Bis(µ-2-cyanoguanidine)-bis[(2-cyanoguanidine)copper(I)], a planar dimeric cation containing co-ordinatively unsaturated copper(I)

The air- and moisture-stable copper(I) salts [Cu2(µ-cnge)2(cnge)2]X·2H2O (cnge = 2-cyanoguanidine; X = S2O62–) or SO42– have been prepared by sodium sulfite reduction of aqueous solutions of copper(II) chloride containing cnge. The two salts have been structurally characterised by single-crystal X-ray diffraction methods. The dithionate structure (triclinic, space group P, a= 7.320, b= 7.315, c= 12.765 A, α= 97.10, β= 118.91, γ= 92.93°) is more symmetrical than that of the sulfate (monoclinic, space group P21/c, a= 6.920, b= 14.951, c= 21.042 A, β= 95.17°). Nonetheless, the two [Cu2(µ-cnge)2(cnge)2]2+ cations are very similar. They contain two trigonal-planar copper(I) atoms bridged by two cnge molecules and terminally co-ordinated by a single cnge molecule. To minimise interaction between the in-plane π systems of the bridging cnge molecules, the latter are separated by ca. 3.1 A and angled such that they lie on two essentially parallel planes separated by ca. 0.75 A. The perturbation of the IR spectrum of cnge on co-ordination is dependent not on the co-ordination type (bridging, terminal) but on the strength of its co-ordination to the metal.