One-step synthesis of ball-shaped metal complexes with a main absorption band in the near-IR region

The design of near-IR materials is highly relevant to energy and pharmaceutical sciences due to the high proportion of near-IR irradiation in the solar spectrum and the high penetration of near-IR light in biological samples. Here, we show the one-step synthesis of hexacoordinated ruthenium and iron complexes that exhibit a main absorption band in the near-IR region. For that purpose, novel tridentate ligands were prepared by condensation of two diimines and four cyanoaryl derivatives in the presence of ruthenium and iron template ions. This method was applied to a wide variety of cyanoaryl, diimine, and metal ion combinations. The relationship between the structure and the optical and electrochemical properties in the resulting complexes was examined, and the results demonstrated that these compounds represent novel near-IR materials whose physical properties can be controlled based on rational design guidelines. The intense absorption bands in the 700–900 nm region were assigned to metal-to-ligand charge transfer (MLCT) transitions, which should allow applications in materials with triplet excited states under irradiation with near-IR light.


Materials
Unless otherwise noted, materials were purchased from Tokyo Kasei Co., Aldrich Inc., and other commercial suppliers and were used after appropriate purification (distillation or recrystallization).
Diimine 5a-c and 6a were synthesized according to published procedures. v

Synthesis of 1,3-diiminoisoindoline derivative 6b
3, 6-Bis(pentylthio)-phthalonitrile vi (3.0 g, 9 mmol) was suspended in ethylene glycol (40 mL) and heated to 115°C. Sodium (ca. 10 mg) was added to the solution and gaseous NH3 was bubbled through the suspension for 2 h. Then the reaction mixture was poured into iced water (350 mL). After the mixture was stirred for 30 min at rt, the precipitate was collected by filteration and washed with water repeatedly. The target compound with some impurities was obtained as a dark yellow powder (3.2 g, 9 mmol, quant yield), and used for the next reaction without further purification.
The mixture was stirred under reflux for 12 h, before it was concentrated. The residue was dissolved and extracted with CHCl3. The organic layer was washed with brine and dried over Na2SO4, filtered and concentrated in vacuo. The product was purified by column chromatography on silica gel (eluent: CHCl3/Hexane = 1/1 v/v) to provide 1a as a moss-green solid (23.6 mg, 18.8 µmol, 15%) after recrystallization from methanol.

Synthesis of 1e
Under Ar atmosphere, diimine 5c (100.5 mg, 0.25 mmol) and ruthenium trichloride (31.6 mg, 0.15 mmol) were dissolved in benzonitrile (1.0 mL, 9.7 mmol) and dimethylaminoethanol (0.5 mL), then DBU (0.1 mL, 0.67 mmol) was added. After the mixture was stirred under reflux for 12 h, the reaction mixture was concentrated. The residue was dissolved and extracted with CHCl3. The organic layer was washed with brine and dried over Na2SO4, filtered and concentrated in vacuo.
After the mixture was stirred under reflux for 20 h, the reaction mixture was concentrated. The residue was dissolved and extracted with CHCl3. The organic layer was washed with brine and dried over Na2SO4, filtered and concentrated in vacuo.          Copying the excited state density for this state as the 1-particle RhoCI density.