Gas-phase ligand binding to Jacobsen's manganese salen catalyst: Functional group and steric effects

Gas-phase equilibrium measurements have been used to determine the relative binding affinities of 18 ligands to Jacobsen's manganese salen catalyst. The group of ligands spans 5.7 kcal/mol and includes seven functional groups (alcohol, ketone, ester, acyclic ether, cyclic ether, epoxide, and amine). The data follow general trends seen in other gas-phase metal cation affinities, but are influenced to a much greater extent by steric effects. For example, a 2° amine is a stronger binder than a 1° amine (as typical for gas-phase cation binding), but a 3° amine is a much weaker binder due to excessive crowding with the bulky salen ligand. The impact of steric effects was also explored with computational modeling at the B3LYP/6-311+G(d,p)/B3LYP/6-31G(d) level. The study demonstrates the utility of using mass spectrometry to probe the ligand binding characteristics of sterically demanding, metal-centered catalysts.

Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (158 K)Download as PowerPoint slideHighlights► Gas-phase binding affinities of 18 ligands to Jacobsen's catalyst. ► Identification of functional group patterns in the binding affinities. ► Identification of strong steric effects in the binding affinities. ► Major differences in binding to atomic cations, proton and lithium.