Jörg Grunenberg

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

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.

Intramolecular heterolytic dihydrogen cleavage by a bifunctional frustrated pyrazolylborane Lewis pair

The reaction of bis(pentafluorophenyl)borane, HB(C(6)F(5))(2), with 3,5-di-tert-butyl-1H-pyrazole (3) affords the zwitterionic pyrazolium-borate trans-5 and, after dehydrogenation by use of the frustrated carbene-borane Lewis pair 1/B(C(6)F(5))(3), the bifunctional pyrazolylborane 6, which is able to cleave dihydrogen heterolytically with the formation of a mixture of cis-5 and trans-5.

Reactivity studies on [Cp′FeI]2: From iron hydrides to P4-activation

Metathesis of [Cp′FeI]2 (1) with KHBEt3 affords the polyhydride iron complexes [Cp′FeH2]2 (2) and [Cp′2Fe2H3] (3). The ratio in which both 2 and 3 are obtained correlates to the applied H2 pressure during synthesis. Complex 2 activates CH- or CD- bonds in aromatic compounds and shows slow H/D exchange in the presence of D2 at room temperature in cyclohexane solvent. [Cp′FeH2]2 acts as a Cp′Fe(I)-synthon when reacted with white phosphorus (P4) to give [Cp′Fe]2(μ-P4) (4) as the only P-containing product. This complex is best described as a triple-decker complex with a planar arrangement of a severely distorted kite-like cyclo-P4 unit. This distortion persists in solution and solid state as evidenced by a small PP coupling constant in the 31P{1H} NMR spectrum and a long P–P distance of 2.53 A. Complex 4 is an isomer to the long-known [{Cp′Fe}2(μ-η4:η4-P4)] (5) with a cis-tetraphosphabutadiene moiety and it thermally rearranges to 5, [{Cp′Fe}2(μ-η3:η3-P3)] and [Cp′Fe(P5)]. All complexes described in this paper have been completely characterized including X-ray crystallography, variable temperature NMR studies and DFT calculations. Relaxed force constants (inverse compliance constants) are used as bond strength descriptors.

MP2 and DFT Calculations on Circulenes and an Attempt to Prepare the Second Lowest Benzolog, [4]Circulene

MP2 and DFT calculations have been carried out for [n]circulenes for n=3 to 20 in order to predict the strain energy and topology of these cyclically condensed aromatic systems. To synthesise [4]circulene (2), 1,5,7,8-tetrakis(bromomethyl)biphenylene (14) was prepared from the corresponding tetramethyl derivative (8) and subjected to various dehalogenation reactions; all attempts to obtain [2.2]biphenylenophane (7) as a precursor for 2 by this route failed. Treatment of 14 with sodium sulfide furnished the thiaphanes 16 and 17, thermal and photochemical desulfurization of which also failed to provide 7. In a second approach [2.2]paracyclophane was converted to the pseudo-geminal dithiol 23, which was subsequently bridged to the thiaphanes 22 and 24. On flash vacuum pyrolysis at 800 degrees C these were converted exclusively into phenanthrene (30). An approach to dehydrochlorinate the commercial product PARYLENE C to the tetrahydro[4]circulene 7 led only to polymerisation. The X-ray structures of the intermediates 8, 14, 17, 23, 24, 26, and 35 are reported.

The strongest bond in the universe? Accurate calculation of compliance matrices for the ions N2H+, HCO+, and HOC+

Compliance matrices of protonated CO and N2 are calculated using coupled cluster methods and basis sets of quadruple zeta quality. Diagonal elements of the compliance matrices are used as unique bond strength descriptors. Going from CO (0.052 A/mdyn) to CO–H+ the C–O bond is weakened (0.062 A/mdyn), while the C–O bond in H–CO+ is getting stronger (0.045 A/mdyn). After protonation, the N–N bond strength is getting stronger (from 0.043 to 0.042 A/mdyn), too. The invariance of compliance matrix elements Cij under completion of (xi,xj) to a complete set (…,xi,…,xj,…) of internal coordinates is demonstrated.

π‐Helicenes Truncated to a Minimum: Access Through a Domino Approach Involving Multiple Carbopalladations and a Stille Coupling

A novel type of π-helicenes is reported, in which the π-system is truncated to an all-s-cis all-Z oligoene chain. A domino sequence was developed, consisting of up to four consecutive carbopalladation reactions and a terminal Stille cross-coupling, to generate these entities in one step from the respective linear oligoynes. Despite the minimal π-system, very high optical rotation values were encountered for the single enantiomers. X-ray crystallography confirmed their screw-shaped structure.

Are compliance constants ill-defined descriptors for weak interactions?

Just as the potential energy can be written as a quadratic form in internal coordinates, so it can also be expanded in terms of generalized forces. The resulting coefficients are termed compliance constants. In this article, the suitability of compliance constants as non-covalent bond strength descriptors is studied (a) for a series of weakly bound hydrogen halide–rare gas complexes applying a configuration interaction theory, (b) for a double stranded DNA 4-mer using approximate density functional methods and finally (c) for a double stranded DNA 20-mer using empirical force fields. Our results challenge earlier studies, which concluded the inappropriateness of compliance constants as soft matter descriptors. The discrepancy may be ascribed, inter alia, to the application of an oversimplified potential function in these earlier studies, assuming a central forces approximation.

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