AN IMPROVED SYNTHESIS OF ARACHNO-4-CB8H14 AND CRYSTALLOGRAPHIC AND AB-INITIO IGLO NMR INVESTIGATIONS OF THE SOLID-STATE AND SOLUTION STRUCTURES OF THE ARACHNO-4-CB8H13- ANION/

An improved synthetic route for the monocarbaborane arachno-4 -CB8H14, as well as the first definitive confirmation Of the structure of its conjugate anion, arachno-4-CB8H13- are reported. Thus, the reaction of the nido-7,9C(2)B(10)H(13)- anion and Me(2)S with the addition of con centrated hydrochloric acid has been found to give the 9-Me(2)S-mu-6,9 -[(HO)BCH2]-arachao-6-CB9H11 zwitterion, in 58% yield. Further treatme nt of this intermediate species with a mixture of hexane and water res ults in the hydrolytic elimination of one CH and two BH vertices to gi ve arachno-4-CB8H14 in a typical yield of 75% (45% based on the starti ng o-carborane). The structure of the arachno-4-CB8H13- conjugate anio n was determined by both a single-crystal X-ray study and ab initio/IG LO/NMR calculations. The X-ray study confirms that in the solid state the anion has a 9-vertex arachno cage-geometry of C-1 symmetry with en do-hydrogens on C4 and B8 and adjacent bridge hydrogens at the B5-B6 a nd B6-B7 edges. This structure is also supported by the ab initio calc ulations which find that this configuration is the lowest in energy am ong those investigated. However, the IGLO calculated B-11 NMR chemical shifts for this structure do not match the reported room temperature solution B-11 NMR data for arachno-4-CB8H13-. Instead, the experimenta l spectra indicate a C, symmetry cage-structure containing three bridg e hydrogens on the open face, suggesting the anion is fluxional in sol ution at room temperature. Good agreement between the experimental and calculated B-11 NMR chemical shifts was obtained by assuming a simple fluxional process involving rapid simultaneous migration of two hydro gens between bridge and endo positions and one hydrogen between two br idging positions. Averaging the IGLO calculated chemical shift values for the borons in the static structure that become equivalent in the f luxional process is found to give good agreement with the room tempera ture experimental B-11 NMR spectrum.