Theoretical studies on how to tune the π‐hole pnicogen bonds by substitution and cooperative effects

Abstract

A systemic investigation of the substitution and cooperative effects on the P…N π‐hole pnicogen bond were performed via theoretical calculations. The structural and energetic properties of the binary complexes between a series of substituted benzonitrile and PO2F have been examined to study the substitution effect. The stability of the binary complexes increases in the order of CN < Br ≈ Cl < F < H < CH3 < NH2, in which all halogen atoms serve as the electron‐withdrawing substituents. The cooperativity between halogen/triel bond with pnicogen bond were also studied in X…NC‐Ph‐CN…PO2F and Y…Br‐Ph‐CN…PO2F complexes with X = F2, Cl2, Br2, FCl, FBr, BrCl, FCN, ClCN, BrCN, BH3, BF3, and Y = NH3, NH2CH3, NHCH2, HCN. It is found that the mutual effect of the interactions is weakened through the phenyl ring relative to the heterocyclic ring. The molecular electrostatic potential, atoms in molecules and natural bond orbital analysis were carried out to unveil the nature of these interactions. Several linear relationships have been established between the geometric parameters and the interaction energies or some electronic properties of the complexes. The distinct differences for the complexes involving BH3 and their BF3 analogs may be attributed to the greater π electron density on the BF3 moiety, and the underlying reasons are discussed in detail.

Dissection of the Origin of π-Holes and the Noncovalent Bonds in Which They Engage

“…Although first-row N is quite reluctant to engage in σ-hole pnicogen bonds, it does so in the context of the π-hole above NFO 2 , with a strength of 5 kcal/mol. (Earlier sets of calculations , had evaluated the π-hole bond formed by the P-analogue PFO 2 , one of which provided an interaction energy with NH 3 of 35.5 kcal/mol.) ZnF 4 engages in a strong spodium bond with NH 3 , with E int equal to 30 kcal/mol.…”

“…Although not as extensive as the research centering on σ-hole bonds, there has been substantial coverage of their π-hole cousins as well, particularly in the past few years. ,,− This work has covered the full gamut of central atoms, from aerogen to pnicogen to triel bonds. In summary, it has been learned that π-hole bonds can be quite strong, competitive with their σ-hole bond sisters, and even stronger in some circumstances.…”

“…The intensive scrutiny of σ-hole bonds has led to a solid understanding of the fundamental phenomena from which they draw their strength. The chain of processes, beginning with the distortion of the σ­(R–X) covalent bond arising from the R–X electronegativity difference, and the resulting σ-hole in the density, which in turn leads to the positive potential, have been characterized. ,,,− Also studied extensively has been the relationship between the magnitude of the σ-hole potential and the strength of the ensuing interaction with a nucleophile and how other factors enter into the equation such as charge transfer and dispersion. ,− Research has also built a body of understanding of the effects of the σ-hole bond on the intermolecular distance and perturbations within the individual monomers, both geometric and spectroscopic. − …”

Proximity Effects of Substituents on Halogen Bond Strength

“…Recent years have witnessed an explosive growth in the study of noncovalent bonds that are close parallels of the venerable H-bond. In each of the chalcogen, pnicogen, tetrel, and triel bonds, an atom of that same particular family of the periodic table replaces the bridging proton of the H-bond. − Among these interactions, the halogen bond (XB) is arguably the one that has been acknowledged for the longest time − and has engendered the greatest amount of scrutiny. Extensive study has demonstrated that the XB owes its stability to several factors. − In the first place, the electron-density cloud surrounding the X atom is quite anisotropic; while the overall charge on the X atom is partially negative, there is a pocket of positive potential that lies along the extension of the covalent C–X bond, which is commonly referred to as a σ-hole.…”

Maximal occupation by bases of π‐hole bands surrounding linear molecules

“…In addition, the electron‐withdrawing O will enhance the lobe of the π* antibonding orbital that lies above the C atom, thereby facilitating charge transfer from the nucleophile in a direct parallel with σ‐hole bonding. Although the majority of the earlier studies of these sorts of noncovalent bonds were focused on σ‐hole bonds, there is a growing emphasis to better explore their π‐hole analogues 3,7,12,33–43 . While this work is continuing, it is important to stress here, that there are strong indications that π‐hole bonds are not necessarily weaker than their σ‐hole counterparts, and can indeed be stronger in numerous instances.…”