Syntheses and Crystal Structures of the closo-Borates M2[B7H7] and M[B7H8] (M = PPh4, PNP, and N(n-Bu4)): the Missing Crystal Structure in the Series [BnHn]2- (n=6-12)

Oxidation of [N(n-Bu-4)](2)[B9H9] with oxygen in a mixture of dimethoxyethane and CH2Cl2 leads to salts of the [B7H7](2-) dianion. This is the first convenient synthesis for a seven-vertex hydro-closo-borate anion. Protonation with NEt3 center dot HCl resulted in salts of the [B7H8](-) monoanion. Both closo-borate anions were isolated and characterized by H-1, H-1{B-11}, B-11, and B-11{H-1} NMR spectroscopy. The temperature-dependent H-1{B-11}, B-11, and B-11{H-1} NMR spectra of [B7H8](-) were also measured. The structure of [B7H7](2-) as well as of [B7H8](-) were determined by single-crystal X-ray diffraction.