The Synthesis of, and Characterization of the Dynamic Processes Occurring in, Pd(II) Chelate Complexes of 2-pyridyldiphenylphosphine

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.