Controlled Access to Four- and Six-Membered Palladacycles Modifying Donor Abilities of β-Ketiminato Ligands ("NacAcs").

The synthesis of Pd complexes of the type [PdBr(Pr-bimy)(NacAc)] (NacAc = β-ketiminate, Pr-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 κ-,-six-membered palladacycles were observed with R = Me and Et, while the unusual κ-,-four-membered palladacycles were isolated with R = Pr, Cy, and 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.