The importance of the electronic and steric features of the ancillary ligands on the rate of cis–trans isomerization of olefins coordinated to palladium(0) centre. A study involving (Z)-1,2-ditosylethene as olefin model

doi:10.1016/j.poly.2019.114144

Polyhedron

Volume 173, 15 November 2019, 114144

https://doi.org/10.1016/j.poly.2019.114144 Get rights and content

Abstract

With the aim of better understanding the mechanisms governing the spontaneous cis–trans isomerization of the olefins promoted by palladium(0) centers, we have synthesized several novel complexes bearing (Z)- or (E)-1,2-ditosylethene and a combination of other two spectator ligands including phosphines (PPh₃ and 1,3,5-triaza-7-phosphaadamantane), isocyanides (i.e. 2,6-dimethylphenylisocyanide) and N-heterocyclic carbenes (NHCs). The nature of ancillary ligands heavily influences the steric and electronic features of the metal centre and consequently its interaction with olefin determines a different capability of supporting the isomerization process. The kinetic analysis and the determination of the rate constants of the isomerization suggested that both the steric and electronic factors play an important role and only an unpredictable combination can guarantee the success of the reaction.

The work is completed by the structural characterization of three different Pd(0) complexes.

Graphical abstract

Introduction

Owing to the importance of palladium(0) olefin complexes as catalysts or pre-catalysts in the field of the homo and hetero cross-coupling processes [1] and alkyne hydrogenation [1b] we have been often involved in the studies on the stability and the related reactivity of such compounds for long time.

In particular, we have determined: (i) the scale of stability imparted by the different olefins to the derived complexes and consequently new synthetic protocols have been proposed [2]; (ii) the nature of the fluxionality affecting the palladium olefin derivatives [3]; (iii) the reactivity of palladium(0) complexes as a function of the their spectator ligands [4] and of the coordinated olefin [2], [5].

In this latter case, we have sometimes noticed that the palladium(0) complexes stabilized by the strong electron-withdrawing (Z)-1,2-ditosylethene (cis-disulf) promoted a spontaneous reaction yielding the corresponding complex bearing the isomerized olefin (E)-1,2-ditosylethene (trans-disulf) which in all cases represents the energetically favored species [2], [5].

Owing to the great importance of olefin isomerization processes in chemistry [6] and biology [7] and the originality of the observed process which is usually promoted by external factors [5 and Refs. therein], we decided to extend our knowledge on this kind of systems and carry out a reasonably exhaustive study on the capability of the different spectator ligands in stabilizing the cis-disulf derivatives or inducing its isomerization. In this respect we decided to synthesize several palladium(0) complexes stabilized by the olefins 1,2-ditosylethene (E and Z) and a selected combination of spectator ligands i.e. phosphines (PPh₃ and 1,3,5-triaza-7-phosphaadamantane), isocyanides (i.e. 2,6-dimethylphenylisocyanide) and N-heterocyclic carbenes (NHCs). Although the strong σ-donating property is a common feature of these ligands, their steric and π-accepting nature can be modulated by a careful choice of the substituents thereby providing some more information on the mechanism involved in the olefin isomerization. Our interpretation of the mechanism of isomerization is discussed in this paper whereas the synthesized and studied complexes are reported in the following Scheme 1.

Section snippets

Synthesis of the olefin complexes with two identical spectator ligands

Complexes 1a–5a are thermodynamically stable whereas in the case of the 1b–4b complexes the isomerization of the coordinated cis-disulf cannot be excluded a priori. Therefore, the preparation of the complexes 1b–4b was checked by NMR in order to establish the rate of the reaction, the work-up conditions and the temperature. However, 1b–4b complexes do not show particular propensity to fast isomerization and only after several hours is a partial olefin rearrangement observed. However, in order

Conclusion

The number of the synthesized and fully characterized palladium(0) complexes coordinating a chosen range of ancillary ligands with different electronic and steric characteristics and the comparative kinetic study allow a more exhaustive interpretation than that advanced before of the mechanism of spontaneous isomerization of the coordinated (Z)-1,2-ditosylethene into the coordinated (E)-1,2-ditosylethene. Thus, the conclusions drawn elsewhere [2], [5] have been partially updated and in

Solvents and reagents

The solvent CH₂Cl₂ and CHCl₃ were distilled over CaH₂, acetone was refluxed over 4 Å molecular sieves and distilled. All other solvents and chemicals were commercial grade products and used as purchased.

IR, NMR, UV–Vis measurements and elemental analysis

The IR, ¹H, ¹³C and ³¹P NMR spectra were recorded on a Perkin-Elmer Spectrum One spectrophotometer and on a Bruker 300 or 400 Advance spectrometers, respectively. The elemental analysis of the synthesized complexes was carried out using an Elemental CHN “CUBO micro Vario” analyzer.

Synthesis of the complexes Sa–c

Complexes Sa,

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Polyhedron

Abstract

Graphical abstract

Introduction

Section snippets

Synthesis of the olefin complexes with two identical spectator ligands

Conclusion

Solvents and reagents

IR, NMR, UV–Vis measurements and elemental analysis

Synthesis of the complexes Sa–c

References (25)

Chem Rev.

Acta Crystallogr., Sect. D

Acta Crystallogr., Sect. A

Acta Crystallogr., Sect. D

J. Appl. Crystallogr.

The PyMOL Molecular Graphics System

Theor. Chem. Acc.

J. Chem. Phys.

J. Chem. Theory Comput.

J. Chem. Phys.

Essentials of Computational Chemistry

Introduction to Computational Chemistry

Gaussian 09

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Biological and Catalytic Applications of Pd(II)-Indenyl Complexes Bearing Phosphine and N-Heterocyclic Carbene Ligands

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