Luciano Canovese

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

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.

Palladium(II) Allyl Complexes with Nitrogen-Sulfur Bidentate Ligands.Substituent Effects in the Mechanism of Allyl Amination.

The reactivity of palladium(II) allyl complexes containing the nitrogen–sulfur bidentate ligand N–SR (N–SR=2-(phenylthiomethyl)pyridine, 2-(phenylthiomethyl)-6-methylpyridine, 2-(tert-butylthiomethyl)pyridine) was studied in CHCl3 in the presence of the activated olefin fumaronitrile (fn). The stepwise mechanism involves a fast pre-equilibrium in which the N–SR ligand is displaced by the amine with formation of an inert bis-amino allyl species and concomitant rate-determining bimolecular attack of the amine on the coordinated allyl moiety to give the allylamine and the olefin-stabilized Pd(0) complexes [Pd(η2-fn)(N–SR)]. The influence of the substituents at the allyl fragment and at the nitrogen–sulfur ligand is rationalized together with the fluxional behavior in solution.

Binding ability of 2,6-bis(methylthiomethyl)pyridine with proton, palladium(II) and copper(II) in aqueous solutions

Abstract The acidity constant of the tridentate ligand 2,6-bis(methylthiomethyl)pyridine (L) and formation constants of its Pd(II) and Cu(II) complexes [PdLTu]2+ and [CuL(H2O)]2+ have been determined in aqueous solutions by potentiometric and spectrophotometric techniques. The acidity constant as determined by potentiometry is log K=4.04±0.04 (4.01±0.02 by spectrophotometry), whereas the formation constants for the Pd(II) and Cu(II) species are log K=28.92±0.09 and 4.6±0.1 (4.41±0.04), respectively. Some preliminary results on the high selectivity for Pd(II) over Cu(II) of a macroporous polystyrene-divinylbenzene resin bearing the same chelating group are also reported.

Novel palladium(II) allyl complexes with nitrogen-sulfur donor bidentate ligands. Mechanism of allyl amination of [Pd(η3-allyl)-(N-SR)]ClO4 (allyl = C3H5; N-SR = C5H4N-2-CH2SR, R = C6H5, C2H5) in the presence of activated olefins. X-ray structure determination and fluxional behavior

Abstract The reactions of [Pd(η3-C3H5)(C5H4-N-2-CH2SR) (R = C6H5, C2H5) with amines in the presence of fumaronitrile (in), involving the formation of allylamines, were studied kinetically in CHCl4, by UV-Vis techniques. The proposed stepwise mechanism involves a fast pre-equilibrium in which the mixed N-SR ligand is displaced by the amine, giving an inert bis-amino allyl species. The concomitant rate-determining bimolecular attack by the amine yields the allylamine and the corresponding Pd(O) complexes [Pdη3-fn) (C3H54N-2-CH2SR)]. The solid state structure of complex was determined by X-ray crystallography. A preliminary 1H-NMR study of the fluxional manifestations was carried out. The apparent rotation of the allyl moiety is operative at the lowest temperature increasing the temperature promotes the breaking and rearangement of the Pd-S bond and, finally, the η′-η′-η′ rearrangement of the allyl fragment.

Palladium(0)–olefin complexes with potentially terdentate nitrogen–sulfur ligands. The role of the chelate in the olefin exchange path

Abstract The synthesis and the reactivity of Pd(0) olefin complexes [Pd(η 2 -olefin)(SNS)] and [Pd(η 2 -olefin)(NSN)] containing potentially terdentate nitrogen–sulfur ligands were studied. The presence of a potentially coordinating atom in the environment of the metal, influences strongly the fluxional behavior in solution but not the overall reactivity with respect to olefin exchange and thermodynamic stability which is very close to that of the corresponding bidentate nitrogen–sulfur complexes. The intimate mechanism of olefin exchange also involves a path promoted by the third dangling coordinating atom which induces olefin dissociation and stabilizes the ensuing Pd(0) three-coordinated species.

The marked influence of steric and electronic properties of ancillary pyridylthioether ligands on the rate of allene insertion into the palladium–carbon bond

Abstract Neutral methyl- and acyl-palladium chloro complexes containing pyridylthioether ancillary ligands (R′NSR) (R′=H, Me, Cl; R=Me, Et, i-Pr, t-Bu, Ph) have been synthesised and characterised by elemental analysis and spectroscopic methods. The reactivity of these complexes toward allene (allene=DMA=1,1-dimethylpropadiene; TMA=1,1,3,3-tetramethylpropadiene) insertion into the palladium–carbon bond has been studied by 1H-NMR and UV–vis techniques. The rate of reaction appears to be strongly influenced by the steric and electronic properties of the ancillary ligand. The distortion induced by the substituent R′ in position 6 of the pyridine ring on the main coordination plane of the substrate (allowed by sulphur sp3 hybridisation) renders the substrate itself more prone to nucleophilic attack by the allene. The rate of allene insertion can further be enhanced by lowering the basicity of the chelating atoms in the NS moiety which results in an increase of electrophilicity of the palladium core, so that the rate constants measured in the case of the complexes containing the ligand 6-chloro-2-phenylthiomethylpyridine (ClNSPh) are by far the greatest observed so far for similar reactions. Furthermore, on the basis of the indications emerging from the exhaustive study on the behaviour of all the related pyridylthioether methyl complexes, an associative asynchronous bond making mechanism for the rate determining nucleophilic attack by allene is proposed.

Palladium(II) allyl complexes with potentially terdentate ancillary ligands. Mechanism of allyl amination by piperidine

Abstract The reactivity of palladium(II) allyl complexes containing potentially terdentate ligands toward allyl amination was studied in CHCl 3 in the presence of the activated olefin fumaronitrile. The influence of different L–L′–L terdentate ligands on the fluxionality in solution and on the reactivity was discussed. Quite surprisingly the behavior of S–N–S, N–S–N and N–N–N ligands is very similar to that of the corresponding N–S and N–N bidentate ligands. In these cases the conventional stepwise mechanism is observed which involves a fast pre-equilibrium in which the terdentate ligand is displaced by the entering amine and the concomitant rate-determining bimolecular attack of the amine itself to give the final allylamine and the olefin-stabilized Pd(0) complex. At variance, the P–N–N ligand imparts to the allyl complex a reactivity similar to that of the corresponding complexes containing a strongly hindered bidentate P–N species, from which the ligand is not displaced thanks to the strongly bound phosphine group.

Kinetics of the displacement of cyclobutane-1,1-dicarboxylate from diammine(cyclobutane-1,1-dicarboxylato)platinum(II) in aqueous solution

The displacement of 1,1-cyclobutanedicarboxylate (cbdca2–) from [Pt(NH3)2(cbdca)] has been studied in aqueous solution. In the presence of acid the process resembles the successive displacement of two monodentate carboxylates. The first (ring-opening) stage follows the rate law kobs.=(k0+k1k0[H+])(1 +k0[H+])–1, k0= 8 × 10–5 s–1, K0= 0.6 dm3 mol–1, k1= 8.0 × 10–4 s–1 at 25 °C, while the second follows the simple relationship kobs.=k[H+], k= 1.61 × 10–4 dm3 mol–1 s–1 at 25 °C. In the absence of acid and other nucleophiles the complex is inert and in the presence of chloride the displacement of ligand follows a first-order dependence on [Cl–], kobs.=kCl[Cl–]. At 80 °C, kCl= 1.32 × 10–4 dm3 mol–1 s–1. The chelate differs from the bis-monodentate carboxylate species in the great importance of the reverse, ring-closing process, which can be prevented in the presence of acid.

Phenylation of cationic allyl palladium(II) complexes by tetraphenylborate. Synthesis of α-diimine olefin palladium(0) complexes and mechanistic aspects

The cationic allyl complexes [Pd(η3-2-R1C3H4)(N–N′)]+(N–N′=α-diimine ligand; R1= H or Me) react with BPh4– in the presence of activated olefins to give [Pd(η2-olefin)(N–N′)](olefin = fumaronitrile, dimethyl fumarate or maleic anhydride) and PhCh2C(R1)CH2. The palladium(0) derivatives can be isolated in good yield and have been characterized by elemental analysis, molecular weight measurements and standard spectroscopic techniques. The reaction rates increase with increasing π-accepting ability of the α-diimine, with decreasing steric requirements of the imino carbon substituents and with decreasing stability towards palladium–nitrogen bond breaking in the parent cationic compounds. The rates also increase with decreasing relative permittivity and co-ordinating properties of the solvent. Kinetic measurements in aqueous (2% v/v) methanol provide pseudo-first-order rate constants that are independent of both BPh4– and olefin concentrations. This has been interpreted on the basis of extensive ion pairing between the cationic substrate and the BPh4– anion, followed by rate-determining phenyl transfer to the palladium centre and fast reductive elimination of allylbenzenes.

A novel mechanism for the fluxional behaviour of [Pd(η2-tetramethylethylenetetracarboxylate)(2-methylthiomethylpyridine)]

Abstract The fluxional behaviour of [Pd(η 2 -tmetc)(NSMe)] (tmetc=tetramethylethylenetetracarboxylate, NSMe=2-methylthiomethylpyridine) in CD 2 Cl 2 is governed by two mechanisms: (i) the concentration independent inversion at sulfur which averages the methylenic signals and collapses the four methyl signals of tmetc to two; (ii) the concentration dependent mechanism occurring via a dimeric intermediate with C S symmetry which collapses the two residual tmetc signals into a singlet.