(Hydr)oxo-bridged heme complexes: From structure to reactivity

Abstract

Iron–porphyrins ([Formula: see text] hemes) are present throughout the biosphere and perform a wide range of functions, particularly those that involve complex multiple-electron redox processes. Some common heme enzymes involved in these processes include cytochrome P450, heme/copper oxidase or heme/non-heme diiron nitric oxide reductase. Consequently, the (hydr)oxo-bridged heme species have been studied for the important roles that they play in many life processes or for their application for catalysis and preparation of new functional materials. This review encompasses important synthetic, structural and reactivity aspects of the (hydr)oxo-bridged heme constructs that govern their function and application. The properties and reactivity of the bridging (hydr)oxo moieties are directly dictated by the coordination environment of the heme core, the nature and ligation of the second metal center attached to the (hydr)oxo group, the presence or absence of a linker, and the degree of flexibility around that linker within the scaffold. Here, we summarize the structural features of all known (hydr)oxo-bridged heme constructs and use those to categorize and thus, provide a more comprehensive picture of structure–function relationships.

Dioxygen Reactivity of Copper(I)/Manganese(II)-Porphyrin Assemblies: Mechanistic Studies and Cooperative Activation of O2

“…This μ -peroxo Mn III /Cu II complex was not stable at higher temperatures, primarily leading to the formation of a μ -oxo complex, [(tmpa)Cu II –O–Mn III (TPP)] + . By analogy to the process shown to occur in similar bridged peroxo systems, it was presumed that this transformation occurred through a disproportionation reaction of two μ -peroxo species that generated two μ -oxo complexes and released O 2 [ 27 , 34 ].…”

Protonation of the oxo-bridged heme/copper assemblies: Modeling the oxidized state of the cytochrome c oxidase active site

“…To date, the pKa ranges have been estimated for only three of these μ-hydroxo-heme/Cu systems [13] as shown in Fig. 2 [ [14], [15], [16], [17]]. In this report, we have used a range of protonated nitrogen bases (i.e., weak acids) that are paired with the bulky and non-coordinating counterion tetrakis(pentafluorophenyl)borate, [B(C6F5)4] − , with known pKa values spanning from 12.30 to 18.46 in MeCN (Table S1) for measuring acid dissociation constants for a series of μ-hydroxo-heme/Cu systems.…”