Proton and Electron Transfer in the Formation of a Copper Dithiolene-Based Coordination Polymer

Metal bis(dithiolene) complexes are promising building blocks for electrically conductive coordination polymers. N-Heterocyclic dithiolene complexes allow their cross-linking via the coordination of N-donor atoms to additional transition metal ions. In this study, we present the formal copper(II) and copper(III) 6,7-quinoxalinedithiolene complexes [Cu(qdt)₂]^- and [Cu(qdt)₂]^2- (qdt^2-: 6,7-quinoxalinedithiolate), as well as the 2D coordination polymer Cu[Cu(Hqdt)(qdt)] (3). The dithiolene complexes were isolated as (Bu₄N)₂[Cu(qdt)₂] (1), Na[Cu(qdt)₂]·4H₂O (2a), [Na(acetone)₄][Cu(qdt)₂] (2b), and [Ni(MeOH)₆][Cu(qdt)₂]₂·2H₂O (2c). Their crystal structures reveal nearly planar complexes with a high tendency of π-stacking. For a better understanding of their coordination behavior, the electronic properties are investigated by UV-vis-NIR spectroscopy, cyclic voltammetry, and DFT simulations. The synthesis of the 2D coordination polymer 3 involves the reduction and protonation of the monoanionic copper(III) complex. A combination of powder X-ray diffraction, magnetic susceptibility measurements, as well as IR and EPR spectroscopy confirm that formal [Cu^II(Hqdt)(qdt)]^- units link trigonal planar copper(I) atoms to a dense 2D coordination polymer. The electrical conductivity of 3 at room temperature is 2 × 10^-7 S/cm. Temperature dependent conductivity measurements confirm the semiconducting behavior of 3 with an Arrhenius derived activation energy of 0.33 eV. The strong absorption of 3 in the visible and NIR regions of the spectrum is caused by the small optical band gap of E_g,opt = 0.65 eV, determined by diffuse reflectance spectroscopy. This study sheds light on the coordination chemistry of N-heterocyclic dithiolene complexes and may serve as a reference for the future design and synthesis of dithiolene-based coordination polymers with interesting electrical and magnetic properties.