A crystal modification of dibenzo[b,i][1,4,8,11]tetraaza[14]annulene: X-ray molecular structure and proton tautomerism of the highly π-conjugated form

Abstract

The crystal structure for one of crystal modifications of the title compound (taa) has been determined by X-ray diffraction at room temperature. The crystal system is monoclinic with a space group of P2₁/a : a=14.926(2), b= 5.2342(8), c=19.570(2)Å, β=112.195(7)°, Z= 4. The final R factor is 0.033 for 1 443 observed reflections to 270 variables. The crystals recrystallized from xylene have been proven to be a different crystal modification from those obtained by vacuum sublimation at 250 °C. In the present crystal, two independent molecules are located on the crystallographic centres of symmetry. Each of them shows positional disorder at the amino hydrogen atoms. This modification does not exhibit the alternation of the bond lengths in the 1,3-propanediiminato linkage beyond the limit of ΔI < 3σ(I), which is the case for a previously reported crystal modification. These findings lend support to dynamic imine-enamine tautomerization rather than static disorder. The difference Fourier map suggests a double-minimum potential for the proton tautomer ism. Rapid tautomerization in [²H₈]toluene solution can be observed by ¹H NMR spectroscopy down to –90 °C, just above the melting point of the solvent.