6533b820fe1ef96bd127a303

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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-CHNR)]+

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Abstract 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.