Three new copper-lead selenite bromides obtained by chemical vapor transport: Pb5Cu+4(SeO3)4Br6, Pb8Cu2+(SeO3)4Br10, and the synthetic analogue of the mineral sarrabusite, Pb5Cu2+(SeO3)4(Br,Cl)4

Abstract

Three new copper-lead selenite bromides were synthesized by chemical vapor transport reactions. Pb₅Cu⁺₄(SeO₃)₄Br₆ is monoclinic, space group C2/m, a = 17.7248(14), b = 5.5484(5), c = 12.7010(10) Å, β = 103.398(2)º, V = 1215.08(17) ų, R₁ = 0.024; Pb₈Cu²⁺(SeO₃)₄Br₁₀ is orthorhombic, space group I222, a = 9.5893(5), b = 12.4484(9), c = 12.7927(6) Å, V = 1527.08(15) ų, R₁ = 0.027; Pb₅Cu²⁺(SeO₃)₄(Br,Cl)₄ is monoclinic, C2/c, a = 24.590(6) Å, b = 5.5786(14) Å, c = 14.248(4) Å, β = 102.883(7)º, V = 1905.3(9) ų, R₁ = 0.026. The crystal structure of Pb₅Cu⁺₄(SeO₃)₄Br₆ consists of two distinct parts: corner- and edge-sharing Cu⁺Br₄ tetrahedra form infinite [Cu⁺₄Br₆]^2- layers which alternate with [Pb₅(SeO₃)₄]²⁺ layers. Pb₈Cu²⁺(SeO₃)₄Br₁₀ contains positively charged unique [Pb₈Cu²⁺(SeO₃)₄]¹⁰⁺ rod-like chains with Cu²⁺ cations in the core. These chains are held together by Br^- anions. Pb₅Cu⁺₄(SeO₃)₄Br₆ and Pb₈Cu²⁺(SeO₃)₄Br₁₀ belong to new structure types. Pb₅Cu²⁺(SeO₃)₄(Br,Cl)₄ is a synthetic analogue of the mineral sarrabusite, Pb₅Cu(SeO₃)₄Cl₄, previously known from an electron diffraction study. The investigation of this synthetic equivalent of sarrabusite by conventional single-crystal X-ray diffraction provides a distinctly improved insight in this crystal structure. Cu atom has well-defined [2O+(2O+2X)] (X = halogen) distorted octahedral coordination. PbO_n and SeO₃ polyhedra interconnect via common oxygen atoms into [Pb₅(SeO₃)₄]²⁺ layers parallel to (001). Cu²⁺ cations interconnect the layers into the framework with the large cavities filled by halide X anions. In all three new compounds described, a common feature is the formation of the selenophile substructure which is terminated by a ‘lone-pair’ shell that faces bromide complexes thus forming the surface of a halophile substructure.