Multiple-Edge XAS Studies of Cyanide-Bridged Iron−Copper Molecular Assemblies Relevant to Cyanide-Inhibited Heme−Copper Oxidases Using Four-Body Multiple-Scattering Analysis

A pair of cyanide-bridged iron−copper molecular assemblies relevant to cyanide-inhibited heme−copper oxidases has been studied by X-ray absorption spectroscopy at both Fe and Cu K-edges. These two complexes, [(py)(OEP)Fe−CN−Cu(Me 6 tren)] 2+ (2) and [(py)(OEP)Fe−CN−Cu(TIM)] 2+ (3), contain a unique four-body Fe−C−N−Cu bridge with an Fe−C−N angle of 179°. They, however, differ significantly in the Cu−N−C angle (174° in 2 and 147° in 3 ). These two complexes provide the opportunity to study long-range multiple scattering (MS) interactions between the Fe and the Cu centers. We have calculated theoretical four-body MS signals for the bridge configuration and other four-body pathways in the structures, and performed least-squares fittings of the theoretical signals to the experimental data using the GNXAS programs. A strong long-range Fe···Cu interaction (4.94 Å) has been observed from both the Fe and Cu K-edge data for 2 . GNXAS analysis shows that this long-range interaction is attributed to the MS amplitude enhancement from the linear four-body Fe−C−N−Cu configuration. This interaction is not observable from the Fe and Cu K-edge data for 3 where the angle deviates significantly from linearity. An angle-dependence study of MS effects on the Cu−N−C angle shows that there is a large enhancement of MS intensity when the angle approaches linearity, and that this MS effect is negligible when the angle is below ∼160°. Essentially no Fe−Cu interaction is detectable for a configuration with such an angle. Comparisons with three-body MS effects found in Fe−O/O(H)−Cu bridged systems are made. The significance of the findings of this study to the structural definition of the binuclear center in the cyanide-inhibited heme-copper oxidases is discussed.