Crystal structure of bis(methylammonium) hexadecaselenidopalladate(II), (CH 3 NH 3 ) 2 PdSe 16

(CH 3 NH 3 ) 2 PdSe 16 , tetragonal, P 4 b 2 (no. 117), a = 12.7821(4) Å, c = 7.1123(2) Å, V = 1162.02(8) Å 3 , Z = 2, R gt ( F ) = 0.0306, w R ref ( F 2 ) = 0.0841, T = 223 K.

3 Comment The structures of Pd polyselenide anions have proven to be highly sensitive to the size and shape of their counterions [5,6].As one of Pd polyselenide compounds, A  [5,10,11].
Compared to the alkali metal cations, organic ammonium cations have the advantage of being removed with less damage to the chalcogenidometallate anion framework by thermal treatment at lower temperatures.Among organic ammonium cations, the smallest cation, CH 3 NH + 3 is known to be in possession of a close ionic radius to those of Cs + and Rb + .Successful replacement of Cs + or Rb + with CH 3 NH + 3 has already been demonstrated for the preparation of organicinorganic hybrid perovskite material such as CH 3 NH 3 PbI 3 [12,13].
(CH 3 NH 3 ) 2 PdSe 16 was prepared by the methanothermal reaction of K 2 PdCl 4 , K 2 Se 4 and CH 3 NH 3 Cl in a molar ratio of 1:4:2.In the case of A 2 PdSe 16 (A = Rb, Cs), the mixture of A 2 Se (A = Na, Cs) and elemental Se served as the source of Se 4 2ligands and Se 8 rings, but K 2 Se 4 was used instead for the preparation of (CH 3 NH 3 ) 2 PdSe 16 .
The structure of (CH 3 NH 3 ) 2 PdSe 16 is isostructural to those of A 2 PdSe 16 (A = Rb, Cs), as all three compounds are crystallized in the same space group P4b2.As with the A 2 PdSe 16 (A = Rb, Cs) analogues, the structure of [PdSe 16 ] 2− in (CH 3 NH 3 ) 2 PdSe 16 consists of an array of Se 8 rings and [Pd(Se 4 ) 2 ] 2n− n layers running through the ab plane.The unit cell volume of (CH 3 NH 3 ) 2 PdSe 16 is 1162.02(8)Å 3 , which is considerably larger than that of A 2 PdSe 16 (A = Rb, Cs) with values of 1112.9(2) and 1118.5(3) Å 3 , respectively.The increase in the unit cell volume of (CH 3 NH 3 ) 2 PdSe 16 , compared to A 2 PdSe 16 (A = Rb, Cs), is attributed not only to the increase in the c parameter but also to changes in both the a and c unit cell parameters.The Pd-Se bond length is 2.4319  also simultaneously sitting at a site with coordinates (0.5 + x, 1.0 − y, 1.0), generating two symmetrically equivalent sites, which are assigned for disordered C and N atoms with 50 % occupancy.Thermal stability and decomposition behaviour of (CH 3 NH 3 ) 2 PdSe 16 was examined by the TGA experiments.TGA results show that there are three decomposition steps upon heating to 900 °C under N 2 flow.Based on the amount of weight loss during the periods of decomposition steps, CH 3 NH 3 + cations were lost in the first step and seleniums were lost in the following two steps to leave PdSe as a final residue.The first decomposition step begins at about 150 °C and ends at about 200 °C.The other two steps occur between 300 and 420 °C and between 460 and 870 °C, respectively.While decomposition of both Rb 2 PdSe 16 and Cs 2 PdSe 16 analogues did not occur up to around 300 °C, (CH 3 NH 3 ) 2 PdSe 16 can lose its organic cation below 200 °C, and start its second decomposition at about 300 °C.When crystals of (CH 3 NH 3 ) 2 PdSe 16 were calcined at 250 °C under N 2 flow for a day, the shape and morphology of the crystals remained intact, but numerous holes formed on the surface.The XRD pattern of the calcined crystals indicates a loss of crystallinity in (CH 3 NH 3 ) 2 PdSe 16 with no prominent crystalline peaks.

Table  :
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å  ).