Computational Evaluation of Me2 TCCP as Lewis Acid

Abstract

Supramolecular adducts between dimethyl-2,2,3,3-tetracyanocyclopropane (Me₂ TCCP) with 21 small (polar) molecules and 10 anions were computed with DFT (B3LYP-D3/def2-TZVP). Their optimized geometries were used to obtain interaction energies, and perform energy decomposition and 'atoms-in-molecules' analyses. A set of 38 other adducts were also evaluated for comparison purposes. Selected examples were further scrutinized by inspection of the molecular electrostatic potential maps, Noncovalent Interaction index plots, the Laplacian, the orbital interactions, and by estimating the Gibbs free energy of complexation in hexane solution. These calculations divulge the thermodynamic feasibility of Me₂ TCCP adducts and show that complexation is typically driven by dispersion with less polarized partners, but by orbital interactions when more polarized or anionic guests are deployed. Most Me₂ TCCP adducts are more stable than simple hydrogen bonding with water, but less stable than traditional Lewis adducts involving Me₃ B, or a strong halogen bond such as with Br₂ . Several bonding analyses showed that the locus of interaction is found near the electron poor sp³ -hydridized (NC)₂ C-C(CN)₂ carbon atoms. An empty hybrid σ*/π* orbital on Me₂ TCCP was identified that can be held responsible for the stability of the most stable adducts due to donor-acceptor interactions.

Keywords: density functional calculation; intermolecular interaction; molecular recognition; noncovalent interactions; tetrel bonding.

Figure 1

Molecular electrostatic potential map (MEP) of Me₂TCCP (a) and illustration of the crystal structure of the [Me₂TCCP⋅⋅⋅THF] co‐crystal (b). ^[19b] The MEP was calculated at the B3LYP‐D3/TZ2P level of theory and the colour scale ranges from −31 kcal mol⁻¹ (red) to 38 kcal mol⁻¹ (blue).

Figure 2

Ball and sticks representations of several charge neutral Me₂TCCP adducts that were geometry optimized with DFT at the B3LYP‐D3/def2‐TZVP level of theory. The thin red lines and small spheres respectively represent the bond paths and bond critical points (b.c.p.) of an ‘atoms‐in‐molecules’ analysis. The b.c.p.’s with an sp³‐C of Me₂TCCP are highlighted in yellow and the density of this point is indicated (ρ in a.u. ×10²).

Figure 3

Ball and sticks representations of several anionic Me₂TCCP adducts that were geometry optimized with DFT at the B3LYP‐D3/def2‐TZVP level of theory. The thin red lines and small spheres respectively represent the bond paths and bond critical points (b.c.p.) of an ‘atoms‐in‐molecules’ analysis. The b.c.p.’s with an sp³‐C of Me₂TCCP are highlighted in yellow and the density of b.c.p.’s are indicated (ρ in a.u. ×10²).

Figure 4

Analyses of the bonding between CH₃CO₂ ⁻ or Cl⁻ and cyclopropane derivatives. Top: noncovalent interaction (NCI) plots, colour coded from 0.003 (red) to 0.015 (blue) a.u. where larger values indicate stronger attraction. Middle: Projection of the Laplacian on the C₂Cl plane colour coded from −0.05 (red) to 0.02 (blue) a.u. in 50 increments. This scaling was chosen based on the [Me₂TCCP⋅⋅⋅n‐hexane] adduct, which has no overlapping lines on this scale (see Figure S7). The AIM analysis is also shown in this figure (see also Figure S1). Bottom: the bonding orbital of the adduct arising from electron donation of a p‐orbital of Cl⁻ (n) into an antibonding (σ*/π*) orbital of the cyclopropane derivative.