Hydrazo-keto and azo-enol tautomerism in organometallic palladacycles. New bidentate monoanionic and tridentate bianionic ligands

Abstract

The action of PdCl₂ on phenylhydrazones derived from 2-oxopropionaldehyde, benzoylformaldehyde or butane-2,3-dione led to dinuclear metallacycles 1 [{Pd[2-(NR⁴NCR³CR²O)-5-R¹C₆H₃]Cl}₂] (R¹ = H, Br, NO₂ or MeO; R² = Me or Ph; R³, R⁴ = H or Me). Compounds 1 reacted with PPh₃ to afford [PdCl{κC,κN-(C–N)}(PPh₃)] 2 or [PdCl{κC-(C–N)}(PPh₃)₂] 3 (C–N being the metallated hydrazone) which contain one or two phosphine molecules per metal atom respectively, depending on the steric hindrance of the N-donor ligands. The structure of [Pd{2-(NHNCHCOMe)-5-O₂NC₆H₃}Cl(PPh₃)] has been determined by X-ray diffraction. The bond distances and angles are similar to those reported for related metallacycles. The distance N(1)· · ·O(1) [2.631(4) Å] indicates the presence of a strong intramolecular hydrogen bond between the NH and COMe groups. The C(8)–O(1) [1.222(5) Å], C(8)–C(7) [1.470(5) Å], C(7)–N(2) [1.304(4) Å] and N(1)–N(2) [1.342(4) Å] bond lengths indicate that the complex 3 exists mainly in the keto-hydrazo form. The action of NaOMe on the PPh₃ or (PPh₃)₂ complexes in MeOH afforded deep violet compounds which do not contain chlorine atoms. The analytical data, IR and NMR spectra showed that deprotonation of the NH group had taken place to give [Pd{2-(NNCR³CR²O)-5-R¹C₆H₃}(PPh₃)], by means of an hydrazo-keto azo-enol tautomerization.