A Redox-Innocent Uranium(IV)-Quinoid Metal-Organic Framework

Quinoid-based ligands constitute
the most common class of redox-active
ligands used to construct electrically conductive and magnetic metal–organic
frameworks (MOFs). Whereas this chemistry is intensively explored
for transition-metal and lanthanide ions, any related actinide compound
has not received attention. In particular, the MOF chemistry of actinide
ions in the lower oxidation states is underexplored. We herein report
the synthesis, and structural and physical property characterization
of a uranium­(IV) quinoid-based MOF, [U­(Cl2dhbq)2(H2O)2]·4H2O (1, Cl2dhbq2– = deprotonated 2,5-dichloro-3,6-dihydroxybenzoquinone). 1 is a rare example of a U­(IV)-based coordination solid and
the first material to incorporate bona fide reducible bridging ligands.
Despite the anticipated thermodynamic driving force, no indications
of valence tautomerism are evident from magnetometry, near-IR spectroscopy,
and X-band electron paramagnetic resonance measurements. These initial
results suggest that reduction potentials alone are insufficient as
guidelines for the prediction of the occurrence of electron transfer
in uranium–quinoid-based materials.