Metallopolymerization as a Strategy to Translate Ligand-Modulated

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Metallopolymerization as a Strategy to Translate Ligand-Modulated Chemoselectivity to Porous Catalysts Wen-Yang Gao,† Andrew A. Ezazi,† Chen-Hao Wang,† Jisue Moon,‡,§ Carter Abney,‡ Joshua Wright,∥ and David C. Powers*,† †

Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States § Department of Chemical Engineering and Materials Science, University of California−Irvine, Irvine, California 92697, United States ∥ Illinois Institute of Technology, Chicago, Illinois 60616, United States

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‡

S Supporting Information *

ABSTRACT: Porous catalysts have garnered substantial interest as potential platforms for group-transfer catalysis due to the ability to noncovalently colocalize substrates in proximity to site-isolated reactive intermediates. In contrast to soluble molecular catalysts, the limited synthetic toolbox available to prepare porous catalysts presents a formidable challenge to controlling the primary coordination sphere of lattice-confined catalysts and thus modulating the electronic structures of reactive catalyst intermediates. Here, we utilize Sonogashira cross-coupling chemistry to prepare a family of porous metallopolymers in the primary coordination sphere of Ru2 sites. The newly synthesized materials are characterized by IR, elemental analysis, gas sorption, powder X-ray diffraction, thermogravimetric analysis, X-ray absorption spectroscopy, and diffuse-reflectance UV−vis-NIR spectroscopy. The resulting porous materials are catalysts for nitrene-transfer chemistry, and the chemoselectivity for allylic amination versus olefin aziridination can be tuned by modulating the primary coordination sphere of the catalyst sites. The demonstration of metallopolymerization as a rational synthetic strategy enables ligandmodulated chemoselectivity to be achieved with porous catalysts and represents a new opportunity to tailor the functionality of heterogeneous catalyst materials.

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INTRODUCTION The ability to control the chemoselectivity of substrate functionalization reactions by systematic ligand perturbation is a hallmark of molecular catalyst design. For example, selection for allylic C−H amination versus aziridination products during Ru2-catalyzed nitrene-transfer reactions intimately depends on the primary ligand sphere of the Ru2 catalyst.1 On the basis of the potential to noncovalently colocalize substrates in proximity to site-isolated catalyst sites, metal−organic frameworks (MOFs) have garnered substantial interest as catalyst platforms for both intra- and intermolecular C−H functionalization. Efforts to exert rational synthetic control over the chemoselectivivity of group transfer reactions effected by MOF catalysts are hampered by the current inability to rationally control the primary coordination sphere of catalyst sites, and thus the electronic structure of latticeconfined reactive intermediates. For example, a single porous Ru2-based MOF with open Ru sites has been reported and the Ru2 sites are supported by carboxylate ligands.2,3 Synthesis of crystalline MOFs is predicated on reversible metal−ligand (M−L) bond-forming chemistry, which simulta© XXXX American Chemical Society

neously assembles the porous network and establishes the primary coordination sphere of the lattice-confined transition metal ions.4−6 The necessity for reversible M−L bond formation to achieve material crystallinity limits the diversity of ligands that can be utilized to generate potential catalyst MOFs. Pyridyl-, carboxylate-, and azolate-derived ligandsall of which are fairly weak-field ligands that engender high-spin electronic configurationsare ubiquitous in the chemistry of MOFs (Figure 1a).7 In contrast, strongly basic donors, such as amido, amidinate, and alkoxide ligandscommon ligands in molecular transition metal catalystsare rarely encountered in the coordination chemistry of MOFs.8,9 Porous organic materials,10,11 such as crystalline covalent organic frameworks (COFs),12−14 amorphous porous organic polymers (POPs),15−19 and organic cage compounds20,21 are comprised of exclusively organic components connected via Special Issue: Organometallic Chemistry within Metal-Organic Frameworks Received: March 11, 2019

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DOI: 10.1021/acs.organomet.9b00162 Organometallics XXXX, XXX, XXX−XXX

Organometallics

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Article

RESULTS AND DISCUSSION Synthesis of Molecular Complexes. A family of molecular Ru2[II,III] complexes bearing iodinated bridging ligands was prepared by thermally promoted ligand exchange with Ru2(OAc)4Cl (Figure 2). Exchange reactions were carried

Figure 1. (a) The pKa values (in DMSO) of a series of proligands (the pKa values of carboxylic acid, amide, and amidine refer to those of benzoic acid, benzamide, and benzamidine, respectively). (b) There is a single Ru2-based MOF, and it is based on bridging carboxylate linkers. This work demonstrates metallopolymerization as a synthetic strategy that allows a family of porous Ru2-based catalysts to be prepared with tuned primary coordination spheres.

Figure 2. Five Ru2L4BF4 complexes, featuring carboxylate (L = 4-IOBz), amidate (L = 4-I-HNBz), 2-oxypyridinate (L = 4-I-hp or 5-Ihp), and 2-aminopyridinate (L = 4-I-ap) ligands, were prepared by a two-step procedure of ligand substitution and anion exchange.

out with 4-iodobenzoic acid (4-I-HOBz), 4-iodobenzamide (4I-H2NBz), 2-hydroxy-4-iodopyridine (4-I-hp), 2-hydroxy-5iodopyridine (5-I-hp), and 2-amino-4-iodopyridine (4-I-ap) to provide Ru2(4-I-OBz)4Cl, Ru2(4-I-HNBz)4Cl, Ru2(4-Ihp)4Cl, Ru2(5-I-hp)4Cl, and Ru2(4-I-ap)4Cl, respectively. In each case, ligand substitution reactions were followed by electrospray ionization mass spectrometry (ESI-MS; negative mode), which provided a value of m/z that corresponded to the mass of [Ru2L4Cl2]−. Structurally, these Ru2[II,III] complexes are comprised of a binuclear core supported by four bridging ligands; charge balance of the Ru2[II,III] mixedvalent core is achieved with an axially bound chloride ligand (see, for example, the structure of Ru2(4-I-HNBz)4Cl derived from single-crystal X-ray diffraction, Figure S1 and Table S1). The electronic absorption spectra of the iodinated Ru2 complexes are well-matched to the spectral data for noniodinated analogues (Figure S2). In addition, the 1H NMR spectrum of Ru2(4-I-OBz)4Cl (Figure S3) displays similar paramagnetically shifted signals as Ru2(OBz)4Cl.35 Ru2(4-Ihp)4Cl, Ru2(5-I-hp)4Cl, and Ru2(4-I-ap)4Cl do not display 1H NMR signals when measured in DMSO at 295 K. The formation of Ru−Cl−Ru chains in the solid-state structures of Ru2L4Cl complexes typically leads to poor solubility in many organic solvents (see, for example, the structure of Ru2(4-I-HNBz)4Cl, Figure S1 and Table S1). To increase the solubility of these complexes, we replaced the axially bound chloride ligand with tetrafluoroborate by treatment with silver tetrafluoroborate. The anion exchange process was monitored by IR spectroscopy, which showed the characteristic broad B−F stretch of BF4− at 1050 cm−1 (Figure S4), and elemental analysis. Single-crystal X-ray diffraction analysis of Ru2(HNBz)4BF4 (Figure S5 and Table S2) reveals the two axial positions of the Ru2 unit are occupied by a water and a THF ligand, respectively. The electronic absorption spectra and 1H NMR spectra of the tetrafluoroborate complexes are similar to those of the related chloride complexes (Figures S2, S6, and S7). Synthesis of Ru2-Based Polymers. A mixture of Ru2(4-Ihp)4BF4 and 1,3,5-triethynylbenzene (A) in 1:1 N,Ndimethylformamide (DMF) and diisopropylamine (DIPA) was heated at 80 °C for 24 h under a N2 atmosphere (Table 1 and details in Table S3). The harvested orange material, i.e.,

covalent bonds. A diverse set of reactions, including imine and boronic ester condensations,22−24 nitrile cyclotrimerizations,25 nucleophilic substitutions,26−28 C−C cross-coupling reactions,29−33 and Huisgen cycloadditions,34 have been applied to the synthesis of porous organic materials. With interest in developing platforms for selective group-transfer catalysis,35−37 we were attracted by the potential to utilize the polymerization strategies pioneered in POPs, in combination with preformed metallomonomers containing the specific metal coordination site of interest, to generate new porous materials in which the primary coordination sphere, and thus the electronic structure of site-isolated catalysts, could be systematically tuned.38,39 Because the envisioned synthetic logic is based on retrosynthetic disconnection of covalent bonds within the organic linkers, and not on the M−L bonds, we speculated that such a synthetic approach would provide access to families of catalyst materials in which the primary coordination sphere of the catalyst sites could be systematically tuned via a common synthetic protocol.40 Here we report the synthesis of a family of porous polymer materials featuring Ru2 sites supported by a diverse family of bridging ligands, including carboxylate, amidate, 2-hydroxypyridinate, and 2-aminopyridinate. The materials are readily accessed by Sonogashira cross-coupling chemistry between preformed molecular Ru2 monomers decorated with aryl iodide functionality and polyalkynes. Consistent with the irreversible Sonogashira reactions utilized to generate these materials, 27 the new Ru 2 -based porous polymers are amorphous. These new porous materials are competent nitrene-transfer catalysts in intramolecular C−H amination. Importantly, in the context of intramolecular nitrene transfer in olefinic substrates, the selectivity for allylic amination versus olefin aziridination displays ligand-dependent selectivity analogous to the selectivity achieved by soluble molecular catalysts. We anticipate that the synthetic strategy described herein, which provides access to porous materials that combine the site isolation of catalyst sites characteristic of MOF chemistry with the atomistic control over the primary coordination sphere characteristic of molecular catalysis, will provide new opportunities to develop porous catalysts for chemoselective group-transfer chemistry. B


Article

Organometallics Table 1. BET (Langmuir) Surface Area Values (m2/g) for Each Obtained Polymeric Materialb

a The synthesis of these materials was not attempted. bRu2-based porous polymers were synthesized from Sonogashira cross-coupling reaction between Ru2 metallomonomers and alkyne-based linkers A, B, and C. The surface areas of these materials were probed by N2 adsorption isotherm at 77 K.

[A4/3(Ru24‑hp)]n, did not dissolve in any common solvents, such as dimethyl sulfoxide (DMSO), methanol (MeOH), and tetrahydrofuran (THF). Soxhlet extraction using MeOH was carried out to ensure complete removal of any unincorporated monomer units. The obtained solid did not display an IR feature of alkyne C−H stretching at 3288 cm−1 (Figure S8a) and showed trace amounts of iodine by elemental analysis, consistent with a high degree of polymerization via Sonogashira cross-coupling.41 The developed polymerization chemistry is modular; both the polyalkyne and the Ru2 monomer can be replaced by different reaction partners. In addition to tris-alkyne A, we have carried out polymerization reactions with 1,3,5-tris(4ethynylphenyl)benzene (B), an extended version of A, and tetra(4-ethnylphenyl)methane (C), which features a tetrahedrally disposed array of terminal alkynes (Table 1). In addition to polymerization with Ru2(4-I-hp)4BF4, we have carried out metallopolymerization reactions with each of the iodinated Ru2 complexes described above. In each case, insoluble materials were obtained and IR analysis (Figure S8) indicated a high degree of polymerization. On the basis of powder X-ray diffraction (PXRD) analysis, all of the prepared materials are amorphous (PXRD data for the five polymers prepared from linker B are collected in Figure S9), which is consistent with previous reports that Sonogashira cross-coupling reactions provide access to amorphous POPs.34,42 Thermogravimetric analysis (TGA) revealed this family of materials is thermally stable to above 200 °C (Figure S10). For example, [B4/3(Ru24‑hp)]n is stable up to 250 °C after which temperature continuous weight loss is observed (Figure S10c). Gas Sorption. N2 adsorption isotherms were collected at 77 K for each of the Ru2-based polymers in order to evaluate the porosity of these materials; data are collected in Table 1

and Figure S11. Pore size distribution analysis is summarized in Figure S11j. Each material was preactivated at 150 °C under reduced pressure ( [B4/3(Ru2HNBz)]n > [B4/3(Ru2OBz)]n. This set of experiments demonstrates that the developed Sonogashira coupling strategy for polymerization of metallomonomers provides a platform to translate the chemoselectivity of molecular catalysts to recyclable porous material platforms. Comparison of the chemoselectivity for 4- and 5-substituted hp complexes suggests that, in addition to the primary coordination sphere, confinement effects within porous E


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Organometallics

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expand the suite of porous catalyst materials that can be evaluated for chemoselective C−H functionalization chemistry.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.9b00162. Experimental details, characterization data including NMR, electronic absorption, and IR spectra, PXRD patterns, TGA plots, N2 adsorption isotherms, and X-ray absorption spectroscopy data (PDF) Accession Codes

CCDC 1873925−1873926 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 Author

*E-mail: [email protected]. ORCID

Wen-Yang Gao: 0000-0002-9879-1634 Andrew A. Ezazi: 0000-0002-3884-3500 Chen-Hao Wang: 0000-0003-3416-4452 Jisue Moon: 0000-0002-1210-6342 Carter Abney: 0000-0002-1809-9577 Joshua Wright: 0000-0002-6217-3084 David C. Powers: 0000-0003-3717-2001 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Texas A&M University, the Welch Foundation (A1907), and the U.S. Department of Energy (DE-SC0018977) for financial support. XAS data were collected at Sector 10, MRCAT, of the Advanced Photon Source at Argonne National Laboratory. MRCAT operations are supported by the Department of Energy and the MRCAT member institutions. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC0206CH11357.

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