COOPERATIVE REDOX REGULATION OF [4FE-4S] FERREDOXIN MODEL ARENETHIOLATE COMPLEXES BY NH-CENTER-DOT-CENTER-DOT-CENTER-DOT-S HYDROGEN-BONDS AND AN AROMATIC C-H-CENTER-DOT-CENTER-DOT-CENTER-DOT-S INTERACTION

A series of the model complexes containing ortho-substituted arenethio lato ligands, ((Et4N)(2)[Fe4S4(S-2RCONHC(6)H(4))(4)] {R=Ph (1), 4-MeO- C6H4 (2), and 4-F-C6H4 (3)} and (Et4N)(2)[Fe4S4{S-2,6-(RCONH)(2)C6H3}( 4)] {R=Ph (4), 4-MeO-C6H4 (5), and 4-F-C6H4 (6)}) was synthesized and characterized by H-1 NMR, IR spectroscopy, and cyclic voltammetry. The solution structures of these complexes are discussed based on their ( HNMR)-H-1 T-1 data and moleculardynamics calculations. Complex 4 has a shorter distance (av. 4.3 Angstrom) between the protons of the benzoy l group and the inorganic sulfur atom of the [4Fe-4S] cluster than the corresponding ones of 1 (av. 6.2 Angstrom). These results indicate th e C-H ... S interaction between the protons of the benzoyl group and t he sulfur atom of the [4Fe-4S] cluster. The [Fe4S4(SAr)(4)](2-)/[FeS4( SAr)(4)](3-) redox potential for 1 and 4 are -0.86 and -0.65 V, respec tively. The difference between 1 and 4 is Delta 0.21 V. This is larger than the value Delta 0.11 V between [Fe4S4(S-2-t-BuCONHC6H4)(4)](2-) (-0.91 V) and [Fe4S4{S-2,6-(t-BuCONH)(2)C6H3}(4)](2-) (-0.80 V), consi dered to be the difference between singly and doubly NH ... S hydrogen -bonded complexes. The redox potentials for 1-6 follow the trend of th e Hammett sigma(m) values, showing that the aromatic ring of the benzo yl group interacts with the [4Fe-4S] cluster directly. A cooperative e ffect between the C-H ... S interaction and the NH ... S hydrogen bond is thus found to regulate the redox potential of the model complexes.