Reactivity of palladacycles with phosphorus donor nucleophiles. The first crystal and molecular structure of a mixed-bridged acetate/phosphine dinuclear species.

Abstract

Treatment of 2,4-(MeO)C₆H₃C(H)=N-[3,5-Cl₂C₆H₃] 1 with palladium (II) acetate in toluene at 60 °C under argon gave the dinuclear acetate-bridged palladacycle 2 as an air-stable solid. Reaction of 2 with the diphosphine Ph₂PCH₂PPh₂ (dppm) in 1:1 molar ratio and NH₄PF₆ yielded complex 3 where only one of the acetate ligands was exchanged by a diphosphine to give a mixed-bridged dinuclear palladacycle bearing charged and neutral linkers spanning between the metal centers, with the organic ligands in a cisfashion. The first crystal structure of one such complex, 3, is now reported herein. Complex 2 could be converted into the corresponding halide-bridged analogues by treatment in acetone with aqueous sodium chloride or sodium bromide to give the chloride-bridged 4, or bromide-bridged 5, compounds, respectively, as air-stable solids. The metallated moieties show a trans conformation of the Schiff base ligands, as opposed to the cis arrangement in 3. Treatment of 4 and 5 with the small bite diphosphine Ph₂PCH₂PPh₂ in 1:1 ratio gave the dinuclear compounds 6 and 7, likewise with two distinct bridging ligands; whereas the large bite diphosphine trans-Ph₂PCH=CHPPh₂ (trans-dppe) yielded 8 and 9 that are symmetric complexes across the C=C double bond as shown by the appearance of only one set of signals in the ¹H NMR spectrum, and by a singlet resonance in the ³¹P-{¹H} spectrum for the two equivalent phosphorus nuclei. Reaction of 4 and 5 with Ph₂PCH₂PPh₂, Ph₂P(CH₂)₂PPh₂ (dppe) cis-Ph₂PCH=CHPPh₂ (cis-dppe), Ph₂P(CH₂)₃PPh₂ (dppp) or Ph₂P(CH₂)₄PPh₂ (dppb), and with PPh₃ in the appropriate molar ratio gave the complexes 10-19 as mono- or dinuclear species. All the compounds in this report were adequately characterized by microanalytical and spectroscopic measurements; the crystal molecular structures of 2, 3, 9 and 12 are reported.