Flexible kappa4-PNN'O-tetradentate ligands: synthesis, complexation and structural studies.

The three-step synthesis of new, air-stable, PNN'O-tetradentate ligands 3a x HH-3c x HH by Schiff base condensation of the primary amines 2a-2c with 2-Ph2PC6H4(CHO) in refluxing EtOH is described. The racemic ligand 3d x 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 x HH-3d x HH display various coordination modes. Reaction of 3a x HH with AuCl(tht) (1:1 molar ratio) affords AuCl(3a x HH), 4a, in which kappa(1)-P-complexation of the functionalised ligand is observed. In contrast, reaction of 3a x HH (or 3d x HH) with MCl2(cod) (M = Pt, Pd) affords MCl2(3a x HH) (M = Pt, 5a; M = Pd, 5b) or MCl2(3d x HH) (M = Pt, 5c; M = Pd, 5d) in which ligand chelation is achieved using both P and imine N donor atoms. Moreover kappa(2)-P,N-chelation was also observed when 3a x HH-3c x HH were separately allowed to react with [PdCl(eta(3)-C3H5)]2 in CH2Cl2 affording new cationic eta(3)-allyl complexes [Pd(eta(3)-C3H5)(3a x HH-3c x HH)]Cl, 6a-6c. Two neutral (methyl)chloropalladium(II) complexes, 7a/7c, were isolated in high yields from 3a x HH or 3c x HH and Pd(CH3)Cl(cod). Elimination of cod and single methyl protonation from Pt(CH3)2(cod) with 1 equiv. of 3a x HH, 3b x HH or 3d x HH in toluene, at room temperature, afforded square-planar complexes Pt(CH3)(kappa(3)-3a x H/3b x H/3d x H), 8a/8b/8d, containing monoanionic kappa(3)-PNN'-tridentate ligands. The kappa(3)-PNN'-tridentate mode was likewise observed for Pd(CH3)(3a x H/3c x H), 10a/10c, upon treatment of a methanolic solution of Pd(CH3)Cl(3a x HH/3c x HH) with tBuOK. Similarly the monohapto (allyl)Pd(II) compounds Pd(CH2CHCH2)(3a x HH-3c x HH), 9a-9c, were obtained cleanly from 6a-6c and tBuOK via an eta(3)-->eta(1) allyl isomerisation. Both amide and phenolic protons in 5a-5d were smoothly deprotonated, with base, to give the kappa(4)-PNN'O-tetradentate complexes 11a/11b and 11d/11e containing the dianionic ligands 3a(2-)/3d(2-) respectively. The Ni(II) complexes 11c and 11f were synthesised directly from NiCl2 x 6 H2O, 3a x HH (or 3d x 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 x HH, 3c x HH, 4a, 7c, 8a, 8b, 8d and 11a-11d.