Synthesis and Alkali Metal Ion-Binding Properties of a Chromium(III) Triacetylide Complex

A simple triacetylide complex of chromium(III) is synthesized for use as a potential precursor to metal-dicarbide clusters. Reaction of Me₃SiCCLi with [(Me₃tacn)Cr(CF₃SO₃)₃] (Me₃tacn = N,N‘,N‘‘-trimethyl-1,4,7-triazacyclononane) in THF generates [(Me₃tacn)Cr(CCSiMe₃)₃], which subsequently reacts with Bu₄NF to supply [(Me₃tacn)Cr(CCH)₃] as an air-stable orange solid. The crystal structure of this unprecedented triacetylide complex reveals octahedral coordination of the chromium center, with linear Cr−C⋮C bond angles and C⋮C bond distances essentially identical to the corresponding distance in acetylene. Crystallization of the complex from a DMF solution containing K(CF₃SO₃) leads to the sandwich complex {[(Me₃tacn)Cr(CCH)₃]₂K}⁺, in which the K⁺ ion is coordinated in a side-on fashion by each of the six C⋮C units. With the larger Cs⁺ cation, a triangular {[(Me₃tacn)Cr(CCH)₃]₃Cs}⁺ complex is instead observed. The magnetic properties of these alkali metal complexes are indicative of weak antiferromagnetic exchange between Cr^III centers, with J = −0.8 and −0.3 cm^-1, respectively.