Synthesis of novel palladium allyl complexes bearing heteroditopic NHC–S ligands. Kinetic study on the carbene exchange between bis-carbene palladium allyl complexes

doi:10.1016/j.jorganchem.2013.01.016

Journal of Organometallic Chemistry

Volume 732, 15 May 2013, Pages 27-39

https://doi.org/10.1016/j.jorganchem.2013.01.016 Get rights and content

Abstract

We have synthesized several novel palladium allyl and 1,1-dimethylallyl complexes bearing different heteroditopic NHC–S ligands giving rise to a five-membered chelate ring with the metal center. We were able to synthesize some homoleptic bis-carbene allyl derivatives by taking advantage of the hemilability of the thioetheric sulfur. Attempts at preparing mixed bis-carbene complexes bearing two different heteroditopic carbenes (i.e. NHC–S and NHC–Py) simultaneously coordinated to the palladium center lead to a carbene transmetalation with the formation of a statistically distributed equilibrium mixture of the two pure homoleptic and of the mixed bis-carbene palladium allyl complexes in solution. In two different cases the rate of the equilibrium reaction was measured and a mechanistic hypothesis provided. Finally, we have determined the solid state structures of a complex bearing only one NHC–S heteroditopic carbene and of the bis-carbene (NHC–S, NHC–Py) palladium allyl derivatives.

Graphical abstract

Novel palladium allyl complexes with NHC–S ligands were synthesized. Preparation of mixed bis-carbene complexes bearing two different carbenes simultaneously coordinated to the palladium center yield the formation of an equilibrium mixture of the two homoleptic and the mixed bis-carbene species. The reaction rates were measured and a mechanistic hypothesis advanced.

Highlights

► Novel allyl palladium complexes bearing two carbene ligands were synthetized. ► The kinetics of carbene exchange were studied in two different cases. ► The solid state structures of two palladium allyl carbene complexes were determined.

Introduction

The family of efficient σ donor N-heterocyclic carbenes (NHC) which impart a remarkable stability and catalytic performances to their transition metal derivatives [1], has been soon extended to include a new class of bidentate ligands carrying another coordinating functionality. The coordination of heteroditopic NHC–E (E = P, N, O) ligands to transition metals yields an important class of new compounds [2]. In particular, the complexes characterized by a secondary, labile heteroatom act as efficient and stable catalysts since the dangling wing can restore the starting complexes by re-coordination of the site made vacant by their catalytic activity [3]. We have been long involved in the study of the synthesis, behavior in solution and reactivity of Pd(0) and Pd(II) complexes with bidentate or terdentate ligands bearing at least one thioether function [4]. Thus, owing to the not particularly high number of heteroditopic bidentate sulfur–carbene ligands in the literature [5], we decided to synthesize some new NHC–S chelating moieties and the corresponding palladium allyl complexes. We undertook such an investigation since the majority of NHC palladium allyl complexes displaying a marked catalytic activity [6] is mainly stabilized by monodentate carbene ligands [7]. On the contrary, the bidentate heteroditopic derivatives are comparatively less common [8] whereas the NHC–S complexes represent a rarity [5](c), [5](d), although the thioether wing might impart peculiar catalytic properties to their complexes owing to the stereogenic nature of the coordinated sulfur [5k].

The NHC–S ligands that can form five-membered C–S ring upon coordination to a metal center, are also rare [5](h), [5](l), [5](n) and no allyl derivatives with this kind of spectator ligands are reported in the literature. In addition, it is known that two different heteroditopic carbene ligands bearing nitrogen or sulfur as secondary atom can simultaneously coordinate to either Pd(0) [5](n), [9] or Pd(II) [10] and this very fact might suggest interesting studies on the possible carbene exchange. As a matter of fact, despite the great deal of papers on the transmetalation reactions involving different metals such as the carbonyl carbene complexes of the group 6 metals [11] or AgBr(NR,NCH₂R′-NHC) substrates [12] with several different transition metal derivatives, to the best of our knowledge no studies on the carbene exchange between Pd(II) compounds have appeared in the literature, although Caddick and Cloke have already demonstrated the feasibility of carbene exchange in Pd(0) complexes, whereas Yamamoto and Espinet have found that the aryl groups can exchange between Pd(II) aryl complexes [13]. In the present study we show that the allyl palladium derivatives bearing five-membered NHC–S ligands can be easily synthesized and that the transmetalation reaction involving exchange of the heterobidentate ligands between different palladium allyl complexes takes place through an associative mechanism that is strongly dependent on the nature of the dangling wing.

Section snippets

Palladium allyl complexes

We have firstly synthesized the imidazolium salts described in Scheme 1 by reacting the suitable R-imidazole with chloromethyl-methyl sulfide, chloromethyl-phenyl sulfide or chloromethyl-pyridine in acetonitrile in the presence of KBr (Scheme 1):

1a and 1c represent newly synthesized ligands, whereas 1b [5l], 1d [5n] are literature compounds. All the bidentate NHC–S ligands in Scheme 1 form a five-membered chelate ring upon complexation with the group 10 metals.

The silver carbene derivatives (2)

Conclusions

We have synthesized and characterized some potentially five-membered NHC–S ligands and the corresponding new chelate palladium allyl derivatives by transmetalation between the silver carbene derivatives AgBr(NR,NCH₂SR′-NHC) and the Pd(II) allyl chloro dimer, followed by dechlorination of the monodentate derivative with NaClO₄. In the case of the reaction involving the dimer [Pd(η²-Me₂-ally)(μ-Cl)]₂ only the complex with the bis-substituted allyl terminus trans to the carbene carbon was obtained

Solvents and reagents

All solvents were purified by standard procedures and distilled under argon immediately before use. 1D- and 2D-NMR spectra were recorded using a Bruker 300 Avance spectrometer. Chemical shifts (ppm) are given related to TMS (¹H and ¹³C NMR). Peaks are labeled as singlet (s), doublet (d), triplet (t), quartet (q), multiplet (m) and broad (b). The proton and carbon assignments were carried out by ¹H–2D COSY, ¹H–2D NOESY, ¹H–¹³C HMQC and HMBC experiments.

IR spectra were recorded on a Perkin–Elmer

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Synthesis of novel palladium allyl complexes bearing heteroditopic NHC–S ligands. Kinetic study on the carbene exchange between bis-carbene palladium allyl complexes

Abstract

Graphical abstract

Highlights

Introduction

Section snippets

Palladium allyl complexes

Conclusions

Solvents and reagents

Curr. Org. Chem.

Eur. J. Inorg. Chem.

Organometallics

Organometallics

Organometallics

Appl. Organomet. Chem.

Dalton Trans.

Dalton Trans.

Organometallics

Coord. Chem. Rev.

Organometallics

Dalton Trans.

Organometallics

Organometallics

Organometallics

Organometallics

Angew. Chem. Int. Ed.

Coord. Chem. Rev.

Eur. J. Inorg. Chem.

Coord. Chem. Rev.

Angew. Chem. Int. Ed.

Angew. Chem. Int. Ed.

Eur. J. Inorg. Chem.

Annu. Rep. Sect. B (Org. Chem.)

Coord. Chem. Rev.

Coord. Chem. Rev.

Angew. Chem. Int. Ed.

Organometallics

Organometallics

Organometallics

Organometallics

Chem. Commun.

Chem. Commun.

Eur. J. Inorg. Chem.

Organometallics

Organometallics

Dalton Trans.

Dalton Trans.

Organometallics

Dalton Trans.

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Organometallics

Dalton Trans.

Acc. Chem. Res.

Organometallics

Coord. Chem. Rev.

Organometallics

Organometallics

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Org. Lett.

Chem. Eur. J.

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