Daniel Kratzert

DSR: enhanced modelling and refinement of disordered structures with SHELXL

The new computer program DSR enables semi-automatic modelling of disordered or well ordered moieties in crystal structures via a placement procedure of molecular fragments and corresponding stereochemical restraints from a database.

Cationic cluster formation versus disproportionation of low-valent indium and gallium complexes of 2,2’-bipyridine

Group 13 MI compounds often disproportionate into M0 and MIII. Here, however, we show that the reaction of the MI salt of the weakly coordinating alkoxyaluminate [GaI(C6H5F)2]+[Al(ORF)4]− (RF=C(CF3)3) with 2,2-bipyridine (bipy) yields the paramagnetic and distorted octahedral [Ga(bipy)3]2+•{[Al(ORF)4]−}2 complex salt. While the latter appears to be a GaII compound, both, EPR and DFT investigations assign a ligand-centred [GaIII{(bipy)3}•]2+ radical dication. Surprisingly, the application of the heavier homologue [InI(C6H5F)2]+[Al(ORF)4]− leads to aggregation and formation of the homonuclear cationic triangular and rhombic [In3(bipy)6]3+, [In3(bipy)5]3+ and [In4(bipy)6]4+ metal atom clusters. Typically, such clusters are formed under strongly reductive conditions. Analysing the unexpected redox-neutral cationic cluster formation, DFT studies suggest a stepwise formation of the clusters, possibly via their triplet state and further investigations attribute the overall driving force of the reactions to the strong In−In bonds and the high lattice enthalpies of the resultant ligand stabilized [M3]3+{[Al(ORF)4]−}3 and [M4]4+{[Al(ORF)4]−}4 salts.

Synthesis of Bridged Benzazocines and Benzoxocines by a Titanium‐Catalyzed Double‐Reductive Umpolung Strategy

A sequence of two titanium(III)-catalyzed reductive umpolung reactions is reported that allows the rapid construction of benzazo- and benzoxozine building blocks. The first step is a reductive cross-coupling of quinolones or chromones with Michael acceptors. This reaction proceeds with complete syn-selectivity for the quinolone functionalization while the anti-diastereomers are obtained as the major products from chromones. With different reaction conditions, the stereochemical outcome can be altered to afford the syn-chromanone products as well. A subsequent reductive ketyl radical cyclization forges the tricyclic title compounds in good yields. A stereochemical model explaining the observed stereoselectivities is provided and the product configurations were unambiguously verified by X-ray analyses and 2Du2005NMR spectroscopic experiments.

Mechanism of the TiIII-Catalyzed Acyloin-Type Umpolung: A Catalyst-Controlled Radical Reaction

The titanium(III)-catalyzed cross-coupling between ketones and nitriles provides an efficient stereoselective synthesis of α-hydroxyketones. A detailed mechanistic investigation of this reaction is presented, which involves a combination of several methods such as EPR, ESI-MS, X-ray, in situ IR kinetics, and DFT calculations. Our findings reveal that C-C bond formation is turnover-limiting and occurs by a catalyst-controlled radical combination involving two titanium(III) species. The resting state is identified as a cationic titanocene-nitrile complex and the beneficial effect of added Et3N·HCl on yield and enantioselectivity is elucidated: chloride coordination initiates the radical coupling. The results are fundamental for the understanding of titanium(III)-catalysis and of relevance for other metal-catalyzed radical reactions. Our conclusions might apply to a number of reductive coupling reactions for which conventional mechanisms were proposed before.

[Ni(cod)2][Al(ORF)4], a source for naked nickel(I) chemistry

The straightforward synthesis of the cationic, purely organometallic Ni(I) salt [Ni(cod)2](+)[Al(OR(F))4](-) was realized through a reaction between [Ni(cod)2] and Ag[Al(OR(F))4] (cod = 1,5-cyclooctadiene). Crystal-structure analysis and EPR, XANES, and cyclic voltammetry studies confirmed the presence of a homoleptic Ni(I) olefin complex. Weak interactions between the metal center, the ligands, and the anion provide a good starting material for further cationic Ni(I) complexes.

Homoleptic Gold Acetonitrile Complexes with Medium to Very Weakly Coordinating Counterions: Effect on Aurophilicity?

A series of gold acetonitrile complexes [Au(NCMe)2 ]+ [WCA]- with weakly coordinating counterions (WCAs) was synthesized by the reaction of elemental gold and nitrosyl salts [NO]+ [WCA]- in acetonitrile ([WCA]- =[GaCl4 ]- , [B(CF3 )4 ]- , [Al(ORF )4 ]- ; RF =C(CF3 )3 ). In the crystal structures, the [Au(NCMe)2 ]+ units appeared as monomers, dimers, or chains. A clear correlation between the aurophilicity and the coordinating ability of counterions was observed, with more strongly coordinating WCAs leading to stronger aurophilic contacts (distances, C-N stretching frequencies of [Au(NCMe)2 ]+ units). An attempt to prepare [Au(L)2 ]+ units, even with less weakly basic solvents like CH2 Cl2 , led to decomposition of the [Al(ORF )4 ]- anion and formation of [NO(CH2 Cl2 )2 ]+ [F(Al(ORF )3 )2 ]- . All nitrosyl reagents [NO]+ [WCA]- were generated according to an optimized procedure and were thoroughly characterized by Raman and NMR spectroscopy. Moreover, the to date unknown species [NO]+ [B(CF3 )3 CN]- was prepared. Its reaction with gold unexpectedly produced [Au(NCMe)2 ]+ [Au(NCB(CF3 )3 )2 ]- , in which the cyanoborate counterion acts as an anionic ligand itself. Interestingly, the auroborate anion [Au(NCB(CF3 )3 )2 ]- behaves as a weakly coordinating counterion, which becomes evident from the crystallographic data and the vibrational spectral characteristics of the [Au(NCMe)2 ]+ cation in this complex. Ligand exchange in the only room temperature stable salt of this series, [Au(NCMe)2 ]+ [Al(ORF )4 ]- , is facile and, for example, [Au(PPh3 )(NCMe)]+ [Al(ORF )4 ]- can be selectively generated. This reactivity opens the possibility to generate various [AuL1 L2 ]+ [Al(ORF )4 ]- salts through consecutive ligand-exchange reactions that offer access to a huge variety of AuI complexes for gold catalysis.

Light- or oxidation-triggered CO release from [MnI(CO)3(κ3-L)] complexes: reaction intermediates and a new synthetic route to [MnIII/IV2(μ-O)2(L)2] compounds

The reaction products and intermediates of the three CO-releasing manganese(i) coordination compounds [Mn(tpm)(CO)3]+, [Mn(bpza)(CO)3] and [Mn(tpa)(CO)3]+ were analysed by combining IR-spectroscopy, electrochemical measurements and single-crystal XRD. The intermediate formation of manganese(i) biscarbonyl compounds and the rather facile oxidation of these species were identified as key reaction steps that accompany CO liberation. For the use of [MnI(CO)3] complexes as light-triggered CO sources, the results indicate that in this case photo- and redox-chemistry seem to be strongly coupled which could be important and potentially even useful in the pharmacological context. Additionally, one has to be aware of the fact that [MnII(κ3-L)(solv)3]2+ complexes, the primary reaction products after CO substitution, are able to bind to proteins, which was demonstrated using bovine serum albumin as a model. And finally it could be shown that the CO-release reactions can be used as a new synthetic route to prepare multinuclear μ-oxido-bridged manganese complexes: the mixed-valence compound [Mn(μ-O)2(tpa)2]3+ could be prepared in a single step from [Mn(tpa)(CO)3]+via photo- or electrochemically induced CO substitution.

Using the Lewis Acid Me3Si−F−Al(ORF)3 To Prepare Phosphino‐Phosphonium Cations with the Least‐Coordinating Anion [(RFO)3Al−F−Al(ORF)3]−

By reaction of two equivalents of Me3 Si-F-Al(ORF )3 1 with an equimolar amount of PPh2 Cl, the salt [Ph2 P-PPh2 Cl]+ [(RF O)3 Al-F-Al(ORF )3 ]- 2 is prepared smoothly in 91u2009% yield (NMR, XRD). The synthesis of [Ph2 P-PPh3 ]+ [(RF O)3 Al-F-Al(ORF )3 ]- 3 is best achieved by a two-step reaction: first, two equivalents of 1 react with one PPh3 to give [Me3 Si-PPh3 ]+ [(RF O)3 Al-F-Al(ORF )3 ]- 4 (NMR, XRD), which, upon reaction with PPh2 Cl, yields pure 3 and Me3 SiCl (NMR, XRD). Typically, a stoichiometry of two equivalents of 1 with respect to one equivalent of the chloride donor should be used. Otherwise, the residual strong Lewis acidity of the [(RF O)3 Al-F-Al(ORF )3 ]- anion in the presence of the [F-Al(ORF )3 ]- anion-that forms with less than two equivalents of 1-leads to further chloride exchange reactions that complicate work-up. This route presents the easiest way to introduce the least-coordinating [(RF O)3 Al-F-Al(ORF )3 ]- anion into a system. We expect a wide use of this route in all areas, in which chloride-bond heterolysis in combination with very weakly coordinating anions is desirable. Additionally, we performed calculations on the bond dissociation mechanisms of [R2 P-PMe3 ]+ and the isoelectronic Me2 P-SiMe3 and Me2 Si-PMe3 in dependence of the solvent permittivity. These calculations show, especially for the neutral reference compounds, a heavy influence of the solvent on the dissociation mechanism, which is why we suggest investigating these properties in solution instead of gas phase.

Dibenzo[a,e]pentalenes with Low-Lying LUMO Energy Levels as Potential n-Type Materials

Ambipolar organic semiconductors are of high interest for organic field-effect transistors. For n-type conduction, low LUMO energies are required. Dibenzo[a,e]pentalenes (DBPs) are promising compounds; however, few derivatives exist with energetically low-lying LUMO levels. Here, we present DBP derivatives with LUMO energies down to -3.73 eV and small bandgaps down to 1.63 eV determined through cyclic voltammetry, UV/vis absorption spectroscopy, and TDDFT calculations. Single-crystal X-ray diffraction analysis revealed a 1D π-stacking mode. The addition of arylalkynyl substituents at the five-membered rings in a facile and versatile synthetic route allowed for tuning of the band gaps and LUMO energies. The synthetic route can easily be modified to access a variety of DBP derivatives. The LUMO energies of the DBP derivatives presented herein make them attractive for an application in n-type or ambipolar field-effect transistors.

Thioether- and sulfone-functionalized dibenzopentalenes as n-channel semiconductors for organic field-effect transistors

Dibenzo[a,e]pentalenes (DBPs) are promising candidates to be used as ambipolar or n-type semiconductors in organic field-effect transistors (OFETs). For n-channel conduction, low LUMO energy levels are required. Furthermore, a close molecular packing in the solid state is advantageous. Here we present thioether-functionalized DBPs with LUMO energies down to −3.47 eV as well as a DBP sulfone with a LUMO energy of −3.94 eV. X-ray crystallography revealed 1-D π-stacking or herringbone packing in the solid state with close intramolecular distances between the DBP cores. Two thio-DBPs and the DBP sulfone showed n-channel conduction in FET measurements on well-aligned crystals with electron mobilities of up to 0.18 cm2 V−1 s−1, to date the highest reported values for n-channel conduction in DBP derivatives. The short synthetic route will in the future allow for facile access to a variety of thio- and sulfone-DBPs for n-channel or ambipolar semiconduction.