Cu(I)/Cu(II) Creutz-Taube Mixed-Valence 2D Coordination Polymers

Graphene-like two-dimensional (2D) coordination polymers (GCPs) have been of central research interest in recent decades with significant impact in many fields. According to classical coordination chemistry, Cu(II) can adopt the dsp2 hybridization to form square planar coordination geometry, but not Cu(I); this is why so far, there has been no 2D layered structures synthesized from Cu(I) precursors. Herein we report a pair of isostructural GCPs synthesized by the coordination of benzenehexathiol (BHT) ligands with Cu(I) and Cu(II) ions, respectively. Various spectroscopic characterizations indicate that Cu(I) and Cu(II) coexist with a near 1:1 ratio in both GCPs but remain indistinguishable with a fractional oxidation state of +1.5 on average, making these two GCPs a unique pair of Creutz-Taube mixed-valence 2D structures. Based on DFT calculations, we further uncovered an intramolecular pseudo-redox mechanism whereby the radicals on BHT ligands can oxidize Cu(I) or reduce Cu(II) ions upon coordination, thus producing isostructures yet with distinct electron configurations. For the first time, we demonstrate that using Cu(I) or Cu(II), one can achieve atomically isostructural 2D structures, indicating that a neutral periodic structure can host a different number of total electrons as ground states, which may open a new chapter for 2D materials.