Studies on Molybdena- and Tungstenacyclobutadiene Complexes Supported by Fluoroalkoxy Ligands as Intermediates of Alkyne Metathesis

The molybdenum and tungsten 2,4,6-trimethylbenzylidynes [MesC​M­{OC­(CF3)2­Ph}3] (12a, M = Mo; 12b, M = W) were prepared and structurally characterized as related complexes to already known [MesC​M­{OC­(CF3)2­Me}3] (MoF6, M = Mo; WF6, M = W). While treatment of 12a with 3-hexyne yielded the propylidyne complex [EtC​Mo­{OC­(CF3)2­Ph}3] (13), the tungsten congener 12b formed isolable metallacyclobutadiene (MCBD) species 14–16 by reaction with 3-hexyne, 1-phenyl-1-propyne, and 2,4-hexadiyne, which can be correlated with the higher electrophilicity of the tungsten complex. Furthermore, the labile MCBD [(C3Et3)­Mo­{OC­(CF3)2­Me}3] (17) was isolated at low temperature from the reaction of the highly active MoF6 catalyst with 3-hexyne and could be characterized by X-ray diffraction analysis. At room temperature, the same reaction afforded [EtC​Mo­{OC­(CF3)2­Me}3] (18), and the equilibrium reaction with 3-hexyne to form 17 was additionally studied by variable temperature NMR spectroscopy, which allowed determining ΔH° and ΔS° for the formation of MCBD 17. The experimental thermodynamic data were used to set the benchmark for DFT calculations. Moreover, the deprotiometallacyclobutadiene complex (DP­MCBD) [{C3­(Mes)­(Ph)}­Mo­{OC­(CF3)3}2] (19), prepared from [MesC​Mo­{OC­(CF3)3}3] (MoF9) and phenylacetylene, was isolated and structurally characterized as a decomposition product of terminal alkyne metathesis and employed in the polymerization of phenylacetylenes.