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A PP(O)P pincer ligand based upon a peri- substituted acenaphthyl (Ace) scaffold and a secondary phosphine oxide, (5-Ph 2P-Ace-6-) 2P(O)H, was prepared and fully charac- terized including a neutron diffraction study. The reaction with [Ni(H2 O)6]Cl2 and PdCl2 produced ionic metal(II) complexes [κ3 -P,P’,P’’((5-Ph 2P-Ace-6-) 2P(OH))MCl]Cl, which upon addi- tion of Et3 N gave rise to zwitterionic metal(II) complexes κ3 - P,P’,P’’((5-Ph 2 P-Ace-6-)2P(O))MCl (M = Ni, Pd). The reaction with Ni(COD) 2 (COD = cyclooctadiene) provided the η3 - cyclooctenyl Ni(II) complex κ3 -P,P’,P’’((5-Ph 2P-Ace-6-) 2P(O))Ni- (η3 -C8H 13 ). A detailed complementary bonding analysis of the P−H, P−O, and P−M interactions was carried out (M = Ni, Pd).

In a combined experimental and computational study, the molecular and electronic structures of the divalent bis(m- terphenyl)element cations [(2,6-Mes2C6H3)2E]+ of group 13 (1, E=B; 2, E=Al; 3, E=Ga; 4, E=In; 5, E=Tl) were investigated. The preparation and characterization of 2, 3 and 5 were previously reported by Wehmschulte’s (Organometallics 2004, 23, 1965– 1967; J. Am. Chem. Soc. 2003, 125, 1470–1471) and our groups (Organometallics 2009, 28, 6893–6901). The indinium ion 4 was prepared and fully characterized for the first time. Attempts to prepare the borinium ion 1 by fluoride or hydride abstraction were unsuccessful. The electronic structures of 1–5 and the stabilization by the bulky m-terphenyl substituents were analyzed using quantum chemical calculations and compared to the divalent bis(m-terphenyl)pnictogenium ions [(2,6- Mes2C6H3)2E]+ of group 15 (6, E=P; 7, E=As; 8, E=Sb; 9, E=Bi) previously investigated by our group (Angew. Chem. Int. Ed. 2018, 57, 10080–10084). The calculated fluoride ion affinities (FIA) of 1–9 are higher than that of SbF5, which classifies them as Lewis superacids.

Boron-nitrogen substitutions in polycyclic aromatic hydrocarbons (PAHs) have a strong impact on the optical properties of the molecules due to a significantly more heterogeneous electron distribution. However, besides these single-molecule properties, the observed optical properties of PAHs critically depend on the degree of intermolecular interactions such as ��-��-stacking, dipolar interactions, or the formation of dimers in the excited state. Pyrene is the most prominent example showing the latter as it exhibits a broadened and strongly bathochromically shifted emission band at high concentrations in solution compared to the respective monomers. In the solid state, the impact of intermolecular interactions is even higher as it determines the crystal packing crucially. In this work, a thiophene-flanked BN-pyrene (BNP) was synthesized and compared with its all-carbon analogue (CCP) in solution and in the solid state by means of crystallography, NMR spectroscopy, UV-vis spectroscopy, and photoluminescence (PL) spectroscopy. In solution, PL spectroscopy revealed the solvent-dependent presence of excimers of CCP at high concentrations. In contrast, no excimers were found in BNP. Clear differences were also observed in the single-crystal packing motifs. While CCP revealed overlapped pyrene planes with centroid distances in the range of classical ��-stacking interactions, the BNP scaffolds were displaced and significantly more spatially separated.

The reaction of (5-Ph2P-Ace-6-)2MeSiH with [(C2H4)2RhCl]2 proceeded via elimination of ethylene and oxidative addition of the Si–H bond at rhodium and provided the rhodium(III) complex (5-Ph2P-Ace-6-)2MeSiRh(H)Cl (1) possessing a reverse agostic interaction of the type Rh–H···Si, which was characterized by Hirshfeld atom refinement (HAR) of single-crystal X-ray diffraction data. This technique allows the precise and accurate determination of hydrogen positions also for heavier transition-metal hydrides. The bonding situation in the Rh–H···Si interaction was further analyzed using a set of complementary bonding indicators, such as the compliance matrix analysis or real-space bonding indicators derived from the electron and electron pair densities. The reaction of (5-Ph2P-Ace-6-)2MeSiH with [(CO)2RhCl]2 occurred via elimination of hydrogen chloride and produced the rhodium(I) complex (5-Ph2P-Ace-6-)2MeSiRh(CO)2 (2).

The Br��nsted acidity of the perfluorinated trialkoxysilanol {(F3C)3CO}3SiOH is more than 13 orders of magnitude higher than that of orthosilicic acid, Si(OH)4, and even more for most previously known silanols. It is easily deprotonated by simple amines and pyridines to give the conjugate silanolates [OSi{OC(CF3)3}3]

Crystal structure determination using X-ray diffraction has become a routine technique for life and materials sciences, chemistry, biology, physics, etc. since its discovery 106 years ago. Despite its long existence, the most widely used model for crystal structure refinement is still the hundred-years old Independent Atom Model (IAM), which neglects chemical information present in the experimental data, such as bonding densities and densities due to lone electron pair regions. Over the past decades, new models have been developed to include this chemical information via aspherical description of atoms either using multipole-based techniques, maximum entropy methods or wavefunction-based techniques. The latter constitute the recently emerged field of quantum crystallography, combining quantum theory and crystallography to provide advanced structure refinements and wavefunctions with embedded information from experiment, giving access to experimental electron density distributions. In this thesis, we show the different models that allow access to these chemical information present in the experimental data, explaining the theory behind the atomic scattering factors, and their different sources according to each different model. These are employed and tested thoroughly for cases where the spherical approach (independent atom model) using X-ray diffraction data fails most dramatically: hydrogen atoms. We show how to obtain accurate atomic parameters for hydrogen atoms in unusual bonding situations using X-ray diffraction data only. These include hydrogen atoms in tautomeric equilibrium (Chapter 8), involved in bridging positions within strong intramolecular hydrogen bonds (Chapters 9, 9.2 and Section 10.2), involved in agostic interactions, bonded to heavy elements (Chapter 11) and in proteins (Chapter 12). Correlations between hydrogen atoms displacement parameters and their positions are evaluated through their congruent refinement using state of the art methods for the estimation of anisotropic displacement parameters (Chapter 10.2). We compare derived X-H bond lengths and ADPs to those obtained from neutron diffraction data as reference. We demonstrate that the effects of the electric field imposed by the crystal environment for ionic systems plays an important role on the location of the mobile hydrogen atom inside strong intramolecular hydrogen bonds for the hydrogen maleate class of compounds (Chapter 9.2). The results obtained here lead to the development of a software (lamaGOET) and a new method, namely the HAR-ELMO method (Chapter 12). Successful applications of Hirshfeld Atom Refinement and HAR-ELMO for systems including transition metal hydrides and proteins are presented for the first time using the developed software and method (Chapter 11 and 12), where quantum crystallography’s powerful tools are tested to obtain properties for molecules which are dependent on accurate determination of their hydrogen atom parameters such as agostic interactions.

The synthesis, characterization, biological activity,and toxicology of sila-ibuprofen, a silicon derivative of the most common nonsteroidal anti-inflammatory drug, is reported. The key improvements compared with ibuprofen are a four times higher solubility in physiological media and a lower melting enthalpy,which are attributed to the carbon−silicon switch. The improved solubility is of interest for postsurgical intravenous administration.A potential for pain relief is rationalized via inhibition experiments of cyclooxygenases I and II (COX-I and COX-II) as well as via a set of newly developed methods that combine molecular dynamics,quantum chemistry, and quantum crystallography. The binding affinity of sila-ibuprofen to COX-I and COX-II is quantified in terms of London dispersion and electrostatic interactions in the active receptor site. This study not only shows the potential of sila-ibuprofen for medicinal application but also improves our understanding of the mechanism of action of the inhibition process.

Resonance-assisted hydrogen bonds (RAHBs) are exploited in chemical synthesis or serve as models for biologically relevant proton-transfer reactions [K. T. Mahmudov and A. J. L. Pombeiro, Chem. – Eur. J., 2016, 22, 16356–16398]. Their properties depend on the position of the hydrogen atom between donor and acceptor atoms. In the intramolecular RAHB of hydrogen maleate salts, this position is strongly influenced by the counter cation and varies from highly asymmetric to perfectly symmetric. This compound class is therefore ideally suited to investigate the factors determining geometry and electron density of intramolecular RAHBs. Here, it is shown that Hirshfeld atom refinements (HARs) of X-ray diffraction data of different hydrogen maleate salts can accurately and precisely match the hydrogen atom positions obtained from neutron diffraction and are independent of the model used for the hydrogen atom displacement parameters. Thus, the simplest, i.e. isotropic, HAR model can be used to locate hydrogen atoms also in bridging positions. In contrast, the determination of electron-density parameters with X-ray constrained wavefunction (XCW) fitting requires anisotropic, highly accurate hydrogen atom displacement parameters.