Noncovalent Interactions at Lanthanide Complexes

Pairing a Global Optimization Algorithm with EXAFS to Characterize Lanthanide Structure in Solution

Ensemble-average sampling of structures from ab initio molecular dynamics (AIMD) simulations can be used to predict theoretical extended X-ray absorption fine structure (EXAFS) signals that closely match experimental spectra. However, AIMD simulations are time-consuming and resource-intensive, particularly for solvated lanthanide ions, which often form multiple nonrigid geometries with high coordination numbers. To accelerate the characterization of lanthanide structures in solution, we employed the Northwest Potential Energy Surface Search Engine (NWPEsSe), an adaptive-learning global optimization algorithm, to efficiently screen first-shell structures. As case studies, we examine two systems: Eu(NO3)3 dissolved in acetonitrile with a terpyridine ligand (terpyNO2), and Nd(NO3)3 dissolved in acetonitrile. The theoretical spectra for structures identified by NWPEsSe were compared to both experimental and AIMD-derived EXAFS spectra. The NWPEsSe algorithm successfully identified the proper solvation structure for both Eu(NO3)3(terpyNO2) and Nd(NO3)(acetonitrile)3, with the calculated EXAFS signals closely matching the experimental spectra for the Eu-ligand complex and showing good similarity for the Nd salt; the better agreement with the ligand-containing structure is attributed to a less dynamic coordination environment due to the rigid ligand. The key advantage of the global optimization algorithm lies in its ability to sample the coordination environment across the potential energy surface and reduce the time required to identify structures from generally a month to within a week. Additionally, this approach is versatile and can be adapted to characterize main-group metal complexes.

Azobenzene as an Effective Ligand in Europium Chemistry-A Synthetic and Theoretical Study

The preparation and characterization of two novel europium-azobenzene complexes that demonstrate the effectiveness of this ligand for stabilizing reactive, redox-active metals are reported. With the family of rare earth metals receiving attention due to their potential as catalysts, critical components in electronic devices, and, more recently, in biomedical applications, a detailed understanding of factors contributing to their coordination chemistry is of great importance for customizing their stability and reactivity. This study introduces azobenzene as an effective nonprotic ligand system that provides novel insights into rare earth metal coordination preferences, including factors contributing to the coordinative saturation of the large, divalent europium centers. The two compounds demonstrate the impact of the solvent donors (tetrahydrofuran (THF) and dimethoxyethane (DME)) on the overall coordination chemistry of the target compounds. Apart from the side-on coordination of the doubly-reduced azobenzene and the anticipated N-N bond elongation due to decreased bond order, the two compounds demonstrate the propensity of the europium centers towards limited metal-π interactions. The target compounds are available by direct metallation in a straightforward manner with good yields and purity. The compounds demonstrate the utility of the azobenzene ligands, which may function as singly- or doubly-reduced entities in conjunction with redox-active metals. An initial exploration into the computational modeling of these and similar complexes for subsequent property prediction and optimization is performed through a methodological survey of structure reproduction using density functional theory.

Crystal structure and Hirshfeld surface analysis of 8-benzyl-1-[(4-methyl-phen-yl)sulfon-yl]-2,7,8,9-tetra-hydro-1H-3,6:10,13-diep-oxy-1,8-benzodi-aza-cyclo-penta-decine ethanol hemisolvate

The asymmetric unit of the title compound, 2C31H28N2O4S·C2H6O, contains a parent mol-ecule and a half mol-ecule of ethanol solvent. The main compound stabilizes its mol-ecular conformation by forming a ring with an R 1 2(7) motif with the ethanol solvent mol-ecule. In the crystal, mol-ecules are connected by C-H⋯O and O-H⋯O hydrogen bonds, forming a three-dimensional network. In addition, C-H⋯π inter-actions also strengthen the mol-ecular packing.

Halogen bonds, chalcogen bonds, pnictogen bonds, tetrel bonds and other σ-hole interactions: a snapshot of current progress

We report here on the status of research on halogen bonds and other σ-hole interactions involving p-block elements in Lewis acidic roles, such as chalcogen bonds, pnictogen bonds and tetrel bonds. A brief overview of the available literature in this area is provided via a survey of the many review articles that address this field. Our focus has been to collect together most review articles published since 2013 to provide an easy entry into the extensive literature in this area. A snapshot of current research in the area is provided by an introduction to the virtual special issue compiled in this journal, comprising 11 articles and entitled `Halogen, chalcogen, pnictogen and tetrel bonds: structural chemistry and beyond.'

A molecular descriptor of intramolecular noncovalent interaction for regulating optoelectronic properties of organic semiconductors

In recent years, intramolecular noncovalent interaction has become an important means to modulate the optoelectronic performances of organic/polymeric semiconductors. However, it lacks a deep understanding and a direct quantitative relationship among the molecular geometric structure, strength of noncovalent interaction, and optoelectronic properties in organic/polymeric semiconductors. Herein, upon systematical theoretical calculations on 56 molecules with and without noncovalent interactions (X···Y, X = O, S, Se, Te; Y = C, F, O, S, Cl), we reveal the essence of the interactions and the dependence of its strength on the molecular geometry. Importantly, a descriptor S is established as a function of several basic geometric parameters to well characterize the noncovalent interaction energy, which exhibits a good inverse correlation with the reorganization energies of the photo-excited states or electron-pumped charged states in organic/polymeric semiconductors. In particular, the experimental ¹H, ⁷⁷Se, and ¹²⁵Te NMR, the optical absorption and emission spectra, and single crystal structures of eight compounds fully confirm the theoretical predictions. This work provides a simple descriptor to characterize the strength of noncovalent intramolecular interactions, which is significant for molecular design and property prediction.

Crystal structure and Hirshfeld surface analysis of (22 RS,23 SR,25 RS,26 SR)-23,25,5-trimethyl-21-(2,2,2-tri-fluoro-acet-yl)-5-aza-2(2,6)-piperidina-1,3(2,5)-di-furana-cyclo-hexa-phan-24-one

The title compound, C20H21F3N2O4, features a main twelve-membered difuryl ring with which the furan rings make dihedral angles of 76.14 (5) and 33.81 (5)°. The dihedral angle between the furan rings is 42.55 (7)°. The six-membered nitro-gen heterocycle has a twist-boat conformation. In the crystal, pairs of mol-ecules are connected by inter-molecular C-H⋯O inter-actions, generating an R 2 2(14) ring motif. These pairs of mol-ecules form zigzag chains along the a-axis direction by means of C-H⋯F inter-actions. Furthermore, C-H⋯π and C-F⋯π inter-actions link the mol-ecules into chains along the b-axis direction, forming sheets parallel to the (001) plane. These sheets are also connected by van der Waals inter-actions.

Crystal structure and Hirshfeld surface analysis of 2-(4-amino-6-phenyl-1,2,5,6-tetra-hydro-1,3,5-triazin-2-yl-idene)malono-nitrile di-methyl-formamide hemisolvate

The title compound, 2C12H10N6·C3H7NO, crystallizes as a racemate in the monoclinic P21/c space group with two independent mol-ecules (I and II) and one di-methyl-formamide solvent mol-ecule in the asymmetric unit. Both mol-ecules (I and II) have chiral centers at the carbon atoms where the triazine rings of mol-ecules I and II are attached to the phenyl ring. In the crystal, mol-ecules I and II are linked by inter-molecular N-H⋯N, N-H⋯O and C-H⋯N hydrogen bonds through the solvent di-methyl-formamide mol-ecule into layers parallel to (001). In addition, C-H⋯π inter-actions also connect adjacent mol-ecules into layers parallel to (001). The stability of the mol-ecular packing is ensured by van der Waals inter-actions between the layers. The Hirshfeld surface analysis indicates that N⋯H/H⋯N (38.3% for I; 35.0% for II), H⋯H (28.2% for I; 27.0% for II) and C⋯H/H⋯C (23.4% for I; 26.3% for II) inter-actions are the most significant contributors to the crystal packing.

Crystal structure and Hirshfeld surface analysis of a new polymorph of (E)-2-(4-bromo-phen-yl)-1-[2,2-di-bromo-1-(3-nitro-phen-yl)ethen-yl]diazene

A new polymorph of the title compound, C14H8Br3N3O2, (form-2) was obtained in the same manner as the previously reported form-1 [Akkurt et al. (2022 ▸). Acta Cryst. E78, 732-736]. The structure of the new polymorph is stabilized by a C-H⋯O hydrogen bond that links mol-ecules into chains. These chains are linked by face-to-face π-π stacking inter-actions, resulting in a layered structure. Short inter-mol-ecular Br⋯O contacts and van der Waals inter-actions between the layers aid in the cohesion of the crystal packing. In the previously reported form-1, C-H⋯Br inter-actions connect mol-ecules into zigzag chains, which are linked by C-Br⋯π inter-actions into layers, whereas the van der Waals inter-actions between the layers stabilize the crystal packing of form-2. Hirshfeld mol-ecular surface analysis was used to compare the inter-molecular inter-actions of the polymorphs.

Crystal structure and Hirshfeld surface analysis of (E)-2-(4-bromo-phen-yl)-1-[2,2-di-bromo-1-(4-nitro-phen-yl)ethen-yl]diazene

The mol-ecule of the title compound, C14H8Br3N3O2, consists of three almost planar groups: the central di-bromo-ethenyldiazene fragment and two attached aromatic rings. The mean planes of these rings form dihedral angles with the plane of the central fragment of 26.35 (15) and 72.57 (14)° for bromine- and nitro-substituted rings, respectively. In the crystal, C-H⋯Br inter-actions connect mol-ecules, generating zigzag C(8) chains along the [100] direction. These chains are linked by C-Br⋯π inter-actions into layers parallel to (001). van der Waals inter-actions between the layers aid in the cohesion of the crystal packing. The most substantial contributions to crystal packing, according to a Hirshfeld surface analysis, are from Br⋯H/H⋯Br (20.9%), C⋯H/H⋯C (15.2%), O⋯H/H⋯O (12.6%) and H⋯H (11.7%) contacts.

The Stibium Bond or the Antimony-Centered Pnictogen Bond: The Covalently Bound Antimony Atom in Molecular Entities in Crystal Lattices as a Pnictogen Bond Donor

A stibium bond, i.e., a non-covalent interaction formed by covalently or coordinately bound antimony, occurs in chemical systems when there is evidence of a net attractive interaction between the electrophilic region associated with an antimony atom and a nucleophile in another, or the same molecular entity. This is a pnictogen bond and are likely formed by the elements of the pnictogen family, Group 15, of the periodic table, and is an inter- or intra-molecular non-covalent interaction. This overview describes a set of illustrative crystal systems that were stabilized (at least partially) by means of stibium bonds, together with other non-covalent interactions (such as hydrogen bonds and halogen bonds), retrieved from either the Cambridge Structure Database (CSD) or the Inorganic Crystal Structure Database (ICSD). We demonstrate that these databases contain hundreds of crystal structures of various dimensions in which covalently or coordinately bound antimony atoms in molecular entities feature positive sites that productively interact with various Lewis bases containing O, N, F, Cl, Br, and I atoms in the same or different molecular entities, leading to the formation of stibium bonds, and hence, being partially responsible for the stability of the crystals. The geometric features, pro-molecular charge density isosurface topologies, and extrema of the molecular electrostatic potential model were collectively examined in some instances to illustrate the presence of Sb-centered pnictogen bonding in the representative crystal systems considered.

Crystal structure and Hirshfeld surface analysis of 2,2,2-tri-fluoro-1-(7-methyl-imidazo[1,2-a]pyridin-3-yl)ethan-1-one

The bicyclic imidazo[1,2-a]pyridine core in the mol-ecule of the title compound, C10H7F3N2O, is planar within 0.004 (1) Å. In the crystal, the mol-ecules are linked by pairs of C-H⋯N and C-H⋯O hydrogen bonds, forming strips. These strips are connected by the F⋯F contacts into layers, which are further joined by π-π stacking inter-actions. The Hirshfeld surface analysis and fingerprint plots reveal that mol-ecular packing is governed by F⋯H/H⋯F (31.6%), H⋯H (16.8%), C⋯H/H⋯C (13.8%) and O⋯H/H⋯O (8.5%) contacts.