An Electrically Conductive Single-Component Donor–Acceptor–Donor

Department of Nanotechnology for Sustainable Energy, Graduate School of Science and Technology, Kansei Gakuin University, Gakuen, Sanda, Hyogo 669-133...
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An Electrically Conductive Single-Component Donor−Acceptor− Donor Aggregate with Hydrogen-Bonding Lattice Mikihiro Hayashi,*,† Kazuya Otsubo,† Mitsuhiko Maesato,† Tokutaro Komatsu,‡ Kunihisa Sugimoto,§ Akihiko Fujiwara,∥ and Hiroshi Kitagawa*,† †

Division of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa-Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan Division of Chemistry, Institute of Liberal Education, Nihon University School of Medicine, Tokyo 173-8610, Japan § Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan ∥ Department of Nanotechnology for Sustainable Energy, Graduate School of Science and Technology, Kansei Gakuin University, Gakuen, Sanda, Hyogo 669-1337, Japan ‡

S Supporting Information *

ABSTRACT: An electrically conductive D−A−D aggregate composed of a single component was first constructed by use of a protonated bimetal dithiolate (complex 1H2). The crystal structure of complex 1H2 has one-dimensional (1-D) π-stacking columns where the D and A moieties are placed in a segregated-stacking manner. In addition, these segregated-stacking 1-D columns are stabilized by hydrogen bonds. The result of a theoretical band calculation suggests that a conduction pathway forms along these 1-D columns. The transport property of complex 1H2 is semiconducting (Ea = 0.29 eV, ρrt = 9.1 × 104 Ω cm) at ambient pressure; however, the resistivity becomes much lower upon applying high pressure up to 8.8 GPa (Ea = 0.13 eV, ρrt = 6.2 × 10 Ω cm at 8.8 GPa). The pressure dependence of structural and optical changes indicates that the enhancement of conductivity is attributed to not only an increase of π−π overlapping but also a unique pressure-induced intramolecular charge transfer from D to A moieties in this D−A−D aggregate.

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and flat D−A−Ds (A−D−As) have the potential to uniformly construct a 1-D column because of their canceled dipole moments in which D and A moieties are placed in a segregatedstacking manner because of their canceled dipole moments (Figure 1). The π−π overlapping and resonance structure of

INTRODUCTION Both electron donor (D) and acceptor (A) molecules are important parts of molecular materials applied to energyconversion, mass-storage, and carrier-transportation systems, such as artificial photosynthesis, ferroelectric memories, and molecular conductors.1 In such systems, charge-transfer interactions between D and A molecules (Dδ+···Aδ− (δ ≤ 1)) play a significant role in electron delocalization or carrier generation. In this delocalization, the alignment of spatial layouts of D and A molecules is dominant, as well as the low energy gap between the highest occupied molecular orbital (HOMO) of D and the lowest unoccupied molecular orbital (LUMO) of A. With regard to their alignment, the segregated stacking is of advantage for transport properties, as seen in a TTF−TCNQ pair, because it affords a one-dimensional (1-D) column of Dδ+···Aδ− working as a conducting pathway.1g However, it is difficult to control the displacement of D and A in molecular solids, such as segregated or mixed stacking. Recently, π-conjugated D−A molecules, in which D and A moieties are directly connected by covalent bonds, have been investigated for a possible application to the solar-conversion system because of the simple modulation of their molecular electronic structure on the basis of the D-based HOMO and Abased LUMO.2 In addition, the aggregation of D−As affords 1D columns in which D and A moieties are arranged in a segregated or mixed manner.2f Among these D−As, symmetric © XXXX American Chemical Society

Figure 1. Construction of a 1-D column with segregated stacking of D and A moieties in D−A−Ds.

D−A−D (Dδ+−Aδ−−Dδ+) in this 1-D column are expected to make D−A−D aggregates electrically conductive without any doping; however, there have been few reports that study their transport properties.3 Recently, D−A−Ds have been widely recognized as materials that show unique optical properties and stable redox character, so that their applications, not only to organic devices but also to Received: October 21, 2016

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DOI: 10.1021/acs.inorgchem.6b02301 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry unimolecular nanoelectronics, have attracted much attention.4 In addition, inorganic−organic hybrids with D−A−D nature are potential materials to construct energy-conversion systems due to the small energy gap between the HOMO and LUMO.5 To develop the potential of D−A−Ds, it is significant to explore the transport properties of D−A−D aggregates in detail. Previously, we reported a low-energy-gap (1.19 eV) D− A−D based on a nickel bimetal complex bridged by a tetrathiooxalate (tto) ligand: tetrabutylammonium (μtetrathiooxalato)bis[(pyrazine-2,3-dithiolato)nickel(II)], (Bu4N)2(tto)[Ni(pdt)]2 (complex 1) (Figure 2a).6 In the

synthetic procedure for complex 1H2 is the method of slow diffusion of an acidic solution (acetone/water (4/1, v/v)) into a solution of complex 1 (DMSO/water (4:1, v/v)) (Scheme 1). Scheme 1. Synthesis Method of Complex 1H2

From this procedure, a fine black powder was obtained. The result of its elemental analysis suggests that this black powder was composed of complex 1H2 and some crystal solvents (see the Supporting Information). The result of thermogravimetric and differential thermal analyses (TG-DTA) of complex 1H2 revealed that it is thermally stable below 200 °C (Figure S3 in the Supporting Information). Electrochemical analysis of complex 1H2 in DMSO solution helps in understanding its electronic structure. As shown in the cyclic voltammograms in Figure 3, complex 1H2 shows one-electron reduction and

Figure 2. (a) Chemical structure of complex 1. (b) Protonation reaction of the pdt ligand. (c) Formation of 1-D columns of D−A−Ds stabilized by hydrogen-bonding interactions.

report, we demonstrated that the tto skeleton of complex 1 works as the A moiety because of its strong electron-accepting ability and the terminal dithiolene parts are recognized as two D moieties derived from the divalent Ni atom as well as because of the redox behavior of the pdt skeleton with electrondonating ability. In view of the geometrical aspects of complex 1, such as flatness and small size, complex 1 is recognized as a planar and compact D−A−D, so that complex 1 is a promising candidate for electrically conductive D−A−D aggregates. With regard to to the pdt ligand, it has the ability to catch and release protons owing to a pyrazine skeleton, as shown in Figure 2b. This protonation reaction induces a reconstruction of π conjugation in the pdt skeleton, so that a protonation of metal complexes with pdt ligands gives rise to the change in their electronic structures on the basis of the energetic difference between pdt and Hpdt.7 From this point of view, a protonated form of complex 1, (tto)[Ni(Hpdt)]2 (complex 1H2), has the potential to attain an electronic structure where the energies of the tto skeleton and terminal dithiolene moieties are close. In addition, it is known that a protonated pyrazine affords direction-aligned 1-D hydrogen-bonding chains in the solid.8 Therefore, complex 1H2 is expect to afford 1-D π-stacking columns with segregated stacking character stabilized by hydrogen-bonding interactions (Figure 2c). Moreover, such metal bis(dithiolene) complexes with extended π conjugation are potential candidates for singlecomponent conductors.9 In this work, we show the transport properties of complex 1H2 as a single-component conductor and consider the mechanism of conduction on the basis of its D−A−D nature.

Figure 3. Cyclic voltammograms of complexes 1 (black) and 1H2 (red) in 0.1 M nBu4NClO4/DMSO solution at 100 mV s−1. Ean and Eca indicate potentials where cathodic and anodic currents were observed, respectively.

multielectron oxidation reactions, as with complex 1. The theoretically calculated frontier orbital of complex 1H2 supports its D−A−D nature; the LUMO is localized at the tto skeleton. In contrast, the HOMO is delocalized through the Ni(Hpdt) parts (Figure 4). In view of the result of the theoretical

Figure 4. Calculated frontier orbitals of complex 1H2: (a) HOMO; (b) LUMO.

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RESULTS AND DISCUSSION Synthesis, Thermal Stability, Electrochemical Properties, and DFT Calculations. The protonation behavior of complex 1 was confirmed by tracing its spectral change using 1 H NMR and the absorption spectra of dimethyl sulfoxide (DMSO) solution (see the Experimental Section and Scheme S1 and Figures S1a and S2 in the Supporting Information). The

calculation, this electrochemical behavior in the reduction region corresponds to the ability to accept an electron into the tto skeleton on the basis of its π* orbital. In comparison with complex 1, complex 1H2 undergoes the first reduction reaction at more positive potential (E°′(1) = −0.71 eV in complex 1H2, −0.98 eV in complex 1 (Fc/Fc+)) because of its neutral charge. On the other hand, in the oxidation region, complexes 1 and B

DOI: 10.1021/acs.inorgchem.6b02301 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

the sum of the van der Waals radii of two N atoms (∼3.1 Å), so that a hydrogen-bonding interaction (N−H···N) between 1-D columns is suggested. In this crystal, it was difficult to identify the position of the proton because of its disordered nature. However, the shorter C−S distance in the pdt ligand of complex 1H2 (dS(1)−C(3) = 1.71(1) Å in Figure 5d) in comparison to that of complex 1 (1.743(5) Å) clearly shows the formation of a Hpdt ligand: a recombination of π conjugation in pdt ligands accompanied by protonation of the N atom (Figure 2b). These results indicate that 1-D πstacking columns are stabilized by hydrogen-bonding interactions. In view of the structure of the tto skeleton, the C−C bond distance (dC(1)−C(2) as shown in Figure 5d) is 1.45(1) Å. This value is intermediate between tthose of he free tto ligand (1.52 Å) and its two-electron-reduced form (1.36 Å), so that a D−A interaction between the tto skeleton and low-valence Ni dithiolate parts is identified in this crystal.12 This structural information indicates that the solid of complex 1H2 is composed of 1-D columns of D−A−Ds with segregatedstacking nature stabilized by hydrogen-bond interactions. The structure of complex 1H2 under ambient conditions was determined by Rietveld refinement of its powder X-ray diffraction pattern (PXRD) (Figure S4 in the Supporting Information). This refined structure was consistent with the result of the SCXRD analysis, and crystal solvents suggested by elemental analysis were contained (Figure S5 in the Supporting Information). In addition, the structural stability of complex 1H2 was confirmed by use of the result of PXRD measurements, as seen in Figure S6 in the Supporting Information. Optical Measurements To Investigate HydrogenBonding Lattice. To investigate the hydrogen-bonding interactions and electronic structure of complex 1H2, several spectroscopic measurements were performed. The infrared (IR) absorption spectrum of complex 1H2 displayed a weak peak around 3200 cm−1 corresponding to the vibration of N− H···N between Hpdt ligands (Figure S7a in the Supporting Information). X-ray photoelectron spectroscopy (XPS) measurements helped in the investigation of the binding state of N atoms in Hpdt ligands. As shown in Figure 6a, a doublet peak

1H2 both undergo precipitation and redissolution reactions on the electrode surface at the same potential (Eca = 0.21 V and Ean = −0.91 V ((vs Fc/Fc+)), as shown in Figure 3.10 This result indicates that the oxidization reaction of complex 1H2 is accompanied by its deprotonation process, so that its ability to donate an electron is affected by the interaction with the proton. Crystal Structure. By use of the above synthetic method, tiny single crystals (∼20 μm) of complex 1H2 were obtained. Figure 5 shows its crystal structure at 100 K analyzed by single-

Figure 5. (a) Crystal structure of complex 1H2. (b) Schematic image of a 1-D column along the a axis (left) and intramolecular interaction in this 1-D column (right). (c) Schematic image of molecular location in the bc plane (left) and intermolecular interaction in this plane (right). (d) Molecular structure of complex 1H2 on the basis of a disordered nature of the position of the proton.

Figure 6. (a) XPS spectrum of complex 1H2. (b) Calculated electron distribution of complex 1H2.

crystal X-ray diffraction (SCXRD) measurements.11 Along the a axis, 1-D columns are constructed of complex 1H2 because of its planar structure. In these columns, the intermolecular distance is 3.34(1) Å between Ni and S atoms (dNi−S; denoted by dashed lines in Figure 5b), so that an orbital overlapping between the pz orbital of the S atom and the dz2 orbital of the Ni atom is expected to occur. With regard to the intercolumn interaction, the distance between two N atoms (dN−N) is 2.83(1) Å, as shown in Figure 5c. This distance is shorter than

of complex 1H2 indicates two kinds of N atoms which form covalent and hydrogen bonds with a proton (N−H and N···H). The binding energy of N−H (400.2 eV) is higher than that of N···H (398.4 eV) because of the difference in electron density, which is supported by the calculated electron density distribution shown in Figure 6b. It is noteworthy that the XPS spectrum of complex 1 displays only one peak corresponding to the N element in the pdt parts (Figure S7b). These results indicate the formation of a hydrogenbonding network where the proton is trapped in a bottom of C

DOI: 10.1021/acs.inorgchem.6b02301 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry the symmetrical double-well potential (Figure S7c). Electronic absorption spectra of complexes 1 and 1H2 displayed intense peaks around 1.1 eV (Figure S8 in the Supporting Information); however, that of complex 1H2 showed peak broadening because of a kind of vibronic interaction based on the hydrogen-bonding lattice. A small value of the estimated optical band gap (0.65 eV) was attributed to π-stacked D−A− Ds with low energy-gap character that are connected by hydrogen-bonding interactions (Figure S9 in the Supporting Information). Band Calculations. To consider the conduction pathway of complex 1H2, a band structure based on its crystal structure was calculated. Figure 7a shows the result of the band calculation.

Figure 8. Temperature dependence of electrical conductivity of complex 1H2 using a compacted pellet at ambient pressure and 8.8 GPa. Conditions: pressure medium, Daphne 7373; pressure apparatus, cubic anvil cell.

calculated value (Ea = (1/2)Eg = 0.24 eV). Note that an expected conduction pathway of complex 1H2 is 1-D π-stacking columns of D−A−Ds. In view of this feature, applying pressure to complex 1H2 is expected to cause an increase in overlapping between π orbitals of D−A−Ds and a possible additional charge transfer in the 1-D columns. To investigate the effect of pressure, the pressure dependence of electrical resistivity of complex 1H2 was measured using a cubic anvil apparatus. As the pressure increased, its resistivity decreased (Figure S12 in the Supporting Information). At 8.8 GPa, the temperature dependence of its electrical conductivity was still semiconductive; however, its activation energy went down to less than half (0.13 eV) and the room-temperature resistivity was 6.2 × 10 Ω cm, which is 3 orders of magnitude lower than the value at ambient pressure (Figure 8 and Figure S11). In this experiment, it is difficult to identify a drastic change in temperature dependence of electrical conductivity, such as an insulator−metal transition. One possible cause is the effect of grain boundaries coming from the compaction pellet. High-Pressure Measurements of X-ray Diffraction and Raman Spectroscopy. To investigate the origin of the decrease in resistivity under high pressure, the crystal structure and electronic state of complex 1H2 were investigated by measuring the pressure dependence of the PXRD pattern and Raman spectrum of complex 1H2. With regard to its structural change, all peaks in the PXRD pattern shifted to higher angle with increasing pressure due to lattice shrinkage (Figure S13 in the Supporting Information). In particular, the shift of the 11− 2 peak was large. The d spacing corresponding to the 11−2 peak is consistent with an intermolecular distance in these 1-D columns, so that this large shift means an increase in π−π overlap. Resonance Raman spectra of complex 1H2 at several pressures are shown in Figure 9a. With application of pressure up to 4.5 GPa, peak 1 greatly shifts to higher energy and peak 2 gradually disappears. The calculated Raman scattering energies of complex 1H2 indicate that peaks 1 and 2 correspond to the frequency of the stretching vibration of the C−C bond and the bond-length alternation between C−S bonds (C−S ↔ CS) in the tto skeleton, respectively, as shown in Figure 9b and Figure S14 in the Supporting Information. These theoretical results indicate that the pressure-dependent behaviors of peaks 1 and 2 are caused by an increase in C−C bond order and an

Figure 7. (a) Calculated band structure of complex 1H2 (left) and the bandwidth values of conduction and valence bands along with Z → Γ and Γ → X directions (right). (b) Superposition of k space and crystal structure of complex 1H2.

As seen in this band structure, the dispersion of momentum becomes large from Γ to X points: 0.08 and 0.18 eV in conduction and valence bands. Superposing k space and the crystal structure of complex 1H2 clearly indicates that the direction of Γ → X corresponds to the 1-D columnar direction along the a axis, as displayed in Figure 7b. This result suggests that a conduction pathway is expected to form along the 1-D πstacking columns of D−A−Ds. In addition, the calculated density of states (DOSs) shows that the conduction band is mainly composed of p orbitals; in contrast, the valence band is constructed of both p and d orbitals (Figure S10 in the Supporting Information). This character is similar to the D− A−D-type frontier orbital of complex 1H2 (Figure 4). A calculated band gap (Eg = 0.48 eV) between these DOSs suggests that the temperature dependence of electrical conductivity of complex 1H2 is semiconductive. Electrical Conductivity Measurements. Figure 8 displays the temperature dependence of electrical conductivity of complex 1H2. In this experiment, the measurement sample is a compaction pellet of complex 1H2 because its single crystal is too small to measure. Under ambient conditions, the resistivity of complex 1H2 was 9.1 × 104 Ω cm, and its temperaturedependent change in electrical conductivity is semiconductive with 0.29 eV as an activation energy (Figure S11 in the Supporting Information). This value is comparable to the D

DOI: 10.1021/acs.inorgchem.6b02301 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 9. (a) Pressure dependence of the resonance Raman spectrum of complex 1H2 (left), with a He−Ne laser (632.8 nm) used as an exciting light, and schematic images of pressure-induced reduction of the tto skeleton caused by an additional intramolecular charge transfer (right). (b) Calculated modes corresponding to peaks 1 and 2 of Raman scattering of complex 1H2.

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CONCLUSION We achieved the construction of an electrically conductive D− A−D aggregate by use of a single-component metal complex: a protonated Ni bimetal dithiolate (complex 1H2). The electrical conduction of complex 1H2 is relatively high as a singlecomponent molecular solid, which is considered to originate from the D−A−D nature, as confirmed by its electrochemical behavior and geometrical character. As suggested by the result of a band calculation, this transport property is attributed to 1D π-stacking columns of D−A−D molecules that are stabilized by the hydrogen-bonding network. Under ambient conditions, the temperature dependence of electrical conduction of complex 1H2 is semiconductive (Ea = 0.29 eV) and its electrical resistivity is 9.1 × 104 Ω cm. By application of pressure, its activation energy becomes less than half (Ea = 0.13 eV) and its conductivity greatly increases (6.2 × 10 Ω cm). The increase in conductivity under high pressure is derived from not only an enhancement of intermolecular interaction between D−A−Ds but also an intramolecular charge transfer in D−A− Ds. This is the first observation of pressure-induced static intramolecular charge transfer in D−A−D aggregates. From our work, D−A−D aggregates with a hydrogen-bonding lattice are new candidates for single-component molecular conductors and potential materials applied not only to optical electronics but also to molecular-based electrical devices, which will open up a new aspect of D−A−Ds.

averaging of C−S bond order in the tto skeleton by applying pressure. This result strongly suggests that the tto skeleton was reduced under pressure by injecting an electron into its π* orbital composed of both bonding and antibonding orbitals on C−C and C−S, respectively (Figure 9a). This reduction is considered to result from an intramolecular charge transfer from the HOMO to the LUMO: in other words, from two D to A moieties of complex 1H2. The reduced form of the tto skeleton may induce an additional change in other molecular vibration modes. Figure 10d shows a Raman peak (peak 3) at

Figure 10. Pressure-dependence energy shift in Peaks 1 and 3.

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each pressure that corresponds to the frequency of the C−S stretching vibration in the tto skeleton (as shown in Figure S14). The pressure-dependent energy shift of peak 3 and that of peak 1 are shown in Figure 10 (see also Figure S15 in the Supporting Information). Peak 1 shows a monotonic increase of frequency with increasing pressure, as is often observed for molecular solids in high-pressure Raman spectra. In contrast, peak 3 is saturated above 3 GPa. This is considered to originate from a competing effect between molecular compression under pressure and pressure-induced intramolecular charge transfer between D and A moieties. These pressure studies on the change in geometric and electronic structures of complex 1H2 indicate that stabilization of the resonance form of D−A−D (Dδ+−Aδ−−Dδ+) and a large π−π overlap between D−A−Ds occur under high pressure, which is attributed to 1-D π-stacking columns of D−A−Ds stabilized by the hydrogen-bonding network. This pressure-dependent feature of complex 1H2 based on its D−A−D nature affords both an enhancement in electrical conductivity and a reduction in activation energy.

EXPERIMENTAL SECTION

Materials. Tetrabutylammonium (μ-tetrathiooxalato)bis[(pyrazine-2,3-dithiolato)nickel(II)], (Bu4N)2(tto)[Ni(pdt)]2 (complex 1), was prepared according to a previous report.6 In the procedure of protonation to complex 1, ethylenediamine dihydrochloride ((C2H6)(NH3)2Cl2; Wako Pure Chemical Industries, Ltd.) was used without any purification. Water was purified by ion exchange columns. Dimethyl sulfoxide (DMSO; Wako Pure Chemical Industries, Ltd.) and acetone (Wako Pure Chemical Industries, Ltd.) of commercially available grade were used. In the NMR measurements, DMSO-d6 (Wako Pure Chemical Industries, Ltd.) was used. In the electrochemical measurements, tetrabutylammonium perchlorate (TBAClO4, special prepared regent; Nacalai Tesque, Inc.), ferrocene (Wako Pure Chemical Industries, Ltd.), and DMSO (infinity Pure grade; Wako Pure Chemical Industries, Ltd.) were adopted. In optical measurements, potassium bromide (KBr; Wako Pure Chemical Industries, Ltd.), calcium fluoride (CaF2; Wako Pure Chemical Industries, Ltd.), and DMSO (for optical measurement grade) were used as dilution media. In the absorption measurement, bis(trifluoromethane)sulfonimide (TFSIH; Sigma-Aldrich Co.) was used as an added acidic agent. E

DOI: 10.1021/acs.inorgchem.6b02301 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Synthesis. Under an ambient atmosphere, complex 1 (315 mg, 0.30 mmol) was dissolved in 40 mL of a mixed solvent (DMSO/H2O (4/1, v/v)). A saturated solution of ethylenediamine dihydrochloride dissolved in 60 mL of mixed solvent (acetone/H2O (4/1, v/v)) was diffused into this prepared solution of complex 1. After a few weeks, fine black crystalline powder was collected by filtration and washed with water. After brief drying, complex 1H2 was obtained in 50% synthetic yield (86 mg, 0.15 mmol). 1H NMR (DMSO-d6, 600 MHz): δ 7.99 (4H, s), as shown in Figure S1b in the Supporting Information. Anal. Calcd for C10H6N4Ni2S8·0.5H2O·0.5(acetone): C, 23.25; H, 1.70; N, 9.43. Found: C, 23.33; H, 1.99; N, 9.39. TG−DTA Measurements. TG and DTA measurements of complex 1H 2 were performed with NETZSCH Japan TGDTA2000SA instrument: heating, 2 °C/min; room temperature to 500 °C; N2 flow. Optical Measurements. NMR spectroscopic measurement in DMSO-d6 was performed with a JEOL spectrometer (600 MHz). Absorption and diffuse reflectance spectroscopic measurements were performed with Jasco V-570 UV/vis spectrometers. Infrared (IR) absorption spectroscopic measurements were performed with a ThermoNicolet NEXUS 670 FT-IR spectrometer. X-ray photoemission spectroscopic (XPS) analysis was performed with an ESCA-3400 instrument (Shimadzu) with a monochromatic Mg Kα source (1253.6 eV). Resonance Raman scattering measurement was performed by using the laser Raman spectrophotometer (Jasco NRS1000) with microscopic apparatus in which a He−Ne laser (λ 632 nm) was used as the excitation light source. In the pressure experiments of resonance Raman spectroscopy, pressure was applied by use of a cubic anvil apparatus in which Daphne 7373 oil was used as a liquid pressure-transmitting media. The value of applied pressure was calibrated by measuring the energy of ruby fluorescence at each pressure. The absorption spectrum of complex 1 was measured by use of its DMSO solution in which the concentration was adjusted to the order of 10 μM. In order to observe the protonation behavior of complex 1, 1 μL of a DMSO solution of TFSIH (30 mM) was added to a DMSO solution of complex 1 (3 mL) until the amount of protons became 7 times as large as that of complex 1. IR and electronic absorption spectra of complex 1H2 were measured by using the KBr method. Diffuse reflectance spectra of complexes 1 and 1H2 were obtained by using each crystalline solid diluted in CaF2. XPS spectra of complex 1H2 were fitted by use of XPS peak fitting. A series of electrochemical measurements was carried out in a standard one-component cell, using 3 mm i.d. glassy carbon (BAS Inc.) as a working electrode, platinum wire (BAS Inc.) as a counter electrode, and an Ag/AgClO4 reference electrode (0.01 M AgClO4 in 0.1 M TBAP/acetonitrile, house made). As a supporting electrolyte tetrabutylammonium perchlorate was used. As an internal standard, ferrocene was added after each measurement. Electrochemical data were acquired with an ALS 650B voltammetric analyzer (BAS). The concentration of the DMSO solution of complex 1H2 was adjusted to 0.1 M. X-ray Diffraction Measurements. For the single crystal X-ray diffraction measurements (SCXRD), the diffraction data of a single crystal of complex 1H2 at 100 K were collected by use of a Bruker SMART APEX II CCD (charge-coupled device) area detector with Mo Kα radiation. Empirical absorption corrections using equivalent reflections and Lorentzian polarization correction were performed using the program Crystal Clear 4.0. The structures were refined against F2 using SHELXL-97. In this crystal, the electron density corresponding to crystal solvents was indicated; however, the temperature factor did not converge because of the highly disordered nature. Powder X-ray diffraction (PXRD) measurements of complex 1H2 under ambient conditions were performed by using a synchrotron X-ray source at the SPring-8 BL02B2 beamline. Refinement of the crystal structure of complex 1H2 under ambient conditions was conducted on the basis of the obtained PXRD data. The structure including crystal solvents was initially defined by using a simulated annealing program (Expo2014) and refined by using a Rietveld refinement program (Topas3). In addition, the pressure-dependent

change in X-ray diffraction pattern of complex 1H2 was measured by a synchrotron X-ray source at the SPring-8 BL10XU beamline. Pressure was applied by use of a diamond anvil apparatus. Daphne 7373 oil was used as a liquid pressure-transmitting media. The value of applied pressure was calibrated by measuring the energy of ruby fluorescence at each pressure. Transport Measurements. The electrical conductivity was measured by use of a compaction pellet of complex 1H2 employing four-probe and two-probe dc methods at ambient pressure and under applied pressure, respectively. Gold wires (10 μm in diameter) were attached to the pellet with carbon paste as electrodes. The temperature dependence of the electrical conductivity at ambient pressure was performed using a Quantum Design Physical Property Measurement System (PPMS). Under pressure, the temperature-dependent conductivity was measured using dc electrical conductivity measurement devices where hydrostatic pressure was applied by use of a cubic anvil apparatus (Rockgate Corp.). Daphne 7373 oil was used as a liquid pressure-transmitting media. Theoretical Calculations. For the density functional theory (DFT) calculations of the frontier orbital and electron density distribution of complex 1H2, the Becke three-parameter hybrid functional combined with Lee−Yang−Parr correlation functional (B3LYP) method was employed. As basis sets, 6-31+G(d,p) (for C, H, N, and S atoms) and Lanl2dz (for Ni atom) were used. This calculation was implemented with the Gaussian 09W program. The generalized gradient approximation (GGA) with Perdew−Burke− Ernzerhof (PBE) functional was employed to calculate the band structure of complex 1H2 on the basis of its crystal structure obtained by the result of SCXRD measurements. This band calculation was performed with the CASTEP program, which uses DFT with a plane wave basis set. The Raman-active modes of complex 1H2 on the basis of its molecular structure defined by SCXRD measurements were simulated by the DFT calculations with the B3LYP method. As basis sets, 6-31+G (d,p) (for C, H, N, and S atoms) and Lanl2dz (for Ni atom) were used. This calculation was implemented with the Gaussian 09W program.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02301. Crystallographic data (CIF) 1 H NMR spectra, electronic and infrared (IR) absorption spectra, TG analysis, powder X-ray diffraction pattern by use of the high-intensity X-ray source in SPring-8, refined crystal structure under ambient conditions, diffuse reflectance spectra, temperature dependence of electrical conductivities under ambient pressure and 8.8 GPa, pressure dependence of powder X-ray diffraction pattern, resonance Raman spectra, and DFT calculations (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for M.H.: [email protected]. *E-mail for H.K.: [email protected]. ORCID

Mikihiro Hayashi: 0000-0002-1386-847X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grants-in-Aid for Scientific Research (A) (Grants 26248019) and Grant-in-Aid for JSPS Fellows (No. 13J07305) from the Japan Society for the F

DOI: 10.1021/acs.inorgchem.6b02301 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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Promotion of Science (JSPS). Synchrotron XRD measurements were supported by the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos. 2011B4907, 2012B1558, 2014B1440, 2014A1406, 2015A1523, 2015A1535). The authors are grateful to Dr. Ohishi and Dr. Hirao of the Japan Synchrotron Radiation Research Institute (JASRI)/SPring-8 for the measurement of powder X-ray diffraction patterns under high pressure.

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DOI: 10.1021/acs.inorgchem.6b02301 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Conductors: A Sturdy Electronic Structure Generated by Bulky Substituents. Inorg. Chem. 2016, 55, 6036−6046. (10) Anjos, T.; Roberts-Bleming, S. J.; Charlton, A.; Robertson, N.; Mount, A. R.; Coles, S. J.; Hursthouse, M. B.; Kalaji, M.; Murphy, P. J. Nickel dithiolenes containing pendant thiophene units: precursors to dithiolene-polythiophene hybrid materials. J. Mater. Chem. 2008, 18, 475−483. (11) Crystal data of 1H2 @ 100 K: C10N4Ni2S8; formula weight 550.04, triclinic, P1̅, a = 3.950(3) Å, b = 10.125(7) Å, c = 13.144(9) Å, α = 101.311(8)°, β = 90.999(8)°, γ = 94.437(8)°, V = 513.6(6) Å3, Z = 1, no. of reflections measured 2140 (no. of unique reflections 1502), R1 (I > 2.00σ(I)) = 0.0651, wR2 = 0.2305 (all data), GOF = 1.053. These data have been deposited at The Cambridge Crystallographic Data Center as publication number CCDC 1455718. (12) (a) Lund, H.; Hoyer, E.; Hazell, R. G. Tetrathiooxalate. Electrochemical Preparation and X-Ray Structure Determination of a Tetrathiooxalate. Acta Chem. Scand. 1982, 36B, 207−209. (b) Holloway, G. A.; Rauchfuss, T. B. Direct Observation of Ligand-Centered Redox in Cp*2Rh2(μ-C2S4)Clx (x = 2, 0). Inorg. Chem. 1999, 38, 3018−3019.

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DOI: 10.1021/acs.inorgchem.6b02301 Inorg. Chem. XXXX, XXX, XXX−XXX