No Switching Cooperativity between Coordinated Azo Ligands on Complexes Having {M^II(phosphane-κ²<i>P</i>)}²⁺ (M = Pd, Pt) Scaffolds

A series of square-planar palladium and platinum compounds with <i>cis</i>-blocking phosphanes and terminal azobenzene ligands [M(dppp)(azo)₂](OTf)₂ (azo = CN(C₆H₄)-N═N-(C₆H₄)CN (<b>iso-cyano</b>), CN(C₆H₄)-N═N-(C₆H₅) (<b>iso-Ph</b>)) and [{M₂(tpbz)}(azo)₄](OTf)₄ (azo = CN(C₆H₄)-N═N-(C₆H₅) (<b>iso-Ph</b>)) have been synthesized and fully characterized. Similarly to the uncoordinated ligands, the new coordination compounds have shown to be photochemically active with respect to their trans-to-cis isomerization process. Their cis-to-trans back spontaneous reaction have been studied as a function of solvent, temperature and pressure and the corresponding activation parameters determined in order to investigate the mechanism of these transformations. The results obtained are indicative of the operation of a rotational mechanism with no cooperativity between the azo ligands attached to the same metal. Density functional theory calculations have been carried out in order to estimate the relative energies of the different photoisomers for the theoretical interpretation of the experimental data.