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

摘要

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-Me(4)C(6)H], lithium 2,4,6-triisopropylbenzenethiolate, Li[S-2,4,6-i-Pr3C6H2] 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-Pr3C6H2)(4)], respectively (2). Compounds 1 and 2 were structurally characterized by X-ray diffraction giving the crystallographic parameters: for 1, tetragonal space group with a=b=12.366(2) c=16.352(4) Angstrom, V=2500 Angstrom(3), Z=2; for 2, triclinic space group with a=14.709(2), b=20.928(5), c=13.901(2) Angstrom, alpha=90.97(3)degrees, beta=105.43(3)degrees, gamma=78.13(3)degrees, V=4033 Angstrom(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-) anion to have S-4 point group symmetry and the [FeS4] core to have D-2d symmetry. [Ph(4)P][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. [References: 74]