A structural comparison of bis(quinoxaline-2,3-dithiolato)cuprate complexes

Abstract

The crystal structures, u.v.–visible spectra, and electrochemical properties of [Cu(qdt)₂]^2– and [Cu(qdt)₂]^–(qdt = quinoxaline-2,3-dithiolate) are reported. The compound [PPh₄]₂[Cu(qdt)₂] crystallizes in space group P2₁/c with a= 9.9320(4), b= 23.8217(10), c= 11.9178(5)Å, β= 95.075(5)°, and Z= 2; [PPh₄][Cu(qdt)₂] crystallizes in space group P2/n with a= 23.709(2), b= 7.192(1), c= 24.188(2)Å, β= 119.170(6)°, and Z= 4. In both anions the CuS₄ moiety is essentially planar, with the mean Cu–S bond length in [Cu(qdt)₂]^–[2.187(1)Å] being significantly shorter than that in [Cu(qdt)₂]^2–[2.260(1)Å], consistent with the unpaired electron of the latter complex occupying an orbital which is antibonding with respect to the Cu–S σ bonds. The [Cu(qdt)₂]^––[Cu(qdt)₂]^2– couple is electrochemically reversible with E_½=–0.18 V vs. saturated calomel electrode in dimethylformamide; a quasi-reversible process with E_½=–1.28 V observed in this solvent is attributed to the [Cu(qdt)₂]^2––[Cu(qdt)₂]^3– couple.