A (MU-OXO)BIS(MU-CARBOXYLATO)DIIRON(III) COMPLEX WITH A TETHERED PHENOXYL RADICAL AS A MODEL FOR THE ACTIVE-SITE OF THE R2 PROTEIN OF RIBONUCLEOTIDE REDUCTASE

We have designed and synthesized a novel bidentate nitrogen donor ligand, 1,1-bis(2-(1-methylimidazolyl))-1-(3,5-di-tert-butyl-4-hydroxyphenyl)ethane (BIDPhEH) (2). The phenoxyl radical form of this ligand, BIDPhE (3), was prepared and isolated as a stable solid. BIDPhEH was used in the synthesis of the mononuclear ferric complex, [Fe(BIDPhEH)(2)Cl-2]X (X = Cl (5a), FeCl4 (5b). The crystallographic characterization of 5b is reported. Use of the solvento complex [Fe2O(XDK)(MeOH)(5)(H2O)](NO3)(2), where XDK is m-xylenediaminebis(Kemp's triacid)-imide, a pre-organized, cleft-shaped dicarboxylate ligand, facilitated preparation of the phenoxyl radical (mu-oxo)-bis(mu-carboxylato)diiron(III) complex [Fe2O(XDK)(BIDPhE)(2)(NO3)(2)] (6), a new model for the tyrosyl radical containing active site of the R2 protein of E. coli ribonucleotide reductase. Magnetic susceptibility studies of 6 revealed overall. magnetic behavior quite similar to that of the protein. The iron atoms are antiferromagnetically coupled, and a theoretical fit of the data determined the J value (H = -2JS(1).S-2) to be -117 cm(-1). Pulsed saturation-recovery EPR experiments conducted on 6 provided an independent measure of J, the value of which was in excellent agreement with the magnetic susceptibility measurements. Comparisons are made to the saturation-recovery EPR results for the R2 protein.