Oxo- and hydroxo-bridged heme-copper assemblies formed from acid-base or metal-dioxygen chemistry

The iron-copper dinuclear active site in heme-copper oxidases (e.g., cyctoc hrome c oxidase) has spurred the development of the inorganic: chemistry of bridged heme-copper complexes, including species possessing (porphyrinate) Fe-III-O(H)-Cu-II-L moieties. We describe here the synthesis, by two routes , of [(F8TPP)Fe-III-O-Cu-II(MePY2)](+) (5) {F8TPP = tetrakis(2,6-difluoroph enyl)porphyrinate; MePY2 = N,N-bis[2-(2-pyridyl)ethyl] methylamine). First, 5-(CF3SO3) was generated by reaction of [(MePY2)Cu-II](CF3SO3)(2) (3-(CF3S O3)(2)) and [(F8TPP)Fe-III-OH] (4) with triethylamine in THF or CH3CN in 65 -70% yield. The complex was also prepared by reduction of O-2 by a 1:1 mixt ure of copper(I) and iron(II) complexes, [(MePY2)Cu-I(CH3CN)](BArF) (1-(BAr F)) (BArF = tetrakis(3,5-bis-trifluoromethylphenyl)borate) and (F8TPP)Fe-II (2) in O-2-saturated THF or acetone, at -80 degrees C with subsequent warm ing to room temperature. Preliminary stopped-flow kinetics on the O-2 react ion with the 1:1 mixture show the formation of at least two intermediates ( i.e., a superoxo species (F8TPP)Fe-O-2 first, and then a presumed peroxo-br idged Fe-O-2-Cu species) prior to the formation of the final mu-oxo complex [(F8TPP)Fe-III-O-Cu-II(MePY2)](+) (5-(BArF)). The H-1 NMR spectrum of 5-(C F3SO3) in CD2Cl2 exhibits a pyrrole peak at 67.7 ppm (corroborated by H-2 N MR), while downfield (23.4 and 18.9 ppm) and dramatically upfield-shifted r esonances (-87.7, -155.4 and -189.4) have been assigned to hydrogens of the MePY2 moiety, by specific deuteration. The mu-hydroxo complex [(F8TPP)Fe-( OH)-Cu(MePY2)](OTf)(2) (6-(CF3SO3)(2)) was synthesized by addition of 3-(CF 3SO3)(2) to 4 in CH3CN, or by protonation of 5-(CF3SO3) with CF3SO3H. In a H-1. NMR-spectroscopic protonation titration (CF3SO3H)(2) the pyrrole 67.7 ppm resonance for 5-(CF3SO3) progressively converts to 70.3 ppm, diagnostic of 6-(CF3SO3)(2). The protonation is slow on the NMR time scale. The H-1 N MR spectral properties are consistent with antiferromagnetically coupled hi gh-spin iron(III) and Cu(II) ions (S = 2 spin state), and the interaction i s weaker in 6-(CF3SO3)(2) (5-(CF3SO3), mu(eff) = 5.05 mu(B); 6-(CF3SO3)(2), mu(eff) = 5.60 mu(B); Evans NMR method). By titration using a series of or ganic acids, the pK(a) for 6-(CF3S03)2 has been estimated to be 16.7 < pK(a ) < 17.6 (CH3CN solvent), or 9.6 +/- 2 (aqueous). Plots of delta vs 1/T for both mu-oxo and mu-hydroxo complexes 5-(CF3SO3) and 6-(CF3SO3)(2) have bee n obtained, showing linear Curie (for downfield resonances) or anti-Curie ( for upfield peaks) behavior.