Study of the crystal and molecular structure of the 9-cyanoanthracene trans dimer and of its monomerisation

Abstract

The structure of the 9-cyanoanthracene trans dimer (9-CNAD) has been solved by direct methods [P1, a= 10.146(1)Å, b= 17.785(1)Å, c= 11.501(1)Å, α= 92.95(1)^°, β= 95.59(1)^°, γ= 89.82(1)^°, Z= 4]. No evidence for X-ray-induced monomerisation was observed, although a 20% fading of the crystal occurred during data collection. On heating a 9-CNAD crystal to 420 K, a yellow single crystal (9-CNADY) was obtained. Its crystal structure [P1, a= 10.217(2)Å, b= 10.235(2)Å, c= 11.594(2)Å, α= 95.20(2)^°, β= 90.37(2)^°, γ= 120.13(1)^°, Z= 2] was solved to show that the sample was still dimeric to an extent of 92%. This single-crystal → single-crystal transition is topotactic, and monomerisation occurs homogeneously. Further heating leads to a complete, heterogeneous and topotactic monomerisation. The monomeric component in 9-CNADY was not located. Final R was 0.0436 for 9-CNAD and 0.1015 for 9-CNADY.