Structures of Clusters Surrounding Ions Stabilized by Hydrogen, Halogen, Chalcogen, and Pnicogen Bonds

Four H-binding HCl and HF molecules position themselves at the vertices of a tetrahedron when surrounding a central Cl-. Halogen bonding BrF and ClF form a slightly distorted tetrahedron, a tendency that is amplified for ClCN which forms a trigonal pyramid. Chalcogen bonding SF2, SeF2, SeFMe, and SeCSe occupy one hemisphere of the central ion, leaving the other hemisphere empty. This pattern is repeated for pnicogen bonding PF3, SeF3 and AsCF. The clustering of solvent molecules on one side of the Cl- is attributed to weak attractive interactions between them, including chalcogen, pnicogen, halogen, and hydrogen bonds. Binding energies of four solvent molecules around a central Na+ are considerably reduced relative to chloride, and the geometries are different, with no empty hemisphere. The driving force maximizes the number of electronegative (F or O) atoms close to the Na+, and the presence of noncovalent bonds between solvent molecules

Competition Between Intra and Intermolecular Triel Bonds. Complexes Between Naphthalene Derivatives and Neutral or Anionic Lewis Bases

A TrF2 group (Tr = B, Al, Ga, In, Tl) is placed on one of the α positions of naphthalene, and its ability to engage in a triel bond (TrB) with a weak (NCH) and strong (NC−) nucleophile is assessed by ab initio calculations. As a competitor, an NH2 group is placed on the neighboring Cα, from which point it forms an intramolecular TrB with the TrF2 group. The latter internal TrB reduces the intensity of the π-hole on the Tr atom, decreasing its ability to engage in a second external TrB. The intermolecular TrB is weakened by a factor of about two for the smaller Tr atoms but is less severe for the larger Tl. The external TrB can be quite strong nonetheless; it varies from a minimum of 8 kcal/mol for the weak NCH base, up to as much as 70 kcal/mol for CN−. Likewise, the appearance of an external TrB to a strong base like CN− lessens the ability of the Tr to engage in an internal TrB, to the point where such an intramolecular TrB becomes questionable

Triel-Bonded Complexes Between TrR3 (Tr = B, Al, Ga; R = H, F, Cl, Br, CH3) and Pyrazine

Complexes between TrR3 (Tr = B, Al, Ga; R= H, F, Cl, Br, CH3)molecules and pyrazine have been characterized at the MP2 and CCSD(T) levels of theory. The adducts can be grouped according to the type of molecular arrangement. The first situation places the Tr atom in the plane of the pyrazine ring and contains a triel bond to the N lone pair. For the boron complexes the orbital interaction energy is almost equal to the electrostatic component, while the former is only half the latter for Tr= Al and Ga. The two monomers are stacked above one another in the second configuration, which depends to a greater degree upon orbital interaction and dispersion. The former complexes are more strongly bonded than the latter. Interaction energies (Eint) for the stronger complexes vary between -50 and -20 kcal/mol for BBr3 and Ga(CH3)3paired respectively with pyrazine. Eint is much smaller for the stacked configurations, ranging from -8 for GaF3 to -1.4 kcal/mol for BF3. The value of the maximum of the electrostatic potential correlates poorly with Eint, attributed in part to monomer distortions upon complexation

Comparison Between Tetrel Bonded Complexes Stabilized by σ and π Hole Interactions

The σ-hole tetrel bonds formed by a tetravalent molecule are compared with those involving a π-hole above the tetrel atom in a trivalent bonding situation. The former are modeled by TH4, TH3F, and TH2F2 (T = Si, Ge, Sn) and the latter by TH2=CH2, THF=CH2, and TF2=CH2, all paired with NH3 as Lewis base. The latter π-bonded complexes are considerably more strongly bound, despite the near equivalence of the σ and π-hole intensities. The larger binding energies of the π-dimers are attributed to greater electrostatic attraction and orbital interaction. Each progressive replacement of H by F increases the strength of the tetrel bond, whether σ or π. The magnitudes of the maxima of the molecular electrostatic potential in the two types of systems are not good indicators of either the interaction energy or even the full Coulombic energy. The geometry of the Lewis acid is significantly distorted by the formation of the dimer, more so in the case of the σ-bonded complexes, and this deformation intensifies the σ and π holes

Regium Bonds Between Mn Clusters (M=Cu,Ag,Au and n=2-6) and Nucleophiles NH3 and HCN

The most stable geometries of the coinagemetal (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 and tend to follow the Au \u3e Cu \u3e Ag order. These interaction energies are in large part derived from Coulombic attraction, with a smaller orbital interaction contribution

Coordination of Anions by Noncovalently Bonded σ-Hole Ligands

Research on σ-hole interactions that include halogen, chalcogen, pnicogen, and tetrel bonding has been accelerating in recent years. These cousins of the H-bond have many similar properties, including geometric preferences and energetics. Most of the work to date has focused on neutral complexes, with less known about these bonds to anions. This review summarizes the current state of knowledge about the complexes of anions with ligands that engage in these sorts of noncovalent bonds. Of particular interest are comparisons with H-bonds, and how the geometry of the fully coordinated complex varies as the number of surrounding ligands increases. A specific application of these ideas is explored in which these noncovalent bonds can be used to selectively bind certain anions in a multidentate arrangement, where a symbiotic interplay of experimental and computational methods has provided some useful insights

Influence of Monomer Deformation on the Competition Between Two Types of σ-Holes in Tetrel Bonds

One of several tetrel (T) atoms was covalently attached to three F atoms and a substituted phenyl ring. A NH3 base can form a tetrel bond with TF3C6H2R3(T = Si, Ge, Sn, Pb; R = H, F, CH3) in a position opposite either an F atom or the ring. The σ-hole opposite the highly electron-withdrawing F (T-F) is more intense than that opposite the ring (T-C). However, when the Lewis base deforms from a tetrahedral to a trigonal bipyramidal shape so as to accommodate the base, it is the T-C σ-hole that is more intense. Accordingly, it is the T-C tetrel-bonded complex for which there is a larger interaction energy with NH3, as high as 34 kcal/mol. Countering this trend, it requires more energy for the TF3C6H2R3 to deform into the geometry it adopts within the T-C complex than within its T-F counterpart. There is consequently a balance between the overall binding energies of the two competing sites. The smaller tetrel atoms Si and Ge, with their larger deformation energies, show a preference for T-F tetrel binding, while the T-C site is preferred by Pb which suffers from a smaller degree of deformation energy. There is a near balance for T=Sn and the two sites show comparable binding energies

Chalcogen Bonding of Two Ligands to Hypervalent YF4 (Y=S, Se, Te, Po)

The ability of two NH3 ligands to engage in simultaneous chalcogen bonds to a hypervalent YF4 molecule, with Y=S, Se, Te, Po, is assessed via quantum calculations. The complex can take on one of two different geometries. The cis structure places the two ligands adjacent to one another ina pseudo-octahedral geometry, held there by a pair of σ-hole chalcogen bonds. The bases can also lie nearly opposite one another, in a distorted octahedron containing one π-hole and one strained σ-hole bond. The cis geometry is favored for Y=S, while Te,and Po tend toward the trans structure; they are nearly equally stable for Se. In either case, the binding energy rises rapidly with the size of the Y atom, exceeding 30 kcal/mol for PoF4

Anion–Anion Interactions in Aerogen-Bonded Complexes. Influence of Solvent Environment

Ab initio calculations are applied to the question as to whether a AeX5− anion (Ae = Kr, Xe) can engage in a stable complex with another anion: F−, Cl−, or CN−. The latter approaches the central Ae atom from above the molecular plane, along its C5 axis. While the electrostatic repulsion between the two anions prevents their association in the gas phase, immersion of the system in a polar medium allows dimerization to proceed. The aerogen bond is a weak one, with binding energies less than 2 kcal/mol, even in highly polar aqueous solvent. The complexes are metastable in the less polar solvents THF and DMF, with dissociation opposed by a small energy barrier

On the Ability of Pnicogen Atoms to Engage in Both σ and π-hole Complexes. Heterodimers of ZF\u3csub\u3e2\u3c/sub\u3eC\u3csub\u3e6\u3c/sub\u3eH\u3csub\u3e5\u3c/sub\u3e (Z = P, As, Sb, Bi) and NH\u3csub\u3e3\u3c/sub\u3e

When bound to a pair of F atoms and a phenyl ring, a pyramidal pnicogen (Z) atom can form a pnicogen bond wherein an NH3 base lies opposite one F atom. In addition to this σ-hole complex, the ZF2C6H5 molecule can distort in such a way that the NH3 approaches on the opposite side to the lone pair on Z, where there is a so-called π-hole. The interaction energies of these π-hole dimers are roughly 30 kcal/mol, much larger than the equivalent quantities for the σ-hole complexes, which are only 4–13 kcal/mol. On the other hand, this large interaction energy is countered by the considerable deformation energy required for the Lewis acid to adopt the geometry necessary to form the π-hole complex. The overall energetics of the complexation reaction are thus more exothermic for the σ-hole dimers than for the π-hole dimers