Beatrice Braun-Cula

Trapping Aluminum Hydroxide Clusters with Trisilanols during Speciation in Aluminum(III)–Water Systems: Reproducible, Large Scale Access to Molecular Aluminate Models

To gain molecular level insights into the properties of certain functions and units of extended oxides/hydroxides, suitable molecular model compounds are needed. As an attractive route to access such compounds the trapping of early intermediates during the hydrolysis of suitable precursor compounds with the aid of stabilizing ligands is conceivable, which was tested for the aluminum(III)/water system. Indeed, trisilanols proved suitable trapping reagents: their presence during the hydrolysis of Al(i) Bu2 H in dependence on the amount of water used allowed for the isolation of tri- and octanuclear aluminum hydroxide cluster complexes [Al3 (μ2 -OH)3 (THF)3 (PhSi(OSiPh2 O)3 )2 ] (1) and [Al8 (μ3 -OH)2 (μ2 -OH)10 (THF)3 (p-anisylSi(OSiPh2 O)3 )4 ] (2). 1 can be regarded as the Al(OH)3 cyclic trimer, where six protons have been replaced by silyl residues. While 2 features a unique [Al8 (μ3 -OH)2 (μ2 -OH)10 ](12+) core. In contrast to most other known aggregates of this type, 1 and 2 can be readily prepared at reasonable scales, dissolve in common solvents, and retain an intact framework even in the presence of excessive amounts of water. This finding paves the way to future research addressing the reactivity of the individual functional groups.

Completing the Heterocubane Family [Cp*AlE]4 (E = O, S, Se, and Te) by Selective Oxygenation and Sulfuration of [Cp*Al]4: Density Functional Theory Calculations of [Cp*AlE]4 and Reactivity of [Cp*AlO]4 toward Hydrolysis

The subvalent aluminum compound [Cp*Al]4 (1) reacts with dioxygen, N2O, or sulfur to yield the heterocubane complexes [Cp*AlX]4 [X = O (2) and S (3)]. Treatment of [Cp*AlO]4 (2) with (tBuO)3SiOH gave [(tBuO)3SiOAlO]4 (6) and Cp*H. The structures and spectroscopic data of the Al clusters are supported by density functional theory (DFT) calculations, which also demonstrate the importance of noncovalent interactions (NCI) in oligomeric Al(I) complexes as well as in [Cp*AlS]4 and the heavier homologues of Se and Te. The computed (27)Al NMR shifts indicate a deshielding at the Al centers with increasing electronegativity of the chalcogen atom as well as significant spin-orbit shielding effects within the heavier heterocubane [Al4E4] cores. Further hydrolysis of 6 with an additional amount of silanol in the presence of water resulted in the formation of [Al4(OH)6(OH2)2(OSiOtBu3)6] (7), which shows a structural motif found in boehmite and diaspore.

Activation of Dioxygen at a Lewis Acidic Nickel(II) Complex: Characterization of a Metastable Organoperoxide Complex

In metal-mediated O2 activation, nickel(II) compounds hardly play a role, but recently it has been shown that enzymes can use nickel(II) for O2 activation. Now a low-coordinate Lewis acidic nickel(II) complex has been synthesized that reacts with O2 to give a nickel(II) organoperoxide, as proposed for the enzymatic system. Its formation was studied further by UV/Vis absorption spectroscopy, leading to the observation of a short-lived intermediate that proved to be reactive in both oxygen atom transfer and hydrogen abstraction reactions, while the peroxide efficiently transfers O atoms. Both for the enzyme and for the functional model, the key to O2 activation is proposed to represent a concomitant electron shift from the substrate/co-ligand.

Taming the High Reactivity of Gold(III) Fluoride: Fluorido Gold(III) Complexes with N-based Ligands

The synthesis of [NMe4 ][AuF4 ] and [NEt4 ][AuF4 ], as well as an improved one-pot synthesis of Cs[AuF4 ], is reported. The new [AuF4 ]- salts were structurally characterized by single crystal X-ray diffraction, NMR spectroscopy, vibrational spectroscopy, and mass spectrometry. These salts are the first gold(III) fluoride salts that can be isolated in pure form and are convenient to be used in usual organic solvents for subsequent synthesis. The formation of molecular AuF3 complexes in solution is further reported, characterized as [F3 Au(NCCH3 )] at low temperature, as [F3 Au(py)] and a binuclear derivative which are stable at room temperature. The stability of these species in common organic solvents was investigated and they showed a satisfying stability.

The Versatile Behavior of Platinum Alkyne Complexes Towards XeF2: Formation of Fluorovinyl and Fluorido Complexes

Reactions of platinum(0) tolane complexes, bearing a chelating ligand with P and N donor atoms, with the electrophilic fluorinating agent XeF2 give facile access to platinum(II) β-fluorovinyl fluorido complexes. A series of new platinum(II) β-fluorovinyl complexes have been synthesized and were structurally characterized. Further oxidation with XeF2 led to ortho-metalated platinum(IV) fluorido compounds. Additional reactions of platinum(0) tolane complexes, bearing a chelating P,P donor ligand, with XeF2 led to a variety of fluorido and fluorovinyl complexes.

A Hexanuclear Iron(II) Layer with Two Square-Planar FeO4 Units Spanned by Tetrasiloxide Ligands: Mimicking of Minerals and Catalysts

A hexanuclear iron(II) siloxide complex has been prepared by reacting an incompletely condensed silsesquioxane first with NaOMe and then with Fe(OTf)2. In the process of product formation, the siloxane framework undergoes a transformation and it was shown that this happens already upon addition of base: Treatment of the ligand precursor with NaOMe leads to a completely condensed silsesquioxane cage with 12 Si atoms that is composed of 2 equiv of the tetrasiloxide ligands found in the product complex. Its iron centers form a two-dimensional array reminiscent of the situations found in minerals and two-dimensional oxide films caused by segregation of FeOx and silica. As the hexairon(II) assembly contains two high-spin square-planar FeO4 units-suggested to represent the active sites in Fe-zeolites, which react with N2O to generate strongly oxidizing sites-it was treated with Me3NO. This led to the oxidation of two of the iron centers to the oxidation state +III and elimination of one iron ion, so that a pentanuclear, mixed valent iron siloxide was formed. All complexes were fully characterized.

A Biomimetic Nickel Complex with a Reduced CO2 Ligand Generated by Formate Deprotonation and Its Behaviour towards CO2

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 Hx COyz- at electrodes and formate oxidation in formate dehydrogenases.

Photochemical Degradation of Various Bridge-Substituted Fluorene-Based Materials

Photochemical degradation is an important issue to be overcome in advancing the lifetime of fluorene-containing conjugated polymers. In order to optimize the inertness of the materials, a quantitative measure for the efficiency of degradation is needed. Here, we introduce a method to measure a relative quantum yield of the photochemical degradation by monitoring the kinetics of the process by means of UV/vis spectroscopy and liquid chromatography (LC) techniques. This method is employed to a set of differently substituted 2,7-diphenylfluorenes, serving as model compounds for polyfluorene materials. Our measurements show that the quantum yield changes by orders of magnitude upon varying the bridge substituents and that altered kinetics indicate changing degradation mechanisms.

Diverse Reactivity of Platinum SF3 and SF2 Complexes towards EtOH and Me3SiOEt

The complex trans-[Pt(F)(SF3 )(PCy3 )2 ] (1) shows a diverse reactivity pattern at the SF3 ligand towards ethanol and Me3 SiOEt. This led by deoxyfluorination or alcoholysis to the generation of platinum complexes bearing a metal-bound S(O)F or S(OEt)2 group. For the latter reaction trans-[Pt(F){S(F)(OEt)2 }(PCy3 )2 ] is suggested as an intermediate. In reactivity studies of trans-[Pt(F)(SF2 )(PCy3 )2 ][BF4 ] the complex [Pt(F){S(F)(OEt)}(PCy3 )2 ][BF4 ] was prepared, which does not react with ethanol.

Bismuthanes as Hemilabile Donors in an O2-Activating Palladium(0) Complex

A xanthene-based bismuthane/phosphane chelating ligand has been accessed that has enabled the synthesis of a palladium(0) bismuthane complex. The bismuthane donor proved to be hemilabile as it switched to a dangling position upon addition of O2 that gave a palladium(II) peroxide complex. Unlike the corresponding 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (Xantphos) palladium peroxide, the bismuth analogue could be employed for catalytic phosphane oxidation and oxidative phenol coupling.