Synthesis and characterisation of neutral mononuclear cuprous complexes based on dipyrrin derivatives and phosphine mixed-ligands

Abstract

Heteroleptic neutral mononuclear cuprous complexes with dipyrrin derivatives and phosphine mixed-ligands including 1,3,7,9-tetramethyldipyrrin (1), 5-phenyl-1,3,7,9-tetramethyldipyrrin (2), 2,8-dibromo-1,3,7,9-tetramethyldipyrrin (3), 1,9-dichloro-5-phenyldipyrrin (4), 1,9-dibromo-5-phenyldipyrrin (5), 5-pentafluorophenyl-1,3,7,9-tetramethyldipyrrin (6) and 1,5,9-triphenyldipyrrin (7) have been synthesized and fully characterized. The central Cu(I) atoms of these complexes in general formulas of Cu(1–6)(PPh₃)₂ (1a–6a) and Cu(1–6)(DPEphos) (1b–6b) [DPEphos = bis(2-diphenylphosphinophenyl)ether] all exhibit a pseudo-tetrahedral geometry, while complex Cu(7)(PPh₃) (7a) is tricoordinated in a pyramidal conformation due to the large steric hindrance of ligand 7. The oxidation potentials assigned to oxidations of Cu(I)–Cu(II) are extraordinarily low in the range of 0.36–1.02 V vs. Ag/AgCl compared with traditional [Cu(phen)(PP)]⁺ analogues. Their emission maxima range from 495 to 595 nm in dichloromethane at room temperature with quantum yields of 0.05–4.03% and lifetimes on the order of nanoseconds. Unlike the characteristic MLCT emission in cationic Cu(I) complexes, the emissions are assigned to the dipyrrin-centered intraligand charge transition (ILCT) based on the fact that the increased conjugation within the dipyrrinato anion leads to a weaker metal–ligand interaction, thus preventing the mixing of π orbitals of ligand and 3d orbitals of Cu(I) atom. This conclusion is also supported by electrochemical data and theoretical calculations.