Unique Thermal Structural Phase Transitions Exhibited by Unsymmetrical Organometallic Gold(III)-Dithiolene Complexes with Pentylthio and Hexylthio Groups

Unsymmetrical gold(III)-dithiolene complexes are potential candidates for molecular materials that exhibit thermal structural phase transitions. In this study, unsymmetrical ppy-gold(III) (ppy(-)=C-deprotonated-2-phenylpyridine(-)) complexes [AuC5] and [AuC6] coordinated by dithiolene ligands containing tetrathiafulvalene (TTF) skeletons with pentylthio (2-{bis(pentylthio)-1,3-dithiol-2-ylidene}-1,3-dithiol-4,5-dithiolate(2-)) and hexylthio groups (2-{bis(hexylthio)-1,3-dithiol-2-ylidene}-1,3-dithiol-4,5-dithiolate(2-)) were synthesized. Both complexes exhibited a large absorption band at approximately 508 nm, owing to intramolecular ligand-to-ligand charge transfer. One-dimensional columnar structures with head-to-tail molecular arrangements around the metal ions were constructed in the crystals. The flexible alkylthio groups were intercalated into crystalline spaces between dithiolene ligands in the columns. [AuC5] exhibits a simple phase transition at 198 degrees C between crystalline and isotropic phases irreversibly. The crystalline phase of [AuC6] observed at 25 degrees C melted at 148 degrees C. Another crystalline phase grew above 148 degrees C with a very slow crystallization rate from the liquid phase and was completely transformed into an isotropic phase at 200 degrees C.

Automated summary

Unsymmetrical ppy-gold(III) complexes coordinated by dithiolene ligands with pentylthio and hexylthio groups were synthesized. The complexes formed one-dimensional columnar structures in crystals with head-to-tail molecular arrangements around the metal ions. [AuC5] exhibited a simple phase transition between crystalline and isotropic phases irreversibly at 198°C, while [AuC6] melted at 148°C and slowly crystallized to a different crystalline phase above 148°C, which transformed into an isotropic phase at 200°C.