Direct, One-Pot Syntheses of MOFs Decorated with Low-Valent Metal

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Direct, One-Pot Syntheses of MOFs Decorated with Low-Valent Metal-Phosphine Complexes Samuel G. Dunning,† Joseph E. Reynolds, III,† Kelly M. Walsh, David J. Kristek, Vincent M. Lynch, Pranaw Kunal,‡ and Simon M. Humphrey* Department of Chemistry, The University of Texas at Austin, 100 East 24th Street, Stop A1590, Austin, Texas 78712, United States

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S Supporting Information *

ABSTRACT: We demonstrate a simple and generalizable method for the one pot, in situ formation of bimetallic metal−organic framework (MOF) materials decorated with crystallographically defined, low-valent metal sites. Solvothermal reactions between poly(carboxylated) aryl(phosphine) ligands and mixtures of higher- and lower-valent metal precursors achieves concomitant P−M complexation and MOF formation via metal−carboxylate network bonding. This method is demonstrated in the preparation of two new phosphine coordination materials, PCM-107 and PCM-74, which feature Ar3P−AuX (X = Cl, Br) and chelated ̂ ̂ PN −Cu 2I2−PN units as the MOF building blocks, respectively. PCM-107 is an active catalyst for alkyne hydroaddition, while PCM-74 shows promise for the crystallization of MOFs bearing kinetically favored isomers.

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INTRODUCTION The field of metal−organic frameworks (MOFs) is now well established, and the potential applications of this topical class of modular crystalline solids are extensive.1−3 Arguably, however, the full scope of MOF structural diversity and reactivity has yet to be fully realized. New and more advanced synthetic strategies hold the potential to provide access to MOFs with previously unrealized solid-state chemical reactivity.4,5 Of particular current interest are materials that can achieve stronger and more highly selective binding of guest adsorbates inside MOF micropores. MOFs are able to reversibly adsorb a wide range of small-molecule substrates via weak physisorption interactions, achieving capacities and selectivities that are superior to classical mesoporous solids commonly utilized in important industrial applications.6,7 In contrast, absorbate chemisorptionwhich could potentially enable adsorbate bond activationremains rare in MOFs.8 Achieving selective chemisorption of small-molecule adsorbates by MOFs (e.g., H2, CO2, N2, O2, light olefins) would pave the way to their application in a myriad of large-scale catalytic processes, including the synthesis of liquid fuels and other value-added products from renewable gas feedstocks. Progress in this regard has been limited thus far by a lack of available synthetic methods and MOF precursors for the systematic installation of low-valent and/or coordinatively unsaturated reactive metal species (i.e., catalyst complexes) into MOFs. Classically, MOFs are synthesized using metal cations that favor coordinative saturation in the presence of “hard” ligand donors (e.g., carboxylates, alkoxides, imidazolates) to yield thermodynamically robust polymeric solids. In © XXXX American Chemical Society

contrast, low-valent metals of catalytic relevance (including 4d and 5d metals) are incompatible with this general synthetic approach because they tend to favor more labile, dative covalent M−L bonds. One potential solution to this problem is to utilize organic building blocks with chemically disparate donor moieties, which include (i) hard donors that favor the formation of multidirectional framework connections by reaction with similarly hard metal cations and (ii) softer donor groups that do not participate in MOF network bonding but can selectively ligate lower-valent metal species inside the MOF pore networks. There are a growing number of examples of MOFs based on this approach. For example, in earlier studies, Cohen developed the use of MOFs with pendant amines for the postsynthetic installation of reactive metal sites9 and Yaghi demonstrated low-valent metal coordination in carbenedecorated MOFs.10 Our ongoing approach in this area is to utilize phosphine- and arsine-based MOFs for the installation of low-valent metal complexes, given that such ligands are ubiquitous in molecular catalysis.11−22 Our group and those of Wade and Lin have shown that MOF building blocks based on preformed bis(phosphine)16,23 and PCP-pincer17,18,24−26 complexes can be used to assemble MOF structures containing reactive metal sites. However, we increasingly disfavor this Special Issue: Organometallic Chemistry within Metal-Organic Frameworks Received: May 14, 2019

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DOI: 10.1021/acs.organomet.9b00319 Organometallics XXXX, XXX, XXX−XXX

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Organometallics approach because the organic chemistry required to prepare the molecular precursors is often arduous and low-yielding. A simpler and potentially more scalable alternative synthetic strategy is to form MOFs with abundant Lewis basic sites using ligands that can be prepared in multigram quantities and fewer steps. The so-called PCMs and ACMs (phosphine and arsine coordination materials) are a family of MOFs decorated with abundant, structurally well-defined Lewis base sites, which provide ideal platforms to study low-valent metal incorporation, often in a single-crystal to single-crystal fashion.19,20 Such materials can be considered as “solid-state ligands” (SSLs), which can be utilized as crystalline catalyst supports (Scheme 1A). We recently reported the first example of a

complexes along with crystallization of the PCM polymers under moderate heating in the presence of secondary 3d metal salts (Scheme 1B). This method can potentially provide access to phosphine complexes in thermodynamically disfavored coordination geometries, in comparison to their geometrically relaxed molecular analogues. Incorporation of phosphines into rigid polymeric arrays can also induce geometric strain (i.e., cone angle or P−Ar π-conjugation perturbations), thus potentially affecting the phosphine electronic properties and coordination behavior.

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RESULTS AND DISCUSSION Synthesis and Characterization of PCM-107: In Situ Formation of Ar3P−AuX-Based MOFs (X = Cl, Br). We initially focused on the synthesis of new PCMs by the in situ P coordination of Au(I) halide complexes. This approach was chosen since we have previously shown that ClAu(SMe2) undergoes clean coordination with preformed PCMs at accessible P: sites inside the micropores.19,20,27 Tris(pcarboxylato)triphenylphosphine (Scheme 2; tctpH3), 1 equiv of the secondary pillaring ligand 1,4-dipyridylbenzene (1,4dpb), 3 equiv of Co(BF4)2, and 1 equiv of XAu(SMe2) (X = Cl, Br) were codissolved in 5 mL of a 5/2/1 (v/v/v) N,N′dimethylformamide (DMF)/methanol/H2O mixture and left to react at 75 °C for 18 h, resulting in the formation of deep purple crystals. When the same reaction was carried out in the absence of XAu(SMe2), a reticular analogue of the previously reported material PCM-101 featuring uncoordinated trans-P2 moieties was obtained instead (Scheme 2, left). Bulk analysis of the new purple solid by powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM) confirmed that the material was free of Au(0) (i.e., clusters or nanoparticles; Figures S1−S3). Single-crystal X-ray diffraction (SCXRD) revealed a new PCM with 3-D microporosity, hereafter referred to as PCM-107. PCM-107 crystallizes in the orthorhombic space group Cmca (Z = 8) with the empirical formula [Co6(tctp−AuX)2(1,4-dpb)(OH2)4]∞ (X = Cl, Br) (Figure 1), in agreement with other bulk characterization data (see the Supporting Information). PCM-107 consists of Ar3P−AuX units assembled into infinite 2-D layers by coordination of all p-carboxylate-O atoms to octahedrally coordinated Co(II) centers (Figure 1A). The [Co3(OCO)6(1,4-dpb)2(OH2)2] nodes are angular trimers (∠Co2−Co1−Co2A = 141.3°) in which the central Co1 atom is coordinated only by bridging carboxylate-O donors, while the symmetrically equivalent peripheral Co2 sites are bound to four carboxylate-O donors and are also coordinated by one 1,4-dpb-N donor and a terminal OH2 molecule (Figure 1A). Overall, each Co3 unit thus acts as an 8connected nodal point in the extended structure of PCM-107. Most notably, the 1,4-dpb ligands are arranged in a trans arrangement at the tips of each node (blue ligands; Figure 1A)

Scheme 1. Alternative Synthetic Routes Currently under Investigation for the Preparation of Crystalline PhosphineBased MOFs Decorated with Low-Valent Metal Sites

MOF containing trans-phosphine coordination pockets, assembled using simple monophosphine ligands;19 the material can be postsynthetically functionalized with low-valent metals with retention of crystallinity, thus permitting single-crystal Xray analysis of the resulting materials. In one example, Cu2Br2 dimers were coordinated under persistent compressive strain between trans-P2 sites within the rigid MOF scaffold.19 We have also reported the first example of an ACM containing cisdiarsine coordination pockets, which were postsynthetically modified with strained Au2Cl2 dimers that displayed exceptionally short aurophilic bonds.20 In an extension of this work, we have recently investigated the in situ (or peri-synthetic) formation of PCMs decorated with low-valent metal complexes, as a complementary route to new functional MOFs (Scheme 1B). Here, we present the synthesis, crystal structures, and solid-state properties of two new PCMs obtained by a simple one-pot approach. To the best of our knowledge, this work is the first demonstration of the direct crystallization of multimetallic MOFs containing low-valent metals selectively complexed at P: sites. The methodology involves the in situ formation of Ar3P−MLn

Scheme 2. Comparative Outcomes in the Formation of Co(II)-Based PCMs with and without the Presence of an Au(I) Precursor

B

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was attributed to the loss of uncoordinated solvents of synthesis confined in the pores. The framework of PCM-107 was found to be thermally stable up to 340 °C, in good agreement with previously reported Co(II) PCMs. To confirm the bulk textural properties of PCM-107, as-synthesized samples were desolvated under high vacuum over 18 h and their sorption isotherms were collected. Gas sorption data for PCM-107 at cryogenic temperatures using a range of probe adsorbates confirmed that the material was permanently porous, with a modest surface area of 254 for CO2 (196 K) and a corresponding pore volume of 0.21 cm3 g−1 (Figures S6−S10 and Table S1). The significant TGA mass loss observed upon desolvation, as well as the calculated solvent-accessible void space in PCM-107 (Vcalc = 10586 Å3 per unit cell), is somewhat contradictory of its measured surface area. This is not uncommon for bipyridine pillared MOFs, which can permit significant, reversible distortions in the solid state upon desolvation, which can reduce the apparent accessible internal voids.19,28 PXRD analysis of an activated sample of PCM-107 showed a loss of some lowerintensity reflections at higher 2θ values, indicating some loss of long-range order upon activation (Figure S11); this is consistent with a material that retains its porosity while becoming partially amorphous in the absence of solvent. We next probed the accessibility of small-molecule substrates to the AuX moieties within the micropores and the potential reactivity of these Au(I) sites to facilitate simple chemical reactions. To achieve these aims, we studied catalytic alkyne hydroaddition using a range of substrates as a relevant model reaction, since it is readily catalyzed by molecular gold(I) phosphine complexes (Scheme 3).29 We recently

Figure 1. (A) Extended asymmetric unit of PCM-107 showing coordination of an AuCl moiety to a single PAr3 group in the framework. The 1,4-dpb pillars are drawn in blue for clarity. (B) Space-filling rendering of PCM-107 with superimposed wire frame structure as viewed in the ac plane, showing oval pore openings. (C) Enlarged region of one pore, showing the spatial orientation of four AuCl moieties per unit cell (distances shown are in Å).

Scheme 3. PCM-107-Catalyzed Hydroaddition of 4-Pentyn1-ol

and act to pillar adjacent Ar3P−AuX sheets into infinite 3D arrays, which support micropore openings in all three crystallographic directions (Scheme 1B and Scheme S4). Interestingly, when XAu(SMe2) was omitted from the synthesis mixture, the alternate PCM-101 materials obtained have the same overall composition, but the Co3 nodes are more highly symmetric triangular clusters that permit the perfectly trans alignment of P: sites in adjacent layers (Scheme 2).19 In PCM-107, presumably since the P: sites are already coordinated by bulky AuX moieties, the Co3 nodal distortion facilitates a slight tilting of each P−AuX group within each layer, thus alleviating potential intralayer steric clashing. This offset orientation of adjacent AuX groups is clearly evident by viewing the contents of a single pore in PCM-107 (Figure 1C). Each pore contains four P−AuX groups in a rectangular configuration. As shown by the Au···Au separation distances in Figure 1C, the AuX groups exist as isolated entities. However, the PCM pore still acts to uniquely constrain four Au(I) centers in a relatively close and structurally well-defined orientation that would only be transient in solution. The 3D microporous structure in PCM-107 has pore windows that are large enough to permit access of small molecules (Scheme 1B and Scheme S4); the largest openings between adjacent 1,4-dpb pillars have van der Waals accessible openings of approximately 20.9 × 18.5 × 5.4 Å. Thermogravimetric analysis (TGA) for PCM-107 (Figure S5) showed a 24% mass loss upon heating to 140 °C, which

showed that this reaction is readily catalyzed by Au(I) sites immobilized within PCM-101 under mild conditions and can generate products that are disfavored in solution. Chloroformexchanged crystals of PCM-107 were suspended (2.5 mol %) in dry CDCl3 and exposed to 1 equiv of 4-pentyn-1-ol and an equimolar amount of C6H6 as an internal NMR standard. The mixtures were left to react at 50 °C in sealed glass vials for 24 h without stirring. The products were monitored by 1H NMR and gas chromatography−mass spectrometry (GC-MS) to confirm the identities of all products. In this convenient model reaction, an alkyne feedstock (4pentyn-1-ol, 1; Scheme 3) becomes activated via alkynyl coordination to the Au(I) center. Subsequent intramolecular nucleophilic attack by the alcohol followed by proton transfer yields the gem-alkene (2). Under normal conditions in solution, 2 israpidly attacked by water to give the corresponding hemiacetal (3). Alternatively, nucleophilic attack at 2 by a second equivalent of 1 under anhydrous conditions leads to the furan product 4. Hydration of 4 then yields the ketone 5 (Scheme 3). We previously showed that C

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Scheme 4. (A) Synthetic route To Obtain the Chelating P,N Ligand and (B) Synthesis of the Cu(I/II) PCM-74 and Schematic Representation of Its Extended Structure

AuCl-functionalized PCM-101 selectively gave only 4 and 5; at longer reaction times, 5 was more favored. This indicated that the initial hydroaddition step occurred predominantly inside the PCM micropores, which are largely hydrophobic; as product 4 migrated out of the PCM crystallites, 5 was formed by reaction with H2O in the solution.19 Mon et al. have also studied this reaction using an Au(I/III) MOF catalyst, which produced exclusively product 3, presumably because the micropores of their MOF contained sufficient H2O. PCM-107 produced exclusively products 4 and 5 and displayed higher overall normalized conversions than PCM101 under the same conditions (Table 1; rows 1 and 2); this is Table 1. Data for the PCM-107-Catalyzed Hydroaddition of 4-Pentyn-1-ol after 24 h catalyst 19

PCM-101 PCM-107 PCM-107 PCM-107 PCM-107 PCM-107

Ra

concn (%)b

SA (%)b

SB (%)b

SC (%)b,c

N/A N/A CH3 C6H5 C6H4-p-OMe C6H4-p-CF3

76 91 87 83 89 88

77 73 14 19 11 9

23 27 11 14 14 11

75 67 76 80

Precipitation of the free acid via addition of HCl to a degassed aqueous solution yielded the target ligand as a solid in gram quantities. Next, we screened suitable conditions for the direct ̂ by reaction with different formation of new PCMs using PN -1 combinations of metal precursors in various solvent systems. ̂ Treatment of a 5/2/1 DMF/methanol/H2O solution of PN -1 with 4 equiv of CuI dissolved in acetonitrile followed by heating of the mixture at 75 °C for 18 h yielded a green crystalline solid (Scheme 4B). Single-crystal X-ray diffraction analysis confirmed the isolation of a new PCM, hereafter referred to as PCM-74. The structure of PCM-74 was solved in the orthorhombic space group Pbcn (Z = 4) and has the framework formula unit ̂ [Cu(OH2)(PN -1−CuI)] ∞. This was further confirmed by elemental analysis and TGA studies (see the Supporting Information and Figure S21). Interestingly in this case, the material had been formed by partial oxidation of the CuI precursor to give a monometallic PCM that incorporates both P,N-chelated Cu(I) ions and carboxylate-coordinated Cu(II) sites (Figure 2A). In adherence to the hard/soft donor amphoteric nature of ̂ as a PCM building block, PCM-74 has an unusual PN -1 structure based on segregation of two discrete types of metal clusters as framework nodes. On one hand, unoxidized tetrahedral Cu(I) centers were directly incorporated into the “soft” P,N chelates in the form of Cu2I2 squares; on the other hand, an equal proportion of Cu(I) ions were oxidized in situ and incorporated as octahedral Cu(II) sites in [Cu2(OCO)4(OH2)2] nodes, which display a classical metal acetate paddlewheel motif (Figure 2A; yellow and cyan Cu atoms, respectively). The expanded structure of PCM-74 consists of 2D sheets that form a trapezoidal-shaped opening (van der Waals accessible size 8.5 × 6.3 Å) as viewed along the crystallographic b axis (Figure 2B). Adjacent sheets are staggered in an ABAB packing fashion (Figure S22). Bulk purity was confirmed on the basis of good agreement with the experimental PXRD pattern vs the simulated pattern (Figure S23). TGA of as-synthesized PCM-74 showed a continual mass loss below 150 °C of 8%, attributed to loosely bound acetonitrile and DMF confined within the voids. Above 165 °C, PCM-74 began to thermally decompose, which is consistent with other Cu(II)-based MOFs. Perhaps most

a 2.5 mol % catalyst loading in all cases. bDetermined by 1H NMR versus C6H6 internal standard. cProducts determined by GC-MS.

most likely due to the fact that the AuCl occupancy in PCM107 is close to unity, owing to the one-pot synthesis method employed in this work. In comparison, postsynthetic coordination of PCM-101 with AuCl only achieved a 63% loading.19 Given the appreciable reactivity of PCM-107, we next experimented with nucleophiles bearing electron-donating and -withdrawing substituents, to investigate if PCM-107 could selectively exclude certain reagents on the basis of steric or electronic effects. In all cases (Table 1, rows 3−6) the reactions proceeded in good yields, yielding the alcoholincorporated furans as the major products (6). Small amounts of products 4 and 5 were also present. Synthesis and Characterization of PCM-74: In Situ ̂ Formation of a [(PN )−CuI] 2 Chelated MOF with MixedValent Cu Sites. Encouraged by this successful result, we next aimed to broaden the scope of peri-synthetic complexation in PCM synthesis. Two further questions were next addressed. (i) Is this synthetic strategy applicable to structurally more complex (i.e., chelating) phosphine linkers? (ii) Can we create symmetrical PCMs using asymmetric linkers via in situ assembly? In order to simultaneously answer these questions, we set out to synthesize a simple asymmetric chelating linker. Ligands with cis-P,N-donors are prevalent in molecular catalysis and are known to incorporate a range of low-valent transition metal ions.30−33 The simplest ligand we could envision to achieve this aim was by the replacement of a single p-benzoic acid group in tctpH3 (Scheme 2) with an ô was thus synthesized substituted amine. The new ligand PN -1 as shown in Scheme 4A. Lithiation of 2-bromo-N,Ndimethylaniline followed by treatment with bis(4bromophenyl)phosphinous chloride afforded the asymmetric ̂ 2 as a crude oil, which was purified by column phosphine PN -Br chromatography (SiO2; CH2Cl2/hexanes, 1/9) to give a colorless solid. Subsequent lithiation by nBuLi followed by ̂ quenching with solid CO2 yielded the dilithium salt of PN -1. D

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of structurally well defined and reagent-accessible low-valent metal sites. This method has been demonstrated for both monophosphine building blocks (PCM-107) and more complex chelating linkers (PCM-74). The method should be applicable to a range of other phosphine ligands in the future. Further, this approach may be useful as a means to trap thermodynamically disfavored isomers, potentially opening a new avenue of application for phosphine-based MOFs.

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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.9b00319. Ligand and material synthetic details, associated NMR spectra, powder X-ray diffraction (PXRD), FT-IR, and additional BET data (PDF) Accession Codes

CCDC 1913703−1913705 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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Figure 2. (A) Twice the asymmetric unit of PCM-74 showing one complete Cu2I2 cluster and two PN ̂ chelating ligands. Cu(I) centers are drawn in yellow; Cu(II) centers within the paddlewheel nodes are drawn in cyan. (B) Space-filling view of PCM-74 normal to the ac plane, with the wire frame structure superimposed.

AUTHOR INFORMATION

Corresponding Author

*E-mail for S.M.H.: [email protected]. ORCID

Simon M. Humphrey: 0000-0001-5379-4623

interestingly, the Cu2I2 clusters in PCM-74 adopt a trans confirmation with respect to the P-donor atoms. A number of other Cu2X2 (X = Cl, Br, I) compounds based on structurally similar P,N-chelating ligands have been reported in the molecular literature over the past three decades.34−41 Crystal structure data obtained from these studies shows that only Cl-based analogues adopt a trans-P2 conformation (similar to what is observed in PCM-74), in the absence of ligand steric effects.35−37 In contrast, Br- and Ibased molecular analogues tend to form complexes with cisoriented P-donors as the thermodynamic products, unless ligand steric bulk is employed to force the trans conformation.38−41 Although molecular Cu2I2 complexes with transoriented P-donors are apparently the kinetic isomers when they are formed in solution, in the MOF setting, it is more likely that the higher symmetry trans isomer would be most readily incorporated into a framework, given the inherent need for symmetric linkages in these materials. In this case, the trans-coordinated Cu2I2 complex provides a C2-symmetric building block that favors the formation of square grids, as observed in the crystal structure of PCM-74. Upon incorporation of each complex into the growing MOF, it then becomes “trapped” in the trans conformation since Cu−L chelate dissociation and ligand reorganization are highly disfavored in the solid state. Further studies to form PCMs ̂ ligand under peri-synthetic conditions are using the new PN -1 ongoing.

Present Address ‡

Oak Ridge National Laboratory, P.O. Box 2008, MS6472, Oak Ridge, TN 37831-6472, USA. Author Contributions †

S.G.D. and J.E.R. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the NSF for funding under grant number DMR-1506694 and the Welch Foundation (F-1738). REFERENCES

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CONCLUSION In summary, we have demonstrated that in situ or perisynthetic phosphine metal complex formation is a viable route toward the formation of new MOFs that feature a high density E

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DOI: 10.1021/acs.organomet.9b00319 Organometallics XXXX, XXX, XXX−XXX