Elsevier

Inorganica Chimica Acta

Volume 378, Issue 1, 30 November 2011, Pages 239-249
Inorganica Chimica Acta

Facile synthesis and reactivity study of mixed phosphane–isocyanide Pd(II) and Pd(0) complexes

https://doi.org/10.1016/j.ica.2011.09.006Get rights and content

Abstract

The reaction between an equimolecular mixture of isocyanide CNR (CNR = di-methylphenyl isocyanide (DIC), tert-butyl isocyanide (TIC), triphenyl phosphane (PPh3) and a dechlorinated solution of the palladium allyl dimers [Pd(η3-allyl)Cl]2 (allyl = 2-Meallyl, 1,1-Me2allyl) in stoichiometric ratio yields the mixed derivative [Pd(η3-allyl)(CNR)(PPh3)] only. Apparently, the mixed derivative represents the most stable species among all the possible ones that might be formed under those experimental conditions. Theoretical calculations are in agreement with the experimental observation and the energy stabilization of the mixed species with respect to the homoleptic derivatives is traced back to an overall pushpull effect exerted by the isocyanide and the phosphane acting synergically. Similar behavior is observed in the case of the synthesis of the palladacyclopentadienyl complexes [Pd(C4(COOMe)4)(CNR)(PPh3)] and of the palladium(0) olefin complexes whose synthesis invariably yields the mixed [Pd(η2-olefin)(CNR)(PPh3)] derivatives. The paper includes studies on the reactivity toward allylamination in the case of the palladium(II) allyl complexes. A diffractometric investigation on the solid state structures of four different palladium isocyanide–phosphane complexes is also included.

Graphical abstract

New mixed complexes of palladium bearing isocyanides and triphenylphosphane as spectator ligands were prepared by a reaction based on the simple mixing of all the reactants in stoichiometric ratio. The reason for the peculiar result was traced back to the considerable stability imparted to the resulting mixed complexes by the synergic pushpull effect exerted by the concomitant presence of one isocyanide and one phosphane group as was also confirmed by theoretical calculations. The reactivity of the mixed species in some standard reactions was also studied together with the X-ray structural determination of four novel isocyanide-phosphane substrates.

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Highlights

► The dechlorinated complex [Pd(η3-allyl)Cl]2 yields only [Pd(η3-allyl)(CNR)(PPh3)]. ► The reactivity toward allylamination of the palladium(II) allyl complexes was studied. ► Diffractometric investigations on four isocyanide–phosphane complexes are included.

Introduction

With the aim of evaluating the electronic characteristics of the spectator ligands from kinetic data, we have been recently involved in a study dealing with the reactivity toward amination of palladium allyl complexes bearing mixed monodentate carbene, phosphite, phosphane and isocyanide ligands. The synthesis of the allyl derivatives bearing phosphanes and isocyanides ([Pd(η3-allyl)(L′)(L″)]ClO4), was indeed trivial since the complexes required were obtained by simple addition of equimolecular amounts of phosphane and isocyanide to a CH2Cl2 solution of the allyl dimer [Pd(η3-allyl)(μ-Cl)]2 dechlorinated with NaClO4 [1], [2]. We have also noticed that a non statistical mixture of homoleptic and mixed complexes was obtained when equimolecular solutions of different ligands (L′ = phosphites, L″ = isocyanides, and phosphanes) were added to the dechlorinated allyl dimer obtained as previously described.1 Moreover, simultaneous addition of different isocyanides, namely di-methylphenyl isocyanide (DIC) and tert-butyl isocyanide (TIC) again gave rise to the statistical mixture (25%:50%:25%) of all the possible species [3] irrespectively of their remarkable different steric hindrance and electron donating characteristics.

Therefore, among the explored ligands, only the equimolecular mixture of phosphanes (L′) and isocyanides (L″) yields the mixed derivatives [Pd(η3-allyl)(L′)(L″)]ClO4 as the sole products of the synthesis. In order to rationalize such a surprising behavior we undertook an accurate literature investigation on complexes of the type cis-[Pd(E-E)(CNR)(PR′3)]n+ (n = 0, 1; E–E = generic coordinating systems imposing cis geometry) [4]. Some derivatives ([Pd(CNR)(PR′3)X2] [5], [Pd(CNR)(PR′3)(R″)X] [8]) bearing the phosphanes trans to isocyanides can also be found in the literature together with complexes of the type trans-[Pd(CNR)2(PR′3)2] [4](h), [6].

From such an investigation it was clear that the mixed complexes were synthesized by means of different synthetic protocols and with different stoichiometric ratios among phosphanes, isocyanides and the metal, suggesting that no generalized approaches has in any case been exploited.

Thus, we thought that an understanding of the energetic reasons governing the formation of the mixed (or homoleptic) complexes and consequently the simplest experimental conditions for the achievement of the former was in order. In this respect, we have undertaken a systematic experimental investigation on the facile synthesis of some of the above mentioned mixed complexes of palladium(0) and palladium(II) and a theoretical study of their thermodynamic stability. Moreover, we have also carried out a diffractometric study on some of these substrates and investigation on the reactivity of the palladium(II) mixed allyl derivatives toward amination.

Section snippets

Result and discussion

The species and the complexes involved in the present study are summarized in Scheme 1.

Conclusion

We have prepared some new mixed complexes of palladium in different oxidation states bearing isocyanides and triphenylphosphane as mixed spectator ligands. The synthetic protocol was based on the reaction with strictly stoichiometric ratios of all the reactants involved and was traced back to the considerable stability imparted to the resulting mixed complexes by the synergic push–pull effect exerted by the concomitant presence of one isocyanide and one phosphane group. Notably such a result

Materials

All solvents were purified by standard procedures and distilled under argon immediately prior to use. Fumaronitrile was purified by sublimation under vacuum and piperidine distilled over NaOH. 1D- and 2D-NMR spectra were recorded using a Bruker 300 Avance spectrometer. Chemical shifts (ppm) are given relative to TMS (1H and 13C NMR) and 85% H3PO4 (31P NMR). UV–Vis spectra were recorded on a Perkin–Elmer Lambda 40 spectrophotometer equipped with a Perkin–Elmer PTP 6 (Peltier temperature

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      It is worth noting that the synthesis of the pure mixed complexes is not always warranted. In the present case, the well defined thermodynamic and kinetic conditions to be met in order to avoid the formation of the bis-NHC and bis-L complexes together with the wanted mixed species, have been apparently fulfilled [8 and Refs. therein]. The identity of the synthesized products can be definitely confirmed by the analysis of their NMR spectra.

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