The synthesis of Pd complexes of the type [PdBr(ⁱPr₂-bimy)(NacAc)] (NacAc = β-ketiminate, ⁱPr₂-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 = ⁱPr, Cy, and ^tBu. 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.

中文翻译：

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通过修改 β-酮亚基配体 (“NacAcs”) 的供体能力来控制对四元和六元钯环的访问

^{[PdBr( i} Pr ₂ -bimy)(NacAc)] (NacAc = β-ketiminate, ⁱ Pr ₂ -bimy = 1,3-diisopropylbenzimidazolin-2-ylidene)类型的 Pd 配合物的合成被尝试，在连续努力使用双齿供体 (HEP2) 的 Huynh 电子参数量化螯合 β-酮亚酸酯配体的供体能力。NacAc 主链上 N-取代基的细微变化被发现会导致首选螯合模式发生剧烈变化，因为观察到常见的 κ ² - N，O -六元钯环，其中 R = Me 和 Et，而不寻常的 κ ² - C , N- 四元 palladacycles 与 R = ⁱ Pr、Cy 和^t Bu 分离。计算研究随后证实了这些发现，其形式为整体放能六到四元环收缩过程和三个更多供电子烷基部分的较低相关活化能。已建立的能量分布中的这种趋势可归因于六元环配合物相应优化结构中 HOMO-LUMO 间隙的减少。