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

Abstract

Factors affecting the FeIFeI/FeIFe0 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.