Structure and Stereochemical Nonrigidity in Pd(F6acac)2P(aryl)3, a New Isomeric Class of Palladium Bis(hexafluoroacetylacetonate) Complexes

Article abstract of DOI:10.1021/ja00388a018

The reaction of palladium bis(hexafluoroacetylacetonate), Pd(F6acac)2, and (aryl)3P produces Pd(F6acac)2P(aryl)3, a new class of 1:1 adducts isomeric with carbon-bonded Pd(F6acac-C)(F6acac-O,O)(ligand).Pd(F6acac)2PPh3 crystallizes in the monoclinic space group P2₁/n with a = 20.106 (3) Angstroem, b = 13.160 (7) Angstroem, c = 23.185 (3) Angstroem, β = 93.52 (3) deg, and V = 6123 (5) Angstroem.The structure solution, which converged at R = 0.071 and R_w = 0.086 with 6536 reflections for which F₀² >= 2.0?(F₀²), showed that the palladium has a distorted, square-pyramidal coordination geometry.The base of the pyramid comprises a phosphorus atom, two oxygen atoms from a bidentate F6acac, and one oxygen atom from a semichelating F6acac.The Pd-O contact involving the apical oxygen, provided by the other terminus of the semichelating F6acac, is long, 2.699 (6) Angstroem.The Pd(F6acac)2P(aryl)3 compounds are stereochemically nonrigid, and the barrier to rearrangement increases as the steric bulk of the (aryl)3P group increases.At low temperatures, the 19F NMR spectrum of Pd(F6acac)2P(o-tolyl)3 displays four singlets of equal area.These simultaneously coalesce at circa -40 deg C, indicative of a single dynamic process involving a C_2c square-pyramidal transition state in which all four CF3 groups are equivalent.Analysis of the DNMR spectra gave ΔH<*> = 7.7 +/- 1 kcal/mol and ΔS<*> = -20 +/- 5 eu.Comparison of the molecular structures of the Ph3P and (o-tolyl)3P adducts of Pd(F6acac)2 suggests that the less bulky triphenylphosphine ligand is associated with geometrical changes that may parallel those associated with formation of the transition state.