Unusual azobenzene/bipyridine palladacycles: structural, dynamical, photophysical and theoretical studies

Two types of Pd(II) azobenzene/bipyridine complexes with unusual coordination mode of azobenzenes, PdCl{(mu-Cl)(mu-R1C6H3N=NC6H3R2)} Pd(bpy) 1a-4a and [(bpy) PdCl(mu-NH2C6H3N=NC6H4)-Pd(bpy)]Cl 3b were formed by the reaction of dicyclopalladated azobenzenes (DMF) PdCl(mu-R1C6H3N=NC6H3R2) PdCl(DMF) with excess of bpy, where bpy=2,2'-bipyridine; R-2=H and R-1=H (1), CH3 (2), NH2 (3) or R-1=N(CH3)(2) and R-2=NO2 (4). Neutral species 1a-4a were obtained in acetone, while in DMSO or MeOH the ionic complex 3b was produced. When dissolved, 3b decomposes to 3a and free bpy; however in DMSO upon addition of bpy 3b crystallizes again. X-ray structures of all complexes confirmed breaking of one Pd-N bond in the initial precursors, thus allowing rotation of one phenyl ring and positioning of both Pd atoms on the same side of the azobenzene ligand. Two Pd atoms are connected by the azobenzene ligand and in neutral complexes additionally by the Cl-bridge. In all complexes in the solid-state azobenzenes act simultaneously as monodentate C- and bidentate C,N-donors while bpy acts as bidentate donor. Variable-temperature H-1 NMR experiments established that structures of 1a-4a in DMF and DMSO at ambient temperature are not consistent with solid-state structures due to the fast exchange of one of the bpy nitrogen atoms bound to the Pd atom with solvent molecules. Theoretical studies confirmed the experimental structures as the most stable isomers. Photoabsorption and photoemission properties of the new complexes have been measured and photoabsorption is rationalized by time dependent DFT calculations. The presence of bpy significantly increases the intensity of fluorescence either in the solution (4a) or in the solid state (3a, 4a, 3b) at ambient temperature.