Selenium makes the difference: protonation of [FeFe]-hydrogenase mimics with diselenolato ligands

The synthetic models of the active site of an [FeFe]-hydrogenase containing a Sn atom in the bridgehead of the diselenato ligand, namely [Fe₂(CO)₆{μ-(SeCH₂Se)SnMe₂}], 3 and [Fe₂(CO)₆{μ-(SeCH₂)₂SnMe₂}], 4 have been synthesized and characterized by different spectroscopic methods. The protonation properties of complex 4 have been investigated by monitoring the IR spectra in the carbonyl stretching region, ¹H NMR in the hydride region as well as the ⁷⁷Se{H} NMR upon addition of strong and moderate acids wherein the protonation of the active site of the [FeFe]-hydrogenase at one of its internal basic sites is considered an essential step in the catalytic cycle. Furthermore, we investigated the redox properties and the catalytic behaviour of complexes 3 and 4 in the presence of AcOH as a source of protons suggesting an ECE (E = electrochemical process, C = chemical process) mechanism.