Models for the iron-sulfur protein rubredoxin: the use of sterically hindered thiolate ligands to stabilize [Fe(SR)4]1− complexes; some considerations of the structure of the [Fe(S-Cys)4] centers in oxidized rubredoxins

Abstract Sterically hindered aromatic thiolate ligands have been used to synthesize the first examples of [Fe III (SR) 4 ] 1− complexes with monodentate ligands. The reaction of FeCl 3 with 4 equiv. of lithium 2,3,5,6-tetramethylbenzenethiolate, Li[S-2,3,5,6-Me 4 C 6 H], lithium 2,4,6-triisopropylbenzenethiolate, Li[S-2,4,6- i -Pr 3 C C 6 H 2 ] and the appropriate cation gives high yields of [Et 4 N][Fe(S-2,3,5,6-Me 4 C 6 H) 4 ] ( 1 ) and [Ph 4 P][Fe(S-2,4,6- i -Pr 3 C 6 H 2 ) 4 ], respectively ( 2 ). Compounds 1 and 2 were structurally characterized by X-ray diffraction giving the crystallographic parameters: for 1 , tetragonal space group I 4 with a = b = 12.366(2), c = 16.352(4) A , V = 2500 A 3 , Z = 2 ; for 2 , triclinic space groups P 1 with a = 14.709(2); b = 20.928(5); c = 13.901(2) A , α = 90.97(3)°, β = 105.43(3)°, γ = 78.13(3)°, V = 4033 A 3 . The crystallographic symmetry of [Et 4 N][Fe(S-2,3,5,6-Me 4 C 6 H) 4 ] requires the entire [Fe(SR) 4 ] 1−1 anion to have S 4 point group symmetry and the [ FeS [ in 4] core to have D 2 d symmetry. [Ph 4 P][Fe(S-2,4,6- i -Pr 3 C 6 H 2 ) 4 ]) possesses no crystallographi or apparent symmetry. It is suggested that the ability of the sterically hindered thiolate ligand to inhibit the autoredox reaction [ described by: Fe(III)SR → Fe(III) + 1 2 RSSR ] is due to the reduced tendency of these thiolates to bridge metal centers. Attention is drawn to the high idealized D 2 d symmetry of the [FeS 4 ] cores and the [Fe(SCH 2 -) 4 ] units of oxidized rubredoxins which are conserved in five different rubredoxins.