Reductive Degradation of nido ‐1‐CB 8 H 12 into Smaller‐Cage Carborane Systems via New Monocarbaboranes [ arachno ‐5‐CB 8 H 13 ] − and closo ‐2‐CB 6 H 8

Treatment of the nido ‐1‐CB 8 H 12 ( 1 ) carborane with NaBH 4 in THF at ambient temperature led to the isolation of the stable [ arachno ‐5‐CB 8 H 13 ] − ( 2 − ), which was isolated as Na + [5‐CB 8 H 13 ] − ⋅ 1.5 THF and PPh 4 + [5‐CB 8 H 13 ] − in almost quantitative yield. Compound 2 − underwent a boron‐degradation reaction with concentrated hydrochloric acid to afford the arachno ‐4‐CB 7 H 13 ( 3 ) carborane in 70 % yield, whereas reaction between 2 − and excess phenyl acetylene in refluxing THF gave the [ closo ‐2‐CB 6 H 7 ] − ( 4 − ) in 66 % yield. Protonation of the Cs + 4 − salt with concentrated H 2 SO 4 or CF 3 COOH in CH 2 Cl 2 afforded a new, highly volatile 2‐CB 6 H 8 ( 4 ) carborane in 95 % yield, the deprotonation of which with Et 3 N in CH 2 Cl 2 leads quantitatively to Et 3 NH + [2‐CB 6 H 7 ] − (Et 3 NH + 4 − ). Both compounds 4 − and 4 can be deboronated through treatment with concentrated hydrochloric acid in CH 2 Cl 2 to yield the carbahexaborane nido ‐2‐CB 5 H 9 ( 5 ) in 60 % yield. New compounds 2 − , 3 , and 4 were structurally characterised by the ab initio/GIAO/MP2/NMR method. The method gave superior results to those carried out using GIAO‐HF when relating the calculated 11 B NMR chemical shifts to experimental data.