NHC-Coordinated Diphosphene-Stabilized Gold(I) Hydride and Its Reversible Conversion to Gold(I) Formate with CO2

Abstract

An NHC-coordinated diphosphene is employed as ligand for the synthesis of a hydrocarbon-soluble monomeric Au^I hydride, which readily adds CO₂ at room temperature yielding the corresponding Au^I formate. The reversible reaction can be expedited by the addition of NHC, which induces β-hydride shift and the removal of CO₂ from equilibrium through the formation of an NHC-CO₂ adduct. The Au^I formate is alternatively formed by dehydrogenative coupling of the Au^I hydride with formic acid (HCO₂ H), thus in total establishing a reaction sequence for the Au^I hydride mediated dehydrogenation of HCO₂ H as chemical hydrogen storage material.

Keywords: carbon dioxide; diphosphene; gold; ligand design; phosphorus.

Scheme 1

Chemical structures of I–IV (R=monoanionic ligand, NHC=N‐heterocyclic carbene).

Scheme 2

Synthesis of NHC${}^{\mathrm{Me}{}_4}$ ‐coordinated diphosphene‐stabilized Au^I‐Cl complex 2 (Ar=2,6‐Mes₂C₆H₃, Mes=2,4,6‐Me₃C₆H₂).

Figure 1

Molecular structure of 2 with ellipsoids set at 50 % probability. All hydrogen atoms and one molecule of co‐crystallized toluene are omitted for clarity.33

Scheme 3

Synthesis of NHC‐coordinated diphosphene‐stabilized Au^I‐hydride 3 and the rearrangement to 4.

Figure 2

Molecular structure of 3 with ellipsoids set at 50 % probability. All H atoms except Au‐H and one co‐crystallized molecule of benzene are omitted for clarity.33

Figure 3

Molecular structure of 4 with ellipsoids set at 50 % probability. All hydrogen atoms and one molecule of co‐crystallized hexane solvent molecule are omitted for clarity.33

Scheme 4

Synthesis of NHC‐coordinated diphosphene‐stabilized Au^I‐formate 5.

Figure 4

Molecular structure of 5 with ellipsoids set at 50 % probability. All hydrogen atoms and one molecule of toluene are omitted for clarity.33

Scheme 5

NHC ${}^{\mathbf{i}\mathbf{P}\mathbf{r}{}_{\mathbf{2}}\mathbf{M}\mathbf{e}{}_{\mathbf{2}}}$ ‐mediated release of CO₂ from Au^I‐formate 3.

Scheme 6

Au^I‐hydride 3 mediated release of H₂ and CO₂ from HCO₂H (inset: Reaction of HCO₂H to CO₂ and H₂).