A series of NHC-containing [C^N]- or [C^C']-type palladacyclic complexes of the general formula [PdBr(iPr2-bimy)(L^X)] (5-8, 11, 12, iPr2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene) have been synthesized and fully characterized. Using these complexes, the donating abilities of monoanionic chelators were probed for the first time. The [C^N]-type palladacycles 5-8 were prepared from acetato-bridged dipalladium complexes [Pd(μ-CH3COO)(C^N)]2 (1-4) and iPr2-bimy·H+Br-as precursors. In the case of the [C^C']-type NHC palladacycles (11, 12), the hetero-bis(NHC) complexes trans-[PdBr2(iPr2-bimy)(trz)] (8, 9, trz = 1,2,3-triazolin-5-ylidene) containing the iPr2-bimy probe were first prepared followed by acetate-assisted cyclopalladations. The 13Ccarbene NMR signals of the iPr2-bimy ligands in all complexes (i.e. HEP and HEP2 values) are found to rationally reflect the donating abilities of the incorporated trz or [L^X]-type chelators with the exception of the Bzpy ligand (Bzpy = 2-(2-pyridinylmethyl)phenyl-C,N). This has been attributed to its larger bite angle, the resulting varied coordination geometry and the lack of electronic delocalization between the two donor units. The donicities of [L^X]-type chelators studied in this work were found to surpass those of all other bidentate ligands evaluated by HEP2 thus far.
The last decade has witnessed the rapid development of high‐valent Pd‐involved organic transformations. This has also led to a steadily growing number of publications concerning the preparation of isolable and characterizable palladium(III) and palladium(IV) complexes. A variety of one‐electron and two‐electron oxidants have been employed to give access to high‐oxidation‐state Pd compounds. Undoubtedly, the study of these stoichiometric reactions has great implications for relevant Pd‐mediated catalysis. In this minireview, the focus is on the synthetic approaches to structurally determined PdIII/IV complexes starting from their PdII precursors, and the advances in this research area from early 2010 to late 2019 will be highlighted. Chemical oxidations exploiting various oxidizing agents including 1) hypervalent iodine reagents; 2) halogens; 3) electrophilic fluorination reagents; 4) alkyl/aryl halides; 5) ferrocenium salts; 6) peroxides/O2; 7) sulfonyl chlorides; and 8) others are covered. A “greener” electrooxidation manner has also been reviewed.
Four pyridine-bridged bis(1,2,3-triazoles) A/B and C/D have been prepared by "click" reactions. Methylations/ethylations of A/B and C/D employing Meerwein's salts led to the formation of the corresponding dicationic salts 1/2 and 3/4 with pyridine moieties surviving from the alkylations. Interestingly, the reactions of "N-linked" salts 1/2 with KCO in the presence of elemental sulfur did not yield the mesoionic carbene-sulfur betaine adducts but unexpectedly afforded 1,5-disubstituted triazoles 5/6, which may produce pyridine thioaldehydes as by-products. In contrast, in the case of "C-linked" salts 3/4, the reactions to synthesize carbene-sulfur zwitterions 9/10 proceeded smoothly. Employing a silver-carbene transfer protocol with salt 3 as the precursor, the [CNC]-type palladium pincer complex 8 was synthesized, the solid-state structure of which was determined by X-ray diffraction analysis. Complex 8 underwent ligand substitutions with silver carboxylates producing the acetato complex 11 and trifluoroacetato complex 12. All newly synthesized pincer complexes were employed to test their catalytic activities in Mizoroki-Heck reactions. Positive mercury drop tests implied that the pincer-type complexes do not survive under the conditions of catalysis, owing to which no conclusions can be drawn regarding the catalyst variation or substrate scope.
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