Structure and conformation of polynitrodiphenylmethanes and related carbanions

Abstract

¹ H and ¹³C n.m.r. data are reported for a series of ortho and para nitro-substituted diphenylmethanes and related diphenylmethyl anions. In the case of the neutral molecules, a predominance of the skew conformation is suggested for mono- and di-o nitro-substituted derivatives, e.g. 2,4,4′-trinitro- and 2,2′4,4′-tetranitro-diphenylmeth anes, while a perpendicular conformation is preferred by the tri-ortho nitro-substituted derivative, e.g. 2,2′,4,4′,6-pentanitrodiphenylmethane. ¹³C shifts and ¹J_cαHα coupling constants indicate that, in all carbanions studied, the exocyclic α-carbon is sp² hybridized and carries only a small fraction of the negative charge. For carbanions with different substituents on the aromatic rings, e.g. 2,4,4′-trinitro- and 2,2′,4,4′,6-pentanitrodiphenylmethyl carbanions, the negative charge appears to be preferentially delocalized in the more substituted aromatic ring.

From arguments based upon the observed n.m.r. and u.v. spectroscopic data, the two rings in diphenylmethyl anions with up to two ortho nitro substituents are essentially coplanar, whereas steric interactions prevent such coplanarity in those with three or four such substituents.