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.