Charge Transfer via the Dative N−B Bond and Dihydrogen Contacts. Experimental and Theoretical Electron Density Studies of Four Deltahedral Boranes

Synthesis of Boronated Amidines by Addition of Amines to Nitrilium Derivative of Cobalt Bis(Dicarbollide)

A series of novel cobalt bis(dicarbollide) based amidines were synthesized by the nucleophilic addition of primary and secondary amines to highly activated B-N+≡C-R triple bond of the propionitrilium derivative [8-EtC≡N-3,3'-Co(1,2-C2B9H10)(1',2'-C2B9H11)]. The reactions with primary amines result in the formation of mixtures of E and Z isomers of amidines, whereas the reactions with secondary amines lead selectively to the E-isomers. The crystal molecular structures of E-[8-EtC(NMe2)=HN-3,3'-Co(1,2-C2B9H10)(1',2'-C2B9H11)], E-[8-EtC(NEt2)=HN-3,3'-Co(1,2- C2B9H10)(1',2'-C2B9H11)] and E-[8-EtC(NC5H10)=HN-3,3'-Co(1,2-C2B9H10)(1',2'-C2B9H11)] were determined by single crystal X-ray diffraction.

GIAO versus GIPAW: Comparison of Methods To Calculate 11B NMR Shifts of Icosahedral Closo-Heteroboranes toward Boron-Rich Borides

In this work, we perform first-principle density functional theory calculations with the Perdew-Burke-Ernzerhof (PBE) exchange correlation functional to compare the results of the gauge-including atomic orbital (GIAO) method with the gauge-including projector-augmented wave (GIPAW) approach for isotropic ¹¹B nuclear magnetic resonance shifts. GIPAW had been used successfully for the theoretical calculation of nuclear magnetic parameters of ¹¹B species in strong ionic solid-phase compounds such as borates but had been applied very rarely to structures where boron is mainly involved in complex covalent bonding situations, for example, in icosahedra of boron-rich borides. Thus, we investigate the accuracy of both well-known methods and reliability of the effective treatment of core electrons on a test set containing 16 experimentally known closo-(hetero)dodecaboranes. In general, we find very good agreement between GIAO and GIPAW when compared to experimental observations. However, accidental degeneracies of the shift values are better predicted by GIPAW. The optimized molecular geometries on the PBE level agree well with gaseous electron diffraction data and lead to theoretical isotropic chemical ¹¹B shifts with root-mean-square errors of 2.1 and 1.0 ppm depending on the used model of converting absolute shieldings to chemical shifts. The comparison with results from hybrid functionals (B3LYP, B3LYP-D2, and PBE0) shows a minor improvement in accuracy, which is in agreement with ¹³C shifts of sp³-hybridized species. In order to prove the reliability of the conversion parameters obtained by PBE, we report the calculated ¹¹B shifts of 1,2-, 1,7-, and 1,12-PCB₁₀H₁₁ with GIAO and GIPAW to our knowledge for the first time. Additionally, Bader's analysis is carried out on the converged electron density for all boron species within the molecular test set, yielding no simple direct relation between charge and isotropic shifts.

Dihydrogen Bonds in Salts of Boron Cluster Anions [BnHn]2− with Protonated Heterocyclic Organic Bases

Dihydrogen bonds attract much attention as unconventional hydrogen bonds between strong donors of H-bonding and polyhedral (car)borane cages with delocalized charge density. Salts of closo-borate anions [B10H10]2− and [B12H12]2− with protonated organic ligands 2,2’-dipyridylamine (BPA), 1,10-phenanthroline (Phen), and rhodamine 6G (Rh6G) were selectively synthesized to investigate N−H...H−B intermolecular bonding. It was found that the salts contain monoprotonated and/or diprotonated N-containing cations at different ratios. Protonation of the ligands can be implemented in an acidic medium or in water because of hydrolysis of metal cations resulting in the release of H3O+ cations into the reaction solution. Six novel compounds were characterized by X-ray diffraction and FT-IR spectroscopy. It was found that strong dihydrogen bonds manifest themselves in FT-IR spectra that allows one to use this technique even in the absence of crystallographic data.

Electron density: historical remarks with a focus on the Berlin school

The history of electron density (ED) research is surveyed and its historical highlights in the 20th century are summarized. The progress, reached at the beginnig of the 21st century, is described including contributions from the Berlin school. Key experimental advances concern X-ray sources and synchrotron radiation, measuring reflection data at very low temperatures (down to ~10 K), as well as the introduction of area detectors. Methodological progress was reached by new software systems for the refinement and analysis of ED data sets. This allowed to obtain quantitative ED properties, making use of Bader’s Quantum Theory of Atoms in Molecules (QTAIM). More recently methods, which combine experimental X-ray data and quantum-chemically derived wave functions, established the rapidly developing new field of “Quantum Crystallography.”

Ground-State Charge-Density Distribution in a Crystal of the Luminescent ortho-Phenylenediboronic Acid Complex with 8-Hydroxyquinoline

This contribution is devoted to the first electron density studies of a luminescent oxyquinolinato boron complex in the solid state. ortho-Phenylenediboronic acid mixed with 8-hydroxyquinoline in dioxane forms high-quality single crystals via slow solvent evaporation, which allows successful high resolution data collection (sin θ/λ = 1.2 Å^-1) and charge density distribution modeling. Particular attention has been paid to the boron-oxygen fragment connecting the two parts of the complex, and to the solvent species exhibiting anharmonic thermal motion. The experiment and theory compared rather well in terms of atomic charges and volumes, except for the boron centers. Boron atoms, as expected, constitute the most electron-deficient species in the complex molecule, whereas the neighboring oxygen and carbon atoms are the most significantly negatively charged ones. This part of the molecule appears to be very much involved in the charge transfer occurring between the acid fragment and oxyquinoline moiety leading to the observed fluorescence, as supported by the time-dependent density functional theory (TDDFT) results and the generated transition density maps. TDDFT calculations indicated that p-type atomic orbitals contributing to the HOMO-1, HOMO, and LUMO play the major role in the lowest energy transitions, and enabled further comparison with the charge density features, which is discussed in details. Furthermore, the results confirmed the known fact the Q ligand character is most important for the spectroscopic properties of this class of complexes.

Superelectrophilic Behavior of an Anion Demonstrated by the Spontaneous Binding of Noble Gases to [B12 Cl11 ]<sup/>

It is common and chemically intuitive to assign cations electrophilic and anions nucleophilic reactivity, respectively. Herein, we demonstrate a striking violation of this concept: The anion [B₁₂ Cl₁₁ ]^- spontaneously binds to the noble gases (Ngs) xenon and krypton at room temperature in a reaction that is typical of "superelectrophilic" dications. [B₁₂ Cl₁₁ Ng]^- adducts, with Ng binding energies of 80 to 100 kJ mol^-1 , contain B-Ng bonds with a substantial degree of covalent interaction. The electrophilic nature of the [B₁₂ Cl₁₁ ]^- anion is confirmed spectroscopically by the observation of a blue shift of the CO stretching mode in the IR spectrum of [B₁₂ Cl₁₁ CO]^- and theoretically by investigation of its electronic structure. The orientation of the electric field at the reactive site of [B₁₂ Cl₁₁ ]^- results in an energy barrier for the approach of polar molecules and facilitates the formation of Ng adducts that are not detected with reactive cations such as [C₆ H₅ ]⁺ . This introduces the new chemical concept of "dipole-discriminating electrophilic anions."

Fundamental relation between molecular geometry and real-space topology. Combined AIM, ELI-D, and ASF analysis of hapticities and intramolecular hydrogen-hydrogen bonds in zincocene-related compounds

Despite numerous advanced and widely distributed bonding theories such as MO, VB, NBO, AIM, and ELF/ELI-D, complex modes of bonding such as M-Cp*((R)) interactions (hapticities) in asymmetrical metallocenes or weak intramolecular interactions (e.g., hydrogen-hydrogen (H···H) bonds) still remain a challenge for these theories in terms of defining whether or not an atom-atom interaction line (a "chemical bond") should be drawn. In this work the intramolecular Zn-C(Cp*(R)) (R = Me, -(CH2)2NMe2, and -(CH2)3NMe2) and H···H connectivity of a systematic set of 12 zincocene-related compounds is analyzed in terms of AIM and ELI-D topology combined with the recently introduced aspherical stockholder fragment (ASF) surfaces. This computational analysis unravels a distinct dependency of the AIM and ELI-D topology against the molecular geometry for both types of interactions, which confirms and extends earlier findings on smaller sets of compounds. According to these results the complete real-space topology including strong, medium, and weak interactions of very large compounds such as proteins may be reliably predicted by sole inspection of accurately determined molecular geometries, which would on the one hand afford new applications (e.g., accurate estimation of numbers, types, and strengths of intra- and intermolecular interactions) and on the other hand have deep implications on the significance of the method.

Peri-substituted (ace)naphthylphosphinoboranes. (Frustrated) Lewis pairs

The synthesis and molecular structures of 1-(diphenylphosphino)-8-naphthyldimesitylborane (1) and 5-(diphenylphosphino)-6-acenaphthyldimesitylborane (2) are reported. The experimentally determined P-B peri distances of 2.162(2) and 3.050(3) Å allow 1 and 2 to be classified as regular and frustrated Lewis pairs. The electronic characteristics of the (non)bonding P-B contacts are determined by analysis of a set of real-space bonding indicators (RSBIs) derived from the theoretically calculated electron and pair densities. These densities are analyzed utilizing the atoms-in-molecules (AIM), stockholder, and electron-localizability-indicator (ELI-D) space partitioning schemes. The recently introduced mapping of the electron localizability on the ELI-D basin surfaces is also applied. All RSBIs clearly discriminate the bonding P-B contact in 1 from the nonbonding P-B contact in 2, which is due to the fact that the acenaphthene framework is rather rigid, whereas the naphthyl framework shows sufficient conformational flexibility, allowing shorter peri interations. The results are compared to the previously known prototypical phosphinoborane Ph3PB(C6F5)3, which serves as a reference for a bonding P-B interaction. The most prominent features of the nonbonding P-B contact in 2 are the lack of an AIM bond critical point, the unaffected Hirshfeld surfaces of the P and B atomic fragments, and the negligible penetration of the electron population of the ELI-D lone pair basin of the P atom into the AIM B atomic basin.

Advances in understanding of chemical bonding: inputs from experimental and theoretical charge density analysis

The development of charge density analysis has undergone a major renaissance in the last two decades. In recent years, the characterization of bonding features associated with atoms in molecules and in crystals has been explored using high-resolution X-ray diffraction data (laboratory or synchrotron) complemented by high level ab initio theoretical calculations. The extraction of one electron topological properties, namely, electrostatic charges, dipole moment and higher moments, electrostatic potential, electric field gradients, in addition to evaluation of the local kinetic and potential energy densities, have contributed toward an understanding of the electron density distributions in molecular solids. New topological descriptors, namely, the source function (SF) and electron localization function (ELF) provide additional information as regards characterization of the topology of the electron density. In addition, delocalization indices have also been developed to account for bonding features pertinent to M-M bonds. The evaluation of these properties have contributed significantly toward the understanding of intra- and intermolecular bonding features in organic, inorganic, and biomolecules in the crystalline phase, with concomitant applications in the understanding of chemical reactivity and material/biological properties. In recent years, the focus has strongly shifted toward the understanding of structure-property relationships in organometallic complexes containing labile M-C bonds in the crystal structure with subsequent implications in catalysis. This perspective aims to highlight the major developments in electron density measurements in the past few years and provides pointers directed toward the potential use of this technique in future applications for an improved understanding of chemical bonding in systems that have been unexplored.

The halogen bond made visible: experimental charge density of a very short intermolecular Cl···Cl donor-acceptor contact

[ZnCl(2)(3,4,5-trichloropyridine)(2)] features short intermolecular Cl···Cl contacts between halogen atoms of different nature, and a charge density study provides experimental evidence for the accepted model of the halogen bonds: an arene-bonded Cl atom acts as a donor of electron density towards the "sigma hole" of a chlorido ligand attached to a neighbouring Zn(II) cation.

Experimental and theoretical electron-density study of three isoindole derivatives: topological and Hirshfeld surface analysis of weak intermolecular interactions

A combined experimental and theoretical study of three isoindole derivatives was made on the basis of a topological analysis of their electron-density distributions. Experimental electron densities were determined from high-resolution X-ray diffraction data sets measured with synchrotron radiation at 100 K, whereas theoretical calculations were performed using DFT methods at the B3LYP\6-311++G(3df,3pd) level of approximation. Both experimental and theoretical models are in good agreement with each other. Since the analysed structures possess a variety of hydrogen-bonding interactions, weak intermolecular contacts of C-H···C(π), C,N(π)···C,N(π) and H···H types were subject to our special interest and are discussed in detail. They were characterized quantitatively and qualitatively by topological properties using Bader's Atoms in Molecules theory and by mapping the electron-density distribution, electrostatic potential and a geometric function on the Hirshfeld surface. This way the forces and directions of intermolecular interactions as present on the molecular surfaces were depicted and described. These interactions not only guide crystal packing, but are likewise important for recognition processes involving (aza)isoindole fragments in a biological environment.

Reactivity differences between α,β-unsaturated carbonyls and hydrazones investigated by experimental and theoretical electron density and electron localizability analyses

It is still a challenge to predict a compound's reactivity from its ground-state electronic nature although Bader-type topological analyses of the electron density (ED) and electron localizability indicator (ELI) give detailed and useful information on electron concentration and electron-pair localization, respectively. Both ED and ELI can be obtained from theoretical calculations as well as high-resolution X-ray diffraction experiments. Besides ED and ELI descriptors, the delocalization index is used here; it is likewise derived from theoretical calculations as well as from experimental X-ray results, but in the latter case, demonstrated here for the first time. We investigate α,β-unsaturated carbonyl and hydrazone compounds because resonance exhibited by these compounds in the electronic ground-state determines their reactive behavior. The degree of resonance as well as the reactivity contrast are quantified with the electronic descriptors. Moreover, competitive mesomeric substituent effects are studied using the two biologically important compounds acrolein and acrylamide. The reactivity differences predicted from the analyses are in line with the known reactivity of these compounds in organic synthesis. Hence, the capability of the ED and ELI for rationalizing and predicting different and competing substituent effects with respect to reactivity is demonstrated.