Iron complexes pentadentate ligands

Iron complexes with the pentadentate ligand 3 derived from pyridyl and prolinol building blocks containing a stereogenic center were reported from the group of Klein Gebbink (Scheme 4) [34]. In alkene oxidations with hydrogen peroxide,... 

Recently, Nam, Fukuzumi, and coworkers succeed in an iron-catalyzed oxidation of alkanes using Ce(IV) and water. Here, the generation of the reactive nonheme iron (IV) 0x0 complex is proposed, which subsequently oxidized the respective alkane (Scheme 16) [104]. With the corresponding iron(II) complex of the pentadentate ligand 31, it was possible to achieve oxidation of ethylbenzene to acetophenone (9 TON). 0 labeling studies indicated that water is the oxygen source. 

In addition to tri- [105] and tetradentate N-ligands, mononuclear and dinuclear iron complexes with pentadentate N,N,N,N,0-ligands were applied to alkane... 

Non-heme Iron Complexes with Tetra- and Pentadentate Ligands... 

Roelfes et al. prepared a non-heme iron(II) complex 26 from pentadentate ligand N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine 25 (Scheme 3.32) [126]. In the presence of H202, complex 26 reacted to a low-spin Fe(III)OOH intermediate, which was cleaved homolytically to an oxo Fe(IV) species and a hydroxy radical. Both species are capable of oxidizing various organic substrates via a radical pathway (Scheme 3.32). Under the catalysis of complex 26, cyclohexene la was oxidized with excess H202 to a mixture of products 2a, 3a and 4a. The TON was found to be solvent dependent, with higher TON in acetonitrile than in acetone (Scheme 3.32). In no case were isolated yields given and, furthermore, the allylic oxidation is limited to cyclohexene la. 

Iron complexes of the new tetrapodal pentadentate ligands highlighted in this contribution are probably the most diverse and depart most strongly from previously established patterns of reactivity (8,21,79). 

The photolabile [Fe—NO] iron nitrosyl complexes with a pentadentate ligand A ,A -bis(2-pyridylmethyl)amine-A -ethyl-2-pyridine-2-carboxamide, PaPyaH, and its derivative [(PaPyalFe (N0)](C104) have been synthesized in which the binding of NO is reversible and the bound NO is released rapidly under irradiation with visible light (78-80). 

G. Roelfes, V. Vrajmasu, K. Chen, R. Y. N. Ho, J.-U. Rohde, C. Zondervan, R. M. la Crois, E. P. Schudde, M. Lutz, A. L. Spek, R. Hage, B. L. Feringa, E. Miinck, L. Que, Jr., End-on and side-on peroxo derivatives of non-heme iron complexes with pentadentate ligands Models for putative intermediates in biological iron/dioxygen chemistry, Inorg. Chem. 42 (2003) 2639. 

From a biological perspective the bleomycin (BLM) family of antibiotics that are used widely in chemotherapy, have stimulated considerable effort in understanding oxygen activated iron complexes. In particular iron bleomycin reacts with oxygen to form the so-called purple species, which exhibits an absorption band around 600 nm that is now known to be characteristic of the Fe -OOH species. In the case of Fe-BLM the active intermediate has been identified spectroscopically as a low-spin Fe -hydroperoxide. Several structural mimics of the metastable purple Fe -OOH species have been prepared, in particular complexes with neutral pentadentate ligands e.g., l,l-di(pyiidin-2-yl)-N,N-bis(pyridin-2-ylmethyl)-ethanamine (N4Py) Resonance Raman spectroscopy has proven... 

Cobalt complexes have been also extensively studied as artificial hydrogenases for the reductive side of water splitting.It has been recently demonstrated that the cobalt(ii) complex of the pentadentate ligand l,4-di(picolyl)-7-(p-toluenesulfonyl)-l,4,7-triazacyclononane (iy2T stacn) displays excellent H2 photoproduction catalytic activity, in the presence of [Ir(ppy)2(bpy)]PF6 as photosensitiser, and EtgN as electron donor. Under the same conditions, the corresponding complex of nick-el(ii) presents low photochemical activity, while the iron(n) analogue is inactive. 

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