A Neutral, Radical-Rich, Cobalt Valence ... - ACS Publications

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Sophisticated Construction of Electronically Labile Materials: A Neutral, Radical-Rich, Cobalt Valence Tautomeric Triangle Bao Li,*,† Xiao-Ning Wang,†,⊥ Angelo Kirchon,‡,⊥ Jun-Sheng Qin,‡ Jian-Dong Pang,‡ Gui-Lin Zhuang,*,§ and Hong-Cai Zhou*,‡,∥

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Key Laboratory of Material Chemistry for Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China ‡ Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, United States § Institute of Industrial Catalysis, College of Chemical Engineering, Zhejiang University of Technology, Zhejiang 310023, People’s Republic of China ∥ Department of Materials Science and Engineering, Texas A&M University, College Station, Texas 77842, United States S Supporting Information *

limited.16−19 VT involves not only electron transfer between the metal and ligands but also the high- and low-spin transition of the metal ions accompanied by the process of charge transfer.20−22 The more rigorous conditions for VT systems have limited their developments on a wider scale and the vast majority of the reported results are similar and mostly concentrated on the mononuclear(metal) systems as well as the effect of guest molecules and counteranions have on the VT transition.23−25 Subsequently, there needs to be more attention given to studying how to effectively prepare and expand the scale of novel VT systems. Viewed from the components of the traditional VT system, the adopted nitrogenous ligands normally have larger spatial steric hindrance, while the counter-anions in the structure can not only balance the valence charge but isolate the effective synergy between the sensing sources. The weaknesses of these components lead to the formation of bistable complexes with low density and intensity. Breaking through the restriction of traditional architecture concepts, the chelating radical-bridges were tentatively selected to construct VT systems with specific electro-active ligands. Thanks to the unique characteristic of one unpaired electron, the radical bridges can not only supply the negative charge to cancel the demand for the additional counter-anions, but effectively transfer the strong exchange coupling to enhance the synergistic effect compared to the diamagnetic bridges. Herein, to the best of our knowledge, the first neutral cobalt VT triangle bearing radical-bridges, [Co3(L↑)3(3,5-DBcat)3] (1) (L↑ = 3,6-bis(pyridyl)-1,2,4,5tetrazine radical, 3,5-DBcat = 3,5-di-tert-butylcatechol), had been constructed, which exhibits a transition with the expected characteristics of high signal-intensity and -density. After the repeated attempts, the crystal samples could be finally obtained, which had been directly used for versatile measurements. Crystal data had been collected at 100 K, listed in Table S1−S3. 1 crystallizes in monoclinic system with space group of C2/c, and the asymmetric unit contains one and half cobalt ions and corresponding ligands. A complete isosceles triangle can be finally presented after the growth of asymmetric

ABSTRACT: Herein, we report the construction of a neutral, radical-rich, cobalt valence tautomeric triangle, which consists of two types of radical groups including tetrazine-based bridges and semiquinone anions at high temperature and has traits of high intensity and density of sensing sites. The mechanism of the Valence Tautomerism process within the triangle has been illustrated as one electron transfer, preceding a two electrons transfer along with the phenomenon of spin flipping.

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n order to satisfy the urgent requirements set forth for new functional materials by the rapidly growing technology community, the investigation on the effective preparation of materials for sensing or memory based applications has been booming.1,2 Normally, the signal transmission of sensory or memory materials is expressed in the form of electronic or magnetic signals, which are fairly simple to integrate into convenient equipment.3,4 To fully explore new sensing or memory materials with the goal of improving the intensity and density of the signal sources, new synthetic methods for the preparation of novel materials must be developed.5,6 Electronically labile materials, also termed as bistable materials, have attracted great attention due to their superior relationship between electronic structures and signal expression.7−9 It is also known that along with possible isomer changes, various events can occur, such as a change in luminescent properties, magnetic properties and even electronic states, which all point to these materials having potential applications as spin, memory and sensing devices. Electronically labile materials normally include different potential systems, such as Spin Crossover (SCO) and Valence Tautomerism (VT). The components of SCO systems usually consist of Fe ions as well as nitrogenous ligands, which facilitate the spin transition between high- and low-spins of the iron centers in different isomers.10−12 The systematic research that has occurred for SCO complexes has been well established for the consideration of the specific coordination environments and the facile device preparation.13−15 Comparably, in the last ten years, the progress in the field of VT has been extremely © XXXX American Chemical Society

Received: August 22, 2018 Published: October 22, 2018 A

DOI: 10.1021/jacs.8b09062 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Communication

Journal of the American Chemical Society components (Figure 1). Each vertex of triangle is occupied by one cobalt ion, which is bonded with one chelating 3,5-DBcat

Figure 1. Perspective view of the triangle structure (H atoms have been omitted for clarity).

unit. The adjacent vertexes are further interconnected by one tetrazine-based radical bridge that acts as the edge to form the triangle skeleton. The Co−N and −O bond distances fall in the range of 1.855 to 1.968 Å, similar to the typical values of Co(III) state,20−25 which could be also validated by the calculation of the analysis of bond valence sum (BVS) (Table S4). For the two electroactive ligands, C−O bond lengths from 1.343(2) to 1.359(1) Å, while C−C bond lengths of 1.383(2) and 1.401(2) Å, are similar to the typical values for catecholato species.25,26 In addition, N−N bond lengths in two tetrazine rings are 1.356(2), 1.360(1) and 1.366(2) Å respectively, which are longer than the average values of 1.33 Å for the neutral form of tetrazine, indicating the radical state.27 Therefore, the formula of the triangle should be of [CoIII3(L↑)3(3,5-DBCat)3]. In such a triangle, the flexible coordination ability of L↑ would release the strain of N−Co−N angles in octahedral environment with the range from 82.14 to 95.71°, which is essential for the formation of the isosceles triangle. The Co···Co distances in the intracluster are 6.222(1) and 6.307(1) Å respectively, while the closest Co···Co distance between interclusters is 8.716(2) Å, which are much smaller than the distances in Cobalt VT systems, indicating the possibility of strong synergistic effect in 1. To explore the possibility of VT phenomenon in 1, variabletemperature magnetic measurements were carried out in the temperature of 2−350 K (Figure 2). The χMT values are approximately constants around 1.00 cm3 K mol−1 in the range of 2−250 K, which are slightly smaller than the typical value of 1.125 cm3 K mol−1 for three spin-only electrons of radical bridges. And then, the χMT values increase gradually up to 3.08 cm3 K mol−1 at 350 K, clearly manifesting the occurrence of VT transition in triangle. The repeatable magnetic curve can be observed along with the temperature cooling, indicating a reversible charge transfer between 2 and 350 K. If three electrons transfer between Co(III) ions and catechol species step by step in triangle, there would be four isomers as [CoIII3(L↑)3(3,5-DBcat)3], [CoIICoIII2(L↑)3(3,5-DBcat)2(3,5DBsq)], [Co I I 2 Co I I I (L ↑ ) 3 (3,5-DBcat)(3,5-DBsq) 2 ], [CoII3(L↑)3(3,5-DBsq)3] (3,5-DBsq = 3,5-di-tert-butylsemiquinone radical), which possess 3, 7, 11 and 15 single electrons (Table S5). The theoretical χMT values then should be calculated as 1.125, 3.375, 5.625 and 7.875 cm3 K mol−1 for each isomer (Table S5). The χMT value at 350 K is just lower than the parameter after the first step, indicating an incomplete transition at this temperature. When the temperature was increased to 390 K, irreversible transition would be presented

Figure 2. Variable-temperature χMT values vs T plots (a) and EPR spectra (b) of as-synthesized samples of 1.

(Figure S5), which might be caused by the loss of crystallinity at high temperatures. Referring to other cobalt systems, strong antiferromagnetic interactions can be effectively transferred via the tetrazine-based radical bridges.27 Therefore, at 390 K, the χMT value around 6.0 cm3 K mol−1 indicates the final state of [CoII3(L↑)3(3,5-DBsq)3], which consists of three tetrazinebased radical bridges and three semiquinone radicals. The VT transition could be also validated by variabletemperature IR spectra (Figure S7). At 250 K, the peaks at 1478 and 1255 cm−1 were assigned to the characteristics of skeletal dioxolene on catechol, while the weak bands at 1581 and 1456 cm−1 were ascribed to the CC stretch vibration of semiquinone. As temperature increasing, the peaks of catechol disappeared gradually, and the bands of semiquinone became more intensely. Up to 350 K, only the semiquinone bands could be observed, indicating the final state of [CoII3(L↑)3(3,5DBsq)3].28 Variable-temperature EPR studies have been also performed to confirm the VT process in 1. Around room temperature, only the signal of a radical with g = 2.00 could be detected, indicating the existence of radical species.29 The intensity of radical signal get stronger gradually with a temperature increase (Figure 2), in consistent with the tendency observed in magnetic results. Interestingly, the anisotropic signal displaying hyperfine splitting to eight lines was present along with the temperature increasing (Figure S6), which must be ascribed to the strong interaction between CoII ions and the B

DOI: 10.1021/jacs.8b09062 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society radical components. 30 The origin of Co(II) ion was hypothesized to only be caused by the charge transfer between Co(III) and catechol species. Differing from the EPR signal that displays hyperfine splitting at low temperature regions in other cobalt-radical systems, the cyclic interactions between CoII ions and six radical components in triangle must be responsible for the interesting results at high temperature region. Because of the difficulty in illustrating the VT mechanism in the triangle via experimental methods, spin-polarized density functional theory calculations were performed by using ORCA software31 to explore the possible mechanism. To reduce computational cost, the effective trinuclear moiety using hydrogen-atoms instead of tert-butyls was taken into account in whole calculations. First, seven different spin-state configurations, consisting of S = 1/2, 3/2, 5/2, 7/2, 9/2, 11/ 2, 13/2, 15/2, were fully relaxed. S = 3/2 and 9/2 are the lowest and highest-energy spin states, respectively (Figure S8). Furthermore, spin population and electronic configuration of different spin states had been calculated (Tables S6 and S7, and Figures S9−S12), and VT process in 1 can be postulated as follows (Figure 3): (1) Inconsistent with the X-ray

the one charge transfers randomly between the remnant Co(III) centers and chelated catechol units’ step by step, but the two charges transfer simultaneously. Different to the conventional postulation of final spin state of S = 15/2, the intermediate state after two charges transfer should be the species of S = 9/2 or 11/2 spin states. The spin population results show that spin state of S = 9/2 should feature three LS Co(II) ions with the electronic configuration (t2g6eg1), [LSCoII3(L↑)3(3,5-DBsq)3], labeled as LSCoII3(L↑)3, while S = 11/ 2 should be characterized as three HS Co(II) states, [HSCoII3(L↓)2(L↑)(3,5-DBsq)3], labeled as HSCoII3(L↓)2(L↑). For the tetrazine-based radical bridges in HSCoII3(L↓)2(L↑), the occurrence of spin flipping could be observed due to the weak coordination field of CoII ions and antiferromagnetic coupling between CoII ions and radical bridges.27 Although there are 39 mathematical types of spin population modes in the state of S = 11/2, viewed form the point of thermodynamic, LS CoIII2HSCoII(L↑)3 is easier transferred to HSCoII3(L↓)2(L↑) than other mathematical types. Therefore, the second transition step should be directly occurred between LS CoIII2HSCoII(L↑)3 and HSCoII3(L↓)2(L↑), indicating the two electrons transfer and spin-flipping of two radical bridges. (4) And then, along with increased temperature, the spin state on two spin-flipping radical bridges could be further flipped to keep consistent with other single electrons, leading to the maximal spin-state S = 15/2, [HS-CoII3(L↑)3(3,5-DBsq)3], labeled as HSCoII3(L↑)3. Referred to the other theoretical systems, endothermic effect of spin-flipping transition during final step might be caused by the strong antiferromagnetic coupling between CoII ions and radical bridges.32,33 In conclusion, the novel neutral radicals-rich cobalt VT triangle has been successfully constructed. The utilization of tetrazine-based radical groups not only increases the synergistic effect but also enhances the intensity and density of the sensing process. With the consideration of the theoretical calculations and versatile experimental results, the mechanism of charge transfer process in the VT triangle could be illustrated as one charge transfer and then two charges transfer along with the phenomenon of spin flipping, but not three charges transfer step by step. The presented results provide the scientific guidance for how to build novel sensing materials, and potentially encourage more research to developing excellent VT systems.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 3. Postulated charge transfer process between different spin states along with the view of the distribution of spin density.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b09062.

diffraction studies at low temperature, the trinuclear features quartet rather than doublet. Spin density of S = 3/2 state mainly concentrates on three radical bridges, while three Co3+ ions feature low-spin (LS) configuration (t 2g 6 ), [LSCoIII3(L↑)3(3,5-DBcat)3], labeled as LSCoIII3(L↑)3. (2) As temperature increases, one LS Co3+ ion (Co2 with half occupancy) obtains one electron from its chelated catechol ligand, leading to the formation of [HS-CoIILS-CoIII2(L↑)3(3,5DBcat)2(3,5-DBsq)] with spin state of S = 7/2, labeled as LS CoIII2HSCoII(L↑)3. The endothermic value of ΔH = 7.65 kcal/ mol also demonstrates that the transition between LS CoIII3(L↑)3 and LSCoIII2HSCoII(L↑)3 is very easy, and spin density mostly concentrate on Co(2), three radical bridges and one semiquinone. (3) Very interestingly, the next step was not

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Detailed experimental methods, crystal data, theoretical calculation methods and results (PDF) Data for C78H82Co3N18O6 (CIF)

AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] *[email protected] ORCID

Bao Li: 0000-0003-1154-6423 Angelo Kirchon: 0000-0003-1082-9739 Hong-Cai Zhou: 0000-0002-9029-3788 C

DOI: 10.1021/jacs.8b09062 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society Author Contributions

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⊥

These two authors contribute equally

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Center for Gas Separations Relevant to Clean Energy Technologies, an Energy Frontier Research Center (EFRC) funded by the U.S. Department of Energy (DOE), Office of Science, and Office of Basic Energy Sciences (DESC0001015), Office of Fossil Energy, the National Energy Technology Laboratory (DEFE0026472), and the Robert A. Welch Foundation through a Welch Endowed Chair to HJZ (A-0030). The authors also acknowledge the financial supports of National Science Foundation of China (21471062).

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DOI: 10.1021/jacs.8b09062 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX