Interplay Between Oxidation State, Intervalence Charge Transfer Transitions and Electronic Conductivity in a Series of Ru‐Based Metal–Organic Frameworks

ABSTRACT Multinuclear Ru complexes connected by multitopic ligands were central in the development of electron transfer theories. Infinite coordination frameworks constructed from these motifs could inherit their unique electron delocalization and spectroscopic properties, allowing to systematically explore their correlation in the solid‐state. However, controlling the crystallization of Ru‐based frameworks continues to present a major hurdle. Herein, four nitrogen‐based heterocyclic ligands of different lengths were used to construct a series of metal–organic frameworks (MOFs) with the general formula [Ru(L) 2 Cl 2 ], L = pyrazine (pz), 2,6‐naphthyridine (naph), 4,4′‐bipyridine (bipy), and 1,4‐di(4‐pyridyl)‐benzene (bpbn). Structural analysis revealed two‐dimensional square grid connectivity with the ligand length governing layer packing and interpenetration. The existence of electronic delocalization through the framework backbones was confirmed using electrochemical and spectroscopic techniques. Mixed‐valent states of Ru‐MOFs were generated by controlled chemical oxidation using tris(4‐bromophenyl)ammoniumyl hexachloroantimonate. The resultant samples featured intervalence charge transfer (IVCT) bands, which were dependent on the oxidation state and were accompanied by 4‐orders‐of‐magnitude enhancement of conductivity. The mixed‐valent states of Ru‐naph and Ru‐bipy exhibited the highest conductivities, reaching 2.35 × 10 −5 and 5.91 × 10 −6 S/cm, respectively. Interrogation of these frameworks offered valuable insights into the interplay between the IVCT transitions and the electronic conductivity.