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.