Unusual Cage Rearrangements in 10-Vertex nido-5,6-Dicarbaborane Derivatives: An Interplay between Theory and Experiment.

The reaction between selected X-nido-5,6-C2 B8 H11 compounds (where X = Cl, Br, I) and "Proton Sponge" [PS; 1,8-bis(dimethylamino)naphthalene], followed by acidification, results in extensive rearrangement of all cage vertices. Specifically, deprotonation of 7-X-5,6-C2 B8 H11 compounds with one equivalent of PS in hexane or CH2 Cl2 at ambient temperature led to a 7 → 10 halogen rearrangement, forming a series of PSH+ [10-X-5,6-C2 B8 H10 ]- salts. Reprotonation using concentrated H2 SO4 in CH2 Cl2 generates a series of neutral carbaboranes 10-X-5,6-C2 B8 H11 , with the overall 7 → 10 conversion being 75%, 95%, and 100% for X = Cl, Br, and I, respectively. Under similar conditions, 4-Cl-5,6-C2 B8 H11 gave ∼66% conversion to 3-Cl-5,6-C2 B8 H11 . Since these rearrangements could not be rationalized using the B-vertex swing mechanism, new cage rearrangement mechanisms, which are substantiated using DFT calculations, have been proposed. Experimental11 B NMR chemical shifts are well reproduced by the computations; as expected δ(11 B) for B(10) atoms in derivatives with X = Br and I are heavily affected by spin-orbit coupling.