Structural importance of secondary interactions in molecules: Origin of unconventional conformations of phosphine-borane adducts

The series of phosphine-borane adducts, Ph-2(H3C-C=C)P-B(C6F5)(3) (8c), Ph(H3C-C=C)(2)P-B(C6F5)(3) (8b) and (H3C-C=C)(3)P-B(C6F5)(3) (8a), was prepared. The X-ray crystal structure analyses revealed close to eclipsed conformations for all members of this series with average dihedral angles theta(C-P-B-C) of 8.1 degrees (8c), 12.3 degrees (8b) and 20.3 degrees (8a). 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.