Anti-Electrostatic Pi-Hole Bonding: How Covalency Conquers Coulombics.

Frank Weinhold

doi:10.3390/molecules27020377

Abstract

Intermolecular bonding attraction at π-bonded centers is often described as “electrostatically driven” and given quasi-classical rationalization in terms of a “pi hole” depletion region in the electrostatic potential. However, we demonstrate here that such bonding attraction also occurs between closed-shell ions of like charge, thereby yielding locally stable complexes that sharply violate classical electrostatic expectations. Standard DFT and MP2 computational methods are employed to investigate complexation of simple pi-bonded diatomic anions (BO−, CN−) with simple atomic anions (H−, F−) or with one another. Such “anti-electrostatic” anion–anion attractions are shown to lead to robust metastable binding wells (ranging up to 20–30 kcal/mol at DFT level, or still deeper at dynamically correlated MP2 level) that are shielded by broad predissociation barriers (ranging up to 1.5 Å width) from long-range ionic dissociation. Like-charge attraction at pi-centers thereby provides additional evidence for the dominance of 3-center/4-electron (3c/4e) nD-π*AX interactions that are fully analogous to the nD-σ*AH interactions of H-bonding. Using standard keyword options of natural bond orbital (NBO) analysis, we demonstrate that both n-σ* (sigma hole) and n-π* (pi hole) interactions represent simple variants of the essential resonance-type donor-acceptor (Bürgi–Dunitz-type) attraction that apparently underlies all intermolecular association phenomena of chemical interest. We further demonstrate that “deletion” of such π*-based donor-acceptor interaction obliterates the characteristic Bürgi–Dunitz signatures of pi-hole interactions, thereby establishing the unique cause/effect relationship to short-range covalency (“charge transfer”) rather than envisioned Coulombic properties of unperturbed monomers.

Highlights

The concept that pi-bonded centers exert a characteristic form of directional binding traces back to pioneering statistical analyses of the Cambridge Structural Database (CSD) by Bürgi and Dunitz [1,2], who recognized the general propensity for nucleophilic groups to adopt a particular orientation with respect to the pi-bond of ketones or aldehydes
As simple examples drawn from a selected set of main-group monatomic (H−, F−) and pi-bonded diatomic (BO−, CN−) anions, we focus on complexes with the more electropositive end (B, C) of each diatomic bonding pattern (:B≡O:, :C≡N:) as principal coordinating center with the other anion
Except for F−···BO− (3rd row), the formal natural Lewis structure (NLS) depiction of each equilibrium structure exhibits dative bond formation that suggests the remarkable strength of anion–anion binding energies in such n-π* complexes compared to known n-σ* species

Summary

Introduction

The concept that pi-bonded centers exert a characteristic form of directional binding traces back to pioneering statistical analyses of the Cambridge Structural Database (CSD) by Bürgi and Dunitz [1,2], who recognized the general propensity for nucleophilic groups to adopt a particular orientation ( termed the “Bürgi–Dunitz angle” [3]) with respect to the pi-bond of ketones or aldehydes. The broader implications of such pi-type stabilizing interactions in protein chemistry (complementing the well-known sigma-type interactions of hydrogen bonding) were subsequently explored by Raines and coworkers [4,5,6,7,8] with combined CSD, NMR, computational and natural bond orbital (NBO) methods [9,10] Results of these studies consistently affirm the conceptual aptness of the “n-π*” orbital picture of Bürgi–Dunitz interaction (lone pair ndonor of the nucleophilic e-donor with the π*acceptor valence antibond of the pi-bonded e-acceptor moiety), consistent with the analogous n-σ* picture of H-bonding interactions [11,12]. ) analogs of H-bonding [15,16] were increasingly recognized as significant features of intermolecular interactions, the alternative “sigma-hole” picture of such interactions was introduced by Politzer and coworkers [17,18] This picture focuses on the electrostatic potential (ESP) and its characteristic depletion region along the C-X bonding axis that suggests a quasi-classical electrostatic rationale for directional attraction to lone pairs of an incoming nucleophile. A variety of computational studies have lent support to such “electrostatically driven” conceptions of pi-hole interactions [20,21,22,23,24,25,26,27,28,29]

Methods

Results

Conclusion