NMR AND X-RAY STUDIES OF PENTABORANE AND HEXABORANE ALKYL DERIVATIVES INVOLVING [3.3.1] AND [3.3.2] RING-SYSTEMS

The reaction of the B5H8- anion with B-chloro-9-borabicyclo[3.3.1]nonane (B-Cl-9-BBN) produces a boron bridge substituted nido-pentaborane, (μ-9-BBN)B5H8 (1). The solid-state structure of 1 reveals that the 9-BBN fragment retains its characteristic chair‒chair conformation. In solution at room temperature the 9-BBN fragment rotates relative to the pentaborane framework. From a dynamic13C NMR experiment the free energy of activation for the rotation has been found to be 14.6 ± 0.1 kcal mol-1. In diethyl ether solution this compound slowly converts to a nido-hexaborane derivative, 4,5-(cycloocta-1,5-diyl)B6H8 (2), in which the bridging boron atom has moved into the cluster. The result is the conversion of the [3.3.1] ring system of 9-BBN to a [3.3.2] bi-cycle in which two adjacent basal boron atoms of a nido-hexaborane cluster provide the two-atom bridge. Compound 2 is the first example of a fused borane cluster–organic bi-cycle. The solid-state structure of 2 reveals a chair–boat conformation for the [3.3.2] bicyclic ring system. In addition, two independent molecules are observed that differ primarily in the placement of the hydrogen atoms bridging the basal edges of the hexaborane fragment. Compounds 1 and 2 were characterized by 11B, 13C, 1H, and 11B–11B COSY NMR spectroscopic studies and by single-crystal X-ray diffraction. Crystal data for 1, C8H22B6, at -150 °C: triclinic, space group P1, a = 9.427 (2) Å, b = 10.380 (2) Å, c = 12.525 (2) Å, α = 87.341 (16)°, β = 80.729 (15)°, γ = 89.86 (2)°, Z = 4. Crystal data for 2, C8H22B6, at -125 °C: orthorhombic, space group P212121, a = 9.077 (3) Å, b = 11.585 (3) Å, c = 22.352 (6) Å, Z = 8. © 1990, American Chemical Society. All rights reserved.