Do the discrete dianions C2B9H112− exist? Characterisation of alkali metal salts of the 11-vertex nido dicarboranes, C2B9H112−, in solution

Abstract

Detailed experimental solution-state NMR data are reported for the nine moisture-sensitive salts of M₂C₂B₉H₁₁ (M = Li, Na, K) 1–9 generated by deprotonation of 7,8-C₂B₉H₁₃, 7,9-C₂B₉H₁₂⁻ and 2,9-C₂B₉H₁₃ by butyllithium, sodium hydride and potassium hydride. Indicative of cation–anion interactions, the ¹¹B, ¹³C and ¹H chemical shifts depend on the identity of the cation and, to a lesser degree, the solvent. Computed NMR shifts generated from MP2-optimised geometries of C₂B₉H₁₁²⁻, LiC₂B₉H₁₁⁻ and NaC₂B₉H₁₁⁻ suggest that intimate ion-pair cluster anions MC₂B₉H₁₁⁻ are present in solutions of M₂C₂B₉H₁₁. As a test of the method of comparing experimental structures and chemical shifts with those from optimised geometries, the optimised geometries of the small carborane alkali metal salts M₂C₂B₄H₄(SiR₃)₂ (M = Li or Na, R = H or Me) were computed and shown to agree well with experimental structures.