Reexamination of a Cytochrome Oxidase Model

Alternative Title

A noncoupled iron-copper binuclear complex


A heterobinuclear complex [Fe(mac)(Bpm)Cu(acac)2]2+ (I) (mac = the macrocyclic adduct of 2,6-diacetylpyridine and hydrazine; Bpm = bipyrimidine), previously postulated to contain a Fe(Bpm)Cu bridge, had been studied as a possible model for cytochrome oxidase. Four important questions arise with this complex. (1) The high-spin state of the Fe(II) conflicts with reports of low-spin Fe(II) in closely analogous environments. Our study removes the discrepancy: the complexes are all low spin except in adducts with M(acac)2. (2) Can mac bend enough to permit coordination of the cis Bpm nitrogens, given that the only reported structural evidence is for planar mac? Our structure of [Fe(mac)bpy](ClO4)2 demonstrates appropriate mac bending, though not quite in the way that had been proposed. (3) Cu(acac)2 is fairly resistant to the addition of an extra donor atom, and even more resistant to two donors as required for Bpm, and the observed magnetic properties can be explained by ligand-exchange reactions. We have eliminated this possibility with FAB mass spectral data. (4) The unpaired electrons of Cu and Fe appear to be coupled in I. To check whether a peculiarity of Bpm sometimes prevents electron coupling, we demonstrate the generality of coupling through M(Bpm)M bridges with M = Ni, Co, and Mn. Thus, binuclear complexes of Cu, Ni, Co, and Mn are now known to be antiferromagnetic, the first finding that fails to support a CuBpmFe bridge. A further experiment virtually eliminates the possibility of a Bpm bridge. Electronic spectra indicate the formation, from [Fe(mac)bpy]2+, that Bpm is not necessary to form the binuclear complex. Possible mechanisms for the binucleation involve hydrogen bonding or, more likely, one of the uncoordinated mac N atoms, in which light the electrochemistry of the complex can now be reexamined. Crystal data for [Fe(mac)(Bpm)Cu(acac)2](ClO4)2: FeCl2-O8N8C28H28, P21/n, Z = 4, a = 8.943 (3) Å, b = 24.332 (8) Å, c = 15.954 (5) Å, β = 100.77 (4)°. © 1984 American Chemical Society.



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© 1984 American Chemical Society (ACS), All rights reserved.