Synthesis and Structure of Co2[Re6Se8(CN)6]⋅12H2O

Abstract

The cluster anions [Re6X8(CN)6] 4− were synthesized not long ago [1-4]. Anions of this kind have six terminal cyano groups. These are topological analogs of hexacyanometallate anions [M(CN)6] n−, reacting with transition metal cations to form polymer compounds. The larger size of cluster anions compared to mononuclear ones lead to unusual structures containing large cavities [5-7]. Another interesting feature of large cluster anions is possible stabilization of oligomer cations. For example, interaction of aqueous solutions of [Cu(H2O)6] 2+ with a solution of the tetranuclear anion cyano complex [Re4Te4(CN)12] 4− leads to formation of [Cu4(OH)4][Re4Te4(CN)12] containing a hydroxo cation tetramer [Cu4(OH)4] 4+ [8]. Here we report on the synthesis and structural study of Co2[Re6Se8(CN)6]⋅12H2O containing cobalt cation dimers bridged by water molecules. Experimental. Co2[Re6Se8(CN)6]⋅12H2O (1) was prepared by diffusion of the starting substances in silica gel. For this, Na2SiO3⋅9H2O (12.25 g) was dissolved in H2O (50 ml) and titrated to pH 7 while vigorously stirring with 1 M hydrochloric acid (∼20 ml). The resulting mixture was poured into a U-tube and allowed to stay for 1-2 days until the gel completely polymerized. An aqueous solution containing K4[Re6Se8(CN)6] [2] was applied to the gel in one knee of the U-tube, and an aqueous solution of CoCl2 was applied in the other knee. After 4 weeks, well-faceted orange prisms crystallized in the middle part of the tube. The structure of compound 1 was established by X-ray diffraction analysis. A yellowish orange prismatic single crystal sized 0.08×0.05×0.05 mm was chosen from the gel. The data were collected at 293 K on a Rigaku AFC5R diffractometer with a rotating anode (MoKα radiation, graphite monochromator). Crystal data: Co2Re6Se8C6N6O12H24, mol. wt. 2749.50, monoclinic, with cell parameters a = 9.875(4), b = 16.375(2), c = 12.1875(30) A, β = 96.00(2)°, Vcell = 1959.96(90) A3, space group P21/c, Z = 4, ρcalc = 3.753 g/cm 3. The number of reflections collected was 6280 (2θmax = 60.06°), including 5972 unique reflections (Rint = 0.09). An absorption correction (μ = 26.45 cm –1) was applied using the DIFABS program [9]. The structure was solved by direct methods with SHELX-86. The full-matrix least-squares refinement was carried out anisotropically for Re, Co, and Se atoms with teXsan program package [10]. The water hydrogen atoms were not located. The final R indices were R = 0.0708, wR = 0.0989 for 3692 reflections with I ≥ 2σ(I). Atomic coordinates are given in Table 1; bond lengths, in Table 2. Discussion of results. The structure of [Re6Se8(CN)6] 4− in compound 1 is shown in Fig. 1. The interatomic distances and the anion geometry are analogous to those found for other compounds with octahedral cluster anions [Re6X8(CN)6] 4− [1-7]. While dimerization via the bridging water molecules is not typical for transition metal cations [11], centrosymmetric dimers of Co2+ cations bridged by two water molecules are formed in 1 (Fig. 2). The distances from the bridging oxygen atom to the cobalt atoms are nonequivalent (2.14 and 2.25 A). This leads to a distortion of the Co2O2 rhomb to a parallelogram. The Co–Co distance is 3.43 A, ruling out the presence of a metal—metal bond. Cobalt has a slightly distorted octahedral environment formed by three water molecules (two bridging O2, O2a and one terminal O1) and three nitrogen atoms of the cyanide ligands belonging to three different anions. For the CoO3N3 coordination unit, edge and face geometrical isomers are possible. In this structure, the face isomer is realized (Fig. 2). The Co–N distance is 2.05-2.09 A; this value is typical for the Co2+–N coordination bond.