Synthetic, Spectroscopic and DFT Studies of Iron Complexes with Iminobenzo(semi)quinone Ligands: Implications for o-Aminophenol Dioxygenases

The oxidative CC bond cleavage of o-aminophenols by nonheme Fe dioxygenases is a critical step in both human metabolism (the kynurenine pathway) and the microbial degradation of nitroaromatic pollutants. The catalytic cycle of o-aminophenol dioxygenases (APDOs) has been proposed to involve formation of an FeII/O2/iminobenzosemiquinone complex, although the presence of a substrate radical has been called into question by studies of related ring-cleaving dioxygenases. Recently, we reported the first synthesis of an iron(II) complex coordinated to an iminobenzosemiquinone (ISQ) ligand, namely, [Fe(Tp)(tBuISQ)] (2 a; where Tp=hydrotris(3,5-diphenylpyrazol-1-yl)borate and tBuISQ is the radical anion derived from 2-amino-4,6-di-tert-butylphenol). In the current manuscript, density functional theory (DFT) calculations and a wide variety of spectroscopic methods (electronic absorption, Mössbauer, magnetic circular dichroism, and resonance Raman) were employed to obtain detailed electronic-structure descriptions of 2 a and its one-electron oxidized derivative [3 a]+. In addition, we describe the synthesis and characterization of a parallel series of complexes featuring the neutral supporting ligand tris(4,5-diphenyl-1-methylimidazol-2-yl)phosphine (TIP). The isomer shifts of about 0.97 mm s−1 obtained through Mössbauer experiments confirm that 2 a (and its TIP-based analogue [2 b]+) contain FeII centers, and the presence of an ISQ radical was verified by analysis of the absorption spectra in light of time-dependent DFT calculations. The collective spectroscopic data indicate that one-electron oxidation of the FeII–ISQ complexes yields complexes ([3 a]+ and [3 b]2+) with electronic configurations between the FeIII–ISQ and FeII–IBQ limits (IBQ=iminobenzoquinone), highlighting the ability of o-amidophenolates to access multiple oxidation states. The implications of these results for the mechanism of APDOs and other ring-cleaving dioxygenases are discussed

Bis(amidophenolato)phosphonium: Si−H Hydride Abstraction and Phosphorus‐Ligand Cooperative Activation of C−C Multiple Bonds

The first bis(amidophenolato)phosphonium salts are prepared and fully characterized. The perfluorinated derivative represents the strongest monocationic phosphorus Lewis acid on the fluoride and hydride ion affinity scale isolable to date. This affinity enables new reactions, such as hydride abstraction from Et3SiH, the first phosphaalkoxylation of an alkyne or a phosphorus catalyzed intramolecular hydroarylation. All properties and reactions are scrutinized by theory and experiment. Substantial σ‐ and π‐acidity provides the required affinity for substrate activation, while phosphorus‐ligand cooperativity substantially enriches the reactivity portfolio of phosphonium ions

A Synthetic Model of the Nonheme Iron–Superoxo Intermediate of Cysteine Dioxygenase

A nonheme Fe(II) complex (1) that models substrate-bound cysteine dioxygenase (CDO) reacts with O2 at −80 °C to yield a purple intermediate (2). Analysis with spectroscopic and computational methods determined that 2 features a thiolate-ligated Fe(III) center bound to a superoxide radical, mimicking the putative structure of a key CDO intermediate

Valence Tautomerism of p-Block Element Compounds – An Eligible Phenomenon for Main Group Catalysis?

Valence tautomerism has had a remarkable impact on several branches of transition metal chemistry. By switching between different valence tautomeric states, physicochemical properties and reactivities can be triggered reversibly. Is this phenomenon transferrable into the p-block – or is it already happening there? This Perspective collects observations of p-block element-ligand systems that might be assignable to valence tautomerism. Further, it discusses occurrences in p-block element compounds that exhibit the related effect of redox-induced electron transfer. As disclosed, the concept of valence tautomerism with p-block elements is at a very early stage. However, given the substantial disparity in the properties of those elements in different redox states, it might offer a valid extension for future developments in main group catalysis

An Electrochemical Study of Frustrated Lewis Pairs: A Metal-free Route to Hydrogen Oxidation

[Image: see text] Frustrated Lewis pairs have found many applications in the heterolytic activation of H(2) and subsequent hydrogenation of small molecules through delivery of the resulting proton and hydride equivalents. Herein, we describe how H(2) can be preactivated using classical frustrated Lewis pair chemistry and combined with in situ nonaqueous electrochemical oxidation of the resulting borohydride. Our approach allows hydrogen to be cleanly converted into two protons and two electrons in situ, and reduces the potential (the required energetic driving force) for nonaqueous H(2) oxidation by 610 mV (117.7 kJ mol(–1)). This significant energy reduction opens routes to the development of nonaqueous hydrogen energy technology

Localized mixed-valence and redox activity within a triazole-bridged dinucleating ligand upon coordination to palladium

The new dinucleating redox-active ligand (LH4), bearing two redox-active NNO-binding pockets linked by a 1,2,3-triazole unit, is synthetically readily accessible. Coordination to two equivalents of PdII resulted in the formation of paramagnetic (S=inline image ) dinuclear Pd complexes with a κ2-N,N′-bridging triazole and a single bridging chlorido or azido ligand. A combined spectroscopic, spectroelectrochemical, and computational study confirmed Robin–Day Class II mixed-valence within the redox-active ligand, with little influence of the secondary bridging anionic ligand. Intervalence charge transfer was observed between the two ligand binding pockets. Selective one-electron oxidation allowed for isolation of the corresponding cationic ligand-based diradical species. SQUID (super-conducting quantum interference device) measurements of these compounds revealed weak anti-ferromagnetic spin coupling between the two ligand-centered radicals and an overall singlet ground state in the solid state, which is supported by DFT calculations. The rigid and conjugated dinucleating redox-active ligand framework thus allows for efficient electronic communication between the two binding pockets

The optical and magnetic properties of redox-active d-block metal complexes with non-innocent ligands

Redox-active metal organic complexes with ‘non-innocent’ ligands are promising candidates when it comes to developing novel homogenous catalysts. The related complexes have gained interest since they could be utilized as affordable base-metal containing compounds instead of expensive and scarce noble metal containing complexes. The complexes with non-innocent ligands undergo readily one- or multielectron oxidation/reduction processes, which are vital in catalytic processes. Therefore, the complexes have been studied for biomimetic model compounds for metalloenzymes. These redox-active complexes have low-lying, intramolecular charge transfer processes that enable the compounds to absorb strongly electromagnetic radiation at low frequency range, namely, in the near-infrared range. In addition, the complexes possess interesting magnetic properties which are potentially influenced by photon excitation. Some of the redox-active complexes bear unpaired spins on the orbitals of the organic ligands. Redox-active complexes, in some cases, do exhibit bistability and are known to go through valence tautomerization process, where the sum of the electrons within the complex remains, while their locations and orientations vary. This may result in differing spin states within the complex and thus differing physical (optical and magnetic) properties between the tautomers. These features are the key in order to use these complexes in various sensing applications or in molecular memory. Because of their intense absorptions, the redox-active complexes have also been considered as dye sensitizers in a titanium dioxide-based dye sensitized solar cells and other photovoltaic applications. The present thesis exhibits the synthesis, characterization and the redox-active, optical and magnetic properties of first-row d-block metal complexes with non-innocent ligands