Chemistry of polynuclear metal complexes with bridging carbene or carbyne ligands. Part 78. Reactions of [MoFe(µ-CC6H4Me-4)(CO)6(η-C5H5)] with alkynes; crystal structures of [MoFe{µ-C(C6H4Me-4)C(O)C(Et)C(Et)}(µ-CO)(CO)4(η-C5H5)] and [MoFe{µ-C(C6H4Me-4)C(Me)C[C(O)Me]}(CO)5(η-C5H5)]

Abstract

The dimetal compound [MoFe(µ-CR)(CO)₆(η-C₅H₅)](R = C₆H₄Me-4) reacts with the alkynes HCCH and HCCEt at room temperature in light petroleum to afford the complexes [MoFe{µ-C(R)CHC(R¹)}(CO)₅(η-C₅H₅)] and [MoFe{µ-C(R)CHC(R¹)C(O)}(µ-CO)(CO)₄(η-C₅H₅)](R¹= H or Et), and with MeCCEt to yield a mixture of [MoFe{µ-C(R)C(R¹)C(R²)}(CO)₅(η-C₅H₅)](two isomers: R¹= Me, R²= Et; R¹= Et, R²= Me) and [MoFe{µ-C(R)C(O)C(R¹)C(R²)}(µ-CO)(CO)₄(η-C₅H₅)](two isomers: R¹= Me, R²= Et; R¹= Et, R²= Me). Hex-3-yne and the molybdenum–iron compound react to give a mixture of [MoFe{µ-C(R)C(Et)C(Et)}(CO)₅(η-C₅H₅)] and [MoFe{µ-C(R)C(O)C(Et)C(Et)}(µ-CO)(CO)₄(η-C₅H₅)]. Treatment of the former species with CO affords the acyl complex [MoFe{µ-C(R)C(Et)C(Et)C(O)}(µ-CO)(CO)₄(η-C₅H₅)], a process which is readily reversed in the presence of a stream of nitrogen. The structure of [MoFe{µ-C(R)C(O)C(Et)C(Et)}(µ-CO)(CO)₄(η-C₅H₅)] has been established by an X-ray diffraction study, which revealed that the asymmetric unit contained two crystallographically independent molecules of similar geometry. The Mo(1)–Fe(1) bond [2.694(1)Å] is spanned by the C(R)C(O)C(Et)C(Et) group in such a manner that the terminal carbon atoms of the chain are each bonded to the molybdenum and the iron atoms but in addition the molybdenum is ligated by the carbon atom of the CEt group adjacent to the ketonic moiety. The iron atom carries three terminal CO ligands, and the molybdenum atom one such group and the C₅H₅ ring. However, another CO group attached to the molybdenum semi-bridges the Mo–Fe bond [Mo(1)–C(9)–O(9) 164.2(8)°]. Reactions between [MoFe(µ-CR)(CO)₆(η-C₅H₅)] and the alkynes MeCCEt and EtCCEt in large excess (ca. 1:60) afforded, in addition to the products mentioned above, the species [MoFe{µ-C(R)C(R¹)C(R²)}(CO)₅(η-C₅H₅)][R¹= Me, R²= C(O)Me and R¹= C(O)Me, R²= Me from MeCCEt, and R¹= Et, R²= C(O)Me and R¹= C(O)Me, R²= Et from EtCCEt]. These products arose from the presence of MeCCC(O)Me and EtCCC(O)Me as impurities in commercial samples of pent-2-yne and hex-3-yne. The structure of [MoFe{µ-C(R)C(Me)C[C(O)Me]}(CO)₅(η-C₅H₅)] was established by X-ray diffraction. The Mo–Fe bond [2.722(1)Å] is bridged by the C(R)C(Me)C{C(O)Me} fragment. The iron atom is ligated by three terminal CO groups, and the molybdenum atom by two such groups and the η-C₅H₅ ring.