Revisiting the origin of the bending in group 2 metallocenes AeCp2 (Ae = Be-Ba).

Metallocenes are well-established compounds in organometallic chemistry, and can exhibit either a coplanar structure or a bent structure according to the nature of the metal center (E) and the cyclopentadienyl ligands (Cp). Herein, we re-examine the chemical bonding to underline the origins of the geometry and stability observed experimentally. To this end, we have analysed a series of group 2 metallocenes [Ae(C5R5)2] (Ae = Be-Ba and R = H, Me, F, Cl, Br, and I) with a combination of computational methods, namely energy decomposition analysis (EDA), polarizability model (PM), and dispersion interaction densities (DIDs). Although the metal-ligand bonding nature is mainly an electrostatic interaction (65-78%), the covalent character is not negligible (33-22%). Notably, the heavier the metal center, the stronger the d-orbital interaction with a 50% contribution to the total covalent interaction. The dispersion interaction between the Cp ligands counts only for 1% of the interaction. Despite that orbital contributions become stronger for heavier metals, they never represent the energy main term. Instead, given the electrostatic nature of the metallocene bonds, we propose a model based on polarizability, which faithfully predicts the bending angle. Although dispersion interactions have a fair contribution to strengthen the bending angle, the polarizability plays a major role.

Maximized axial helicity in a Pd2L4 cage: inverse guest size-dependent compression and mesocate isomerism.

Helicity is an archetypal structural motif of many biological systems and provides a basis for molecular recognition in DNA. Whilst artificial supramolecular hosts are often helical, the relationship between helicity and guest encapsulation is not well understood. We report a detailed study on a significantly coiled-up Pd2L4 metallohelicate with an unusually wide azimuthal angle (∼176°). Through a combination of NMR spectroscopy, single-crystal X-ray diffraction, trapped ion mobility mass spectrometry and isothermal titration calorimetry we show that the coiled-up cage exhibits extremely tight anion binding (K of up to 106 M-1) by virtue of a pronounced oblate/prolate cavity expansion, whereby the Pd-Pd separation decreases for mono-anionic guests of increasing size. Electronic structure calculations point toward strong dispersion forces contributing to these host-guest interactions. In the absence of a suitable guest, the helical cage exists in equilibrium with a well-defined mesocate isomer that possesses a distinct cavity environment afforded by a doubled Pd-Pd separation distance.

Dispersion forces in chirality recognition - a density functional and wave function theory study of diols.

In the discussion of chirality recognition, steric considerations and strongly directed interactions such as hydrogen bonds are primarily discussed. However, given the sheer size of biomolecules, it is expected that dispersion forces could also play a determining role for aggregate formation and associated chirality recognition. With the example of diol molecules, we explore different factors in the formation of homo- and hetero-dimers as well as their relative stability. By comparing density functional results with the analysis of local correlation methods, we infer the impact of dispersion not only on the energies but also on the structures of such chiral aggregates. A local orbital based scheme is used to calculate wave function dispersion-free gradients and compare to uncorrected density functional structures.

All That Binds Is Not Gold-The Relative Weight of Aurophilic Interactions in Complex Formation.

Often have you heard that complexes containing close Au(I) contacts are strongly influenced by what has come to be known as aurophilic interactions. In this work, local orbital analysis is carried out to separate competing metallophilic and other weak interactions at the correlated level in three selected molecular crystals. We carefully separate and discuss the different contributions to the total interaction energy of dimers and trimers according to their spatial location, and identify the relative weight in binding. Changes according to the orientation of the monomers have also been computed. Our results show that although metallophilic contacts contribute to the overall stability and the structure of the crystals, they are not at all dominant, and ligand-ligand interactions can easily outweigh the latter.

The phenyl vinyl ether-methanol complex: a model system for quantum chemistry benchmarking.

The structure of the isolated aggregate of phenyl vinyl ether and methanol is studied by combining a multi-spectroscopic approach and quantum-chemical calculations in order to investigate the delicate interplay of noncovalent interactions. The complementary results of vibrational and rotational spectroscopy applied in molecular beam experiments reveal the preference of a hydrogen bond of the methanol towards the ether oxygen (OH∙∙∙O) over the π-docking motifs via the phenyl and vinyl moieties, with an additional less populated OH∙∙∙P(phenyl)-bound isomer detected only by microwave spectroscopy. The correct prediction of the energetic order of the isomers using quantum-chemical calculations turns out to be challenging and succeeds with a sophisticated local coupled cluster method. The latter also yields a quantification as well as a visualization of London dispersion, which prove to be valuable tools for understanding the role of dispersion on the docking preferences. Beyond the structural analysis of the electronic ground state (S0), the electronically excited (S1) state is analyzed, in which a destabilization of the OH∙∙∙O structure compared to the S0 state is observed experimentally and theoretically.

The furan microsolvation blind challenge for quantum chemical methods: First steps.

Herein we present the results of a blind challenge to quantum chemical methods in the calculation of dimerization preferences in the low temperature gas phase. The target of study was the first step of the microsolvation of furan, 2-methylfuran and 2,5-dimethylfuran with methanol. The dimers were investigated through IR spectroscopy of a supersonic jet expansion. From the measured bands, it was possible to identify a persistent hydrogen bonding OH-O motif in the predominant species. From the presence of another band, which can be attributed to an OH-π interaction, we were able to assert that the energy gap between the two types of dimers should be less than or close to 1 kJ/mol across the series. These values served as a first evaluation ruler for the 12 entries featured in the challenge. A tentative stricter evaluation of the challenge results is also carried out, combining theoretical and experimental results in order to define a smaller error bar. The process was carried out in a double-blind fashion, with both theory and experimental groups unaware of the results on the other side, with the exception of the 2,5-dimethylfuran system which was featured in an earlier publication.

Influence of size, shape, heteroatom content and dispersive contributions on guest binding in a coordination cage.

A halide-triggered metallosupramolecular host was systematically studied for the uptake of small neutral molecules using NMR and MS experiments. Starting from benzene, cyclic guests were screened with respect to size (ring count), shape (flatness, 3D structure, substitution pattern, flexibility) and hetero atom content (number, position, donor character). 5-Rings and substituted 5/6-rings bind only weakly, while oversized (e.g. naphthalene, adamantane, ferrocene) and linear alkanes do not bind at all. Bridged 6-rings of the norbornane type and in particular DABCO bind strongly, likewise other guests with oppositely arranged hetero atoms. For the DABCO complex, a single crystal X-ray structure was obtained. The contribution of dispersive interactions to binding was derived from electronic structure calculations. Together, experimental and theoretical data deepen the understanding of guest selectivity and encapsulation driving force towards application of the host as a switchable receptor and reaction chamber.

Manganese(I)-Catalyzed Dispersion-Enabled C-H/C-C Activation.

C-H/C-C Functionalizations were achieved with the aid of a versatile manganese(I) catalyst. Thus, an organometallic manganese-catalyzed C-H activation set the stage for silver-free C-H/C-C transformations with ample substrate scope and excellent levels of chemo-, site-, and diastereo-selectivities. The robust nature of the manganese(I) catalysis regime was reflected by the first C-H/C-C functionalization on amino acids under racemization-free reaction conditions. Detailed experimental and computational mechanistic studies provided strong evidence for a facile C-H activation and a rate-determining C-C cleavage, with considerable contribution from London dispersion interactions.

Visualizing dispersion interactions through the use of local orbital spaces.

The interpretation of chemical properties/phenomena can often be aided through the use of imagery. The mapping of molecular electrostatic potentials is a prime example, serving as a guideline in the design of novel compounds or understanding transition state stabilization effects. It is today a common tool for theoreticians and experimentalists alike. With the emergence of concepts such as dispersion energy donors, and the overall importance of dispersion in chemical systems, representations targeting such a class of interactions are warranted. In this work, we make use of local orbital analysis to extract dispersion interactions and represent them in a scalar quantity, the Dispersion Interaction Density (DID). A particular advantage of the method is the possibility to represent at the same footing intermolecular and intramolecular interactions in a straightforward fashion from wave function calculations. We present examples for the benzene dimer, several substituted benzenes and a coupled diamondoid molecule. © 2016 Wiley Periodicals, Inc.

Internal dynamics and guest binding of a sterically overcrowded host.

Substituent control in self-assembled host systems allows for a fine-tuning of structure, dynamics and guest preference. Flat banana-shaped ligands L1 assemble with Pd(ii) cations into the interpenetrated coordination cage dimer [3BF4@Pd4L18], capable of sequential guest uptake. In contrast, the introduction of bulky adamantyl groups in ligand L2 prevents dimerization and results in the clean formation of monomeric cage species [Pd2L24]. Owing to steric crowding, the adamantyl substituent is considerably bent sideways with respect to the ligand backbone, and is rapidly flipping between both faces of the free ligand giving rise to two energetically degenerate conformers. Surprisingly, the flipping is preserved in the cage, albeit at a lower rate due to entropic reasons. Despite the very dense packing within the self-assembled structure, the cage is able to encapsulate a series of bis-anionic guests in an induced-fit fashion. Electronic structure calculations revealed a substantial contribution from dispersion interactions between the guest and the surrounding adamantyl groups that stabilize the host-guest complex. Guest exchange kinetics were quantified and the influence that encapsulated guests imparted on the ligand flipping dynamics was examined by a series of 2D NMR experiments. Four synchrotron X-ray structures of the cage and its host-guest complexes are presented, allowing for unprecedented insight into the host-guest interactions of a sterically overcrowded host and its guest-induced distortion.