From an antiferromagnetic insulator to a strongly correlated metal in square-lattice MCl2(pyrazine)2 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 VCl₂(pyrazine)₂ is an electrical insulator, TiCl₂(pyrazine)₂ displays the highest room-temperature electronic conductivity (5.3 S cm^–1) for any metal-organic solid involving octahedrally coordinated metal ions. Notably, TiCl₂(pyrazine)₂ 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. Achieving high conductivity in metal-organic solids can be challenging, due to the difficulty of obtaining a good overlap between the d-orbitals of the metal and the π-orbitals of the organic molecule. Here, the authors present two coordination solids, VCl₂(pyrazine)₂ and TiCl₂(pyrazine)₂, with remarkably different electrical conductivity. While the former is an insulator, the latter displays the highest conductivity of any octahedrally coordinated metal ions based metal-organic solid. [ABSTRACT FROM AUTHOR]