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

摘要

Pd(ii) complexes in which 2-pyridyldiphenylphosphine (Ph_2Ppy) chelates the Pd(ii) centre have been prepared and characterized by multinuclear NMR spectroscopy and by X-ray crystallographic analysis. trans- [Pd(κ~1-Ph_2Ppy)_2Cl_2] is transformed into [Pd(κ~2-Ph_2Ppy)(κ~1- Ph_2Ppy)Cl]Cl by the addition of a few drops of methanol to dichloromethane solutions, and into [Pd(κ~2-Ph _2Ppy)(κ~1-Ph_2Ppy)Cl]X by addition of AgX or TlX, (X = BF_4~-, CF_3SO_3~- or MeSO_3~-). [Pd(κ~1-Ph _2Ppy)_2(p-benzoquinone)] can be transformed into [Pd(κ~2-Ph_2Ppy)(κ~1-Ph _2Ppy)(MeSO_3)][MeSO_3] by the addition of two equivalents of MeSO_3H. Addition of further MeSO_3H affords [Pd(κ~2-Ph_2Ppy)(κ~1-Ph _2PpyH)(MeSO_3)][MeSO_3]_2. Addition of two equivalents of CF_3SO_3H, MeSO_3H or CF _3CO_2H and two equivalents of Ph_2Ppy to [Pd(OAc)_2] in CH_2Cl_2 or CH_2Cl _2-MeOH affords [Pd(κ~2-Ph_2Ppy) (κ~1-Ph_2Ppy)X]X, (X = CF_3SO _3~-, MeSO_3~- or CF_3CO _2~-), however addition of two equivalents of HBF _4·Et_2O affords a different complex, tentatively formulated as [Pd(κ~2-Ph_2Ppy)_2]X _2. Addition of excess acid results in the clean formation of [Pd(κ~2-Ph_2Ppy)(κ~1-Ph _2PpyH)(X)]X_2. In methanol, addition of MeSO_3H and three equivalents of Ph_2Ppy to [Pd(OAc)_2] affords [Pd(κ~2-Ph_2Ppy)(κ~1-Ph _2Ppy)_2][MeSO_3]_2 as the principal Pd-phosphine complex. The fluxional processes occuring in these complexes and in [Pd (κ~1-Ph_2Ppy)_3Cl]X, (X = Cl, OTf) and the potential for hemilability of the Ph_2Ppy ligand has been investigated by variable-temperature NMR. The activation entropy and enthalpy for the regiospecific fluxional processes occuring in [Pd(κ~2- Ph_2Ppy)(κ~1-Ph_2Ppy)_2][MeSO _3]_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 Ph_2Ppy ligand. X-Ray crystal structure determinations of trans-[Pd(κ~1- Ph_2Ppy)_2Cl_2], [Pd(κ~2-Ph _2Ppy)(κ~1-Ph_2Ppy)Cl][BF_4], [Pd(κ~1-Ph_2Ppy)_2(p-benzoquinone)], trans-[Pd(κ~1-Ph_2PpyH)_2Cl _2][MeSO_3]_2, and [Pd(κ~1-Ph _2Ppy)_3Cl](Cl) are reported. In [Pd(κ~2- Ph_2Ppy)(κ~1-Ph_2Ppy)Cl][BF_4] a donor-acceptor interaction is seen between the pyridyl-N of the monodentate Ph_2Ppy ligand and the phosphorus of the chelating Ph_2Ppy resulting in a trigonal bipyramidal geometry at this phosphorus.