Copper ion salts of arylthiotetrathiafulvalenes: synthesis, structure diversity and magnetic properties

The combination of CuBr2 and arylthio-substituted tetrathiafulvalene derivatives (1–7) results in a series of charge-transfer (CT) complexes. Crystallographic studies indicate that the anions in the complexes, which are derived from CuBr2, show diverse configurations including linear [Cu(I)Br2]–, tetrahedral [Cu(II)Br4]2–, planar [Cu(II)2Br6]2–, and coexistence of planar [Cu(II)Br4]2– and tetrahedral [Cu(II)Br3]– ions. On the other hand, the TTFs show either radical cation or dication states that depend on their redox potentials. The central TTF framework on most of TTFs is nearly planar despite the charge on them, whereas the two dithiole rings on molecule 4 in complex 4·CuBr4 are significantly twisted with a dihedral angle of 38.3°. The magnetic properties of the complexes were elucidated. The temperature-dependent magnetic susceptibility of complex 5·Cu2Br6 shows the singlet–triplet transition with coupling constant J = −248 K, and that of 3·(CuBr4)0.5·CuBr3·THF shows the abrupt change at 270 K caused by the modulation of intermolecular interactions. The thermo variation of magnetic susceptibility for the other complexes follows the Curie–Weiss law, indicating the weak antiferromagnetic interaction at low temperature.


Synthesis
The donor molecules (1-7, Scheme 1) were synthesized according to our previous report [37,38], and their electrochemical activities as well as the crystal structures have been fully elucidated [38,39]. Since the redox potentials of TTFs are very important in the formation of complexes, particularly on the charge-transfer degree, the first (E 1/2 1 ) and the second (E 1/2 2 ) redox potentials of 1-7 are summarized in Table 1. As reported in the following section, TTFs 1-5 have the E 1/2 2 < 0.90 V and form the dicationic salts by reaction with CuBr 2 . On the contrary, the E 1/2 2 values of 6 and 7 are higher than 0.90 V, and these two donor molecules form the radical cation salts by reaction with CuBr 2 . The reaction of 1-7 with CuBr 2 was performed in the mixed solvent of tetrahydrofuran-acetonitrile (THF-CH 3 CN; v/v, 1:1) at room temperature. In the low concentration (<10 −4 mol L −1 ), a dark green solution was formed, indicating the oxidation of 1-7 by CuBr 2 . When the concentration of the reaction system was increased to higher than 10 −3 mol L −1 , TTFs 1-7 afforded the ionic salts showing the same phase as those of the corresponding single crystalline ones. The single crystalline salts were obtained by a conventional two-phase diffusion method. In a typical procedure, the CuBr 2 solution in CH 3 CN and the solution of TTFs in THF were placed in two different chambers of an H-shape cell, respectively. After several weeks, black single crystalline salts were formed. The compositions of the salts were determined by X-ray single crystal diffraction analyses, as summarized in Table 2.

Crystal structure
The single crystals for all of the present salts were suitable for the X-ray single crystal diffraction analyses. Herein, we report the crystal structures of the typical salts ( Figures 1-5), and those of the others are supplied in Supporting Information File 1. As mentioned above, the molecular geometries of Ar-S-TTF are sensitive to the environmental variations, especially the 28 mg (0.03 mmol) 7·CuBr 2 16 mg (47%) black cuboid a TTFs were dissolved in 4 mL of THF, and CuBr 2 (100 mg, dissolved in 4 mL of CH 3 CN) was applied in the synthesis. b The compositions were determined by X-ray single crystal diffraction analyses. c See the photographs of the crystals in Figure S1 in Supporting Information File 1. guest components are included in their solid-state matrix. Besides, the bond lengths and the conformation of the central TTF core are sensitive to the charge variation. The charge on TTFs can be estimated according to an empirical formula suggested by Day et al. [55], that is The calculated δ values and the conformation of TTFs 1-7 in neutral state and salts are summarized in Table 3. These results indicate that 1-5 have the charge of +2, whereas 6 and 7 are radical cations. The central TTF cores on the neutral TTFs show various conformations including chair, planar, and boat confor-mations. However, the central TTF cores of TTFs in the present salts are planar except that of 4, where the two dithiole rings are twisted with a dihedral angle of 38.3°. In the following, we will discuss the crystal structures of these salts, including the molecular geometry of TTFs, the structure of anions, and the packing motifs.
1·CuBr 4 crystallizes in the orthorhombic Pbcn space group with half of molecule 1 and half of CuBr 4 crystallographically unique (Figure 1a). The central TTF core on 1 is nearly planar, which is different from the chair conformation in the neutral state. Moreover, the spatial alignment of peripheral phenyls is modulated (see Figure S2 in Supporting Information File 1). The calculated δ value of 1 is 0.608 in the salt, indicating it has the charge of +2 according to the criteria proposed by Day [55].  (Figure 1b). Donor molecules form a zig-zag chain alignment along the c-axis (Figure 1c), and the [CuBr 4 ] 2− ion locate at the cavity formed by 1. The spin exchange interaction between Cu(II) on [CuBr 4 ] 2− ions would take place as mediated by the π-orbitals of 1. The crystal structure of 2·CuBr 4 is reminiscent to that of 1·CuBr 4 as shown in Figure S3 and Figure S4 in Supporting Information File 1.
The crystal structure of 3·(CuBr 4 ) 0.5 ·CuBr 3 ·THF at room temperature is shown in Figure 2. This salt crystallizes in the triclinic P−1 space group, and the asymmetric unit contains one molecule 3, half of CuBr 4 , one CuBr 3 , and one THF. The central TTF core of 3 takes the planar conformation similar to its neutral state, whereas the spatial alignment of the 4-tolyl groups is altered ( Figure S6 in     Figure 4b. There is no atomic close contact between the organic and inorganic components in a stacking column, whereas one S···S contact (3.57 Å) is observed between the neighbouring molecules of 5 along their longitudinal axis. 6·CuBr 2 ·CH 3 CN crystallizes in the triclinic P−1 space group, and the asymmetric unit contains half of molecule 6, half of CuBr 2 , and half of a CH 3 CN solvent molecule (Figure 5a). The central TTF core of 6 has a pseudo-planar conformation, and the calculated δ value of 6 in the salt is 0.744, indicating that 6 is in the radical cation form. The inorganic component CuBr 2 is linear, and the Cu-Br bond length is 2.54 Å, which is close to that of a typical Cu(I)-Br bond [51][52][53][56][57][58], indicating that CuBr 2 has the charge of −1. The organic and inorganic components form the mixed stacks along the a-axis as shown in Figure 5b. Moreover, the peripheral aryl groups form the cavity to accommodate a CH 3 CN solvent molecule, thus a supramolecular framework is formed in this salt. In the salt of 7 with CuBr 2 , molecule 7 is also oxidized to the radical cation form and the counter anion is [CuBr 2 ] − as shown in Supporting Information File 1 ( Figure S11).

Magnetic properties
The temperature-dependent magnetic susceptibilities of the salts were measured on the polycrystalline samples. In the salts of 1-5, the spin susceptibility comes from Cu(II) (S = 1/2), because the TTFs in these salts are oxidized to the dication form and the inorganic components contain Cu(II). On the other hand, spin susceptibility on the salts of 6 and 7 originates from the radical cation, as the inorganic components in these salts contain Cu(I). Figure 6 depicts the temperature-dependent magnetic susceptibilities of the representative salts.
1·CuBr 4 , 2·CuBr 4 , 4·CuBr 4 , and 7·CuBr 2 show the similar magnetic properties. The temperature dependence of the magnetic susceptibility follows the Curie-Weiss law, and the spins in these salts show the antiferromagnetic interaction at low temperature. The antiferromagnetic interactions of Cu(II) in 1·CuBr 4 , 2·CuBr 4 , and 4·CuBr 4 arise from the d-π-d pathway, as discussed in the crystal structure section. On the other hand, the antiferromagnetic interaction of radical cations in 7·CuBr 2 could be due to the π-π interactions, because the neighbouring donor molecules have a S···S contact (3.30 Å) along the a-axis. Figure 6a shows the magnetic susceptibility of 1·CuBr 4 by varying temperature, and the best-fitting parameters for this salt are C = 0.382 emu K mol −1 and θ = −5.4 K.
In the case of 3·(CuBr 4 ) 0.5 ·CuBr 3 ·THF, the temperature dependence of magnetic susceptibility shows the monotonic decrement upon cooling in the temperature range of 300-270 K. Furthermore, an abrupt jump of the magnetic susceptibility is observed at 270 K (see Figure 6b). This abrupt jump could be attributed to the variation of intermolecular interactions as discussed in the crystal structure section. Below 270 K, the temperature dependence of magnetic susceptibility follows the Curie-Weiss law with C = 0.379 emu K mol −1 and θ = −4.6 K.
As mentioned in the crystal structure section, two Cu(II) atoms in 5·Cu 2 Br 6 are connected by two bromine bridges, which result in the strong spin interaction between Cu(II) atoms. The temperature-dependent magnetic susceptibility of 5·Cu 2 Br 6 is shown in Figure 6c, which can be well-fitted by the singlet-triplet model [60]. The best-fitting parameters are: J = −243 K which is consistent with the significant magnetic susceptibility dropping at 245 K, f = 0.993, and A = 3.21 × 10 −4 emu mol −1 . The latter two terms reflect the non-zero magnetic susceptibility originated from the crystal defects (the Curie term) and the residue paramagnetic impurities.

Conclusion
We have reported the synthesis, structures, and magnetic properties of the copper ion salts of Ar-S-TTFs 1-7. The present salts show a wide variety of solid state structures and magnetic
The electrochemical properties of 1-7 were recorded on a RST 5000 electrochemical workstation at a scan rate of 50 mV s −1 , with glassy carbon discs as the working electrode, Pt wire as the counter electrode, and a SCE electrode as the reference electrode. The concentration was 5 × 10 −4 mol L −1 in CH 2 Cl 2 , and the supporting electrolyte was (n-Bu) 4 N·PF 6 (0.1 mol L −1 ). The measurement was performed at 20 °C after bubbling the solution with N 2 gas for 15 min.
The X-ray diffraction measurement was carried out on Super-Nova (Agilent) type diffractometer. The crystal structure was solved by a direct method SIR2004 [61] and refined by a fullmatrix least-squares method on F 2 by means of SHELXL-97 [62]. The X-ray powder diffraction (XRPD) pattern was recorded on X'Pert PRO (PANalytical). The temperature dependence of the magnetic susceptibility was measured on a SQUID magnetometer of Quantum Design MPMS-XL applying a magnetic field of 1 kOe. The data were corrected for core diamagnetism estimated from the sum of the Pascal constants [63].