Selenium-Rich Octanuclear Iridium Clusters with Mixed-Valent Selenium Ligands in the Densest Known Packing of Ellipsoids

摘要

Dark-red air-stable crystals of Ir_8Se_(28B)r_14·2H_2O were obtained by reacting iridium in an excess of selenium and selenium tetrabromide at 300 °C for one week. The crystal structure is monoclinic (space group P2_1) with a = 1274.66(3) pm, b = 1211.48(3) pm, c = 1743.82(4) pm, and β = 91.227(1)° at 296(1) K. The structure consists of uncharged clusters Ir_8Se_(28B)r_()14 = (Ir~(3+))_8(Se_2~(2-))(Se_4~(2-))_6(SeBr_2)_2(Br~-)_(10) with two water molecules attached to each of them. Selenium adopts three different oxidation states, -I in the diselenide dumbbell, -I and ±0 in the tetraselenide chains, and +II in the SeBr_2 units. The iridium(III) cations are coordinated octahedrally by selenium atoms and bromide ions. The octahedra share common vertices, while the oligoselenide anions provide high intra-cluster connectivity: μ6-η~2 in the case of Se_2~(2-) and μ_4-η~3 for Se_4~(2-). In parts the cluster resembles a cut-out of the rhombohedral Ir_3Se_8 framework structure. The ellipsoidal clusters form an excellent approximation of the mathematically predicted densest known packing of ellipsoids, featuring the coordination number of 14.