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 Tl-2[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 P2(1)/c (a = 685.64(3) pm, b = 1978.21(9) pm, c = 1006.89(5) pm, beta = 132.918(3)degrees 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-H delta+ center dot center dot center dot H delta--B interactions near to non-classical hydrogen bridges ("dihydrogen bonds") and exhibits considerably different luminescence properties compared to regular closo-hydroborates, such as Tl-2[B10H10], Tl-2[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]4f(14)5d(10)6s(2) (P-3(1)) to [Xe]4f(14)5d(10)6s(1)6p(1) (S-1(0)) 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.