Flexible κ4-PNN′O-tetradentate ligands: synthesis, complexation and structural studies

Abstract

The three-step synthesis of new, air-stable, PNN′O-tetradentate ligands 3a·HH–3c·HH by Schiff base condensation of the 1° amines 2a–2c with 2-Ph₂PC₆H₄(CHO) in refluxing EtOH is described. The racemic ligand 3d·HH, isolated in 79% yield, was successfully prepared from 2-C₆H₄(OH){C(O)NHCH₂CH(Me)NH₂} 2d and 2-Ph₂PC₆H₄(CHO) in absolute EtOH. Upon careful choice of metal precursor, ligands 3a·HH–3d·HH display various coordination modes. Reaction of 3a·HH with AuCl(tht) (1 : 1 molar ratio) affords AuCl(3a·HH), 4a, in which κ¹-P-complexation of the functionalised ligand is observed. In contrast, reaction of 3a·HH (or 3d·HH) with MCl₂(cod) (M = Pt, Pd) affords MCl₂(3a·HH) (M = Pt, 5a; M = Pd, 5b) or MCl₂(3d·HH) (M = Pt, 5c; M = Pd, 5d) in which ligand chelation is achieved using both P and imine N donor atoms. Moreover κ²-P,N-chelation was also observed when 3a·HH–3c·HH were separately allowed to react with [PdCl(η³-C₃H₅)]₂ in CH₂Cl₂ affording new cationic η³-allyl complexes [Pd(η³-C₃H₅)(3a·HH–3c·HH)]Cl, 6a–6c. Two neutral (methyl)chloropalladium(II) complexes, 7a/7c, were isolated in high yields from 3a·HH or 3c·HH and Pd(CH₃)Cl(cod). Elimination of cod and single methyl protonation from Pt(CH₃)₂(cod) with 1 equiv. of 3a·HH, 3b·HH or 3d·HH in toluene, at room temperature, afforded square-planar complexes Pt(CH₃)(κ³-3a·H/3b·H/3d·H), 8a/8b/8d, containing monoanionic κ³-PNN′-tridentate ligands. The κ³-PNN′-tridentate mode was likewise observed for Pd(CH₃)(3a·H/3c·H), 10a/10c, upon treatment of a methanolic solution of Pd(CH₃)Cl(3a·HH/3c·HH) with ^tBuOK. Similarly the monohapto (allyl)Pd^II compounds Pd(CH₂CHCH₂)(3a·HH–3c·HH), 9a–9c, were obtained cleanly from 6a–6c and ^tBuOK via an η³→η¹ allyl isomerisation. Both amide and phenolic protons in 5a–5d were smoothly deprotonated, with base, to give the κ⁴-PNN′O-tetradentate complexes 11a/11b and 11d/11e containing the dianionic ligands 3a²⁻/3d²⁻ respectively. The Ni^II complexes 11c and 11f were synthesised directly from NiCl₂·6H₂O, 3a·HH (or 3d·HH) and ^tBuOK in CH₃OH. All new compounds were characterised by multinuclear NMR, FT–IR, mass spectrometry and microanalysis. Single crystal X-ray studies have been undertaken on the compounds 3a·HH, 3c·HH, 4a, 7c, 8a, 8b, 8d and 11a–11d.