Regium-π bonds: An Unexplored Link between Noble Metal Nanoparticles and Aromatic Surfaces

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).

Regium Bonds between Silver(I) Pyrazolates Dinuclear Complexes and Lewis Bases (N2, OH2, NCH, SH2, NH3, PH3, CO and CNH)

A theoretical study and Cambridge Structural Database (CSD) search of dinuclear Ag(I) pyrazolates interactions with Lewis bases were carried out and the effect of the substituents and ligands on the structure and on the aromaticity were analyzed. A relationship between the intramolecular Ag–Ag distance and stability was found in the unsubstituted system, which indicates a destabilization at longer distances compensated by ligands upon complexation. It was also observed that the asymmetrical interaction with phosphines as ligands increases the Ag–Ag distance. This increase is dramatically higher when two simultaneous PH3 ligands are taken into account. The calculated 109Ag chemical shielding shows variation up to 1200 ppm due to the complexation. Calculations showed that six-membered rings possessed non-aromatic character while pyrazole rings do not change their aromatic character significantly upon complexation.

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.

Adsorption of Hydrophobic and Hydrophilic Ionic Liquids at the Au(111) Surface

Electrode-electrolyte microscopic interfacial studies are of great interest for the design and development of functional materials for energy storage and catalysis applications. First-principles-based simulation methods are used here to understand the structure, stability, energetics, and microscopic adsorption mechanism of various hydrophilic and hydrophobic ionic liquids (ILs; 1-butyl 3-methylimidazolium [BMIm]+[X]-, where X = Cl, DCA, HCOO, BF4, PF6, CH3SO3, OTF, and TFSA) interacting with a metallic surface. We have selected the Au(111) surface as a potential electrode model, and our computations show that ILs (anions and cations) adsorb specifically at some selective adsorption sites. Indeed, hydrophilic anions of ILs are strongly adsorbed on the gold surface (via Au-Cl and Au-N bonds at Au(111)), whereas hydrophobic anions are weakly bonded. The [BMIm]+ is always found to be stabilized parallel to the metal surface, irrespective of the nature of the anion, through various kinds of noncovalent interactions. Mulliken, Löwdin, and Hirshfeld charge analyses reveal that there is significant charge transfer between ILs and the surface that may enhance the charge transfer mechanism between the surface and electrolytes for electrochemical applications. Our study shows that the electrostatic and van der Waals interactions are in action at these interfaces. Moreover, we show that there are several covalent and noncovalent interactions between ILs and the metal surface. These interactions play an essential role to maintain the electrostatic behaviors at the solid-liquid interface. The present findings can be helpful to predict specific selectivity and subsequent design of materials for energy harvesting and catalysis applications.

Regium bonds between Mn clusters (M = Cu, Ag, Au and n = 2-6) and nucleophiles NH3 and HCN

The most stable geometries of the coinage metal (or regium) atom (Cu, Ag, Au) clusters Mn for n up to 6 are all planar, and adopt the lowest possible spin multiplicity. Clusters with even numbers of M atoms are thus singlets, while those with odd n are open-shell doublets. Examination of the molecular electrostatic potential (MEP) of each cluster provides strong indications of the most likely site of attack by an approaching nucleophile, generally one of two positions. A nucleophile (NH3 or HCN) most favorably approaches one particular M atom of each cluster, rather than a bond midpoint or face. In the closed-shell clusters, the interaction energies are highly dependent upon the intensity of the MEP, but this correlation fades for the open-shell systems studied in this work. The strength of the interaction is also closely related to the basicity of the nucleophile. Regium bond energies can be more than 30 kcal mol-1 and tend to follow the Au > Cu > Ag order. These interaction energies are in large part derived from Coulombic attraction, with a smaller orbital interaction contribution.

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.

Lone pair–π vs. σ-hole–π interactions in bromine head-containing oxacalix[2]arene[2]triazines

New bromine head-containing oxacalix[2]arene[2]triazines were synthesized. Owing to the bromine head and complementary V-shaped cavity, the solid state structure showed an intriguing and unique 1D-supramolecular chain-like self-assembly.