Palladium complexes with a new hemilabile bis(oxazoline)phenylphosphonite ligand.: Characterization of an unprecedented chloro palladium(II)-(η1-allyl) complex

The coordination chemistry of the new, structurally characterized ligand bis(oxazoline)phenylphosphonite (I, abbreviated NOPONMe2), shows its flexibility which is due to the possible formation of six-membered chelate rings. In the Pd(II) complexes [Pd(NCMe)-(NOPONMe2-N,P,N)] (BF4)(2), 1 (characterized by X-ray diffraction in 1.0.5Et(2)O.0.33MeCN), and [PdCl(NOPONMe2-N,P,N)] (PF6), 2, this ligand behaves in a static tridentate manner, whereas in [Pd(Me)Cl(NOPONMe2-N,P)],3, [PdI2(NOPONMe2-N,P)],4, [PdCl2(NOPONMe2-N,P)], 5, and the allyl complex [Pd(eta (3)-C3H5)(NOPONMe2-N,P)](PF6), 6, it displays fluxional bidentate behavior, as shown by variable-temperature NMR studies. In 3, only the isomer in which the methyl ligand is trans to nitrogen is formed. In the related complex [Pd(eta (3)-C3H5)(NOPONMe2-N,P)]Cl, 7, an equilibrium has been evidenced between 7a and 7b, which involves coordination of the chloride and isomerization of the allyl ligand from eta (3) to eta (1). The latter isomer is quantitatively formed in toluene at 259 K and in the solid state. This was established using NMR spectroscopy by combined variable-temperature solution and solid-state studies. Isomer 7b was also characterized by X-ray diffraction, a rare example of a fully characterized allyl yl-bonding mode for Pd complexes and the first in transition metal chemistry for a mutual cis arrangement of eta (1)-allyl and chloride ligands, a situation relevant to intermediates involved in catalytic transformations. The tridentate coordination mode of I found in complexes 1 or 2 never occurred in the related alkyl or allyl complexes. This is consistent with the antisymbiotic effect between carbon and phosphorus donors, and this finding was confirmed by theoretical calculations. To understand whether the mutually cis disposition in 3 and 7b of the chloride ligand (trans to P) and of a sigma -donor ligand such as the methyl or the eta (1)-allyl ligand (trans to N) is intrinsic to the nature of these ligands or related in one way or another to the P,N heterobidentate nature and resulting asymmetry of the NOPONMe2 ligand, DFT-B3LYP calculations were carried out on a series of isomeric structures of four- and three-coordinate chloro, methyl, and eta (1)-allyl Pd(II) complexes. The existence of an energetic barrier against the formation of a compound where the phosphorus atom of tridentate NOPONMe2 is trans to an alkyl or eta (1)-allyl ligand was established.