Ligand-enabled stabilisation of elusive copper metallocenes

Abstract

The synthesis and isolation of copper metallocenes have remained a long-standing challenge in organometallic chemistry due to the inherent propensity of copper to promote carbon–carbon bond formation rather than sustaining in the sandwich coordination. Evans and coworkers addressed this obstacle and reported a ligand-centric strategy for isolating bis(cyclopentadienyl) copper complexes using sterically encumbered tri-tert-butylcyclopentadienyl (Cp^ttt) ligands. Computational and spectroscopic analyses suggest that, despite a change in formal oxidation states of copper from +1 to +3, the electronic charge at the copper centre remains largely invariant, highlighting the dominant role of ligand-based charge delocalisation and dynamic coordination. This work establishes a conceptual framework for stabilising electronically flexible, low-coordinate metal centres, and provides new insights into the interplay between steric confinement and electronic structure in transition-metal complexes.

Graphical abstract

A missing member of the 3d metallocene series has been realised through the synthesis and structural characterisation of Cp^ttt₂Cu, the first neutral bis(cyclopentadienyl) copper complex, providing a true ‘cuprocene’ analogue and redefining bonding at copper. Importantly, the ligand has the flexibility of coordination to stabilise Cu in this set of three metallocenes, both in terms of structure and electron delocalisation.

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