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

Abstract

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 O^O, O^N or N^S ligands comprise the series under study, [{Pd(μ-L) (C^N)}₂] (C^N = 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(C^N)(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(C^N)–Pd(C^N) 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 Å and π–π and C–H⋯π 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.