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Article Cite This: Inorg. Chem. 2019, 58, 8657−8664

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Concomitant Use of Tetrathiafulvalene and 7,7,8,8Tetracyanoquinodimethane within the Skeletons of Metal−Organic Frameworks: Structures, Magnetism, and Electrochemistry Hai-Ying Wang,†,‡ Jian Su,‡ Jian-Ping Ma,§ Fei Yu,‡ Chanel F. Leong,∥ Deanna M. D’Alessandro,∥ Mohamedally Kurmoo,⊥ and Jing-Lin Zuo*,‡ †

College of Chemistry and Materials Science, Sichuan Normal University, Chengdu 610066, PR China State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210023, PR China § School of Chemistry, Chemical Engineering and Materials Science, Shandong Normal University, Jinan 250014, PR China ∥ School of Chemistry, The University of Sydney, Sydney, New South Wales 2006, Australia ⊥ Université de Strasbourg, Institut de Chimie de Strasbourg, CNRS-UMR7177, 4 rue Blaise Pascal, Strasbourg 67008, France

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S Supporting Information *

ABSTRACT: In search of multifunctional metal−organic frameworks (MOFs), redox-active donors and acceptors, namely, tetrathiafulvalene (TTF) and 7,7,8,8-tetracyanoquinodimethane (TCNQ), were concomitantly used as skeletal components with diamagnetic metal nodes (Cd and Zn) to construct unique framework materials. Six isostructural frameworks were synthesized by diffusion of metal salts, TTF(py)4, and either paramagnetic Li(TCNQ) or diamagnetic H2TCNQ. They were characterized by single-crystal X-ray diffraction and FT-IR and UV−vis−NIR spectroscopy, and their physical properties were studied, including two postsynthetic modifications involving crystal-to-crystal transformations following a solid-solution reaction with I2. The highly colored crystals of two isostructural Zn and Cd frameworks contain undulating Cd-TTF(py)4 layers entwined with TCNQ in a chicken-wire net as part of the skeleton of the MOF as well as TCNQ intercalated within the channels, while nitrate anions are occluded within the cavities formed by the pyridine moieties. Reaction with I2 replaces each intercalated TCNQ•− within the channels with I3−. The optical properties and the electron paramagnetic resonance (EPR) spectra indicate the presence of only radical TCNQ•− in the parent compounds, while the magnetic susceptibilities enabled an estimation of the amount of TCNQ•− (S = 1/2) leading to almost paramagnetic behavior. Solid-state electrochemistry provides evidence of several one-electron redox states corresponding to the electroactive cores.

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redox-active TTF ligand were reported.18−26 These CPs may retain the redox activity arising from the TTF moiety and their conductive, magnetic, and adsorptive properties can be tuned through the redox-state of TTF. 7,7,8,8-Tetracyanoquinodimethane (TCNQ) is known as a multiredox-active organic molecule with a large π-surface and can act as a good acceptor (Scheme 1). In addition, the presence of four cyanido groups can act as coordination sites allowing TCNQ to potentially function as a multidentate bridging ligand to form strong dative bonds with several metals.27−30 Moreover, TCNQ is easily reduced to TCNQ•− and TCNQ2−, and radical anions can serve as additional spin carriers to enhance magnetic exchange interactions as compared to diamagnetic linkers.31,32 Because of the unique structural and electronic diversities of

INTRODUCTION

Metal−organic frameworks (MOFs), a family of compounds with extended structures formed by metal ions or clusters and functionalized organic ligands via coordination bonds, have been intensively investigated in recent years.1−7 Although many strategies have been used to add or improve the functionality of MOFs, introducing ligands or auxiliary ligands with specific structures and functions as “building blocks” has proven to be one of the most straightforward and effective means to construct novel functional coordination compounds.8−11 Tetrathiafulvalene (TTF) and its derivatives, known as πelectron donors capable of forming stable cation radical and dication species sequentially and reversibly upon oxidation, are good candidates to be used as electron-rich donors and effective linkers to construct novel functional materials.12−17 Recently, some coordination polymers (CPs) built up from the © 2019 American Chemical Society

Received: April 6, 2019 Published: June 12, 2019 8657

DOI: 10.1021/acs.inorgchem.9b01000 Inorg. Chem. 2019, 58, 8657−8664

Article

Inorganic Chemistry

(TTF(py)4)(TCNQ2−)1/2](TCNQ•−)1/2(NO3)1/2·CH2Cl2·1/ 2CH 3 OH; 1-Cd-I 3 , [Cd(TTF(py) 4 )(TCNQ •− ) 1/2 ](I 3 )(NO3)1/2·1/2(C6H12·CH3OH); and 2-Cd-Air-I3, [Cd(TTF(py)4)(TCNQ2−)1/2](TCNQ•−)1/2(NO3)1/2·2C6H12. The results are compared to those of the starting materials, TCNQ0, Li+(TCNQ•−), H2TCNQ, and TTF(py)4.

Scheme 1. Organic Building Blocks Used in the MOF Skeleton

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EXPERIMENTAL SECTION

All starting materials and solvents were commercially available and were used without further purification. TTF(py)4 and H2TCNQ were synthesized according to literature methods.30,39 [Zn(TTF(py)4)(TCNQ•−)1/2](TCNQ•−)1/2(NO3)·2CH3OH (1-Zn). All reactions were performed under a dry N2 atmosphere. A methanol solution (5 mL) of Zn(NO3)2·6H2O (12 mg, 0.04 mmol) was layered onto a CH2Cl2/CH3OH (1:1 v/v) solution (5 mL) of TTF(py)4 (5 mg, 0.01 mmol) and Li(TCNQ) (4.2 mg, 0.02 mmol). The solution was left at room temperature for 7 days. Black block crystals were obtained. Yield: 42%. Anal. Calcd for ZnC40H28N9O5S4 (908.37 g/ mol, wt %): C, 52.89; H, 3.10; N, 13.88. Found: C, 52.98; H, 3.04; N, 13.37%. IR (cm−1): 3063 (w), 2176 (s), 2127 (w), 1693 (w), 1600 (s), 1541 (m), 1502 (m), 1441 (m), 1416 (m), 1347 (s), 1320 (m),1276 (m), 1213 (m), 1187 (m), 1172 (m), 1102 (w), 1063 (m), 1013 (m), 986 (w), 848 (m), 823 (m), 787 (m), 766 (w), 732 (w), 695 (w), 657 (s), 628 (s), 610 (m), 539 (m), 505 (w), 478 (w), 447 (w), 424 (w). [Cd(TTF(py)4)(TCNQ•−)1/2](TCNQ•−)1/2(NO3)·CH2Cl2 (1-Cd). A procedure similar to that for 1-Zn was employed, where Cd(NO3)2· 4H2O was used instead of Zn(NO3)2·6H2O. Yield: 40%. Anal. Calcd for CdC39H22N9O3S4Cl2 (976.22 g/mol, wt %): C, 47.98; H, 2.27; N, 12.91. Found: C, 48.17; H, 2.39; N, 12.83%. IR (cm−1): 3356 (w), 2943 (w), 2830 (w), 2176 (s), 2150 (w), 2127 (w), 1682 (w), 1600 (s), 1568 (m), 1542 (w), 1503 (w), 1440 (w), 1418 (m), 1344 (m), 1318 (m), 1275 (m), 1216 (w), 1187 (w), 1171 (m), 1103 (w), 1063 (w), 1012 (s), 848 (m), 824 (m), 799 (m), 767 (w), 733 (w), 698 (w), 656 (s), 628 (s), 610 (w), 537 (m), 505 (w), 479 (w), 441 (w). [Cd(TTF(py)4)(TCNQ2−)1/2](NO3)·CH3OH (2-Cd-N2). The reaction was performed under an N2 atmosphere. A methanol solution (5 mL) of Cd(NO3)2·4H2O (25 mg, 0.08 mmol) and Li(OAc)·2H2O (17 mg, 0.01 mmol) was layered onto a CH2Cl2/DMF (1:1 v/v)

TCNQ, there has been considerable interest in the incorporation of TCNQ into MOFs where it may be expected to act as a multifunctional bridging ligand.33−35 With the aim of obtaining multifunctional materials, we have focused on the mixed-ligand approach for the synthesis of a MOF incorporating both electroactive TCNQ (metrics given by N···N distances of 4.4 and 8.4 Å) and tetra(4-pyridyl)tetrathiafulvalene (TTF(py)4, N···N distances of 6.8 and 13 Å) (Scheme 1). To avoid complications in the synthesis and study of physical properties, diamagnetic transition metals were selected, i.e., Cd2+ and Zn2+. Two different TCNQ precursors (Li(TCNQ) and H2TCNQ) were employed for their different charge and spin states and the reactions were performed under anaerobic and aerobic conditions followed by postsynthetic reactions with molecular iodine.36−38 We anticipated that a neutral framework may result owing to the balance of charge between the metal cation and the negatively charged TCNQ while the TTF(py)4 is neutral. A unique framework structure was indeed obtained for both Zn and Cd systems which differ in their pore contents. Herein, we report the syntheses, structures, magnetic, optical, and electrochemical properties of the four frameworks and two derivatives obtained by reaction with iodine: 1-Zn, [Zn(TTF(py) 4 )(TCNQ • − ) 1 / 2 ](TCNQ •− ) 1/2 (NO 3 )·2CH 3 OH; 1-Cd, [Cd(TTF(py) 4 )(TCNQ•−)1/2](TCNQ•−)1/2(NO3)·CH2Cl2; 2-Cd-N2, [Cd(TTF(py)4)(TCNQ2−)1/2](NO3)·CH3OH; 2-Cd-Air, [Cd-

Table 1. Crystal Data of 1-Zn, 1-Cd, 1-Cd-I3, 2-Cd-N2, 2-Cd-Air, and 2-Cd-Air-I3 T (K) empirical formula Mr wavelength (Å) crystal system space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z crystal size (mm) dcalcd(g cm−3) μ (mm−1) reflns collected unique reflns [(R(int)] S R1a, wR2b (I > 2σ(I)) CCDC numbers

1-Zn

1-Cd

1-Cd-I3

2-Cd-N2

2-Cd-Air

2-Cd-Air-I3

153 C38H20N9O3S4Zn 844.24 0.71073 monoclinic C2/m 15.8714(15) 19.4255(19) 15.4965(15) 105.571(3) 4602.4(8) 4 0.23 × 0.24 × 0.26 1.218 0.757 12555 4175(0.045)

153 C38H20N9O3S4Cd 891.27 0.71073 monoclinic C2/m 15.7117(19) 19.697(2) 15.8085(19) 105.896(3) 4705.1(10) 4 0.22 × 0.25 × 0.26 1.258 0.683 16450 4255(0.043)

153 C64H36I6N13O3S8Cd2 2277.74 0.71073 monoclinic C2/c 15.679(2) 19.2806(16) 30.7917(13) 103.770(3) 9051.3(15) 4 0.22 × 0.24 × 0.26 1.671 2.745 18253 8161(0.025)

153 C32H18N7O3S4Cd 789.17 0.71073 monoclinic C2/m 15.345(4) 19.790(5) 15.888(3) 106.595(6) 4623.7(18) 4 0.26 × 0.18 × 0.05 1.134 0.685 11974 4526(0.0468)

153 C76H40N17O3S8Cd2 1720.53 0.71073 monoclinic C2/c 15.8627(9) 19.7621(9) 31.3526(15) 103.6961(19) 9549.0(8) 4 0.27 × 0.19 × 0.13 1.197 0.668 30807 9667(0.0429)

153 C76H60N13O3S8Cd2 1684.65 0.71073 orthorhombic Pbcn 16.7361(14) 19.6712(17) 31.371(3) 90.00 10327.9(16) 4 0.23 × 0.15 × 0.07 1.083 0.615 68421 9604(0.0491)

0.96 0.0793, 0.2464

1.073 0.0958, 0.3002

1.072 0.0587, 0.1452

1.193 0.0674, 0.2173

1.060 0.0600, 0.1918

1.093 0.1072, 0.2599

1855245

1855243

1855244

1855248

1855246

1855247

R1 = ∑||Fo| − |Fc||/∑|Fo|. bwR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2.

a

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DOI: 10.1021/acs.inorgchem.9b01000 Inorg. Chem. 2019, 58, 8657−8664

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Inorganic Chemistry

Figure 1. (a) View of the MOF along the a-axis with octahedral Cd2+ nodes (red), organic TTF(py)4 (yellow), and TCNQ•− (blue) as framework components and guest TCNQ•− (green) occupying the channels. (b) c-Axis view of the entwined undulating layers of Cd-TTF(py)4 and chickenwire Cd-TCNQ. solution (5 mL) of TTF(py)4 (5 mg, 0.01 mmol) and H2TCNQ (20 mg, 0.02 mmol). The solution was left at room temperature for 7 days. Red block crystals of 2-Cd-N2 were obtained. Yield: 31%. Anal. Calcd for CdC33H22N7O4S4 (821.24 g/mol, wt %): C, 48.26; H, 2.70; N, 11.93. Found: C, 48.37; H, 2.63; N, 12.05%. IR (cm−1): 3364 (w), 2927 (w), 2171 (s), 2099 (s), 1651 (s), 1602 (s), 1571 (w), 1543 (w), 1504 (s), 1435 (w), 1418 (w), 1384 (m), 1326 (w), 1283 (w), 1253 (w), 1220 (w), 1174 (w), 1095 (w), 1065 (w), 1015 (w), 847 (w), 823 (m), 795 (w), 733 (w), 658 (m), 631 (w), 571 (w), 538 (m), 494 (m), 429 (w). [Cd(TTF(py) 4 )(TCNQ 2− ) 1/2 ](TCNQ •− ) 1/2 (NO 3 ) 1/2 ·CH 2 Cl 2 ·1/ 2CH3OH (2-Cd-Air). Black crystals of this compound were obtained from the same reagents as for 2-Cd-N2 under similar reaction conditions except under air for 7 days. Yield: 31%. Anal. Calcd for Cd2C77.5H44N7O3.5S8Cl2 (910.765 g/mol, wt %): C, 51.10; H, 2.43; N, 13.07. Found: C, 51.02; H, 2.36; N, 13.15%. IR (cm−1): 3379 (w), 2184 (m), 2027 (w), 1652 (m), 1592 (s), 1543 (w), 1505 (w), 1473 (w), 1439 (m), 1418 (m), 1336 (m), 1275 (m), 1217 (w), 1185 (s), 1100 (w), 1064 (w), 1011 (m), 848 (w), 827 (m), 798 (w), 732 (w), 699 (w), 657 (s), 628 (s), 610 (w), 574 (w), 537 (m), 489 (m), 440 (w). [Cd(TTF(py)4)(TCNQ•−)1/2](I3)(NO3)1/2·1/2(C6H12·CH3OH) (1Cd-I3). Crystals of 1-Cd were reacted with a cyclohexane solution of iodine (0.05 M) at room temperature for 2 days. Dark crystals were washed with cyclohexane to remove surface iodine. The iodine content was estimated by thermogravimetric analysis (TGA) (Figure S17) and elemental analyses. Anal. Calcd for 1-Cd-I 3 (CdC35.5H26I3N6.5O2S4) (1197.01 g/mol, wt %): C, 35.62; H, 2.18; N, 7.60. Found: C, 35.71; H, 2.09; N, 7.52%. Selected FT-IR data (cm−1): 3055 (w), 2921 (w), 2323 (w), 2282 (w), 2176 (m), 2147 (w), 2121 (w), 1981 (w), 1942 (w), 1692 (w), 1600 (s), 1567 (m), 1541 (w), 1502 (w), 1439 (w), 1417 (m), 1344 (s), 1320 (m), 1274 (m), 1215 (m), 1187 (w), 1171 (m), 1101 (w), 1063 (m), 1012 (m), 985 (w), 847 (m), 825 (m), 798 (m), 767 (w), 732 (m), 699 (w), 657 (s), 628 (s), 610 (w), 538 (m), 504 (w), 478 (w), 429 (w). [Cd(TTF(py)4)(TCNQ2−)1/2](TCNQ•−)1/2(NO3)1/2·2C6H12 (2-CdAir-I3). Crystals of 2-Cd-Air were soaked in a cyclohexane solution of iodine and allowed to stand at room temperature for 2 days. The crystals underwent solvent exchange in a crystal-to-crystal transformation. Anal. Calcd for CdC56H56N8.5O1.5S4 (1112.77 g/mol, wt %): C, 60.44; H, 5.07; N, 10.69. Found: C, 60.52; H, 5.01; N, 10.74%. IR (cm−1): 3410 (w), 2921 (w), 2845 (w), 2324 (w), 2184 (m), 1980 (w), 1720 (w), 1599 (s), 1542 (w), 1474 (w), 1439 (m), 1417 (m), 1336 (m), 1283 (m), 1216 (m), 1186 (s), 1113 (w), 1064 (m), 1011 (m), 895 (w), 847 (m), 824 (m), 798 (m), 731 (m), 700 (w), 656 (m), 628 (m), 610 (w), 537 (w), 478 (w), 444 (w).

backbone is comprised of TTF(py)4 and TCNQ bound to octahedral mononuclear metal centers (Zn or Cd). Although they share the same connectivity, the MOFs differ in their space groups (C2/m for 1-Zn, 1-Cd and 2-Cd-N2, C2/c for 1Cd-I3 and 2-Cd-Air, and Pbcn for 2-Cd-Air-I3) and pore contents (Table 1). Each framework can be described as a (4,4,6)-connected 3-nodal network with the Schläfli symbol of {42·84}{44·62}2{48·66·8}2, which corresponds to a “fsy”-type topology (Table S1 and Figure S1).40 Each TTF(py)4 and TCNQ ligand is coordinated to four metal centers whereby each metal node is coordinated to six nitrogen atoms: four from the pyridyl groups on TTF(py)4 and two from the nitrile groups of TCNQ (Figure S2a). The structure is formed from planar 2D chicken-wire Cd-TCNQ nets which run parallel to the ab plane and orthogonal undulating Cd-TTF(py)4 sheets parallel with the ac plane which give rise to paired TTF moieties, stacked in a bond-over-ring fashion, resulting in close S···S contacts (Figures 1 and S2b,c). Intermolecular π···π contacts are present with a C3···C3 distance of 3.42 Å between neighboring TTFs and a S···S distance of 3.61 Å (Figure S 2d). Little difference was observed between the 1-Cd and 1-Zn analogues apart from the slight difference of ionic radii of the metals (Figure S3). The framework contains channels along the a-axis housing linear chains of H-bonded TCNQ (CN···H of 1.982 Å, Figure 2a), as well as nitrate ions and solvent molecules in a smaller pore (Figures 2b and S4). The interstitial TCNQ molecules lie along the ac plane, orthogonal to both the Cd-

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RESULTS AND DISCUSSION Crystal Structures. Crystal structures obtained for 1-Zn, 1-Cd, 2-Cd-N2, 2-Cd-Air, 1-Cd-I3, and 2-Cd-Air-I3 revealed an isostructural series of 3D MOFs in which the framework

Figure 2. Part of the framework structure showing (a) a channel housing the intercalated TCNQ•− and (b) a cavity housing the nitrate ion. The nitrate is disordered on two positions. 8659

DOI: 10.1021/acs.inorgchem.9b01000 Inorg. Chem. 2019, 58, 8657−8664

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Inorganic Chemistry TCNQ (ab) and Cd-TTF(py)4 (bc) linkers. The nitrate is disordered over two positions and lies within a cavity shelled by eight pyridine moieties and two TCNQs. The solvents are heavily disordered and have been removed prior to the final refinement using the SQUEEZE function in PLATON.41,42 Given the number of redox states that TCNQ and TTF may possess, close inspection of the charges present in the frameworks was required. The asymmetric unit contains 1/2 Cd or Zn, both being 2+, one TTF(py)4 which we assume to be neutral, 1/4 TCNQ making up the framework backbone is assumed to be 1−, and 1/2 nitrate; for charge balance, the remaining 1/4 TCNQ molecules forming the chains will each be 1−. Thus, it is unlikely that a charged anion was omitted using the SQUEEZE tool. The central CC bond lengths of the TTF(py)4 moiety are 1.34 Å (1-Zn) and 1.36 Å (1-Cd) which are close to those reported for the neutral unit (1.33− 1.39 Å), indicating that TTF(py)4 can be regarded as neutral (Table S2, Figure S5).43 Intermolecular π···π and S···S interactions between the adjacent TTF moieties in the framework can be observed and the shortest S···S and C···C contacts are 3.63 and 3.44 Å (1-Zn) and 3.61 and 3.42 Å (1Cd), respectively (Figures S2 and S3). The reaction of H2TCNQ, TTF(py)4, Cd(NO3)2·4H2O and Li(CH3CO2) under anaerobic conditions leads to crystals of 2Cd-N2.30 Its structure reveals an isostructural framework to 1Cd where the nitrate occupies the same position, but the chain of intercalated TCNQ is absent and is replaced by disordered solvent molecules (Figure S6). The neutral TTF moiety also adopts a bent configuration with a dihedral angle of 21.6° folding at S1−S2 and a dihedral angle of 16.6° folding at S3− S4, which are slightly smaller than those angles found in 1-Cd (22.4 and 17.3°, respectively). The shortest C···C and S···S distances between two neighboring TTF cores are 3.47 and 3.63 Å, respectively. The central CC bond length of the TTF(py)4 core is 1.34 Å which again leads to the conclusion that TTF(py)4 is neutral. The absence of the charged TCNQ chains in the channels requires, for charge balance, the framework TCNQ to be in its 2− anionic state. This is consistent with the marked difference in color, i.e., dark red instead of blue-green (Figure S7). When the reaction was conducted in air, blue-green crystals (2-Cd-Air) were obtained which exhibited the same framework structure with the nitrate occluded within the cavities formed by the pyridine moieties. The asymmetric unit contains one Cd which is 2+, one TTF(py)4 in its neutral form, 1/2 TCNQ which is part of the framework (assumed to be 2−), 1/ 2 TCNQ which sits in the channel (assumed to be 1−), and 1/ 2 nitrate for charge balance (Figure S8). However, for these crystals, the interstitial TCNQ and solvents are highly disordered and could not be resolved. In an attempt to oxidize the TTF(py)4, the crystals of 1-Cd were soaked in a cyclohexane solution of I2 for 2 days at room temperature in air (Figure S9). Crystal structure determination of an iodine reacted crystal, denoted 1-Cd-I3, revealed the absence of the interstitial TCNQ•− chain, and in its place, the I3− ion was found (Figure 3, Figure S10). As a result, charge balance was retained, and the TTF(py)4 ligand was not oxidized. Soaking the blue-green crystals of 2-Cd-Air in a cyclohexane solution of I2 also led to dark crystals (2-Cd-Air-I3), and structure determination found little change in the framework. No iodine was found, and the guest solvents CH2Cl2 and CH3OH were replaced by cyclohexane molecules (Figure

Figure 3. Structure of 1-Cd before and after reaction with iodine to 1Cd-I3 showing the replacement of the intercalated TCNQ•− by I3−.

S11). Furthermore, the TCNQ in the channels was highly disordered, and its position could not be refined. Thus, the structure was treated with SQUEEZE within PLATON.40,41 The central CC bond length of the TTF(py)4 core is 1.34 Å which is close to that of 2-Cd-N2 (1.35 Å) and strongly suggests that TTF(py)4 is neutral (Table S2). Magnetic Properties. Due to the variable spin states of TCNQ (S = 0), TCNQ•− (S = 1/2), H2TCNQ (S = 0), and TCNQ2− (S = 0), as well as TTF(py)4 (S = 0), TTF(py)4+ (S = 1/2), and TTF(py)42+ (S = 0), magnetic susceptibility measurements and electron paramagnetic resonance (EPR) spectroscopy were employed to discern the charges in each material. Thus, the susceptibilities of the compounds were measured in an applied field of 1 kOe on cooling over the temperature range of 300−2 K (Figure 4).

Figure 4. Temperature dependence of the magnetic susceptibilities for 1-Zn, 1-Cd, and 1-Cd-I3 (circles) and their fits (solid lines; see text).

Owing to the large diamagnetic contribution and the limited knowledge of the temperature-independent paramagnetism of the different states of TCNQ, estimation of the exact susceptibilities was difficult. This was further complicated by the weakness of the signals obtained on a SQUID magnetometer and the care required to estimate the correction for the sample holder (which should sum to one constant for all compounds). However, without correcting for any of the above, a clear picture emerged where the susceptibilities of 1Zn and 1-Cd at room temperature were higher than that of 1Cd-I3. This is consistent with the higher magnetic moment of the interstitial TCNQ•− chains compared to I3−. Therefore, the temperature dependence of 1-Cd-I3 fits well with a simple Curie−Weiss model with a very small Weiss constant, which is consistent with distant noninteracting spins. In the case of 1Zn and 1-Cd, we consider two contributions: a Curie−Weiss 8660

DOI: 10.1021/acs.inorgchem.9b01000 Inorg. Chem. 2019, 58, 8657−8664

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Inorganic Chemistry model for the TCNQ•− forming the framework and either a Curie−Weiss or a one-dimensional Bonner−Fisher model for the intercalated TCNQ•− chains.44,45 Electron Paramagnetic Resonance. EPR spectra were collected on polycrystalline samples at 110 K (Figure 5). The

frameworks was expected to shed light on the charge of the TCNQ moieties (Figure S13). The spectrum of H2TCNQ does not have any peaks in the CN region but exhibits a strong, sharp peak at 2890 cm−1 due to the (C−H) mode. Li(TCNQ) exhibits three peaks at 2205, 2195, and 2167 cm−1, whereas TCNQ0 has a single sharp peak at 2223 cm−1. 1-Cd and 1-Zn have identical bands at 2210, 2176, 2150, and 2128 cm−1 which are due to both the framework and intercalated TCNQ•−. In contrast to H2TCNQ, 2-Cd-N2 displays two strong peaks at 2173 and 2100 cm−1 which suggests that the charge of the TCNQ moiety is different to TCNQ0, TCNQ•−, and TCNQ2−. 2-Cd-Air has a weak peak at 2185 cm−1, and similarly, the iodine-reacted 2-Cd-Air-I3 and 1-Cd-I3 also have a weak peak at 2177 cm−1 suggesting that these compounds have TCNQ•−, as seen in their crystal structures. Thus, we can conclude that the TCNQ forming the skeleton of the frameworks can be either TCNQ•− or TCNQ2−, while those in the channels are exclusively TCNQ•−, consistent with the magnetic studies. Solid-State Cyclic Voltammetry. Solid-state cyclic voltammetry (CV) was performed on 1-Zn and 1-Cd in {(nBu)4N}PF6/CH3CN electrolyte to investigate the potential for redox modulation of the frameworks (Figure 6 and S14). The

Figure 5. EPR spectra for 1-Zn, 1-Cd, 1-Cd-I3, 2-Cd-N2, 2-Cd-Air, and 2-Cd-Air-I3.

spectra of 1-Zn, 1-Cd, and 1-Cd-I3 are dominated by a sharp non-Lorentzian peak (Hpp ∼ 5 Oe) at g = 2.0013. The spectrum of 1-Zn is a superposition of three resonances: a sharp peak (Hpp = 3 Oe) at g = 2.0013 flanked symmetrically by two slightly broad peaks (Hpp = 11 Oe) at g = 2.0054 and 1.9972. The latter two resonances are shifted by ±0.0006 for 1Cd-I3 and ±0.0012 for 1-Cd. The width of the peaks and their g-values are indicative of organic radicals, suggesting that the spins originate principally from TCNQ•−. Given the different alignments of the two types of TCNQ species within the structures of 1-Zn and 1-Cd, and considering the powder averaging, the origin of the splitting may not be due to g-tensor anisotropy. Since the spectrum for 1-Cd-I3 is the same as those of 1-Zn and 1-Cd, the signals cannot be associated with the two different types of TCNQ moieties in the frameworks. A splitting is often observed in EPR spectra of complexes containing the TCNQ anion radical at low temperature, which may be interpreted as arising from spin exchange between mobile excitons.46 The intensities of the peaks for 2-Cd-Air and 2-Cd-Air-I3 are significantly lower than those of 1-Zn, 1Cd, and 1-Cd-I3 owing to the presence of the nonmagnetic TCNQ2− state. Due to the neutral TTF(py)4 and the absence of radical TCNQ•−, 2-Cd-N2 is EPR silent. Resonance Raman and Infrared Spectroscopies. The Raman spectra for 1-Zn, 1-Cd, 1-Cd-I3, Li(TCNQ), and TTF(py)4 were obtained on crystals or powders using two different excitation wavelengths (623.8 and 785.0 nm) to avoid fluorescence. That of H2TCNQ was dominated by a high fluorescence background. From Figure S12, it is clear that the peaks at 1660, 1699, and 1763 cm−1 in the framework spectra originate from the TCNQ•− vibrations, and the peak at 1601 cm−1 may be attributed to TTF(py)4. The vibrations of the triiodide ion, however, could not be detected in 1-Cd-I3. The infrared spectra were recorded in ATR mode to probe both the vibrational modes and low-energy electronic transitions of the framework materials. The low background in each case confirms that these compounds do not have any electronic component in this spectral region; thus, only the vibrational structures will be discussed. The spectra are characterized by a plethora of peaks originating from the different building blocks, namely, a comparison of the cyanide region in the spectra of the starting materials and those of the

Figure 6. Solid-state cyclic voltammograms for 1-Zn and 1-Cd in {(nBu)4N}PF6/CH3CN electrolyte obtained at 200 mV s−1. Arrow indicates the direction of the forward scan.

CV of 1-Zn exhibited four distinct redox processes. The reversible redox peak at 0.1 V and the quasi-reversible oxidation peak at 0.36 V can be assigned to the TTF/TTF•+ and TTF•+/TTF2+ redox couples, respectively. These values correspond well to the oxidation potentials of the ligand TTF(py)4 itself (E1/2= 0.18 and 0.36 V vs Fc/Fc+).24 The processes at −0.70 and −1.38 vs. Fc/Fc+ observed upon the cathodic sweep can be assigned to the reduction of the TCNQ ligand to its corresponding radical anion and dianion forms, respectively. At more cathodic potentials, a strong spike in current results in dissolution of the framework material, likely due to reduction of the pyridyl rings on TTF(py)4. CV also revealed a change in the current over multiple cycles which may be attributed to the quasi-reversible nature of the second oxidation and reduction processes (Figure S14a). Electro8661

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localization due to the significant distance between TCNQ and TTF molecules, as well as their orthogonal alignment.

chemical measurements of 1-Cd showed a reversible oxidation at E1/2 = −0.07 V and a quasi-reversible oxidation process at 0.34 V which correspond well with the oxidation of TTF to its radical cation and dication states, respectively. The third irreversible process was observed at 0.66 V and was tentatively assigned to oxidation of the electrochemically generated mixed-valence state TTF•+/TTF2+ formed from the second oxidation process to its fully oxidized species TTF2+. The reversible and quasi-reversible reduction processes at −0.66 and −1.76 V, respectively, are due to the sequential reduction of the TCNQ ligand to its radical anion and dianion states. The latter exhibits a sharp spike in current which suggests leaching of the ligand into solution and accounts for the drop in current over subsequent cycles (Figure S14b). If the cycle is performed over a narrower potential window, i.e., −0.25 to −1.25 V vs Fc/Fc+, then the cyclability of the material was better preserved; this strongly suggests decomposition of 1-Cd as a result of 2e− reduction of the TCNQ unit. UV−Vis−NIR Spectroscopy. Diffuse reflectance spectra collected on powdered samples of the MOFs and starting materials were obtained in the region 200−2000 nm (6−0.6 eV) (Figures 7 and S15). The spectra of the MOFs appear to

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CONCLUSION A new series of MOFs comprising diamagnetic Zn and Cd, neutral TTF(py)4, and radical TCNQ•− or TCNQ2− have been synthesized and fully characterized. The frameworks 1Zn, 1-Cd, 2-Cd-Air, and 2-Cd-Air-I3 were found to contain intercalated TCNQ•− within the channels. Solid-state cyclic voltammetry established several one-electron redox processes characteristic of the TTF units and TCNQ. Attempts to oxidize the TTF(py)4 ligand in 1-Cd with iodine led to a single-crystal to single-crystal transformation whereby each TCNQ•− present in the channels was replaced by I3−, but the TTF in the skeleton was still in its neutral state. Depending on the source of TCNQ as either TCNQ•− or diamagnetic TCNQ2−, magnetic measurements showed that the behaviors of frameworks were almost paramagnetic. Although these radical-containing MOFs exhibit low conductivities, these materials have potential applications in many fields such as dual-ion battery systems, catalysts, and photothermal therapy.47−49 Work at introducing paramagnetic metal ions is currently under investigation in our group.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b01000. Measurements, X-ray structure determination, TOPOS results, summary of selected structural parameters, additional pictures of 1-Cd, 1-Zn, 2-Cd-N2, 2-Cd-Air, 1-Cd-I3, and 2-Cd-Air-I3, TTF(py)4 conformations, the color of the crystals, evolution of the I2 reaction with 1Cd, Raman spectra, infrared spectra, figures of CV, UV− vis−NIR reflectance spectra, PXRD patterns, and TGA plots (PDF)

Figure 7. UV−vis−NIR reflectance spectra for 1-Zn, 1-Cd, 2-Cd-N2, 2-Cd-Air, 1-Cd-I3, 2-Cd-Air-I3, and TTF(py)4.

Accession Codes

be a superposition of those of their constituents. Since Zn(II) and Cd(II) do not have any electronic transitions in this range, the colors observed by eye originate from the organic components. Most spectra display a couple of vibrational overtones below 7000 cm−1. The spectrum of TCNQ0 (green) has characteristic bands at 16 200, 21 900, and 23 800 cm−1, while that for TCNQ•− (purple) has bands at 7400 and 13 500 cm−1. H2TCNQ (white with a trace of green) has bands at 15 600 and 24 200 cm−1. In contrast, TTF(py)4 is a deep red color, and its spectrum is characterized by bands at 8500, 11 500, and 17 400 cm−1. The framework compounds have similar spectra with peaks at 11 500 and 14 300 cm−1 which may be attributed to the TTF(py)4 ligand present in the framework backbone. The iodine reacted frameworks exhibit broad low energy bands, possibly due to the contribution of the additional tri-iodide in the structure. There is no spectral evidence for additional donor−acceptor or intervalence charge transfer in these solids that would indicate longer range charge transfer interactions. Electrical Conductivity. The room-temperature electrical conductivities measured by the two-probe method from pelletized samples of 1-Zn, 1-Cd, 1-Cd-I3, 2-Cd-Air, 2-CdAir-I3, and 2-Cd-N2 are 2.48 × 10−8, 2.63 × 10−8, 2.16 × 10−7, 4.77 × 10−8, 5.97 × 10−8, and 1.05 × 10−8 S/cm, respectively. The low conductivities are indicative of strong electronic

CCDC 1855243−1855248 contain 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 emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jian-Ping Ma: 0000-0002-5300-3307 Fei Yu: 0000-0001-9721-271X Mohamedally Kurmoo: 0000-0002-5205-8410 Jing-Lin Zuo: 0000-0003-1219-8926 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (2018YFA0306004), the National Natural Science Foundation of China (Nos. 21875099, 21801127, and 21631006), and the Australian Research Council’s Future 8662

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Fellowship funding scheme (FT170100283). M.K. was funded by the Centre National de la Recherche Scientifique (CNRS), France.

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