1−Au2 distance (3.4382(3) Å) is slightly shorter earlier findings and empirical evidences.[1-6] Since then, than the corresponding Ag–Ag separation in 1 (3.4512(5) Å), substantial efforts have been made in order to understand the and a minor variance in the spatial positioning of the side arms nature of such interactions,[7-12] further extended to other M···M is likewise observed. contacts, such as argentophilicity.[13] In spite of the relatively low The 1H NMR spectra of 2 (Figure 2) reveal a noteworthy stabilization energy, in the rank of hydrogen bonds, numerous difference when compared to that of 1: Methylene bridging examples in which metallophilic interactions efficiently modify protons (denoted as C31 and C61 in Figure 1) split into two chemical and physical properties of metal-based compounds signals, drawing an AB system. These protons are non- and materials have been described. Such interactions are strong equivalent owed to the static environment. In other words, cation enough to maintain aggregates and host-guest systems,[14-16] 2 exists in solution as a single U-conformation, and no dynamic induce formation of nanoparticles,[17-20] cooperate in catalysis,[21- 23] behavior (U⇋Z conversion) is detected at moderate or to drive metal-directed self-assembly,[24-27] among others. temperatures. In addition, proton signals corresponding to the N- Depending on the nature of solvents, ligands and counterions, CH2- groups directly attached to the N-imidazole atoms (C16, binding energies between closed-shell metals have been C26, C46, C56 in Figure 1) suffer hindered rotation, and estimated to be in the range 7–12 kcalmol–1 for gold and 5–15 therefore build a second order multiplet, instead of a triplet. This kcalmol–1 for silver.[10,13,28,29] Although it could be assumed that fact represents an empirical confirmation of the stronger aurophilic bonds are stronger than their argentophilic analogues, character of aurophilicity in comparison to argentophilicity. Even there is a lack of conclusive evidences to support this fact.[13] though the distance between both gold centers in the solid state In the past we discussed the effect of strong electron donor (Au1–Au2, 3.4382(3) Å) exceeds two times van der Waals ligands on closed-shell metal ions, and postulated that M-M radius of gold (3.32 Å), cation 2 exhibits rigidity in solution owed interactions may surpass the van der Waals radii.[30-31] In exclusively to intramolecular Au···Au contacts. particular, the cationic [Ag2(bisNHC)2]2+ system behave as host for Ag+ ions and revealed argentophilic interactions in solution. As a matter of fact, an intriguing cation-cation inclusion complex was isolated: {Ag+Ì[Ag2(bisNHC)2]2+}. Within this arrangement, argentophilicity appears as host-guest stabilization force, overcoming their electrostatic repulsion. One interesting feature [a] Dr. A. Vellé, Dr. P. J. Sanz Miguel Departamento de Química Inorgánica, Instituto de Síntesis Química y Catálisis Homogénea (ISQCH), Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain. E-mail: pablo.sanz@unizar.es [b] Dr. L. Rodríguez-Santiago, Prof. M. Sodupe Figure 1. View of cation [Au2(bisMeOEtIm)2]2+ (2) (left), and assignment of 1H Departament de Química, Universitat Autònoma de Barcelona, NMR resonances (right). 08193 Bellaterra, Barcelona, Spain Supporting information for this article is given via a link at the end of the document. COMMUNICATION The affinity of cation 2 toward Ag+ ions was monitored by 1H NMR spectroscopy. Figure 4 shows the resulting spectra after addition of increasing amounts of AgNO3 to 0.025 M solutions of 2. There, the chemical shifts of the H5 signals (Δδ = 0.25 ppm) are particularly affected by the presence of silver ions —which is indicative of local heterometallic Ag···Au contacts—, and to a lesser extent, the bridging CH2 protons (Δδ = 0.18 ppm). This experiment was analogous to that performed for 1,[30] which shows lower affinity toward Ag+ ions, based on the smaller Figure 2. 1H NMR spectra (CD3CN, room temperature, 0.025 M) of [Ag2(bisMeOEtIm)2]2+ (1)[30] and [Au2(bisMeOEtIm)2]2+ (2). resulting Δδ values (0.11 and 0.05 ppm, respectively). A noteworthy difference is that, in contrast with cation 1, the conformational U⇋Z switch rate in 2 seems to be not perturbed We examined by 1H NMR the behavior of cation 2 in solution by the presence of Ag+ ions in solution, even at high (CD3CN) at different temperatures (Figure 3). This study concentrations (Supporting Information). At this point, we cannot revealed that by warming up the sample, methylene bridge conclude that heterometallic Ag–Au interactions are stronger signals become wider, and at 340 K they coalesce. The reported than the corresponding Ag–Ag contacts,[35] as the stability of the cation [Au2(bisNHC)2]2+,[34] bearing methyl groups as side arms Au2 platform may significantly influence the host-guest (R = CH3 in Figure 1), coalesced at 383 K (in DMSO). In contrast, intermetallic connectivity. coalescence temperature in 1 was found to occur at 250 K (in By addition of AgBF4 to a solution of 2 in acetonitrile, we CD3CN). The effect of the temperature on the N-CH2- signals in isolated [Ag(CH3CN)2 Ì Au2(bisMeOEtIm)2](BF4)3·2CH3CN (3), 2 is likewise substantial: At low thermal readings, they split which can be considered as a formal modification of cation forming a dense multiplet, whereas at moderate temperatures [Ag(CH3CN)2 Ì Ag2(bisMeOEtIm)2]3+ (4).[30] Cation 3 (Figure 5, (333–348 K), become equivalent appearing as a pseudotriplet, left) displays on average longer Au–Ag intermetallic lengths whereas neighboring -CH2-O protons show minor variations (2.9522(8) and 3.0705(8) Å) than the corresponding Ag–Ag throughout the temperature range. No significant chemical shift separations in 4 (2.8231(4) and 2.9952(4) Å) (Table 1). changes are observed. Switch rate constants of 2 were Furthermore, separation between both hosting gold ions in 3 determined by 1H NMR line-shape method, with values ranging (3.8304(7) Å) is considerably lengthier if compared to the from 2.6 s–1 (233 K) to 369 s–1 (348 K). Enthalpy (ΔH‡, 5.7±0.5 precursor 2 (3.4382(3) Å). In essence, cation 3 represents an kcalmol–1) and entropy of activation (ΔS‡, –31.4±1.7 calK–1mol–1) interesting cation-cation inclusion complex stabilized by ligand- were estimated using the Eyring analysis. unsupported Au–Ag heterometallophilic interactions. A further contrast regarding the behavior of 1 and 2 is revealed by concentration-dependent 1H NMR studies (Figure S9). Lowly (0.33×10–3 M) and highly (25×10–3 M) concentrated samples of cation 2 displayed no significant differences in chemical shifts or shape, including the CH2-bridge resonances, associated to the U⇋Z switch. Thus, in contrast to the flexibility and propensity to form aggregates exhibited by cation 1, the Au···Au environs of 2 remain virtually unaltered, and we have not found any evidence of intermolecular aurophilicity in solution. It is worth emphasizing here that the lack of U⇋Z interconversion rate is directly associated with the difficulty of separating both gold atoms in the skeleton of 2, namely, to circumvent their aurophilic attraction. Figure 4. 1H NMR spectra of [Au2(bisMeOEtIm)2]2+ (2) in CD3CN (0.025 M) with different amounts of AgNO3 added. Figure 3. 1H NMR spectra of 2 in CD3CN (0.025 M) recorded at different Figure 5. View of the cation-cation host-guest systems 3 and 5. temperatures (see also the Supporting Information). COMMUNICATION Table 1. Intermetallic bond distances [Å] and angles [º] in cations 2, 3 and 5. 2 3 5 Au1–Ag1 – 3.0705(8) 2.8802(11) Ag1–Au2 – 2.9522(8) 2.8584(12) Au1–Au2 3.4382(3) 3.8304(7) 3.5252(14) Au1–Ag1–Au2 – 78.96(2) 75.80(3) Figure 6. View of a [Cu2(bisMeOEtIm)2]2+ cation in the asymmetric unit of 6. In a further experiment, we aimed to modify the environment of the guest, and therefore, the nature of the host-guest system. For that, a mixture of tetrabutylammonium nitrate and AgNO3 In order to evaluate metallophilic bond strengths, we was added to a solution of 2 in CH3CN. In principle, this would calculated Wiberg bond indices (WBIs) on compounds 1–5, from facilitate the interaction of Ag+ with nitrate anions. As a result, an experimental X-ray crystal data and from optimized models at intriguing host-guest system was isolated: [Ag(CH3CN)(NO3) Ì the B3LYP-D3 level (see SI for details). The WBI is a measure Au2(bisMeOEtIm)2](NO3)2·CH3CN (5). In this new organization, a of the density between atoms A and B and often similar in nitrate formally replaces one of the acetonitrile ligands (Figure 5, magnitude to the bond order expected from valence bond theory. right). The inclusion of nitrate in the coordination sphere of silver Resulting values (Table 2) evidence relatively strong bonding provokes a considerable effect in the geometry of the host-guest interactions between metals, being the highest WBI values those system: The electrostatic repulsion between host and guest is that correspond to compounds 3 and 4 (0.17), and 5 (0.20). partially diminished in 5, and hence the M–M distances are in turn shortened (Table 1). In addition, bond lengths involving the Table 2. WBIs for metallophilic bonds in cations 1–5 at experimental and Ag+ ion are not particularly short, neither with acetonitrile (Ag–N, optimized structures. Bond distances [Å] are shown in square brackets. 2.283(4) Å), nor with nitrate (Ag–O, 2.414(4) Å). Both Ag1–Ag2 Ag1–Ag3 Ag2–Ag3 circumstances may reasonably be interpreted as the result of an exp opt exp opt exp opt important contribution in the stability of the host-guest system of the high affinity of the silver ion toward the AuI centers, despite 1 0.101 0.113 – – – – of the stabilization offered by the donor ligands. Inclusion of a [3.451] [3.328] – – – – second NO3– in the coordination sphere of silver was not 4 0.069 0.080 0.151 0.102 0.174 0.103 successful, probably due to steric and repulsion reasons. [3.659] [3.572] [2.995] [3.408] [2.823] [3.404] In order to discard any kind of rigidification imposed by the Au1–Au2 Au1–Ag1 Au2–Ag1 ligands, we studied our system with the smallest metal of the group (CuI), and prepared [Cu2(bisMeOEtIm)2](BF4)2 (6). In exp opt exp opt exp opt contrast to the behavior of the Ag2 and Au2 complexes, cation 6 2 0.143 0.133 – – – – proved unstable after contact with water or air, evolving rapidly [3.438] [3.512] – – – – to the characteristic green patina color. The asymmetric unit of 6 3 0.091 0.106 0.158 0.138 0.171 0.135 contains two different Cu2 entities. Intermetallic Cu–Cu distances [3.830] [3.679] [3.071] [3.295] [2.952] [3.261] found in 6 (3.0927(9) and 2.9476(10) Å) are largely shorter than 5 0.122 0.119 0.204 0.129 0.201 0.128 those of 1 and 2. There are two reported examples of related [3.525] [3.604] [2.880] [3.312] [2.858] [3.272] constructions —same skeleton, different wings— (R = tBu,[36] CH3[37] in Figure 1), in which the flexibility of the bisNHC ligands allows for Cu–Cu separations of 2.87 and 2.90 Å. These Existence of metallophilic Au–Au, Ag–Ag and Ag–Au partial distances are slightly longer than the sum of twice the van der bonds in cations 1–5 is corroborated by computation, at Waals radii for Cu (2.80 Å). This flexibility is also patent in our experimental geometries, of electron delocalization indexes (DI) system: For instance, mutual positioning of the cross-linked from atoms in molecules (AIM) theory, and the identification of imidazole rings attached to Cu3 (83.4(2)°, Figure 6) is nearly bond critical points (BCP) between neighboring metals. The perpendicular, whereas the corresponding dihedral angles in 1 delocalization index DI(A,B) measures the average number of and 2 are roughly coplanar. Besides, the 1H NMR spectrum of 6 electrons delocalized (shared) between atoms A and B. The (Figure S12) shows the methylene bridge signal at 6.37 ppm as obtained results suggest the presence in our systems of robust a singlet (rapid U⇋Z conversion). Thus, any constriction metallophilic interactions with values up to 0.3 (Table 3). It is imposed by the bisNHC ligands can be definitely ruled out as interesting to point out a particular situation: at this level of meaningful stabilization factor. An additional conclusion can be theory, both gold atoms in 2 (Au1–Au2, 3.4382(3) Å) exhibit a drawn: the rapid U⇋Z conversion observed for the Cu2 implies value of 0.18, while, their separation surpasses the van der that Cu–Cu contacts are weaker than argentophilic interactions. Waals limits. A similar situation occurs with cation 1. This underpins further the existence of interactions beyond the van der Waals sphere. COMMUNICATION Table 3. Electron delocalization indexes (DI) and bond critical points (BCP) for orders. More important, although M–M separations in solid state metallophilic bonds in cations 1–5. may lead to the assumption that Au···Au and Ag···Ag Ag1–Ag2 Ag1–Ag3 Ag2–Ag3 interactions are similar, the solution behavior reveals a stronger character of aurophilicity. DI BCP DI BCP DI BCP 1 0.12 yes – – – – 4 0.07 no 0.26 yes 0.18 yes Experimental Section Au1–Au2 Au1–Ag1 Au2–Ag1 Experimental details, NMR measurements, and X-ray diffraction data are DI BCP DI BCP DI BCP given in the Supporting Information. CCDC-1954651 (2), 1954652 (3), 2 0.18 yes – – – – 1954653 (5), and 1954654 (6), contain the crystallographic data. 3 0.08 no 0.24 yes 0.29 yes 5 0.14 no 0.30 yes 0.28 yes Acknowledgements To get an estimation of the difference between Au–Au and Financial support from the University of Zaragoza (UZ2018-CIE- Ag–Au contacts in compounds 1 and 2, we computed interaction 06), the Aragón Government (E42_17R, and A.V. predoctoral energies of models of 1 and 2 using the spin-component scaling fellow), the MINECO (CTQ2017-89132-P) and the Generalitat second order Møller–Plesset perturbation method (SCS-MP2) de Catalunya (2017SGR1323) is kindly acknowledged. method[38] with extrapolation to complete basis set. This method has proven to produce accurate results for other Au–Au Keywords: Aurophilicity • Argentophilicity • Host-Guest Systems dimers.[29] Models were built deleting C31 and C61 bridges and replacing them by H atoms (see details in the SI). Table 4 shows • van der Waals radii • Metal-Metal Interactions the computed values for the interaction energies between the two resulting monomers. [1] H. Schmidbaur, Gold Bull. 1990, 23, 11-21. [2] M. Jansen, Angew. Chem. Int. Ed. Engl. 1987, 26, 1098-1110, and references therein. Table 4. Computed interaction energies [kcalmol–1] for models of 1 and 2. [3] Y. Jiang, S. Alvarez, R. Hoffmann, Inorg. Chem. 1985, 24, 749-757. SCS-MP2 / cc-pVDZ SCS-MP2 / cc-pVTZ SCS-MP2 / CBS [4] F. Scherbaum, A. Grohmann, B. Huber, C. Krüger, H. Schmidbaur, Angew. Chem. Int. Ed. Engl. 1988, 27, 1544-1546. 1 –3.0 –3.2 –3.9 [5] W. S. Rapson, Gold Bull. 1989, 22, 19-20. 2 –6.6 –6.6 –7.2 [6] P. K. Mehrotra, R. Hoffmann, Inorg. Chem. 1978, 17, 2187-2189. [7] H. Schmidbaur, A. Schier, Chem. Soc. Rev. 2012, 41, 370-412. [8] P. Pyykkö, Chem. Rev. 1997, 97, 597-636. It can be observed that the interaction energy (in absolute [9] S. Sculfort, P. Braunstein, Chem. Soc. Rev. 2011, 40, 2741-2760. value) at the complete basis set limit for 2 is 3.3 kcal mol-1 larger [10] Q. Zheng, S. Borsley, G. S. Nichol, F. Duarte, S. L. Cockroft, Angew. than that obtained for 1. Since the distance between monomers Chem. Int. Ed. 2019, 58, 12617-1262. (Au–Au, 3.438 Å; Ag–Ag, 3.451 Å) and dispersion energy (25.2 [11] A. K. Singh, M. Usman, G. Sciortino, E. Garribba, S. P. Rath, Chem. and 26.0 kcalmol–1, respectively) are very similar in both 1 and 2 Eur. J. 2019, 25, 10098-10110. complexes the differences between Au and Ag complexes [12] M. B. Brands, J. Nitsch, C. Fonseca Guerra, Inorg. Chem. 2018, 57, 2603-2608. should be attributed to the difference in the metallophilic [13] H. Schmidbaur, A. Schier, Angew. Chem. Int. Ed. 2015, 54, 746-784. interaction. [14] T. Osuga, T. Murase, M. Fujita, Angew. Chem. Int. Ed. 2012, 51, In summary, cation [Au2(bisMeOEtIm)2]2+ (2) exhibits 12199-12201. considerably stronger metallophilic interactions in solution than [15] S.-L. Zheng, C. L. Nygren, M. Messerschmidt, P. Coppens, Chem. its Ag2 (1) analogue. This fact represents an empirical evidence Commun. 2006, 3711-3713. that aurophilicity is stronger than argentophilicity. While cation 1 [16] A. Deák, T. Tunyogi, Z. Károly, S. Klébert, G. Pálinkás, J. Am. Chem. exhibits two different conformers in solution, with dynamic U⇋Z Soc. 2010, 132, 13627-13629. switch and an aggregation tendency based on intermolecular [17] M. J. MacLeod, J. A. Johnson, J. Am. Chem. Soc. 2015, 137, 7974- 7977. Ag···Ag interactions, cation 2 exists in solution as a single [18] J. Vignolle, T. D. Tilley, Chem. Commun. 2009, 7230-7232. conformer (U) due to the robust Au···Au interactions, which [19] X. Ling, S. Roland, M.-P. Pileni, Chem. Mater. 2015, 27, 414−423. prevent to a large degree its conversion to the Z-conformer and [20] S. Wang, Y. Song, S. Jin, X. Liu, J. Zhang, Y. Pei, X. Meng, M. Chen, P. intermolecular approaches even at high concentrations. Both 1 Li, M. Zhu, J. Am. Chem. Soc. 2015, 137, 4018-4021. and 2 cations act as excellent hosts for Ag+ cations: two unusual [21] P. Buchwalter, J. Rose, P. Braunstein, Chem. Rev. 2015, 115, 28-126. cation-cation inclusion complexes were isolated [Ag(CH3CN)2 Ì [22] D. L. J. Broere, D. K. Modder, E. Blokker, M. A. Siegler, J. I. van der M2(bisMeOEtIm)2]3+ (M = Ag, 3; Au, 4). Interestingly, when Vlugt, Angew. Chem. Int. Ed. 2016, 55, 2406-2410. electrostatic repulsions are partially weakened by incorporating [23] X.-L. Pei, Y. Yang, Z. Lei, S.-S. Chang, Z.-J. Guan, X.-K. Wan, T.-B. Wen, Q.-M. Wang, J. Am. Chem. Soc. 2015, 137, 5520-5525. a nitrate in the coordination sphere of the guest, metallophilic [24] V. J. Catalano, M. A. Malwitz, A. O. Etogo, Inorg. Chem. 2004, 43, contacts become stronger and therefore intermetallic Ag···Au 5714-5724. distances considerably shorter. This is the case of [25] G. Kampf, P. J. Sanz Miguel, M. M. Cerda, M. Willermann, A. [Ag(CH3CN)(NO3) Ì Au2(bisMeOEtIm)2]2+ (5). Electron Schneider, B. Lippert, Chem. Eur. J. 2008, 14, 6882-6891. delocalization indexes reveal appreciable metallophilic bond COMMUNICATION [26] W.-X. Ni, Y.-M. Qiu, M. Li, J. Zheng, R. W.-Y. Sun, S.-Z. Zhan, S. W. [33] A. Bayler, A. Schier, G. A. Bowmaker, H. Schmidbaur, J. Am. Chem. Ng, D. Li, J. Am. Chem. Soc. 2014, 136, 9532-9535. Soc. 1996, 118, 7006-7007. [27] Q. Wan, J. Xia, W. Lu, J. Yang, C.-M. Che, J. Am. Chem. Soc. 2019, [34] P. J. Barnard, M. V. Baker, S. J. Berners-Price, B. W. Skelton, A. H. 141, 11572-11582. White, Dalton Trans. 2004, 1038-1047. [28] H. Schmidbaur, Gold Bull. 2000, 33, 3-10. [35] T. P. Pell, D. J. D. Wilson, B. W. Skelton, J. L. Dutton, P. J. Barnard, [29] E. Andris, P. C. Andrikopoulos, J. Schulz, J. Turek, A. Ruzicka, J. Inorg. Chem. 2016, 55, 6882-6891. Roithova, J. Rulisek, J. Am. Chem. Soc. 2018, 140, 2316-2325. [36] E. Kühnel, I. V. Shishkov, F. Rominger, T. Oeser, P. Hofmann, [30] A. Vellé, A. Cebollada, M. Iglesias, P.J. Sanz Miguel, Inorg. Chem. Organometallics 2012, 31, 8000-8011. 2014, 53, 10654-10659. [37] K. Matsumoto, N. Matsumoto, A. Ishii, T. Tsukuda, M. Hasegawab, T. [31] A. Cebollada, A. Vellé, M. Iglesias, L. B. Fullmer, S. Goberna-Ferrón, M. Tsubomura, Dalton Trans. 2009, 6795-6801. Nyman, P. J. Sanz Miguel, Angew. Chem. Int. Ed. 2015, 54, 12762- [38] S. Grimme, J. Chem. Phys. 2003, 118, 9095-9102. 12766. [32] J. Rieb, B. Dominelli, D. Mayer, C. Jandl, J. Drechsel, W. Heydenreuter, S. A. Sieber, F. E. Kühn, Dalton Trans. 2017, 46, 2722-2735. COMMUNICATION Entry for the Table of Contents COMMUNICATION A. Vellé, L. Rodríguez-Santiago, M. Sodupe, P. J. Sanz Miguel* Page No. – Page No. Enhanced Metallophilicity in Metal– Carbene Systems: Stronger Character of Aurophilic Interactions in Solution Although they seem to be almost identical in solid state, Au···Au interactions in solution proved to be much stronger than Ag···Ag bonding in our unsupported system. Such interactions take place even beyond the van der Waals limits.