Synthesis and Characterization of Homoleptic Zintl Anions [(Pn$_7$)M(Pn$_7$)]$^{4−}$ (M/Pn = Pb/As, Zn/Bi)

The formation and follow-up chemistry of P$_7^{3−}$ has been intensely studied in the recent decades. In contrast, corresponding investigations of its heavier polycyclic polypnictide congeners Pn$_7^{3−}$ (Pn=As, Sb, Bi), especially Bi$_7^{3−}$, have been done to a much lesser extent. In our report, we add to these rare studies, by reporting new coordination compounds with As$_7^{3−}$ and Bi$_7^{3−}$, respectively. Addition of PbI$_2$ to a solution of K$_3$As$_7$ in ethane-1,2-diamine (en) afforded the heterometallic cluster anion [(As$_7$)Pb(As$_7$)]$^{4−}$ (with an 8 % co-crystallization of [PbAs$_{15}$]$^{3−}$ on the same site), in compound [K$_{3.92}$(crypt-222)$_4$][(As$_7$)Pb(As$_7$)]$_{0.92}$[PbAs$_{15}$]$_{0.08}$ ⋅ 3en (1). A similar reaction of [ZnPh$_2$] with a pyridine (py) solution of the ternary solid ‘K$_5$Ga$_2$Bi$_4$’, which was previously shown to release Ga and polybismuthide units in situ under certain reaction conditions, resulted in the formation of [K(crypt-222)]$_4$[(Bi$_7$)Zn(Bi$_7$)] ⋅ 2.5py (2), comprising the related heterometallic anion [(Bi$_7$)Zn(Bi$_7$)]$^{4−}$. The former represents the first Pb−As binary Zintl anions obtained via solution chemistry, and the latter adds one more piece to the remarkably rare coordination compounds reported for Bi$_7^{3−}$.