[Ni(dap)3]4[As10Cu2S18]: a new thioarsenate containing the rare [As3CuS6] cluster with mixed-valence As2+/As3+ ions.

A new mixed-valence thioarsenate(II,III) [Ni(dap)3]4-[As10Cu2S18] (1, dap = diaminopropane) has been solvothermally synthesized and structurally characterized. 1 features a new heterometallic thioarsenate [As10Cu2S18](8-) with mixed valence As(2+)/As(3+) ions, which is constructed by combination of the chair-shaped tetramer cyclo-[As4S8] and the rare heterometallic cluster [As3CuS6]. 1 is the first example of transition metal ions encapsulated within mixed-valent thioarsenates(II,III). The optical properties of 1 have also been investigated.