Polymorphs of 2,3-diphenyl maleic acid anhydride and 2,3-diphenyl maleic imide: Synthesis, crystal structures, lattice energies and fluorescence

Abstract

2,3-Diphenyl maleic acid anhydride (DPMA), (C₆H₅)₂C₄O₃, is a “historical” compound for which polymorphism, crystal morphology and fluorescence were first reported by Drugman in 1912. For 2,3-diphenyl maleic imide (DPMI), (C₆H₅)₂C₄O₂NH, crystallographic data have not been reported previously. In this work the polymorphs of DPMA have been crystallised from various solvents, from the supercooled melt and from the vapour phase. Small single crystals of the stable, orthorhombic α-phase (m.p. = 156 °C, ρ = 1.334 g/cm³) and of the metastable, monoclinic β-phase (m.p. = 146 °C, ρ = 1.350 g/cm³) were obtained, as described by Drugman. In addition, a fine powder of a previously unknown second metastable phase γ(m.p. = 149 °C) was crystallised. For DPMI, a stable, triclinic α-phase (m.p. = 216 °C, ρ = 1.307 g/cm³) and a metastable, monoclinic β-phase (m.p. = 212 °C, ρ = 1.296 g/cm³) were observed. The structures of the α- and β-polymorphs of DPMA and DPMI were determined by single-crystal X-ray diffraction. Space groups are Pbca for α-DPMA, P2₁/c for β-DPMA, P-1 for α-DPMI and C2/c for β-DPMI. In both DPMA polymorphs van der Waals interactions dominate. In both phases of DPMI, pairs of molecules form centrosymmetric dimers via H—N…O hydrogen bonds. Between the dimers significant van der Waals interactions exist. Lattice energy calculations based upon the Dreiding force-field using individual molecules as the building blocks for the crystal lattice yield reasonable energy values for DPMI. For DPMA, however, the calculated lattice energy is significantly higher for the stable phase than for the metastable phase. This observation correlates with the observed violation of the density rule for polymorphs, with ρ_α < ρ_β, and is discussed here in terms of the different conformational energies of the molecules. The results of fluorescence and Raman spectroscopic measurements on the α- and β-polymorphs of both DPMA and DPMI are reported.