Comparing the Acidity of (R3P)2BH-Based Donor Groups in Iridium Pincer Complexes

Leon Maser,Lukas Alig,Christian Schneider,Robert Langer

doi:10.3390/inorganics7050061

Abstract

In the current manuscript, we describe the reactivity of a series of iridium(III) pincer complexes with the general formulae [(PEP)IrCl(CO)(H)]n (n = +1, +2) towards base, where PEP is a pincer-type ligand with different central donor groups, and E is the ligating atom of this group (E = B, C, N). The donor groups encompass a secondary amine, a phosphine-stabilised borylene and a protonated carbodiphosphorane. As all ligating atoms E exhibit an E–H bond, we addressed the question of wether the coordinated donor group can be deprotonated in competition to the reductive elimination of HCl from the iridium(III) centre. Based on experimental and quantum chemical investigations, it is shown that the ability for deprotonation of the coordinated ligand decreases in the order of (R3P)2CH+ > R2NH > (R3P)2BH. The initial product of the reductive elimination of HCl from [(PBP)IrCl(CO)(H)]n (1c), the square planar iridium(I) complex, [(PBP)Ir(CO)]+ (3c), was found to be unstable and further reacts to [(PBP)Ir(CO)2]+ (5c). Comparing the C–O stretching vibrations of the latter with those of related complexes, it is demonstrated that neutral ligands based on tricoordinate boron are very strong donors.

Highlights

Tricoordinate boron compounds, BR3, are typically Lewis acids and stabilise their electron deficiency by π-donating substituents, hyperconjugation or dimerisation and formation of two-electron three-centre bonds
Protonated carbodiphosphoranes of the type (R3 P)2 CH+ (II) can be deprotonated by strong bases and form their deprotonated analogues when coordinated to a metal centre [12]
The deprotonation of 1a–1c can take place at several positions in the complex, but commonly either the central donor group E is deprotonated or the hydrido ligand is abstracted in a reductive elimination (Figure 2)

Summary

Introduction

Tricoordinate boron compounds, BR3 , are typically Lewis acids and stabilise their electron deficiency by π-donating substituents, hyperconjugation or dimerisation and formation of two-electron three-centre bonds. In consequence, they can accept electron donation from electron rich metal centres and serve as Z-type ligands [1,2]. Lewis-base with the general formulae L2 BR (III) [3,4,5,6,7,8,9] Such compounds are able to serve as electron-donating or L-type ligands, but the coordination chemistry of such nucleophilic boron compounds is rather unexplored [8,9,10]. Protonated carbodiphosphoranes of the type (R3 P) CH+ (II) can be deprotonated by strong bases and form their deprotonated analogues when coordinated to a metal centre [12]

Methods

Results

Conclusion