Dynamic Axial Ligand-Site Exchange in Facially Discriminated Ruthenium (II) Carbonyl and Rhodium (III) Halide Metalloporphyrins

For a series of six-coordinate Ru''(CO)L or Rh'''(X-)L porphyrins, which are facially differentiated by having a naphthoquinol- or hydroquinol-containing strap across one face, we show that ligand migration from one face to the other can occur under mild conditions and that ligand-site preference is dependent on the nature of L and X-. For bulky nitrogen-based ligands, the strap can be displaced sideways to accommodate the ligand on the same side as the strap. For the ligand pyrazine, we show ¹H NMR evidence for monodentate and bridging binding modes on both faces, dependent on ligand concentration and metalloporphyrin structure, and that interfacial migration is rapid under normal conditions. For monodentate substituted pyridine ligands, there is a site-dependence on structure, and we show clear evidence of dynamic ligand migration through a series of ligand-exchange reactions.