On the role of hydrogen bond acceptors in electrocatalytic hydride formation

<h2>Summary</h2> Carefully designed mechanistic studies often provide the most important insights for future advances in catalysis. Here, we study the role of hydrogen-bond (H-bond)-accepting functional groups in a catalyst microenvironment. This approach provides quantitative information on the rate of electrochemically driven catalyst-hydride formation from protons. Ligand substitution of Na(diglyme)₂[Fe₄N(CO)₁₂] was used to access a series of six clusters with various H-bond-accepting secondary coordination sphere (SCS) functional groups that were chosen to span a wide range of H-bond-acceptor strengths (<i>B</i>). We demonstrate that the rate of formation of [H-Fe₄N(CO)₁₂]⁻ increases linearly with <i>B</i>, because the H-bond acceptor modulates the reactivity of water that serves as a proton source for the formation of [H-Fe₄N(CO)₁₂]⁻. Specifically, the substrate water likely accumulates near the catalyst active site and/or has lower p<i>K</i>_a when the SCS functional groups are stronger H-bond acceptors. The rate of [H-Fe₄N(CO)₁₂]⁻ formation is not correlated directly with p<i>K</i>_a of the SCS H-bond-accepting functional group.