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Oxygenases have been postulated to utilize d4 FeIV and d8 CuIII oxidants in proton-coupled electron transfer (PCET) hydrocarbon oxidation. In order to explore the influence the metal ion and d-elec...

The complex [FeIII2(μ-O2)(L3)4(S)2]4+ (L3 = 2-(4-thiazolyl)benzimidazole, S = solvent) forms upon reaction of [FeII(L3)2] with H2O2 and is a functional model of peroxo-diiron intermediates invoked during the catalytic cycle of oxidoreductases. The spectroscopic properties of the complex are in line with those of complexes formed with N-donor ligands. [FeIII2(μ-O2)(L3)4(S)2]4+ shows both nucleophilic (aldehydes) and electrophilic (phenol, N,N-dimethylanilines) oxidative reactivity and unusually also electron transfer oxidation., A bidentate ligand based iron complex shows nucleophillic and electrophillice reactivity in the oxidation of organic substrates.

Oxygenases have been postulated to utilize d

The effect of a minor difference in ligand structure is shown to have a large effect on the photochemical pathways followed by two ruthenium(II) polypyridyl based complexes [Ru(CH3CN) (LL)](2+), 1 and 2, where LL is MeN4Py (1,1-di(pyridin-2-yl)-N,N-bis (pyridin-2-yl-methyl) ethan-1-amine) or N4Py (1,1-di (pyri din-2-yl)-N,N-bis (pyridin-2-yl-m ethyl) methanamine), respectively. In our earlier report we demonstrated near completely reversible two-way photochromism of 1, in which a pyridyl ring dissociated on irradiation with visible light to form the thermally stable 1P, [Ru(CH3CN)(2)(MeN4Py)](2+). Complex 1 was recovered upon irradiation in the near-UV. Here, we show that the methyl group in the ligand backbone is critical to the reversibility by impeding the dissociation of one of the two sets of pyridyl rings. Irradiation of 2, which does not bear the methyl group, with visible light results in formation of two thermally stable isomers 2a and 2b, which are characterized by UV-vis absorption, FTIR, H-1 NMR spectroscopy, ESI mass spectrometry, and X-ray crystallography. In contrast to 1P, in both 2a and 2b, a different pyridyl moiety is dissociated. Whereas UV irradiation returns 2a to its original state (2), the overall reversibility is limited by the relative stability of 2b. The changes to the structure of 2 made possible by the increased freedom for all four pyridyl moieties to dissociate allows access to coordination modes that are not accessible thermally opening opportunities toward new catalysts for oxidation chemistry, photochromism and photoswitching.

Phenolato moieties impart redox flexibility to metal complexes due their accessible (oxidative) redox chemistry and have been proposed as functional ligand moieties in redox non-innocent ligand based transition metal catalysis. Here, the electro- and spectroelectrochemistry of phenolato based μ-oxodiiron(III) complexes [(L1)Fe(μ-O)Fe(L1)]2+ (1) and [(L2)Fe-(μ-O)Fe(L2)]2+ (2), where L1 = 2-(((di(pyridin-2-yl)methyl)-(pyridin-2-ylmethyl)amino)methyl)phenol and L2 = 3, 5-di-tert-butyl-2-(((di(pyridin-2-yl)methyl)(pyridin-2-ylmethyl)amino)-methyl)phenol, is described. The electrochemical oxidation of 1 in dichloromethane results in aryl C-C coupling of phenoxyl radical ligand moieties to form tetra nuclear complexes, which undergo subsequent oxidation to form iron(III) phenolato based polymers (poly-1). The coupling is blocked by placing tert-butyl groups at para and ortho positions of phenol units (i.e., 2). Poly-1 shows two fully reversible redox processes in monomer free solution. Assignment of species observed during the electrochemical and chemical {(NH4)2[CeIV(NO3)6]} oxidation of 1 in acetonitrile is made by comparison with the UV-vis-NIR absorption and resonance micro-Raman spectroelectrochemistry of poly-1, and by DFT calculations, which confirms that oxidative coupling occurs in acetonitrile also. However, in contrast to that observed in dichloromethane, in acetonitrile, the oligomers formed are degraded in terms of a loss of the Fe(III)-O-Fe(III) bridge by protonation. The oxidative redox behavior of 1 and 2 is, therefore, dominated by the formation and reactivity of Fe(III) bound phenoxyl radicals, which considerably holds implications in regard to the design of phenolato based complexes for oxidation catalysis The European Research Council (StG, no. 279549, D.U.,W.R.B.) and the Ministry of Education, Culture and Science(Gravity program 024.001.035, W.R.B.), the Ministerio de Economia y Competitividad (MINECO, projects CTQ2014-59212-P and CTQ2015-70851-ERC, M.S.), the DIUE of the Generalitat de Catalunya (project 2014SGR1202, M.S.), and the European Fund for Regional Development (FEDER, UNGI10-4E-801, M.S.) are acknowledged for financial support. A. van Dam (ERIBA, University of Groningen) is thanked for assistance with a recording of ESI-MS spectra. L. Kortekaas (University of Groningen) is acknowledged for discussion. This work was performed in the framework of the COST action CM1305 “Explicit Control Over Spin-states in Technology and Biochemistry (ECOSTBio)” (with support of an STSM to W.R.B., COST-STSM-CM1305-29045)

The oxidation of the C-H and C=C bonds of hydrocarbons with H2O2 catalyzed by non-heme iron complexes with pentadentate ligands is widely accepted as involving a reactive Fe-IV=O species such as [(N4Py)Fe-IV=O](2+) formed by homolytic cleavage of the O-O bond of an Fe-III-OOH intermediate (where N4Py is 1,1-bis(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine). We show here that at low H2O2 concentrations the Fe-IV=O species formed is detectable in methanol. Furthermore, we show that the decomposition of H2O2 to water and O-2 is an important competing pathway that limits efficiency in the terminal oxidant and indeed dominates reactivity except where only sub-/near-stoichiometric amounts of H2O2 are present. Although independently prepared [(N4Py)Fe-IV=O](2+) oxidizes stoichiometric H2O2 rapidly, the rate of formation of Fe-IV=O from the Fe-III-OOH intermediate is too low to account for the rate of H2O2 decomposition observed under catalytic conditions. Indeed, with excess H2O2, disproportionation to O-2 and H2O is due to reaction with the Fe-III-OOH intermediate and thereby prevents formation of the Fe-IV=O species. These data rationalize that the activity of these catalysts with respect to hydrocarbon/alkene oxidation is maximized by maintaining sub-/near-stoichiometric steady-state concentrations of H2O2, which ensure that the rate of the H2O2 oxidation by the Fe-III-OOH intermediate is less than the rate of the O-O bond homolysis and the subsequent reaction of the Fe-IV=O species with a substrate.

The oxidation of the C–H and C=C bonds of hydrocarbons with H2O2 catalyzed by non-heme iron complexes with pentadentate ligands is widely accepted as involving a reactive FeIV=O species such as [(N4Py)FeIV=O]2+ formed by homolytic cleavage of the O–O bond of an FeIII–OOH intermediate (where N4Py is 1,1-bis(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine). We show here that at low H2O2 concentrations the FeIV=O species formed is detectable in methanol. Furthermore, we show that the decomposition of H2O2 to water and O2 is an important competing pathway that limits efficiency in the terminal oxidant and indeed dominates reactivity except where only sub-/near-stoichiometric amounts of H2O2 are present. Although independently prepared [(N4Py)FeIV=O]2+ oxidizes stoichiometric H2O2 rapidly, the rate of formation of FeIV=O from the FeIII–OOH intermediate is too low to account for the rate of H2O2 decomposition observed under catalytic conditions. Indeed, with excess H2O2, disproportionation to O2 and H2O is due to reaction with the FeIII–OOH intermediate and thereby prevents formation of the FeIV=O species. These data rationalize that the activity of these catalysts with respect to hydrocarbon/alkene oxidation is maximized by maintaining sub-/near-stoichiometric steady-state concentrations of H2O2, which ensure that the rate of the H2O2 oxidation by the FeIII–OOH intermediate is less than the rate of the O–O bond homolysis and the subsequent reaction of the FeIV=O species with a substrate.

Non-heme iron(II) complexes of pentadentate N4Py (N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) type ligands undergo visible light-driven oxidation to their iron(III) state in the presence of O2 without ligand degradation. Under mildly basic conditions, however, highly selective base catalyzed ligand degradation with O2, to form a well-defined pyridyl-imine iron(II) complex and an iron(III) picolinate complex, is accelerated photochemically. Specifically, a pyridyl-CH2 moiety is lost from the ligand, yielding a potentially N4 coordinating ligand containing an imine motif. The involvement of reactive oxygen species other than O2 is excluded; instead, deprotonation at the benzylic positions to generate an amine radical is proposed as the rate determining step. The selective nature of the transformation holds implications for efforts to increase catalyst robustness through ligand design., Photoaccelerated oxidation of an aminopyridyl ligand bound to an Fe(III) ion to a well-defined imine-based Fe(II) complex involves initial alkyl C−H deprotonation followed by reaction with O2 to form an alkyl peroxyl radical. Intramolecular C−H abstraction followed by C−N bond cleavage yields picoline aldehyde and a pyridyl-imine complex. The selectivity of the reaction prevents further oxidation and holds implications for ligand degradation under conditions used in oxidation catalysis with peroxides.

An efficient and simple method for selective oxidation of secondary alcohols and oxidation of alkanes to ketones is reported. An insitu prepared catalyst is employed based on manganese(II) salts, pyridine-2-carboxylic acid, and butanedione, which provides good-to-excellent conversions and yields with high turnover numbers (up to 10000) with H2O2 as oxidant at ambient temperatures. In substrates bearing multiple alcohol groups, secondary alcohols are converted to ketones selectively and, in general, benzyl CH oxidation proceeds in preference to aliphatic CH oxidation.

The formation of an Fe-III-OOH species by reaction of complex 1 ([(MeN3Py)Fe-II(CH3CN)(2)](2+)) with H2O2 at room temperature is reported and is studied by a combination of UV/vis absorption, EPR, and resonance Raman spectroscopies. The formation of the Fe-III-OOH species, and its subsequent conversion to relatively inert Fe-III-O-Fe-III species, is shown to be highly dependent on the concentration of water, with excess water favoring the formation of the latter species, which is studied by UV/vis absorption spectroelectrochemistry also. The presence of acetic acid increases the rate and extent of oxidation of 1 to its iron(III) state and inhibits the wasteful decomposition of H2O2 but does not affect significantly the spectroscopic properties of the Fe-III-OOH species formed.

Fully reversible photoswitching of the coordination mode of the ligand MeN4Py (1,1-di(pyridin-2-yl)-N,N'-bis(pyridin-2-yl-methyl)-ethan- 1-amine) in its ruthenium(II) complex with visible light is reported. Irradiation with visible light results in dissociation of a pyridyl moiety, which is reversed by irradiation at 355 nm.