Halogen Complexes of Anionic N-Heterocyclic Carbenes

Abstract

The lithium complexes [(WCA-NHC)Li(toluene)] of anionic N-heterocyclic carbenes with a weakly coordinating anionic borate moiety (WCA-NHC) reacted with iodine, bromine, or CCl₄ to afford the zwitterionic 2-halogenoimidazolium borates (WCA-NHC)X (X=I, Br, Cl; WCA=B(C₆ F₅ )₃ , B{3,5-C₆ H₃ (CF₃ )₂ }₃ ; NHC=IDipp=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene, or NHC=IMes=1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene). The iodine derivative (WCA-IDipp)I (WCA=B(C₆ F₅ )₃ ) formed several complexes of the type (WCA-IDipp)I⋅L (L=C₆ H₅ Cl, C₆ H₅ Me, CH₃ CN, THF, ONMe₃ ), revealing its ability to act as an efficient halogen bond donor, which was also exploited for the preparation of hypervalent bis(carbene)iodine(I) complexes of the type [(WCA-IDipp)I(NHC)] and [PPh₄ ][(WCA-IDipp)I(WCA-NHC)] (NHC=IDipp, IMes). The corresponding bromine complex [PPh₄ ][(WCA-IDipp)₂ Br] was isolated as a rare example of a hypervalent (10-Br-2) system. DFT calculations reveal that London dispersion contributes significantly to the stability of the bis(carbene)halogen(I) complexes, and the bonding was further analyzed by quantum theory of atoms in molecules (QTAIM) analysis.

Keywords: London dispersion; N-heterocyclic carbenes; halogen bonding; hypervalency; iodine.

Scheme 1

Examples of hypervalent iodine(I) species (10‐I‐2).

Scheme 2

Synthesis of 2‐iodo‐, 2‐bromo‐, and 2‐chloroimidazolium borates. Dipp=2,6‐diisopropylphenyl, Mes=2,4,6‐trimethylphenyl.

Figure 1

ORTEP diagram of 2 a⋅C₆H₅Cl and 2 a⋅ONMe₃ with thermal displacement parameters drawn at the 50 % probability level; hydrogen atoms are omitted for clarity. Pertinent structural data of all compounds 2 are assembled in Table 1.

Figure 2

ORTEP diagrams of 5 a⋅C₆H₅Cl and 5 b⋅C₆H₅Cl⋅ n‐hexane with thermal displacement parameters drawn at the 50 % probability level; hydrogen atoms and solvent molecules are omitted for clarity. 5 a must be regarded with care, owing to the usage of SQUEZZE. Pertinent structural data of all compounds 5 are assembled in Table 2.

Scheme 3

Synthesis of neutral bis(carbene)iodine(I) complexes. Dipp=2,6‐diisopropylphenyl, Mes=2,4,6‐trimethylphenyl.

Scheme 4

Synthesis of anionic bis(carbene)iodine(I) complexes. Dipp=2,6‐diisopropylphenyl, Mes=2,4,6‐trimethylphenyl.

Figure 3

ORTEP diagram of 6 with thermal displacement parameters drawn at the 50 % probability level; the PPh₄ ⁺ counterion and the hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]: C1−N1 1.3739(18), C1−N2 1.3586(19), N1−C2 1.4164(17), C2−C3 1.3512(19), C3−N2 1.3853(17), C2−B1 1.6416(19); N1‐C1‐N2 101.93(11), C1‐N1‐C2 113.89(11), N1‐C2‐C3 102.97(11), C2‐C3‐N2 109.05(12), C3‐N2‐C1 112.15(11), N1‐C2‐B1 133.29(11), C3‐C2‐B1 123.56(12).

Figure 4

ORTEP diagrams of 7 a⋅2 THF and 7 b with thermal displacement parameters drawn at the 50 % probability level; the PPh₄ ⁺ counterions, the hydrogen atoms, THF molecules, and a second molecule in the asymmetric unit of 7 b are omitted for clarity. Pertinent structural data of all compounds 7 are assembled in Table 2.

Figure 5

ORTEP diagram of 8⋅C₆H₅Cl with thermal displacement parameters drawn at the 50 % probability level; the PPh₄ ⁺ counterion, the hydrogen atoms, and the chlorobenzene molecule are omitted for clarity. Pertinent structural data of all compounds 8⋅C₆F₅Cl are assembled in Table 2.

Figure 6

Contour maps of the Laplacian of the electron density, ∇² ρ(r), along the C‐I‐C axis in [(WCA‐IDipp)I(IDipp)] (5 a, top) and [(WCA‐IDipp)₂I]⁻ (as in 7 a, bottom). Red dashed lines and solid black lines indicate positive (electronic charge depletion) and negative (electronic charge concentration) ∇² ρ(r) values, respectively. Solid blue points mark (3, −1) bond critical points (bcp) along the bond path (solid blue line). The solid brown lines separate the atomic basins.

Figure 7

Calculated electrostatic potential (ESP) surface of 2 a (X=I), 3 (X=Br), and 4 (X=Cl) based on the B97‐D geometries; the blue discs on the surface in the forefront represent the σ‐hole (see Figure S28 in the Supporting Information for further details).