Cytochrome Oxidase Models. 2. μ-Bipyrimidyl Mixed-Metal Complexes as Synthetic Models for the Fe/Cu Binuclear Active Site of Cytochrome Oxidase

Article abstract of DOI:10.1021/ja00522a028

μ-Bipyrimidyl (bipym) mixed-metal complexes, with II(bipym)M^II> cores (M^II = Cu and Zn), have been synthesized to model the proposed imidazolate-bridged 3³⁺(imid)Cu_U²⁺> active site structure of cytochrome oxidase where -J(Fe^III-Cu^II) > 200 cm^-1.The binuclear compounds have been prepared from a six-coordinate II(C18H18N6)(bipym)>²⁺ species (B) (C18H18N6 = a folded macrocycle) by reaction with the appropriate bis(acetylacetonato)M(II) compound in CH2Cl2 to yield II(C18H18N6)(bipym)Cu^II(acac)2>²⁺ (C) and II(C18H18N6)(bipym)Zn^II(acac)2>²⁺ (D) as ClO4^- salts.Compound B contains low-spin iron(II), whereas C and D are high-spin species in both the solid and solution states at room temperature.Comparative variable-temperature (10-300 K) magnetic susceptibility measurements for C and D indicate C to contain magnetically isolated S = 2 (Fe^II) and S = 1/2 (Cu^II) centers with J ca. 0 through the bipym bridge.The 57Fe Moessbauer spectra of C and D and the Cu^II EPR spectrum of C at 8 K are also supportive of this interpretation.In solution, the redox activity of B, C, and D has been examined by cyclic voltammetry in CH2Cl2 where E_1/2 for the Fe^II/Fe^III couple of high-spin C and D are identical at +0.60 V (SCE) and 700 mV lower in potential than for the low-spin monomer compound (B).The Cu^II/Cu^I couple for C appears to occur at E_1/2 = -0.24 V (SCE).Finally, the μ-bipyrimidyl Cu2 compound, <(hfa)2Cu^II(bipym)Cu^II(hfa)2> (hfa^- = hexafluoroacetylacetonato anion), has been prepared and found to exhibit an antiferromagnetic exchange interaction with -J = 7.9 cm^-1.The ramifications of these results as they pertain to the magnetically coupled 3³⁺-Cu_u²⁺> active site of resting cytochrome oxidase are discussed, and an oxo-bridged alternative to the imidazolate-bridge possibility is also considered in view of the findings from the present model study.