π-Pimer, π-Dimer, π-Trimer, and 1D π-Stacks in a Series of Benzene Triimide Radical Anions: Substituent-Modulated π Interactions and Physical Properties in Crystalline State

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE24 Jun 2022π-Pimer, π-Dimer, π-Trimer, and 1D π-Stacks in a Series of Benzene Triimide Radical Anions: Substituent-Modulated π Interactions and Physical Properties in Crystalline State De-Hui Tuo, Shuxuan Tang, Pengfei Jin, Jikun Li, Xinping Wang, Chuang Zhang, Yu-Fei Ao, Qi-Qiang Wang and De-Xian Wang De-Hui Tuo Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular Recognition and Function, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 , Shuxuan Tang State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210023 , Pengfei Jin University of Chinese Academy of Sciences, Beijing 100049 Beijing National Laboratory for Molecular Sciences, Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 , Jikun Li Beijing National Laboratory for Molecular Sciences, Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 , Xinping Wang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210023 , Chuang Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Beijing National Laboratory for Molecular Sciences, Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 , Yu-Fei Ao Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular Recognition and Function, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 , Qi-Qiang Wang Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular Recognition and Function, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 and De-Xian Wang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular Recognition and Function, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 https://doi.org/10.31635/ccschem.022.202202167 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail We herein present the efficient syntheses, crystal structures, and physical properties of a series of novel benzene triimide (BTI) radical anions, emphasizing the relationship between molecular structure with spin–spin intermolecular interactions and physical properties in the crystalline state. BTI radical crystals were prepared by reducing various substituted BTIs ( 1) with cobaltocene (CoCp2), followed by sequential crystallization processes. Two isolable radical species, that is, neutral molecule-radical anion pair ( 2) and monoradical species ( 3), were obtained from BTIs bearing small substituents (ethyl and propyl), while for those bearing larger substituents (butyl, hexyl, and benzyl) sole monoradicals ( 3) were produced. The radical species showed diverse intermolecular stackings in the crystalline state referred to as π-pimer, π-dimer, alternating π-trimer, and one-dimensional (1D) slipped π-stacks depending on the length and size of the substituents. Different spin–spin interactions arising from the substituent-tuned radical packings were demonstrated by electron spin resonance (ESR) spectra. The structure–property relationship was studied in terms of magnetism and conductivity. The radical π-pimers and π-stacks with effective charge transport exhibited paramagnetic property and conductivity feasibly at room temperature, while π-dimers and π-trimers featuring thermally populated spin-triplet states were antiferromagnetic and nonconductive due to their strong radical–radical pairing effect. Download figure Download PowerPoint Introduction The design and synthesis of novel, persistent and isolable radicals play crucial roles in spin and material sciences.1–3 Many current interests in persistent radicals probably arise from intermolecular self-association of neutral π-radicals or π-radical ions because they are important in understanding spin–spin interaction,4–17 developing new materials with magnetism and conductivity,15–26 and fabricating functional supramolecular systems.27,28 Many of these applications rely on the specific packing of aromatic radicals in solid materials. However, two crucial challenges exist. First, the competitive σ-associations cannot be avoided in some systems due to the existence of reactive carbon in the molecular skeleton.29–34 Second, the scarcity of structural diversity to systematically modulate and control the intermolecular interactions that govern the physical properties makes it difficult to predict the structure–property relationship.35 Benzene triimide (BTI) or mellitic triimide, a planar electron-deficient unit with C3-symmetry, is capable of sequentially accepting three electrons.36–39 Recently, we prepared and isolated the first BTI-radical anion, associated with its parent neutral molecule to form radical pimer ([BTI-3C]2−•, referred to as π-pimer40).41 The intriguing coupling between three N-propyl substituents and the planar backbone produced unprecedented magnetic multistability in this pimeric crystalline material. BTI-radical anion represents a new and persistent π radical species. Its peculiar physical property (magnetism) deserves deep exploration of other BTI-based radicals for advancing their future applications in material science. Here, we emphasized on substitute-modulated BTI radical systems for three critical considerations: (1) bulky protecting groups are not necessary for this building unit to stabilize the radical; thus, a diverse radical library could be readily established, (2) no reactive carbon exists in the extended aromatic framework, which allows exclusive π-association, and (3) three substituents can be attached to the nitrogen atoms, and a rich substituent-BTI core interplay is readily accessible. In this work, we report the synthesis, structure, and property of a series of BTI-radical anions. Of particular interest are the different packing modes of π-radicals tuned by substituents and their related magnetic and conductivity properties. Experimental Methods Detailed synthetic procedures of compounds [ 2]2−•CoCp2+ and 3−•CoCp2+ are listed in Supporting Information Scheme S1. Their structures are characterized by elemental analysis and single-crystal X-ray diffraction (see Supporting Information Figures S1–S6 and Table S1). For X-ray diffraction measurement, the radical crystal samples were coated with polyisobutene in a glove box under an Ar atmosphere. The protected samples were stable for several days in the air and suitable for X-ray diffraction measurements. X-ray intensity data were collected on an XtaLAB Synergy R HyPix diffractometer for the structures using CuKα (λ = 1.54184 Å) at set temperatures. The intensity data were collected by the omega scans techniques, scaled, and reduced with the CrysAlisPro software ( https://www.rigaku.com/products/crystallography/crysalis). Also, the correction of absorption intensities was performed using the CrysAlisPro program. The structures were solved by direct methods using SHELXT and refined using full-matrix least-squares methods in ShelXL. All non-hydrogen atoms were refined anisotropically depending on the occurrence of disorder in the structures. All hydrogen atoms were placed geometrically with a riding model for their isotropic temperature factors (see Supporting Information Table S1). For superconducting quantum interference device (SQUID) measurement, the radical crystal samples were wrapped in PE film under an Ar atmosphere of the glove box, and magnetic measurements were acquired on a Quantum Design SQUID VSM magnetometer with a field of 0.1 T. For electron spin resonance (ESR) measurement, the crystal powder or solution samples of BTI radicals were sealed in a capillary under an Ar atmosphere and loaded into a paramagnetic tube for ESR measurements (see Supporting Information Figures S10–S19). For conductivity measurements, radical crystals were fixed on a carefully cleaned glass substrate in a glove box, followed by placing linear shelter perpendicular to the long axis of the crystal. The electrode layer was fabricated through aluminum thermal evaporation under a high vacuum at 10−10 bar and the resultant electrical conduction channel was ∼110 μm. After putting two-probe on aluminum electrodes, current–voltage (I–V) measurements were acquired by sweeping voltages across a range of −10 V ≤ V ≤ +10 V. The temperature variable conductivity measurements were carried out under vacuum at 10−4 bar. The sample for UV–vis–near-infrared (NIR) spectral measurement was prepared in the glove box. Dissolving the BTI radical crystal in CH2Cl2, the prepared sample solution was injected into a sealed cuvette, and the absorption was recorded at room temperature (see Supporting Information Figure S9). For titrations, the neutral 1b solution was added with a microsyringe to the sealed cuvette containing 3b−• radical solution in the glove box (see Supporting Information Figure S7). Results and Discussion Synthesis The synthesis of BTI radical anions was achieved by chemical reduction of the corresponding neutral molecules 1a– 1f with CoCp2 as a reducing agent. Mixing equimolar 1a– 1f and CoCp2 in dimethylacetamide (DMA) at room temperature resulted in dark green solutions. Slow diffusion of ethyl ether to the solutions produced radical crystals in high yields (Scheme 1). The synthetic outcome showed close dependency on the bulkiness of substituents. For 1b and 1c bearing ethyl or propyl chains, each parent molecule gave two isolable radical products by a two-step crystallization process.41 The first step afforded green needle crystals, which were identified as pairs of one radical anion and one neutral molecule, that is, [ 2b]2−•CoCp2+ and [ 2c]2−•CoCp2+. Further crystallization of the remaining filtrate (second step) yielded monoradical products ( 3b−•CoCp2+ and 3c−•CoCp2+) as bright blue block crystals. For BTI molecules bearing larger groups, that is, butyl, hexyl, and benzyl ( 1d– 1f), sole monoradical products 3d−•CoCp2+, 3e−•CoCp2+, and 3f−•CoCp2+ were obtained in 59–84% yields using a similar procedure. The use of equal excess of 1d– 1f in the reacting mixtures also failed to produce radical-neutral molecule pairs. As an exception, the methyl-substituted 1a only gave a trace amount of [ 2a]2−•CoCp2+ (yield <1%) due to its poor solubility, and no 2a−•CoCp2+ monoradical product was detected. All the radical crystals obtained were thermally stable under a nitrogen or argon atmosphere and could be stably stored at room temperature. Once exposed to an air atmosphere, the radical crystals quickly turned brown, and the spins were quenched. The radical samples were characterized by X-ray single-crystal diffraction ( Supporting Information Table S1) and elemental analysis (except for [ 2a]2−•CoCp2+, for details, see Supporting Information). Scheme 1 | Synthesis of BTI radical anions. Download figure Download PowerPoint Structure and crystal packings The structures of the isolated radical species in a crystalline state were studied by X-ray crystallography analysis. Substituent-modulated π-pimer, π-dimer, alternating π-trimer, and slipped π-stacks were revealed for radicals 2 and 3 (except for 3e−•CoCp2+ due to severe disorder). The structural details are depicted in Figure 1 and Supporting Information Figures S1–S6. Figure 1 | X-ray single-crystal structures of BTI radicals (a) [2a]2−•CoCp2+, (b) [2b]2−•CoCp2+, (c) 3b−•CoCp2+, (d) 3c−•CoCp2+, (e) 3d−•CoCp2+, and (f) 3f−•CoCp2+. Blue and red dots in (a) and (b) represent the centroid of BTI core. Download figure Download PowerPoint π-Pimer The radical species 2 ([ 2a]2−•CoCp2+, [ 2b]2−•CoCp2+, and [ 2c]2−•CoCp2+) form similar stackings, where we noticed that each of the two BTIs corresponded to one cobaltocenium (BTI:CoCp2+ = 2:1) (Figures 1a and 1b). The mean interplanar distances of the two BTIs were 3.23∼3.33 Å and less than the sum of the van der Waals radii of sp2 carbon atoms (∼3.4 Å). The average bond lengths for sp2 C–C and C=O bonds fell between those found in monoradical and neutral molecules ( Supporting Information Table S2), suggesting an almost equal distribution of the single electron on each BTI skeleton (0.5e/0.5e). The general structural features of radicals 2 represent the formation of charge-transfer complexes (π-pimer40). Scrutiny of the structures revealed that the packing details in crystals were slightly regulated by the substituents. In the methyl-bearing [ 2a]2−•CoCp2+, the pimeric BTI cores were stacked in an antiparallel fashion along a C2 axis, and the interplanar distance was 3.24 Å. For the ethyl-substituted [ 2b]2−•CoCp2+ radical, the BTI cores stacked staggeringly with mean interplanar distances of 3.26∼3.32 Å, resembling the stacking modes of previously reported [ 2c]2−•CoCp2+.41 Moreover, the pimers served as repeating units and were linked in an eclipsed pattern by weak π–π stacking (secondary driving force), leading to infinite layered self-assemblies. The pimer–pimer separation distances followed an increasing order of [ 2a]2−•CoCp2+ (3.24 Å) < [ 2b]2−•CoCp2+ (3.39 Å) < [ 2c]2−•CoCp2+ (3.51 Å), in line with the lengthening of alkyl chains. Along with the π interactions of BTI cores, the ethyl and propyl groups adopted an up–up–down (or down–down–up) array relative to the plane of the BTI core in order to match the steric demand of the π interactions (Figure 1b and Supporting Information Figure S2). π-Dimer The monoradical species 3b−•CoCp2+ and 3c−•CoCp2+ exist as π-dimers in a crystalline state; however, two distinctive types of π-associations (bonding) were observed. In 3b−•CoCp2+, from the side view, the two BTI radicals were stacked back-to-back, and from the top view, they overlapped antiparallelly along a C2 axis (Figure 1c). The interplanar distance of 2.90 Å and the shortest intermolecular Csp2–Csp2 distance of 3.11 Å were substantially less than the sum of van der Waals radii, indicative of π-associations driven by singly occupied molecular orbital (SOMO)–SOMO overlap. To facilitate the π-associations, the three ethyl chains in each BTI radical adopted an all-up array. Additionally, CoCp2+ countercation localized between the dimers and separated the dimers at a long distance of ∼ 6.70 Å, suggesting the formation of discrete π-dimers. In stark contrast to 3b−•CoCp2+, the BTI cores in the dimeric 3c−•CoCp2+ showed a complete staggered packing (top view),41 resembling a 2e/12C pancake bonding (d = 3.09 Å) that originated from a large SOMO–SOMO overlap (Figure 1d). Discrete π-dimers for 3c−•CoCp2+ were also suggestive of a dimer–dimer distance being ∼ 4.70 Å, in spite of the enhanced van der Waals interplay between the propyl chains and less involvement of countercation in the packing than that of 3b−•CoCp2+. π-Trimer The 3d−•CoCp2+ monoradical in the crystalline state showed interesting trimeric contacts in an alternative way. As shown in Figure 1e, BTI-radical plane B (middle) used each of its sides to stack with A and C in a slipped way, forming a "spiral staircase" structure (C6 helix). The interplanar distances were not identical either, showing that B and C formed a more close contact (d B-C = 3.00 Å) than A and B (d A-B= 3.19 Å), implying a possible "dimers in trimer" structural feature. Besides, four adjacent sp2 carbon atoms (C1–C4) of B show interplay with two bonded sp2 carbon atoms of A and C with C–C distances in the range of 3.20–3.28 Å, respectively, suggesting 3e/8C intermolecular trimeric π-associations. Beyond the π-trimer, A or C was also stacked with a second BTI-radical through π–π stackings. The longer intertrimer planar–planar (∼3.33 Å) and sp2 C–C (3.37–3.84 Å) distances than that within the trimer suggested that the inter-trimer driving force was mainly van der Waals contribution. Such π–π stackings linked the π-trimers into infinite layered self-assembly (see Supporting Information Figure S5). To favor the packings, the butyl chains adopted an up–up–down array to overcome steric hindrance. It is worth addressing that although extensive examples of radical π-dimers have been reported as a result of SOMO–SOMO bonding, the radical π-trimer is extremely rare.42 To the best of our knowledge, this is the first example of a radical π-trimer with interplanar or intermolecular carbon–carbon distances smaller than van der Waals radii. 1D π-stacks The crystal structure of 3f−•CoCp2+ showed distinct one-dimensional (1D) slipped π-stacks (Figure 1f). A short interplanar distance of 3.27 Å was observed from the side view; however, the top view showed marginal overlaps between the BTI cores. Such structural features implied that the intermolecular interactions were mainly contributed by dispersion force rather than SOMO–SOMO overlap. The distinct stacks of 3f−•CoCp2+ were probably caused by benzyl substituents. From the side view, the three benzyl groups adopted an up–up–down array, and the two up–up arrayed benzyl groups formed a V-shape cavity where a CoCp2+ cation was incorporated through C–H…π or cation…π interactions. The bulkiness of benzyl groups and Coulomb repulsion between CoCp2+ cations probably inhibited the close overlap of the BTI cores and led to slipped packings. The structure of radical anions in solution was investigated by UV–vis–NIR measurements. The spectrum of [ 2b]2−•CoCp2+ shows similar absorption band as [ 2c]2−•CoCp2+ ([ 2a]2−•CoCp2+ was not investigated) in the NIR region (1000–2200 nm) (Figure 2b). The NIR absorption was assigned to a charge-transfer transition from SOMO of the radical to the lowest unoccupied molecular orbital (LUMO) of the neutral molecule, according to the time-dependent density functional theory (TD-DFT) calculation (Figure 2d), characterizing the formation of the pimer in solution. The dimeric association constant of [ 2b]2−•CoCp2+ (Kdimer = 19.0 M−1, ε = 7.28 × 104 M−1 cm−1, Supporting Information Figures S7 and S8) is smaller than that of [ 2c]2−•CoCp2+ (Kdimer = 66.7 M−1, ε = 5.75 × 104 M−1 cm−1),41 indicative of substituent effect. The monoradical species showed similar UV–vis–NIR spectra with absorption bands centered at 350–450 nm (band I) and 700–1000 nm (band II) (Figure 2a). The maximum absorption wavelength (λmax) located at 908 nm was assigned as SOMO to LUMO+1 transition (energy gap 1.37 eV) of the BTI radical anion (Figure 2c).43 Figure 2 | (a) UV–vis–NIR spectrum of 3b−•CoCp2+ (2.5 × 10−4 M in DCM) and (b) spectral changes of [2b]2−•CoCp2+ formation upon addition of 1b to the solution of 3b−•CoCp2+ (3.75 × 10−4 M in DCM). TD-DFT calculations of the spectra at UB3LYP/6-31G(d,p) level for (c) 3b−•and (d) [2b]2−•. Download figure Download PowerPoint ESR study The intriguingly different stackings of monoradicals with short distances (<3.4 Å) encouraged us to study the spin–spin interactions, particularly, the thermally excited high-spin states. The ESR spectra of the crystalline powder samples at room temperature are shown in Figure 3, where different ESR signals for individual radicals were observed. Although ( 3b−•)2(CoCp2+)2 and ( 3c−•)2(CoCp2+)2 both formed π-dimers in the crystalline state, their ESR spectra are quite different. In the former case, besides the sharp central signal in the doublet state, an additional broad signal was observed (Figure 3a), which could be assigned to dimeric aggregate arising from strong spin-spin (dipole–dipole) interactions, consistent with the dimeric stacking in Figure 1c. The absence of typical state and signal is probably due to the large spin–spin distance caused by the slipped array within the discrete ( In the ESR spectrum of ( 3c−•)2(CoCp2+)2 showed state signals corresponding to the = in addition to an central signal from monoradical species. that the were = = = = = = and = The an of the spin–spin in an excited state of the A weak signal at room temperature for ( 3c−•)2(CoCp2+)2 (Figure was The different ESR spectra of the two π-dimers are probably the result of their stacking modes in the crystalline state, that is, eclipsed staggered stackings. In other the overlap of central benzene in the is important in the of the spin–spin Figure 3 | ESR spectra of (a) 3b−•CoCp2+, (b) 3c−•CoCp2+, (c) 3d−•CoCp2+, and (d) 3f−•CoCp2+. The central signals in are radical Download figure Download PowerPoint the ESR spectrum of 3d−•CoCp2+ crystalline powder exhibited similar with 3c−•CoCp2+ and showed state signals (Figure The obtained from the spectral were = = = = = = and = signal of = transition was observed at room temperature (Figure were of the overlaps in of the BTI central cores in the crystal the calculation on the trimeric radicals observed in crystal structure overlap of SOMO at two the of "dimers in trimer" states in radicals (see Supporting Information Figure As π-trimer stacking was observed in the crystal structure of 3d−•CoCp2+, we that an = state probably the signal intensity corresponding to the state was and the signal of = transition was less of the state or weak intensity of the To the of the high-spin state of the dimers and ESR were performed for the crystalline powder samples of 3b−•CoCp2+, 3c−•CoCp2+, and The state signals with and at of for 3b−•CoCp2+, for 3c−•CoCp2+, and for 3d−•CoCp2+ (see Supporting Information Figures suggesting that these signals from thermally excited high spin states. the ESR spectra of spin–spin interactions in the π-dimers and π-trimer 3f−•CoCp2+ exhibited 1D π-stacks in the crystalline state, a broad and ESR A signal corresponding to the state was not observed (Figure a signal of = transition was observed at room temperature (Figure In contrast to the ESR spectra in the crystalline state, BTI radical anions in solution exhibited and broad signals = suggesting the of spin the aromatic unit (see Supporting Information Figure For the monoradical anions, ESR measurements were performed in = × to the association properties in the solution (see Supporting Information Figures the ESR intensities upon temperature from to in the range of and a or slightly at Such an outcome the absence of π-associations in solution under the temperature In other the formation of π-associations was only in a crystalline state or under temperature To we performed UV–vis–NIR DCM) for the monoradical anions, as changes with increasing due to the formation of As in various from × 10−4 M to × the BTI radical anion band (band as an showed a linear on the result demonstrated the absence of radical π-associations in the solution (see Supporting Information Figure S9). SQUID measurements of the radical samples were studied by magnetic using SQUID in the temperature range of with a field of 0.1 T. The magnetic are depicted in Figure a small from ([ to ([ 2c]2−•CoCp2+) BTI pimeric radicals caused in magnetic properties. and [ 2c]2−•CoCp2+ exhibited multistability with a thermal in the range of and a smaller at The of [ 2b]2−•CoCp2+ was at which upon an antiferromagnetic coupling at temperatures. The of [ 2b]2−•CoCp2+ was using the The were obtained with = = cm−1, and = × 10−4 where the constant revealed the antiferromagnetic Figure | (a) of for the radical crystal samples and (b) of the conductivity measurements for radical crystals. Download figure Download PowerPoint The for 3b−•CoCp2+, 3c−•CoCp2+ and 3d−•CoCp2+ were small and only slightly in the range of < The small in these three radicals strong antiferromagnetic interactions, in line with their short stackings in crystalline states. the of 3f−•CoCp2+ showed different in different temperature The was at and upon to a at the The increasing of obtained at the coupling in 3f−•CoCp2+. that the crystal structure of 3f−•CoCp2+ exhibited 1D slipped chains, the of 3f−•CoCp2+ were using the 1D magnetic The best were = = cm−1, = cm−1, and = × 10−4 where is the and is the that 3f−•CoCp2+ features a 1D structure, with a weak antiferromagnetic between the 1D chains. measurements single-crystal conductivity measurements at room temperature were to the radical crystals. The of radical pimers [ 2b]2−•CoCp2+ and [ 2c]2−•CoCp2+ with average conductivity being × ([ and × ([ 2c]2−•CoCp2+) (Figure The conductivity property of the radical pimers could be to the charge between the pimeric Such was by a in upon (see Supporting Information Figure For radical π-dimers 3b−•CoCp2+, 3c−•CoCp2+, and π-trimer 3d−•CoCp2+, the are small × 10−10 implying that an effective charge transport was probably by the strong spin–spin interactions in the crystalline

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