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

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.