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

The Creutz–Taube ion, [(NH3)5RuII(μ-pz)­RuIII(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.