Luminescent mononuclear and binuclear cyclometalated palladium(II) complexes of 6-phenyl-2,2 '-bipyridines: Spectroscopic and structural comparisons with platinum(II) analogues

The mononuclear cyclometalated Pd(II) complexes [Pd(L-1)X] (HL1 = 6-phenyl- 2,2'-bipyridine; X = Cl, 1a; Br, 1b; I, 1c), [Pd(L-1)PPh3](+) (1d), [Pd(L2- 5)Cl] [2a-5a, HL2-5 = 4-(aryl)-6-phenyl-2,2'-bipyridine; aryl = phenyl (2), 4-chlorophenyl (3), 4-tolyl (4), 4-methoxyphenyl (5)] and the binuclear de rivatives [Pd-2(L1-5)(2)(mu-dppm)](2+) (1e-5e, dppm = bis(diphenylphosphino )methane) and [Pd-2(L-1)(2)(mu-dppC(5))](2+), (1f, dppC(5) = 1,5-bis(diphen ylphosphino)pentane) were prepared. The crystal structures of 1d(ClO4), 1e( ClO4)(2).DMF, and 2e(ClO4)(2) have been determined by X-ray crystallography . The magnitude of the Pd-Pd distances in 1e and 2e (3.230(1) and 3.320(2) Angstrom, respectively) suggest minimal metal-metal interaction, although p i-stacking of the aromatic ligands (interplanar separations 3.34 and 3.35 A ngstrom, respectively) is evident. All complexes display low-energy UV abso rptions at lambda similar to nm, which are tentatively assigned to (MLCT)-M -1 transitions; red shifts resulting from Pd-Pd interactions in the binucle ar species are not apparent. The complexes in this work are non-emissive at 298 K, but the cationic derivatives exhibit intense luminescence at 77 K. The structured emissions of 1d and 1f in MeOH/EtOH glass (lambda(max) 467-5 86 nm) and all cationic species in the solid state (lambda(max) 493-578 nm) are assigned to intraligand excited states. Complexes 1e-5e display dual e missions in MeOH/EtOH glass at 77 K, and the broad structureless bands at l ambda(max) 626-658 nm are attributed to pi-pi excimeric IL transitions. A c omparison between the photophysical properties of Pd(II) and Pt(II) congene rs is presented.