Gavino Chessa

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.

Synthesis and characterization of square-planar platinum(II) and palladium(II) complexes with pyridine-2,6-dicarboxylic acid (H2dipic). X-Ray crystal structure of trans-Na2[Pt(dipic)2]6H2O and K[Pt(dipic)I]1/2H2O

Abstract The tetra-coordinate complexes trans-[Pt(Hdipic)2]·2H2O (1) and trans-[Pd(Hdipic)2]·2H2O (2) as well as their potassium and sodium derivatives trans-K2[Pt(dipic)2] (3), trans-Na2[Pt(dipic)2]·6H2O (4) and trans-Na2[Pd(dipic)2]·2H2O (5) have been obtained in high yield by reacting aqueous solutions of K2[PtCl4] and K2[PdCl4] with pyridine-2,6-dicarboxylic acid (H2dipic) or its potassium or sodium salts, respectively, in 1:2 molar ratio. 1 and 2 are also formed on reacting equimolar amounts of reagents in water (pH 1.4) leaving in solution unreacted K2[PtCl4] and K2[PdCl4]. The resulting mixtures brought to pH 5 with KOH undergo a redistribution reaction with formation of the monoanionic species K[Pt(dipic)Cl]·H2O (6) and K[Pd(dipic)Cl]·H2O (7), respectively. The iodo derivative K[Pt(dipic)l]·1/2H2O (8) was obtained from the chloro species by replacement of the coordinated chloride with iodide in aqueous solution. All the complexes have been characterized by a number of physico-chemical measurements including the X-ray analysis of trans-Na2[Pt(dipic)2]·6H2O (4) and K[Pt(dipic)I]·1/2H2O (8). Crystals of 4 are monoclinic, space group P21/c, Z=2, with unit-cell dimensions a=11.912(1), b=6.544(1), c=12.699(1) A, β=91.56(1)°. Crystals of 8 are triclinic, space group P 1 , Z=4, with unit- cell dimensions a=11.018(1), b=11.585(2), c=9.946(1) A, a=97.99(1), β=115.07(1), γ=73.14(1)°. The structures were solved from three dimensional counter data by Patterson and Fourier methods and refined by full-matrix least-squares to R=0.019 and 0.030 for 1556 and 4118 observed reflections, respectively. The crystal of 4 is built up by centrosymmetric Pt(dipic)22− anions and Na+ cations hexacoordinated by four water molecules and two carboxylate oxygens belonging to the ligands. Each dipicolinate ion behaves as a bidentate ligand, and the resulting coordination around the platinum is square planar. The asymmetric unit of compound 8 contains two independent square planar Pt(dipic)l− anions, in each of them the dipicolinate ion acts as a tridentate ligand and the fourth position is occupied by an iodine atom. The planar complex anions Pt(dipic)l− are stacked but not parallel and Pt atoms form infinite zigzag chains.

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.

Nucleophilic reactivity in substitution reactions at planar tetra-coordinate monocationic platinum(II) complexes. Kinetics of displacement of chloride from chloro[2,6-bis(methylthiomethyl)pyridine]platinum(II) cation

Abstract The kinetics of replacement of chloride in the [Pt(SNS)Cl]+ cation [SNS = 2,6-bis(methylthiomethyl)pyridine] by a wide variety of nucleophiles [H2O, OH−, Me2SO, NO2−, NH3, pyridine (py), C5H10NH (pip), N3−, Br−, (C2H5)2S, (CH2)4S, I−, N,N,N′,N′-tetramethylthiourea (tmtu), SCN−, Ph3As, SO32−, thiourea (tu), SeCN− and CN−] have been studied in water at 25°C, μ = 0.50 mol dm−3 (LiClO4). The log k2° values (k2° is the second-order rate constant at μ = 0) do not follow the usual dependence upon the nPt° scale, while there is a good linear relationship between log k2° for the present mono-cationic substrate and log k2° for the substitution reactions of a smaller number of nucleophiles with other mono-cationic platinum(II) substrates previously studied. A new more extensive nucleophilicity scale, based on [Pt(SNS)Cl]+ as the standard substrate, that is appropriate to mono-cationic platinum(II) substrates, is proposed.