Secondary coordination sphere interactions introduced through installation of hydrogen bond donor/acceptor motifs onto transition metal complexes have been shown to facilitate bond forming and breaking steps, stabilize reactive intermediates, or act as proton shuttles during the reaction. A well-known example in the field of hydrogen evolution catalysis is the inclusion of secondary sphere pendant amines in the Ni(P2N2)22+ class of electrocatalysts developed by DuBois et al. which demonstrate increased turnover frequencies for the hydrogen evolution reaction. Previous work from the Darensbourg group has explored expansion of the coordination sphere in the M-N2S2 metallodithiolate class of biomimetics through sulfur alkylation. A recent report was the acetylation of a tridentate Ni-N2S unit whose thermodynamic product is a structurally interesting molecular square. Under electrochemical conditions, this compound, featuring a carboxylic acid moiety in the secondary sphere, was shown to catalyze the reduction of protons to dihydrogen with a weak acid such as AcOH. The catalysis has been initially ascribed to the ability of the carboxylate group to act in a proton shuttle in the overall mechanism. These works are actively being expanded through the inclusion of other secondary sphere motifs, such as pendant amines, which can also act as proton delivery vehicles during small molecule activation. Here we report the synthesis, characterization, and electrochemical activity for a family of pendant amine containing metallodithiolates, which have been shown to be highly active proton reduction catalysts in the presence of a relatively weak acids, such as CH3COOH. The catalysis occurs with low overpotential (ca. -1.7 V vs. Fc+/0) and turnover frequencies significantly greater than the parent metallodithiolate. Additionally, we compare between Ni(N2S2) complexes featuring primary and secondary amines, which serve to modulate the pKa of the catalyst in order to explore a structure/function relationship within this family of complexes. The role of these pendant bases during catalysis is consistent with the demonstrated enhancement of electrochemical activity toward the hydrogen evolution reaction.

Tuning the reactivity of nickel metallodithiolate complexes through secondary sphere pendant bases

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