Syntheses, Crystallographic/Computational Characterizations, and Reactions of the First 10-Vertex arachno- and nido-Phosphamonocarbaboranes

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Abstract
A synthetic sequence involving the initial reaction of a substituted phosphorus dihalide (RPCl2, R = CH3, C6H5) with the arachno-CB8H13- (1 -) monoanion followed by an in situ dehydrohalogenation reaction initiated by Proton Sponge, resulted in phosphorus cage insertion to yield the first 10-vertex arachno- and nido-phosphamonocarbaboranes, exo-6-R-arachno-6,7-PCB8H12 (2a, 2b) and PSH+6-R-nido-6,9-PCB8H9- (PSH +3a -, PSH +3b -) (R = C6H5 (a), CH3 (b)). Alternatively, 2a and 2b were synthesized in high yield as the sole product of the reaction of the arachno-4-CB8H122- (1 2-) dianion with RPCl2. Crystallographic determinations of PSH +3a - and PSH +3b - in conjunction with DFT/GIAO computational studies of the anions have confirmed the expected nido cage framework based on an octadecahedron missing the six-coordinate vertex. DFT/GIAO computational studies have also shown that while the gross cage geometries of the exo-6-R-arachno-6,7-PCB8H12 compounds 2a and 2b resemble the known isoelectronic arachno-6,9-SCB8H12, the phosphorus and carbon atoms are in thermodynamically unfavorable adjacent positions on the six-membered puckered face. They also each have an endo-hydrogen at the P6-position arising from proton transfer to the basic phosphorus during the cage-insertion reaction. Possible stepwise reaction pathways that can account for the formation of both the arachno and nido products are discussed. Deprotonation of 2a and 2b resulted in the formation of their corresponding conjugate monoanions, 6-R-arachno-6,7-PCB8H11- (2a -, 2b -), in which the proton that had been attached to the P6 atom was removed. Reactions of 2a -with O2, S8, BH3·THF, or Br2 further demonstrated the basicity of the P6-phosphorus yielding the new arachno-substituted compounds, endo-6-O-exo-6-(C6H5)-arachno-6,7-PCB8H11- (4a -), endo-6-S-exo-6-(C6H5)-arachno-6,7-PCB8H11- (5a -), endo-6-BH3-exo-6-(C6H5)-arachno-6,7-PCB8H11- (6a -), and endo-6-Br-exo-6-(C6H5)-arachno-6,7-PCB8H11 (7a), respectively, in which the O, S, BH3, and Br substituents are bound to the phosphorus at the endo position.

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