New ligands in the 2,2′-dipyridylamine series and their Re(I) complexes; synthesis, structures and luminescence properties

Abstract

A series of new derivatives of the di(2-pyridyl)amine ligand has been prepared, having the general formula (py)₂NR, where R is one of a wide range of groups including naphthyl, pyrenyl, benzo-18-crown-6, pyrenyl, oxazolyl and bromo-biphenyl. All ligands are luminescent, with emission maxima between 370 and 520 nm, and with quantum yields in fluid solution of up to 0.74. Reaction of these ligands with Re(CO)₅Cl afforded complexes of the type [Re(CO)₃ClL] in which the di(2-pyridyl)amine ligand binds as an N,N-bidentate chelate via the two pyridyl units; five such complexes have been structurally characterised. With one exception the complexes are essentially non-luminescent, which we ascribe to the energetic availability of metal-based d-d excited states which can quench any MLCT or ligand-centred excited states. The exception is [Re(CO)₃Cl(tppd)] [where tppd = N,N,N′,N′-tetrakis(2-pyridyl)-p-phenylenediamine] in which one di(2-pyridyl)amine site is occupied by a {Re(CO)₃Cl} unit but the other is vacant; this luminesces in fluid solution at 516 nm (ϕ = 0.04, τ = 42 ns).