Luminescent mononuclear and binuclear cyclometalated palladium(II) complexes of 6-Phenyl-2,2'-bipyridines: Spectroscopic and structural comparisons with platinum(II) analogues1,2

The mononuclear cyclometalated Pd(II) complexes [Pd(L¹)X] (HL¹ = 6-phenyl-2,2'-bipyridine; X = Cl, 1a; Br, 1b; I, 1c), [Pd(L¹)PPh₃]⁺ (1d), [Pd(L^2-5)Cl] [2a-5a, HL^2-5 = 4-(aryl)-6-phenyl-2,2'-bipyridine; aryl = phenyl (2), 4-chlorophenyl (3), 4-tolyl (4), 4-methoxyphenyl (5)] and the binuclear derivatives [Pd₂(L¹-5)₂(μ-dppm)]²⁺ (1e-5e, dppm = bis(diphenylphosphino)methane) and [Pd₂(L¹)₂(μ-dppC₅)]²⁺, (1f, dppC₅ = 1,5-bis(diphenylphosphino)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) Å, respectively) suggest minimal metal-metal interaction, although π-stacking of the aromatic ligands (interplanar separations 3.34 and 3.35 Å, respectively) is evident. All complexes display low-energy UV absorptions at λ ~ 390 nm, which are tentatively assigned to ¹MLCT transitions; red shifts resulting from Pd-Pd interactions in the binuclear 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 (λ(max) 467-586 nm) and all cationic species in the solid state (λ(max) 493-578 nm) are assigned to intraligand excited states. Complexes 1e-5e display dual emissions in MeOH/EtOH glass at 77 K, and the broad structureless bands at λ(max) 626-658 nm are attributed to π-π excimeric IL transitions. A comparison between the photophysical properties of Pd(II) and Pt(II) congeners is presented.