NEW SYNTHETIC ROUTES TO AZACARBORANE CLUSTERS - NITRILE INSERTION REACTIONS OF NIDO-5,6-C2B8H11- AND NIDO-B10H13-

A synthetic sequence involving the initial nucleophilic attack of the isoelectronic nido-5,6-C2B8H11- or nido-B10H13- anions at a nitrile ca rbon, followed by nitrile hydroboration and cage-insertion, has been f ound to yield new azacarborane clusters in good yields. Thus, the reac tion of nido-5,6-C2B8H11- with refluxing acetonitrile gave the azatric arbaborane anion arachno-7-CH3-5,7,14,12-C3NB8H11- (1(-)) in 65% yield , while nido-B10H13- reacted with acetonitrile, benzyl cyanide, or (CH 3CN)-C-13 to give the azamonocarbaborane anions arachno-7-CH3-7,12-CNB 10H13- (2a(-)), arachno-7-Bn-7,12-CNB10H13- (2b(-)), and arachno-7-CH3 -7,12-(CNB10H13-)-C-13 (2a(-)-C-13), respectively. Single crystal X-ra y studies of the isoelectronic clusters, 1(-) and 2a(-), showed that h ydroboration of the nitrile occurred, with the resulting imino group i nserting 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 exopo lyhedral bridging imine substituents. Acidification of 1- resulted in loss of one boron and the imine nitrogen and formation of the known tr icarbaborane 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 o pen faces, that is based on a 14-vertex close 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 co nnected 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 yiel d the isomeric anion hypho-12-R-12,11-CNB9H14- (4(-)). A single-crysta l X-ray study of 4(-) confirmed that it has an Ii-vertex hypho structu re with one seven-membered and one five-membered open face. Subsequent acidification of 4(-) resulted in loss of one additional boron to giv e hypho-8-R-8,13-CNB8H14 (5). Deprotonation with Proton Sponge gave th e new anion hypho-8-R-8,13-CNB8H13- (5-). Based upon spectral and comp utational data, 5 and 5(-) are proposed to have the CN unit incorporat ed into a 10-vertex hypho structure.