Room temperature stable E,Z-diphosphenes: their isomerization, coordination, and cycloaddition chemistry†

E,Z-isomers display distinct physical properties and chemical reactivities. However, investigations on heavy main group elements remain limited. In this work, we present the isolation and X-ray crystallographic characterization of N-heterocyclic vinyl (NHV) substituted diphosphenes as both E- and Z-isomers (L] CH-P]P-CH]L, E,Z-2b; L = N-heterocyclic carbene). E-2b is thermodynamically more stable and undergoes reversible photo-stimulated isomerization to Z-2b. The less stable Z-isomer Z-2b can be thermally reverted to E-2b. Theoretical studies support the view that this E -> Z isomerization proceeds via P]P bond rotation, reminiscent of the isomerization observed in alkenes. Furthermore, both E,Z-2b coordinate to an AuCl fragment affording the complex [AuCl(eta(2)-Z-2b)] with the diphosphene ligand in Z-conformation, exclusively. In contrast, E,Z-2b undergo [2 + 4] and [2 + 1] cycloadditions with dienes or diazo compounds, respectively, yielding identical cycloaddition products in which the phosphorus bound NHV groups are in trans-position to each other. DFT calculations provide insight into the E/Zisomerisation and stereoselective formation of Au(I) complexes and cycloaddition products.

Automated summary

In this study, N-heterocyclic vinyl (NHV) substituted diphosphenes were isolated and characterized as both E- and Z-isomers. The thermodynamically more stable E-2b undergoes reversible photo-stimulated isomerization to Z-2b, which can be thermally reverted back to E-2b. The study also reveals the coordination of both E- and Z-2b to an AuCl fragment, as well as their different cycloaddition reactions.