Secondary Coordination Sphere Design to Modify Transport of Protons and CO₂

An exploration of secondary coordination sphere (SCS) functional groups is presented with a focus on proton transport to a metal hydride active site for H₂ formation and transport of CO₂ so that formate can be obtained. In MeCN-H₂O, pK_a(AH) and steric bulk of the SCS groups are discussed along with their influence on each step in the mechanism for CO₂ to formate catalysis and along with the influence of the proton source, which is MeCN-H₂O or (MeCN)₂H₂O in MeCN-H₂O (95:5) under N₂ atmosphere. Under CO₂, carbonic acid is also available. Catalysts containing various SCS groups were synthesized from [Fe₄N(CO)₁₂]^- and have the form [Fe₄N(CO)₁₁L]^- where L is Ph₂P-SCS. Hydride formation rates are distinct under N₂ versus CO₂, and that variation is dependent on the size of the SCS group. Under CO₂, larger SCS groups inhibit access of the MeCN-H₂O adducts to the active site and formate formation is observed, whereas smaller SCS groups allow transport of these adducts. This is best illustrated by catalysts containing the small SCS group pyridyl and the large SCS group N,N-dimethylaniline which both have the same pK_a(AH) value. The smaller pyridyl group promotes selective H₂ evolution, whereas larger N,N-dimethylaniline supports selective formate formation by slowing the transport of large MeCN-H₂O adducts, allowing hydride transfer to the smaller substrate CO₂.