Meisenheimer complex from picric acid and diisopropylcarbodiimide
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)Angstrom, b=15.47(2)Angstrom, c=17.74(2)Angstrom, alpha=110.43(9)degrees, beta=99.85(10)degrees, gamma=92.78(10)degrees, V=2629(6)Angstrom(3) for Z=6 (three molecules per asymmetric unit). Crystals of (6) are orthorhombic, Pna2(1) (#33), with a=24.12(4), b=9.011(19)Angstrom, c=10.87(2)Angstrom, V=2362(8)Angstrom(3) for Z=4. Nitro groups in the Meisenheimer complex are twisted 2-8degrees from the mean formerly aryl ring plane; in (5), nitro groups are twisted out of the aryl plane by 11-62degrees. 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.