Synthesis, Gas-Phase Photoelectron Spectroscopic, and Theoretical Studies of Stannylated Dinuclear Iron Dithiolates

Stannylated dinuclear iron dithiolates (μ-SSnMe2CH2S)[Fe(CO)3]2, (μ-SCH2SnMe2CH2S) [Fe(CO)3]2, and (μ-SCH2SnMe3)2[Fe(CO)3]2, which are structurally similar to the active site of iron-only hydrogenase, were synthesized and
studied by gas-phase photoelectron spectroscopy. The orbital origins of ionizations were assigned by comparison
of He I and He II photoelectron spectra and with the aid of hybrid density functional electronic structure calculations.
Stannylation lowers the ionization energy of sulfur lone pair orbitals in these systems owing to a geometry-dependent
interaction. The Fe−Fe σ bond, which is the HOMO in all these systems, is also substantially destabilized by
stannylation due to a previously unrecognized geometry-dependent interaction between axial sulfur lone pair orbitals
and the Fe−Fe σ bond. Since cleaving the Fe−Fe σ bond is a key step in the mechanism of action of iron-only
hydrogenase, these newly recognized geometry-dependent interactions may be utilized in designing biologically
inspired hydrogenase catalysts.