A design strategy for stable light-sensitive palladium complexes

We describe the important determinants of stabilizing/destabilizing azobenzene-based palladium complexes capable of undergoing repeated light-triggered conformation changes. Inspired by highly warped palladium complexes with distorted azobenzene ligands, three different types of azobenzene ligands (1–3) were chosen to modify both the molecular structure (from a ubiquitous planar to a twisted trans-azobenzene) and the solubility of the azobenzenes in organic solvents. Single-crystal X-ray structure analyses were carried out for ortho-alkylated azobenzene (ortho-Az: 1b), meta-alkylated azobenzene (meta-Az: 2b) and PdCl2(ortho-Az)2 (1-Pd). Whereas both 2b and 3 lacking the meta and ortho substituents had planar structures, ortho-alkylated 1b was highly distorted by 68.06° from planarity. In stark contrast to PdCl2(meta-Az)2 (2-Pd) and PdCl2(Az)2 (3-Pd), which quickly dissociated in organic solvents, 1-Pd was very stable, especially in nonpolar solvents, so that it was possible to successfully purify and characterize them. The results suggest that unusually distorted trans-azobenzene is hardly influenced by the complexation reaction which requires considerable distortion of the azobenzene unit, thus stabilizing mononuclear palladium complexes. In nonpolar solvents, 1-Pd underwent repeated conformation changes under alternating UV and visible light irradiation. However, in polar solvents, the UV-triggered conformation change was accompanied by facile light-assisted breaking of the N:→Pd bond. Even dark incubation in polar organic solvents caused the dissociation of azobenzene ligands from the complexes. The breaking rate of the N:→Pd bond increased in the order of benzene ≈ dichloromethane < acetone < DMF, with more polar solvents inducing faster dissociation. The solvent polarity effect on the stability of azobenzene-based complexes can be interpreted in terms of the degree of polarization of the metal–ligand bond formed as a consequence of interactions between the palladium ion as a soft acid and azobenzene nitrogen as a hard base.