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 B-11, C-13 and H-1 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.