Elsevier

Journal of Organometallic Chemistry

Volume 794, 1 October 2015, Pages 288-300
Journal of Organometallic Chemistry

Synthesis and characterization of palladacyclopentadiene complexes with N-heterocyclic carbene ligands

https://doi.org/10.1016/j.jorganchem.2015.07.014Get rights and content

Highlights

  • Novel Palladacyclometallate complexes with NHC ligands were prepared.

  • Requirements and procedures to obtain κ1 or κ2 coordination mode were discussed.

  • Structures of the complexes were described in solid state and solution.

  • A kinetic study of the equilibration between two structural isomers was reported.

Abstract

New palladacyclopentadiene compounds containing different chelate NHC-thioether and NHC-pyridine ligands have been prepared by transfer of the functionalized carbenes from the respective silver complexes to the polymeric precursors [PdC–COOR)4]n (R = Me, t-Bu). Their dynamic behaviour in solution was discussed and the solid-structure of 2c was determined by X-ray crystallography.

The treatment of [Pd(C–COOCH3)4]n with two equivalents of the carbene silver complexes led to the (NHC)2Pd(C4–COOCH3)4 derivatives (3c–i), a new class of compounds with only Pd–C bonds. A serious limitation to this synthetic procedure is an excessive steric crowding around the metal centre.

The complexes 3 are present in solution as a mixture of two atropoisomers, due to restricted rotation around the Carbene–Pd bond. The kinetics of equilibration between the two configurational isomers was studied for complex 3c, which was also structurally defined by X-ray crystallography (anti isomer).

Finally a synthetic protocol was set up for the synthesis of mixed NHC-Phosphine and NHC-Isocyanide palladacyclopentadiene complexes. In this procedure the order of addition of the reactants is of great importance.

Introduction

The palladacyclopentadiene fragment Pd(C–COOR)4 is an interesting organometallic building unit that is obtained by oxidative homocoupling of acetylenic esters to Pd(0) substrates [1], [1](a), [1](b), [1](c), [1](d), [1](e), [1](f), [1](g) and the mechanistic study of this process was the subject of some previous publications by our group [2], [2](a), [2](b). Its synthetic importance arises from the fact that this organometallic functional group was involved in the Pd(0)-catalysed [2+2+2] alkyne cyclotrimerization and cocyclotrimerization of acetylenes with alkenes, dienes or allenes [1], [2], [3]. Furthermore its reactivity toward organic halides or molecular halogens resulted in the production of (σ-dienyl)palladium compounds formed by a sequence of oxidative addition/reductive elimination through a transient Pd(IV) intermediate (Scheme 1) [4]. Remarkably such process was stereospecific and usually only the cis-arrangement of the esteric functions at the double bonds was obtained. Since the addition of molecular halogens [4] or organotin [5] reagents allowed the displacement of the dienyl fragment from the metal, it was possible to take advantage of this synthetic strategy to produce dienes with a Z–Z configuration. In this respect only a limited number of methods are available for the stereospecific preparation of dienes from acetylenes [6], [6](a), [6](b), [6](c), [6](d), [6](e), [6](f).

In this context the features of spectator ligands are of great importance; it was proved that the ability to couple two alkynes decreases with the π-acceptor capability and steric bulkiness of the ancillary ligands of the starting Pd(0) compounds. In some cases the monoalkyne-Pd(0) complexes were isolated instead of the palladacyclopentadiene derivatives even with excess of alkynes (i.e. with L = P(OPh)3 [1b], or L–L = 2,9-dimethylphenanthroline [7]). In any case several mononuclear complexes [L2Pd(C-COOR)4] are reported in the literature with monodentate or bidentate supporting ligands [1], [2](a), [3](d), [8]. Among them N-heterocyclic carbenes have been scantily utilized in spite of their ubiquitous utilization as spectator ligands in palladium organometallic chemistry [9], [9](a), [9](b), [9](c), [9](d) and to the best of our knowledge the two examples shown in Scheme 2 only have been described so far.

Complex a was observed by Elsevier and co-workers as a by-product in the Pd(NHC)-catalysed semihydrogenation of alkynes [10] whereas complex b was detected by our group when studying the reaction of [(Me–NHC–CH2Py)Pd(η2-MA)] (MA = maleic anhydride) with an excess of dimethylbutynedioate [11].

In the present paper we intend to partially fill this gap by describing the general procedures for the synthesis of some different classes of palladacyclopentadienes stabilized by NHC ligands. Furthermore the structural features and solution behaviour of these new complexes will be discussed in detail.

The monodentate or heteroditopic N-heterocyclic carbene ligands employed are summarized in Scheme 3.

Section snippets

NHC-pyridine and NHC-thioether chelate palladacyclopentadiene complexes

The silver-mediated transfer of functionalized carbene ligands onto appropriate palladium precursors was the synthetic strategy employed to achieve the title complexes [12]. In more detail these compounds were prepared by stoichiometric treatment of the respective silver complexes 1 [13] with the polymeric precursors [Pd(C-COOR)4]n [2a] (R = Me, t-Bu), in dichloromethane at room temperature. The precipitation of silver bromide was observed almost immediately and the final products could be

Conclusions

In this contribution we have proposed the synthesis and full characterization of a scarcely explored class of palladium(II) complexes containing the organometallic building unit Pd(C-COOR)4 and N-heterocyclic carbene ligands with various substituents in the wingtip position. The presence of a second coordinating function on the ligands has allowed us to obtain κ2-C,N or κ2-C,S chelate structures with one pyridyl-carbene or one thioethereal-carbene moiety per metal upon coordination at the

Materials

Unless otherwise stated, all operations were carried out under an argon atmosphere using standard Schlenk techniques. All solvents were purified by standard procedures and distilled under argon immediately prior to use. 1D and 2D-NMR spectra were recorded on a Bruker 300 Avance spectrometer. Chemical shifts (ppm) are given relative to TMS (1H and 13C NMR) and 85% H3PO4 (31P NMR).

Peaks are labelled as singlet (s), doublet (d), triplet (t), quartet (q), multiplet (m) and broad (br). The proton

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