Combination of Argentophilic and Perfluorophenyl-Perfluorophenyl

Dec 23, 2014 - C6F5−C6F5 centroid−centroid distances being generally less than 4.9 Å.4. Received: October 23, 2014. Revised: December 22, 2014...
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Combination of Argentophilic and Perfluorophenyl-Perfluorophenyl Interactions Supports a Head-to-Head [2 + 2] Photodimerization in the Solid State Michael A. Sinnwell,† Jonas Baltrusaitis,‡ and Leonard R. MacGillivray*,† †

Department of Chemistry, University of Iowa, Iowa City, Iowa, United States Department of Chemical and Biomolecular Engineering, Lehigh University, Bethlehem, Pennsylvania, United States

‡

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S Supporting Information *

ABSTRACT: Face-to-face perfluorophenyl−perfluorophenyl interactions (C6F5···C6F5) are achieved in a disilver metal− organic complex. The C6F5···C6F5 interactions along with argentophilic forces support trans-pentafluorostilbazole to undergo a head-to-head [2 + 2] photodimerization to form a cyclobutane that sustains a fluorinated two-dimensional metal−organic framework.

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functional supramolecular materials. Moreover, the use of C6F5···C6F5 forces to support a [2 + 2] photodimerization wherein stacking olefins lie within the critical distance of Schmidt (i.e., < 4.20 Å) has not been reported.19 Here, we report a crystalline metal−organic complex in the form of [Ag2(5F-stilbz)4][ClO3]2 (where 5F-stilbz = trans-1-(4pyridyl)-2-(pentafluorophenyl)ethylene) (1) with perfluorophenyl groups engaged in face-to-face stacked geometries defined by C6F5···C6F5 forces. Specifically, argentophilic forces (Ag···Ag) assemble and align 1 to exhibit proper topochemical geometry20 for a [2 + 2] photodimerization to generate rctt-1,2-bis(pentafluorophenyl)3,4-bis(4-pyridyl)cyclobutane (2) regioselectively and quantitatively. In essence, the photoreaction covalently captures the C6F5···C6F5 interactions achieved in the solid. Computational studies based on the X-ray data comparing our head-to-head (hh) and the head-to-tail (ht) geometry of 5F-stilbz as a pure solid confirm the face-to-face C6F5···C6F5 forces to contribute to the close-range hh stacking geometry of 5F-stilbz in reactive 1. We also demonstrate the photoproduct 2 to support a novel metal−organic framework (MOF) decorated with F atoms (FMOF).21 Interactions between π-faces of perfluorinated phenyl groups may seem counterintuitive given that the driving force for such interactions is considered to involve complementary electrostatic forces of stacked atoms. 2,3 However, C 6F 5 ···C 6F 5 interactions involving offset C6F5 rings are relatively common in coordination and organometallic crystalline solids, with C6F5−C6F5 centroid−centroid distances being generally less than 4.9 Å.4

n a classic paper, Coates and Grubbs reported the use of perfluorophenyl−phenyl (C6F5···C6H5) interactions to noncovalently control the packing of olefinic molecules to undergo [2 + 2] photocycloaddition reactions in the solid state.1 In this context, perfluorophenyl−perfluorophenyl interactions (C6F5··· C6F5) have also emerged as means to control supramolecular architectures and frameworks (Scheme 1a).2−18 There still remains uncertainty, however, concerning the nature and utility of C6F5···C6F5 interactions (e.g., electrostatics, hydrophobic effects, van der Waals forces). Indeed, there is limited evidence of C6F5···C6F5 interactions having been employed to design and control crystal packing, particularly for the construction of Scheme 1. Photodimerization of 1

Received: October 23, 2014 Revised: December 22, 2014 Published: December 23, 2014 © 2014 American Chemical Society

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DOI: 10.1021/cg501571u Cryst. Growth Des. 2015, 15, 538−541

Communication

Crystal Growth & Design

Ag(1)−N(1) 2.173(2), Ag(1)−N(2) 2.170(2)] in a linear geometry [N−Ag−N (o): 168.4(1)]. The ClO3− ions lie in close proximity to the Ag(I) ions [Ag−O (Å): Ag(1)−O(2) 2.833(3), Ag(1)−O(3) 2.780(3)] yet are noncoordinating (Figure 2a). The face-to-face stacking of C6F5 rings is offset with centroid−centroid and centroid-to-plane distances of 3.97 and 3.37 Å, respectively (Figure 2b). The C6F5 planes are nearly coplanar, lying at an angle of 6.12°. The metrics place the C6F5 groups within the criteria for C6F5···C6F5 forces.4−18 Moreover, the face-to-face C6F5···C6F5 interactions support the CC bonds to be parallel and separated by 3.89 Å, which conforms to the postulate of Schmidt for a [2 + 2] photodimerization.23,24 The complex packs offset with CC bond distance between assemblies of 4.76 Å [C(20)···C(19) 4.76 Å] (Figure 2c). The pendant C6F5 groups interact between assemblies via a combination of C···F and F···F contacts.25−31 To determine the photoreactivity of 1, a powdered sample was exposed to broadband UV irradiation (450 W mediumpressure Hg-lamp) for a period of 130 h. 1H NMR spectroscopy revealed a [2 + 2] photodimerization in quantitative yield. The formation of a cyclobutane ring was evidenced by disappearances of the olefinic peaks at 7.39 ppm and appearances of cyclobutane peaks at 5.03 and 4.90 ppm (DMSO-d6). To confirm the hh stereochemistry of 2, photoreacted 1 was dissolved in EtOH, which was followed by the addition of NaCl and H2O. The solution was filtered and allowed to slowly evaporate. Single crystals of 2 formed as colorless needles over a period of 1 day.a A single-crystal X-ray analysis of 2 enabled us to structurally authenticate the stereochemistry of the photoproduct. The Xray data revealed the hh and rctt stereochemistry of 2 (Figure 3a). The bipyridine crystallizes in the monoclinic space group

We sought to determine whether a combination of argentophilic and C6F5···C6F5 forces could align 5F-stilbz hh in the solid state to support a [2 + 2] photodimerization. As a pure form, crystalline 5F-stilbz reacts to give the corresponding ht dimer,22 with the packing also involving C−H···N and C− H···F forces [C(5)···N(1) 3.44 Å, C(4)···F(5) 3.57 Å, C(6)··· C(7) 4.19 Å, C(6)···C(6) 5.86 Å]. The carbon−carbon double (CC) bonds lie parallel in a sheet and separated by 4.19 Å in the solid (Figure 1).

Figure 1. X-ray structure of 5F-stilbz: (a) 2D sheet and (b) ht stacking of CC bonds.

Single crystals of 1 as colorless needles were obtained via slow evaporation over a period of 24 h of a solution of 5F-stilbz (40 mg) and AgClO3 (14 mg, 2:1 molar ratio) in CH3CN (5 mL). The composition of 1 was confirmed by 1H NMR spectroscopy and single-crystal X-ray diffraction.a Perspectives of the X-ray structure of 1 are shown in Figure 2. The components crystallize in the triclinic space group P1̅. The Ag(I) ions and 5F-stilbz form a dinuclear complex sustained by Ag···Ag forces that involve parallel and face-to-face stacking of 5F-stilbz [Ag···Ag: 3.10(1) Å]. Each Ag(I) ion is coordinated to two trans N atoms of 5F-stilbz [Ag−N (Å):

Figure 3. X-ray structure of 2: (a) hh and rctt stereochemistry and (b) face-to-face C6F5···C6F5 interactions of rings. Minor occupancy sites for pyridine ring omitted for clarity.

P21/n, being propagated along the b-axis as one-dimensional chains via C−H···N forces [C(9)···N(2) 3.658(6) Å, C(14)··· N(2) 3.461(5) Å]. One pyridine ring lies disordered over two sites (occupancies: 0.60/0.40). Adjacent molecules participate in offset and face-to-face C6F5···C6F5 forces (centroid-centroid 4.80 Å, F(3)···F(7) 2.923(4) Å). We have determined that the photoproduct 2 supports the formation of a novel two-dimensional (2D) FMOF.21 When photoreacted 1 was recrystallized from CH3CN, colorless single crystals of [Ag2(2)2][ClO3]2 (3) formed after a period of 4 days.a A single-crystal X-ray analysis of 3 revealed the components to crystallize in the monoclinic space group P21/c. Each Ag(I) ion is coordinated by two N atoms of two 4-pyridyl groups of 2 [Ag−N (Å): Ag(1)−N(1) 2.17(1), Ag(1)−N(2) 2.15(1)] that act as double bridges (Ag···Ag 6.35 Å) and two O atoms of μ2-

Figure 2. X-ray structure of 1: (a) stacked CC bonds, (b) offset face-to-face stacking of C6F5 rings (inset: centroid distance), and (c) extended packing (inset: interactions of F atoms (Å): [F(4)···F(4) 2.74, F(3)···F(8) 2.75, F(4)···F(9) 2.89, C(5)···F(8) 3.06, C(12)··· F(9) 3.16, C(25)···F(4) 3.10]). 539

DOI: 10.1021/cg501571u Cryst. Growth Des. 2015, 15, 538−541

Communication

Crystal Growth & Design ClO3− ions [Ag−O (Å): Ag(1)−O(2) 2.69(1), Ag(1)−O(3) 2.61(2)] that act as single bridges (Ag···Ag 5.32 Å). As a consequence of the assembly process, the components form dinuclear rhomboids32 that self-assemble to form a 2D MOF via the ClO3− ions and double-bridging cyclobutanes (Figure 4b). The C6F5 rings decorate the peripheries of the rhomboids,

between the stacked pyridine rings increased from 3.67 to 3.78 Å, while the separation between the stacked C6F5 rings decreased from 3.97 to 3.61 Å. The energies of the optimized structures were comparable, differing by 1.2 kcal/mol with the ht geometry being most energetically favorable. Collectively, these observations are consistent with the C 6 F 5 ···C 6 F 5 interactions supporting the face-to-face stacking and photodimerization in 1, with the Ag(I) ions also serving to orient the olefins. We are unaware of a system composed of both C6F5 and pyridine rings being studied computationally.37 In conclusion, we have shown that C6F5···C6F5 interactions support a hh [2 + 2] photodimerization in the solid state via a disilver complex. Relevance of the face-to-face C6F5···C6F5 forces has been realized using DFT calculations. We are now focusing on the integration of the covalently captured geometry involving C6F5 units into fluorinated organic materials.

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ASSOCIATED CONTENT

* Supporting Information S

Details of syntheses, 1H NMR studies, computational methods, and X-ray structure solutions. CCDC 1024697−1024699. For ESI and crystallographic data in CIF or other electronic format see http//dx.doi.org/10.1039/c000000x. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 4. Structure of 3: (a) rhomboid, (b) 2D MOF via bridging ClO3− ions, (c) stacking of sheets with F atoms space-filling (anions omitted for clarity) and pores [selected F···F contacts (Å): F(2)···F(8) 2.86(1), F(3)···F(7) 2.74(1), F(3)···F(5) 2.82(1), F(4)···F(8) 2.88(1)], and (d) simplified hcb network (blue).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +1 319-335-1270. Tel: +1 319-335-3504.

interdigitating via F···F contacts of adjacent networks (Figure 4c).33 The assembly of the units generates a honeycomb net of hcb topology defined by central Ag(I) nodes and based on distorted Ag···Ag edges (Figure 4d).34 Alternating channels of the dinuclear rhomboids propagate within the plane and contain highly disordered solvent molecules. The assembly of the components thus defines a fluorinated framework with bridging units that introduce F atoms at termini. Gas phase density-functional theory (DFT) calculations show that the interactions between the C6F5 rings contribute to the assembly of the components in reactive 1. When the coordinates of the stacked olefins directly from the X-ray coordinates of pure 5F-stilbz and 1 were optimized at the RIBLYP/def-TZVPP level,35 the face-to-face ht and hh stackings, respectively, underwent significant reorganizations (Figure 5). Specifically, when optimization was performed ht stacked 5F-stilbz in the pure solid,22 the centroid-to-centroid distance between the stacked pyridine and C6F5 rings underwent a significant contraction from 4.24 to 3.59 Å. The resulting shorter separation is in line with reported values.36 In contrast, when the same calculation was performed on the hh stacked olefins of reactive 1, the centroid-to-centroid distance

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Foundation (LRM DMR1408834) for financial support, Prof. Davide M. Proserpio for help with verification of topology and using TOPOS, and Dr. Eric W Reinheimer for guidance on crystal structure refinement.

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ADDITIONAL NOTE

a

Crystal data for 1 (CCDC 1024697): M = 733.70 g/mol, triclinic, P1̅, a = 8.9733(9) Å, b = 10.3938(10) Å, c = 15.2887(15) Å, α = 108.464(5)°, β = 96.474(5)°, γ = 100.257(5)°, V = 1308.9(2) Å3, Z = 2, T = 298(2) K, μ(MoKα) = 0.977 mm−1, 21430 reflections measured, 4608 unique (Rint = 0.0198), R1(obs) = 0.0304, wR1(obs) = 0.0752, R2(all) = 0.0385, wR2(all) = 0.0816. Crystal data for 2 (CCDC 1024698): M = 542.38 g/mol, monoclinic, P21/n, a = 8.2581(15) Å, b = 11.394(2) Å, c = 25.103(5) Å, β = 96.980(5)°, V = 2344.5(8) Å3, Z = 4, T = 293(2) K, μ(MoKα) = 0.147 mm−1, 16975 reflections measured, 3049 unique (Rint = 0.0280), R1(obs) = 0.0579, R2(all) = 0.0761, wR2(all) = 0.1600. Crystal data for 3·0.25(CH3CN)·0.5H2O (CCDC 1024699): M = 751.20 g/mol, monoclinic, P21/c, a = 19.91(5) Å, b = 10.19(3) Å, c = 14.84(4) Å, β = 106.78(3)°, V = 2883(14) Å3, Z = 4, T = 296.15 K, μ(MoKα) = 0.891 mm−1, 20552 reflections measured, 3729 unique (Rint = 0.1379), R1(obs) = 0.0704, R2(all) = 0.1260, wR2(all) = 0.2106.

Figure 5. DFT calculations involving superimposed hh stacked dimers of 5F-stilbz derived from reactive 1: (a) overhead view, (b) side-view, and (c) end-on view (blue optimized structure; gray/green X-ray structure). 540

DOI: 10.1021/cg501571u Cryst. Growth Des. 2015, 15, 538−541

Communication

Crystal Growth & Design

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