Influence of the methylene group between azadithiolate nitrogen atom and phenyl moiety on the protophilic properties of [FeFe]-hydrogenase model complexes

Abstract

An [FeFe]-hydrogenase mimic 4 with functional benzyl moiety covalently linked to the azadithiolate ligand was synthesized. The structure, protonation, and electrochemical properties of 4 and a phenyl substituted analogue (coded as 3) were simultaneously studied to explore the influence of the methylene group between the bridgehead nitrogen atom and functional phenyl moiety on the protophilic properties of the model complexes. X-ray single crystal diffraction analysis revealed that the nitrogen atoms of 3 and 4 possessed sp 2 and sp 3-hybridization, respectively. Although the light-driven electron transfer was prevented in the molecule of 4, the sp 3-hybridized nitrogen atom of 4 could be protonated in the presence of the proton acid to give the [4(NH)]+ cation. The generated positive charge could be reduced at ca. −1.2 V versus Fc/Fc+ with a distinctly electrocatalytic proton reduction activity, whereas the proton reduction catalysed by 3 occurred at ca. −1.45 V. The catalytic proton reductions of 3 and 4 followed ECCE and CECE mechanisms, respectively. It was noteworthy that the potential of 4 was remarkably anodic shifted and closer to that of the proton reduction catalysed by natural enzymes.