The synthesis of, and characterization of the dynamic processes occurring in Pd(ii) chelate complexes of 2-pyridyldiphenylphosphine

Abstract

Pd(II) complexes in which 2-pyridyldiphenylphosphine (Ph₂Ppy) chelates the Pd(II) centre have been prepared and characterized by multinuclear NMR spectroscopy and by X-ray crystallographic analysis. trans-[Pd(κ¹-Ph₂Ppy)₂Cl₂] is transformed into [Pd(κ²-Ph₂Ppy)(κ¹-Ph₂Ppy)Cl]Cl by the addition of a few drops of methanol to dichloromethane solutions, and into [Pd(κ²-Ph₂Ppy)(κ¹-Ph₂Ppy)Cl]X by addition of AgX or TlX, (X = BF₄⁻, CF₃SO₃⁻ or MeSO₃⁻). [Pd(κ¹-Ph₂Ppy)₂(p-benzoquinone)] can be transformed into [Pd(κ²-Ph₂Ppy)(κ¹-Ph₂Ppy)(MeSO₃)][MeSO₃] by the addition of two equivalents of MeSO₃H. Addition of further MeSO₃H affords [Pd(κ²-Ph₂Ppy)(κ¹-Ph₂PpyH)(MeSO₃)][MeSO₃]₂. Addition of two equivalents of CF₃SO₃H, MeSO₃H or CF₃CO₂H and two equivalents of Ph₂Ppy to [Pd(OAc)₂] in CH₂Cl₂ or CH₂Cl₂–MeOH affords [Pd(κ²-Ph₂Ppy)(κ¹-Ph₂Ppy)X]X, (X = CF₃SO₃⁻, MeSO₃⁻ or CF₃CO₂⁻), however addition of two equivalents of HBF₄·Et₂O affords a different complex, tentatively formulated as [Pd(κ²-Ph₂Ppy)₂]X₂. Addition of excess acid results in the clean formation of [Pd(κ²-Ph₂Ppy)(κ¹-Ph₂PpyH)(X)]X₂. In methanol, addition of MeSO₃H and three equivalents of Ph₂Ppy to [Pd(OAc)₂] affords [Pd(κ²-Ph₂Ppy)(κ¹-Ph₂Ppy)₂][MeSO₃]₂ as the principal Pd-phosphine complex. The fluxional processes occuring in these complexes and in [Pd (κ¹-Ph₂Ppy)₃Cl]X, (X = Cl, OTf) and the potential for hemilability of the Ph₂Ppy ligand has been investigated by variable-temperature NMR. The activation entropy and enthalpy for the regiospecific fluxional processes occuring in [Pd(κ²-Ph₂Ppy)(κ¹-Ph₂Ppy)₂][MeSO₃]₂ have been determined and are in the range −10 to −30 J mol⁻¹ K⁻¹ and ca. 30 kJ mol⁻¹ respectively, consistent with associative pathways being followed. The observed regioselectivities of the exchanges are attributed to the constraints imposed by microscopic reversibility and the small bite angle of the Ph₂Ppy ligand. X-Ray crystal structure determinations of trans-[Pd(κ¹-Ph₂Ppy)₂Cl₂], [Pd(κ²-Ph₂Ppy)(κ¹-Ph₂Ppy)Cl][BF₄], [Pd(κ¹-Ph₂Ppy)₂(p-benzoquinone)], trans-[Pd(κ¹-Ph₂PpyH)₂Cl₂][MeSO₃]₂, and [Pd(κ¹-Ph₂Ppy)₃Cl](Cl) are reported. In [Pd(κ²-Ph₂Ppy)(κ¹-Ph₂Ppy)Cl][BF₄] a donor–acceptor interaction is seen between the pyridyl-N of the monodentate Ph₂Ppy ligand and the phosphorus of the chelating Ph₂Ppy resulting in a trigonal bipyramidal geometry at this phosphorus.