Regium-π vs Cation-π Interactions in M2 and MCl (M = Cu, Ag and Au) Complexes with Small Aromatic Systems: An ab Initio Study

Wolfium bonds in homodimers of MX4Y (M = Mo, W; X = F, Cl, Br; Y = O, S, Se)

The term "wolfium bond" has been recently introduced to describe the noncovalent attraction between an atom of group 6 and a nucleophile via a σ-hole binding site. Crystal structures commonly contain a motif wherein two MX4Y units are arranged in close proximity, where M represents either Mo or W, and X and Y refer to halogen and chalcogen atoms respectively. DFT calculations were thus applied to a wide range of homodimers of these molecules so as to assess their preferred arrangements, and to characterize the types of bonding that are present in each in a systematic manner. The most stable Dual-X configuration is symmetric and contains a pair of equivalent M⋯X bonds. The interaction energies range from -8 to -29 kcal mol-1, and are largest for X = F, Y = O, and M = W. The X electron donor is replaced by Y, and the two wolfium bonds are reduced to one, in the less stable Mono-Y structure, with interaction energies between -2 and -10 kcal mol-1. There is some question as to whether the weaker bonds of this type constitute true wolfium bonds.

Gold(III) derivatives as the noncovalent interaction donors: theoretical study of the π-hole regium bonds

Except for the well-known σ-hole regium bonds formed by metal nanoparticles and M(I) (M = Cu, Ag, and Au) derivatives, the existence of π-hole regions located above and below the Au atom in gold(III) derivatives suggests that gold(III) also functions as an efficient electrophilic site. In this study, a comprehensive analysis was conducted on the electrophilicity of trichloro-(p-toluonitrilo-N)-gold(III) derivatives AuL3(NCC6H4X) (L = Cl, Br, CN; X = NH2, CH3, CF3, NC, and CN) and the nature of π-hole regium bonds in the AuL3(NCC6H4X)⋯LB (LB = NH3, N(NH3)3, CH2O, C2H2, C2H4, C6H6) and (AuCl3(NCC6H4Y))n (Y = Cl, CN, NC, NO2; n = 2, 3)) complexes. The characteristics of the π-hole regium bonds were studied with respect to the influence of ligands and substituents, the strength of intermolecular interactions between Au(III) derivatives and Lewis bases, and those in the polymers. In the case of the AuL3(NCC6H4X)⋯NH3 complexes, the strength of the regium bonds increases gradually in the order of L = Cl < Br < CN and X = NH2 < CH3 < CF3 ≈ NC < CN. The ligands (L) attached to the Au atom exert a significant effect on the strength of the π-hole regium bonds in comparison to the substituents (X) on the benzene ring. The regium bonds are primarily dominated by electrostatic interaction, accompanied by moderate contribution from polarization. Linear relationships were identified between the electrostatic energies and the local most positive potentials over the Au atom, as well as between the polarization energies and the amount of charge transfer. Most of the π-hole regium bonds in the AuL3(NCC6H4X)⋯LB complexes exhibit the characters of closed shell noncovalent interactions. In the polymers (AuCl3(NCC6H4Y))n, weak face-to-face π-π stacking interactions are also present, in addition to regium bonds. The trimers displayed a slightly negative cooperativity in comparison to the dimers.

Ligand and Substituent Effect on Regium-π Bonding in Cu and Ag π-Conjugated Complexes: A Density Functional Study

Density functional theory investigation of regium (Rg)-π bonding using the RgL-X model system, where Rg = Cu and Ag; L = CN, NO2, and OH; X = π-conjugated system (benzene, cyanobenzene, benzoic acid, pyridine, 2-methoxy aniline, 1,4-dimethoxy benzene, and cyclophane), has been performed. Conclusive evidence of the Rg-π bond has been provided by analysis of molecular electrostatic potential surfaces, Rg-π bond length, interaction energy (ΔE), second-order perturbation energy (E2), charge transfer (Δq), quantum theory of atom in molecules, and noncovalent interaction plots for 42 structural arrangements with varying ligands and the substituted aromatic ring. The Rg-π bond length in the optimized model systems varies from 2.03 to 2.12 Å in Cu complexes (1-21) and from 2.26 to 2.38 Å in Ag complexes (22-42) at the PBE0-D3 functional. While the ligand (L) attached to the Rg metal has a bargaining effect on the strength of the Rg-π bond (in the order of -OH > -CN = -NO2), the π-conjugated systems have a diminutive effect. Two X-ray crystal structures (CUCSOI and AHIDQU) having the Rg-π bond, accessed from Cambridge Crystallographic Data Centre (CCDC), are discussed here to signify the influence of Rg-π bonding on the crystal structure.

Why much of Chemistry may be indisputably non-bonded?

In this compendium, the wide scope of all intermolecular interactions ever known has been revisited, in particular giving emphasis the capability of much of the elements of the periodic table to form non-covalent contacts. Either hydrogen bonds, dihydrogen bonds, halogen bonds, pnictogen bonds, chalcogen bonds, triel bonds, tetrel bonds, regium bonds, spodium bonds or even the aerogen bond interactions may be cited. Obviously that experimental techniques have been used in some works, but it was through the theoretical methods that these interactions were validate, wherein the QTAIM integrations and SAPT energy partitions have been useful in this regard. Therefore, the great goal concerns to elucidate the interaction strength and if the intermolecular system shall be total, partial or non-covalently bonded, wherein this last one encompasses the most majority of the intermolecular interactions what leading to affirm that chemistry is debatably non-bonded.

Importance of Cu and Ag regium-π bonds in supramolecular chemistry and biology: a combined crystallographic and ab initio study

Identifying and characterizing new binding events between electron donor and acceptor counterparts represents a crucial step to complete the molecular recognition and aggregation picture, which is key to chemistry and biology. In this study we interrogated both the PDB (Protein Data Bank) and CSD (Cambridge Structural Database) for the presence of Cu and Ag regium-π (Rg-π) bonds (an attractive noncovalent force between elements from group 11 and π-systems). Concretely, we found evidence of the plausible biological role of the interaction in protein-DNA systems, bacterial Ag extrusion processes and Heme group redox functionality. Furthermore, we also highlighted the implications of Rg-π bonds in the crystal packing of two host-guest systems, where this interaction is key for the binding and recognition of small organic molecules as well as for the encapsulation of organometallic complexes. Theoretical models were used to analyse the strength of the interaction (RI-MP2/def2-TZVP level of theory) together with QTAIM (Quantum Theory of Atoms in Molecules), NBO (Natural Bonding Orbital) and NCIplot (Non Covalent Interactions plot) analyses, which further assisted in the characterization of the regium-π interactions described herein. We expect the results from this study will be useful to attract the attention of chemical biologists as well as to expand the potential of the interaction to the supramolecular chemistry and crystal engineering communities.

Pyridine interaction with γ-CuI: synergy between molecular dynamics and molecular orbital approaches to molecule/surface interactions

We have used a synergistic computational approach merging Molecular Dynamics (MD) simulations with density functional theory (DFT) to investigate the mechanistic aspects of chemisorption of pyridine (Py) molecules on copper iodide. The presence of both positive and negative ions at the metal halide surface presents a chemical environment in which pyridine molecules may act as charge donors and/or acceptors. Computational results reveal that Py molecules interact with the γ-CuI(111) surface owing to a combination of noncovalent Cu⋯N, Cu/I⋯π/π*, and hydrogen bonding interactions as determined via Natural Bonding Orbitals (NBO). Introduction of surface defect sites alters the interaction dynamics, resulting in a "localizing effect" in which the Py molecules clump together within the defect site. Significant enhancement of hydrogen bonding between C-H σ* and I 6p orbitals results in more tightly surface-bound Py molecules. Our findings provide a platform for understanding the interaction between Py and Py-derivative vapors and metal-based surfaces that contain both electron acceptor and donor atoms.

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.

Theoretical investigation on the nature of substituted benzene⋯AuX interactions: covalent or noncovalent?

The nature of interactions between AuX (X = F, Cl, Br, CN, NO2, CH3) and aromatic moieties with different electronic properties has been investigated for possible tuning of coinage–metal bonds by varying the substituents.

Substituent effects on the regium-π stacking interactions between Au6 cluster and substituted benzene

The regium-π stacking interactions in the Au₆···PhX (X = H, CH₃, OH, OCH₃, NH₂, F, Cl, Br, CN, NO₂) complexes are studied using quantum chemical methods. The present study focuses on the different effects of electron-donating and electron-withdrawing substituent. The structure and binding strength of the complexes are examined. The interactions between Au₆ cluster and various substituted benzene become strengthened relative to the Au₆···benzene complex. The interaction region indicator analysis was performed, and the interaction region and interaction between the substituent and Au₆ cluster are discussed. It is found that the substituent effects on the regium-π stacking interactions between Au₆ cluster and substituted benzene are different from π···π interactions of benzene dimer. Energy decomposition analysis was carried out to study the nature of regium-π stacking interactions, and the substituent effects are mainly reflected on the electrostatic interaction and dispersion.

σ-Hole and σ-lump interactions between gold clusters Aun (n = 2-8) and benzene

In this study, the non-covalent interactions between gold cluster and benzene have been evaluated at the PBE0-D3/def2-TZVP level of theory. Gold clusters Au_n (n = 2-8) were used as σ-hole and σ-lump donors, and benzene was the corresponding electron-donating and -accepting molecule. The molecular electrostatic potential of Au clusters was analyzed, and the optimized structures and interaction energies of the Au_n (n = 2-8) Bz complexes with σ-hole or σ-lump interaction were studied. Strong σ-hole and relative weak σ-lump interactions exist between Au cluster and benzene. With the help of atoms-in-molecules analysis and plotting of non-covalent interaction map, the interaction zones of the complexes were found out. The nature of these interactions was revealed through energy decomposition analysis by using the symmetry-adapted perturbation theory. σ-Hole interactions are dominated by electrostatic interaction, while σ-lump interactions are mainly driven by dispersion. This study can enrich the knowledge of interaction between Au cluster and π-systems and design of new materials based on coinage metal of σ-hole and σ-lump interactions.

Study of Beryllium, Magnesium, and Spodium Bonds to Carbenes and Carbodiphosphoranes

The aim of this article is to present results of theoretical study on the properties of C⋯M bonds, where C is either a carbene or carbodiphosphorane carbon atom and M is an acidic center of MX2 (M = Be, Mg, Zn). Due to the rarity of theoretical data regarding the C⋯Zn bond (i.e., the zinc bond), the main focus is placed on comparing the characteristics of this interaction with C⋯Be (beryllium bond) and C⋯Mg (magnesium bond). For this purpose, theoretical studies (ωB97X-D/6-311++G(2df,2p)) have been performed for a large group of dimers formed by MX2 (X = H, F, Cl, Br, Me) and either a carbene ((NH2)2C, imidazol-2-ylidene, imidazolidin-2-ylidene, tetrahydropyrymid-2-ylidene, cyclopropenylidene) or carbodiphosphorane ((PH3)2C, (NH3)2C) molecule. The investigated dimers are characterized by a very strong charge transfer effect from either the carbene or carbodiphosphorane molecule to the MX2 one. This may even be over six times as strong as in the water dimer. According to the QTAIM and NCI method, the zinc bond is not very different than the beryllium bond, with both featuring a significant covalent contribution. However, the zinc bond should be definitely stronger if delocalization index is considered.

Interaction between Trinuclear Regium Complexes of Pyrazolate and Anions, a Computational Study

The geometry, energy and electron density properties of the 1:1, 1:2 and 1:3 complexes between cyclic (Py-M)3 (M = Au, Ag and Cu) and halide ions (F-, Cl- and Br-) were studied using Møller Plesset (MP2) computational methods. Three different configurations were explored. In two of them, the anions interact with the metal atoms in planar and apical dispositions, while in the last configuration, the anions interact with the CH(4) group of the pyrazole. The energetic results for the 1:2 and 1:3 complexes are a combination of the specific strength of the interaction plus a repulsive component due to the charge:charge coulombic term. However, stable minima structures with dissociation barriers for the anions indicate that those complexes are stable and (Py-M)3 can hold up to three anions simultaneously. A search in the CSD confirmed the presence of (Pyrazole-Cu)3 systems with two anions interacting in apical disposition.

Rivalry between Regium and Hydrogen Bonds Established within Diatomic Coinage Molecules and Lewis Acids/Bases

A theoretical study of the complexes formed by Ag₂ and Cu₂ with different molecules, XH (FH, ClH, OH₂ , SH₂ , HCN, HNC, HCCH, NH₃ and PH₃ ) that can act as hydrogen-bond donors (Lewis acids) or regium-bond acceptors (Lewis bases) was carried out at the CCSD(T)/CBS computational level. The heteronuclear diatomic coinage molecules (AuAg, AuCu, and AgCu) have also been considered. With the exception of some of the hydrogen-bonded complexes with FH, the regium-bonded binary complexes are more stable. The AuAg and AuCu molecules show large dipole moments that weaken the regium bond (RB) with Au and favour those through the Ag and Cu atoms, respectively.

Synergistic and Diminutive Effects between Regium and Aerogen Bonds

The aerogen bond is formed in complexes of HCN-XeF₂ O and C₂ H₄ -XeF₂ O. The lone pair on the N atom of HCN is a better electron donor in the aerogen bond than the π electron on the C=C bond of C₂ H₄ . The coinage substitution strengthens the aerogen bond in MCN-XeF₂ O (M=Cu, Ag, and Au) and its enhancing effect becomes larger in the Au<Cu<Ag pattern. The aerogen bond is further enhanced by the regium bond in C₂ H₂ -MCN-XeF₂ O and C₂ H₄ -MCN-XeF₂ O, but is weakened by the regium bond in MCN-C₂ H₄ -XeF₂ O and C₂ (CN)₄ -MCN-XeF₂ O. Simultaneously, the regium bond is also strengthened or weakened in these triads. The synergistic and diminutive effects between regium and aerogen bonds have been explained by means of charge transfer and electrostatic potentials.

Regium-π Bonds Are Involved in Protein-Gold Binding

The regium-π interaction is an attractive noncovalent force between group 11 elements (Cu, Ag, and Au) acting as Lewis acids and aromatic surfaces. Herein, we report for the first time experimental (Protein Data Bank analysis) and theoretical (RI-MP2/def2-TZVP level of theory) evidence of regium-π bonds involving Au(I) and aromatic amino acids (Phe, Tyr, Trp, and His). These findings might be important in the field of drug design and for retrospectively understanding the role of gold in proteins.

Not Only Hydrogen Bonds: Other Noncovalent Interactions

In this review, we provide a consistent description of noncovalent interactions, covering most groups of the Periodic Table. Different types of bonds are discussed using their trivial names. Moreover, the new name “Spodium bonds” is proposed for group 12 since noncovalent interactions involving this group of elements as electron acceptors have not yet been named. Excluding hydrogen bonds, the following noncovalent interactions will be discussed: alkali, alkaline earth, regium, spodium, triel, tetrel, pnictogen, chalcogen, halogen, and aerogen, which almost covers the Periodic Table entirely. Other interactions, such as orthogonal interactions and π-π stacking, will also be considered. Research and applications of σ-hole and π-hole interactions involving the p-block element is growing exponentially. The important applications include supramolecular chemistry, crystal engineering, catalysis, enzymatic chemistry molecular machines, membrane ion transport, etc. Despite the fact that this review is not intended to be comprehensive, a number of representative works for each type of interaction is provided. The possibility of modeling the dissociation energies of the complexes using different models (HSAB, ECW, Alkorta-Legon) was analyzed. Finally, the extension of Cahn-Ingold-Prelog priority rules to noncovalent is proposed.

Synthesis, X-ray characterization and regium bonding interactions of a trichlorido(1-hexylcytosine)gold(iii) complex

Herein we report the synthesis and X-ray characterization of a gold(iii) complex of 1-hexylcytosine via N(3). The AuCl₃N complexes stack on top of each other by reciprocal [AuCl] regium bonding interactions. After the first example 35 years ago, this is the second available structure of a cytosine nucleobase model complexed to gold(iii).

Coinage-Metal Bond between [1.1.1]Propellane and M2/MCl/MCH3 (M = Cu, Ag, and Au): Cooperativity and Substituents

A coinage-metal bond has been predicted and characterized in the complexes of [1.1.1]propellane (P) and M₂/MCl/MCH₃ (M = Cu, Ag, and Au). The interaction energy varies between −16 and −47 kcal/mol, indicating that the bridgehead carbon atom of P has a good affinity for the coinage atom. The coinage-metal bond becomes stronger in the Ag < Cu < Au sequence. Relative to M₂, both MCl and MCH₃ engage in a stronger coinage-metal bond, both -Cl and -CH₃ groups showing an electron-withdrawing property. The formation of coinage-metal bonding is mainly attributed to the donation orbital interactions from the occupied C-C orbital into the empty metal orbitals and a back-donation from the occupied d orbital of metal into the empty C-C anti-bonding orbital. In most complexes, the coinage-metal bond is dominated by electrostatic interaction, with moderate contribution of polarization. When P binds simultaneously with two coinage donors, negative cooperativity is found. Moreover, this cooperativity is prominent for the stronger coinage-metal bond.

Understanding Regium Bonds and their Competition with Hydrogen Bonds in Au2 :HX Complexes

A theoretical study of the regium and hydrogen bonds (RB and HB, respectively) in Au₂ :HX complexes has been carried out by means of CCSD(T) calculations. The theoretical study shows as overall outcome that in all cases the complexes exhibiting RB are more stable that those with HB. The binding energies for RB complexes range between -24 and -180 kJ ⋅ mol^-1, whereas those of the HB complexes are between -6 and -19 kJ ⋅ mol^-1 . DFT-SAPT also indicated that HB complexes are governed by electrostatics, but RB complexes present larger contribution of the induction term to the total attractive forces. ¹⁹⁷ Au chemical shifts have been calculated using the relativistic ZORA Hamiltonian.

Boron triel bonding: a weak electrostatic interaction lacking electron-density descriptors

In an effort to describe π-hole interactions, we undertook accurate high-resolution X-ray diffraction analyses of single crystals of 1,4-dinitrobenzene, a co-crystal of cis-tartaric acid and bis-pyridine N-oxide and the hydrochloride of B-4-pyridinylboronic acid. We selected these three compounds owing to the π-hole accessibility features that the sp² hybridized B, C and N atoms provide, thus allowing us to compare the fundamental characteristics of π-hole interactions using Bader's Atom in Molecules (AIM) theory. This particular study required extremely accurate experimental diffraction data, because the interaction of interest is weak. As shown by the experimental charge density maps of the -YO₂ (Y = B, C, N) units, we assign the depletion of electron-density present in the central boron, carbon and nitrogen atoms (electrophilic π-holes) as the main origin for the establishment of intermolecular Lewis acid-Lewis base attractive interaction with complementary electron-rich regions. Unexpectedly, the Bader's analyses of both experimentally and theoretically calculated charge distribution maps for the solid involving the - BO₂H₂ group do not show the presence of bond paths, neither of the bond critical points, between the interacting electron rich sites and the boron or carbon atoms featuring the electron hole. In contrast, these topological descriptors of chemical interactions for the AIM theory were easily located in the solid-state structures of the compounds involving the carboxylic and the nitro groups.

Crystal structures of N6-modified-aminoacid/peptide nucleobase analogs: hybrid adenine–glycine and adenine–glycylglycine molecules

Anion–π interactions in crystal structures of N⁶-modified-aminoacid and dipeptide adenine analogs are investigated using X-ray crystallography and DFT calculations.