PnictogenIII Dications Supported by BZIMPY Ligands

Abstract

Two homologous series of pnictogen(III) dications, stabilized by 2,6-bis(benzimidazole-2-yl)pyridine ligands have been prepared. Both series contain Pn^III-X moieties (Pn = P, As, Sb, Bi; X = Cl or Ph) and have been fully characterized using spectroscopic methods including X-ray crystallography. The Lewis acidity of these compounds has also been probed by computational methods; the results suggest that the dictations are strong Lewis acids, with the PnCl²⁺ compounds being more acidic than the PnPh²⁺ compounds, and with Lewis acidity increasing from P to Bi, in both series. The PhP²⁺-containing compound was also found to be a versatile P^III transfer reagent, leading to new synthetic routes for various PhP-containing compounds. The redox chemistry of all compounds has also been probed using cyclic voltammetry and chemical reductions. In some cases the resulting Pn^I moieties could be trapped using diazabutadienes.

Keywords: BZIMPY; Crystal structures; Dications; Lewis acids; Pnictogen; Transfer reactions; Tridentate ligands.

Scheme 1

Cationic pnictogen compounds discussed herein.

Scheme 2

Meridional binding by ligand (blue) generates ambiphilic dicationic Pn^III site.

Scheme 3

Synthesis of [ClPn(Bn‐BZIMPY)][OTf]₂ (1Pn) (Pn=P, As, Sb, Bi; Bn=benzyl).

Figure 1

¹H NMR spectra of 1P (blue), 1As (green), 1Sb (orange), 1Bi (purple) in CD₃CN.

Figure 2

Solid‐state structures of the formal dicationic core in 1As (left), 1Sb (middle), and 1Bi (right) and their short interactions with the triflate anions (bottom). Hydrogen atoms, and disorder if present, have been omitted for visual clarity, as are the anions from the images on the top. Thermal ellipsoids are drawn at the 50 % probability level.

Scheme 4

Synthesis of [PhPn(Bn‐BZIMPY)][OTf]₂ (4Pn) (Pn=P, As, Sb, Bi; Bn=benzyl) (Top). Structures of the P^III dications incorporating CF₃ groups (Bottom).

Figure 3

Solid‐state structures of 4P (left; cation 1 only), 5 (middle), and 6 (right). Hydrogen atoms, disorder where present, and the triflate anions have been omitted for visual clarity. Thermal ellipsoids are drawn at the 50 % probability level.

Figure 4

¹H NMR spectra of 4P (blue), 4As (green), 4Sb (orange), 4Bi (purple) in CD₃CN.

Figure 5

Solid‐state structures of the formal dicationic core in 4As (left), 4Sb (middle), and 4Bi (right) and their short interactions with the triflate anions (bottom). Hydrogen atoms, and disorder where present, have been omitted for visual clarity, as are the anions from the images on the top. Thermal ellipsoids are drawn at the 50 % probability level.

Scheme 5

Transfer and reduction reactions of the PhP²⁺ synthon.

Figure 6

General numbering scheme for the assignment of chemical shifts in 1Pn and 4Pn.