Suprachannel as a Radical Trap: Crystal Structure of Single Carbon

M 3 L 2 zinc( ii ) complex containing 1,3,5-tris(dimethyl(pyridin-3-yl)silyl)benzene: selective photoluminescence recognition of diiodomethane. Sa...
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Suprachannel as a Radical Trap: Crystal Structure of Single Carbon Radicals Yoonjung Cho,‡ Tae Hwan Noh,‡ Jeong Gyun Kim, Haeri Lee, and Ok-Sang Jung* Department of Chemistry, Pusan National University, Pusan 609-735, Korea S Supporting Information *

ABSTRACT: We report proof-of-concept experiments on the unprecedented crystal structure of an iodomethyl radical (· CH2I) nestled within an ensemble’s suprachannel via single crystal-to-single crystal photoreaction without destruction of the suprachannel. The trapped nonconjugated carbon radical was additionally confirmed by EPR, 13C MAS, UPS, and Raman spectra.

C

nestled within a unique suprachannel via single crystal-to-single crystal photoreaction23−25 and to efficiently trap the radical species in the suprachannel crystal for X-ray crystal-structural determination. In this communication, we report proof-ofconcept observations on the single crystal-to-single crystal transformation of CH2I2@[ZnI2L]2·2CH2I2 to (0.3 ·CH2I + 0.85 I2)@[ZnI2L]2·2CH2I2 via UV irradiation along with the Xray crystal structure of the single-carbon-radical species, ·CH2I, in the solid state. This suprachannel system safely stores unstable CH2I2 molecules and activates the CH2I2 within the suprachannel architecture, resulting in the formation of transient ·CH2I radicals via photoreaction in the crystalline state. A zinc(II) metallacyclodimeric suprachannel ensemble consisting of CH2I2@[ZnI2L]2·2CH2I2 was constructed by self-assembly of ZnI2 with L (L = 2,3-bis(isonicotinoyloxy)naphthalene) in a mixed solvent containing CH2I2, yielding pale-yellow crystals suitable for single-crystal X-ray diffraction (Scheme 1). IR and elemental analyses confirmed the formation of the proposed products. Even though the crystals were prepared carefully in the dark owing to the visible-light instability of CH2I2 during self-assembly, the CH2I2 solvate molecules in the resulting crystals were in fact very stable under visible light. The reaction afforded the metallacyclodimeric ensemble instead of coordination polymeric species, irrespective of the reactant mole ratio, solvents, concentration, and halides, indicating that the cyclodimeric ensemble is thermodynamically stable. The ensemble’s formation might be attributable to the intrinsic properties of the stable hemicircle-type conformation of L containing a potential π···π

rystal engineering on molecular behavior within a confined space has been directed toward practical and timely host-material applications such as gas storage and separation, stabilization of reactive intermediates, drug delivery, biomolecular recognition, catalysis, medical imaging, ion conduction, energy transfer, and membrane structures.1−7 Recently, such porous materials that do not change their underlying skeleton have been utilized as new task-specific trapping-matrices of liquid molecules and unstable molecules for single-crystal X-ray crystallography.8,9 Characterization technology based on confined molecules within available channels, for example, is an exciting emerging focus of research, as single-crystal diffraction can clearly reveal three-dimensional structural information on gas or liquid products that cannot be solved by the conventional X-ray crystallographic method. Meanwhile, diiodomethane (CH2I2) is an unsafe molecule with respect to both human health and (in its destruction of ozone molecules) the atmosphere’s oxidative capability.10 In contrast, it is also a useful reagent for determination of mineralsample densities and as an optical contact liquid.11 Thus, clear and comprehensive structural information on the recognition, capture, and decomposition of CH2I2 species is an urgent issue. To date, excitation of CH2I2 in condensed phases via UV, direct photoionization, and radiolysis has been intensively carried out,12−15 and its photodecomposed species such as trapped electrons, the cation of CH2I2, radicals, and the isomer of CH2I2 are hot issues.12−14,16 In particular, a close investigation of the nonconjugated carbon radical species ·CHnX3−n (X = F, Cl, Br, I; n = 0−2) is a fascinating field.17−19 Even though the structural determination of delocalized diphenyl carbenes was reported,20−22 crystal-structural analysis of single-carbon-radical species remains an unfulfilled ambition. In this context, two crucial aims of the present research were to explore the photodegradation of the CH2I2 molecules © 2016 American Chemical Society

Received: February 26, 2016 Revised: April 14, 2016 Published: May 9, 2016 3054

DOI: 10.1021/acs.cgd.6b00318 Cryst. Growth Des. 2016, 16, 3054−3058

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Scheme 1. Schematic Diagram of Storage and Photodissociation of CH2I2 Molecules within the Metallacyclodimer Ensemble’s Suprachannela

a

Before and after 1 week 350 nm-UV-irradiation along with the radical formation procedure of “inside” CH2I2. Both “inside” and “outside” CH2I2 are depicted in the scheme. The top pictures show microscopic images of the corresponding single crystals and bulk crystalline solids.

interacting naphthyl group. The interdigitated π···π interaction between the naphthyl moieties, significantly, is ascribable to the formation of the suprachannel ensemble. Similarly, the single crystals of the dichloromethane- and dibromomethane-solvated cyclodimeric ensemble, CH2Cl2@[ZnI2L]2 and 1.5CH2Br2@[ZnI2L]2·2CH2Br2, respectively, were obtained from the same self-assembly reaction in the mixed ethanol/ CH2Cl2 and ethanol/CH2Br2, respectively. The crystalline products are insoluble in water and common organic solvents, but are dissociated in polar organic solvents such as dimethyl sulfoxide and N,N-dimethylformamide. As is apparent in the plotted thermogravimetric analyses of CH2I2@[ZnI2L]2·2CH2I2 (Figure S1, Supporting Information), the skeletal structure was thermally stable up to 380 °C and collapsed in the 390−700 °C range. The ensemble structure’s high-temperature collapse seems attributable to the stable, interdigitated π···π interactionbased arrangement. The CH2I2 molecules began to evaporate at 105−190 °C in two steps corresponding to the “outside” CH2I2 and “inside” CH2I2. UV (350 nm) irradiation of the pale-yellow crystals of CH2I2@[ZnI2L]2·2CH2I2 for 1 week transformed them into dark-brown, (0.3 ·CH2I + 0.85 I2)@[ZnI2L]2· 2CH2I2, without destruction of crystallinity in the solid state, which will be discussed in detail. In contrast, the analogues, CH2Cl2@[ZnI2L]2 and 1.5CH2Br2@[ZnI2L]2·2CH2Br2, were not photoreacted under the same condition at 350 nm irradiation. The thermal properties of the (0.3 ·CH2I + 0.85 I2)@[ZnI2L]2·2CH2I2 are similar to those of CH2I2@[ZnI2L]2· 2CH2I2, except the slightly different solvate molecule portion. X-ray diffraction measurement of CH2I2@[ZnI2L]2·2CH2I2 and (0.3 ·CH2I + 0.85 I2)@[ZnI2L]2·2CH2I2 revealed that the skeletal structure of CH 2I2@[ZnI2L]2·2CH2I2 is a 30membered centrosymmetric metallacyclodimer with the intramolecular Zn···Zn separation of 10.568(2) Å (Figures 1 and S2, Supporting Information). The coordination geometry around the Zn(II) ion is a slightly distorted tetrahedral arrangement with two nitrogen donors from two L (Zn−N = 2.067(8) and 2.074(7) Å; N−Zn−N = 102.2(3)°) and two iodides (Zn−I = 2.532(1) and 2.565(1) Å; I−Zn−I = 122.04(5)°). Figure S3 in Supporting Information shows the metallacyclodimers to be stacked in an eclipsed mode along the crystallographic a-axis,

Figure 1. Crystal structure of CH2I2@[ZnI2L]2·2CH2I2 showing suprachannels via interdigitated π···π interactions (a). Solvate CH2I2 molecules were omitted for clarity. Highlighted representation showing photodissociation of “inside” CH2I2 for CH2I2@[ZnI2L]2· 2CH2I2 (b) to (0.3 ·CH2I + 0.85 I2)@[ZnI2L]2·2CH2I2 (c) via 350 nm UV-irradiation for 7 days. Corresponding electron density maps of CH2I2@[ZnI2L]2·2CH2I2 (d) and (0.3 ·CH2I + 0.85 I2)@[ZnI2L]2· 2CH2I2 (e) showing “inside” CH2I2, ·CH2I, and I2 along with relevant distances (Å).

thereby forming 1D suprachannel ensembles of 3.6 × 4.8 Å2 dimensions. The eclipsed stacking exists as the result of the presence of interdigitated π···π interactions between the adjacent naphthyl moieties (3.47(9) Å; 0.0(3)°). The most interesting feature is that one CH2I2 is safely nestled in a zigzag fashion within the suprachannels (I−CH2−I···I−CH2−I = 3.077(8) Å). By contrast, two “outside” CH2I2 solvate molecules remain in the vacancy with the shortest intermolecular I···I distance of 4.263(2) Å, and thus, the present Zn(II) compound can be described as composed of CH2I2@[ZnI2L]2· 2CH2I2. For the photoreacted dark-brown crystals consisting of (0.3 ·CH2I + 0.85 I2)@[ZnI2L]2·2CH2I2, the cyclodimeric skeleton and “outside” CH2I2 molecules are coincident with those of CH 2 I2 @[ZnI2 L]2 ·2CH 2 I2 . However, the CH 2 I2 molecules nestled within the suprachannel exhibit a drastic change after the photoreaction. That is, the intermolecular I···I distance is significantly shortened (2.835(7) Å) compared with that (3.077(8) Å) of CH2I2@[ZnI2L]2·2CH2I2 (Table 1). This shortened distance is comparable to the bond distances of I2 (2.715 Å) and CH2I−I (3.02 Å).26,27 Moreover, the shortened C−I distance (2.12(8) Å) corresponding to ·CH2I is similar to the value (2.044−2.103 Å) of ·CH2I, as suggested by a computational method.18,28 Concomitantly, the dissociated distance between iodine and the iodomethyl radical (I−I··· 3055

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Table 1. Relevant Bond Lengths (Å) and Angles (deg) of CH2I2@[ZnI2L]2·2CH2I2 and (0.3 ·CH2I + 0.85 I2) @[ZnI2L]2·2CH2I2a CH2I2@[ZnI2L]2·2CH2I2

a

C(24)−I(5) C(24)−I(6) I(5)···I(5)[1] Zn(1)···I(5) Zn(1)[1]···I(5)

2.173(4) 2.212(5) 3.077(8) 5.604(4) 5.604(4)

I(5)−C(24)−I(6)

109.1(3)

(0.3 ·CH2I + 0.85 I2)@-[ZnI2L]2· 2CH2I2 C(24)···I(5) C(24)−I(6) I(5)−I(5)[2] I(7)−I(7)[3] Zn(1)···I(5) Zn(1)[2]···I(5)

2.38(9) 2.12(8) 2.835(7) 2.655(8) 5.584(3) 5.315(3)

I(5)···C(24)−I(6)

107(4)

[1] −x+1,−y,−z+1. [2] −x+3,−y+1,−z+1. [3] −x+2,−y+1,−z+1.

CH2I = 2.38(9) Å) is much longer than the C−I distances of the initial “inside” CH2I2 molecule (2.173(4) Å) and the “outside” CH2I2 molecule (2.08(2); 2.14(2) Å). In particular, it should be pointed out that a new electron density map corresponding to the iodine molecules (I(7)−I(7)′ = 2.655(8) Å, Figure 1e) is very similar to that of the isolated I2 molecule (2.666 Å).29 The UV (350 nm) irradiation of the single crystal makes the confined CH2I2 molecules dissociate to form iodine (I2) and the iodomethyl radical (·CH2I). Then, partial successive decomposition of ·CH2I gives rise to vaporizing carbene (: CH2) formation along with further I2 without significant change of the crystallinity.30,31 Between the two kinds of “inside” and “outside” CH2I2 in the crystalline state, only “inside” CH2I2 is photoreacted to form ·CH2I and I2, indicating that the “inside” CH2I2 is activated superiorly to the “outside” CH2I2 in this system. In this regard, the different intermolecular I−CH2−I··· I−CH2−I distance between the “inside” and “outside” CH2I2 can concretely affect the discriminated photoreactivity. That is, the short intermolecular I···I contact of “inside” CH2I2 with the optimum orientation within the confined suprachannel relative to that (4.263(2) Å) of “outside” CH2I2 is the driving force behind the selective generation of the radical species. Absorption spectra of the samples before and after photoreaction further confirmed the formation of I2 upon the UV irradiation (Figure S4, Supporting Information). The EPR (electron paramagnetic resonance) spectrum at Xband microwave frequency (9.64 GHz) obtained on (0.3 ·CH2I + 0.85 I2)@[ZnI2L]2·2CH2I2 are plotted in Figure 2a. For the crystalline samples there is an EPR signal with the g value = 2.0712 in contrast to EPR silent CH2I2@[ZnI2L]2·2CH2I2, but no distinct hyperfine coupling at 5 K in 10 G. The g value can be comparable to the free electron (g = 2.0023) and ·CH3 (g = 2.0026).32 The extraordinary broad EPR spectrum can be explained by the existence of interspecies magnetic interaction within the confined suprachannel and weak interaction between the channel and the radical species in the solid state,33 even though hyperfine EPR spectra for the 5 mol % solution of CF3X•− radical anions (X = Cl, Br, I) in tetramethylsilane (TMS) have been observed at 77 K.34 As plotted in Figure S5 in Supporting Information, the UPS (UV photoelectron spectroscopy) spectrum signal at 9.08 eV is an additional evidence of the presence of the ·CH2I species, which is comparable with the value (8.52 eV) of gaseous ·CH2I free radical.35 The single crystal-to-single crystal phototransformation of CH2I2@[ZnI2L]2·2CH2I2 to (0.3 ·CH2I + 0.85 I2)@[ZnI2L]2·

Figure 2. EPR spectra (a), Raman spectra along with assignments of relevant species (b), and partial 13C CP-MAS-TOSS spectra showing “inside” and “outside” CH2I2 in the solid state for CH2I2@[ZnI2L]2· 2CH2I2 (black) and (0.3 ·CH2I + 0.85 I2)@[ZnI2L]2·2CH2I2 (red). The full spectra of (b) and (c) are plotted in Figures S6 and S7 in Supporting Information, respectively.

2CH2I2 could be confirmed by the characteristic Raman bands (Figure 2b). CH2I2@[ZnI2L]2·2CH2I2 shows two signals at 86 and 122 cm−1 corresponding to CH2I2 whereas (0.3 ·CH2I + 0.85 I2)@[ZnI2L]2·2CH2I2 shows several characteristic signals at 91, 113, 120, 169, and 195 cm−1. The two very strong modes at 113 and 169 cm−1 can be assigned by the stretching modes of I2 with symmetries Ag and B3g, respectively,36,37 indicating the formation of iodine (I2) via the photoreaction. The sharp peak at 195 cm−1 is a new signal, which can be explained by · CH2I,38 while the other two peaks are close to the signals of the remaining “outside” CH2I2. The other peaks of the spectrum corresponding to the cyclodimeric skeleton remain virtually unchanged (Figure S6, Supporting Information). The 113 and 169 cm−1 bands are red-shifted relative to 180 and 189 cm−1 of bulk I2,36 owing presumably to the confinement effect within the crystalline suprachannel. 13 C CP-MAS-TOSS experiments were performed for identification of the main structural changes taking place in the crystalline samples as a consequence of the photoirradiation treatments.18 As depicted in Figures 2c and S7 in Supporting Information, CH2I2@[ZnI2L]2·2CH2I2 gives rise to two signals at −61.0 and −57.9 ppm corresponding to the “inside” and “outside” CH2I2, respectively, whereas (0.3 ·CH2I + 0.85 I2) @[ZnI2L]2·2CH2I2 shows a single signal at −58.2 ppm from just the “outside” CH2I2. Such a disappearance of the ·CH2I carbon signal can be ascribed to its extreme broadening owing to quadruple interaction. Furthermore, in order to check the relationship between the population of the ·CH2I species and the irradiation time, the 1H NMR spectra of each irradiated sample according to the photoreaction time were measured in Me2SO-d6 (Figure S8, Supporting Information), which is coincident with the following eq 1. The intensity of CH2I2 slowly decreases with increasing photoreaction time, reflecting the evaporation of carbene species via further decomposition of radical species. According to the 1H NMR spectra, the “inside” 3056

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emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

CH2I2 signal was decreased by half, evaporated upon UVirradiation for 4 days. After 2 weeks of irradiation, the population corresponding to “inside” CH2I2 disappears. In this regard, the second photodissociation process contributed to the 70% reduced electron density corresponding to ·CH2I (C(24)−I(6)) and the appearance of the new electron density corresponding to iodine molecule (I(7)−I(7)′) (Figure 1e). CH 2I 2 → ·CH 2I + 1/2I 2 → : CH 2( ↑ ) + I 2

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Corresponding Author

*E-mail: [email protected]. Fax: +83-52-516-7421. Tel: +82-51-510-2591.

(1)

Author Contributions

For CH2Cl2@[ZnI2L]2, the solvate molecules could be removed at 50 °C in vacuum, as confirmed by reference to the IR, elemental analysis, and 1H NMR results. When the evacuated [ZnI2L]2 species were immersed in the respective CH2Cl2, CH2Br2, and CH2I2, the sample absorbed each solvent, thus forming their original chemical formulas. Furthermore, when the evacuated [ZnI2L]2 was immersed in a mixture of CH2Cl2, CH2Br2, and CH2I2 (v/v/v = 1:1:1) for 1 day, it absorbed CH2Cl2:CH2Br2:CH2I2 in the ratio of 1:2:6 (Figure S9, Supporting Information). In fact, the total solvent-accessible volumes for the desolvated skeleton after removal of the solvate molecules were estimated to be 35.7% for [ZnI2L]2 (559.5 Å3/ 1569.0 Å3), as calculated by the PLATON.39 Notably in this regard, [ZnI2L]2 predominantly adsorbs CH2I2 molecules, which is consistent with the “like-attracts-like” model. Thus, this suprachnnel system can be applied as a tailored storage or stabilizer for unstable CH2I2, but what is the critical driving force behind the CH2I2 container? We attributed the reversible solvate molecules to the felicitous “size influence” and the “weak non-covalent halogen interaction” aspects. CH2I2 is commonly used as a reagent in cyclopropanation reactions with alkenes (e.g., Simmons-Smith reaction).40 In conclusion, proof-of-concept experiments on the first crystal structure of a nonconjugated carbon radical within a suprachannel via single crystal-to-single crystal photoreaction were accomplished. The present metallacyclodimer’s ensemble constituting a suprachannel, via interdigitated π···π interactions, efficiently nestles and activates CH2I2 molecules. UV (350 nm) irradiation of the single crystal discriminates the “inside” and “outside” CH2I2 of the suprachannel along with the color change of light yellow to dark brown. That is, only the “inside” CH2I2 is changed to ·CH2I and I2 (0.3 ·CH2I + 0.85 I2) without destruction of the channel skeleton in the crystalline state. The most remarkable feature is the information afforded on the unprecedented crystal structure of a nonconjugated carbon radical species formed via single crystal-to-single crystal transformation. Further investigations of this type of reaction are currently underway in our laboratory. The research results on selective photodecomposition reactions of CH2I2 might indeed represent a landmark in various scientific fields including synthetic methodologies for organic reactions.

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AUTHOR INFORMATION

‡

Yoonjung Cho and Tae Hwan Noh contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean Government [MEST] (2013R1A2A2A07067841).

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REFERENCES

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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.cgd.6b00318. Experimental details and characterization data, 1H NMR spectra showing adsorption of dihalomethane molecules to the evacuated [ZnI2L]2 (PDF) Accession Codes

CCDC 1427847−1427850 contains 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 3057

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