Tridentate Assembling Ligands Based on Oxazoline and Phosphorus Donors in Dinuclear Pd(I)−Pd(I) Complexes

To examine the bonding preferences of potentially tridentate phosphorus, nitrogen donor ligands on a dinuclear metal core, we have studied the coordination of the oxazoline-based ligands bis(4,5-dihydro-2-oxazolylmethyl)phenylphosphine (NPN) and bis(4,4-dimethyl-2-oxazolyl dimethylmethoxy)phenylphosphine (NOPON^Me2) toward the dinuclear d⁹-d⁹ Pd(I) complex [Pd₂(NCMe)₆][BF₄]₂. In the dinuclear product [Pd₂(NPN-N,P,N)₂](BF₄)₂ (1), in which the Pd−Pd bond length of 2.5489(7) Å is rather short, the two interacting metal centers are P,N bridged by two molecules of the NPN ligand, forming two six-membered rings. The other oxazoline ring of each ligand further chelates a Pd center through its nitrogen atom, forming five-membered chelates, as in the mononuclear complex [PdCl₂(NPN-N,P)] (5). In contrast, the reaction between [Pd₂(NCMe)₆](BF₄)₂ and NOPON^Me2 in the presence of LiCl afforded the mononuclear cationic complex [Pd(NOPON^Me2-N,P,N)Cl](BF₄) (3), which is also obtained by halide abstraction from [Pd(NOPON^Me2-N,P)Cl₂] with NaBF₄. When this reaction was performed in the presence of 1 equiv of t-BuNC, the new dinuclear Pd(I)−Pd(I) complex [Pd₂Cl₂(CNt-Bu)(NOPON^Me2-N,P,N)] (4) was isolated, which can also be obtained from a comproportionation reaction between Pd(II) and Pd(0) complexes. The oxazoline in the P,N bridge is involved in a seven-membered ring moiety, a situation rarely encountered in Pd(I)−Pd(I) chemistry. Its nitrogen atom is coordinated trans to the isonitrile ligand whereas that of the P,N chelate at Pd(1) is trans to Pd(2). The fluxional processes involving the oxazoline moieties of the NPN and NOPON^Me2 ligands in 1 and 4, respectively, were examined by variable-temperature NMR spectroscopy. The crystal structures of 1, 3·0.5CH₃CN, and 4 have been determined by X-ray diffraction. Prior to this work, relatively few complexes have been reported in the literature in which a potentially tridentate functional phosphorus ligand is simultaneously chelating and bridging a dinuclear Pd(I)−Pd(I) system.