Unveiling Unusual Reactivity of SO2 and Unusual Type of S-X Long Bonds

Abstract

A new reactivity of SO₂ to form unusual stable M-X-SO₂ (X = F, Cl, H) complexes is unveiled in this study. Moreover, a new type of S-X long bonds, which are significantly longer than traditional S-X covalent bonds, has been discovered. The P,N-ligated Ni-F complex model 1A can bind a SO₂ molecule through the new F-S long bond (2.207 Å), and a stable Ni-F-SO₂ complex 1B is generated, being exergonic by 2.2 kcal/mol. According to natural localized molecular orbital analysis, the new S-F long bond has a unique p(F) → π*(O═S═O) bonding interaction, which is shown to arise from the long S-F length. In comparison, the strength of the new F-S long bond (-2.2 kcal/mol) is found to be significantly stronger than common noncovalent interactions such as the hydrogen and halogen bond. The substituent modulations suggest that the electron-donating groups can increase the strength of new F-S bonds and enhance binding free energies ΔG_bind. The scope of possible M-X complexes was explored, and various metals and X (F, Cl, and H) ligands were found to form stable M-X-SO₂ complexes. Specifically, the anionic M-X complexes display much higher ΔG_bind values, ranging from -8 to -10 kcal/mol. The study paves the way for a green, recyclable, and adjustable SO₂ absorption method.

1. Comparison of SO₂ Absorption Methods

2. HOX···SO₂ Interaction Model

3. Previous Studies on the Reactivities of Metal-Sulfonyl Complexes in (a) Desulfonative Suzuki–Miyaura Cross-Coupling Reaction; (b) α-Alkylation of Sulfones and Sulfonyl Dissociation

1

Reversible absorption of SO₂ by the Ni–F complex through the formation of a new S–F long bond. The relative Gibbs free energies (ΔG) and potential energies (ΔE) are given in kcal/mol. n/a denotes that the structure is not available.

2

Structural comparisons of SO₂, FSO₂ ^– anion, 1A _, and 1B.

3

Natural localized molecular orbitals (NLMOs) for the S–F bonding interactions in (a) Ni–F–SO₂ complex 1B, (b) free FSO₂ ^– anion, (c) S–F bond restrained FSO₂ ^– anion, and (d) HOF···SO₂.

4

Formation of weak hydrogen bond and halogen bond complexes for structure 1A. The relative Gibbs free energies (ΔG) and potential energies (ΔE) are in kcal/mol.

5

Modulation of (a) R substituents and (b) X ligands in 1B and 1C for the formation of a new S–F long bond.

6

M–X complexes (X = F, Cl, H) with different ligand skeletons for the formation of stable M–X–SO₂ complexes. The relative Gibbs free energies (ΔG) and potential energies (ΔE) are in kcal/mol.