Reduction of CO2 and Related Compounds with a Low-Valent CaI Synthon

The reduction of CO₂ with low-valent s-block metal complexes is limited to Mg^I reducing agents. Herein, the reduction of a series of CX₂ reagents (X = O, S, NiPr) with the Ca^I synthon [(BDI)Ca(THP)]₂(N₂) (VI) is described, which is a Ca^II complex that reacts like the hypothetical Ca^I complex (BDI)CaCa(BDI); BDI = HC[(Me)-N(DIPeP)]₂, DIPeP = 2,6-Et₂CH-phenyl, THP = tetrahydropyran. Reaction of VI with excess CO₂ led to crystallization of a carbonate complex with a Ca₄(CO₃)₂⁴⁺ core. The stabilizing BDI ligands also react by nucleophilic addition of CO₂ at the backbone γ-C atom. Reaction with only two equivalents of CO₂ gave the presumed carbonite (CO₂²⁻) intermediate, but this could not be confirmed by X-ray diffraction. Reaction with CS₂ gave the first unique s-block metal complex with a CS₂²⁻ anion. This anion bridges between two (BDI)Ca⁺ cations by CaC and CaS bonding. The reduction of iPrN=C=NiPr with VI gave C–C coupling to a [(iPrN)₂C–C(NiPr)₂]²⁻ dianion. An intermediate with a C(NiPr)₂²⁻ dianion could not be isolated.