Synthesis of [Tl₄Se₁₆]^4-: a novel tetranuclear Tl³⁺ polyselenide

(Ph₄P)₄[Tl₄Se₁₆] was prepared hydrothermally in a sealed pyrex tube by the reaction of TlCl, K₂Se₄ and Ph₄PCl in a 1:1:1 molar ratio at 110 °C for one day. The red crystals were obtained in 50% yield. Crystals of (Ph₄P)₄[Tl₄Se₁₆]: triclinic P1 (No. 2), Z=1, a=12.054(9), b=19.450(10), c=11.799(6) Å, α=104.63(4), β=98.86(6), γ=101.99(6)° and V=2555(3) ų at 23 °C, 2θ_max=40.0°, μ=120.7 cm^-1, D_calc=2.23. The structure was solved by direct methods. Number of data collected: 5206. Number of unique data having F_o²>3σ(F_o²): 1723. Final R=0.075 and R_w=0.089. [Tl₄Se₁₆]^4- consists of four, almost already linearly arranged, tetrahedral thallium centers which are coordinated by two chelating Se₄^2-, two bridging Se₂^2- and four bridging Se^2- ligands. [Tl₄Se₁₆]^4- sits on an inversion center and possesses a central {Tl₂Se₂}²⁺ planar core. The Tl(1)-Tl(1)′ distance in this core is 3.583(6) Å. These two thallium atoms are then each linked to two cyclic Tl(Se₄) fragments via bridging Se₂^2- and Se^2- ligands forming Tl₂Se(Se₂) five-membered rings.