Kai Brandhorst

Preparation of Imidazolin‐2‐iminato Molybdenum and Tungsten Benzylidyne Complexes: A New Pathway to Highly Active Alkyne Metathesis Catalysts

The reaction of [PhC[triple bond]MBr(3)(dme)] (dme=1,2-dimethoxyethane) with the hexafluoro-tert-butoxides LiX or KX [X=OC(CF(3))(2)Me] afforded the benzylidyne complexes [PhC[triple bond]MX(3)(dme)] (2a: M=W, 2b: M=Mo), which further reacted with the lithium reagent Li(Im(tBu)N), generated with MeLi from 1,3-di-tert-butylimidazolin-2-imine (Im(tBu)NH), to form the imidazolin-2-iminato complexes [PhC[triple bond]MX(2)(Im(tBu)N)] (3a: M=W, 3b: M=Mo). The propylidyne complex [EtC[triple bond]MoX(2)(NIm(tBu))] (4) was obtained by treatment of 3b with an excess of 3-hexyne. Complexes 3a and 3b are able to efficiently catalyse alkyne cross metathesis of various 3-pentynyl benzyl ethers 5 and benzoic esters 7 at room temperature, to afford 2-butyne and the corresponding diethers 6 and diesters 8. The tungsten complex 3a proved to be a superior catalyst for ring-closing alkyne metathesis, and the [10]cyclophanes 10 and 12 were synthesised in high yield from 1,3-bis(3-pentynyloxymethyl)benzene (9) and bis(3-pentynyl) phthalate (11), respectively. The molecular structures of compounds 2a, 2b, 3a, 3b, 4, and 12 were determined by single-crystal X-ray diffraction. DFT calculations have been carried out for catalyst systems based on the imidazolin-2-iminato tungsten and molybdenum complexes 3a and 3b by choosing the alkyne metathesis of 2-butyne as the model reaction; the studies revealed a lower activation barrier for the tungsten system.

Efficient computation of compliance matrices in redundant internal coordinates from Cartesian Hessians for nonstationary points

We present an extension to the theory of compliance matrices, which is valid for arbitrary nonstationary points on the potential energy hypersurface. It is shown that compliance matrices computed as the inverse of the covariant Hessian matrix obey the same invariance properties with respect to different internal coordinate systems as they do for stationary points. Furthermore, we demonstrate how the computation of compliance matrices in arbitrary sets of redundant internal coordinates starting from a Cartesian Hessian can be achieved efficiently, and we discuss their potential usefullness in geometry optimization processes

Sulfur-bridged BODIPY DYEmers.

Reactions of BODIPY monomers with sulfur nucleophiles and electrophiles result in the formation of new BODIPY dimers. Mono- and disulfur bridges are established, and the new dyestuff molecules were studied with respect to their structural, optical, and electrochemical properties. X-ray diffraction analyses reveal individual angulated orientations of the BODIPY subunits in all cases. DFT calculations provide solution conformers of the DYEmers which deviate in a specific manner from the crystallographic results. Clear exciton-like splittings are observed in the absorption spectra, with maxima at up to 628 nm, in combination with the expected weak fluorescence in polar solvents. A strong communication between the BODIPY subunits was detected by cyclic voltammetry, where two separated one-electron oxidation and reduction waves with peak-to-peak potential differences of 120-400 mV are observed. The qualitative applicability of the exciton model by Kasha for the interpretation of the absorption spectral shape with respect to the conformational state, subunit orientation and distance, and conjugation through the different sulfur bridges, is discussed in detail for the new BODIPY derivatives. This work is part of our concept of DYEmers, where the covalent oligomerisation of BODIPY-type dye molecules with close distances is leading to new functional dyes with predictable properties.

Preparation of cyclophanes by room-temperature ring-closing alkyne metathesis with imidazolin-2-iminato tungsten alkylidyne complexes.

Room-temperature ring-closing alkyne metathesis of 1,2-, 1,3-, and 1,4-bis(3-pentynyloxymethyl)benzenes has been investigated in the presence of catalytic amounts of an imidazolin-2-iminato tungsten alkylidyne complex. The m- and p-diynes selectively form the respective [10]metacyclophane or [10.10]paracyclophane, respectively, whereas a mixture of monomeric and dimeric cycloalkynes is obtained in the case of the o-diyne. DFT calculations reveal that the different selectivities can be attributed to the relative thermodynamic stability of the emerging cyclophanes.

The Corrole Radical

The reaction of 5,10,15-trimesitylcorrole (H3 cor) with tungsten hexachloride and tungsten hexacarbonyl resulted in the unexpected formation of the 3,17-dichloro-5,10,15-trimesitylcorrole radical (H2 cor*) as an air-stable product. X-ray crystallography proved the planarization of the corrole radical structure, which was rationalized by the reduced steric hindrance of two versus three hydrogen atoms inside the N4 cavity. Although the aromaticity was lost, no specific changes in C-C or C-N bond distances could be observed. The regioselectivity of the two-fold chlorination is the result of the nucleophilic attack of chloride ions to an oxidized corrole macrocycle, and is supported by DFT results. The corrole radical acts as a dianionic ligand and allows the insertion of the divalent zinc(II) cation, which usually does not form neutral corrole complexes.

Probing Noncovalent Interactions in Biomolecular Crystals with Terahertz Spectroscopy

The far-infrared vibrational spectra of molecular crystals are dominated by intramolecular (internal) modes, which are also present in the isolated molecule, and noncovalent intermolecular modes, which arise from the interaction of the nearest neighbours (external modes). Conceptually this has long been understood and early experiments using Fourier transform infrared (FTIR) spectroscopy confirmed the existence of a rich vibrational spectrum in polypeptides in the low-energy region. Yet, the assignment of the experimentally observed peaks in the low-energy region is often unsatisfying. Recently, the advent of terahertz time-domain spectroscopy (THz TDS) has revitalized the field. Using this convenient room-temperature technique, noncovalent interactions between several smalland medium-sized molecules have been investigated, including nucleobases and nucleosides, short-chain polypeptides, cystine and glutathione, retinal, and saccharides. Chen et al. and Nagai et al. could clearly distinguish between intramolecular and intermolecular modes by comparing crystalline structures and solvated molecules. Furthermore, it was shown that different isomers as well as diverse crystalline forms show distinctively different terahertz spectra. Most of these experiments have benefited from a comparison with quantum-mechanical calculations of the normal modes, which are nowadays feasible due to advances both in computer technology and software development. Herein, we study two types of molecular crystals—one with weak [dimethyluracil (DMU), see Figure 1] and one with strong hydrogen bonds [thymine (THY), see Figure 2]. In order to gain insight into the chemical nature of the THz signals by assigning individual low-frequency modes, we compare our data to a

Synthesis of unsymmetrical 1,3-diynes via alkyne cross-metathesis.

The tungsten benzylidyne complex [PhC≡W{OSi(OtBu)3}3] (1) efficiently catalyses the metathetic conversion between symmetrical and unsymmetrical 1,3-diynes, which provides the opportunity to prepare the latter species directly from terminal alkynes by a combination of copper-catalysed homocoupling and catalytic alkyne cross-metathesis (ACM).

Fast Sparse Cholesky Decomposition and Inversion using Nested Dissection Matrix Reordering.

Here we present an efficient, yet nonlinear scaling, algorithm for the computation of Cholesky factors of sparse symmetric positive definite matrices and their inverses. The key feature of this implementation is the separation of the task into an algebraic and a numeric part. The algebraic part of the algorithm attempts to find a reordering of the rows and columns which preserves at least some degree of sparsity and afterward determines the exact nonzero structure of both the Cholesky factor and its corresponding inverse. It is based on graph theory and does not involve any kind of numerical thresholding. This preprocessing then allows for a very efficient implementation of the numerical factorization step. Furthermore this approach even allows use of highly optimized dense linear algebra kernels which leads to yet another performance boost. We will show some illustrative timings of our sparse code and compare it to the standard library implementation and a recent sparse implementation using thresholding. We conclude with some comments on how to deal with positive semidefinite matrices.

A stable enol from a 6-substituted benzanthrone and its unexpected behaviour under acidic conditions

Summary Treatment of benzanthrone (1) with biphenyl-2-yl lithium leads to the surprisingly stable enol 4, which is converted by dehydrogenation into the benzanthrone derivative 7. Under acidic conditions 4 isomerises to the spiro compound 11 and the bicyclo[4.3.1]decane derivative 12. Furthermore, the formation of 7 and the hydrogenated compound 13 is observed. A mechanism for the formation of the reaction products is proposed and supported by DFT calculations.

Viking Helmet Corroles: Activating Inert Oxidometal Corroles

Chemically inert oxidometal(V) corrols of molybdenum and rhenium undergo clean ligand-exchange reactions upon the action of SiCl4 . The resulting dichlorido complexes show trigonal prismatic coordination of the metal ion with the chlorine atoms residing in a cis configuration, and were studied by optical and resonance spectroscopy as well as DFT calculations. In situ reactivity studies with carbon nucleophiles indicate high reactivity for chlorine replacement. Treatment with sodium cyclopentadienide paves the way to robust molybdenum corrolocene half-sandwich complexes. These organometallic compounds are the first corrole species that stabilize an air-stable and diamagnetic low spin d2 -MoIV center. Structural, spectroelectrochemical, and chemical investigations prove a reversible MoIV /MoV redox couple close to the Fc/Fc+ potential for these systems. The high stability of the compounds in both redox states calls for future applications in catalysis and as redox switch.