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Chiral, Hemilabile Palladium(II) Complexes of Tridentate Oxazolidines, Including C2-Symmetric “Pincers”

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posted on 17.11.2008, 00:00 by Elizabeth T. J. Strong, Steven A. Cardile, Allison L. Brazeau, Michael C. Jennings, Robert McDonald, Nathan D. Jones
Two classes of diastereomerically enriched chiral tridentate ligands incorporating either two oxazolidine and one pyridine (1) or two pyridine and one oxazolidine (2a-c) donor groups have been made in a high-yielding modular fashion from readily available enantiopure amino alcohols and aldehydes. Both ligand classes readily formed metal complexes via 1:1 reactions with trans-PdCl2(PhCN)2. The compounds Pd(1)Cl2 and Pd(2a-c)Cl2 were formed as mixtures of 3 and 2 diastereomers, respectively, owing to indeterminate absolute configuration at the C2 position of their constituent oxazolidine rings. Within each diastereomeric manifold, the metal complexes existed as equilibrium mixtures of bi- and tridentate isomers in solution, the interconversion between which was very rapid even at −50 °C. The fluxional nature of the compounds was inferred from a combination of 1H and 15N NMR spectroscopic and solution conductivity data. Substitution of one chloride ligand for hexafluorophosphate gave as mixtures of diastereomers the salts [Pd(1N2N′)Cl]PF6 (8) and [Pd(2a-cNN′N′′)Cl]PF6 (12a-c) in which the ligands were coordinated through all three N-donors. A single recrystallization of 8 gave in optically pure form the major diastereomer 8maj, which was characterized crystallographically. Complexes of 2a-c differed substantially from those of the bis(pyridylmethyl)amine (bpma) ligand family with which they shared direct atom-for-atom connectivity in the coordinating groups. The amines are known to form exclusively the static tridentate complexes [PdCl(bpma-κN2N′)]Cl; the difference was attributed to torsional strain associated with the rigid oxazolidine ring in tricoordinated 2a-c.