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, pKa(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 pKa(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.