A temperature-dependent phase transformation of (E)-2-[(4-chlorophenyl)imino]acenaphthylen-1-one

The crystal structure determination based on 90 K data of the title imine ligand revealed non-merohedral twinning with three twin domains. In our experience, this is an indication of an ordering phase transition. Consequently, the structure was redetermined with higher temperature data, and a reversible phase transition was discovered.

The crystal structure determination based on 90 K data of the title imine ligand, C 18 H 10 ClNO, revealed non-merohedral twinning with three twin domains. In our experience, this is an indication of an ordering phase transition. Consequently, the structure was redetermined with higher temperature data, and a reversible phase transition was discovered. The higher temperature phase is indeed an ordered structure. At the higher temperature, the 4-chlorophenyl group has rotated by ca 7 into a crystallographic mirror plane. Warming the crystal from 90 K to 250 K changes the space group from triclinic P1, to monoclinic P2 1 /m. Diverse non-classical interactions are present in the crystal packing, and these are described for the phase change reported in this work. The crystal structure of the title imine ligand, measured at 100 K, has been reported on previously [Kovach et al. (2011). J. Mol. Struct. 992, 33-38].

Chemical context
Transition metal complexes that can photochemically release carbon monoxide upon exposure to visible light have been reported recently (Chakraborty et al., 2014;Stenger-Smith et al., 2017). Facile release of carbon monoxide has been observed in manganese carbonyls containing acenaphthalene derivatives (Carrington et al., 2015) including the ligand imino]acenapthylen-1-one}, the subject of this study, shown in the Scheme. Our crystal structure determination of MIAN at 90 K agrees with the structure reported by Kovach et al. (2011) at 100 K. In particular, the structure occurs in the triclinic space group P1 and it is found to be a twin. In the NMR study of MIAN by Kovach et al., major and minor species were detected in CDCl 3 at room temperature and a single species at 388 K in DMSO-d 6 . They suggested that an E to Z equilibration with the E form dominant takes place at the elevated temperature. The occurrence of a low-symmetry space group and twinning are indicative of a solid-solid phase change, and we were curious about the structure at higher temperatures. While a change of conformation from E to Z would be a very large solid-state change, an alternative structural change would be possible. At 250 K, a small solid-state change was indicated and the new space group is P2 1 /m ( phase). The only difference, aside from small differences in unit-cell dimensions, is a rotation of the iminoacenapthylen-1-one group into a crystallographic mirror plane. In each phase, the molecule remains in the E conformation.

Structural commentary
The crystal structure was initially determined at 90 K. Three twin domains were found, with relative contributions of 0.441 (2), 0.058 (3), 0.060 (3). Redetermination of the structure at higher temperatures validated our suspicion that the structure was temperature-sensitive. In order to more easily compare the low-temperature and room-temperature crystal structures, a non-standard setting for the triclinic form was selected. In this setting the shortest axis is the b axis. The b axis is then the unique axis in the monoclinic setting of P2 1 /m. Since minor changes in unit-cell dimensions occur, the exact temperature of the phase change was difficult to determine, but examination of the diffraction images revealed obvious twinning between 90 and 208 K, coalescence of diffraction spots occurring at 230 K, and by 250 K it was clear that the twinning had vanished and the space-group symmetry had changed. Solution of the two structures showed that the structural effect of the temperature change goes from triclinic, P1 with Z = 2 (Z 0 = 1) to monoclinic, P2 1 /m with Z = 2 (Z 0 = 0.5). The most obvious structural change involves rotation and a change in the dihedral angle between the two molecular planes that brings the acenapthyl group into the crystallographic mirror plane. At 250 K the dihedral angle is 90 while at 90 K it is 83.16 (4) . The unit-cell volume is 2.5% larger at the higher temperature. As would be expected, thermal motion is greater at high temperature, with U eq averaging 0.047 Å 2 vs 0.017 Å 2 at low temperature. Thermal motion in the 4-chlorophenyl ring is slightly greater than the acenapthyl group at both temperatures, 13.5% greater in the -phase (90 K) and 10.0% in the -phase (250 K). Figs. 1 and 2, depict the high (-phase) and low (-phase) temperature structures, respectively. The similarity in the packing is evident from Figs. 3 and 4.

Supramolecular features
The two rings are perpendicular within each polymorph, likely due to a steric effect between H9, bonded to C9, and one of the ortho hydrogen atoms on the 4-chlorophenyl ring (with centroid Cg). Molecular structure of the title compound at 90 K (-phase), showing 50% thermal displacement parameters and the atom-numbering scheme. Table 1 Hydrogen-bond geometry (Å , ) for the -phase.

Figure 1
Molecular structure of the title compound at 250 K (-phase), showing 50% thermal displacement parameters and the atom-numbering scheme. Atoms C14 and C15 are related related to atoms C14A and C15A, respectively, by mirror symmetry.
the two ring systems, there is an intramolecular H9Á Á ÁCg distance of 2.90 Å in the 250 K structure and 2.85 Å in the 90 K structure (Tables 1 and 2). Neither structure has solventaccessible voids. We looked for intra-and intermolecular interactions that might be influential in the structural change.
The only significant non-classical hydrogen bond of the C-HÁ Á ÁA type present is found in the crystal structure of the lowtemperature structure (-phase), with a C-HÁ Á ÁCl i hydrogen bond linking neighbouring molecules to form chains along the c-axis direction (Table 2). There is, however,stacking between the acenapthyl groups in each case: the interplanar distance is 3.438 Å at 250 K and 3.409 Å at 90 K. In both phases there is a C-HÁ Á Á interaction on both sides of the phenyl ring, one intramolecular and one intermolecular (Tables 1 and 2 A view of the packing of the room temperature structure (-phase). The crystallographic mirror planes are shown in blue. Orange dots indicate the crystallographic centers of inversion.

Figure 4
A view of the packing of the low temperature structure (-phase). Orange dots indicate the crystallographic centers of inversion.

(E)-2-[(4-Chlorophenyl)imino]acenaphthylen-1-one (alpha)
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Hydrogen-bond geometry (Å, º)
Cg is the centroid of the 4-chlorophenyl ring (C13-C16/C14A/C15A). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.33 e Å −3 Δρ min = −0.24 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refined as a 4-component twin.