Synthesis and characterization of PdII complexes with bis-pyridinium and isoquinolinium N-ylides:: Moderate C-H•••O=C intramolecular hydrogen bonds as source of conformational preferences

The bis(N-ylide) Pd-II complexes cis-[PdX2{eta(2)-[C(H)NC(x)Hy](2)CO}] (X = 1, NCxHy = NC5H5, 2a; X = Br, NCxHy = NC5H5 and NC5H3-2,3-Me-2, 2c, isoquinolinium NC9H7, 2d) have been prepared by reaction of the corresponding bis-pyridinium salts with Pd(OAc)(2) (1:1 molar ratio). Compounds 2 react with AgClO4 and TI(acac) (1:1:1 molar ratio) to give the acetylacetonato derivatives [Pd(acac-O, O'){eta(2)-[C(H)NCxHy](2)CO}]ClO4 (3a, c, d). In compounds 2 and 3, the bis-ylide is bonded as a C,C-chelate ligand through the two ylidic C-alpha atoms. The reaction is stereoselective, and only one diastereoisomer is observed (meso form, RS/SR). The origin of the observed stereoselectivity lies with the establishment of intramolecular C-H...O=C hydrogen bonds between the ortho protons of the pyridine or isoquinoline fragments and the carbonyl oxygen, as it has been shown by density functional theory (DFT) calculations (B3LYP level) and Bader analysis of the electron density on model pyridinium ylides. Despite the inherent weakness of the C-H...O=C bonds, the results show that in these N-ylides the hydrogen bonds are stronger than expected and should be classified as moderate H bonds.