New mono- and di-nuclear complexes of Pd^II, Pt^II and Ni^II of PNNP ligands with a 2,2′-biaryl bridging unit

Abstract

The Schiff bases 2,2′-bis{N-[(2-diphenylphosphino)phenyl]formimidoyl}biphenyl (L¹) and 2,2′-bis{[(2-diphenylphosphino)benzylidene]amino}biphenyl (L²) were synthesized from the appropriate amine and aldehyde. Their co-ordination behaviour towards Pd^II, Pt^II and Ni^II has been studied. Dinuclear complexes were formed when treating L¹ with 2 equivalents of [PdCl(Me)(cod)] (cod = cycloocta-1,5-diene) and Pd(O₂CMe)₂. In the case of the reaction of L¹ with Pd(O₂CMe)₂ an unexpected double cyclopalladated complex was formed. However, treatment of L¹ with 2 equivalents of [PtCl₂(MeCN)₂] resulted in hydrolysis of the ligand and a mononuclear complex of (2-diphenylphosphino)benzenamine was obtained. The structure of [Pd₂L¹Cl₂Me₂] 1 has been established by an X-ray diffraction study. The dihedral angle between the biphenyl rings is 49.2(8)° and the intramolecular Pd  · · ·  Pd distance is 7.165(3) Å. The biphenyl bridging moiety shows a remarkable bending of 169.2(8)°. Treatment of 1 with carbon monoxide resulted in a double CO insertion, yielding the bis(acetyl)palladium compound [Pd₂L¹Cl₂{C(O)Me}₂] 2. The structure of [Pd₂L¹(O₂CMe)₂]·4CH₂Cl₂ 3 was also determined by X-ray diffraction. The dihedral angle between the biphenyl rings is 75.9(14)° while the intramolecular Pd  · · ·  Pd distance is 8.655(2) Å. The reaction of L² with 2 equivalents of anhydrous [PdCl₂(MeCN)₂] or NiCl₂ afforded mononuclear complexes. The crystal and molecular structure of [PdL²]Cl₂·3.5CH₂Cl₂ was determined. The compound has an approximately square-planar P₂N₂ geometry.

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