Meisenheimer complex from picric acid and diisopropylcarbodiimide

Abstract

Reaction of picric acid with diisopropylcarbodiimide produces a pale yellow 1:1 adduct N-(2′,4′,6′-trinitrophenyl)-N,N′-di(isopropyl)urea (5) and a 1:2 adduct fluorescent red zwitterionic Meisenheimer complex (6) as approximately N′,N”,N′”-tri(isopropyl)-4-oxo-6-(isopropyliminio)-2-s-(2H)triazinespiro-1′-2′,4′,6′-trinitro cyclohexadienylide. Crystals of (5) are triclinic, P-1 (#2), with a = 10.444(13) Å, b = 15.47(2) Å, c = 17.74(2) Å, α = 110.43(9)°, β = 99.85(10)°, γ = 92.78(10)°, V = 2629(6) ų for Z = 6 (three molecules per asymmetric unit). Crystals of (6) are orthorhombic, Pna2₁ (#33), with a = 24.12(4), b = 9.011(19) Å, c = 10.87(2) Å, V = 2362(8) ų for Z = 4. Nitro groups in the Meisenheimer complex are twisted 2–8° from the mean formerly aryl ring plane; in (5), nitro groups are twisted out of the aryl plane by 11–62°. In the Meisenheimer complex, cationic charge is distributed over an iminium/guanidinium group; anion charge is distributed over the cyclohexadienide ring and attached nitro groups.