Oligomeric allyl-palladium(ii) complexes of β-substituted ethylselenolates: Syntheses, structures and thermal decomposition

Abstract

Tri-nuclear allyl-palladium complexes, [Pd(μ-SeCH₂CH₂COOR)(η³-C₃H₄R′)]₃ (R = H, Me, Et and R′ = H, Me), have been synthesized by the reaction of [Pd₂(μ-Cl)₂(η³-C₃H₄R′)₂] with NaSeCH₂CH₂COOR. These complexes exist in a dynamic equilibrium with a dimeric form in solution and are fluxional at room temperature as shown by variable temperature ¹H NMR spectroscopy. The DFT calculations indicate that there is a negligible energy difference between the dimer and the trimer, and suggest that the delicate balance between the steric factors and angular strain decides the reaction products. These complexes (with R′ = H) on treatment with [Pd₂(μ-Cl)₂(η³-C₃H₅)₂] afforded hetero-bridged complexes [Pd₂(μ-Cl)(μ-SeCH₂CH₂COOR)(η³-C₃H₅)₂] (R = Me, Et). All the complexes have been characterized by NMR (¹H, ¹³C, ⁷⁷Se) spectroscopy. The molecular structure of [Pd(μ-SeCH₂CH₂COOEt)(η³-C₃H₅)]₃ revealed a chair conformation of the six-membered Pd₃Se₃ ring, in which all the allyl groups lie at one side of the ring (similar to three axial 1,3,5-hydrogens of cyclohexane). Thermolysis of [Pd(μ-SeCH₂CH₂COOEt)(η³-C₃H₅)]_n in diphenyl ether or hexadecylamine (HDA) yielded Pd₇Se₄ as characterized by powder XRD.