Ligand-Enabled Disproportionation of 1,2-Diphenylhydrazine at a PV -Center

Abstract

We present herein the synthesis of a nearly square-pyramidal chlorophosphorane supported by the tetradentate bis(amidophenolate) ligand, N,N'-bis(3,5-di-tert-butyl-2-phenoxy)-1,2-phenylenediamide. After chloride abstraction the resulting phosphonium cation efficiently promotes the disproportionation of 1,2-diphenylhydrazine to aniline and azobenzene. Mechanistic studies, spectroscopic analyses and theoretical calculations suggest that this unprecedented reactivity mode for P^V -centres is induced by the high electrophilicity at the cationic P^V -center, which originates from the geometry constraints imposed by the rigid pincer ligand, combined with the ability of the o-amidophenolate moieties to act as electron reservoir. This study illustrates the promising role of cooperativity between redox-active ligands and phosphorus for the design of organocatalysts able to promote redox processes.

Keywords: Cooperative Effects; Lewis Acids; Non-Innocent Ligands; Organocatalysis; Redox Chemistry.

Figure 1

a) Strategies available for the construction of organophosphorus redox catalysts and selected examples. b) Design of P‐catalysts through redox cooperation between the ligand and a central P^V atom.

Scheme 1

Relevant oxidation levels and labels for the bis(amidophenolate) ligand 9 and synthesis of of hydrophosphorane 10 and chlorophosphorane 11.

Figure 2

X ray structures of 10 and 11. Ellipsoids are set at 50 % probability; solvent molecules were removed for clarity.

Scheme 2

Chloride ligand exchange at chlorophosphorane 11.

Figure 3

One electron oxidation of 14 and 15. The X band EPR spectra of 16 and 17 were recorded at 200 K in CH₂Cl₂ (0.5 mM). Mulliken spin density distributions calculated at the UB3LYP‐D3(BJ)/def2‐TZVP level and contour plot of spin‐density distribution depicted at iso‐density value of 0.0004 au.

Figure 4

Synthesis and structure of phosphonium‐DMAP adduct 18 and estimated Lewis acidity for 19–23. Ellipsoids are set at 50 % probability; solvent molecules were removed for clarity. Estimated Lewis acidity for 19–23 by the Gutmann–Beckett method and computed fluoride and hydride ion affinities (kJ mol⁻¹) at the PW6B95‐D3(BJ)/def2‐QZVPP//B3LYP‐D3(BJ)/def2‐TZVP level of theory. Values for 21–23 obtained from reference [20d].

Figure 5

Free energy profile for the disproportionation of 1,2‐diphenylhydrazine catalyzed by 19 calculated at the B3LYP‐D3(BJ)/def2‐TZVP(C‐PCM)//PBE‐D3(BJ)/def2‐SVP level of theory.

Scheme 3

a) Trapping of the nitrenoide intermediate. b) Detection of 17(Mes) through X band EPR. Spectra recorded at 298 K in CH₂Cl₂ (0.5 mM).