Concise syntheses of tridentate PNE ligands and their coordination chemistry with palladium(ii) : a solution- and solid-state study

Abstract

A straightforward methodology for the high-yielding synthesis of the di-functionalised phosphines {Ph₂P(CH₂)₂NC₄H₈E, E = NMe (1), O (2), S (3)} via base-catalysed Michael addition is described. Reaction of the functionalised tertiary phosphines 1–3 with PdCl₂(MeCN)₂ affords complexes in which the ligands are bound in a tridentate fashion, namely [PdCl(κ³-PNE)]Cl (6a, 8) as the predominant products. A κ²-PN coordination mode was also identified crystallographically for ligand 1 following its reaction with PdCl₂(MeCN)₂, which afforded [PdCl₂(1-κ²-PN)] (6b) in ca. 5% yield. Conductivity studies of solutions of 6a are consistent with an ionic formulation, however the poor solubility of 7 and 8 precluded their study in a similar fashion. Analysis of bulk samples of [PdCl₂(1)] (6) and [PdCl₂(3)] (8) by ¹⁵N and ³¹P NMR spectroscopy in the solid state as consistent with exclusive tridentate binding of the PNE ligands. An X-ray crystallographic study has probed the coordination of 1 in the unusual salt [PdCl(1-κ³-PNN)]₂[Mg(SO₄)₂(OH₂)₄] (10) prepared by treating a methanolic solution of 6 with excess MgSO₄. No data could be obtained to support the transformation of 6a into 6b on addition of excess chloride. In contrast, 6a reacts regioselectively with the water-soluble phosphine Cy₂PCH₂CH₂NMe₃Cl to afford the cis-diphosphine complex cis-[PdCl(Cy₂PCH₂CH₂NMe₃Cl)(1-κ²-PN)]Cl₂ (9). Reaction of 1 with PdCl(Me)(COD) results in the formation of the κ²-PN dichloride complex [PdCl(Me)(1-κ²-PN)] (11). Attempts to prepare [Pd(Me)(MeCN)(1-κ²-PN)][PF₆] (12) through reaction of 11 with NaPF₆ in MeCN led to decomposition. Treatment of PdMe₂(TMEDA) with 1 at low temperature initially affords [PdMe₂(1-κ²-NN)], which isomerises to afford [PdMe₂(1-κ²-PN)] (13); at temperatures greater than 10 °C complex 13 decomposes rapidly.