The effect of salt formation on the molecular structure and charge distribution in imines: the crystal and molecular structures and the ultraviolet, infrared, and the 1H and 13C nuclear magnetic resonance spectra of 2,6-dimethyl-4-(p-nitrophenylimino)pyran and its hydrobromide and methobromide salts

Abstract

The crystal structures of the title compound (1) and its hydrobromide (2) and methobromide (3) salts have been determined by X-ray crystallography in order to investigate the changes which take place in structure (1) on salt formation. Crystals of both (1) and (2) are monoclinic, with space group P2₁/c and Z= 4, and having unit cell dimensions respectively a= 12.021(1), b= 8.117(1), c= 13.562(2)Å, β= 115.72(1)°, and a= 10.133(2), b= 16.965(4), c= 8.277(2)Å, β= 102.55(2)°. Crystals of (3) are triclinic, space group P, and with Z= 2 in a unit cell of dimensions a= 7.929(2), b= 14.480(2), c= 6.549(1)Å, α= 90.12(1), β= 98.56(2), γ= 95.37(2)°. Structure (1) was solved by direct methods, and (2) and (3) by the heavy-atom Patterson method; refinements were to R 0.041 for 2 165 independent refelctions in (1), to R 0.047 for 1 855 in (2), and to R 0.032 for 2 754 in (3). There is a systematic increase in CN bond length along the series; all such distances are longer, however, than normal values, as substantiated by v_CN frequencies below 1 600 cm^–1 in the i.r. spectra. Bond parameters for the pyran ring show an increase in pyrylium character on salt formation, the effect being largest in (3). Increased positive charge on the pyran rings in (2) and (3) is confirmed by shifts in their ¹H and ¹³C n.m.r. signals relative to (1), though these reveal no obvious distinction between the two salts. There is a relatively short [2.34(4)Å] linear N–H ⋯ Br hydrogen bond in the hydrobromide. Structural and spectroscopic data support an inversion mechanism for isomerisation about the CN bond in the imine (1), consistent with the observed low ΔG^‡ value, while isomerisation in the two salts (with higher ΔG^‡ values) is probably by a different mechanism.