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-MeC 3H 4)(py-2-CHNR)] +

Abstract

The reactions of pyridine-2-carbaldimines, py-2- CHNR (R = C 6H 4OMe- p, Me), with allylic dimers [MCl( n 3-2-MeC 3H 4)] 2 give rise to stoichiometry, concentration, solvent and temperature dependent equilibria, in which the cationic complexes [M( n 3- 2-MeC 3H 4)(py-2-CHNR)] + and the anion [MCl 2( n 3- 2-MeC 3H 4)] - or Cl - are involved. In general, the ligand/dimer reaction (1/1 molar ratio) yields the ionic products [M( n 3-2-MeC 3H 4)(py-2-CHNR)]- [MCl 2( n 3-2-MeC 3H 4)], which can be isolated as solids, whereas the same reaction in a 1/0.5 molar ratio yields the species [M( n 3-2-MeC 3H 4 )(py-2-CH NR)] Cl, which can be studied only in solution, but are easily converted into [M( n 3-2-MeC 3H 4)(py-2-CH NR)]X in the presence of an excess of NaX (X  ClO 4, BF 4, BPh 4). 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  ClO 4 are slightly dissociated in acetonitrile, with the following order of dissociation constants; Pd >> Pt and py-2-CHNC 6H 4OMe- 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 [MCl 2( n3 -2- MeC 3H 4)] -, are responsible for the solvent and anion dependent 1H NMR chemical shifts of the chelate py 2-CHNC 6H 4OMe- p ligand. For [Pd( n 3-2-MeC 3H 4)- (py-2-CHNC 6H 4OMe- p)] [PdCl 2( n 3-2-MeC 3H 4)], but not for the platinum analogue, a rather fast scrambling of the Pd( n 3-2-MeC 3H 4) unit between the cation and anion is observed at ambient temperature in CDCl 3. 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.