Synthesis, characterization and X-ray structural determination of palladium(0)–olefin complexes containing pyridin-thioethers as ancillary ligands. Equilibria and rates of olefin and ligand exchange

https://doi.org/10.1016/S0022-328X(00)00014-0Get rights and content

Abstract

The synthesis of Pd(0)–olefin complexes with pyridin-thioether ligands R′NSR is reported. X-ray structure determinations of selected species are described. The dynamic behavior was studied by variable-temperature 1H-NMR spectrometry. Equilibrium constants for olefin and chelate ligand exchange were determined by UV–vis spectrophotometry in chloroform at 25°C. The following metal–olefin stability order was observed: tetramethylethylenetetracarboxylate (tmetc)≈naphthoquinone (nq)<fumaronitrile (fn)≈maleic anhydride (ma)≪tetracyanoethylene (tcne). The ligand exchange equilibrium constants indicate that α-diimines and pyridin-thioethers affect the stability of the metal–bidentate ligand arrangement to a similar extent, as found in similar Pd(II) complexes. When the entering olefin is tmetc, the approach to equilibrium is slow so that both second-order rate constants k2 and k−2 could be determined along with their activation parameters for the reversible reaction of [Pd(η2-nq)(HNSiPr)] with tmetc. The results indicate an associative mechanism to be operative in these olefin exchange processes.

Introduction

We have been systematically studying the mechanism of nucleophilic attack at the allyl moiety in η3-allyl palladium(II) cationic complexes containing bi- and terdentate (NN, NP, NS, NSN, SNS, PNN, NNN) ligands in the presence of activated olefins (ol) by tetraphenylborate ion or amines. The reactions involve allylphenylation or -amination with concomitant formation of palladium(0) olefin complexes of the type [Pd(ol)(polydentate ligand)] [1]. We have also studied the reverse of the amination reaction, i.e. the oxidative allyl transfer from allylammonium cations to [Pd(η2-ol)(α-diimine)] [2]. It has become apparent that the stability of these zero-valent metal–olefin complexes, either as final products or reacting substrates, is of prime importance in the mechanistic pattern, being dictated by the nature of both the bidentate ligand and the olefin. In this context, we investigated equilibria and rates of olefin displacement in Pd(0) α-diimine complexes by activated, electron-poor olefins [3]. Equilibrium constants increased with increasing electron affinity of the entering olefin, whereas an associative pathway is operating for the slow, kinetically monitored reaction. These studies are among the few described so far concerning the stability and reactivity of zero-valent olefin complexes. Recently it has been shown that back donation of electron density from the metal to the olefin is the major factor affecting stability. Substitution of the olefin may occur via either associative or dissociative mechanisms, depending on the steric requirements of the bidentate ligand [4]. Associative paths had also been proposed for olefin exchange in [Pt(C2H4)(PPh3)2] [5] and [Pd(olefin)(PMePh2)2] [6] and for substitution of coordinated alkynes in [Pt(alkyne)(PPh3)2] complexes [7]. An interesting dynamic behavior is displayed by recently described palladium(0) complexes containing chelating diimines [4], [8], P,N- [9], and P,S-donor ligands [10].

Several of these palladium–olefin substrates have interesting applications in some catalytic reactions either as precursors or as active species [8], [11]. In view of these promising features and of the paucity of quantitative equilibrium and kinetic data available for these systems, we have investigated the thermodynamics and mechanism of substitution of the coordinated olefin by other, electron-poor olefins in complexes of N,S-donor chelating ligands R′NSR of the type [Pd(ol)(R′NSR)] (Scheme 1) in chloroform. We hoped to relate the resulting equilibrium and kinetic data to steric and electronic properties of the chelating ligand, the coordinated olefin, and the entering one.

Section snippets

X-ray diffraction studies

The molecular structure of complex [Pd(η2-ma)(HNStBu] has been confirmed by X-ray crystallography. The ortep diagram is shown in Fig. 1, while the selected bond distances and angles are listed in Table 1.

The bidentate HNStBu ligand is coordinated to palladium and the coordination plane also comprises the double bond (C(11)C(12)) of maleic anhydride. The palladium is situated on the least-squares plane defined by N,S,C(11) and C(12) atoms within a deviation of 0.026 Å from the plane. The

Olefin substitution rate constants

The rate of approach to equilibrium (1)Pdη2-nqR′NSRA+tmetcBPdη2-tmetcR′NSRC+nqDin chloroform at 25°C was determined by UV–vis spectrophotometric techniques. Spectral changes with time were monitored in the range 540–300 nm in the presence of a constant excess of tmetc over the metal substrate ([Pd]0=1×10−4 mol dm−3), in order to provide pseudo-first order conditions and to minimize the contribution of the reverse reaction (k−2) to the overall spectral changes. Under these conditions the

Experimental

The pyridin-thioether ligands (R′NSR) [1g] and the complex Pd2(DBA)3·CHCl3 [24] were prepared according to published procedures. All other chemicals were commercial grade and were purified or dried, where required, by standard methods [25].

IR and NMR measurements

The IR, 1H-, and 13C{1H}-NMR spectra were recorded on a Nicolet Magna™ 750 spectrophotometer and on a Bruker AC™ 200 spectrometer, respectively. The temperature-dependent 1H-NMR spectra were analyzed using the swan program [26] and the first-order rate constants determined were fitted to the appropriate temperatures by a reparametrized Eyring–Polanyi equation [27].

Kinetic and equilibrium measurements

The kinetics of olefin substitution were studied by addition of known aliquots of tmetc solutions to solutions of the complex under study in CHCl3 ([Pd]0≈10−4 mol dm−3) in the thermostatted cell compartment of a Lambda 40 Perkin–Elmer spectrophotometer at the designed temperature. The reactions were followed by recording spectral changes in the wavelength range of 300–540 nm or at a suitable fixed wavelength.

Equilibrium studies were performed by addition of microaliquots of a solution of the

X-ray structural analysis

Data collection, crystal, and refinement parameters are collected in Table 5. Accurate values for the unit-cell dimensions were determined from the angular setting of 50 reflections with Θ between 9 and 14° and intensity data were measured using the ω–2Θ scan technique. The structures were solved using heavy-atom methods, completed by subsequent difference Fourier synthesis, and refined on F2 by full-matrix least-squares procedures. All non-hydrogen atoms were refined with anisotropic

Supplementary material

Tables of atomic coordinates and isotropic thermal parameters, bond lengths and angles, anisotropic thermal parameters, and H-atom coordinates are available upon request. Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre for compound X. Copies of this information may be obtained free of charge from: The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (Fax: +44-1223-336033; e-mail: [email protected] or www:

References (29)

  • L. Canovese et al.

    J. Organomet. Chem.

    (1996)
  • F. Ozawa et al.

    J. Organomet. Chem.

    (1979)
  • R. Van Asselt et al.

    Tetrahedron

    (1994)
  • P. Uguagliati et al.

    Comput. Chem.

    (1984)
  • T. Ukai et al.

    J. Organomet. Chem.

    (1974)
  • P. Uguagliati et al.

    J. Organomet. Chem.

    (1979)
  • (a) B. Crociani, F. Di Bianca, P. Uguagliati, L. Canovese, A. Berton, J. Chem. Soc. Dalton Trans. (1991) 71. (b) B....
  • L. Canovese, F. Visentin, P. Uguagliati, B. Crociani, J. Chem. Soc. Dalton Trans. (1996)...
  • R. Van Asselt et al.

    Inorg. Chem.

    (1994)
  • P.T. Cheng et al.

    Inorg. Chem.

    (1971)
  • C.D. Cook et al.

    Inorg. Chem.

    (1971)
  • F. Gomez-de la Torre, F.A. Jalon, A. Lopez-Agenjo, B.R. Manzano, A. Rodriguez, T. Sturm, W. Weissensteiner, M....
  • M. Tschoerner et al.

    Organometallics

    (1997)
  • (a)R. Van Asselt, C.J. Elsevier, J. Mol. Catal. 65 (1991) L13. (b) R. Van Asselt, C.J. Elsevier, Organometallics 11...
  • Cited by (0)

    View full text