π-Complexes of lignols with manganese(I) and ruthenium(II)

In view of contemporary interest in lignocellulose, we report the synthesis, structures, and preliminary reactivity of the first π-complexes of the lignin monomers (lignols). Initial studies with guaiacol demonstrated that it binds only weakly to Mo(CO) ₃ but [Mn(CO) ₃(guaiacol)] BF ₄ is robust. Related adducts with the cinnamyl alcohol derivatives, which are more closely related to the lignols, were unstable. From [Cp*Ru(MeCN) ₃]PF ₆, however, we generated the complete series of π-lignol complexes, including those from coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol as well as derivatives of ferulic and cinnamic acids. From a model dilignol derived from guaiacol and coniferyl alcohol was generated the expected mixture of π-complexes. Competition studies suggest that the allyl alcohol group on lignols does not influence the rate of π-complexation. Competition studies showed that [Cp* Ru(methylferulate)] ⁺ hydrolyzes faster than the free ester.