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

A series of new derivatives of the di(2-pyridyl) amine ligand has been prepared, having the general formula (py)(2)NR, 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)(5)Cl afforded complexes of the type [Re(CO)(3)ClL] 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)(3)Cl(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)(3)Cl} unit but the other is vacant; this luminesces in fluid solution at 516 nm (phi = 0.04, tau = 42 ns).