Synthesis, reactivity and characterization of mono-, bi- and tri-nuclear Schiff-base palladacycles with aromatic N-heterocycles

Reactions of μ2-Chloro bridged cyclometallated Pd(II) complexes [Pd{(4-R) C6H3CHNC6H3-2,6-i-Pr2}(μ-Cl)]2 (R = H, OMe) with aromatic N-heterocycles such as 1-methylimidazole, 4,4′-bipyridyl (bpy), 1,2-bis(4-pyridyl)-ethene (bpe), 2,5-bis(4-pyridyl)-1,3,4-thiadiazole (bpt) and 2,4,6-tris-(4-pyridyl)-1,3,5-triazine (tpt), generated mononuclear complexes [Pd{(4-R)C6H3CHNC6H3-2,6-i-Pr2}(L)(Cl)] (L = 1-methylimidazole, R = H, OMe), binuclear complexes [Pd{(4-R)C6H3CHNC6H3-2,6-i-Pr2}(Cl)]2(μ-Y) (R = H, OMe; Y = bpy, bpe, bpt) and trinuclear complexes [Pd{(4-R) C6H3CHNC6H3-2,6-i-Pr2}(Cl)]3(tpt) (R = H, OMe). In contrast, the reaction of μ2-Chloro bridged cyclometallated Pd(II) complexes [Pd{(4-R)C6H3CHNC6H3-2,6-i-Pr2}(μ-Cl)]2 (R = H, OMe) with 2 equiv of AgNO3 and followed with bpy, bpe, bpt to produce binuclear cyclometallated Pd(II) nitrato complexes [Pd{(4-R) C6H3CHNC6H3-2,6-i-Pr2}(ONO2)]2(μ-Y) (R = H, OMe; Y = bpy, bpe, bpt). Treatment of [Pd{(4-R)C6H3CHNC6H3-2,6-i-Pr2}(Cl)]3(tpt) (R = H, OMe) with an excess AgNO3 produced trinuclear cyclometallated Pd(II) nitrato complexes [Pd{(4-R) C6H3CHNC6H3-2,6-i-Pr2}(ONO2)]3(tpt) (R = H, OMe). All these complexes were fully characterized by FT-IR, NMR spectroscopy, elemental analysis and/or X-ray crystallography.

Graphical abstractMono-, bi- and tri-nuclear Schiff base palladacycles with aromatic N-heterocycles were synthesized. We found that bi- and tri-nuclear cyclometallated Pd(II) nitrato complexes possess a different cis conformation comparing with corresponding bi- and tri-nuclear cyclometallated Pd(II) chloro complexes.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Mono-, bi- and tri-nuclear Schiff-base palladacycles with aromatic heterocycle were synthesized. ► Bi- and tri-nuclear Schiff-base palladacycles possess an unusual cis conformation. ► Two kinds of interconversion isomers occurred in solution of bi- and tri-nuclear cyclometallated Pd(II) chloro complexes.