SYNTHESIS AND CHARACTERIZATION OF AN ASYMMETRIC BRIDGED ASSEMBLY CONTAINING THE UNSUPPORTED [FE(III)-O-CU(II)] BRIDGE - AN ANALOG OF THE BINUCLEAR SITE IN OXIDIZED CYTOCHROME-C-OXIDASE

A synthetic analogue of the antiferromagnetically coupled S = 2 Fe/Cu binuclear site in oxidized (as-isolated) cytochrome c oxidase has been sought. Initial Cu(II) precursor compound [Cu(Me6tren)(OH2)](ClO4)2 containing the trigonal bipyramidal (TBP) complex [1]2+ upon treatment with methanolic KOH yields [Cu(Me6tern)(OH)]+ ([2]+) with TBP stereochemistry and a terminal hydroxo group. In acetone solution, reaction of [1]2+ with 2 equiv of the hindered base lithium 2,6-di-tert-butyl-4-methylphenolate and 1 equiv of [Fe(OEP)(OClO3)] affords (via the presumed intermediate [2]+) the bridged assembly [(OEP)Fe-O-Cu(Me6tren)]+ ([3]+) in 67% isolated yield as the perchlorate salt (3). A coproduct is the previously unknown high-spin enolate complex [Fe(OEP)(OC(Me)=CH2)], identified by H-1 NMR. The effects of solvent and base on the formation of [3]+ are considered. Compounds 3-MeCN and 3.THF crystallize in monoclinic and orthorhombic space groups, respectively, with the former having two inequivalent molecules in the asymmetric unit; the structures of [3]+ in three different crystalline environments are essentially invariant. Tetragonal five-coordinate Fe(III) is linked to TBP Cu(II) by the asymmetric, unsupported, nearly linear Fe-O-Cu bridge (mean angle 176.6-degrees). The presence of high-spin Fe(III) is supported by structural and Mossbauer spectroscopic parameters. Magnetic susceptibility data at 4-300 K for polycrystalline 3 can be fitted well to Curie-Weiss behavior corresponding to S = 2, which must arise from antiferromagnetic coupling between S = 5/2 and 1/2 centers. Retention of this bridge in acetone solution is indicated by diastereotopic methylene proton resonances whose isotropic shifts are smaller than those of high-spin Fe(III) hemes. Bridged assembly [3]+ reproduces the most prominent electronic feature of the oxidized enzyme site and in this sense is the first synthetic analogue of that site.