PREPARATION OF SMALLER 9-VERTEX AND 10-VERTEX MONOCARBABORANE CAGES BY THE SELECTIVE DISMANTLING OF 12-VERTEX DICARBABORANES VIA THE [NIDO-7,9-C2B10H13]- ANION - SYNTHESIS OF THE MONOCARBABORANE BASE ADDUCTS ENDO-9-ME-8-(NME3)-ARACHNO-6-CB9H12, 9-ME-8-L-NIDO-6-CB9H10 (L = NME3, OH-), AND EXO-6-L-ARACHNO-4-CB8H12 (L = SME2, NME3, UROTROPINE, PYRIDINE, QUINOLINE, MECN, PPH3, MENC)

The closo-1,2-C2B10H12 dicarbaborane (1) undergoes regiospecific clust er degradation reactions via the [nido-7,9-C2B10H13]- anion (2) in the presence of Lewis bases L. When L = NMe3, the reaction is found to pr oceed via carbon-vertex loss to isolate the ten-vertex intermediate en do-9-Me-8-(NMe3)-arachno-6-CB9H12 (3). Further boron-vertex loss occur s to yield a series of arachno nine-vertex compounds exo-6-L-4-CB8H12 (4) [where L = NMe3, 4a; pyridine (py), 4b; urotropine (uro), 4c; 1/2 uro, 4d; and SMe2, 4e]. Ligand-exchange reactions between 4e and stron ger Lewis bases, such as py, uro, quinoline (quin), MeCN, PPh3, and Me NC, afforded compounds 4b-d and further species of type 4 (where L = q uin, 4f, MeCN, 4g; PPh3, 4h; and MeNC 4i). These all have clusters tha t are isostructural with the unsubstituted parent dicarbaborane arachn o-4,6-C2B7H13, as documented by X-ray diffraction analysis on 4c. Crys tal data for 4c: mol wt 251; triclinic; space group P1BAR; Z = 4; a = 581.78(4), b = 1079.87(6), c = 1181.40(7) pm; alpha = 105.331(4), beta = 94.854(4), gamma = 92.797(4)degrees; U = 0.71127(8) nm3; R = 0.0423 and R(w) = 0.0503 for 2056 reflections with F(o) > 3sigma(F(o)). Comp ound 3 can be oxidized to 9-Me-8-(NMe3)-nido-6-CB9H10 (5b) by acetone, and a similar anionic species, [9-Me-8-(HO)-nido-6-CB9H10]-[PPh4]+ (5 b), was obtained directly from anion 2 by treatment with acetone. All compounds have been characterized by B-11 and H-1 NMR spectroscopy, an d the unambiguous assignment of the B-11 and H-1 resonances permits co mparisons with the shielding patterns of the structurally related pare nt analogs such as neutral arachno-4,6-C2B7H13, the anion [arachno-6-C B9H14]-, and the anion [nido-6-CB9H12].