Polymorphs of Picryl Bromide

Polymorphism is a phenomenon that has been observed often during the rich history of energetic materials development. Different polymorphs of the same compound can have different properties, which, in turn, can lead to substantial alterations in their stability and performance. Two polymorphs of the common energetic precursor, 2,4,6-trinitrobromobenzene (picryl bromide), were discovered in 1933; however, no X-ray crystal structures were reported then or have since been reported. Structural details of the two known polymorphs, the α form [triclinic, space group = P1̅, a = 14.8480(17) Å, b = 14.8614(17) Å, c = 15.3318(17) Å, α = 72.762(2)°, β = 80.350(2)°, γ = 60.225(2)°, V = 2803.8(6) ų, T = 93(2) K] and the β form [hexagonal, space group = P6₅, a = 148812(1) Å, c = 22.5449(4) Å, V = 4323.69(9) ų, T = 294(2) K] are detailed here. In addition, three new polymorphs are also presented, the γ form [triclinic, space group = P1̅, a = 14.9166(2) Å, b = 14.9424(2) Å, c = 30.3888(3) Å, α = 94.729(1)°, β = 93.754(1)°, γ = 119.925(1)°, V = 5805.61(12) ų, T = 294(2) K], the δ form [trigonal, space group = P3₁, a = 14.8610(7) Å, c = 44.007(3) Å, V = 8416.9(8) ų, T = 93(2) K], and the ε form [triclinic, space group = P1, a = 14.9086(1) Å, b = 14.9083(1) Å, c = 22.6784(2) Å, α = 95.245(1)°, β = 90.116(1)°, γ = 119.896(1)°, V = 4344.89(6) ų, T = 293(2) K]. Two different “triad” motifs, consisting of C−H···O hydrogen bonds and N−O···Br dipole-induced interactions, universal to all five forms of picryl bromide will be discussed, as well as apparent nitro−π associations between adjacent layers of molecules.