Investigation of Bridgehead Effects on Reduction Potential in Alkyl and Aryl Azadithiolate-Bridged (µ-SCH₂XCH₂S) [Fe(CO)₃]₂ Synthetic Analogues of [FeFe]-H₂ase Active Site

Factors affecting the Fe^IFe^I/Fe^IFe⁰ reduction potentials (E⁰) in a series of propane and azadithiolate bridged (µ-SRS)[Fe(CO)₃]₂ synthetic model complexes of diiron hydrogenases are investigated. The E⁰ is found to vary from 60 to 390 mV depending on the nature of the substituent (Alkyl or Aryl) on the nitrogen atom at the bridgehead of the azadithiolate. The E⁰ is found to shift to more negative values as the average C-O stretching vibrations ν(CO)_avg of the CO ligands decreases, i.e., with increasing backbonding from the iron centers. This trend is accompanied by a linear dependence of the E⁰ on the σ Hammett parameter of the para substituents in a series of Arylamine bridged complexes and on the gas phase proton affinities (PA) of a series of Alkylamine bridged complexes. The unexpected dependence of E⁰ on the substituents on the bridgehead nitrogen is addressed by computational chemistry. Density functional theory (DFT) calculations accurately reproduce the geometric structures and the correlations between E⁰, ν(CO)_avg, σ and PA observed in the experimental data. It is concluded that while for Alkyl-amine bridged complexes an interaction between the nitrogen lone pair with C–S σ* affects ca. 60 mV shift in the E⁰, the inductive and mesomeric effects of the substituents in the Aryl ring are responsible for ca. 300 mV shift in the E⁰ of the Aryl-amine bridged complexes.