Reactivity of Schiff base-[C,N,S] pincer palladacycles: hydrolysis renders singular trinuclear, tetranuclear, and heteropentanuclear Pd3W2 coordinated complexes

Abstract

Treatment of the Schiff base ligands a–f with Li₂[PdCl₄]/NaAcO in methanol under reflux gave the single nuclear palladacycles 1a–1f, with the metal atom bonded to a terdentate monoanionic [C,N,S] iminic ligand and to a chloride ligand that completes the palladium coordination sphere. Reaction of 1a–1c with silver perchlorate/triphenylphosphine in acetone at room temperature yielded the single nuclear complexes 2a–2c as the perchlorate salts, after substitution of the chloride ligand by a triphenylphosphine. However, reaction of a–c with Na₂[PdCl₄]/NaAcO in methanol at room temperature also gave compounds 1a–1c albeit contaminated with small amounts of the corresponding free aldehyde (mixture A). Reaction of mixture A with silver perchlorate/triphenylphosphine in acetone at room temperature gave analogously 2a–2c with some of the corresponding free aldehyde (mixture B). Attempts to purify mixtures A and B via recrystallization produced single crystals of 5 and 6 respectively: two serendipitously formed complexes, bearing thiomethyl aniline and/or acetate ligands, and void of aldehyde or iminic residue; the structures contain eight- and six-membered rings of alternating palladium and nitrogen atoms, respectively. To clarify this situation the aniline itself was reacted with palladium(II) acetate or with Na₂[PdCl₄]; in the latter case after recrystallization a unique behavior is revealed, giving rise to a tetranuclear complex containing a Pd₄N₄ ring with three differing coordination environments on the palladium atoms. Treatment of 1d with Ph₂PCH₂PPh₂ (dppm)/AgClO₄ or with Ph₂PCH₂(PPh₂)W(CO)₅/AgClO₄ gave 3d, with a mono-coordinated dppm ligand, and 4d, respectively; complex 3d could not be converted into 4d by reaction with W(CO)₅(THF). Recrystallization of 4d gave a still further noticeable species, complex 8: a pentanuclear trans-configured heterometallic mixed valent Pd(II)/W(0) linear complex with the palladium atoms supported by two acetate and two thiomethyl aniline bridging ligands. The complexes were fully characterized by microanalysis, IR, ¹H, and ³¹P NMR spectroscopies, as appropriate. The X-ray single-crystal analyses for compounds 1b, 5, 6, 7 and 8 are described.