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

Article abstract of DOI:10.1016/0020-1693(96)04924-9

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 FeCl₃ with 4 equiv. of lithium 2,3,5,6-tetramethylbenzenethiolate, Li[S-2,3,5,6-Me₄C₆H], lithium 2,4,6-triisopropylbenzenethiolate, Li[S-2,4,6-i-Pr₃C₆H₂] and the appropriate cation gives high yields of [Et₄N][Fe(S-2,3,5,6-Me₄C₆H)₄] (1) and [Ph₄P][Fe(S-2,4,6-i-Pr₃C₆H₂) ₄], respectively (2). Compounds 1 and 2 were structurally characterized by X-ray diffraction giving the crystallographic parameters: for 1, tetragonal space group I4? with a = b= 12.366(2) , c = 16.352(4) ?, V = 2500 ?³, Z = 2; for 2, triclinic space group P1? with a = 14.709(2), b = 20.928(5), c = 13.901(2) ?, α = 90.97(3)°, β = 105.43(3)°, γ = 78.13(3)°, V= 4033 ?³. The crystallographic symmetry of [Et₄N][Fe(S-2,3,5,6-Me₄C₆H)₄] requires the entire [Fe(SR)₄]^1- anion to have S₄ point group symmetry and the [FeS₄] core to have D_2d symmetry. [Ph₄P][Fe(S-2,4,6-i-Pr₃C₆H₂) ₄] possesses no crystallographic 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(II) + 1/2RSSR] is due to the reduced tendency of these thiolates to bridge metal centers. Attention is drawn to the high idealized D_2d symmetry of the [FeS₄] cores and the [Fe(S-CH₂-)₄] units of oxidized rubredoxins which are conserved in five different rubredoxins.