To complete the set of synthetic analogues corresponding to the three known types of active sites in iron-sulfur redox proteins, bis(o-xylyl-α, α'-dithiolato)ferrate(II, III) anions, [Fe(S2-o-xyl)2]2-, -, have been prepared and their structures and certain electronic properties determined. These complexes are shown to be related to the active sites of the 1-Fe rubredoxin (Rd) proteins. (Et4N)[Fe(S2-o-xyl)2] crystallizes in the orthorhombic space group C2v9-Pn21a of dimensions a=13.413 (8), b=29.554 (17)and c=13.448 (10) Å with Z=8. Na(Ph4As) [Fe(S2-o-xyl)2] acetonitrile solvate crystallizes in the triclinic space group Ci1-P1 of dimensions a=13.67 (1), b=14.60 (1), c=11.59 (1) Å, α=109.3 (l), β=105.6 (1)and γ=71.6 (1)° with Z=2. Both complexes contain high-spin mononuclear anions whose Fe-S4 coordination units are degraded from Td symmetry by rhombic distortions. Solution absorption spectra of [Fe(S2-o-xyl)2]- (λmax 354, 486, ~640-690 nm) and [Fe(S2-o-xyl)2]2- (λmax 322, ~355, 1800 nm) resemble those of Rdox and Rdred, respectively. The same comment applies to Mössbauer parameters (isomer shift, quadrupole splitting) at 77 K: [Fe(S2-o-xyl)2]-, 0.13, 0.57 mm/s; [Fe(S2-o-xyl)2]2-, 0.61, 3.28 mm/s. Internal hyperfine magnetic field data are also presented for these two complexes. As for Rdred the ground state of [Fe(S2-o-xyl)2]2- has dz2 symmetry and is separated from the next orbital state by ~900 cm-1. Comparison of structural and collective electronic properties with those of the proteins indicates an extent of correspondence sufficient to designate [Fe(S2-o-xyl)2]- and [Fe(S2-o-xyl)2]2- as analogues of the active sites of Rdox and Rdred, respectively. Analogue structures are considered to represent close approaches to unconstrained protein site stereochemistries; further comparison between [Fe(S2-o-xyl)2]- and Rdox awaits final refinement of the protein structure. Also prepared in this work are salts of [M2(S2-o-xyl)3]2-, M=Fe(II) and Ni(II)and [Co(S2-o-xyl)2]2-. The spectrum of the latter together with published data is used to demonstrate the weak ligand field nature of S2-o-xyl2-, which is comparable to that of N3-, OH-and -NCO-. Comparison of the two analogue structures allows an estimate of structural change at the Rd site in the absence of protein constraints accompanying the reaction Rdox+e-År1 ^ Rdred- An Fe-S bond distance increase of 0.1 Å upon reduction is indicated. Factors relevant to the enthalpic structural barriers to electron transfer in Rd proteins and analogues are briefly discussed.
ASJC Scopus subject areas
- Colloid and Surface Chemistry