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 CrIII centers, with J = −0.8 and −0.3 cm-1, respectively.