The effects of introducing sterically demanding aryl substituents in [Cu(N^N)(P^P)]⁺ complexes

The syntheses and characterizations of six [Cu(N^N)(POP)][PF₆] and [Cu(N^N)(xantphos)][PF₆] compounds (POP = bis(2-(diphenylphosphino)phenyl)ether, xantphos = 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene), in which N^N is a bpy ligand (1-Naphbpy, 2-Naphbpy, 1-Pyrbpy) bearing a sterically hindered 1-naphthyl, 2-naphthyl or 1-pyrenyl substituent in the 6-position, are reported. Single-crystal structure determinations of five complexes confirm a distorted tetrahedral environment for copper(i) and a preference for the N^N ligand to be oriented with the sterically-demanding aryl group being remote from the (C₆H₄)₂O unit of POP or the xanthene 'bowl' of xantphos. The angle between the ring planes of the bpy range from 5.8 to 26.0° and this is associated with interactions between the aryl unit and the phenyl substituents of the P^P ligand. In solution at room temperature, the complexes undergo dynamic behaviour which has been investigated using variable temperature 2D NMR spectroscopy. The [Cu(N^N)(xantphos)]⁺ complexes exist as a mixture of conformers which interconvert through inversion of the xanthene bowl-shaped unit; the preference for one conformer over the other is significantly changed on going from N^N = Phbpy to 1-Pyrbpy (Phbpy = 6-phenyl-2,2'-bipyridine). The electrochemical and photophysical properties of the [Cu(N^N)(POP)][PF₆] and [Cu(N^N)(xantphos)][PF₆] compounds are presented; the compounds are orange emitters but the introduction of the 1-naphthyl, 2-naphthyl or 1-pyrenyl substituents result in poor photoluminescence quantum yields.