Hydrogen Transfer Catalysis beyond the Primary Coordination Sphere

In this Perspective, we outline recent examples of homogeneous transition-metal hydrogen transfer catalysts for which functionality within the complex's outer coordination sphere influences the outcome of a reaction. Secondary sphere groups are often applied to hydrogen transfer reactions, but their specific role during catalysis is not always well understood. New experimental and theoretical work details the complexity associated with predicting secondary-sphere interactions and therefore designing improved catalysts. This Perspective highlights examples of catalysts containing secondary-sphere groups that (1) accelerate a key turnover limiting step such as H-2 heterolysis or hydride transfer, (2) limit competing catalytic cycles, (3) prevent catalyst decomposition, and/or (4) provide access to new catalysts through postmetalation modifications. The examples described herein emphasize numerous roles of the secondary sphere in hydrogen transfer catalysis and illustrate how the optimal use of these interactions is predicated on the analyses of key reaction intermediates in a catalytic reaction.