Crystal Structure, Raman Spectrum and Tl+ Lone-Pair Luminescence of Thallium(I) Dodecahydro-Monocarba-closo-Dodecaborate Tl[CB11H12]

Tl[CB11H12] was prepared with a reaction of Tl2[CO3] with the acid of the monocarba-closo-dodecaborate anion (H3O)[CB11H12] in aqueous solution as prismatic colorless single crystals by isothermal evaporation from the clear brine. It crystallizes in a monoclinic primitive structure with the space group P21/c (a = 685.64(3) pm, b = 1978.21(9) pm, c = 1006.89(5) pm, β = 132.918(3)° for Z = 4), which can be derived from the halite-type arrangement if the closo-carbaborate cages are considered as spheres. Due to the different atoms in the [CB11H12]− anion, Tl[CB11H12] features interesting C–Hδ+ ∙∙∙ δ−H–B interactions near to non-classical hydrogen bridges (“dihydrogen bonds”) and exhibits considerably different luminescence properties compared to regular closo-hydroborates, such as Tl2[B10H10], Tl2[B12H12] and Tl3Cl[B12H12]. Tl[CB11H12] shows strong photoluminescence (PL) at 390 nm, while the excitation bands for this broad band are located at 245 and 280 nm. It is caused by an interconfigurational [Xe]4f145d106s2 (3P1) to [Xe]4f145d106s16p1 (1S0) transition, which is also known as lone-pair luminescence. The quantum yield is rather low (<10 %), which is likely caused by the rather large Stokes shift. In addition, temperature-dependent emission spectra were recorded to determine the thermal quenching curve and the respective quenching temperature.