Designing for Intramolecular H-Bonding in Palladium(II) Dichlorides

Strong and synergistic hydrogen bonds (H-bonds) add many possibilities for organizing components around a transition metal ion. However, each additional H-bond introduces a new conformational constraint; mismatched geometric preferences for the metal, supporting ligand, and H-bond donor/acceptor groups can result in unintended contacts. Here, we explore using principles of conformational analysis to design intramolecular H-bonding in square-planar Pd(II) dichlorides. Comparisons between diprotic (L1)PdCl2 and monoprotic analogues, (L2)PdCl2 and (L3)PdCl2, illustrate that unsaturated six-membered metallacycles can “flex” to support intramolecular H-bonding. Analyzing the individual H-bonds with a distortion-interaction model suggests that this flexibility originates from small energies of rearrangement. Our analysis unifies other cases of intramolecular H-bonding under a common framework.