Vitaly Shteiman

Irreversible single-crystal to polycrystal and ­reversible single-crystal to single-crystal phase transformations in cyanurates

4,6-Dimethoxy-3-methyldihydrotriazine-2-one (1) undergoes a single-crystal to single-crystal reversible phase transformation at 319 K. The low-temperature phase crystallizes in monoclinic space group P2(1)/n with two crystallographically independent molecules in the asymmetric unit. The high-temperature phase is obtained by heating a single crystal of the low-temperature phase. This phase is orthorhombic, space group Pnma, with the molecules occupying a crystallographic mirror plane. The enthalpy of the transformation is 1.34 kJ mol(-1). The small energy difference between the two phases and the minimal atomic movement facilitate the single-crystal to single-crystal reversible phase transformation with no destruction of the crystal lattice. On further heating, the high-temperature phase undergoes methyl rearrangement in the solid state. 2,4,6-Trimethoxy-1,3,5-triazine (3), on the other hand, undergoes an irreversible phase transformation from single-crystal to polycrystalline material at 340 K with an enthalpy of 3.9 kJ mol(-1); upon further heating it melts and methyl rearrangement takes place.

Methyl Rearrangement of Methoxy-Triazines in the Solid- and Liquid-State

Abstract 2,4,6-Trimethoxy-1,3,5-triazine 11, and 6-methoxy-3,5-dimethyl-tetrahydrotriazine-2,4-dione 13 undergo intermolecular O→N methyl rearrangement in the liquid-state to 1,3,5-trimethyl 2,4,6-trioxohexahydro-s-triazine 14. 4,6-Dimethoxy-3-methyl-dihydro-triazine-2-one 12 was found to exhibit a different thermal behavior, and the methyl rearrangement takes place in the solid-state. The thermal behavior of each was investigated by calorimetry and high-temperature X-ray diffraction. It was found that 11 undergo phase transition to 11′, and the methyl rearrangement takes place in the melt. The solid-state methyl rearrangement of 12 is topochemically controlled. Two courses of methyl migration in the solid-state of 12 are proposed. A quantitative analysis of samples of 12 heated to different temperatures proves the existence of the two courses. A computer simulation was used to rationalize the reaction routes in 12 and 13.

Synthesis, structural, and magnetic characterization of substituted benzoimidazole-l-yl N,N′-dioxides

The crystal structures, EPR spectra and magnetic properties of the novel halogen- and cyano-substituted nitronyl nitroxide radicals 2-(2,6-dichlorophenyl)benzimidazolyl N,N′-dioxide, 6, 2-(2,6-difluorophenyl)benzimidazolyl N,N′-dioxide, 7, 2-(2-chloro-6-fluorophenyl)benzimidazolyl N,N′-dioxide, 8, 2-(2,3,6-trichlorophenyl)benzimidazolyl N,N′-dioxide, 9, 2-(2,3,4,5,6-pentafluorophenyl)benzimidazolyl N,N′-dioxide, 10, and 2-(3-cyanophenyl)benzimidazolyl N,N′-dioxide, 11, are reported. Compound 6 crystallizes in the triclinic crystal system in space group P. The molecules of 6 are arranged in pairs with short intermolecular distances between the NO groups. 7 crystallizes in two different modifications: polymorph α is orthorhombic, space group Pbca; polymorph β is monoclinic, space group P21/c. 8 crystallizes in two modifications: the α polymorph is monoclinic, space group P21/c; and the β polymorph is monoclinic, space group P21/n. 9 crystallizes in the monoclinic system, space group P21/c. 10 crystallizes in the monoclinic system, space group C2/c. The molecules of 10 are packed in pairs of two types that form a chain perpendicular to the c-axis. 11 crystallizes in the monoclinic crystal system in space group P21/c. The rotation angle between the two rings in compounds 6–10 is 54.2–76.7°. The rotation angle between the two rings is only 21.0° in 11 and it strongly affects the packing of the molecules that adopt the stacking mode. The magnetic measurements show that 6, 7, 10 and 11 exhibit large magnetic coupling. The best fitting with the experimental data for 6 and 11 was obtained using the Bleaney–Bowers singlet–triplet model plus the Curie–Weiss spin impurity (S = 1/2; H = −2JS1·S2) J/kB = −84.2 K and θimp = 0.3 K and J/kB = −95.3 K, θimp = 1.8 K, respectively. A Pade expression for 7 revealed Jintra/kB = 66.0 K and zJinter/kB = −14.0 K. Compound 10 shows evidence for large antiferromagnetic spin coupling (θ = −37.0 K Curie–Weiss model).

Alkyl Rearrangement of Derivatives of 2-Anilino-4,6-dialkoxy-1,3,5-triazines in the Solid- and Liquid-state

Abstract 2-Anilino-4,6-dimethoxy-1,3,5-triazine (13), 2-anilino-4,6-diethoxy-1,3,5-triazine (14), 2-(2′-nitoanilino) 4,6-dimethoxy-1,3,5-triazine (15) undergo alkyl rearrangement in the liquid-state, while 2-(4′-nito-anilino) 4,6-dimethoxy-1,3,5-triazine (16) undergoes methyl rearrangement in the solid-state. The crystal structure and thermal behavior of these compounds are described. 13 crystallizes in monoclinic P21/c space group, a = 11.030(4), b = 6.345(4), c = 16.315(4) Å, β = 90.76(3)°. The calculated density for Z = 4 is 1.351 Mg/m3. The number of unique reflections collected is 2092, and the final R = 0.0643 [I > 2σ(I)]. 14 crystallizes in triclinic P-1 space group, a = 7.700(2), b = 9.723(3), c = 10.154(3) Å, α = 78.78(3), β = 70.32(3), γ = 73.67(3)°. The calculated density for Z = 2 is 1.266 Mg/m3. The number of unique reflections collected is 2401, and the final R = 0.0561 [I > 2σ(I)]. 15 crystallizes in monoclinic P21/m space group, a = 11.020(3), b = 6.600(2), c = 8.409(3) Å, β = 99.72(3)°. The calculated density for Z = 2 is 1.527 Mg/m3. The number of unique reflections collected is 1153, and the final R = 0.0502 [I > 2σ(I)]. 16 crystallizes in monoclinic P21/c space group, a = 7.499(3), b = 21.846(5), c = 7.895(3) Å, β = 115.42(3)°. The calculated density for Z = 4 is 1.576 Mg/m3. The number of unique reflections collected is 2036, and the final R = 0.0757 [I > 2σ(I)].

Topochemically controlled solid-state methyl ­rearrangement in thiocyanurates

4,6-Dimethoxy-3-methyl-1,3,5-triazine-2(3H)-thione crystallizes in two polymorphic forms, needles and plates. In the needle-shaped crystals (9a) the molecules occupy the crystallographic mirror plane, thus the layers are stacked along the b axis. The molecules of the other polymorph [plate-shape crystals, (9b)] are packed in a herringbone packing mode. Upon heating, (9b) undergoes a phase transition to form (9a). At 378 K the needles undergo O --> S topochemically controlled methyl transfer in the solid state to produce 1-methyl-4-methoxy-6-methylthio-1,3,5-triazine-2(1H)-one in 75% yield. The enthalpy of the rearrangement is estimated to be -39.1 kJ mol(-1). 1-Methyl-6-methoxy-4-methylthio-1,3,5-triazine-2(1H)-thione crystallizes in space group P2(1) with two crystallographically independent molecules in the asymmetric unit. Compound (9b) undergoes O --> S methyl transfer in the solid state at 373 K. The rearrangement is topochemically assisted and the product, 1-methyl-2,4-bismethylthio-1,3,5-triazine-6(1H)-one, is obtained in quantitative yield. The enthalpy of the rearrangement is estimated to be -58.8 kJ mol(-1). The crystal structures of the compounds as well as their DSC thermographs are described and discussed. Energy calculation by ab initio methods shows that the driving force for the reactions is the difference between the molecular energies of the pre-rearranged compounds and their products, 54.2 and 59.3 kJ mol(-1) in the two cases, respectively.

3,5-Dimethyl-6-(4-nitroanilino)-2,3,4,5-tetrahydro-1,3,5-triazine-2,4-dione, (II), 3,5-dimethyl-6-(N-methyl-4-nitroanilino)-2,3,4,5-tetrahydro-1,3,5-triazine-2,4-dione, (III), and 1,3,5-trimethyl-6-(4-nitrophenylimino)-1,3,5-triazinane-2,4-dione, (IV)

Methyl rearrangement in the solid state of 2,4-dimethoxy-6-(4-nitroanilino)-1,3,5-triazine reveals three different products: (II), C 11 H 11 N 5 O 4 H 2 O, (III), C 12 H 13 N 5 O 4 , and (IV), C 12 H 13 N 5 O 4 . (II) crystallizes with a molecule of water and (IV) crystallizes in two polymorphic forms. The conformation of the molecules is expressed by the torsion about the bond joining the triazine and the aniline moieties. The two moieties are almost coplanar in (II) due to an intramolecular hydrogen bond between a triazine-N atom and a phenyl-H atom. The methyl groups of the triazine exert severe steric repulsion in the other two compounds, affecting not only the conformation but also the exocyclic bond angles.