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

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)PdII 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 NiII 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.