Synthesis, Crystal Structure, and Electroconducting Properties of a 1d Mixed-valence Cu(i)–cu(ii) Coordination Polymer with a Dicyclohexyl Dithiocarbamate Ligand

A new mixed-valence Cu(I)–Cu(II) 1D coordination polymer, [Cu


Description of Crystal Structure
Complex 1 was synthesized as black single crystals by the reaction of the acetonitrile/acetone solution of CuBr•S(CH3)2 with the CHCl3 solution of Cu(Cy2dtc)2 at room temperature (Scheme 1).The molar ratio of CuBr•S(CH3)2 to Cu(Cy2dtc)2 was kept to 1:1.Single-crystal X-ray diffraction analysis of the complex revealed the formation of the coordination polymer [Cu I 4Cu II Br4(Cy2dtc)2]n with an infinite 1D chain structure (Figure 1).The mononuclear Cu(II) units (Cu(Cy2dtc)2) were found to be linked by the tetranuclear Cu(I) units (Cu I 4Br4) consisting of four trigonal pyramidal Cu(I) ions and four bridging Br − ions.The selected bond distances are listed in Table 1.The Cu(I) ions interact with the slightly shorter Cu I -Cu I or Cu I -Cu II distances (Cu1-Cu2 = 2.7998(6) Å, Cu2-Cu3 = 2.7567(5) Å and Cu3-Cu3* = 2.7816(5) Å) between the nearest neighboring copper ions than the sum of the van der Waals radii for Cu-Cu (2.8 Å).The Cu1 ions in the mononuclear units have distorted square planar coordination geometries, in which the Cy2dtc − ligands are coordinated with the Cu1 ions, leading to four-membered chelate rings.The other Cu ions, Cu2 and Cu3, have trigonal pyramidal S1Br2 coordination geometries, wherein the nearest neighbor copper ions for Cu2 and Cu3 are located close to the Cu ions, resulting in a pseudo tetrahedral geometry for the Cu ions.The oxidation state of Cu complexes with dithiocarbamate ligands can be determined from the Cu-S distances.In the mononuclear Cu(Cy2dtc)2 units, the average Cu1-S distance was found to be 2.3152(9) Å, which is close to the typical Cu(II)-S distance for Cu(II)-dithiocarbamate complexes such as Cu II (Et2dtc)2 (av.2.312(1) Å), Cu II (i-Pr2dtc)2 (av.2.2884(7) Å), and Cu II (n-Bu2dtc)2 (av.2.308(1) Å) [30,31], but longer than the Cu(III)-S distances in Cu(III)-dithiocarbamate complexes such as [Cu(Me2dtc)2](ClO4) (2.234 Å av.) and [Cu(Et2dtc)2](FeCl4) (2.208 Å av.) [32].Hence, the structural data for Cu1-S distances indicate that the Cu1 ions are divalent.In order to confirm the oxidation states of the Cu2 and Cu3 ions, bond valence sum (BVS) calculations [33] were carried out.The estimated BVS values for the Cu2 and Cu3 ions were 0.99 and 1.00, respectively, pointing on the monovalent oxidation states of the Cu atoms.Based on the charge neutrality of the whole polymer, it was concluded that this complex is in the mixed-valence oxidation state, wherein the square-planar Cu1 ions are divalent, and the Cu2 and Cu3 ions with trigonal pyramidal coordination geometries are monovalent.The stoichiometry of the complex could be represented by the formula [Cu I 4Cu II Br4(Cy2dtc)2]n.Figure 2

Spectroscopic Properties of 1
Diffuse reflection spectra of Cu(Cy2dtc)2 and 1 are shown in Figure 3a.Prior to the measurements, the samples (0.01 mmol) were mixed with MgO powder (80 mg) and pelletized.The spectra obtained were further converted from reflectance (R) using the Kubelka-Munk function, as follows: 3b) [34].The spectrum of the mononuclear complex Cu(Cy2dtc)2 revealed the presence of intense absorption bands around 450 nm due to the ligand-to-metal charge transfer (LMCT) [35,36], and broad absorption bands around 630 nm due to the d-d transition in the Cu(II) ion.On the other hand, the spectrum of 1 showed broader adsorption bands that extended to the NIR region.For the calculation of the HOMO-LUMO gaps (Eg), Kubelka-Munk plots of (f(R)•E) 1/2 versus E were employed (Figure 3b) [37].Eg corresponds to the intersection point between the baseline along the energy axis and the line extrapolated from the linear part of the threshold.Thus, the Eg values for Cu(Cy2dtc)2 and 1 were found to be 1.47 and 1.06 eV, respectively.The estimated Eg value of the coordination polymer was lower than that of the mononuclear complexes, which could be explained by the formation of an energy band structure in the infinite chain.

Electroconducting Properties of 1
Complex impedance spectroscopy (CIS) gives the electrical conductivity properties of materials as a function of temperature and frequency.Impedance measurements were carried out with a powder-pressed pellet sample sandwiched between brass electrodes (diameter: 13 mm); the thickness of the pellet for sample 1 was 0.123 mm.The complex impedance (Z*) and modulus (M*) spectra over a wide range of frequencies and in the temperature range 340-380 K are shown in Figure 4a,b, respectively.These parameters are related to each other as follows [38][39][40][41][42]: where ω = 2πfr (fr is the resonance frequency), j = (−1) 1/2 , and C0 is the vacuum capacitance of the circuit elements.In addition, the complex modulus (M*) is given by the inverse of the complex dielectric permittivity (ε*).The complex modulus diagram shows two well-defined regions: a semicircle in the high-frequency region (right arc) due to the bulk (interior), and a part of the semicircle in the low-frequency region associated with the electrode interface.The diameter of the right arcs did not change with an increase in temperature, implying temperature-independent capacitance and conductivity relaxation, thus suggesting the impedance behavior in this system.Figure 4c shows the change in the imaginary part of the electrical modulus (M″) as a function of frequency at different temperatures.
Increase in the temperature leads to the shift of M″max toward the high-frequency region, pointing on the correlation between the motions of mobile charge.To distinguish the parameters of bulk conductivity from the total AC conductivity of 1, we used the equivalent circuit model of the ZView software [43], which comprises series resistances (R1 and R2), a constant phase element (CPE), and capacitance (C, as shown in the inset of Figure 4d. Figure 4d shows the temperature dependence of the bulk conductivities of 1, as calculated from resistances R1 with small time constants.It was observed that the bulk conductivity increases with an increase in temperature.The value of the bulk conductivity at room temperature (300 K), estimated by the extrapolation of the straight line in Figure 4d, was 7.83 × 10 −10 S/cm.Transport of charge carriers through the 1D chain plays the key role in the electrical conductivity of 1.
The activation energy (Ea) of bulk conductivity, calculated from the slope of the Arrhenius plots (lnσ versus 1000/T), was 0.36 eV, which is slightly smaller than half the value of the optical bandgap Eg.Generally, the activation energies Ea for intrinsic semiconductors would be half of the optical bandgaps Eg.The smaller activation energy in this coordination polymer might be due to the existence of the trap levels in the valence and/or conduction bands.

Materials
The mononuclear metal complex Cu(Cy2dtc)2 was prepared via a procedure similar to that described in the literature [30,31].The reagents were purchased from Tokyo Kasei Kogyo Co., Ltd.(Tokyo, Japan) and Aldrich Chemical Co., Inc. (St. Louis, MO, USA).All the chemicals were used without further purification.

X-Ray Structure Determination
XRD measurements of a single crystal of 1 were carried out using a Rigaku R-AXIS RAPID imaging plate area detector with graphite-monochromated Mo-Kα radiation.The data were collected at 100 K up to a maximum 2θ value of 54.9°.Among the 16,790 reflections that were collected, 4005 were unique (Rint = 0.0486); equivalent reflections were merged.The linear absorption coefficient m for Mo-Kα radiation was 77.986 cm −1 .The data were corrected to Lorentz and polarization effects.All calculations were carried out using the Crystal Structure crystallographic software package [44]; for refinement, the SHELXL-97 package [45] was used.The non-hydrogen atoms were refined anisotropically, whereas hydrogen atoms were refined using a riding model.The crystal parameters and experimental details of data collection are summarized in Table 2.

Physical Measurements
UV-Vis-NIR spectra were monitored on a U-4100 UV/VIS/NIR Spectrophotometer (HITACHI, Tokyo, Japan).The DC conductivities were investigated with an Ultra High Resistance Meter R8340 (ADVANTEST, Tokyo, Japan) on a pellet of the powder sample sandwiched by 13 mm diameter brass electrodes.The impedance and dielectric measurements were carried out on a pellet of the powder sample sandwiched by 13 mm diameter brass electrodes with a 6440B (Wayne Kerr Electronics, London, UK) Series Precision Component Analyzer in a frequency range 100 to 3 MHz.

Conclusions
In summary, new mixed-valence Cu(I)-Cu(II) coordination polymers with 1D infinite chain structures were synthesized and characterized by XRD analysis.The coordination polymer comprised mononuclear Cu(II) units and tetranuclear Cu(I) units of Cu I 4Br4, resulting in 1D Cu(II)•••I chains linked by the weak interaction of I − ions at the axial sites of the Cu(II) ions in the mononuclear units.

Figure 4 .
Figure 4. (a) Complex impedance Z′-Z″ plots of 1 at selected temperatures.The solid lines represent the fit result using the equivalent circuit shown in the inset of (d); (b) Complex modulus plots of 1 at selected temperatures; (c) Imaginary parts of the electric modulus as a function of frequency at selected temperatures; (d) Temperature dependence of bulk conductivity, estimated by fitting of the impedance data and the equivalent circuit model.

Table 2 .
Crystal Data and Structure Refinement Parameters for 1.