V. Molecular structures of acetate-bridged dimers of a 2-arylpyridine and a 2,6-diarylpyridine cyclometallated by palladium(II)

Abstract

The crystal and molecular structures of dimeric cyclopalladated acetato(OAc)-bridged complexes of a 2-arylpyridine ( 1) and a 2,6-diarylpyridine ( 2), [Pd( 1)OAc] 2 ( 3) and [Pd( 2)OAc] 2 ( 4) have been determined by X-ray diffraction. In each. two planar acetates, nearly perpendicular to each other, cis-bridge two Pd II atoms, while the other two square planar sites on each metal are occupied by the cyclometallated aryl-substituted pyridine. The two metal coordination planes are in near parallel registry with one another, giving a “boat” molecule shape of exact ( 3 or near ( 4) two-fold symmetry. The non-bonding Pd⋯Pd distance (δ) strongly correlates with the angle (α) subtended by the metal coordination planes, which in turn seems related to the steric requirements of the non-bridging ligands. In 3, with the planar nitrophenylpyridine ligand, α 25.7° and δ 2.822(6) Å, the shortest Pd⋯Pd distance yet observed in dimers of this type. The PdC lengths, 1.94 Å in 3 and 1.944 Å in 4, are significantly shorter than the calculated length, 2.05 Å, suggesting partial multiple bond character in these shortest PdC (aryl) bonds so far observed. PdN bond lengths are near expected single bond lengths. Failure of trans-dimetallation to occur in 4, analogous to that recently observed with certain 2,6-dialkylpyridines or to the tridentate behavior of terpyridine in [Pd terpy X] + compounds, appears due to a combination of the shortened PdC bond in the first-formed metallated ring and the larger constraining angles (near 120° for sp 2 carbons) in the remaining (unmetallated) ring.