Reactions of Alkynes with Quasi-Linear 3d Metal(I) Silylamides of Chromium to Cobalt: A Comparative Study.

This report describes a series of rare low-coordinate 3d transition metal alkyne complexes resulting from the reaction of quasi-linear metal(I) silylamides, K{18c6}[MX] (18c6 = 18-crown-6; X = -N(SiMe)), -N(Dipp)SiMe; Dipp = 2,6-diisopropylphenyl), of chromium, manganese, iron, and cobalt with aliphatic and aromatic alkynes. We evaluated the interaction of alkynes with quasi-linear metal complexes in dependence of the metal and the alkyne substituents. Whereas only a weak and reversible alkyne coordination is observed for cobalt, the formation of side-on alkyne complexes of the type [M(L)(η-RCCR)] takes place readily for iron. In the case of manganese, we report the first example of a low-coordinate manganese alkyne complexes and, depending on the substrate, unique examples for the manganese mediated reduction of the alkyne to their dianions or even alkyne trimerization. For chromium, alkyne coordination or reduction to the respective alkyne dianions is also observed. Computational analysis of the series of [M(N(SiMe))(η-PhCCPh)] complexes (Cr-Co) using DFT and CASSCF methods reveals a partial reduction of the alkyne by the metal. This leads to the description of the electronic situation of all these complexes as formal metal(II) bound alkynyl radical anions. In the case of chromium, indications for further contributions of a metal(III) cyclopropene resonance structure were found. The computational analysis rationalizes the facile reduction to bis-metalated alkene dianions due to the radical anion character of the alkyne π-complexes.