μ-Nitrido Diiron Macrocyclic Platform: Particular Structure for Particular Catalysis

The ultimate objective of bioinspired catalysis is the development of efficient and dean chemical processes. Cytochrome P450 and soluble methane monooxygenase enzymes efficiently catalyze many challenging reactions. Extensive research has been performed to mimic their exciting chemistry, aiming to create efficient chemical catalysts for functionalization of strong C-H bonds. Two current biomimetic approaches are based on (i) mononuclear metal porphyrin-like complexes and (ii) iron and diiron non-heme complexes. However, biomimetic catalysts capable of oxidizing CH4 are still to be created. In the search for powerful oxidizing catalysts, we have recently proposed a new bioinspired strategy using N-bridged diiron phthalocyanine and porphyrin complexes. This platform is particularly suitable for stabilization of Fe(IV)Fe(IV) complexes and can be useful to generate high-valent oxidizing active species. Indeed, the possibility of charge delocalization on two iron centers, two macrocyclic ligands, and the nitrogen bridge makes possible the activation of H2O2 and peracids. The ultrahigh-valent diiron-oxo species (L)Fe-IV-N-Fe-IV(L+center dot)=O (L = porphyrin or phthalocyanine) have been prepared at low temperatures and characterized by cryospray MS, UV-vis, EPR, and Mossbauer techniques. The highly electrophilic (L)Fe-IV-N-Fe-IV(L+center dot)=O species exhibit remarkable reactivity. In this Account, we describe the catalytic applications of mu-nitrido diiron complexes in the oxidation of methane and benzene, in the transformation of aromatic C-F bonds under oxidative conditions, in oxidative dechlorination, and in the formation of C-C bonds. Importantly, all of these reactions can be performed under mild and dean conditions with high conversions and turnover numbers. mu-Nitrido diiron species retain their binuclear structure during catalysis and show the same mechanistic features (e.g., O-18 labeling, formation of benzene epoxide, and NIH shift in aromatic oxidation) as the enzymes operating via high-valent iron-oxo species. mu-Nitrido diiron complexes can react with perfluorinated aromatics under oxidative conditions, while the strongest oxidizing enzymes cannot. Advanced spectroscopic, labeling, and reactivity studies have confirmed the involvement of high-valent diiron-oxo species in these catalytic reactions. Computational studies have shed light on the origin of the remarkable catalytic properties, distinguishing the Fe-N-Fe scaffold from Fe-C-Fe and Fe-O-Fe analogues. X-ray absorption and emission spectroscopies assisted with DFT calculations allow deeper insight into the electronic structure of these particular complexes. Besides the novel chemistry involved, iron phthalocyanines are cheap and readily available in bulk quantities, suggesting high application potential. A variety of macrocyclic ligands can be used in combination with different transition metals to accommodate M-N-M platform and to tune their electronic and catalytic properties. The structural simplicity and flexibility of mu-nitrido dimers make them promising catalysts for many challenging reactions.