Substituent effects in π-hole regium bonding interactions between Au(p-X-Py)2 complexes and Lewis bases: an ab initio study

For the first time, long range substituent effects in regium bonding interactions involving Au(I) linear complexes are investigated. The Au(I) atom is coordinated to two para -substituted pyridine ligands. The interaction energy (RI-MP2/def2-TZVP level of theory) of the π-hole regium bonding assemblies is affected by the pyridine substitution. The Hammett's plot representations for several sets of Lewis bases have been carried out and, in all cases, good regression plots have been obtained (interaction energies vs. Hammett's σ parameter). The Bader's theory of "atoms-in-molecules" has been used to evidence that the electron density computed at the bond critical point that connects the Au-atom to the electron donor can be used as a measure of bond order in regium bonding. Several X-ray structures retrieved from the Cambridge Structural Database (CSD) provide some experimental support to the existence of regium π-hole bonding in [Au(Py) 2 ] + derivatives.

Glutamate carboxypeptidase II as a model system for designing host-guest units: a theoretical approach

Through the combination of X-ray structure analysis and density functional theory (DFT) theoretical calculations at the B3LYP-D3/def2-TZVP level of theory, we designed and investigated two novel host units for the recognition of neutral (e.g. halobenzenes) and charged (e.g. chloride and nitrate) guests inspired by a glutamate carboxypeptidase II (GCPII) system and its inhibitors.

Influence of 2-Amino-4-methylpyridine and 2-Aminopyrimidine Ligands on the Malonic Acid-Cu(II) System: Insights through Supramolecular Interactions and Photoresponse Properties

Two Cu(II)-malonate complexes with 2-amino-4-methylpyridine (complex 1) and 2-aminopyrimidine (complex 2) auxiliary ligands were synthesized, and their single-crystal X-ray diffraction structures were established. Change in the auxiliary ligand exhibits substantial structural variation in the present complexes. Complex 1 shows a one-dimensional anionic copper-malonate moiety connected by the malonate bridge, whereas complex 2 is a mononuclear one. For both the complexes, auxiliary ligands are attached with the Cu-malonate moiety through various noncovalent interactions. Optical band gap, electrical conductivity, and photosensitivity of complexes 1 and 2 were measured, but the values of electrical parameters of the complexes significantly differ from each other. However, the magnitudes of electrical parameters increase several times for both the complexes when they are exposed under visible light, though the values of light sensing parameters of complex 1 were found to be higher than those of complex 2. Density functional theory calculations for complex 1 were carried out to support the experimental result.

Supramolecular assemblies involving salt bridges: DFT and X-ray evidence of bipolarity

Three new aminopyridinium/4,4′-oxydibenzoate salts have been synthesized and structurally characterized. A common feature of these compounds is the formation of antiparallel π-stacked salt bridges.

New 1D diorganotin(iv) dithiolate coordination polymers: crystallographic, computational, Hirshfeld surface and thermal analyses

Three new 1D coordination polymers of diorganotin(iv) dithiolates synthesized and the nature of their weak interactions addressed using computational techniques.

What is the driving force behind molecular triangles and their guests? A quantum chemical perspective about host–guest interactions

The physical nature of host–guest interactions occurring between molecular triangles and linear anions was explored using DFT calculations combined with energy decomposition analyses, nuclear independent chemical shift, and non-covalent interactions.

Tuning Stacking Interactions between Asp-Arg Salt Bridges and Heterocyclic Drug Fragments

Stacking interactions can play an integral role in the strength and selectivity of protein-drug binding and are of particular interest given the ubiquity and variety of heterocyclic fragments in drugs. In addition to traditional stacking interactions between aromatic rings, stacking interactions involving heterocyclic drug fragments and protein salt bridges have also been observed. These "salt-bridge stacking interactions" can be quite strong but are not well understood. We studied stacked dimers of the acetate···guanidinium ion pair with a diverse set of 63 heterocycles using robust ab initio methods. The computed interaction energies span more than 10 kcal mol^-1, indicating the sensitivity of these salt-bridge stacking interactions to heterocycle features. Trends in both the strength and preferred geometry of these interactions can be understood through analyses of the electrostatic potentials and electric fields above the heterocycles. We have developed new heterocycle descriptors that quantify these effects and used them to create robust predictors of the strength of salt-bridge stacking interactions both in the gas phase and a protein-like dielectric environment. These predictive tools, combined with a set of qualitative guidelines, should facilitate the choice of heterocycles that maximize salt-bridge stacking interactions in drug binding sites.

The Importance of CH···X (X = O, π) Interaction of a New Mixed Ligand Cu(II) Coordination Polymer: Structure, Hirshfeld Surface and Theoretical Studies

In this study, a new equimolar (1:1:1) mixed ligand Cu(II) polymer, [Cu(IDA)(ImP)]n (1) with iminodiacetato (IDA) and imidazo[1,2-a]-pyridine (ImP) was synthesized and characterized by single crystal X-ray diffraction analysis. X-ray crystallography reveals that compound (1) consists of polymeric zigzag chain along [010] the carboxylate carbonyl oxygen atom by two-fold symmetry screw axis. The solid-state structure is stabilized through C–H···O hydrogen bonds and C–H···π interactions that lead the molecules to generate two-dimensional supramolecular assemblies. The intricate combinations of hydrogen bonds and C–H···π interactions are fully described along with computational studies. A thorough analysis of Hirshfeld surface and fingerprint plots elegantly quantify the interactions involved within the structure. The binding energies associated with the noncovalent interactions observed in the crystal structure and the interplay between them were calculated using theoretical DFT calculations. Weak noncovalent interactions were analyzed and characterized using Bader’s theory of ‘‘atoms-in-molecules’’ (AIM). Finally, the solid-state supramolecular assembly was characterized by the “Noncovalent Interaction” (NCI) plot index.

Screening polymorphism in a Ni(ii) metal–organic framework: experimental observations, Hirshfeld surface analyses and DFT studies

A new polymorphic form of diaquaiminodiacetatonickel(ii) is structurally characterized by a detailed analysis of Hirshfeld surface and further analysed by DFT and QTAIM calculations.

Bipolar behaviour of salt-bridges: a combined theoretical and crystallographic study

In this manuscript, we study the bipolar behaviour of salt-bridges by combining theoretical calculations with an X-ray crystallographic study of succinate and aminopyridinium salts.

Quantifying conventional C–H⋯π(aryl) and unconventional C–H⋯π(chelate) interactions in dinuclear Cu(ii) complexes: experimental observations, Hirshfeld surface and theoretical DFT study

Conventional C–H⋯π(aryl) and unconventional C–H⋯π(chelate) interactions are explored in detail.

Pb⋯X (X = N, S, I) tetrel bonding interactions in Pb(ii) complexes: X-ray characterization, Hirshfeld surfaces and DFT calculations

We report the synthesis and X-ray characterization of four new Pb(ii) complexes of nicotinoylhydrazone and picolinoylhydrazone-based ligands and three different anionic co-ligands (acetate, thiocyanate and iodide) exhibiting relevant tetrel bonding interactions.

Structural and energetic study of cation–π–cation interactions in proteins

Statistical and energetic analysis of cation–π–cation motifs in protein structures suggests a potential stabilizing role in the protein fold.

Complexation of different transition metals with 4,4'-dimethyl-2,2'-bipyridine: Crystal structure, UV spectra and Hirshfeld surfaces

Four new complexes, [Co(dmbpy)2(dca)2]·CH3OH (1), [Ni(dmbpy)2(dca)2]·CH3OH (2), [Zn(dmbpy)2(dca)2]·(3) and [Cu(dmbpy)2(OH)2]·5H2O (4) (dca=dicyanamide), derived from 4,4'-dimethyl-2,2'-bipyridine (dmbpy) have been synthesized and characterized by elemental analysis, TGA and single-crystal X-ray diffraction. Crystal structures and Hirshfeld surfaces analysis revealed that the complexes 1-3 were mainly supported by OH⋯N, CH⋯N and π⋯π intermolecular interactions, and for complex 4, the uncoordinated water molecules play a key role in the construction of the 3D stacking motif. UV spectrum measurements demonstrate that all of the complexes show typical metal to ligand charge transfer (MLCT) absorption bands between 301 and 306nm. Moreover, after complexation, the absorption maximum bands about intraligand π→π* transitions similarly show slightly red shift compared to dmbpy ligand, consisting with the DFT calculations.

Unorthodox Interactions at Work

This Perspective elaborates on the currently unfolding interest in integrating unorthodox non-covalent interactions into functional systems. Initial emphasis is on anion-π interactions at work, particularly in catalysis. Recent highlights are described in comparison to a coinciding renaissance of the more conventional, charge-inverted cation-π catalysis. Progress with these complementary aromatic systems is then compared to recent efforts to integrate halogen and chalcogen bonds, the unorthodox counterparts of hydrogen bonds, into functional systems. General focus is on catalysis, pertinent examples on self-assembly, transport, sensing, and templation are covered as well.

Experimental investigation of anion–π interactions – applications and biochemical relevance

Anion–π interactions, intuitively repulsive forces, turned from controversial to a well-established non-covalent interaction over the past quarter of a century.

Combined analysis of chemical bonding in a CuIIdimer using QTAIM, Voronoi tessellation and Hirshfeld surface approaches

Interaction of 1-(1H-pyrazol-5-yl)ethanone oxime (H2PzOx) with copper(II) chloride in the presence of pyridine afforded a binuclear discrete [Cu2(HPzOx)2Cl2py2] complex, which was characterized by Fourier transform–IR and electron paramagnetic resonance (EPR) spectra, magnetochemistry and high-resolution X-ray diffraction experiments. Multipole refinement of X-ray diffraction data and density-functional theory (DFT) calculations of an isolated molecule allowed charge and spin distributions to be obtained for this compound. Magnetochemistry data, EPR spectra and DFT calculations of an isolated molecule show antiferromagnetic coupling between copper(II) ions. The spin distribution suggests an exchange pathwayviathe bridging pyrazole ring in the equatorial plane of the CuN4Cl coordination polyhedron, thus providing support for the classical superexchange mechanism; the calculated value of the magnetic coupling constant −2Jis equal to 220 cm−1, which compares well with the experimental value of 203 ± 2 cm−1. Chemical connectivity was derived by Bader's `quantum theory of atoms in molecules' and compared with Voronoi tessellation and Hirshfeld surface representations of crystal space. All methodologies gave a similar qualitative and semi-quantitative description of intra- and intermolecular connectivity.

Exploration of CH⋯π interactions involving the π-system of pseudohalide coligands in metal complexes of a Schiff-base ligand

The role of non-covalent interactions in the self-assembly of Schiff-base complexes of Zn^II, Cu^II and Ni^II has been investigated experimentally and theoretically with especial attention to unconventional C–H⋯π interactions involving pseudohalide coligands.

New Ni(ii) 1,2-bis(diphenylphosphino)ethane dithiolates: crystallographic, computational and Hirshfeld surface analyses

Three new Ni(ii) dppe dithiolates have been synthesized and the nature of weak interactions have been addressed using computational and Hirshfeld surface analyses.

Intermolecular π/π and H/π interactions in dimers researched by different computational methods

The analysis of π/π and H /π interactions in complexes are a challenging aspect of theoretical research. Due to the different approximations of different levels of theory, results tend to be inconsistent. We compared the reliabilities of HF, SVWN, M06L, PW91, BLYP, B3LYP, BHandHLYP, B97D, MP2, and DFTB-D approaches in researching π/π and H /π interactions by calculating the binding energies of five benzene-containing dimers. The effects of 6-31+G**, 6-311++G** and 6-311++G(2df,2p) basis sets on the results were analyzed too. We found that the DFTB-D and B97D methods combined with the 6-311++G** basis set perform well for dimers that contain π/π and H /π interactions. With high efficiency and satisfactory precision, DFTB-D is helpful for the calculation of complexes containing π/π and H /π stacking. We further calculated the structures and properties of phenylalanine-containing dimers using the DFTB-D and B97D methods. The properties of low energy conformers such as rotational constants, dipole moments and molecular orbitals were also analyzed. These data should be helpful for research into systems that contain π/π and H /π stacking.

Effect of non-covalent interaction on the diastereoselective self-assembly of Cu(ii) complexes containing a racemic Schiff base in a chiral self-discriminating process

The effects of π⋯π interactions between benzene–pyridine rings on the diastereoselective self-assembly of Cu(ii) compounds in the chiral self-discrimination process are studied.

On the importance of unprecedented lone pair-salt bridge interactions in Cu(II)-malonate-2-amino-5-chloropyridine-perchlorate ternary system

A Cu(II)-malonate complex with formula {(C5H6N2Cl)12[Cu(1)(C3H2O4)2][Cu(2)(C3H2O4)2(H2O)2][Cu(4)(C3H2O4)2][Cu(3)(C3H2O4)2(H2O)2](ClO4)4}n (1) [C5H6N2Cl = protonated 2-amino-5-chloropyridine, C3H4O4 = malonic acid, ClO4(-) = perchlorate] has been synthesized from purely aqueous media simple by mixing the reactants in their stoichiometric ratio, and its crystal structure has been determined by single-crystal X-ray diffraction. In 1, copper(II) malonate units form infinite 1D polymeric chains, which are interlinked by hydrogen bonds to generate 2D sheets. These 2D sheets are joined side by side primarily by various hydrogen bonds to form a 3D structure. A multitude of salt bridges are formed in this structure, connecting the protonated 2-amino-5-chloropyridines and the malonate ligands of the polymeric polyanion. Examining this characteristic of the solid-state architecture, we noticed several salt-bridge (sb)···π interactions and an unexplored interaction between the lone pair (lp) of one malonate oxygen atom and a planar salt bridge. The combination of this interaction with various other weak intermolecular forces results in a remarkably extended supramolecular network combining a wide variety of interactions involving π-systems (Cl···π, π···π) and salt bridges (sb···π and lp···sb). We describe the energetic and geometric features of this lone pair-salt-bridge interaction and explore its impact on the resultant supramolecular organization using theoretical DFT-D3 calculations.

The stabilization effect of dielectric constant and acidic amino acids on arginine-arginine (Arg-Arg) pairings: database survey and computational studies

Database survey in this study revealed that about one-third of the protein structures deposited in the Protein Data Bank (PDB) contain arginine-arginine (Arg-Arg) pairing with a carbon···carbon (CZ···CZ) interaction distance less than 5 Å. All the Arg-Arg pairings were found to bury in a polar environment composed of acidic residues, water molecules, and strong polarizable or negatively charged moieties from binding site or bound ligand. Most of the Arg-Arg pairings are solvent exposed and 68.3% Arg-Arg pairings are stabilized by acidic residues, forming Arg-Arg-Asp/Glu clusters. Density functional theory (DFT) was then employed to study the effect of environment on the pairing structures. It was revealed that Arg-Arg pairings become thermodynamically stable (about -1 kcal/mol) as the dielectric constant increases to 46.8 (DMSO), in good agreement with the results of the PDB survey. DFT calculations also demonstrated that perpendicular Arg-Arg pairing structures are favorable in low dielectric constant environment, while in high dielectric constant environment parallel structures are favorable. Additionally, the acidic residues can stabilize the Arg-Arg pairing structures to a large degree. Energy decomposition analysis of Arg-Arg pairings and Arg-Arg-Asp/Glu clusters showed that both solvation and electrostatic energies contribute significantly to their stability. The results reported herein should be very helpful for understanding Arg-Arg pairing and its application in drug design.