The crystal and molecular structure of 4-Bromo-1,1,3,3,5,7,7,9,9-nonamethylbicyclo[4,4,0]pentaborophane

Abstract

Crystals of 4-bromo-1,1,3,3,5,7,7,9,9- nonamethylbicyclo[4,4,0]pentaborophane, C₉H₃₅B₅P₅Br, are monoclinic, with a = 9.502(3), b = 20.185(15), c = 13.604(4) Ǻ, β = 116.49(2)°. The space group is P2₁/c, with four molecules in the cell. Intensity data were collected by means of an automated diffractometer. The atomic positions have been determined by least-squares refinement of 2859 observed reflections. The final residual, R, is 0.077. The molecule contains a framework of alternat- ing boron and phosphorus atoms constituting a decalin-like ring system. The two cis-fused cyclo- hexane type rings are both in the chair conformation. The bromine atom is equatorially bonded to the boron atom in the 4-position. The Br-B bond length is 2.039(10) Ǻ. The average P-B and P-C distances are 1.943 and 1.828 Ǻ respectively. Average internal ring angles are 112.2° for P-B-P, and 113.6° for B-P-B. This geometry indicates only very slight charge delocalization in the cyclic system.