Fabiano Visentin

The role of the non-participating groups in substitution reactions at cationic Pt(II) complexes containing tridentate chelating nitrogen donors. Crystal structure of {Pt[bis(2-pyridylmethyl)amine](py)}(CF3SO3)2

Abstract Kinetic measurements on the displacement of chloride with the nucleophiles Br−, I− and N (N=a number of isosteric pyridines and morpholine) from the substrates [Pt(NNN)Cl]+ [NNN=bis(2-pyridylmethyl)amine (bpma); 2,6-bis(aminomethyl)pyridine (dap); diethylenetriamine (dien)] have been carried out in methanol at 25°C. The results, compared with those previously obtained on the complex [Pt(terpy)Cl]+ (terpy=2,2′:6′,2′′-terpyridine), are discussed in terms of reactivity and discrimination ability of the reaction centre. The significant differences in kinetic behaviour along the series are particularly related to the presence of pyridine rings in the non-participating chelate ligand and steric effects. The study of the reverse process, i.e. the displacement of N with a chloride ion from the complexes [Pt(NNN)(N)]2+, allows the determination of the equilibrium constants from the ratio of the rate constants. The crystal structure of [Pt(bpma)(py)](CF3SO3)2 has been determined by the X-ray diffraction technique. It consists of essentially SP (square-planar) [Pt(bpma)(py)]2+ cations. The plane through the pyridine ring makes an angle of 86.1(3)° with that of Pt and the three nitrogen atoms of bpma. The packing is characterised by a hydrogen bond between the NH of the ligand and one oxygen of a triflate anion.

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.

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.

Kinetics and mechanism of regioselective amination of the 1-phenylallyl group in cationic palladium(II) complexes bearing bidentate ligands

Abstract The complexes [Pd(η3-1-PhC3H4)(L–L′)]+ [L–L′=2-(PPh2)C6H4-1-CHNR (R=Me (1a), i-Pr (1b), t-Bu (1c), (R)-bornyl (1d), C6H4OMe-4 (1e), C6H3Me2-2,6 (1f), C6H3(i-Pr)2-2,6 (1g)), 6-MeC5H3N-2-CHNC6H4OMe-4 (2a), C5H4N-2-CHN-t-Bu (2b) and C5H4N-2-CH2S-t-Bu (3a)] are generally present in solution as two geometrical isomers, the relative abundance of which depends essentially on the steric requirements of the L–L′ ligand. In the presence of fumaronitrile the cationic complexes undergo a regioselective amination by secondary amines HY at the CH2 allyl terminus, yielding [Pd(η2-fn)(L–L′)] and the allylamines (E)-PhCHCHCH2Y. Under pseudo-first-order conditions the amination rates (kobs) are found to depend on the k2[HY] term for 2a and 3a, and on the sum k2[HY]+k3[HY]2 for the other complexes. The second-order term k2 is related to direct nucleophilic attack on the CH2 allyl terminus of the substrate whereas the third-order term k3 is ascribed to parallel attack by a further HY molecule on the intermediate [Pd(1-PhC3H4)(L–L′)(HY)]+. The k2 values depend on the steric and electronic properties of both the amine HY and the ligand L–L′. For complexes 1a–1g, the relatively higher k2 values and their increase with increasing steric crowding at the nitrogen-bonded carbon of substituent R are interpreted in terms of a greater reactivity of the isomer with the CH2 allyl terminus trans to phosphorus and cis to the NR group. The high amination rate of 2a, as compared with that of 2b, is related to substantial steric interaction of the CH2 allyl terminus with the 6-Me pyridine group in close proximity in the predominant isomer.