Inke Siewert

Low-Molecular-Weight Analogues of the Soluble Methane Monooxygenase (sMMO): From the Structural Mimicking of Resting States and Intermediates to Functional Models

The active centre of sMMO contains a diiron core ligated by histidine and glutamate residues, which is capable of catalysing a remarkable reaction: the oxidation of methane with O(2) yielding methanol. This review describes the results of efforts to prepare low-molecular-weight analogues of this active site directing towards 1) the assignment of the spectroscopic signatures identified for certain intermediates of the sMMO catalytic cycle to structural features and 2) the synthesis of molecular compounds that can mimic the reactivity. The historical development of the model chemistry, which is subdivided into structural and functional mimicking, is illustrated and achievements reached so far are highlighted.

A Trispyrazolylborato Iron Malonato Complex as a Functional Model for the Acetylacetone Dioxygenase

Acetylacetone dioxygenase (Dke1) is an enzyme that can be isolated from Acinetobacter johnsonii, and it catalyses the degradation of acetylacetone, which is toxic to various mammals and to marine creatures and organisms; to achieve this process it consumes one equivalent of dioxygen. Dke1 is also capable of cleaving a whole series of other b diketones and b ketoesters that are derived from acetylacetone by substitution at the 1, 3, or 5 position to yield the corresponding carboxylic acids and a-ketoaldehydes. A 1,3-carbonyl structural motif is decisive for the activity; thus, for example, related 1-keto-3-hydroxo compounds are not converted. Dke1 has been investigated with the aid of single crystal Xray diffraction and fluorescence and UV/Vis spectroscopy, and from the results obtained its active site is assumed to contain iron(II) coordinated by three histidine residues and water. The substrate cleavage mechanism is however still largely unsettled. After preliminary studies based on isotopic labeling experiments, an initial deprotonation of acetylacetone followed by attack of dioxygen or superoxide at the Ca postion has been suggested. This mechanism would lead to an organoperoxide unit, which, after nucleophilic attack of its terminal oxygen atom at the carbonyl carbon atom, should decompose via a dioxetane species to give the cleavage products. In the course of further investigations, it was proposed that the role of the metal is restricted to breaking the spin forbiddance in the reaction of triplet dioxygen with the singlet substrate (by interaction of the HOMOs involved), so that the organoperoxo species can be generated in a concerted step (Scheme 1). Thus, a step which plays a decisive role in conversions of many oxygenating heme and non-heme iron enzymes, the binding of dioxygen at the iron(II) center under formation of a Fe-O2C entity, has not been considered. On the other hand, it has been shown that a substitution of iron(II) by various other metal ions, such as Zn, Co, Mn, Cu, or Ni, is accompanied by a loss in activity. Model compounds can provide valuable support in answering mechanistic questions arising for the reactions of active sites in metalloenzymes. a-Keto acid-dependent nonheme iron enzymes may serve as an example. In these proteins (similarly to the acteylacetone dioxygenase), an Fe center coordinated by three amino acid residues (two histidines and one asparagine) binds a cofactor through two oxygen donors for a subsequent oxidation with O2. Model complexes containing the Tp ligand (Tp= hydridotrispyrazol1-ylborato) have contributed significantly to understanding the protein function. After precoordination of a bidentate substrate analogue to a {TpFe} unit, one coordination site remains available, as is the case in the enzyme, and it was shown that dioxygen is activated at the free site for an attack at the cofactor. Herein we investigate whether a similar scenario is conceivable for acetylacetone dioxygenase with the aid of {TpFe} complexes. Kitajiama et al. prepared [Tp2Fe(acac)] (acac= acetylacetonato, Tp2 = hydridotris(3,5-ipropylpyrazol-1-yl)borato) without any biomimetic motivation. On exposure to air, this compound decomposes within one week to yield a trinuclear iron(III) complex containing m-oxobis(m-acetato) and m-hydroxobis(m-acetato) ligand constellations between the metal centers. The bridging acetato ligands no doubt have their origin in the acetylacetonato ligands employed, and the question arises as to whether these acetato ligands were generated by a dioxygenating cleavage according to the enzyme reactivity. We therefore synthesized an analogue in which the isopropyl groups at the ligand are replaced by methyl groups to achieve a higher reactivity. This complex, [Tp*Fe(acac)], Tp*= hydridotris(3,5-dimethylpyrazol-1yl)borato, was then treated with dry dioxygen. The observed color change from yellow to brown hinted at the formation of iron(III) compounds, and subsequent ESI-MS studies detected the species [FeTp*2] , [Tp*Fe(acac)], and [(Tp*Fe)2O(acac)] , that is, the acetylacetonato ligand had Scheme 1. Proposed mechanism for the cleavage of b-dicarbonyl compounds promoted by acetylacetone dioxygenase.

Electrocatalytic Dihydrogen Production with a Robust Mesoionic Pyridylcarbene Cobalt Catalyst

A Co(III) complex with a mesoionic pyridylcarbene ligand is presented. This complex is an efficient electrocatalyst for H2 production at very low overpotential and high turnovers when using a (glassy carbon) GC electrode. The corresponding triazole complexes display no catalytic activity whatsoever under identical conditions. The remarkable robustness of the Co-C(carbene) bond towards acids is likely responsible for the high efficiency of this catalyst. The present results thus open new avenues for carbene-based ligands for generating functional models for hydrogenases.

A Dinuclear Iron Complex Based on Parallel Malonate Binding Sites : Cooperative Activation of Dioxygen and Biomimetic Ligand Oxidation

A ligand that offers two parallel malonate binding sites linked by a xanthene backbone, namely, Xanthmal2-, has been utilised to synthesise dinuclear FeII complex [Fe2(Xanthmal)2] (1). The reactivity of 1 in contact with O2 was investigated at -40 degrees C and room temperature. After activation of O2 through interaction with both iron centres the ligand is oxidised: at the Calpha position monooxygenation and peroxide formation occur, partially accompanied by C-C bond cleavage to yield alpha-keto ester groups. To reveal mechanistic details investigations concerning 1) peroxide decomposition, 2) the reactivity of a corresponding mononuclear complex, 3) the influence of monooxygenation of the ligand on the reactivity and 4) product formation in dependence on time were carried out. The results can be explained by postulating formation of high-valent Fe intermediates and ligand-to-metal electron transfer, and the mechanistic scheme derived includes several steps that mimic the (suggested) functioning of non-heme iron enzymes. In agreement with this proposal, ligand oxidation can also be performed catalytically. Furthermore, we show that via a competitive route [(Xanthmal)2Fe2O] (2) is formed, which is unreactive towards O2 and thus is a dead end with respect to ligand oxidation. Both compounds 1 and 2 were fully characterised, and their properties are discussed.

Cobalt Catalyst with a Proton‐Responsive Ligand for Water Oxidation

Herein, we report the synthesis, the thermochemical data, and the catalytic reactivity of a new mononuclear cobalt complex, which has four NH protons in the ligand sphere. The combination of the redox-active metal ion and NH units enabled the coupling of proton and electron-transfer steps, which we exploited in the electrocatalytic water oxidation.

Mechanism of Chemical and Electrochemical N2 Splitting by a Rhenium Pincer Complex

A comprehensive mechanistic study of N2 activation and splitting into terminal nitride ligands upon reduction of the rhenium dichloride complex [ReCl2(PNP)] is presented (PNP– = N(CH2CH2PtBu2)2–). Low-temperature studies using chemical reductants enabled full characterization of the N2-bridged intermediate [{(PNP)ClRe}2(N2)] and kinetic analysis of the N–N bond scission process. Controlled potential electrolysis at room temperature also resulted in formation of the nitride product [Re(N)Cl(PNP)]. This first example of molecular electrochemical N2 splitting into nitride complexes enabled the use of cyclic voltammetry (CV) methods to establish the mechanism of reductive N2 activation to form the N2-bridged intermediate. CV data was acquired under Ar and N2, and with varying chloride concentration, rhenium concentration, and N2 pressure. A series of kinetic models was vetted against the CV data using digital simulations, leading to the assignment of an ECCEC mechanism (where “E” is an electrochemical step and “C” is a chemical step) for N2 activation that proceeds via initial reduction to ReII, N2 binding, chloride dissociation, and further reduction to ReI before formation of the N2-bridged, dinuclear intermediate by comproportionation with the ReIII precursor. Experimental kinetic data for all individual steps could be obtained. The mechanism is supported by density functional theory computations, which provide further insight into the electronic structure requirements for N2 splitting in the tetragonal frameworks enforced by rigid pincer ligands.

A Xanthene-based Ligand with Two Adjacent Malonate Binding Sites

The syntheses of two functionalized xanthenes are described which after deprotonation represent ligands for dinuclear metal complexes. For the previously prepared [RXanthdim]H2 - which after deprotonation leads to a ligand with two adjacent β -diiminato binding sites - a significantly improved synthetic procedure is described involving the Pd catalyzed coupling of two diethyl malonate moieties to the xanthene backbone. Deprotonation of the resulting compound [Xanthmal]H2 provides a ligand with two adjacent diethyl malonate functions. To demonstrate this, [Xanthmal]H2 was reacted exemplarily with two equivalents of LDA to obtain the lithium salt {Li2[Xanthmal]}2 (4) which can be treated with ZnBr2 to yield the zinc complex [Xanthmal]2Zn2 (5). Alternatively, 5 can be obtained directly from [Xanthmal]H2, when ZnEt2 is chosen as the metal precursor. The crystal structures of 4 and 5 are discussed. In summary, the results show that [Xanthmal]2− is a suitable ligand for the preparation of novel dinuclear metal complexes.

Evidence for a Single Electron Shift in a Lewis Acid–Base Reaction

The Lewis acid-base reaction between a nucleophilic hafnocene-based germylene and tris-pentafluorophenylborane (B(C6F5)3) to give the conventional B-Ge bonded species in almost quantitative yield is reported. This reaction is surprisingly slow, and during its course, radical intermediates are detected by EPR and UV-vis spectroscopy. This suggests that the reaction is initiated by a single electron-transfer step. The hereby-involved germanium radical cation was independently synthesized by oxidation of the germylene by the trityl cation or strong silyl-Lewis acids. A perfluorinated tetraarylborate salt of the radical cation was fully characterized including an XRD analysis. Its structural features and the results of DFT calculations indicate that the radical cation is a hafnium(III)-centered radical that is formed by a redox-induced electron transfer (RIET) from the ligand to the hafnium atom. This valence isomerization slows down the coupling of the radicals to form the polar Lewis acid-base product. The implications of this observation are briefly discussed in light of the recent finding that radical pairs are formed in frustrated Lewis pairs.

Electrocatalytic Azide Oxidation Mediated by a Rh(PNP) Pincer Complex

Abstract One‐electron oxidation of the rhodium(I) azido complex [Rh(N3)(PNP)] (5), bearing the neutral, pyridine‐based PNP ligand 2,6‐bis(di‐tert‐butylphosphinomethyl)pyridine, leads to instantaneous and selective formation of the mononuclear rhodium(I) dinitrogen complex [Rh(N2)(PNP)]+ (9 +). Interestingly, complex 5 also acts as a catalyst for electrochemical N3 − oxidation (E p≈−0.23 V vs. Fc+/0) in the presence of excess azide. This is of potential relevance for the design of azide‐based and direct ammonia fuel cells, expelling only harmless dinitrogen as an exhaust gas.