Boron trifluoride complexes. Part VII. Ground-state properties of complexes of aromatic aldehydes

Abstract

Aromatic aldehydes form stable σ–σ complexes with boron trifluoride. The i.r., ¹H n.m.r., ¹³C n.m.r., ¹⁹F n.m.r., and dynamic n.m.r. spectra of a series of such complexes establish the CHO:BF₃ group as a pseudo-substituent. Very good correlations were observed between the i.r. carbonyl frequencies and the ¹H n.m.r. chemical shifts of the formyl proton with the respective Brown σ⁺ substituent constants emphasizing the dependence of these quantities on the charge density on the carbonyl carbon atom of the complexes. The positive charge densities in the formyl carbon atom of the BF₃ complexes were found to be significantly higher than in the respective free aldehydes.