Synthesis, characterisation and structure of square-planar palladium(II) complexes with phosphine–pyridine hybrid ligands o-Ph2PC6H4CH2O(CH2)nC5H4N-2 (n= 1–3). Isolation of the first transition-metal complex with a trans-chelating bidentate PN ligand

Abstract

The reaction of Na₂[PdCl₄] or [PdCl₂(PhCN)₂] with an equimolar amount of new bidentate hybrid ligands having a P and a N donor atom capable of trans chelation, o-Ph₂PC₆H₄CH₂O(CH₂)_nC₅H₄N-2 (n= 1–3), gave mainly the 1 : 1 complex [PdCl₂{o-Ph₂PC₆H₄CH₂O(CH₂)_nC₅H₄N-2}]. Depending on the length of the backbone connecting the phosphino and the pyridyl groups, cis- or trans-co-ordinated complexes were mainly formed; when n= 1 the cis complex 1 was formed, but the ligands with longer bridges gave the trans complexes 2 and 3 respectively as the main products. From the reaction of the palladium(II) complex with o-Ph₂PC₆H₄CH₂O(CH₂)_nC₅H₄N-2 (n= 2 or 3) the trinuclear complexes [Pd₃Cl₆{o-Ph₂PC₆H₄CH₂O-(CH₂)_nC₅H₄N-2}₂]4(n= 2) and 5(n= 3) were also obtained as minor products and the cis-chelated complexes analogous to 1 could not be isolated. The reaction of Na₂[PdCl₄] with 2 equivalents of o- Ph₂PC₆H₄CH₂O(CH₂)₃C₅H₄N-2 gave quantitatively the 1 : 2 complex trans-[PdCl₂{o-Ph₂PC₆H₄CH₂O-(CH₂)₃C₅H₄N-2}₂]7 in which the ligand is bound only through the P atom. This complex is in equilibrium with the trans 1 : 1 complex 3 in solution, dissociating one molecule of the ligand (K_eq= 5.6 × 10^–3 mol dm^–3, 35 °C). On heating in CH₂Cl₂-tetrahydrofuran or diethyl ether, 3 partially isomerised to the dinuclear complex [Pd₂Cl₄{µ-o-Ph₂PC₆H₄CH₂O(CH₂)₃C₅H₄N-2}₂]6, in which the hybrid ligands act as bridges. The crystal structures of 1, 3 and 6 were determined; 3 is the first example of a transition-metal complex with a trans-spanning chelate phosphine-pyridine hybrid ligand. The reaction of the ligand o-Ph₂PC₆H₄CH₂-O(CH₂)₃C₅H₄N-2 with K₂[PtCl₄], in contrast to the palladium complexes, always gave the 1 : 2 complex trans-[PtCl₂{o-Ph₂PC₆H₄CH₂O(CH₂)₃C₅H₄N-2}₂]8 in good yield regardless of the ligand: Pt ratio. It did not dissociate in solution and the trans-chelated complex analogous to 3 could not be isolated even from the reaction of K₂[PtCl₄] with an equimolar amount of the ligand o-Ph₂PC₆H₄CH₂O(CH₂)₃C₅H₄N-2.