Structural Importance of Secondary Interactions in Molecules: Origin of Unconventional Conformations of Phosphine–Borane Adducts



The series of phosphine–borane adducts, Ph2(H3C[BOND]C[TRIPLE BOND]C)P[BOND]B(C6F5)3 (8 c), Ph(H3C[BOND]C[TRIPLE BOND]C)2P[BOND]B(C6F5)3 (8 b) and (H3C[BOND]C[TRIPLE BOND]C)3P[BOND]B(C6F5)3 (8 a), was prepared. The X-ray crystal structure analyses revealed close to eclipsed conformations for all members of this series with average dihedral angles θ(C-P-B-C) of 8.1° (8 c), 12.3° (8 b) and 20.3° (8 a). Quantum chemical analysis of these compounds revealed the importance of a subtle interplay between competing attractive and repulsive secondary interactions, causing the surprising eclipsed conformational preference for systems of this degree of complexity. Some cyclic phosphine–borane adducts were studied for comparison.