Argentophilic Interactions in Solution: An EXAFS Study of Silver(I

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Argentophilic Interactions in Solution: An EXAFS Study of Silver(I) Nitrene Transfer Catalysts Choi L. Mak,† Benjamin C. Bostick,‡ Nadine M. Yassin,† and Michael G. Campbell*,† †

Department of Chemistry, Barnard College, New York, New York 10027, United States Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, United States

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S Supporting Information *

series of carefully chosen case studies, we demonstrate that this technique can be used to clearly distinguish between several potential scenarios, including structures that are dimeric in the solid state but monomeric in solution. In 2003, the He group reported alkene aziridination catalyzed by the complex [(t-Bu3terpy)Ag(NO3)]2 (1).3a SCXRD analysis revealed the dinuclear structure 1a (Figure 1a, left), featuring a close silver−silver interaction of 2.842(2) Å. Subsequent work by the Schomaker group used DOSY NMR to support the theory that 1 retains a dimeric structure in solution;8 however, a dimeric structure for a complex featuring bridging ligands does not

ABSTRACT: Silver(I) catalysts have been developed for nitrene transfer reactions such as aziridination and C−H insertion. For some catalysts, structures determined by Xray crystallography reveal dimers with silver−silver interactions, leading to mechanistic speculation about the potential role of dinuclear silver complexes in catalysis. However, it is often unclear if the silver−silver interactions persist in solution. Here we use EXAFS to directly interrogate the solution-phase structures of several silver(I) nitrene transfer catalysts. Retention or loss of the silver−silver interaction in solution can be clearly observed.

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ynthetically useful homogeneous catalysis with silver(I) has historically been limited primarily to Lewis acid catalysis,1 but in the past 2 decades there has been a significant increase in redox catalysis with silver. One area that has seen substantial growth is silver-catalyzed nitrene transfer, including alkene aziridination and C−H insertion reactions.2 Well-defined silver(I) catalysts for nitrene transfer have been developed, and many have been structurally characterized by single-crystal X-ray diffraction (SCXRD). For some of the structurally characterized catalysts, dimeric complexes with silver−silver interactions were observed, which led to mechanistic speculation about the potential role of dinuclear silver complexes in redox catalysis.3 A key problem, however, is that the relationship between solidand solution-phase structures for silver(I) complexes is often ambiguous because of the flexible coordination chemistry of d10 silver centers. The Schomaker group has demonstrated the use of NMR techniques [variable-temperature 1H and diffusion-ordered spectroscopy (DOSY) NMR studies] to elucidate the dynamic solution-phase behavior of silver nitrene transfer catalysts, including ligand (hemi)lability and dimer/monomer equilibria.4 These studies have provided insight into how dynamic behavior in solution can impact selectivity in catalysis. However, to date, no study has experimentally addressed the question of whether argentophilic interactions, as observed in the solid state,5 persist in solution for any of the silver(I) nitrene transfer catalysts. Silver−silver coupling in 109Ag NMR spectra can be used to examine dinuclear silver complexes in solution,6 but this technique does not provide structural information such as silver−silver distances. Here we use extended X-ray absorption fine structure (EXAFS) analysis to directly interrogate the presence of silver−silver interactions in solution.7 Through a © XXXX American Chemical Society

Figure 1. Terpyridyl silver(I) complex 1 displays a dimeric structure with silver−silver interactions in both the solid state and solution: (a) two distinct structures (1a and 1b) observed by X-ray crystallography; (b) R-space EXAFS spectra for redissolved crystals of 1 in a CH2Cl2 solution. Received: April 10, 2018

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DOI: 10.1021/acs.inorgchem.8b00934 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry necessarily imply the presence of a close silver−silver contact (vide infra). A computational study from Schomaker, Berry, and co-workers also proposed a mechanistic pathway in which the dinuclear structure of 1 is maintained during nitrene transfer catalysis.9 In our hands, we have found that 1 can give two distinct structures upon crystallization. In addition to the reported structure 1a, we have isolated and characterized the new dimeric structure 1b (CCDC 1830204), which features a more symmetric coordination environment compared to 1a (Figure 1a, right). SCXRD analysis shows that the silver−silver distance in 1b is 2.9567(6) Å, slightly longer than for 1a (Figure 2, left). Our combined data suggest that 1b is best described as

Figure 3. Complex 2 displays a monomeric structure in both the solid state and solution, based on a combination of X-ray crystallography and solution-phase EXAFS data.

The data for complexes 1 and 2 demonstrate that EXAFS can establish a correlation between the solid- and solution-phase structures for both dimeric and monomeric silver(I) complexes. We also sought to distinguish between more challenging cases, in which the solid- and solution-phase structures significantly differ. 4,7-Diphenylphenanthroline silver triflate complex 3 was originally reported by the He group and is a competent catalyst for intermolecular nitrene insertion into C−H bonds.3c The solid-state structure of 3, determined by SCXRD analysis, shows a dimeric structure with a silver−silver interaction of 3.386(1) Å (Figure 4a, left); however, DOSY NMR data suggest that 3 exhibits a monomeric structure in solution.8 Further complicating matters, silver(I) phenanthroline complexes can also exhibit triflate-bridged structures that are dimeric but lack silver−silver interactions.4 These factors render the question more difficult

Figure 2. X-ray structures for new complexes 1b and 2, plotted with 50% probability ellipsoids (H atoms, solvent molecules, and noncoordinated counterions omitted for clarity).

the thermodynamic isomer, and we find that 1a converts to 1b after repeated crystallization (see the Supporting Information for a more detailed discussion). We have performed solution-phase EXAFS measurements at the silver K-edge for both 1a and 1b dissolved in CH2Cl2, which is a common solvent for silver-catalyzed nitrene transfer. The data for both forms of 1 clearly show that the silver−silver contact is maintained in solution (Figure 1b). The solutionphase silver−silver distances are 2.91(1) Å (1a) and 2.83(1) Å (1b), which are comparable to the distances observed for the solid-state structures for 1. This is the first experimental demonstration that 1 features a close silver−silver contact in solution, and provides additional support for the mechanistic proposal that dimeric 1 participates in nitrene transfer catalysis. For comparison to dimer 1, we have used EXAFS to examine the solution-phase structure of the 2,9-diphenylphenanthroline silver triflate complex 2 (CCDC 1830203). While phenanthroline-based ligands have successfully been used for silver-catalyzed nitrene transfer (vide infra), it has been shown that 2,9disubstituted phenanthroline ligands shut down nitrene transfer reactivity.3c We have determined the solid-state structure of 2 using SCXRD (Figure 2, right). Complex 2 features a distorted tetrahedral geometry at silver and a noncoordinated triflate counteranion. We have found that both 1:1 and 2:1 mixtures of 2,9-diphenylphenanthroline and silver triflate result in the formation of 2:1 complex 2 as the major isolated product. In a CH2Cl2 solution, the silver K-edge EXAFS spectrum shows that the solution-phase structure is also monomeric, and no silver− silver contact is observed (Figure 3). From modeling of the EXAFS data, it can be seen that the silver coordination sphere in solution matches well with the solid-state structure, indicating that the structure observed by SCXRD is maintained in solution. This is consistent with the lack of nitrene transfer reactivity observed for 2 because the silver center appears to remain coordinatively saturated in solution.

Figure 4. Complex 3 displays a dimeric structure with silver−silver interactions in the solid state but a monomeric structure in solution: (a) structures of dimeric and monomeric forms observed for 3; (b) R-space EXAFS spectrum for 3 in a CH2Cl2 solution. B

DOI: 10.1021/acs.inorgchem.8b00934 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Department of Chemistry and the Office of the Provost at Barnard College for financial support. SCXRD for 1b was performed at the Shared Materials Characterization Laboratory at Columbia University. We thank the Parkin group at Columbia University for assistance with SCXRD data collection for 2. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-76SF00515.

than simply dimer versus monomer, and we felt that a direct probe of the silver−silver interaction in solution would be valuable. EXAFS analysis was performed in a CH2Cl2 solution, which is the reported solvent for C−H amination catalyzed by 3. In contrast to the solid-state structure, solutions of 3 show the complete absence of silver−silver interactions (Figure 4b). Modeling of the EXAFS data suggests that in solution silver can be either three- or four-coordinate: fluxional coordination of the triflate ligand and/or solvent is likely (one model structure is shown in Figure 4a, right). All observed features in the UV−vis spectrum for 3 exhibit linear Beer−Lambert plots, suggesting that aggregation of monomeric 3 into dimers does not occur as a function of increasing concentration (data and analysis given in the Supporting Information).10 The combined data indicate that 3 favors a monomeric structure in a CH2Cl2 solution but can crystallize in a dimeric form that features argentophilic interactions in the solid state. These observations may have significant mechanistic consequences, for example, when nitrene transfer reactivity is compared between complexes 1 and 3. In conclusion, we have reported the first use of EXAFS to experimentally interrogate argentophilic interactions in solution for silver(I) complexes relevant to nitrene transfer catalysis. Scenarios in which the solid- and solution-phase structures are the same or significantly different can be clearly distinguished. Our results are complementary to the existing characterization data for these complexes and provide information that is directly relevant to the mechanistic hypothesis that silver−silver interactions can play a role in homogeneous redox catalysis with silver. We anticipate that EXAFS will be useful as a routine technique for solution-phase structure determination of silver(I) complexes, especially in cases where common methods such as SCXRD and NMR provide inconclusive results.

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(1) (a) Silver in Organic Chemistry; Harmata, M., Ed.; John Wiley & Sons, Inc.: Hoboken, NJ, 2010. (b) Naodovic, M.; Yamamoto, H. Asymmetric Silver-Catalyzed Reactions. Chem. Rev. 2008, 108, 3132. (c) Weibel, J.-M.; Blanc, A.; Pale, P. Ag-Mediated Reactions: Coupling and Heterocyclization Reactions. Chem. Rev. 2008, 108, 3149. (d) Á lvarez-Corral, M.; Muñoz-Dorado, M.; Rodríguez-García, I. Silver-Mediated Synthesis of Heterocycles. Chem. Rev. 2008, 108, 3174. (e) Yamamoto, Y. Silver-Catalyzed Csp−H and Csp−Si Bond Transformations and Related Processes. Chem. Rev. 2008, 108, 3199. (2) (a) Li, Z.; He, C. Recent Advances in Silver-Catalyzed Nitrene, Carbene, and Silylene-Transfer Reactions. Eur. J. Org. Chem. 2006, 2006, 4313. (b) Maestre, L.; Sameera, W. M. C.; Díaz-Requejo, M. M.; Maseras, F.; Pérez, P. J. A General Mechanism for the Copper- and Silver-Catalyzed Olefin Aziridination Reactions: Concomitant Involvement of the Singlet and Triplet Pathways. J. Am. Chem. Soc. 2013, 135, 1338. (c) Alderson, J. M.; Corbin, J. R.; Schomaker, J. M. Tunable, Chemo- and Site-Selective Nitrene Transfer Reactions through the Rational Design of Silver(I) Catalysts. Acc. Chem. Res. 2017, 50, 2147. (3) (a) Cui, Y.; He, C. Efficient Aziridination of Olefins Catalyzed by a Unique Disilver(I) Compound. J. Am. Chem. Soc. 2003, 125, 16202. (b) Cui, Y.; He, C. A Silver-Catalyzed Intramolecular Amidation of Saturated C−H Bonds. Angew. Chem., Int. Ed. 2004, 43, 4210. (c) Li, Z.; Capretto, D. A.; Rahaman, R.; He, C. Silver-Catalyzed Intermolecular Amination of C−H Groups. Angew. Chem., Int. Ed. 2007, 46, 5184. (4) Huang, M.; Corbin, J. R.; Dolan, N. S.; Fry, C. G.; Vinokur, A. I.; Guzei, I. A.; Schomaker, J. M. Synthesis, Characterization, and VariableTemperature NMR Studies of Silver(I) Complexes for Selective Nitrene Transfer. Inorg. Chem. 2017, 56, 6725. (5) Schmidbaur, H.; Schier, A. Argentophilic Interactions. Angew. Chem., Int. Ed. 2015, 54, 746. (6) (a) Tate, B. K.; Wyss, C. M.; Bacsa, J.; Kluge, K.; Gelbaum, L.; Sadighi, J. P. A Dinuclear Silver Hydride and an Umpolung Reaction of CO2. Chem. Sci. 2013, 4, 3068. (b) Tate, B. K.; Jordan, A. J.; Bacsa, J.; Sadighi, J. P. Stable Mono- and Dinuclear Organosilver Complexes. Organometallics 2017, 36, 964. (7) Nelson, R. C.; Miller, J. T. An introduction to X-ray absorption spectroscopy and its in situ application to organometallic compounds and homogeneous catalysis. Catal. Sci. Technol. 2012, 2, 461. (8) Rigoli, J. W.; Weatherly, C. D.; Alderson, J. M.; Vo, B. T.; Schomaker, J. M. Tunable, Chemoselective Amination via Silver Catalysis. J. Am. Chem. Soc. 2013, 135, 17238. (9) Dolan, N. S.; Scamp, R. J.; Yang, T.; Berry, J. F.; Schomaker, J. M. Catalyst-Controlled and Tunable, Chemoselective Silver-Catalyzed Intermolecular Nitrene Transfer: Experimental and Computational Studies. J. Am. Chem. Soc. 2016, 138, 14658. (10) (a) Pugh, D.; Giles, C. H.; Duff, D. G. Determination of the Aggregation Number of Anionic Dyes by Studies of Deviation from Beer’s Law. Trans. Faraday Soc. 1971, 67, 563. (b) Sigal, I. S.; Gray, H. B. Characterization of Cationic Rhodium Isocyanide Oligomers in Aqueous Solutions. J. Am. Chem. Soc. 1981, 103, 2220. (c) Rawashdeh-Omary, M. A.; Omary, M. A.; Patterson, H. H. Oligomerization of Au(CN)2− and Ag(CN)2− Ions in Solution via Ground-State Aurophilic and Argentophilic Bonding. J. Am. Chem. Soc. 2000, 122, 10371.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00934. Experimental procedures, spectroscopic data, and EXAFS analysis (PDF) Accession Codes

CCDC 1830203−1830204 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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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Benjamin C. Bostick: 0000-0002-7513-6469 Michael G. Campbell: 0000-0002-4174-0174 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Christian Rojas for helpful discussions and assistance with NMR spectroscopy. C.L.M., N.M.Y., and M.G.C. thank the C

DOI: 10.1021/acs.inorgchem.8b00934 Inorg. Chem. XXXX, XXX, XXX−XXX