Trigonal Bipyramidal M2Ch32- (M = Sn, Pb; Ch = S, Se, Te) and TlMTe33- Anions: Multinuclear Magnetic Resonance, Raman Spectroscopic, and Theoretical Studies, and the X-ray Crystal Structures of (2,2,2-crypt-K+)3TlPbTe33-·2en and (2,2,2-crypt-K+)2Pb2Ch32-·0.5en (Ch = S, Se)

Abstract

The series of group 14 metal trigonal bipyramidal anions has been extended to the mixed group 13/group 14 metal TlMTe₃^3- anions (M = Sn, Pb), obtained by the reaction of Tl₂M₂Te₃ and K₂Te in en or in en/ethylamine mixtures and a stoichiometric excess of 2,2,2-crypt with respect to K⁺. The thallium anions were characterized in solution by ¹¹⁹Sn, ²⁰⁵Tl, ²⁰⁷Pb, and ¹²⁵Te NMR spectroscopy. The small magnitudes of the relativistically corrected reduced coupling constants, ¹(K_M-Ch)_RC and ¹(K_Tl-Ch)_RC, observed for the previously reported M₂Ch₃^2- (Ch = Se, Te) and the TlMTe₃^3- anions are consistent with predominantly p-bonded cages, and this observation is supported by local and nonlocal density functional theory (DFT) calculations. Theory indicates M−M and Tl−M interactions of high s character corresponding to Mayer bond orders of 0.13−0.32. The (K_M-M)_RC and (K_Tl-M)_RC couplings are unusually large compared to those of the butterfly-shaped Tl₂Ch₂^2- anions and likely arise from higher M−M and Tl−M bond orders, a larger number of coupling pathways, and smaller M−Ch−M and M−Ch−Tl bond angles. The TlPbTe₃^3- anion has also been structurally characterized by X-ray crystallography in (2,2,2-crypt-K⁺)₃TlPbTe₃^3-·2en [monoclinic system, space group P2₁/c, Z = 4, a = 15.256(5) Å, b = 26.087(9) Å, c = 20.984(8) Å, and β = 93.03(3)°] along with Pb₂Ch₃^2- (Ch = S, Se) in (2,2,2-crypt-K⁺)₂Pb₂Ch₃^2-·0.5en [Pb₂S₃^2-: triclinic system, space group P1̄, Z = 2, a = 10.189(2) Å, b = 11.329(2) Å, c = 23.194(4) Å, α = 95.439(14)°, β = 92.562(14)°, and γ = 90.549(14)°; Pb₂Se₃^2-: triclinic system, space group P1̄, Z = 2, a = 10.187(2) Å, b = 11.403(2) Å, c = 23.360(6) Å, α = 95.26(2)°, β = 92.17(2)°, and γ = 90.89(2)°]. Density functional theory calculations show that the experimental structures for the M₂Ch₃^2- and TlPbTe₃^3- anions are true minima and reproduce the experimental bond distances and angles. The vibrational frequencies determined by DFT calculations are in good agreement with those determined by Raman spectroscopy and have been used in their assignment.