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Complexes [L2Fe][Li(DME)]2, 1(DME), {[L2Fe][Na2(DME)3]}∞, 2(DME) and [L2Fe][K(DME)2]2, 3(DME) were synthesized by deprotonation of LH2 (LH2=O(SiPh2OH)2) with the respective alkali metal tert‐butoxides followed by recrystallization from DME. It turned out that upon crossing over from Li+ via Na+ to K+ counterions the structures of the high‐spin iron(II) complexes are increasingly distorted from a square planar towards a tetrahedral structure so that 3(DME) represents a borderline case, as indicated by the τ‐values. The distortions are also reflected in the Mössbauer spectra through the quadrupole splittings. The compounds behave inert in contact with O atom transfer reagents but react rapidly with dioxygen. The reaction rates are too high to be determined even by stopped‐flow measurements quantitatively, but qualitatively it emerged that the rates increase from Li to Na to K. Using NO as an O2 surrogate an NO adduct with an S = 3/2 ground state was isolated where NO is coordinated in an end‐on binding mode, formally as a NO− ligand, with a significantly weakened NO bond. [ABSTRACT FROM AUTHOR]

Bearing in mind the potential which tris(pyrazolyl) hydroborate ligands have demonstrated in various different fields of coordination chemistry, the behavior of a more rarely employed representative, namely bis(3‐mesitylpyrazol‐1‐yl)(5‐mesitylpyrazol‐1‐yl) hydroborate (TpMes,H*), towards a series of divalent metal ions has been investigated. Thus, the synthesis of two new heteroleptic metal chlorido [TpMes,H*MCl] complexes, 2, (Mn2+, 2‐a, and Cu2+, 2‐e) are described, which correspond to the typical precursor compounds that would be needed for subsequent research. Structural and spectroscopic properties of the two new and four published members of this series (M=Fe2+, 2‐b, Co2+, 2‐c, Ni2+, 2‐d, Zn2+, 2‐f) are discussed in detail for the solid and solution states. Furthermore, synthetic routes to five homoleptic [(TpMes,H*)2M] complexes, 3, bearing Mn2+, 3‐a, Fe2+, 3‐b, Co2+, 3‐c, Ni2+, 3‐d, and Cu2+, 3‐e, ions as well as an alternative preparation method for [(κ2‐TpMes,H*)2Zn], 3‐f, are described. Their structures and spectroscopic parameters are compared to those of their heteroleptic counterparts. Additionally, the complex [(TpH,Mes*)2Cu], 4, is reported, representing the second example of a complex with the (3‐mesitylpyrazol‐1‐yl)bis(5‐mesitylpyrazol‐1‐yl) hydroborate ligand (TpH,Mes*). [ABSTRACT FROM AUTHOR]

A calix[4]arene, in which two of the phenol functions are replaced by pyrazole units, [H2(bpzCal)], was investigated as a ligand for Cu+, Ag+ and Au+ ions. Using [Cu(MeCN)4]BF4 and AgSbF6 as the precursors, complexes [MH2(bpzCal)]X (M = Cu, X = BF4; M = Ag, X = SbF6) were formed, where the calixarene ligands adopt a 1,3‐alternate structure and the metal ions are coordinated linearly by the two pyrazolyl donors. [CuH2(bpzCal)]BF4 displayed a – for copper(I) complexes – unusual stability towards O2, which is due to the steric protection of the CuI center. By contrast a dinuclear copper(I) complex [Cu2(bpzCal)] that was obtained through treatment of [H2(bpzCal)] with two equivalents of Cu(HMDS) is rather sensitive towards O2. The preparation of a gold complex required the employment of a gold precursor, which contains one labile and one stabilizing neutral ligand, namely [(PPh3)Au(NCMe)]SbF6, which led to the formation of [(PPh3)AuH2(bpzCal)]SbF6. In this complex [H2(bpzCal)] acts only as a monodentate ligand for the gold center. Taken together, the results demonstrate the potential of [H2(bpzCal)] in providing rather different coordination spheres for metal ions. [ABSTRACT FROM AUTHOR]

Aiming at structural and functional mimics of the active site of the NiII containing quercetin‐2,4‐dioxygenase NiII flavonolate complexes Tp*NiX [Tp* = hydrotris(3,5‐dimethyl)pyrazolylborate, X = 3‐hydroxy flavonolate (Fla), 3‐hydroxy thioflavonolate (SFla), 3‐hydroxy selenoflavonolate (SeFla)] were synthesized and characterized by spectroscopic methods and X‐ray crystallography. The complex Tp*NiFla reacts with O2 via dioxygenation of bound flavonolate to benzoic acid and salicylic acid as one should expect for a functional model of the enzyme. Modification of the carbonyl function of the flavonolate to the corresponding C=S and C=Se compounds retained dioxygenase like reactivity, but did not lead to an increase of reaction rate as had been anticipated due to a weaker interaction of S/Se with the central nickel atom. [ABSTRACT FROM AUTHOR]

Abstract: Reduzierte CO2‐Spezies sind wichtige Intermediate in einer Vielzahl von natürlichen und synthetischen Prozessen. In den meisten Systemen entziehen sie sich jedoch aufgrund ihrer Reaktivität oder eingeschränkten Zugänglichkeit (heterogene Systeme) der Isolierung oder Charakterisierung und bleiben daher oft spekulativ oder nur theoretisch begründet. Hier wird ein Ni‐CO22−‐Komplex vorgestellt, der in vielerlei Hinsicht einzigartig ist. Während seine strukturellen und elektronischen Eigenschaften helfen, den CO2‐gebundenen Zustand in Ni,Fe‐haltigen CO‐Dehydrogenasen zu verstehen, gibt seine Reaktivität Aufschluss darüber, wie CO2 durch Nickelkomplexe in CO/CO32− umgewandelt werden kann. Darüber hinaus wurde der Komplex durch eine selten beschriebene β‐Deprotonierung von Formiat erzeugt, einen mechanistischen Schritt, der für die nickelkatalysierte Umwandlung von HxCOyz− an Elektroden und für die Oxidation von Formiat in Formiat‐Dehydrogenasen relevant ist. [ABSTRACT FROM AUTHOR]

Abstract: Reduced CO2 species are key intermediates in a variety of natural and synthetic processes. In the majority of systems, however, they elude isolation or characterisation owing to high reactivity or limited accessibility (heterogeneous systems), and their formulations thus often remain uncertain or are based on calculations only. We herein report on a Ni−CO22− complex that is unique in many ways. While its structural and electronic features help understand the CO2‐bound state in Ni,Fe carbon monoxide dehydrogenases, its reactivity sheds light on how CO2 can be converted into CO/CO32− by nickel complexes. In addition, the complex was generated by a rare example of formate β‐deprotonation, a mechanistic step relevant to the nickel‐catalysed conversion of HxCOyz− at electrodes and formate oxidation in formate dehydrogenases. [ABSTRACT FROM AUTHOR]

Abstract: Reduced CO2 species are key intermediates in a variety of natural and synthetic processes. In the majority of systems, however, they elude isolation or characterisation owing to high reactivity or limited accessibility (heterogeneous systems), and their formulations thus often remain uncertain or are based on calculations only. We herein report on a Ni−CO22− complex that is unique in many ways. While its structural and electronic features help understand the CO2‐bound state in Ni,Fe carbon monoxide dehydrogenases, its reactivity sheds light on how CO2 can be converted into CO/CO32− by nickel complexes. In addition, the complex was generated by a rare example of formate β‐deprotonation, a mechanistic step relevant to the nickel‐catalysed conversion of HxCOyz− at electrodes and formate oxidation in formate dehydrogenases. [ABSTRACT FROM AUTHOR]

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The synthesis of two new unsymmetric dinucleating ligand systems, L1, combining a bidentate binding site with a tridentate binding site linked through a dibenzofuran spacer, and L2, with two tridentate binding sites bridged by an ethylene spacer, is reported. The ligands L1 and L2 were used for the preparation of the dicopper(I) complexes [Cu2(L1)(NCCH3)3](X)2, {[Cu2(L1)Cl2]} n and [Cu2(L2)(NCCH3)2](Y)2 (X = PF6, BF4, OTf; Y = ClO4, PF6, BF4), in which the two respective copper ions possess two different coordination environments. All complexes reacted with O2, but in the case of the L1 series, no primary O2 adduct could be identified by UV/Vis spectroscopy. Reaction of the [Cu2(L2)(NCCH3)2](Y)2 complexes with pure oxygen led to the formation of an O2 adduct that proved rather unstable. [ABSTRACT FROM AUTHOR]

The design of biomimetic model complexes for the cysteine dioxygenase (CDO) and cysteamine dioxygenase (ADO) is reported, where the 3-His coordination of the iron ion is simulated by three pyrazole donors of a trispyrazolyl borate ligand (Tp) and protected cysteine and cysteamine represent substrate ligands. It is found that the replacement of phenyl groups-attached at the 3-positions of the pyrazole units in a previous model-by mesityl residues has massive consequences, as the latter arrange to a more spacious reaction pocket. Thus, the reaction with O 2 proceeds much faster and afterwards the first structural characterization of an iron(II) η 2 -O,O-sulfinate product became possible. If one of the three Tp-mesityl groups is placed in the 5-position, an even larger reaction pocket results, which leads to yet faster rates and accumulation of a reaction intermediate at low temperatures, as shown by UV/Vis and Mössbauer spectroscopy. After comparison with the results of investigations on the cobalt analogues this intermediate is tentatively assigned to an iron(III) superoxide species., (© 2020 The Authors. Published by Wiley-VCH GmbH.)

A calix[4]arene ligand, in which two of the phenol functions are replaced by pyrazole units has been employed to mimic the His 2 -Tyr 2 (His: histidine, Tyr: tyrosine) ligand sphere within the active site of the galactose oxidase (GO). The calixarene backbone forces the corresponding copper(II) complex into a see-saw-type structure, which is hitherto unprecedented in GO modelling chemistry. It undergoes a one-electron oxidation that is centered at the phenolate donor leading to a copper-coordinated phenoxyl radical like in the GO. Accordingly, the complex was tested as a functional model and indeed proved capable of oxidizing benzyl alcohol to the respective aldehyde using two phenoxyl-radical equivalents as oxidants. Finally, the results show that the calixarene platform can be utilized to arrange donor functions to biomimetic binding pockets that allow for the creation of novel types of model compounds., (© 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

In recent years, it has become clear that the presence of redox-inactive Lewis acidic metal ions can decisively influence the reactivity of metal-dioxygen moieties that are formed in the course of O 2 activation, in molecular complexes, and metalloenzymes. Superoxide species are often formed as the primary intermediates but they are mostly too unstable for a thorough investigation. We report here a series of chromium(III) superoxide complexes [L 2 Cr]M 2 O 2 (THF) y (L= - OSiPh 2 OSiPh 2 O - , M + =Li + , Na + , K + and y=4, 5), which could be accessed, studied spectroscopically and partly crystallized at low temperatures. They only differ in the two incorporated Lewis acidic alkali metal counterions (M + ) and it could thus be shown that the nature of M + determines considerably its interaction with the superoxide ligand. This interaction, in turn, has a significant influence on the stability and reactivity of these complexes towards substrates with OH groups. Furthermore, we show that stability and reactivity are also highly solvent dependent (THF versus nitriles), as donor solvents coordinate to the alkali metal ions and thus also influence their interaction with the superoxide moiety. Altogether, these results provide a comprehensive and detailed picture concerning the correlation between spectroscopic properties, structure, and behavior of such superoxides, that may be exemplary for other systems., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Reduced CO 2 species are key intermediates in a variety of natural and synthetic processes. In the majority of systems, however, they elude isolation or characterisation owing to high reactivity or limited accessibility (heterogeneous systems), and their formulations thus often remain uncertain or are based on calculations only. We herein report on a Ni-CO 2 2- complex that is unique in many ways. While its structural and electronic features help understand the CO 2 -bound state in Ni,Fe carbon monoxide dehydrogenases, its reactivity sheds light on how CO 2 can be converted into CO/CO 3 2- by nickel complexes. In addition, the complex was generated by a rare example of formate β-deprotonation, a mechanistic step relevant to the nickel-catalysed conversion of H x CO y z- at electrodes and formate oxidation in formate dehydrogenases., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Novel conjugated, pyridyl-functionalised triazaphospholes with either tBu or SiMe3 substituents at the 5-position of the N3 PC heterocycle have been prepared by a [3+2] cycloaddition reaction and compared with structurally related, triazole-based systems. Photoexcitation of the 2-pyridyl-substituted triazaphosphole gives rise to a significant fluorescence emission with a quantum yield of up to 12 %. In contrast, the all-nitrogen triazole analogue shows no emission at all. DFT calculations indicate that the 2-pyridyl substituted systems have a more rigid and planar structure than their 3- and 4-pyridyl isomers. Time-dependent (TD) DFT calculations show that only the 2-pyridyl-substituted triazaphosphole exhibits similar planar geometry, with matching conformational arrangements in the lowest energy excited state and the ground state; this helps to explain the enhanced emission intensity. The chelating P,N-hybrid ligand forms a Re(I) complex of the type [(N^N)Re(CO)3 Br] through the coordination of nitrogen atom N(2) to the metal centre rather than through the phosphorus donor. Both structural and spectroscopic data indicate substantial π-accepting character of the triazaphosphole, which is again in contrast to that of the all-nitrogen-containing triazoles. The synthesis and photophysical properties of a new class of phosphorus-containing extended π systems are described., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The synthesis of asymmetrically substituted 2,2':6',2''-terpyridines is reported. First, palladium-catalyzed C-H arylation of pyridine N-oxides with substituted bromopyridines gave 2,2'-bipyridine N-oxides, which were further arylated in a second step to form 2,2':6',2''-terpyridine N-oxides. Yields of up to 77% were obtained with N-oxides bearing an electron-withdrawing ethoxycarbonyl substituent in the 4-position. Pd(OAc)2 with either P(tBu)3 or P(o-tolyl)3 was used as the catalyst. Cyclometalated complexes derived from Pd(OAc)2 and these phosphines were also effective. K3PO4 as the base gave better results than K2CO3. Subsequent deoxygenation with H2 and Pd/C as the catalyst gave the asymmetrically substituted 2,2':6',2''-terpyridines in near quantitative yield. This reaction sequence significantly reduces the number of steps required in comparison with known cross-coupling methods and therefore allows convenient and scalable access to substituted terpyridines., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)