The reaction of 1,1,3,3-tetraphenyl-1,3-disiloxandiol (LH₂) with n-butyllithium and CrCl₂ results in a mononuclear chromium(II) complex (1) that further reacts with O₂ at low temperatures to yield a mononuclear chromium(III) superoxide complex [L₂CrO₂(THF)][Li₂(THF)₃] (2). The crystal structure revealed that the chromium superoxido entity is stabilized by the coordination to an adjacent lithium cation. Complex 2 thus contains an unprecedented heterobimetallic [Cr^III(μ-O₂)Li⁺] core; beyond this it is the first chromium superoxide for which a temperature-dependent magnetic characterization could be achieved, and the first structurally characterized representative with chromium in an exclusive O-donor environment.

New tripodal ligand L2 featuring three different pyridyl/imidazolyl-based N-donor units at a bridgehead C atom, from which one of the imidazolyl units is separated by a phenylene linker, was synthesized and investigated with regards to copper(I) complexation. The resulting complex [(L2)Cu]OTf (2^OTf), the known complex [(L1)Cu]OTf (1^OTf; L1 differs from L2 in that it lacks the phenylene spacer) and [(L3)Cu]OTf (3^OTf), prepared from a known chiral, tripodal, N-donor ligand featuring pyridyl, pyrazolyl, and imidazolyl donors, were tested as catalysts for the oxidation of sodium 2,4-di-tert-butylphenolate (NaDTBP) with O₂. Indeed, they mediated NaDTBP oxidation to give mainly the corresponding catecholate and quinone (Q). None of the complexes 1^OTf, 2^OTf, and 3^OTf is superior to the others, as yields were comparable and, if the presence of protons is guaranteed by concomitant addition of the phenol DTBP, the oxidation can also be performed catalytically. For all complexes stoichiometric oxidations under certain conditions (concentrated solutions, high NaDTBP content) were found to also generate products typical for metal-mediated intradiol cleavage of the catecholate with O₂. As shown representatively for 1^OTf this dioxygenation sets in at a later stage of the reaction. Initially a copper species responsible for the monooxygenation must form from 1^OTf/NaDTBP/O₂, and only thereafter is the copper species responsible for dioxygenation formed and consumes Q as substrate. Hence, under these circumstances complexes 1^OTf–3^OTf show both monooxygenase and catechol dioxygenase activity.

Durch die Reaktion von 1,1,3,3-Tetraphenyl-1,3-disilanol (LH₂) mit n-Butyllithium und CrCl₂ bildet sich ein einkerniger Cr^II-Komplex 1, der bei niedrigen Temperaturen mit O₂ zu einem einkernigen Cr^III-Superoxid-Komplex [L₂CrO₂(THF)][Li₂THF₃] (2) weiterreagiert. Durch Kristallstrukturanalyse konnte gezeigt werden, dass die Chromsuperoxid-Einheit durch die Koordination an ein benachbartes Lithiumkation stabilisiert wird. 2 weist damit eine neuartige heterobimetallische [Cr^III(μ-O₂)Li]-Einheit auf; des Weiteren handelt es sich bei 2 um das erste Chromsuperoxid, dessen magnetische Eigenschaften in Abhängigkeit von der Temperatur bestimmt werden konnten, und um den ersten strukturell beschriebenen Vertreter, in dem Chrom ausschließlich von O-Donoren umgeben ist.

The β-diketiminato nickel(II) hydrazido(–1) complex [L^tBuNi(η²-N₂H₃)], (IV) was investigated with respect to its deprotonation/protonation behavior. Deprotonation with KOtBu led to [L^tBuNi(μ,η²:η²-N₂H₂)K(solv)] (1), and this process was proven to be reversible. Spectroscopic and DFT studies revealed an electronic structure intermediate between nickel(II) hydrazido(–2) and nickel(0) diazene. On the other hand, protonation of IV with [LutH]OTf reversibly generated the hydrazine complex [L^tBuNi(η²-N₂H₄)]OTf (2). Warming a solution of IV led to N–N bond cleavage yielding the nickel(I) ammine complex [L^tBuNi(NH₃)], (3). Hence, L^tBuNi moieties were shown to effectively activate N_xH_y species for diverse conversions.

A three-step synthetic route for a new tripodal branched trisilanol ligand PhSi(OSiPh₂OH)₃ (LH₃) was developed. X-ray diffraction analysis revealed that the trisilanol crystallizes as a dimer with a cyclic hydrogen-bonding network. The reaction of LH₃ with three equivalents of Cu_nMes_n (Mes = mesityl) led to a hexanuclear compound [L₂Cu₆] (1), which was characterized by single-crystal X-ray diffraction analysis as well as by solution NMR spectroscopy. The crystal structure of 1 revealed that the compound features a hexagonal planar Cu^I₆ ring, which is the first of its kind in an oxygen environment. Deprotonation of the ligand with n-butyllithium and subsequent reaction with CuBr₂ resulted in the dinuclear Cu^II complex [L′₂Cu₂][Li(THF₂)]₂ (2, THF = tetrahydrofuran), which contains a new siloxide ligand formed from L^3– by the elimination of a SiOPh₂ unit, as evidenced by X-ray diffraction analysis. To check if the “Ph₂SiO” elimination is a general behavior of this trisilanol, the reaction with ZnBr₂ was investigated under analogous conditions. However, this led to the isolation of [L₂Zn₂][Li(OEt)]₂ (3) without any rearrangement of the siloxide ligand.

Iron centres incorporated in silicate frameworks or located in their pores have been shown to possess unique catalytic properties. As compared to aluminosilicates this area of zeolite chemistry is much younger and in the first part of this review the findings made so far are summarised. Molecular model compounds may help to understand the formation, corrosion and reactivity of such materials or to even develop new ones. Hence, the subsequent parts deal with molecular iron siloxides, the number of which is still quite limited, and their behaviour also in relation to the iron-modified zeolites is outlined. At first, compounds based on an incompletely condensed cubic silsequioxane are discussed, before iron(III) complexes of more basic siloxide ligands with varying steric demands are described. Finally, recent developments based on branched polydentate siloxides are presented.

Research on O₂ activation at ligated Cu^I is fueled by its biological relevance and the quest for efficient oxidation catalysts. A rarely observed reaction is the formation of a Cu^II-O-Cu^II species, which is more special than it appears at first sight: a single oxo ligand between two Cu^II centers experiences considerable electron density, and this makes the corresponding complexes reactive and difficult to access. Hence, only a small number of these compounds have been synthesized and characterized unequivocally to date, and as biological relevance was not apparent, they remained unappreciated. However, recently they moved into the spotlight, when Cu^II-O-Cu^II cores were proposed as the active species in the challenging oxidation of methane to methanol at the surface of a Cu-grafted zeolite and in the active center of the copper enzyme particulate methane monooxygenase. This Minireview provides an overview of these systems with a special focus on their reactivity and spectroscopic features.

The greenhouse gas sulfur hexafluoride is the common standard example in the literature of a very inert inorganic small molecule that is even stable against O₂ in an electric discharge. However, a reduced β-diketiminate nickel species proved to be capable of converting SF₆ into sulfide and fluoride compounds at ambient standard conditions. The fluoride product complex features an unprecedented [NiF]⁺ unit, where the Ni atom is only three-coordinate, while the sulfide product exhibits a rare almost linear [Ni(μ-S)Ni]²⁺ moiety. The reaction was monitored applying ¹H NMR, IR and EPR spectroscopic techniques resulting in the identification of an intermediate nickel complex that gave insight into the mechanism of the eight-electron reduction of SF₆.

Die Forschung zur O₂-Aktivierung an ligierten Cu^I-Ionen wird durch ihre biologische Relevanz und die Suche nach effizienten Oxidationskatalysatoren getrieben. Ein in diesem Zusammenhang selten beobachteter Reaktionspfad ist die ungewöhnliche Bildung von Cu^II-O-Cu^II-Einheiten: Ein einzelner, zwischen zwei Cu^II-Ionen gebundener Oxoligand erfährt eine beträchtliche Elektronendichte, die Cu₂O-Komplexe sehr reaktiv und somit nur schwer zugänglich macht. Daher – und auch wegen der scheinbar fehlenden biologischen Bedeutung – wurden bisher nur wenige solche Verbindungen synthetisiert und charakterisiert. Kürzlich jedoch wurden Cu^II-O-Cu^II-Einheiten als aktive Spezies für die selektive Oxidation von Methan zu Methanol sowohl auf der Oberfläche eines kupferhaltigen Zeoliths als auch im aktiven Zentrum der Methanmomooxygenase diskutiert. Dieser Kurzaufsatz soll einen Überblick über den aktuellen Wissensstand bezüglich niedermolekularer Systeme bieten, mit einem Fokus auf ihren spektroskopischen Eigenschaften und Reaktivitäten.

Das Klimagas Schwefelhexafluorid ist in der Literatur ein viel verwendetes Standardbeispiel für ein sehr reaktionsträges anorganisches Molekül, das selbst durch Disauerstoff in einer elektrischen Entladung nicht angegriffen wird. Dennoch sind reduzierte β-Diketiminato-Nickel-Komplexe unter Standardbedingungen in der Lage, SF₆ in Fluorido- und Sulfido-Verbindungen zu überführen. Das Fluorido-Produkt besitzt dabei eine präzedenzlose [NiF]⁺-Einheit, bei der das Nickelatom nur dreifach koordiniert ist. Der Sulfido-Komplex beinhaltet eine seltene nahezu lineare [Ni(μ-S)Ni]²⁺-Anordnung. Die Reaktion wurde mittels ¹H-NMR-, IR- und ESR-spektroskopischen Methoden untersucht. Diese führten zur Identifizierung einer Zwischenstufe und gewährten so einen Einblick in den Mechanismus der 8-Elektronen-Reduktion von SF₆ .

A novel redox-active ligand, H₄^Ph2SL^AP (1) which was designed to be potentially pentadentate with an O,N,S,N,O donor set is described. Treatment of 1 with two equivalents of potassium hydride gave access to octametallic precursor complex [H₂^Ph2SL^APK₂(thf)]₄ (2), which reacted with FeCl₃ to yield iron(III) complex [H₂^Ph2SL^APFeCl] (3). Employing Fe[N(SiMe₃)₂]₃ for a direct reaction with 1 led to ligand rearrangement through CS bond cleavage and thiolate formation, finally yielding [HL^APFe] (5). Upon exposure to O₂, 3 and 5 are oxidized through formal hydrogen-atom abstraction from the ligand NH units to form [^Ph2SL^SQFeCl] (4) and [L^SQFe] (6) featuring two or one coordinated iminosemiquinone moieties, respectively. Mössbauer measurements demonstrated that the iron centers remain in their +III oxidation states. Compounds 3 and 5 were tested with respect to their potential as models for the catechol dioxygenase. Thus, they were treated with 3,5-di-tert-butyl-catechol, triethylamine and O₂. It turned out that the iron–catecholate complexes react with O₂ in dichloromethane at ambient conditions through CC bond cleavage mainly forming extradiol cleavage products. Intradiol products are only side products and quinone formation becomes negligible. This observation has been rationalized by a dissociation of two donor functions upon coordination of the catecholate.

The reaction of a tripodal trisilanol with n-butyllithium and CrCl₂ results in a dinuclear Cr^II complex (1), which is capable of cleaving O₂ to yield in a unique complex (2) with an asymmetric diamond core composed of two Cr^IVO units. Magnetic susceptibility data reveal significant exchange coupling of Cr^II (S=2) in 1 and large zero-field splitting for Cr^IV (S=1) in 2 owing to strong spin–orbit coupling of the ground state. The Cr^IVO compound can also be generated using PhIO, and evidence was gathered that although it is the stable product isolated after excessive O₂ treatment, it further activates O₂ to yield an intermediate species that oxidizes THF or Me-THF. By extensive ¹⁸O labeling studies we were able to show, that in the course of this process ¹⁸O₂ exchanges its label with siloxide O atoms of the ligand via terminal oxido ligands.

The reaction of a tripodal trisilanol with n-butyllithium and CrCl₂ results in a dinuclear Cr^II complex (1), which is capable of cleaving O₂ to yield in a unique complex (2) with an asymmetric diamond core composed of two Cr^IVO units. Magnetic susceptibility data reveal significant exchange coupling of Cr^II (S=2) in 1 and large zero-field splitting for Cr^IV (S=1) in 2 owing to strong spin–orbit coupling of the ground state. The Cr^IVO compound can also be generated using PhIO, and evidence was gathered that although it is the stable product isolated after excessive O₂ treatment, it further activates O₂ to yield an intermediate species that oxidizes THF or Me-THF. By extensive ¹⁸O labeling studies we were able to show, that in the course of this process ¹⁸O₂ exchanges its label with siloxide O atoms of the ligand via terminal oxido ligands.

After the lithiation of PYR-H₂ (PYR²⁻=[{NC(Me)C(H)C(Me)NC₆H₃(iPr)₂}₂(C₅H₃N)]²⁻), which is the precursor of an expanded β-diketiminato ligand system with two binding pockets, its reaction with [NiBr₂(dme)] led to a dinuclear nickel(II)–bromide complex, [(PYR)Ni(μ-Br)NiBr] (1). The bridging bromide ligand could be selectively exchanged for a thiolate ligand to yield [(PYR)Ni(μ-SEt)NiBr] (3). In an attempt to introduce hydride ligands, both compounds were treated with KHBEt₃. This treatment afforded [(PYR)Ni(μ-H)Ni] (2), which is a mixed valent Ni^Iμ-HNi^II complex, and [(PYR-H)Ni(μ-SEt)Ni] (4), in which two tricoordinated Ni^I moieties are strongly antiferromagnetically coupled. Compound 4 is the product of an initial salt metathesis, followed by an intramolecular redox process that separates the original hydride ligand into two electrons, which reduce the metal centres, and a proton, which is trapped by one of the binding pockets, thereby converting it into an olefin ligand on one of the Ni^I centres. The addition of a mild acid to complex 4 leads to the elimination of H₂ and the formation of a Ni^IINi^II compound, [(PYR)Ni(μ-SEt)NiOTf] (5), so that the original Ni^II(μ-SEt)Ni^IIX core of compound 3 is restored. All of these compounds were fully characterized, including by X-ray diffraction, and their molecular structures, as well as their formation processes, are discussed.

Reaction of the siloxane-diols HO(Ph₂SiO)₂H, 1,1,3,3-tetraphenyldisiloxane-1,3-diol (L¹H₂), HO(iPr₂SiO)₂H, 1,1,3,3-tetraisopropyldisiloxane-1,3-diol (L²H₂), and HO(Ph₂SiO)₃H, 1,1,3,3,5,5-hexaphenyltrisiloxane-1,5-diol (L³H₂) with two equivalents of Cu_nMes_n led to octanuclear compounds [Cu₈L¹₄] (1), [Cu₈L²₄] (2), and [Cu₈L¹₂L*₂] [L* = O(Ph₂SiO)₄] (3), which were characterized by single-crystal X-ray diffraction analysis as well as by solution NMR spectroscopy. The crystal structures revealed that all the compounds are composed of Cu₄O₄ moieties featuring short Cu–Cu distances that can be discussed in terms of cuprophilic interactions. Two such units are linked via four disiloxane units in 1 and 2, in 3 they are connected by two equivalents of (L¹)^2–. Formation of 3 required disproportion of a trisiloxane-1,5-diolate to give a disiloxane-1,3-diolate and a tetrasiloxane-1,7-diolate, and thus indicates that siloxane units are not inert under the synthetic conditions employed. NMR spectroscopic investigations of the structure of 1 in solution revealed an equilibrium, presumably with [Cu₄L²₂] fragments. Crystallization of a structural isomer of 3 indicates a shallow potential energy surface for this compound.

¹H NMR exchange spectroscopy of a reaction mixture of [Cp*Ir(H)₄] (1; Cp*=1,2,3,4,5-pentamethylcyclopentadienyl) and ammonia suggests an exchange of hydrogen atoms between the hydrido ligands and ammonia. Treatment of 1 with ND₃ led to an H/D exchange between ND₃ and the hydrido ligands of 1. Subsequent studies showed that photolysis of 1 isolated in frozen argon matrices leads to the formation of the iridium compounds [Cp*Ir(H)₂] (2) and [Cp*Ir(H)₃] (4), as it was confirmed by IR spectroscopy. In the presence of water the aqua complex [Cp*Ir(H)₂(OH₂)] (3) was generated simultaneously. Accordingly, photolysis of 1 in an argon matrix doped with ammonia gave rise to the ammine complex [Cp*Ir(H)₂(NH₃)] (5). IR assignments were supported by calculations of the gas-phase IR spectra of 1–5 by DFT methods.

A series of complexes of the type [(Tp^R1,R2)M(X)] (Tp=trispyrazolylborato) with R¹/R² combinations Me/tBu, Ph/Me, iPr/iPr, Me/Me and for M=Mn or Fe coordinating [Pz^Me,tBu]⁻ (Pz=pyrazolato) or Cl⁻ as co-ligand X has been synthesised. Although the chloride complexes were very unreactive and stable in air, the pyrazolato series was far more reactive in contact with oxidants like O₂ and tBuOOH. The [(Tp^R1,R2)M(Pz^Me,tBu)] complexes proved to be active pre-catalysts for the oxidation of cyclohexene with tBuOOH, reaching turnover frequencies (TOFs) ranging between moderate and good in comparison to other manganese catalysts. Cyclohexene-3-one and cyclohexene-3-ol were always found to represent the main products, with cyclohexene oxide occasionally formed as a side product. The ratios of the different oxidation products varied with the reaction conditions: in the case of a peroxide/alkene ratio of 4:1, considerably more ketone than alcohol was obtained and cyclohexene oxide formation was almost negligible, whereas a ratio of 1:10 led to a significant increase of the alcohol proportion and to the formation of at least small amounts of the epoxide. Pre-treatment of the dissolved [(Tp^R1,R2)M(Pz^Me,tBu)] pre-catalysts with O₂ led to product distributions and TOFs that were very similar to those found in the absence of O₂, so that it may be argued that tBuOOH and O₂ both lead to the same active species. The results of EPR spectroscopy and ESI-MS suggest that the initial product of the reaction of [(Tp^Me,Me)Mn(Pz^Me,tBu)] with O₂ contains a Mn^III(O)₂Mn^IV core. Prolonged exposure to O₂ leads to a different dinuclear complex containing three O-bridges and resulting in different TOFs/product distributions. Analogous findings were made for other complexes and formation of these overoxidised products may explain the deviation of the catalytic performances if the reactions are carried out in an O₂ atmosphere.

By employing the 2,2′-thiobis(2,4-di-tert-butylphenolate) ligand (^SL²⁻) a novel oxovanadium(V) complex, (PPh₄)₂[^SLV(O)(μ₂-O)₂V(O)^SL] (1), was synthesised that exhibits haloperoxidase activity: on addition of H₂O₂ a sequence of successive peroxide formation and intramolecular thioether oxidation events (sulfoxide and sulfone) led to a mixture of five products, which were all identified unambiguously, partly through an independent synthesis and characterisation. It was shown that internal thioether oxidation proceeds through peroxide formation, but the sulfoxidation of external thioether functions requires further activation of the peroxide function by protons or alkyl cations. Consistently, the employment of tBuOOH instead of H₂O₂ led to a very active system for the catalytic sulfoxidation of thioethers.

On the basis that thiacalix[4]arene (H₄T4A) complex (PPh₄)₂[H₂T4A(VO₂)]₂ (Ia) was found to be an adequate functional model for surface species occurring on vanadium oxide based catalysts and itself catalyses the oxidative dehydrogenation (ODH) of alcohols, an analogue containing 2,2′-thiobis(2,4-di-tert-butylphenolate), ^SL²⁻, as ligand, namely, (PPh₄)₂[^SLVO₂]₂ (II) was investigated in the same context. Despite the apparent similarity of Ia and II, studies on II revealed several novel insights, which are also valuable in connection with surfaces of vanadia catalysts: 1) For Ia and II similar turnover numbers (TONs) were found for the ODH of activated alcohols, which indicates that the additional OH units inherent to Ia do not contribute particularly to the activity of this complex, for instance, through prebinding of the alcohol. 2) On dissolution II enters into an equilibrium with a monomeric form, which is the predominant species in solution; nevertheless, ODH proceeds exclusively at the dimeric form, and this stresses the need for cooperation of two vanadium centres. 3) By omitting O₂ from the system during the oxidation of 9-fluorenol, the reduced form of the catalyst could be isolated and fully characterised (including single-crystal X-ray analysis). The corresponding intermediate had been elusive in case of thiacalixarene system Ia. 4) Reoxidation was found to proceed via a peroxide intermediate that also oxidises one alcohol equivalent. As the peroxide can also perform mono- and dioxygenation of the thioether group in II, after a number of turnovers the active catalyst contains a sulfone group. The reduced form of this ultimate catalyst was also isolated and structurally characterised. Possible implications of 1)–4) for the function of heterogeneous vanadia catalysts are discussed.

2,6-Bis(1,2,3-triazol-4-yl)pyridine (btp) ligands with substitution patterns ranging from strongly electron-donating to strongly electron-accepting groups, readily prepared by means of Cu-catalyzed 1,3-dipolar cycloaddition (the “click” reaction), were investigated with regard to their complexation behavior, and the properties of the resulting transition-metal compounds were compared. Metal–btp complexes of 1:1 stoichiometry, that is, [Ru(btp)Cl₂(dmso)] and [Zn(btp)Br₂], could be isolated and were crystallographically characterized: they display octahedral and trigonal-bipyramidal coordination geometries, respectively, and exhibit high aggregation tendencies due to efficient π–π stacking leading to low solubilities. Metal–btp complexes of 1:2 stoichiometry, that is, [Fe(btp)₂]²⁺ and [Ru(btp)₂]²⁺, could also be synthesized and their metal centers show the expected octahedral coordination spheres. The iron compounds exhibit quite a complex magnetic behavior in the solid state including spin crossover near room temperature, and hysteresis and locking into high-spin states on tempering at 400 K, depending on the substituents on the btp ligands. Cyclic voltammetry studies of [Ru(btp)₂]²⁺ reveal strong modulation of the oxidation potentials by more than 0.6 V and a clear linear correlation to the Hammett constant (σ_para) of the substituent at the pyridine core. Isothermal titration calorimetry was used to measure the thermodynamics of the Fe^II–btp complexation process and enabled accurate determination of the complexation enthalpies, which display a linear relationship with the σ_para values for the terminal phenyl substituents. Detailed NMR spectroscopic studies finally revealed that in the case of Fe^II complexation, dynamics are rapid for all investigated btp derivatives in acetonitrile, while replacing Fe^II by Ru^II or changing the solvent to dichloromethane effectively slows down ligand exchange. The results nicely demonstrate the utility of substituent parameters, originally developed for linear free-energy relationships to explain reactivity in organic reactions, in coordination chemistry, and to illustrate the potential to custom-design btp ligands and complexes thereof with predictable properties. The fast equilibration of the [Fe(btp)₂]²⁺ complexes together with their tunable stability and interesting magnetic properties should enable the design of dynamic metallosupramolecular materials with advantageous properties.

Silsesquioxane dioxovanadate(V) complexes were investigated with respect to their potential as a catalyst for the oxidative dehydrogenation of alcohols with O₂ as an oxidant. The turnover frequencies determined were comparatively low, but during the oxidation of cinnamic alcohol an increase in activity was observed in the course of the process, which was inspected more closely. It turned out that during the oxidation of cinnamic alcohol, not only was the aldehyde formed but also cinnamic acid, which in turn reacts with the silsesquioxane complex employed to give NBu₄[O₂V(O₂CC₂H₂Ph)₂], which can also be obtained from NBu₄VO₃ and cinnamic acid and represents a far more active catalyst, not only for cinnamic alcohol but also for other activated alcohols and hydrocarbons. The rate-determining step of the conversion corresponds to an hydrogen-atom abstraction from the CH units, as shown by the determination of the kinetic isotope effect in case of 9-hydroxyfluorene, and the reoxidation of the reduced catalyst proceeds via a peroxo intermediate, which is also capable of oxidizing one alcohol equivalent. Furthermore the influence of the organic residues at the carboxylate ligands on the catalyst performance was investigated, which showed that the activity increases with decreasing pK_s value. Moreover, it was found that during the oxidation the catalyst slowly decomposes, but can be regenerated by addition of excessive carboxylic acid.

Striving for new solar fuels, the water oxidation reaction currently is considered to be a bottleneck, hampering progress in the development of applicable technologies for the conversion of light into storable fuels. This review compares and unifies viewpoints on water oxidation from various fields of catalysis research. The first part deals with the thermodynamic efficiency and mechanisms of electrochemical water splitting by metal oxides on electrode surfaces, explaining the recent concept of the potential-determining step. Subsequently, novel cobalt oxide-based catalysts for heterogeneous (electro)catalysis are discussed. These may share structural and functional properties with surface oxides, multinuclear molecular catalysts and the catalytic manganese–calcium complex of photosynthetic water oxidation. Recent developments in homogeneous water-oxidation catalysis are outlined with a focus on the discovery of mononuclear ruthenium (and non-ruthenium) complexes that efficiently mediate O₂ evolution from water. Water oxidation in photosynthesis is the subject of a concise presentation of structure and function of the natural paragon—the manganese–calcium complex in photosystem II—for which ideas concerning redox-potential leveling, proton removal, and OO bond formation mechanisms are discussed. The last part highlights common themes and unifying concepts.

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 O₂ 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.

Preparation routes for several sodium salts of diphenylbis(pyrazolyl)borates with bulky substituents are described. For a potential modelling of α-keto-acid-dependent nonheme iron enzymes, mononuclear iron compounds containing those bis(pyrazolyl)borates as ligands were synthesised and structurally characterised. The iron centres are four-coordinate (in one case only three-coordinate) thus offering room for the binding of exogenous ligands. However, reactivity studies with carboxylates and O₂ have revealed nonuniform behaviour, and the identification of a side product in one case, combined with subsequent ESI-MS studies, demonstrated that a pronounced sensitivity of the B–N bonds towards oxo nucleophiles could be a possible reason.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

A ligand that offers two parallel malonate binding sites linked by a xanthene backbone, namely, Xanthmal²⁻, has been utilised to synthesise dinuclear Fe^II complex [Fe₂(Xanthmal)₂] (1). The reactivity of 1 in contact with O₂ was investigated at −40 °C and room temperature. After activation of O₂ through interaction with both iron centres the ligand is oxidised: at the C_α position monooxygenation and peroxide formation occur, partially accompanied by CC bond cleavage to yield α-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.