Activation of Coordinated Carbon Monoxide toward Alkyl and Aryl Migration (CO Insertion) by Molecular Lewis Acids and X-ray Structure of the Relative Intermediate Mn(C(OAlBrBr2)CH3)(CO)4

Article abstract of DOI:10.1021/ja00535a048

In the absence of carbon monoxide, the strong Lewis acid AlBr3 induces a rapid alkyl migration in the compounds L_nM(R)(CO) = Mn(CH3)(CO)5, Mn(CH2C6H5)(CO)5, (η⁵-C5H5)Fe(CH3)(CO)2, and (η₅-C5H5)Mo(CH3)(CO)3 to produce acyl complexes in which Al is coordinated to the acyl oxygen, L_nM(C(OAlBrBr2)R).In the X-ray structure of Mn(C(OAlBrBr2)CH3)(CO)4 a Br aton from the AlBr3 fills the sixth coordination site of the metal, thereby creating the five-membered ring Mn-C-O-Al-Br = 6.835(2) Angstroem, b = 16.374(6) Angstroem, c = 12.278(4) Angstroem, β = 94.18(2) deg, monoclinic space group P2₁/c>.Analogous acetyl complexes are formed also by the reaction of Mn(CH3)(CO)5 with AlCl3 and BF3.Each of these cyclic adducts react with 1 equiv. of CO at subatmospheric pressures to produce the corresponding simple adduct L_nM(C(OMX3)R)(CO) in which the incoming CO ligand has replaced the Lewis acid halogen ligand in the coordination sphere of the metal.These reactions occur more rapidly and under much milder conditions than do the reactions between the parent alkyl complexes, L_nM(R)(CO) and CO.Lewis acid promoted CO insertion into a metal-aryl bond was observed with W(C6H5)(CO)5^- in the presence of Al(C6H5)3.In addition to the rate enchancement of CO insertion, the Lewis acid can also exert a strong stabilizing effect on unstable acetyl compounds.Thus a stable complex (η⁵-C5H5)Mo(C(OAlBr3)CH3)(CO)3 is readily formed in the presence of CO and AlBr3.By contrast, the simple acetyl (η⁵-C5H5)Mo(C(O)CH3)(CO)3 is unstable with respect to CO evolution.