Syntheses, Vibrational Spectra, and Theoretical Studies of the Adamantanoid Sn4Ch104- (Ch = Se, Te) Anions: X-ray Crystal Structures of [18-Crown-6-K]4[Sn4Se10]·5en and [18-Crown-6-K]4[Sn4Te10]·3en·2THF

Abstract

The salts [18-crown-6-K]₄[Sn₄Se₁₀]·5en and [18-crown-6-K]₄[Sn₄Te₁₀]·3en·2THF were isolated upon addition of THF to the ethylenediamine (en) extracts of the alloys KSn_0.90Se_1.93 and K₄Sn₄Te₁₀ that had been extracted in the presence of 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane). The Sn₄Te₁₀^4- anion has been structurally characterized for the first time by a single-crystal X-ray diffraction study of [18-crown-6-K]₄[Sn₄Te₁₀]·3en·2THF: P2₁/n, a = 22.420(5) Å, b = 19.570(4) Å, c = 24.680(5) Å, β = 96.90(3)^o, Z = 4, and R₁ = 0.0468 at −183 °C. In addition to Si₄Te₁₀^4- and Ge₄Te₁₀^4-, the Sn₄Te₁₀^4- anion represents the only other known group 14 adamantanoid telluride. The X-ray crystal structure determination of the related [18-crown-6-K]₄[Sn₄Se₁₀]·5en salt has also been determined: P2₁/n, a = 22.003(2) Å, b = 18.966(2) Å, c = 24.393(2) Å, β = 97.548(8)^o, Z = 4, and R₁ = 0.0843 at −123 °C. The anion geometries are of the adamantanoid type where the Sn^IV atoms occupy the bridgehead positions and the chalcogen atoms occupy the bridging and terminal sites. The energy minimized geometries of Sn₄Ch₁₀^4- have also been determined using density functional theory (DFT). Mayer bond order analyses, Mayer valencies, and empirical bond valencies indicate that the terminal Sn−Ch bonds have significant multiple bond character, with the terminal Sn−Se bond having more multiple bond character than the terminal Sn−Te bond. The vibrational frequencies of the Sn₄Se₁₀^4- and Sn₄Te₁₀^4- anions have been calculated using DFT methods, allowing the Raman spectrum of Sn₄Se₁₀^4- to be fully assigned.