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