C−H Activation of Inert Arenes using a Photochemically Activated Guanidinato‐Magnesium(I) Compound

UV irradiation of solutions of a guanidinate coordinated dimagnesium(I) compound, [{(Priso)Mg}(2)] 3 (Priso = [(DipN)(2)CNPr2i](-), Dip = 2,6-diisopropylphenyl), in either benzene, toluene, the three isomers of xylene, or mesitylene, leads to facile activation of an aromatic C-H bond of the solvent in all cases, and formation of aryl/hydride bridged magnesium(II) products, [{(Priso)Mg}(2)(mu-H)(mu-Ar)] 4-9. In contrast to similar reactions reported for beta-diketiminate coordinated counterparts of 3, these C-H activations proceed with little regioselectivity, though they are considerably faster. Reaction of 3 with an excess of the pyridine, p-NC5H4But (py(But)) gave [(Priso)Mg(py(But)H)(py(But))(2)] 10, presumably via reduction of the pyridine to yield a radical intermediate, [(Priso)Mg(py(But center dot))(py(But))(2)] 11, which then abstracts a proton from the reaction solvent or a reactant. DFT calculations suggest two possible pathways to the observed arene C-H activations. One of these involves photochemical cleavage of the Mg-Mg bond of 3, generating magnesium(I) doublet radicals, (Priso)Mg-center dot. These then doubly reduce the arene substrate to give "Birch-like" products, which subsequently rearrange via C-H activation of the arene. Circumstantial evidence for the photochemical generation of transient magnesium radical species includes the fact that irradiation of a cyclohexane solution of 3 leads to an intramolecular aliphatic C-H activation process and formation of an alkylbridged magnesium(II) species, [{Mg(mu-Priso(-H))}(2)] 12. Furthermore, irradiation of a 1:1 mixture of 3 and the beta-diketiminato dimagnesium(I) compound, [{((Dip)Nacnac)Mg}(2)] ((Dip)Nacnac = [HC(MeCNDip)(2)](-)), effects a "scrambling" reaction, and the near quantitative formation of an unsymmetrical dimagnesium(I) compound, [(Priso)Mg-Mg((Dip)Nacnac)] 13. Finally, the EPR spectrum (77 K) of a glassed solution of UV irradiated 3 is dominated by a broad featureless signal, indicating the presence of a doublet radical species.