Coordination Chemistry at the Hard–Soft Interface: Phosphine Oxide-Based Rare Earth/Transition Metal Complexes

A series of monometallic Al(III), Sm(III), Dy(III), Er(III), and Yb(III) complexes, featuring tetraphenyldiphosphine monoxide (PPO) as a ligand, were synthesized and characterized. These complexes served as precursors for the construction of heterobimetallic rare earth (RE)/transition metal (TM) assemblies. Attempts to introduce soft TMs, such as Cu(I) and Au(I), into the preformed RE–PPO synthons predominantly afforded equilibrium-driven TM-based POP species, underscoring the challenges of incorporating hard and soft metal centers directly. This observation led to an alternative route employing a presynthesized Mo–PPO synthon, which led to the successful formation of RE/Mo heterobimetallic complexes upon addition of the RE ions. The molecular structures of both mono- and heterobimetallic species were strongly influenced by the solvent environment. Notably, reactions in dichloromethane, a noncoordinating solvent, yielded RE2 complexes featuring chloride bridges, a motif absent when coordinating solvents, such as THF or MeCN were employed. This solvent-dependent structural divergence offers a straightforward strategy for modulating the metal nuclearity within the complexes.