On the Ambiphilic Reactivity of Geometrically Constrained Phosphorus(III) and Arsenic(III) Compounds: Insights into Their Interaction with Ionic Substrates.

Thomas P Robinson,Daniel De Rosa,Simon Aldridge,Jose M Goicoechea,Siu‐Kwan Lo

doi:10.1002/chem.201603135

Abstract

The ambiphilic nature of geometrically constrained Group 15 complexes bearing the N,N‐bis(3,5‐di‐tert‐butyl‐2‐phenolate)amide pincer ligand (ONO3−) is explored. Despite their differing reactivity towards nucleophilic substrates with polarised element–hydrogen bonds (e.g., NH3), both the phosphorus(III), P(ONO) (1 a), and arsenic(III), As(ONO) (1 b), compounds exhibit similar reactivity towards charged nucleophiles and electrophiles. Reactions of 1 a and 1 b with KOtBu or KNPh2 afford anionic complexes in which the nucleophilic anion associates with the pnictogen centre ([(tBuO)Pn(ONO)]− (Pn=P (2 a), As (2 b)) and [(Ph2N)Pn(ONO)]− (Pn=P (3 a), As (3 b)). Compound 2 a can subsequently be reacted with a proton source or benzylbromide to afford the phosphorus(V) compounds (tBuO)HP(ONO) (4 a) and (tBuO)BzP(ONO) (5 a), respectively, whereas analogous arsenic(V) compounds are inaccessible. Electrophilic substrates, such as HOTf and MeOTf, preferentially associate with the nitrogen atom of the ligand backbone of both 1 a and 1 b, giving rise to cationic species that can be rationalised as either ammonium salts or as amine‐stabilised phosphenium or arsenium complexes ([Pn{ON(H)O}]+ (Pn=P (6 a), As (6 b)) and [Pn{ON(Me)O}]+ (Pn=P (7 a), As (7 b)). Reaction of 1 a with an acid bearing a nucleophilic counteranion (such as HCl) gives rise to a phosphorus(V) compound HPCl(ONO) (8 a), whereas the analogous reaction with 1 b results in the addition of HCl across one of the As−O bonds to afford ClAs{(H)ONO} (8 b). Functionalisation at both the pnictogen centre and the ligand backbone is also possible by reaction of 7 a/7 b with KOtBu, which affords the neutral species (tBuO)Pn{ON(Me)O} (Pn=P (9 a), As (9 b)). The ambiphilic reactivity of these geometrically constrained complexes allows some insight into the mechanism of reactivity of 1 a towards small molecules, such as ammonia and water.

Highlights

Over the last decade significant advances have been made in the development of main-group species that are capable of activating small molecules.[1]
Theoretical calculations at the density functional theory (DFT) level revealed that the pyramidal Cs isomer is the most stable, but relatively close in energy to the C2v symmetric species
It is highly likely that, in solution, there is a dynamic, and concerted, pyramidal inversion at the phosphorus and nitrogen atoms resulting in a wing-like "flapping" of the ligand backbone

Summary

Introduction

Over the last decade significant advances have been made in the development of main-group species that are capable of activating small molecules.[1]. We recently reported a phosphorus(III) compound bearing the N,N-bis(3,5-di-tert-butyl-2-phenolate)amide ligand (P(ONO); 1 a).[26] This species was found to react with ammonia and water, activating the EÀH bonds in both substrates through a formal oxidative addition to afford the corresponding phos-

Results

Conclusion