K₂PtCl₄ was obtained from Tanaka Kikinzoku Co. PyCN was obtained from Tokyo Kasei Industrial Co. IR spectra were recorded with samples prepared with KBr.

An aqueous solution (2 mL) of K₂PtCl₄ (83 mg, 0.2 mmol) was stirred with 2 equiv of PyCN (42 mg, 0.4 mmol) at room temperature for 5 min. The crystals of 1 were obtained after slow evaporation of the solvent with the addition Me₂CO (2 mL) for a few days at room temperature. Elemental analysis of calcd for C₁₂H₈Cl₂N₄Pt: C, 30.39%; H, 1.70%; N, 11.81%, found: C, 30.35%; H, 1.60%; N, 11.56%.

Measurements were carried out on a Bruker SMART APEX CCD diffractometer equipped with a normal focus Mo-target X-ray tube (λ = 0.71073 Å) operated at 2000 W power (50 kV, 40 mA) and a CCD two-dimensional detector. A total of 1315 frames were collected with a scan width of 0.3˚ in ω with an exposure time of 5 sec for each frame. The frames were integrated with the SAINT software package with a narrow frame algorithm [12]. Absorption correction was applied by using SADABS [13]. The structures were solved by the direct method [14] with the subsequent difference Fourier syntheses and the refinement with the SHELX-97 [15] operated by YadokariXG software package [16]. Non-hydrogen atoms were refined anisotropically and all hydrogen atoms were treated as riding atoms. The crystal data and structure refinement results are summarized in Table 1. In 1-lt, PyCN ligands (N1-3, N5, C1-3, C6-7, C9-10, C12, C15, C17-18, and C20-21) were isotropically refined.

Figure 1 shows the crystal structure of cis-[PtCl₂(PyCN)₂] (1-ht, measured at 200 K). The structure consists of discrete molecules, where the platinum atom is coordinated to two Cl^– ions and two PyCN ligands with cis fashion. The PyCN ligands are coordinated at the pyridine sites with the distances of Pt-N = 2.002(9) Å, and Cl^– ions are coordinated with Pt-Cl = 2.287(3) and 2.288(3) Å, which are in agreement with analogous distances reported previously [17-20]. The dihedral angles of the pyridine rings with respect to the coordination plane are 54˚ and 75˚. The molecules pack in the crystal to be pairs with centrosymmetrically relationship (Figure 1), as similarly found in cis-[PtCl₂(NH₃)₂] [17], cis-[PtCl₂(en)] (en = ethylenediamine) [21], cis-[PtCl₂(py)₂] (py = pyridine) [18], cis-[PtCl₂(pyCH₃)₂] [20], and cis-[PtCl₂(pyC₂H₃)₂] [19], which probably minimizes the potential energy between the permanent dipoles of the molecules. In 1-ht the Pt-Pt distance in the pair is 4.52 Å, which is longer than the sum of the van der Waals radii (4.1 Å) of two Pt atoms [22]. In contrast, they are ca. 3.37 and 3.39 Å in cis- [PtCl₂(NH₃)₂] [17] and cis-[PtCl₂(en)] [21], respectively. The longer Pt-Pt distance in 1-ht originates from a precluding a closer approach by the pyridine rings, which are rotated with respect to the coordination plane (54˚ and 75˚) in order to minimize intramolecular repulsions. Actually, in cis-[PtCl₂(py)₂], each pyridine ring leans with respect to the coordination plane (56˚ and 62˚), to afford Pt-Pt distance of ca. 4.95 Å [18]. Furthermore, the CN moieties in PyCN are interacted with neighboring pyridine rings with the distance of N-C = 3.24 Å in 1-ht.

Figure 2 shows the X-ray diffraction photograph of 1 taken with a fixed-film, a same irradiated angle to the single-crystal, and a same exposure time, at the temperature from 260 to 80 K. In the whole temperatures clear Laue spots are observed, however, there are obvious differences between 160 and 140 K, where the crystal system transforms: 1-ht, at 200 K, monoclinic, P2₁/c, a = 10.0521(19) Å, b = 12.296(2) Å, c = 11.759(2) Å, b = 90.762(3)˚; and 1-lt, at 120 K, triclinic, , a = 10.059(8) Å, b = 11.596(9) Å, c = 12.398(9) Å, a = 91.301(14)˚, b = 92.811(14)˚, g = 90.224(13))˚. In both ranges of 260 - 160 K (high temperature, ht) and 140 - 80 K (low temperature, lt), each spot becomes clearer as temperature decreases. The original reflection spots appeared again when the cooled single crystal was returned to the high temperature, indicating that this single-crystal-to-singlecrystal transformation is reversible. Figure 3 shows the lattice parameters of 1 at various temperatures. As lower temperature, the cell volume becomes reduced, which is attributed to the reduction of both a and b axes. While, the c axis does not depend on temperature to induce distorted angle of b. Because the platinum molecules are not aligned along the particular axis, further discussion based on the crystal axes is difficult.

Table 1. Crystallographic data and structure refinements for cis-[PtCl₂(PyCN)₂] (1).

Several porous compounds containing solvent molecules as guests show structural changes induced by the guest transformation or adsorption/desorption of guest molecules [8,9]. Whereas, despite no solvent molecules in the crystal, compound 1 exhibits the temperature dependent phase transition. Figure 4 shows the packing structures of five Pt molecules in 1-ht and 1-lt. The obvious difference is Pt-Pt distances: in 1-ht, Pt(1)-Pt(1’) = 4.52 Å; and in 1-lt, Pt(1)-Pt(1’) = 4.33 Å, Pt(2)-Pt(2’) = 4.68 Å, where the number of independent atoms measured at 120 K doubles compared with that at 200 K. The shorter and longer distances of Pt-Pt in 1-lt are accompanied with the changes of pyridine ring inclination: 47˚ and 65˚ in Pt(1) molecule; 66˚ and 90˚ in Pt(2) molecule, showing that the stacking each other is obstructed by the steric hindrance of the pyridine ligands. This transformation is caused by some interactions among molecules, where CN moieties and Cl^– ions are close to m-positions of pyridine as shown in Figure 4. Particularly, the distances between CN moieties and pyridine (N-C = 3.19 Å) in 1-lt are shorter than those (N-C = 3.24 Å) in 1-ht.

Figure 5 shows IR spectra of 1 over the temperature range from 77 to 300 K. Although the phase transition is observed around 140 K, the significant difference of profile is not observed. Although the crystal structures show the significant difference, the wavenumber values of CN stretching bands do not change, indicating that this transition is caused in small strains.