MX-Chain Compounds with ReO4 Counterions: Exploration of the

Mar 15, 2018 - Synopsis. A series of isostructural mixed-valence MX-chain compounds with ReO4−, [M(en)2][M(en)2X2](ReO4)4 (X = Br for M = Pd and X =...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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MX-Chain Compounds with ReO4 Counterions: Exploration of the Robin−Day Class I−II Boundary Shohei Kumagai,†,⊥ Shinya Takaishi,*,† Mengqi Gao,† Hiroaki Iguchi,*,† Brian K. Breedlove,† and Masahiro Yamashita*,†,‡,§ †

Department of Chemistry, Graduate School of Science, Tohoku University, 6-3 Aza-Aoba, Aramaki, Aoba-ku, Sendai, 980-8578 Japan ‡ Advanced Institute of Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, 980-8577 Japan § School of Materials Science and Engineering, Nankai University, Tianjin, 300350 China S Supporting Information *

ABSTRACT: MX chains have been widely studied as a 1D mixed-valence system. Although there have been a large number of studies on the boundary between class II and III materials of the Robin−Day classification, there are few studies of compounds at the boundary between classes I and II. In this study, we synthesized a series of Pt- and Pd- MX-chain compounds with perrhenate counterions, [M(en)2][M(en)2X2](ReO4)4 (X = Br for M = Pd and X = Cl, Br, and I for M = Pt). All compounds were isostructural, and the metal−metal distances within the chain exceed 6 Å, which is the longest among MX-chain compounds thus far reported. For [Pt(en)2][Pt(en)2Cl2](ReO4)4 (PtCl), an intervalence charge transfer (IVCT) transition was observed in the UV region at 335 nm (3.7 eV), which is the shortest wavelength for the MX-chain compounds thus far reported, indicating that PtCl is the closest to the Robin−Day class I limit.

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INTRODUCTION

Quasi-one-dimensional halogen-bridged Ni, Pd, and Pt compounds (MX chains) have interesting chemical and physical properties, such as an intense and dichroic chargetransfer band,1 progressive overtones in the resonance Raman spectra,2 midgap absorptions attributable to solitons and polarons,3 gigantic third-order nonlinear optical susceptibility,4 a spin Peierls transition,5 and so forth. These compounds are isolated one-dimensional (1D) electronic systems with −M− X−M−X− 1D linear chain structures composed of the dz2 orbitals of the metal ions (M) and the pz orbitals of the bridging halide ions (X). These MX chains are in a PeierlsHubbard system, where the electron−lattice interaction (S), the transfer integral (T), and the on-site and nearest-neighbor Coulomb repulsion energies (U and V, respectively) strongly compete with each other.6 All Pt and Pd compounds form MII− MIV mixed valence (MV) states where the bridging halide ions are periodically displaced from the midpoints between two neighboring metal ions as a consequence of a large S value.7 This Peierls distorted 1D structure is in the charge-densitywave (CDW) state, and it is represented as −X···M3−ρ···X− M3+ρ−X··· (0 < ρ < 1), where ρ is the degree of charge disproportionation. In this article, however, we represent MII and MIV for M3−ρ and M3+ρ for clarity, respectively. This state corresponds to class II of the Robin−Day classification (Figure 1b).8 In these compounds, a characteristic intervalence charge transfer (IVCT) transition from MII to MIV species is observed. © XXXX American Chemical Society

Figure 1. Schematic representation of classes I to III of MX-chain compounds.

On the other hand, the Ni complexes form averaged valence (AV) Ni3+ states where the bridging halide ions are located at the midpoints between the two neighboring Ni atoms, represented as −X−Ni3+−X−Ni3+−X−, due to a large U value.9 These Ni compounds are Robin−Day class III complexes (Figure 1a).8 To date, there have been a large number of studies on the boundary between class II and III compounds in which the metal−metal distance (L(M···M)) in palladium and platinum compounds has been shortened. We have recently synthesized Received: December 7, 2017

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DOI: 10.1021/acs.inorgchem.7b03051 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Crystallographic Data for PdBr, PtCl, PtBr, and PtI empirical formula formula weight crystal system space group crystal size (mm3) a (Å) b (Å) c (Å) α β γ(Å) V (Å3) Z T (K) μ (mm−1) ρcalcd F(000) GOF on F2 R1, wR2 [I > 2σ] R1, wR2 [all data] reflns. measured CCDC no.

PdBr

PtCl

PtBr

PtI

C4H16BrN4O8PdRe2 806.92 triclinic P−1 0.13 × 0.06 × 0.03 6.0474(3) 7.5700(4) 8.7795(5) 79.8085(18) 72.4445(18) 86.0686(18) 377.10(4) 1 293(2) 19.876 3.553 363 1.127 0.0195, 0.0451 0.0221, 0.0457 1312 1588082

C4H16ClN4O8PtRe2 851.15 triclinic P−1 0.20 × 0.02 × 0.01 6.1189(5) 7.5103(6) 8.7875(7) 79.3410(10) 72.094(2) 85.393(2) 377.52(5) 1 250(2) 25.448 3.744 377 1.087 0.0298, 0.0839 0.0385, 0.0891 2963 1588084

C4H16BrN4O8PtRe2 895.61 triclinic P−1 0.14 × 0.09 × 0.01 6.0602(3) 7.5744(4) 8.7987(5) 79.802(2) 72.3990(10) 86.422(2) 378.88(4) 1 296(2) 27.818 3.925 395 1.101 0.0222, 0.0558 0.0250, 0.0581 1308 1588085

C4H16IN4O8PtRe2 942.60 triclinic P−1 0.21 × 0.03 × 0.02 6.0523(12) 7.6407(12) 8.7879(19) 80.011(4) 72.800(4) 87.121(3) 382.33(13) 1 293(2) 26.973 4.094 413 1.114 0.0198, 0.0401 0.0242, 0.0410 1329 1588083

several compounds exhibiting Pd3+ AV states as well as MV-toAV phase transitions.10 However, the boundary between class I (discrete limit) and II has been rarely studied. In this research, we explored the class I−II boundary by elongating L(M···M) in Pd and Pt compounds. MX-chain compounds have hydrogen bonds between amino-protons of the in-plane ligands (L) and counterions (Y) to stabilize the 1D chain structure. To date, more than 300 derivatives, which possess halide, perchlorate, alkylsulfonate, alkylphosphate, sulfate, and so forth, have been synthesized. In this study, we used the perrhenate ion (ReO4−) instead of the perchlorate ion (ClO4−), which also has tetrahedral geometry, because the Re−O bond distance (≈ 1.7 Å) is much longer than the Cl−O bond distance (≈ 1.4 Å). In this article, we report the syntheses, structure, and physical properties of palladium (X = Br) and platinum (X = Cl, Br, and I) MX chains with Y = ReO4−.

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electron microscope. Time dependence density functional theory (TD-DFT) calculations were performed using Gaussian 09 software.12 The local geometry of PtIV(en)2Cl2 was optimized under the D2 point group constraint. [PdIV(en)2Br2][PdII(en)2][PdIV(en)2Br2] trimer units and platinum analogues with linear structures were built based on the optimized structure using Gaussview 5.0.9 software. PtIV−Cl, PtIV−Br, PtIV−I, and PdIV−Br distances were fixed to 2.32, 2.48, 2.71, and 2.47 Å, respectively, which were the typical MIV−X distances. The LANL2DZ13 basis set was used. Syntheses. [Pd(en)2][Pd(en)2Br2](ReO4)4 (PdBr). [Pd(en)2]Br2 (40 mg, 0.103 mmol) and NH4ReO4 (10 mg; 0.037 mmol) were dissolved in 2.0 mL of water. Into the solution was diffused Br2 gas from a MeOH solution, affording shiny-brown plates within 5 min. Br2 diffusion was allowed to continue overnight, yielding bronze plateshape crystals. They were collected by suction filtration, washed with MeOH, and dried in air. The yield was 5−15 mg, typically. Anal. calcd for C4H16BrN4O8PdRe2: C, 5.95; H, 2.00; N, 6.94. Found: C, 6.24; H, 1.91; N, 6.97. [Pt(en)2][Pt(en)2Cl2](ReO4)4 (PtCl). [Pt(en)2][Pt(en)2Cl2](ClO4)414 (100 mg, 0.091 mmol) was dissolved in 5 mL of water. Several drops of conc HReO4 (90 wt %) were added dropwise. After the mixture was allowed to stand overnight, a pale-yellow precipitate was collected. The solid was dissolved in hot water and slowly cooled to room temperature, affording pale-yellow needle-shaped crystals. The crystals were collected by filtration, washed with MeOH, and dried in air, yielding 130 mg of the title compound (0.076 mmol; 84%). Anal. calcd for C4H16ClN4O8PtRe2: C, 5.64; H, 1.89; N, 6.58. Found: C, 5.84; H, 2.10; N, 6.71. [Pt(en)2][Pt(en)2Br2](ReO4)4 (PtBr). [Pt(en)2]Br2 (43 mg, 0.099 mmol) and [PtBr2(en)2]Br2 (63 mg, 0.099 mmol) were dissolved in 7.5 mL of water to afford a yellow solution. Solid NH4ReO4 (26 mg; 0.097 mmol) was added to the solution, and the mixture was heated until all solids dissolved. The solution was slowly cooled to room temperature and allowed to stand overnight. Orange plate-shape crystals were collected, washed with MeOH, and dried in air, yielding 30 mg of the title compound (0.017 mmol; 68%). Anal. calcd for C4H16BrN4O8PtRe2: C, 5.36; H, 1.80; N, 6.26. Found: C, 5.07; H, 1.74; N, 6.25. [Pt(en)2][Pt(en)2I2](ReO4)4 (PtI). To 3.4 mL of a hot aqueous solution of [Pt(en)2][Pt(en)2I2](ClO4)414 (22 mg, 0.017 mmol) was added NH4ReO4 (28 mg; 0.104 mmol). Water (6.6 mL) was added,

EXPERIMENTAL SECTION

Instrumental Procedures. Single crystal X-ray diffraction (SXRD) measurements were carried out on a Bruker APEX II diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.7107 Å) equipped with a PHOTON1000 CMOS detector. Crystals were attached to a glass capillary using Apiezon H grease. The initial structures were determined by using direct methods, and the refinements were performed using least-squares methods with the SHELX program11 within the Yadokari-XG 2009 interface.11 UV− Vis−NIR diffuse reflectance spectra were recorded at room temperature on a Shimadzu UV-3100 spectrometer equipped with an integrating sphere. Powder BaSO4 was used as a 100% reflectance standard. Samples were diluted with BaSO4. Thermogravimetric/ differential thermal analysis (TG/DTA) measurements were performed on a Shimadzu DTG-60H. Measurements were carried out in the temperature range of 30−550 °C at a scan rate of 5 K min−1 under a nitrogen atmosphere. Differential scanning calorimetry (DSC) measurements were performed on a Shimadzu DSC-60 Plus at a scan rate of 20 K min−1 for all measurements. α-Al2O3 was used as the reference material for both measurements. Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy measurements were performed using a Hitachi S-4300 scanning B

DOI: 10.1021/acs.inorgchem.7b03051 Inorg. Chem. XXXX, XXX, XXX−XXX

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is longer than the sum of the ionic radii of the Pd and Br ions.15 The neighboring Pd−Pd distance in the chain (L(Pd···Pd)) was 6.0474(3) Å, which is the longest among bromo-bridged palladium compounds so far reported. In Figure 2c,d, N−H···O hydrogen bonds formed between the en ligand and the ReO4− counterion are depicted as broken lines. O1, O2, and O3 atoms in the ReO4− moiety contribute to hydrogen bonds with one, two, and two amino-protons of the ethylenediamine ligand, respectively. Moreover, we obtained platinum analogues, [Pt(en)2][Pt(en)2Cl2](ReO4)4 (PtCl), [Pt(en)2][Pt(en)2Br2] (ReO4)4 (PtBr), and [Pt(en)2][Pt(en)2I2](ReO4)4 (PtI) and determined their crystal structures, which are shown in the Supporting Information (Figure S1 to S3). From Table 1, we found that they were isostructural. Selected distances in the present compounds together with those of MX chains with Y = ClO4− are summarized in Table 2. Usually, L(M···M) is dependent on the size of the bridging halide; namely, the Pt−Pt distance increases with an increase in the ionic radius of the halide ions (Cl < Br < I) because the dz2 orbital of M and the pz orbital of X have greater overlap. As can be seen, the MX chains with Y = ClO4− obey this order. On the other hand, in the MX chains with Y = ReO4−, L(M···M) is longer than that of the compounds with Y = ClO4− due to the larger size of the ReO4− ions. It is noteworthy that L(M···M) of the present compounds is almost the same, independent of M and X. This result suggests that the hydrogen bond between the N−H protons of the en ligand and the oxygen atoms of the ReO4− ion has the largest effect on L(M···M) rather than the orbital overlapping. In other words, the hydrogen bonds prevent MII(en)2 and MIV(en)2X2 moieties from approaching each other, meaning that they act as a negative chemical pressure. Furthermore, to the best of our knowledge, no MXchain compounds with L(M···M) > 6 Å have been reported thus far, indicating that the present compounds are the closest to the Robin−Day class I limit of all reported compounds. Figure 3ashows l1(MII···X) and l2 (MIV−X) as a function of L(M···M) together with those of a series of MX chains. The relationship does not depend on M, but it does depend on X. This finding is due to the ionic radii of Pd and Pt being almost the same as a result of the lanthanoid contraction, whereas those of halide ions largely differ (Cl < Br < I). PtCl has the longest l1(MII···X) among all MX chains thus far reported. Okamoto et al. have defined the following displacement (d) parameter to estimate the degree of halide displacement quantitatively:21

and a shiny-gold precipitate immediately formed. The mixture was heated again. The solid residue was separated by filtering the solution through a cotton plug. The yellow filtrate was slowly cooled to room temperature, and the solution was allowed to stand overnight. Gold needle-shape crystals were collected, affording 23 mg (0.012 mmol). The yield based on the number of Pt ions was 70%. Anal. calcd for C4H16IN4O8PtRe2: C, 5.10; H, 1.71; N, 5.94. Found: C, 5.32; H, 1.66; N, 5.96.

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RESULTS AND DISCUSSION Crystal Structure. The crystallographic data for the compounds is summarized in Table 1. An X-ray single crystal structure of [Pd(en)2][Pd(en)2Br2](ReO4)4 (PdBr) is shown in Figure 2. The structure was determined at room temperature

Figure 2. Perspective drawings of the crystal structure of PdBr with 50% probability. Crystal packings viewed from the (a) a and (b) b axis. The brown broken lines represent the unit cell. In these figures, hydrogen atoms are omitted for clarity. In (c) and (d), the N−H···O hydrogen bonds are depicted as blue dotted lines. In (c) and (d), hydrogen atoms bonding to carbon atoms are omitted for clarity. C, gray; H, pink; Br, brown; N, blue; O, red; Pd, olive; and Re, magenta. Black dashed lines illustrate the PdII···Br bonds.

in the triclinic P1̅ space group. 1-D Pd−Br chains are constructed along the a axis. The site occupancy of the Br atom was fixed at 0.5 so that only one Br exists between the neighboring Pd sites. The 1D chain is not linear but slightly zigzag with a Pd−Br−Pd bridging angle, ∠(Pd−Br−Pd), of 172.56(5)°. The longer Pd−Br distance is 3.5903(11) Å, which

Table 2. Selected Distances and Displacement Parameters for a Series of MX-Chain Compounds with Y = ReO4− Together with Those for Compounds with Y = ClO4− for Comparisona Y = ReO4

−

Y = ClO4−

PdBr PtCl PtBr PtI PdClb PdBrc PtCld PtBre PtIf

L(M···M)/Å

l1(MII···X)/Å

l2 (MIV−X)/Å

∠(Pd−Br−Pd)/°

d

6.0474(3) 6.1189(5) 6.0602(3) 6.0523(12) 5.357 5.386 5.403 5.470 5.827(4)

3.5903(11) 3.808(3) 3.5948(10) 3.3434(10) 3.033(3) 2.914(2) 3.085(7) 2.996(1) 3.036(8)

2.4694(11) 2.324(3) 2.4727(10) 2.7104(9) 2.324(3) 2.472(2) 2.318(7) 2.473(1) 2.791(8)

172.56(5) 172.22(12) 174.31(5) 177.50(6) 180 180 180 180 180

0.185 0.243 0.185 0.105 0.132 0.082 0.142 0.096 0.042

d value is defined as d ≡ { l1(MII···X) − l2 (MIV−X)}/L(M··M). bReference 16. cReference 17. dReference 18. eReference 19 (orthorhombic phase). fReference 20. a

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DOI: 10.1021/acs.inorgchem.7b03051 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. Optical absorption spectra of PdBr, PtCl, PtBr, and PtI.

PtBr, and PtCl, respectively. Details for the peak assignment are noted in the Supporting Information (page S5). The IVCT energy of PtCl (3.70 eV) is the largest among the MX chains thus far reported, which is consistent with the fact that PtCl has the longest L(M···M). At the same time, PtCl still shows an IVCT band, although L(M···M) is longer than 6 Å. This finding suggests that there is still orbital overlap between [PtII(en)2]2+ and [PtIV(en)2Cl2]2+, indicating that PtCl is a class II material. Thermal Properties. TG/DTA Profiles. Figure 5 shows the TG/DTA profiles for PdBr, PtCl, PtBr, and PtI, and the estimated decomposition temperatures (Tdec) are summarized in Table 3. We evaluated Tdec from the temperature at which either the first weight loss or thermal anomaly commenced. PdBr, PtCl, PtBr, and PtI decomposed at 474, 564, 522, and 531 K, respectively, under a nitrogen atmosphere. Although the decomposition process has not been clarified at present, for all compounds, the first weight loss involves an endothermic reaction. However, the present compounds do not have any lattice solvent molecules. Thus, the first weight loss is accompanied by an enthalpy-unfavorable chemical reaction. Moreover, the decomposition process of PtCl is different from that of PdBr, PtBr, and PtI. In the case of PdBr, PtBr, and PtI, the final weight fractions (Wtfr) of the residue after heating to 550 °C were 13.4, 20.5, and 22.3%, respectively, which are close to the expected values (13.1, 21.9, and 20.7%, respectively) assuming that the residue is a Pd or Pt metal. It is known that Re2O7 is volatile (boiling point is 360 °C).23 Thus, we believe that two ReO4− ions condense to form a volatile Re2O7 species at the beginning of the decomposition process. In the case of PtCl, on the other hand, the final weight fraction was 63.6%, which is close to 66.7% for a residue of PtRe2. This composition is supported by the energy dispersive X-ray (EDX) spectrum of the residue (see Supporting Information (Figure S4)). Although we think this difference is probably due to the redox properties of the halide ion, further study is necessary to clarify it. Figure 6 shows the DSC profiles. For each compound, a pair of reversible DSC anomalies was observed in the range of 338− 385 K due to a first-order phase transition. The phase-transition temperatures (TC) are summarized in Table 3, in which TC↑ and TC↓ stand for TC upon heating and cooling, respectively. For PtCl, PtBr, and PtI, a first-order phase transition occurred in the ranges of 352−360, 354−360, and 379−385 K, respectively, with the competitive transition enthalpies (ΔH↑/↓) and entropies (ΔS↑/↓), and TC varies depending on X. On the other hand, PdBr exhibited similar but broad and weak DSC anomalies. In particular, the anomaly was hardly visible in the heating process. This is possibly due to the

Figure 3. Correlations between (a) the M−X distance (l(M···X)) and M−M distance (L(M···M)) and (b) between L(M···M) and the d. Circle, M = Pt; triangle, M = Pd; and square, M = Ni. Green, X = Cl; magenta, X = Br; and cyan, X = I. Diamond and inverted triangles are attributed to the Pd and Pt complexes, respectively, in this study (Y = ReO4−; circled by black solid lines). In (a), the black broken line indicates the line for the M3+ state. Green, red, and blue lines are guides for the eyes to highlight the compounds with X = Cl, Br, and I, respectively. The numbers described in (b) correspond to MX-chain complexes previously reported. 1: [Pt(chxn)2 ][Pt(chxn)2Cl2](ClO4)4;21 2: [Pt(en)2][Pt(en)2Cl2](ClO4)4;21 3: [Pt(chxn)2][Pt(chxn)2Cl2]Cl4;21 4: [Pt(en)2][Pt(en)2Br2](ClO4)4 (polymorph I);21 5: [Pt(en)2][Pt(en)2Br2](ClO4)4 (polymorph II);21 6: [Pt(en)2][Pt(en)2Br2](C6−Y)4·H2O;10 7: [Pt(chxn)2][Pt(chxn)2Br2]Br4;21 8: [Pt(chxn)2][Pt(chxn)2Br2](ClO4)4;21 9: [Pt(chxn)2][Pt(chxn)2I2]I4;2210: [Pt(en)2][Pt(en)2I2](ClO4)4;21 11: [Pt(en)2][Pt(en)2Cl2](ClO4)4;21 12: [Pd(en)2][Pd(en)2Br2](ClO4)4;21 13: [Pd(en)2][Pd(en)2Br2](C5−Y)4·2H2O (293 K);10 14: [Pd(en)2Br](C5−Y)2·H2O (130 K);10 15: [Pd(chxn)2][Pd(chxn)2Br2]Br4;21 16: [Ni(chxn)2Cl]Cl2;21 and 17: [Ni(chxn)2Br]Br2.21

d=

l1(MII ··· X) − l 2(MIV − X) L(M ··· M)

Figure 3b shows a relationship between d and L(M···M). d = 0 (l1(MII···X) = l2 (MIV−X)) corresponds to the AV state, namely, the Robin−Day class III limit, and d = 1 (l1(MII···X) ≈ L(M···M) = ∞) corresponds to the discrete (class I) limit. However, the actual d value corresponding to the discrete limit should be much smaller than 1 because there should be no intervalence interaction when l1(MII···X) is much longer than the summation of the ionic diameters of M and X. The d value for PtCl was determined to be 0.243, which is the largest value among the reported MX-chain compounds to the best of our knowledge, suggesting that there is little orbital overlap between PtII and PtIV moieties. Optical Properties. Figure 4 shows the optical absorbance spectra of the present compounds. In the spectra of these mixed-valence compounds, intervalence charge transfer (IVCT) transitions from MII to MIV units were observed. It has been known that the IVCT band shows a hypsochromic shift with lengthening L(M···M). In the present compounds, IVCT bands were observed at 2.05, 2.46, 3.04, and 3.70 eV for PtI, PdBr, D

DOI: 10.1021/acs.inorgchem.7b03051 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. TG−DTA profiles for (a) PdBr, (b) PtCl, (c) PtBr, and (d) PtI at a scan rate of 5 K min−1. Red and blue lines represent weight percent based on the initial weight and DTA profiles corresponding to the left and right vertical axes, respectively. DTA curves are shown only for heating processes.

shows the crystal structure of PdBr at 372 K. At 372 K, the space group was P1̅, and no obvious changes in the unit cell were observed. However, there was a marked difference in the temperature factor of the carbon atoms of the en ligand; the temperature factor was larger along the chain direction. We analyzed it as a disorder of the methylene group and obtained a best-fit site-occupancy ratio of 71(4):29(4) as shown in Figure 7. Thus, we attributed the phase transition to an order− disorder transition of the methylene moiety in the en ligand. Here the relationship between the SXRD results and thermal analyses is discussed. Usually, phase transitions related to thermally induced disordered molecular motion can be linked to the transition entropy estimated from DSC analyses. On the basis of Boltzmann′s equation, ΔS = R ln(WHT/WLT), where R stands for the gas constant (= 8.314 J mol−1 K−1) and WHT and WLT are the numbers of microscopic states in the high- and low-T phases related to the focused phase transition. From this equation, WHT/WLT, which corresponds to the ratio of the number of microscopic states across the transition, was estimated. Considering an order−disorder type phase transition of the methylene moiety of the en ligand, WLT is 1 because the local geometry of en ligands is fixed as (δλ), as shown in Figure 7b. In the high-T phase, on the other hand, WHT is 4 because there are four possibilities, which are (δδ, δλ, λδ, and λλ), if the two en ligands have no correlation. In this case, ΔS should be ΔS = R ln 4 = 11.5 J mol−1 K−1, which is much larger than the measured value. Although this cannot be clarified by using conventional SXRD analysis, the motion of the en ligand should be constrained because each ligand is connected by hydrogen bonds through the ReO4− counterion. Thus, we concluded that the methylene moiety of the en ligand was partially disordered above TC.

Table 3. Decomposition Temperature (Tdec), Transition Temperature (TC↑/↓), Enthalpy (ΔH↑/↓), and Entropy (ΔS↑/↓) Estimated from TG and DSC Profiles compound

PdBr

PtCl

PtBr

PtI

Tdec (K) Wtfr after 550 °C (%) TC↑ (K) ΔH↑ (kJ·mol−1) ΔS↑ (J·mol−1·K−1) TC↓ (K) ΔH↓ (kJ·mol−1) ΔS↓ (kJ·mol−1·K−1)

474 13.4 343.8 0.54 1.57 338.8 1.22 3.59

564 63.6 360.3 1.02 2.83 352.5 1.11 3.13

522 20.5 359.5 2.01 5.60 354.3 2.02 5.71

531 22.3 384.6 2.09 5.44 379.1 2.10 5.54

Figure 6. DSC profiles for PdBr (black), PtCl (green), PtBr (red), and PtI (blue) with a temperature scan rate of 20 K min−1. Upper and lower panels show heating and cooling data, respectively. Positive and negative directions of the vertical axis show exothermic and endothermic processes, respectively.

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difference in M species; however, the reason remains unclear. Since their crystal structures are isomorphic, the observed anomalies can be attributed to a phase transition related to their crystal structures. In other words, a structural phase transition may occur at TC. Next, SXRD investigations were performed on PdBr above TC. Since the equipment used had an upper limit of the temperature of 373 K, the data was collected 372 K. Figure 7

CONCLUSIONS We synthesized a series of MX-chain compounds with perrhenate counterions. The metal-to-metal distance in these compounds was longer than 6 Å, which is the longest distance among MX chains so far reported. In addition, the crystal packings of the compounds were all similar and independent of M and X. The similar structures independent of the chemical E

DOI: 10.1021/acs.inorgchem.7b03051 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 7. (a) ORTEP diagram of PdBr at 372 K with thermal ellipsoids at 50% probability, highlighting the thermally induced disorder of en ligands. Hydrogen atoms and perrhenate counterions are omitted for clarity. C, black and white for part 1 (occupancy = 71%) and part 2 (occupancy = 29%); Br, brown; N, blue; and Pd, olive. Brown dashed lines represent the unit cell. (b) Structural configuration of the ethylenediamine moiety of PdBr at room temperature (upper) and 372 K (lower).

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ACKNOWLEDGMENTS This work was partially supported by the Asahi Glass Foundation, a JSPS KAKENHI grant (A) 26248015, grant (C) 16K05713, CREST (JST), and Tohoku University Molecule and Material Synthesis Platform in Nanotechnology platform project sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

components will help to determine the elemental effects on the electronic states.

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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.7b03051. Crystal structures of PtCl, PtBr, and PtI. SEM image and EDX spectrum of the residue of PtCl after TG measurement, calculated UV−vis spectra of trimer units, experimental UV−vis spectra of [Pt(en)2X2]X2 (X = Cl, Br, and I) monomer in aqueous solution (PDF)

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Accession Codes

CCDC 1588082−1588085 contain the supplementary crystallographic data for this article. 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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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*S.T. E-mail: [email protected]. *H.I. E-mail: [email protected]. *M.Y. E-mail: [email protected]. ORCID

Shohei Kumagai: 0000-0002-1554-054X Shinya Takaishi: 0000-0002-6739-8119 Hiroaki Iguchi: 0000-0001-5368-3157 Present Address

⊥ Department of Advanced Materials Sciences, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan.

Author Contributions

The manuscript was written through contributions of all of the authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. F

DOI: 10.1021/acs.inorgchem.7b03051 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.7b03051 Inorg. Chem. XXXX, XXX, XXX−XXX