The Elusive Chemistry of Pyrroloquinoline Quinone Dimethyl Ester Lanthanide Complexes in Biomimetic Alcohol Oxidation

Since its discovery, pyrroloquinoline quinone (PQQ) has been under constant investigation regarding its efficiency in biomimetic complexes for alcohol dehydrogenation. The discovery of lanthanide (Ln) dependent methanol dehydrogenases has led to the use of lanthanide complexes bearing PQQ derivatives to oxidize alcohols. However, the mechanism of these oxidations is still a subject of debate. Herein, La³⁺ and Lu³⁺ complexes of PQQ dimethyl ester (PQQDME), that are able to stoichiometrically oxidize an alcohol substrate, are reported. In the presence of air, some catalytic turnover is observed, but less than with other, more heavily modified PQQ biomimetics known in the literature. To investigate the reason for this low turnover, the reduced counterpart, PQQDMEH₂, is synthesized. It is shown that in the presence of atmospheric oxygen, the complexes of the reduced form undergo oxidation and can then also convert alcohol to aldehyde. Additionally, the involvement of radicals in the alcohol oxidation reaction and the origin and nature of these radicals is investigated.