Ion Mobility Studies of Pyrroloquinoline Quinone Aza-Crown Ether-Lanthanide Complexes

Lanthanide-dependent enzymes and their biomimetic complexes have arisen as an interesting target of research in the past decade. These enzymes, specifically, pyrroloquinoline quinone (PQQ)-bearing methanol dehydrogenases, efficiently convert alcohols to the respective aldehydes. To rationally design bioinspired alcohol dehydrogenation catalysts, it is imperative to understand the species involved in catalysis. However, given the extremely flexible coordination sphere of lanthanides, it is often difficult to assess the number and nature of the active species. Here, we show how such questions can be addressed by using a combination of ion mobility spectrometry, mass spectrometry, and quantum-chemical calculations to study the test systems PQQ and lanthanide-PQQ-crown ether ligand complexes. Specifically, we determine the gas-phase structures of [PQQH₂]^-, [PQQH₂+H₂O]^-, [PQQH₂+MeOH]^-, [PQQ-15c5+H]⁺, and [PQQ-15c5+Ln+NO₃]²⁺ (Ln = La to Lu, except Pm). In the latter case, a trend to smaller collision cross sections across the lanthanide series is clearly observable, in line with the well-known lanthanide contraction. We hope that in the future such investigations will help to guide the design and understanding of lanthanide-based biomimetic complexes optimized for catalytic function.