Francesca Di Bianca

Reactions of pyridine-2-carbaldimines with chloro-bridged palladium(II) and platinum(II) 2-methylallyl dimers. Solution behaviour of the cationic complexes [M(n3-2-MeC3H4)(py-2-CHNR)]+

The reactions of pyridine-2-carbaldimines, py-2- CHNR (R = C6H4OMe-p, Me), with allylic dimers [MCl(n3-2-MeC3H4)]2 give rise to stoichiometry, concentration, solvent and temperature dependent equilibria, in which the cationic complexes [M(n3- 2-MeC3H4)(py-2-CHNR)]+ and the anion [MCl2(n3- 2-MeC3H4)]- or Cl- are involved. In general, the ligand/dimer reaction (1/1 molar ratio) yields the ionic products [M(n3-2-MeC3H4)(py-2-CHNR)]- [MCl2(n3-2-MeC3H4)], which can be isolated as solids, whereas the same reaction in a 1/0.5 molar ratio yields the species [M(n3-2-MeC3H4 )(py-2-CH NR)] Cl, which can be studied only in solution, but are easily converted into [M(n3-2-MeC3H4)(py-2-CH NR)]X in the presence of an excess of NaX (X  ClO4, BF4, BPh4). In the cationic complexes, the α- diimino ligand is σ,σ′-N,N′ chelate to the central metal. The combined conductivity measurements and electronic, IR, and 1H NMR spectral data show that (i) the cationic complexes are greatly stabilized in methanol solution; (ii) extensive ion-pairing occurs in chlorinated solvents, such as dichloromethane, chloroform, and 1,2-dichloroethane; (iii) the complexes with X  ClO4 are slightly dissociated in acetonitrile, with the following order of dissociation constants; Pd >> Pt and py-2-CHNC6H4OMe-p) py-2-CHNMe; (iv) various dynamic processes take place in solution at different rates depending on the temperature, solvent, central metal, and counteranion. In general, a low-energy process involving syn-syn, anti-anti exchange of the allylic protons occurs, which in some cases cannot be frozen out and which is interpreted in terms of formation of stereochemically non-rigid five-coordinate intermediates by association of the cationic complexes with the solvent or the counteranion. Cation-anion interactions and, probably, formation of five-coordinate species with the more coordinating anions, Cl-and [MCl2(n3-2- MeC3H4)]-, are responsible for the solvent and anion dependent 1H NMR chemical shifts of the chelate py 2-CHNC6H4OMe-p ligand. For [Pd(n3-2-MeC3H4)- (py-2-CHNC6H4OMe-p)] [PdCl2(n3-2-MeC3H4)], but not for the platinum analogue, a rather fast scrambling of the Pd(n3-2-MeC3H4) unit between the cation and anion is observed at ambient temperature in CDCl3. This and other differences in the solution behaviour between palladium and platinum derivatives can be rationalized on the basis of a higher stability (toward dissociation) of the five-membered metallacycle M(py-2-CHNR) on going from M  Pd to M  Pt.

Protonation and methylation reactions of 2-pyridyl-palladium(II) and -platinum(II) complexes

The reactions of strong acids HX and HClO4 with the 2-pyridyl complexes [PdX(μ-C5H4N-C2,N)(PPh3)]2 (X = Cl, Br), trans-[PdCl(C5H4N-C2)(PEt3)2] and [PdCl(C5H4N-C2)(dppe)] yield the N-protonated derivatives cis-[PdX2-(C5H5N-C2)(PPh3)], trans-[PdCl(C5H5N-C2)(PEt3)2]ClO4 and [PdCl-(C5H5N-C2)(dppe)]ClO4, respectively. The terminal 2-pyridyl group of trans-[PdCl(C5H4N-C2)(PEt3)2] and [PdCl(C5H4N-C2)(dppe)] also reacts with Me2SO4/NaClO4 to give trans-[PdClC5H4(l-Me)N-C2(PEt3)2]ClO4 and [PdClC5H4(l-Me)N-C2(dppe)]ClO4. Analogous N-protonation or N-methylation reactions occur with trans-[PtBr(C5H4N-C2)(L)2] (L = PEt3, PPh3). The complexes trans-[MX(C5H5N-C2)(PMe2Ph)2]ClO4 (M = Pd, X = Cl and Br; M = Pt, X = Br) exhibit restricted rotation of the protonated 2-pyridyl group around the MC bond. This and other chemical results and spectral data, such as the 13C NMR data of the PEt3 derivatives, are interpreted in terms of a significant contribution of a carbene-like limiting structure to the electronic configuration of this new type of ligand.

Preparation and reactions of palladium(II) complexes with C2-bonded heteroaromatic ligands trans[PdCl(RN)(PPh3)2] (RN = 2-pyridyl, 2-pirazyl, 2-pyrimidyl group). A new reaction pathway in the insertion of isocyanides into the Pdc bond of trans-[PdXR(L)2] compounds

Abstract The complexes trans -[PdCl(R N )(PPh 3 ) 2 ] (I) [R N = 2-pyridyl (2-Py), 2-pyrazyl (2-pyz), 2-pyrimidyl (2-pym) group] have been prepared in high yield by deprotonation with NEt 3 of the corresponding cationic compounds trans [PdCl(R N H) (PPh 3 ) 2 ] + (R N H = N -protonated C 2 -heteroaromatic ligand) in the presence of an excess of PPh 3 . In chlorinated solvents, complexes I undergo a slow reversible dimerization into the binuclear derivatives [PdCl(μ-R N )(PPh 3 )] 2 (II) (μ-R N = C 2 , N 1 -bridging ligand). From the 31 P NMR spectra in 1,2-dichloroethane the following dissociation constants were obtained: 1.9 mol 1 −1 (R N = 2-py), 5.1 × 10 −2 (2-pym), 6.6 × 10 −3 (2-pyz). The dimerization becomes fast and quantitative if the PPh 3 , involved in the equilibrium is removed by oxidation or by reaction with [PdCl(η 3 -2-MeC 3 H 4 )] 2 . Only the 2-pyridyl complex Ia reacts (slowly) with CO yielding the migratory insertion product trans -[PdCl{C(2-py)O}(PPh 3 ) 2 ], together with the dimer IIa. All the complexes I undergo migratory insertion of t-butylisocyanide with formation of trans -[PdCl{C(R N ) = NCMe 3 }(PPh 3 ) 2 ]] at rates which depend on the heterocyclic group (R N = 2-py > 2-pyz ⪢ 2-pym). The reaction of the 2-pyrazyl complex Ib with CNCMe 3 has been studied in detail by conductivity measurements and by IR and 31 P NMR spectroscopy. The data suggest a complex mechanism in which insertion occurs through rearrangement of a four-coordinate intermediate [PdCl(2-pyz)(CNCMe 3 )(PPh 3 )], and through interaction of a cationic intermediate trans -[Pd(2-pyz)(CNCMe 3 )(PPh 3 ) 2 ] + (Vb) with Cl − and with the free isocyanide of the initial equilibria. The occurrence of the latter reactions is confirmed by independent experiments in which the cationic complex Vb (isolated as perchlorate salt) is treated with an equimolar amount of [AsPh 4 ]Cl or CNCMe 3 . The isocyanide-promoted insertion step represents a new mechanistic pathway for isocyanide insertion into the PdC bond of trans -[PdXR(L) 2 ] complexes.

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.

Mechanism of nucleophilic attack by diethylamine on cationic palladium(II) allyl complexes containing α-diimine ligands

The reactions of the cationic complexes [Pd(η3-allyl)(N–N′)]ClO4(allyl = 4-methoxycyclohexenyl, allyl or 2-methylallyl; N–N′= 1, 2-bis(imino)ethanes or pyridine-2-carbaldimines) with diethylamine, in the presence of an activated olefin, in chloroform at 25 °C have been studied. They involve a fast equilibrium displacement of the co-ordinated α-diimine to yield [Pd(η3-allyl)(NHEt2)2]+, accompanied by slow nucleophilic attack at the allyl ligand of the [Pd(η3-allyl)(N–N′)]+ substrate producing [Pd(η2-olefin)(N–N′)](olefin = dimethyl fumarate or fumaronitrile) and allyldiethylamine. As shown by the stereochemical course of the reaction with [Pd(1–3-η3-C6H8OMe)(C5H4N-2-CHNC6H4OMe4)]ClO4, the nucleophilic attack takes place on the allyl face opposite the metal. The equilibrium constants for α-diimine displacement have been determined. They are strongly affected by the structure of the N–N′ ligand and decrease in the order RNCH–CHNR RNC(Me)–C(Me)NR ≈ C5H4N-2-CHNR > C5H4N-2-CHNCMe3(R = C6H4OMe-4). Kinetic studies showed that the pseudo-first-order rate constants (kobs) for the slow amination path display both a first- and second-order dependence on the NHEt2 concentration of type kobs=k2[NHEt2]+k2′[NHEt2]2. The k2 term is related to direct bimolecular attack of NHEt2 on the terminal allyl carbon, whereas the k2′ term is ascribed to a parallel nucleophilic attack by a hydrogen-bonded diethylamine dimer arising from amine self-association.

N-protonated 2-pyridylnickel(II) complexes insertion of isocyanides into the nickel2-pyridyl bond

The reaction of the binuclear complex [NiCl(μ-2-py)(PPh3)]2 (μ-2-py = μ-C5H4N-C2,N) with the phosphines L (L = PPh3, PMePh2, PMe2Ph or PEt3) or dppe (= 1,2-bis(diphenylphosphino)ethane) in the presence of HClO4 yields the N-protonated 2-pyridyl derivatives trans-[NiCl(2-pyH)(L)2]ClO4 or [NiCl(2-pyH)(dppe)] ClO4 (2-pyH = C5H5N-C2) with a square-planar coordination around the nickel(II) center. These products are largely associated through hydrogen bonding between the NH group and the perchlorate anion, both in the solid and in chlorinated solvents. The configuration in solution has been studied by 1H and 31P NMR spectroscopy. In the complex trans-[NiCl(2-pyH)(PMe2Ph)2]ClO4, the planar 2-pyH ligand is oriented perpendicularly to the coordination plane, with restricted rotation about the NiC2 bond. The reaction of rans-[NiCl(2-pyH)(PMePh2)2]ClO4 with CNC6H4OMe-p and then with NEt3 yields the unstable compound trans-[NiCl{C(2-py)NC6H4OMe-p}(PMePh2)2] through a fast migratory insertion of the isocyanide into the nickel2-pyridyl bond. This product can be isolated and characterized only when the imino(2-pyridyl)methyl group is σ,σ′-N,N′-chelated to ZnCl2.

Preparation and reactions of 1-methylpyrid-6-one-2-yl compounds of palladium(II) and platinum(II)

Abstract The compounds trans -[MCl{(1-Me)C 5 H 3 (6-O)N- C 2 }(L) 2 ] (M = Pd, Pt; L = PPh 3 , PMe 2 Ph), can be prepared from the reaction of the corresponding 1-methyl-6-chloro-2-pyridylium cationic complexes, trans -[MCl{(1-Me)C 5 H 3 (6-Cl)N- C 2 }(L) 2 ]ClO 4 , with a mixture of acetic acid, ethanol, and triethylamine in the molar ratio M/MeCO 2 H/EtOH/NEt 3 of 1/3/3/4. The rate is slow compared to that of the 1-methyl-2-chloropyridinium cation under similar conditions, and is markedly affected by the steric and electronic effects of the trans -MCl(L) 2 unit. The novel 1-methylpyrid-6-one-2-yl derivatives have been characterized by conventional spectroscopic techniques and by reactions involving either protonation and methylation of the carbonyl group or migratory insertion of isocyanides into the Pd C 2 bond.

Solution behaviour and relative stability of the complexes [MCl2(RNCHCHNR)] and [MCl2(py-2-CHNR)] (M=Pd, Pt;R=C6H4OMe-p)

Abstract Even though the α-diimino complexes [MCl 2 (RNCHCHNR)] and [MCl 2 (py-2-CHNR)] (M=Pd, Pt;R=C 6 H 4 OMe- p ) are poorly soluble in chlorinated solvents, such as chloroform and 1,2-dichloroethane, or in acetonitrile, the electronic and 1 H NMR spectra indicate that these compounds are generally present as undissociate monomers with σ, σ′- N,N′ chelate N-ligands in dilute solutions. Only for [PdCl 2 (RNCHCHNR)], some dissociation of the α-diimine occurs in acetonitrile. In dimethylsulfoxide, where the solubility is much higher, no dissociation is observed for the pyridine-2-carbaldimine complexes [MCl 2 (py-2-CHNR)], whereas the 1,2-bis(imino) ethane derivatives [MCl 2 (RNCHCHNR)] are extensively dissociated through a step-wise process involving intermediates with a σ- N monodentate α-diimino group. As is shown by the course of substitution reactions with 2,2′-bipyridine, the higher stability of [MCl 2 (py-2-CHNR)] in dimethylsulfoxide is mainly due to thermodynamic factors (ground state stabilization for the presence of stronger MN bonds) rather than by kinetic factors (higher activation energy for steric strain in the activation states or transients).

Nucleophilic attack by 2-pyridylpalladium(II) and platinum(II) complexes on the organic chlorides ClCH2R (R COMe, CN, Ph, Cl)

Abstract The 2-pyridyl complexes trans-[MCl(C5H4NC2)(PPh3)2] (M = Pd, 1a; M = Pt, 1b), [MCl(C5H4NC2)(dppe)] (M = Pd, 2a; M = Pt, 2b) and [M(dmtc)(C5H4NC2) (PPh3)] (M = Pd, 3a; M = Pt,3b) react with the chlorides ClCH2R(R  COMe, CN, Ph) to give cationic products containing the 2-pyridylium ligands (1-CH2R)C5H4NC2. The rate of nucleophilic displacement of the chloride ion from ClCH2R depends on the central metal (Pt > Pd), on the coordination geometry (1 ~ 3) and on the substituent R (COMe > CN > Ph for the reactions with 3b). Like 1b and 2b, complex 3b also reacts with dichloromethane to give [Pt(dmtc){(1-CH2Cl)C5H4NC2}(PPh3)]+. The reaction of the binuclear compound [{PdCl(μ-C5H4NC2,N)(PPh3)}2] with chloroacetone in the presence of chloride ion yields the zwitterionic derivative cis-[PdCl2{(1-CH2COMe)C5H4NC2}(PPh3)]. The cationic products have been isolated and characterized as perchlorate salts.

Mechanism of oxidative allyl transfer from allylic ammonium cations to palladium(0) α-diimine complexes

Abstract The palladium(0) complex [Pd(η2-fn)(NN′)] (1, fn = fumaronitrile; NN′ = C5H4N-2-CH=NC6H4OME-4) reacts slowly and reversibly with A +  CH 2  CH=CH 2 (2a, A = NEt 3 ; 2 b , A = C 5 H 5 N ) to yield the cationic η3-allypalladium(II) derivative [ Pd (η[ 3 - C 3 H 5 )( N  N ′)] + (3) the free amine A and fn. The equilibrium constant Ke is (2.6 ± 0.1) × 10−3 for 2a and 1.0 ± 0.4 for 2b. Kinetic studies of these oxidative allyl-transfer reactions show that the rates increase with increasing concentration of 2 and with decreasing concentration of fn. A stepwise mechanism is proposed which involves slow and reversible displacement of fn by 2 to give a labile intermediate [ Pd (η 2 - CH 2 = CH  CH 2  A + ( N  N ′)] . This undergoes slow and reversible intramolecular allyl transfer through nucleophilic attack by the palladium(0) metal centre on the nitrogen-bound allyl carbon. The kinetic parameters evaluated by a steady-state treatment satisfactorily generate the observed equilibrium constant Ke. The rate of formation of the intermediate and the relative rate of its decay to the starting reactants and final products are virtually independent of the nature of the amine A.