ETHER CARBABORANES - SYNTHESIS AND CHARACTERIZATION OF MONO-ETHER AND DI-ETHER CARBABORANES WITH 1,2-CLOSO-C2B10 STRUCTURES, AND THEIR MONO-ANIONIC AND DI-ANIONIC NIDO-DERIVATIVES

A series of new closo ether carbaboranes 1-R1-2-R2-1,2-closo-C2B10H10 [1; R1 = R2 = (CH2)OCH3: 2; R1 = (CH2)OCH3, R2 = H:3; R1 = (CH2)2OCH3, R2 = H: 4; R1 = (CH2)3OCH3, R2 = H] have been prepared and characterized. Trends in B-11 NMR chemical shifts of 1, 2 and 1,2-closo-C2B10H12 are discussed in terms of the electron-withdrawing properties of the CH2OCH3 group. A single crystal X-ray diffraction study on 2 reveals dimer formation in the solid state via reciprocal C(cage)-H...O bonding. Partial cage degradation of 1 and 2 affords derivatives of [7,8-nido-C2B9H12]- and of [7,8-nido-C2B9H11]2-(compounds 5-10). Crystallographic study of one of these, [BTMA] [7-(CH2OCH3)-7,8-nido-C2B9H11] (6), shows that the conformation of the pendant ether group is such that its oxygen atom lies to maximum elevation above the open C2B3 polyhedral face, and this conformation is shown to be close to the minimum energy one by the results of molecular orbital calculations at the extended Huckel level.