Reductive degradation of nido-1-CB8H12 into smaller-cage carborane systems via new monocarbaboranes [arachno-5-CB8H13]- and closo-2-CB6H8

Treatment of the nido-1-CB8H12 (1) carborane with NaBH, in THF at ambient temperature led to the isolation of the stable [arachno-5-CB8H13](-) (2(-)), which was isolated as Na+[5-CB8H13](-).1.5THF and PPh4+[5-CB8H13](-) in almost quantitative yield. Compound 2- underwent a boron-degradation reaction with concentrated hydrochloric acid to afford the arachno-4-CB7H13 (3) carborane in 70% yield, whereas reaction between 2- and excess phenyl acetylene in refluxing THF gave the [closo-2-CB6H7](-) (4-) in 66% yield. Protonation of the Cs(+)4(-)salt with concentrated H2SO4 or CF3COOH in CH2Cl2 afforded a new, highly volatile 2-CB6H8 (4) carborane in 95 % yield, the deprotonation of which with Et3N in CH2Cl2 leads quantitatively to Et3NH+[2-CB6H7](-)(Et(3)NH(+)4(-)). Both compounds 4- and 4 can be deboronated through treatment with concentrated hydrochloric acid in CH2Cl2 to yield the carbahexaborane nido-2-CB5H9 (5) in 60% yield. New compounds 2-, 3, and 4 were structurally characterised by the ab initio/GIAC/MP2/NMR method. The method gave superior results to those carried out using GIAO-HF when relating the calculated B-11 NMR chemical shifts to experimental data.