Mixed-Valence Cu^I /Cu^III Metal-Organic Frameworks with Non-innocent Ligand for Multielectron Transfer

We report two novel three-dimensional copper-benzoquinoid metal-organic frameworks (MOFs), [Cu₄ L₃ ]_n and [Cu₄ L₃  ⋅ Cu(iq)₃ ]_n (LH₄ =1,4-dicyano-2,3,5,6-tetrahydroxybenzene, iq=isoquinoline). Spectroscopic techniques and computational studies reveal the unprecedented mixed valency in MOFs, formal Cu(I)/Cu(III). This is the first time that formally Cu(III) species are witnessed in metal-organic extended solids. The coordination between the mixed-valence metal and redox-non-innocent ligand L, which promotes through-bond charge transfer between Cu metal sites, allows better metal-ligand orbital overlap of the d-π conjugation, leading to strong long-range delocalization and semiconducting behavior. Our findings highlight the significance of the unique mixed valency between formal Cu(I) and highly-covalent Cu(III), non-innocent ligand, and pore environments of these bench stable Cu(III)-containing frameworks on multielectron transfer and electrochemical properties.