Preparation of phthalocyanines with eight benzylchalcogeno substituents from 5,6-dibromo-4,7-diethylbenzo[1,2,3]trichalcogenoles

Benzo[1,2,3]trichalcogenoles with two bromine atoms on the benzene ring, 5,6-dibromo-4,7-diethylbenzo[1,2,3]trichalcogenoles (1a) and (1b) (chalcogen: 1a = S; 1b = Se), were first prepared by treating 2,3,5,6-tetrabromo-1,4- diethylbenzene (TBDEB) with elemental sulfur or amorphous selenium in DBU at 140 °C (for 1a) and 100 °C (for 1b) for 24 h. The structures of 1a and 1b were verified by NMR spectroscopy, mass spectrometry, and elemental analysis. X-ray crystallographic analysis ultimately showed that the substitution reactions of TBDEB proceeded at the two adjacent bromine atoms. To apply 1a and 1b to construction of phthalocyanine derivatives with sulfur or selenium functional groups, 4,5-bis(benzylchalcogeno)-3,6-diethylphthalonitriles (5a) and (5b) as key intermediates were prepared by way of introduction of alkyl groups (2-cyanoethyl or 4-nitrophenethyl groups) on two chalcogen atoms, substitution of two bromine atoms with nitrile groups, and subsequent exchange of alkyl groups with benzyl groups. Compound 5a was treated with lithium in n-pentanol at 100 °C for 1 h to produce 2,3,9,10,16,17,23,24-octakis(benzylthio)-1,4,8, 11,15,18,22,25-octaethylphthalocyanine (6a). A similar treatment of 5b in n-hexanol at 100 °C for 2 h gave phthalocyanine 6b. The structures of 6a and 6b were determined by ¹H NMR spectroscopy and MALDI-TOFMS. X-ray crystallographic analysis of 6a was also performed. The Q-band absorptions (λ_max) for 6a and 6b in UV-vis spectra were observed at 755 nm (log ε = 5.1) and 757 nm (log ε = 5.1), respectively, and their electrochemical properties were verified by cyclic voltammetry in dichloromethane with Ag/AgNO₃ as a reference electrode. Compounds 6a and 6b were further treated with lithium in THF/NH₃ at -78 °C and then with dibutyltin dichloride to produce phthalocyanine derivatives 8a and 8b with four dichalcogenastannole rings by way of octachalcogenate phthalocyanines 7a and 7b.