Non-covalent stacking interactions directing the structural and photophysical features of mono- and dinuclear cyclometalated palladium(II) complexes

The solution/solid state luminescence properties of selected orthometalated palladium complexes have been investigated in parallel with the relevant structural information provided by their X-ray crystal structures and theoretical calculations. Two cyclometalated backbones with different stacking abilities and a selection of bridging (OO)-O-boolean AND, (ON)-N-boolean AND or (NS)-S-boolean AND ligands comprise the series under study, [{Pd(mu-L) ((CN)-N-boolean AND)}(2)] ((CN)-N-boolean AND = N-phenylpyrazole (Phpz) I; N-benzylideneaniline (Bza) II; L = acetate (Aco) 1, succinimidate (succ) 2, phthalimidate (phthal) 3, 1-methylimidazoline-2 thionate (Smeimid) 4), completed with mononuclear [ Pd((CN)-N-boolean AND)(N-pClPhsal)] 5 (N-pClPhsal = chlorophenylsalycilaldiminate) complexes. New compounds I3, II4 and II5 were synthesized and the X-ray structures of I3, I5, II3, II4 and II5 have been elucidated in order to examine and compare solid-state Pd((CN)-N-boolean AND)-Pd((CN)-N-boolean AND) and ligand-ligand interactions with the rest of the series. The molecular structures of the complexes reveal intramolecular Pd. Pd distances between 2.842 and 2.999 angstrom and pi-pi and C-H center dot center dot center dot pi interactions. All complexes studied show emission in the solid state at room temperature and a relationship is observed between emission energy, the nature of the lowest energy excited state, and metal-metal interactions. DFT calculations are undertaken to gain insight into the relationship between the structure and photophysical properties of the complexes.