Cooperative redox regulation of [4Fe-4S] ferredoxin model arenethiolate complexes by NH· · ·S hydrogen bonds and an aromatic C-H···S interaction

Article abstract of DOI:10.1246/bcsj.70.1077

A series of the model complexes containing ortho-substituted arenethiolato ligands, ((Et₄N)₂[Fe₄S₄(S-2- RCONHC₆H4)₄] {R=Ph (1), 4-MeO-C₆H4 (2), and4-F-C₆H₄ (3)} and (Et₄N)₂[Fe ₄S₄{S-2,6-(RCONH)₂C₆H ₃}₄] {R=Ph (4), 4-MeO-C₆H₄ (5), and 4-F-C₆H₄ (6)}) was synthesized and characterized by ¹NMR, IR spectroscopy, and cyclic voltammetry. The solution structures of these complexes are discussed based on their ¹NMR T₁ data and moleculardynamics calculations. Complex 4 has a shorter distance (av. 4.3 A) between the protons of the benzoyl group and the inorganic sulfur atom of the [4Fe-4S] cluster than the corresponding ones of 1 (av. 6.2 A). These results indicate the C-H · · · S interaction between the protons of the benzoyl group and the sulfur atom of the [4Fe-4S] cluster. The [Fe₄S₄(SAr)₄] ^2-/[Fe₄S₄(SAr)₄]^3- redox potential for 1 and 4 are-0.86 and-0.65 V, respectively. The difference between 1 and 4 is Δ0.21 V. This is larger than the value Δ0.11 V between [Fe₄S₄(S-2-t-BuCONHC₆H₄)4] ^2- (-0.91 V) and [Fe₄S₄{S-2,6-(t-BuCONH) ₂C6H₃}4]^2- (-0.80 V), considered to be the difference between singly and doubly NH· · ·S hydrogen-bonded complexes. The redox potentials for 1-6 follow the trend of the Hammett σ _mvalues, showing that the aromatic ring of the benzoyl group interacts with the [4Fe-4S] cluster directly. A cooperative effect between the C-H· · ·S interaction and the NH ·· · S hydrogen bond is thus found to regulate the redox potential of the model complexes.