Chemistry of Unsaturated Group 6 Metal Complexes with Bridging Hydroxy and Methoxycarbyne Ligands. 3. Formation and Cleavage of C−C and C−O Bonds in the Reactions of the Complexes [Mo2Cp2(µ-COMe)(µ-COR)(µ-PCy2)]BF4(R = Me, Et)

Abstract

The reactions of the 30-electron complexes [Mo2Cp2(µ-COMe)(µ-COR)(µ-PCy2)]BF4 (Cp = η5-C5H5; R = Me, Et) with CO, CNtBu, or tetraethylpyrophosphite (tedip) lead to the new 32-electron complexes [Mo2Cp2{µ-η2:η2-C(OMe)C(OR)}(µ-PCy2)(CO)2]BF4, [Mo2Cp2{µ-η2:η2-C(OMe)C(OMe)}(µ-PCy2)(CNtBu)2]BF4, and [Mo2Cp2{µ-η2:η2-C(OMe)C(OMe)}(µ-PCy2)(µ-tedip)]BF4, respectively. These products contain a bonded dialkoxyacetylene molecule resulting from the coupling of two alkoxycarbyne ligands, induced by the addition of the donor molecules to the unsaturated dimetal center of the starting material, and this coupling can be fully reversed in the dicarbonyl product upon photochemical treatment. The structure of the diisocyanide and tedip complexes, both displaying quite short intermetallic lengths (ca. 2.66 Å), was confirmed crystallographically. The structure of the dicarbonyl derivative, which in solution exhibits cis and trans isomers, was optimized using DFT methods, which also led to the prediction of short intermetallic distances (2.75 Å). Demethylation of the dicarbonyl complexes leads to the new carboxycarbyne derivatives [Mo2Cp2{µ-C(CO2R)}(µ-PCy2)(CO)2] (R = Me, Et) as a result of the unexpected 1,2-shift of the methoxyl group occurring in the ketenyl intermediate (not detected) presumably formed first. This proposal is supported by DFT calculations, these predicting a moderate activation barrier (ΔG⧧298 = 78 kJmol−1) for the spontaneous transformation (ΔG298 = −51 kJ mol−1) of the ketenyl intermediate into the carbyne complex finally isolated. The bis(methoxycarbyne) complex is also reactive toward the 16-electron fragment Ru(CO)4, to give the 46-electron cluster [Mo2RuCp2(µ-COMe)2(µ-PCy2)(CO)4]BF4, which concentrates most of its electronic unsaturation at the dimolybdenum center (Mo−Mo = 2.686(1) Å), in spite of the rearrangement of the carbyne ligands, both of them now bridging one of the Mo−Ru edges, according to an X-ray diffraction study.