Elsevier

Inorganica Chimica Acta

Volume 363, Issue 11, 10 August 2010, Pages 2375-2386
Inorganica Chimica Acta

Review
Synthesis, stability and reactivity of palladium(0) olefin complexes bearing labile or hemi-labile ancillary ligands and electron-poor olefins

Dedicated to Prof. Fred Basolo
https://doi.org/10.1016/j.ica.2010.04.017Get rights and content

Abstract

An overview of the general features of electron-poor olefin stabilized palladium(0) complexes bearing labile and hemi-labile ancillary ligands is presented. In particular, we have summarized the synthetic methodologies, the ligands commonly used, and the characterization of such complexes. The behavior of these species in solution is also described with particular attention to their fluxional rearrangements and reactivity. Thus, olefin exchange reactions are described and a comprehensive order of coordinative capability of the most widely used electron-poor alkenes is presented. The reactions of the title complexes dealing with olefin isomerization, oxidative addition, and formation of palladacyclopentadiene derivatives are eventually reported together with their main structural characteristics.

Graphical abstract

An overview of the general features of electron-poor olefin stabilized palladium(0) complexes bearing labile and hemi-labile ancillary ligands is presented. In particular, we have summarized the synthetic methodologies, the ligands commonly used, and the characterization of such complexes. The behavior of these species in solution is also described with particular attention to their fluxional rearrangements and reactivity.

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Introduction

The importance of palladium and its complexes as catalysts is of the utmost interest and a general attempt at reviewing exhaustively such a topic might represent an almost impossible challenge [1]. Therefore, any investigation on particular aspects of the characteristics and of the reactions promoted by palladium complexes needs to be specifically addressed. Among all the palladium catalysts, the zero-valent derivatives bearing coordinated olefins are particularly important since they are often used or identified as the active catalytic species in a variety of cross-coupling reactions [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. Moreover, the bonding attitude of olefins toward palladium(0) was extensively studied from theoretical and structural point of view [17] and were developed on the basis of the original work of Dewar [18] and Chatt and Duncanson [19]. At the best of our knowledge, however, the intrinsic characteristics of the olefin palladium(0) complexes bearing labile and hemi-labile ancillary ligands were never analyzed in detail. As a matter of fact, these complexes display interesting features since a coordinating site might be easily available upon partial or total displacement of the ancillary ligand.

We therefore, think that a thorough investigation on the synthesis and the reactivity of these complexes could be useful in rationalizing and planning their design and use. Thus, the synthetic approach, the stability, the peculiar fluxional rearrangements in solution and some stoichiometric reactions of Pd(0) alkene derivatives with ancillary bi- and ter-dentate ligands bearing only labile (N–N, N–S, N–S–N, S–N–S) or hemi-labile (P–N, P–S, P–N–N) coordinating atoms and electron-poor olefins will be discussed in this paper.

Section snippets

The olefins in the synthesis of palladium(0) complexes

Irrespectively of the nature of the ancillary ligands, the stability and hence the synthesis of palladium(0) olefin derivatives strongly depends on the electronic characteristics of the olefins themselves. Olefins bearing electron withdrawing groups at unsaturated carbons (deactivated olefins) stabilize the low oxidation state of palladium since they favour the electron back donation from the electron rich metal centre to the π antibonding orbitals of the alkenes and therefore the separation

Synthesis of the palladium(0) olefin complexes

The synthesis of the palladium(0) olefin complexes of general formula [Pd(η2-ol)(L–L′–L″)] where L–L′–L″ represents a generic labile or hemi-labile bi- or ter-dentate ligand (L = L′ = L″, or L = L″  L′) and ol the stabilizing olefin can be achieved by means of some different protocols.

Protocol (a): This protocol, consisting in reacting the complex Pd2(DBA)3·CHCl3 [23] (DBA = dibenzyliden-acetone) with the appropriate ligand and olefin in anhydrous acetone under inert atmosphere, represents the first

The labile and hemi-labile ligands

The most common labile and hemi-labile ligands used in the synthesis of palladium(0) olefin complexes are summarized in the following Scheme 2.

In the case of the ligand PNN, no palladium(0) olefin derivatives were isolated. Vrieze and co-workers however, were able to separate a complex containing the PNN ligand and palladium(0) without any stabilizing coordinated olefins (naked palladium) [45]. Similar species could be employed as starting materials for the subsequent synthesis of olefin

Characterization of the complexes

The remarkable metal–alkene π-back donation becomes apparent when the chemical shifts of the alkene protons and carbons are compared with those of the uncoordinated moieties. As a matter of fact, the chemical shifts of the protons and carbons ascribable to the coordinated olefins resonate at 2–3.5 and 80–120 ppm, respectively, upfield with respect to those of the free uncoordinated molecules. As expected, as a further confirmation a shift to lower frequency of the νCdouble bondO and νCtriple bondN stretching (when

Fluxional rearrangements

In complexes of general formula [Pd(η2-olefin)(L–L′)] fluxionality can entail metal-olefin and/or metal-ancillary ligand rearrangements which are usually described by the following reported processes and depicted in Scheme 4:

  • (1)

    propeller-like olefin rotation. This is the earliest and most frequently proposed mechanism [30], [42], [43], [46], [47], [48];

  • (2)

    olefin–metal dissociation followed by recombination [30];

  • (3)

    intermolecular associative process with free alkene, via the intermediacy of a [M(η2

Olefin exchange

The stability of the Pd(0) olefin complexes is of remarkable importance in modulating the reactivity of these species especially when they are employed in catalysis. It is also apparent that their stability is governed by the steric and electronic characteristics of the olefins themselves. Therefore, in order to assess the coordinative capability of different olefins toward Pd(0) complexes, reaction (1) has been studied in detail:[Pd(η2-ol1)(LL)]+ol2[Pd(η2-ol2)(LL)]+ol1

As a matter of fact,

Ancillary ligand exchange

At the best of our knowledge the exchange reactions among ancillary ligands in the case of olefin derivatives of palladium(0) were quantitatively studied only in the case of the exchange between the ligands 2-pyridylmethanimine (PyN2) and 2-methylthio-t-butyl pyridine HN-StBu [32].[Pd(η2-ol)(PyN2)]+HN-St-BuKexc[Pd(η2-ol)(HN-St-Bu)]+PyN2

The complexes [Pd(η2-ol)(PyN2)] (ol = fn, tmetc, nq, ma) were spectrophotometrically titrated with a CHCl3 solution of the ligand HN-St-Bu. The ensuing Kexc span

Alkene isomerization

In some cases the palladium(0) olefin complexes induce steric rearrangement of the coordinated olefin. It was experimentally and theoretically demonstrated that dimethyl maleate and (Z)-1,2-bis[(4-methylphenyl)sulfonyl]ethene) undergo isomerization into their trans counterparts catalyzed by the complexes [Pd(η2-dmfu) (DPPQ)] and [Pd(η2-dmfu) (DPPQ-Me)]. In order to rationalize such behavior, it was surmised that such rearrangement is due to the cooperative influence of the two coordinating

Complexes bearing mono- bi- and ter-dentate nitrogen ligands

A significant number of palladium(0) olefin compounds bearing the title ligands was synthesized by several authors [30], [31], [35], [37], [75], [78], [79], [80], [81], [82], [83], [84], [85], [86]. All the structures display the typical trigonal arrangement around the palladium centre with the Pd–N bond lengths being scarcely influenced by the olefin nature, spanning within 2.10 and 2.18 Å. The Cdouble bondC bond length of the variety of used olefins is also scarcely affected, being confined in a narrow

Luciano Canovese was born in Padua in 1950. He graduated in Chemistry from Padua University in 1973. He joined the Chemistry department of Ca’ Foscari Venice University in 1976 where he is now Associate Professor in Chemistry. He followed a post-doctoral research at the University College London (UK) in 1982–1984. His research interest concerns mechanisms in Organometallic Chemistry.

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    Luciano Canovese was born in Padua in 1950. He graduated in Chemistry from Padua University in 1973. He joined the Chemistry department of Ca’ Foscari Venice University in 1976 where he is now Associate Professor in Chemistry. He followed a post-doctoral research at the University College London (UK) in 1982–1984. His research interest concerns mechanisms in Organometallic Chemistry.

    Fabiano Visentin graduated in Industrial Chemistry in 1990, received his Ph.D in Chemistry from Ferrara and Venice Universities under the supervision of Prof. P. Uguagliati. After a post-doctoral study at the C.N.R. of Padua with Prof. R. Michelin and R. Bertani he joined the Chemistry Department of Venice University first as Researcher and then as Associated Professor. In 2004 he was in ETH Zurich (Switzerland) as Visiting Scientist in the group of Prof. A. Togni.

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