Controlled Access to Four- and Six-Membered Palladacycles via Modifying Donor Abilities of β‑Ketiminato Ligands (“NacAcs”)

The synthesis of Pd complexes of the type [PdBr­( i Pr2-bimy)­(NacAc)] (NacAc = β-ketiminate, i Pr2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene) was attempted, in a continuing effort to quantify donor abilities of chelating β-ketiminate ligands using the Huynh electronic parameter for bidentate donors (HEP2). Subtle variation of N-substituents on the NacAc backbone was discovered to induce a drastic change in the preferred chelating mode, in that the commonly encountered κ2-N,O-six-membered palladacycles were observed with R = Me and Et, while the unusual κ2-C,N-four-membered palladacycles were isolated with R = i Pr, Cy, and t Bu. Computational studies subsequently corroborated these findings, in the form of an overall exergonic six-to-four-membered ring contraction process and a lower associated activation energy for the three more electron-donating alkyl moieties. This trend in the established energy profiles can be attributed to a reduced HOMO–LUMO gap in the corresponding optimized structures of the six-membered ring complexes.