From an antiferromagnetic insulator to a strongly correlated metal in square-lattice MCl(pyrazine) coordination solids.

Electronic synergy between metal ions and organic linkers is a key to engineering molecule-based materials with a high electrical conductivity and, ultimately, metallicity. To enhance conductivity in metal-organic solids, chemists aim to bring the electrochemical potentials of the constituent metal ions and bridging organic ligands closer in a quest to obtain metal-d and ligand-π admixed frontier bands. Herein, we demonstrate the critical role of the metal ion in tuning the electronic ground state of such materials. While VCl(pyrazine) is an electrical insulator, TiCl(pyrazine) displays the highest room-temperature electronic conductivity (5.3 S cm) for any metal-organic solid involving octahedrally coordinated metal ions. Notably, TiCl(pyrazine) exhibits Pauli paramagnetism consistent with the specific heat, supporting the existence of a Fermi liquid state (i.e., a correlated metal). This result widens perspectives for designing molecule-based systems with strong metal-ligand covalency and electronic correlations.