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(3-PNE)]Cl (6a, 8) as the predominant products. A 2-PN coordination mode was also identified crystallographically for ligand 1 following its reaction with PdCl2(MeCN)2, which afforded [PdCl2(1-2-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(1-3-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)(1-2-PN)]Cl2 (9). Reaction of 1 with PdCl(Me)(COD) results in the formation of the 2-PN dichloride complex [PdCl(Me)(1-2-PN)] (11). Attempts to prepare [Pd(Me)(MeCN)(1-2-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(1-2-NN)], which isomerises to afford [PdMe2(1-2-PN)] (13); at temperatures greater than 10 °C complex 13 decomposes rapidly.