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

## TL;DR

This paper explores the synthesis and structural behavior of rare earth and transition metal complexes using phosphine oxide ligands, highlighting the influence of solvent conditions on their formation.

## Contribution

The study introduces a novel approach to forming rare earth/transition metal complexes using a presynthesized Mo–PPO synthon.

## Key findings

- Attempts to incorporate soft metals like Cu(I) and Au(I) into RE–PPO complexes mainly produced TM-based POP species.
- Using a presynthesized Mo–PPO synthon enabled successful formation of RE/Mo heterobimetallic complexes.
- Solvent choice significantly affected the molecular structures, with noncoordinating solvents yielding chloride-bridged RE2 complexes.

## Abstract

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.