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Putting Cocrystal Stoichiometry to Work: A Reactive HydrogenBonded ‘Super-Assembly’ Enables Nanoscale Enlargement of a Metal-Organic Rhomboid via a Solid-State Photocycloaddition Qianli Chu, Andrew J. E. Duncan, Giannis S. Papaefstathiou, Tamara D. Hamilton, Manza B. J. Atkinson, S.V. Santhana Mariappan, and Leonard R. MacGillivray J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b01775 • Publication Date (Web): 15 Mar 2018 Downloaded from http://pubs.acs.org on March 16, 2018

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Journal of the American Chemical Society

Putting Cocrystal Stoichiometry to Work: A Reactive HydrogenBonded ‘Super-Assembly’ Enables Nanoscale Enlargement of a Metal-Organic Rhomboid via a Solid-State Photocycloaddition Qianli Chu,‡† Andrew J. E. Duncan,† Giannis S. Papaefstathiou,§ Tamara D. Hamilton,|| Manza B. J. Atkinson, S. V. Santhana Mariappan, and Leonard R. MacGillivray* Department of Chemistry, University of Iowa, Iowa City, IA 52245. KEYWORDS (Word Style “BG_Keywords”). If you are submitting your paper to a journal that requires keywords, provide significant keywords to aid the reader in literature retrieval.

ABSTRACT: Enlargement of a self-assembled metal-organic rhomboid is achieved via the organic solid state. The solid-state synthesis of an elongated organic ligand was achieved by a template directed [2+2] photodimerization in a cocrystal. Initial cocrystals obtained of resorcinol template and reactant alkene afforded a 1:2 cocrystal with the alkene in a stacked yet photostable geometry. Cocrystallization performed in the presence of excess template resulted in 6:4 cocrystal composed of novel discrete 10component hydrogen-bonded ‘super-assemblies’ wherein the alkenes undergo a head-to-head [2+2] photodimerization. Isolation and reaction of elongated photoproduct with Cu(II) ions afforded a metal-organic rhomboid of nanoscale dimensions that hosts small molecules in the solid state as guests.

INTRODUCTION Coordination-driven self-assembly is widely employed to develop metal-organic architectures and materials (MOMs). Synergy between the metal and organic components, as well as the shape of the architecture, define attractive properties of such self-assembled frameworks. Applications in host-guest chemistry with MOMs are realized as sensors,1-2 vessels for chemical reactivity3-4 and structure determination,5 as well as incorporation into catalytically-, electronically-, and photochemically-active materials.6-7 Despite advances, however, there remains a need to develop novel synthetic pathways to design the building blocks of MOMs so as to enable properties to be fully realized. In this context, cyclobutanes formed in the solid state are attractive and novel bridging ligands of MOMs. Cocrystallizations of bifunctional hydrogen-bond-donors (e.g. resorcinol or res) with pyridyl-decorated olefins (e.g. trans1,2-bis(4-pyridyl)ethylene or 4,4’-bpe) have, thus, been shown to afford hydrogen-bonded assemblies wherein carbon-carbon double (C=C) bonds are preorganized for intermolecular [2+2] photodimerizations. The resulting photoproducts (e.g. rctt-

tetrakis(4-pyridyl)cyclobutane or 4,4’-tpcb) are multitopic, possessing, cyclobutane rings with radially-extended pyridyl groups that bridge metal centers in one- (1D), two- (2D), and three- (3D) dimensions. The cyclobutanes formed in the solid state have been applied to construct metal-organic frameworks (MOFs),6-7 metal-organic polygons/polyhedral (MOPs),8-10 and metal-organic gels (MOGs).11-12 Here, we describe work to employ a templated solid-state organic synthesis to deliberately enlarge the cavity of a metalorganic polygon (MOP) to approach nanoscale dimensions. We have described the ability of rctt-1,2-bis(2-pyridyl)-3,4bis(4-pyridyl)cyclobutane (2,4’-tpcb) to support the formation of the rhomboid [Cu4(2,4’-tpcb)2(hfacac)8] (where: hfacac = hexafluoro-acetylacetonate) (Scheme 1).11 Given that only single pyridyl rings (blue, Scheme 1) define edges of the polygon, we sought to enlarge the cavity of the MOP by adding an aromatic linker between the pyridyl group and cyclobutane ring (red, Scheme 2). Cavity enlargement by lengthening a bridging ligand enables MOMs to accommodate guest species and, thus, participate in molecular recognition and host-guest chemistry. While the process of ligand lengthening is invariably practiced in the liquid phase, the

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Scheme 2. Supramolecular pathway to [Cu4(2-pyr-4-ppyr-cb)2(hfacac)8] via solid-state synthesis of elongated ligand.

process has not been attempted via the organic solid state. For a cocrystal that supports a template-directed synthesis of a lengthened ligand, the self-assembly process involving a template and alkene would be required to tolerate the lengthened structure of the olefin and generation of the product cyclobutane, lest perturbation of the crystal packing render the cocrystal unreactive.

Scheme 1. Small rhomboid [Cu4(2,4’-tpcb)2(hfacac)8]. Specifically, we show that while initial attempts to form a reactive cocrystal based on the lengthened alkene 2-pyr-4ppyr in (4,6-diCl-res)·2(2-pyr-4-ppyr) (where: 2-pyr-4-ppyr = trans-1-(2-pyridyl)-2-(4-(4-phenyl)pyridyl)ethylene) was initially unsuccessful (Scheme 2), the lengthened cyclobutane 2-pyr-4-ppyr-cb (where: 2-pyr-4-ppyr-cb = rctt-(1,2-bis(2pyridyl)-3,4-bis(4-(4-phenyl)pyridyl)]cyclobutane) can be synthesized in the cocrystal of modified stoichiometry 3(4,6diCl-res)·2(2-pyr-4-ppyr). We show the change to the stoichiometry of the cocrystal to afford an unprecedented 10component hydrogen-bonded 'super-assembly' wherein the alkenes lie stacked head-to-head (HH) and undergo a photocycloaddition to generate 2-pyr-4-ppyr-cb. The larger MOP is then realized as the rhomboid [Cu4(2-pyr-4-ppyrcb)2(hfacac)8]. In addition to providing a novel entry to

generate large organic building blocks of MOMs, the process of modifying cocrystal stoichiometry increases the synthetic flexibility of the solid-state approach.

RESULTS AND DISCUSSION The relatively small size of the cavity of [Cu4(2,4’tpcb)2(hfacac)8] prompted us to attempt to construct a isostructural rhomboid with a cavity spacious enough to accommodate molecular species as guests.13 We expected that a cyclobutane with a phenyl group (red, Scheme 2) inserted between the 4-pyridyl group and the cyclobutane ring would allow us to generate [Cu4(2-pyr-4-ppyr-cb)2(hfacac)8], which would possess a significantly larger cavity (ca. dimensions 9.0 × 9.2 Å). Unreactive 1:2 cocrystal. To construct [Cu4(2-pyr-4-ppyrcb)2(hfacac)8],14-15 a cocrystallization of a res with 2-pyr-4ppyr was expected to generate the four-component hydrogenbonded assembly 2(res)·2(2-pyr-4-ppyr) with two olefins positioned HH for an intermolecular [2+2] photodimerization (Scheme 3).16-21 In the design, a ditopic res template would accommodate the added phenyl group to enable access to the lengthened cyclobutane 2-pyr-4-ppyr-cb.22 Reaction of 2pyr-4-ppyr-cb with Cu(hfacac)2·H2O would then afford [Cu4(2-pyr-4-ppyr-cb)2(hfacac)8] with a cavity similar in size to those MOPs (i.e. squares) based on 4,4’-bipyridine16-17, 19-21, 23 used as hosts of molecular guests. The lengthened alkene 2-pyr-4-ppyr was synthesized via Kumada coupling and a subsequent Heck reaction. A solution of 2-pyr-4-ppyr (0.015 g, 0.058 mmol) and 4,6-diCl-res (0.011g, 0.058 mmol) was then prepared by combining separate solutions of each component in hot MeCN (1 mL). Upon cooling, pink rectangular crystals suitable for singlecrystal X-ray diffraction analysis formed after a period of approximately 1 day (yield: 93%). The composition of the single crystals as 2(2-pyr-4-ppyr)·(4,6-diCl-res) was confirmed using single-crystal X-ray diffraction and 1H NMR spectroscopy.


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Scheme 3. Expected hydrogen-bonded assembly 2(res)·2(2-pyr4-ppyr) to generate 2-pyr-4-ppyr-cb.

A view of the X-ray crystal structure of 2(2-pyr-4ppyr)·(4,6-diCl-res) shows the asymmetric unit to consist of a single res and two molecules of 2-pyr-4-ppyr (Fig. 1). The res and bipyridine, in contrast to our design, form a threecomponent assembly sustained by two O-H···N hydrogen bonds [O···N separations (Å): O(2)···N(3) 2.733(3), O(1)···N(2) 2.722(3)] (Fig. 1a).24 In the arrangement, 4,6diCl-res interacts with two 4-pyridyl groups of 2-pyr-4-ppyr such that alkenes adopt a HH stacked geometry supported by edge-to-face π-forces. As a consequence of the assembly process, C=C bonds lie beyond the distance of Schmidt for a [2+2] photodimerization (4.566(4) [C(7)···C(36)] and 5.493(4) Å [C(6)···C(37)]).24 Nearest-neighbor assemblies pack as embraced centrosymmetric π-stacked pairs (Fig. 1b)25 with 2pyr-4-ppyr in a HT geometry. The C=C bonds between the assemblies lie beyond the distance for a photoreaction (8.861(4) Å [C(6)···C(7)]) (Fig. 1b). The three-component assemblies pack herringbone within the ab-plane to form a 2D structure (Fig. 1c). To determine the photoactivity of 2(2-pyr-4-ppyr)·(4,6diCl-res), a powdered crystalline sample was placed between Pyrex glass plates and exposed to broadband UV-irradiation (medium pressure Hg vapor lamp) for a period of 24 hours. The solid was determined to be photostable, as evidenced by 1 H NMR spectroscopy. Reactive ‘super-assembly’ of 3:2 cocrystal. To form a reactive cocrystal based on 2-pyr-4-ppyr and 4,6-diCl-res, we turned to modify the stoichiometry of the components. We hypothesized that a cocrystallization using excess 4,6-diCl-res may support the crystallization of a photoactive solid in the form of the targeted 2(res)·2(2-pyr-4-ppyr) stoichiometry or an alternative cocrystal with excess res present in the lattice.

Figure 1. X-ray structure 2(2-pyr-4-ppyr)·(4,6-diCl-res): (a) three-component assembly with edge-to-face stacking, (b) face-toface stacking of embraced assemblies (one assembly highlighted as yellow space-filling), and (c) herringbone packing (embraced assemblies outlined).

A reactive four-component assembly based on 2(2-pyr-4ppyr)·2(4,6-diCl-res) would, thus, form or be supported to form wherein excess diCl-res may cocrystallize with the reactive hydrogen-bonded structure. Any excess res would, essentially, act as a lattice ‘filler’26 by participating in secondary hydrogen bonds with excess acceptor sites involving copies of 4,6-diCl-res (i.e. not with alkene) to support the structure of a photoactive solid. We note that a solid with excess res would be a kind of cocrystal pseudopolymorph,27-31 whereby variation in stoichiometry32 is exploited in a novel manner to activate chemical reactivity. When a solution of 4,6-diCl-res (0.97 g) and excess 2-pyr-4ppyr (0.35 g) (4:1 ratio) in MeCN (30.0 mL) was heated to boil and allowed to cool, light pink crystals formed within a period of approximately 1 day (yield: 95%). Single-crystal Xray diffraction and 1H NMR spectroscopy confirmed the formulation of the solid as 3(4,6-diCl-res)·2(2-pyr-4-ppyr). Views of 3(4,6-diCl-res)·2(2-pyr-4-ppyr) show the components to assemble to form a discrete, 10-component ‘super-assembly’ that sits around a crystallographic center of inversion. The assembly is sustained by a total of 12

Figure 2. X-ray crystal 3(4,6-diCl-res)2(2-pyr-4-ppyr): (a) 10-component 'super-assembly' and (b) one of the four-component subunits with stacked olefins preorganized in HH geometry.



Figure 3. X-ray structure [Cu4(2-pyr-4-ppyr-cb)2(hfacac)8]·8CHCl3: (a) enlarged rhomboid (solvent omitted for clarity) and (b) CHCl3 molecules occupy cavity (additional solvent molecules omitted). hydrogen bonds (Fig. 2). In the arrangement, four of the six molecules of 4,6-diCl-res assemble two pairs of 2-pyr-4-ppyr into HH and face-to-face stacked geometries via a total of eight O-H···N hydrogen bonds [O···N separations (Å): O(2)···N(1) 2.765(3), O(1)···N(3) 2.715(3), O(4)···N(2) 2.609(3), O(3)···N(4) 2.616(3)] (Fig. 2a). The two additional molecules of 4,6-diCl-res of the ‘super-assembly’ bridge the two stacked hydrogen-bonded pairs via four O-H···O hydrogen bonds [O···O separations (Å): O(5)···O(4) 2.739(3), O(6)···O(3)b 2.802(3)].22 The stacking of the olefins places the pairs of C=C bonds in close proximities, being separated by 3.696(4) Å [C(18)···C(36)] and 3.900(4) Å [C(19)···C(37)] (Fig. 2b).33 Thus, in contrast to 2(2-pyr-4-ppyr)·(4,6-diClres), the stacked geometry satisfies the criteria of Schmidt for [2+2] photodimerization. Adjacent super-assemblies stack such that C=C bonds of nearest-neighbor assemblies lie offset and separated by > 9.0 Å (Fig. S1). In line with the structure of 3(4,6-diCl-res)·2(2-pyr-4-ppyr), UV-irradiation of a powdered crystalline sample of 3(4,6diCl-res)·2(2-pyr-4-ppyr) (40.0 mg) for a period of 72 h afforded 2-pyr-4-ppyr-cb, as determined by 1D 1H NMR spectroscopy, and a supporting suite of 2D NMR data (yield: 60%) (See SI). The formation of 2-pyr-4-ppyr-cb was evidenced by the emergence of cyclobutane signals at 4.95 and 4.71 ppm. Isolation and purification of 2-pyr-4-ppyr-cb was achieved in a basic/solvent extraction using NaOH and CH2Cl2 followed by silica gel chromatography using 2% MeOH in CH2Cl2. Enlarged rhomboid. To form the targeted enlarged rhomboid, a solution of 2-pyr-4-ppyr-cb (0.010 g) and Cu(hfacac)2·H2O (0.019 g) (1:2 ratio) in CHCl3 (2.0 mL) was prepared. Green-blue crystals of [Cu4(2-pyr-4-ppyrcb)2(hfacac)8]·8CHCl3 formed within a period of ca. 2 days (yield: 68%). [Cu4(2-pyr-4-ppyr-cb)2(hfacac)8]·8CHCl3 was characterized by single-crystal X-ray diffraction and IR spectroscopy. Views of [Cu4(2-pyr-4-ppyr-cb)2(hfacac)8]·8CHCl3 show the components to form a tetranuclear polygon, which sits around a crystallographic center of inversion, with a topology that conforms to a rhomboid (Fig. 3). Each metal ion adopts an octahedral coordination environment with two identical pyridines in cis positions and two chelating hfacac ions (Fig. 3a). Moreover, the 4-(4-phenyl)pyridyl groups, which are twisted approximately orthogonal to the plane of the polygon,34 define the edges10, 35 of a rhombus-shaped cavity

with dimensions of ca. 9.0 × 9.2 Å (van der Waals radii) and corner angles of about 73° and 107° (Cu(1)···Cu(1)c 1.3 nm, cyclobutane···cyclobutane 1.8 nm, Cu(2)···Cu(2)c 2.5 nm, Cu(2)···Cu(1) 1.4 nm, Cu(2)···Cu(1)c 1.4 nm) (Fig. 3b). The rhomboid [Cu4(2-pyr-4-ppyr-cb)2(hfacac)8]·8CHCl3 stacks, along the crystallographic c-axis, to form 1D arrays wherein, in contrast to [Cu4(2,4’-tpcb)2(hfacac)8], guest CHCl3 solvent molecules, along with two coordinated anions of two nearestneighbor complexes, occupy the central cavity (Fig. 3b). The included solvent interacts with the anions by way of C-H···O interactions (C(59)···O(7) 3.14(1) Å, C(59’)···O(6) 3.47(1) Å). Similar to [Cu4(2,4’-tpcb)2(hfacac)8], CHCl3 molecules are located between the stacked rhomboids.

CONCLUSIONS In this report, we have demonstrated how an enlarged MOP can be constructed via the organic solid state. Enlargement of a targeted cyclobutane ligand afforded access to a rhomboid of nanoscale dimensions that accommodates molecules as guests.16-17, 19-21, 23 The elongated cyclobutane was obtained by way of a templated solid-state organic synthesis.13 An initial attempt at cyclobutane formation was complicated by undesirable packing of 2-pyr-4-ppyr, leading to a photostable cocrystal. Modification of cocrystal stoichiometry by introducing excess template generated a novel 10-component ‘super-assembly’ with photoactive olefins that react to form 2pyr-4-ppyr-cb. Studies are underway to expand the utility of the self-assembly process that generates reactive olefins and determine the range of guests that may be included within the rhomboid. Further studies are also underway to understand those factors that influence the formation of cocrystals of increasing structural variation based on changes to stoichiometry.

ASSOCIATED CONTENT Supporting Information Full experimental details including materials, methods, synthesis, and analysis along with characterization data from 1D and 2D NMR spectroscopy, powder X-ray diffraction, singlecrystal X-ray diffraction, and high-resolution mass spectrometry have been provided in PDF format. Full crystallographic data have been provided in CIF format. The Supporting Information is available free of charge on the ACS Publications website.


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AUTHOR INFORMATION Corresponding Author *[email protected]

Present Addresses ‡

Current address: University of North Dakota, Department of Chemistry, Grand Forks, ND 58202. § Current address: Laboratory of Inorganic Chemistry, Department of Chemistry, National and Kapodistrian University of Athens, Panepistimiopolis, Zografou, 157 71, Greece. || Current address: Barry University, Department of Physical Sciences, Miami Shores, FL 33161.

Author Contributions †

These authors contributed equally.

Notes

ACKNOWLEDGMENT The National Science Foundation (L.R.M. DMR-1708673) is acknowledged for support of the work.

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