A synthetic sequence involving the initial nucleophilic attack of the isoelectronic nido-5,6-C2B8H11 - or nido-B10H13 - anions at a nitrile carbon, followed by nitrile hydroboration and cage-insertion, has been found to yield new azacarborane clusters in good yields. Thus, the reaction of nido-5,6-C2B8H11 - with refluxing acetonitrile gave the azatricarbaborane anion arachno-7-CH3-5,7,14,12-C3NB8H11 - (1 - ) in 65% yield, while nido-B10H13 - reacted with acetonitrile, benzyl cyanide, or CH3 13CN to give the azamonocarbaborane anions arachno-7-CH3-7,12-CNB10H13 - (2a - ), arachno-7-Bn-7,12-CNB10H13 - (2b - ), and arachno-7-CH3-7,12-13CNB10H13 - (2a - -13C), respectively. Single-crystal X-ray studies of the isoelectronic clusters, 1 - and 2a - , showed that hydroboration of the nitrile occurred, with the resulting imino group inserting into the cage framework in a position bridging the B2 and B11 borons. Consistent with their arachno skeletal electron counts, 1 - and 2a - have cage frameworks containing two six-membered open faces that may be derived from a bicapped hexagonal antiprism by removal of two non-adjacent five-coordinate vertices. Alternatively, 1 - and 2a - may be considered as having 10-vertex arachno frameworks with exopolyhedral bridging imine substituents. Acidification of 1 - resulted in loss of one boron and the imine nitrogen and formation of the known tricarbaborane nido-6-CH3-5,6,9-C3B7H10. In contrast, acidification of 2 - led to loss of only one boron to yield the new azamonocarbaborane hypho-12-R-12,13-CNB9H15 (3). A single-crystal X-ray study confirmed that 3 has an 11-vertex hypho structure, containing two six-membered open faces, that is based on a 14-vertex closo polyhedron missing three vertices. In 3, further hydroboration of the CN group occurred, such that the carbon contains an additional hydrogen and the nitrogen is connected to two borons. Deprotonation of 3 with Proton Sponge, 1,8-bis(dimethylamino)naphthalene, initially yielded hypho-12-R-12,13-CNB9H14 - (3 - ), which subsequently underwent a skeletal rearrangement to yield the isomeric anion hypho-12-R-12,11-CNB9H14 - (4 - ). A single-crystal X-ray study of 4 - confirmed that it has an 11-vertex hypho structure with one seven-membered and one five-membered open face. Subsequent acidification of 4 - resulted in loss of one additional boron to give hypho-8-R-8,13-CNB8H14 (5). Deprotonation with Proton Sponge gave the new anion hypho-8-R-8,13-CNB8H13 - (5 - ). Based upon spectral and computational data, 5 and 5 - are proposed to have the CN unit incorporated into a 10-vertex hypho structure.

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The reaction of arachno-6,8-C2B7H13 (1) with methyl propynoate in the presence of only 0.1 equivalent of 1,8-bis(dimethylamino)naphthalene (Proton Sponge®, PS) results in the one-step formation of the neutral tricarbaborane 6-(MeOOCCH2)-arachno-5,6,7-C3B7H12 (2). Similarly, only 0.1 equivalent of PS initiates the reaction of the isomeric arachno-6,7-C2B7H13 (3) carborane with methyl propynoate to yield the new tricarbaborane, 6-(MeOOCCH2)-nido-5,6,10-C3B7H10 (4). These results suggest a cyclic reaction sequence involving initial deprotonation of the starting dicarbaborane by PS, followed by reaction of the resulting anion with methyl propynoate to produce a new anion. Protonation of this second anion by another equivalent of the starting dicarbaborane then produces the final neutral tricarbaborane product and the cycle begins again. While 4 is unreactive toward NaH or KH, it readily added n-BuLi to give, following acidification with HCl-Et2O, a new product tentatively identified as 6-(MeOOCCH2)-Bu-arachno-5,6,10-C3B7H11 (6). The proposed structures of 4 and 6 are strongly supported by ab initio/IGLO nuclear shielding calculations.

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-7,9-C2B10H13 - anion and Me2S with the addition of concentrated hydrochloric acid has been found to give the 9-Me2S-μ-6,9-[(HO)BCH2]-arachno-6-CB9H11 zwitterion, in 58% yield. Further treatment of this intermediate species with a mixture of hexane and water results in the hydrolytic elimination of one CH and two BH vertices to give arachno-4-CB8H14 in a typical yield of 75% (45% based on the starting o-carborane). The structure of the arachno-4-CB8H13 - conjugate anion was determined by both a single-crystal X-ray study and ab initio/IGLO/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 endo-hydrogens on C4 and B8 and adjacent bridge hydrogens at the B5−B6 and B6−B7 edges. This structure is also supported by the ab initio calculations which find that this configuration is the lowest in energy among those investigated. However, the IGLO calculated 11B NMR chemical shifts for this structure do not match the reported room temperature solution 11B NMR data for arachno-4-CB8H13 -. Instead, the experimental spectra indicate a C s symmetry cage-structure containing three bridge hydrogens on the open face, suggesting the anion is fluxional in solution at room temperature. Good agreement between the experimental and calculated 11B NMR chemical shifts was obtained by assuming a simple fluxional process involving rapid simultaneous migration of two hydrogens between bridge and endo positions and one hydrogen between two bridging positions. Averaging the IGLO calculated chemical shift values for the borons in the static structure that become equivalent in the fluxional process is found to give good agreement with the room temperature experimental 11B NMR spectrum.