Synthesis and Characterization of Ruthenium and Osmium Metal–Organic Frameworks with a Pyrazine Ligand: Two-Dimensional Analogues of the Creutz–Taube Ion

Abstract

The Creutz–Taube ion, [(NH₃)₅Ru^II(μ-pz)Ru^III(NH₃)₅]⁵⁺ (pz = pyrazine) is a prominent example of multinuclear Ru systems, which exists in a mixed-valence state. Previous research efforts to interrogate the properties of this complex uncovered important knowledge regarding intramolecular charge transfer between strongly interacting metal centers. In this work, the Ru coordination with pyrazine was extended into two dimensions to create a [Ru(pz)₂Cl₂]_n (Ru-pz) metal–organic framework (MOF). Both solvothermal and solvent-free synthetic methodologies were explored to optimize crystallinity and phase-purity. The detailed structural analysis using single-crystal and powder X-ray diffraction techniques, as well as X-ray absorption spectroscopy and total X-ray scattering confirmed the layered framework structure. In addition, an isostructural Os analogue could also be obtained. The spectroscopic and electrochemical properties of the resultant materials were assessed, which confirmed the existence of mixed-valence states and electronic delocalization between metal centers. This phenomenon facilitated electron propagation in solid-state with conductivity values of some samples reaching the semiconductor range of 10^–7–10^–4 S/cm. These findings expand the well-established chemistry of discrete mixed-valent Ru complexes into the field of MOFs providing useful insights for further development of these systems.