The properties and reactivities of transition metal complexes are rooted in the stereoelectronic properties of their ligands. While the bulk of a ligand can be easily evaluated and compared by the drawing of its Lewis structure, prediction on the electronic contributions is often less straightforward. Thus, several electronic parameters have been developed for the experimental evaluation of ligands throughout the years. This article accounts for the most recent one developed by the Huynh group, which employs C NMR spectroscopy to determine ligand donor strengths using N-heterocyclic carbene complexes. This parameter not only proves to be safer, more convenient and accurate in comparison to existing methodologies, but it also provides, in certain cases, more intuitive and reliable results. Furthermore, it is currently the only one that allows the direct comparison of various Werner-type and organometallic ligands on a unified scale.
A series of 15 mononuclear complexes [PdBr((i)Pr2-bimy)(L2)]PF6 (1-15) ((i)Pr2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene, L2 = aromatic 1,2-diimines, diazabutadienes, or methylene-, ethylene- and propylene-bridged di-N-heterocyclic carbenes) and two dicarbene-bridged, dinuclear complexes [Pd2Br4((i)Pr2-bimy)2(diNHC)] (16 and 17) were synthesized and characterized by multinuclear NMR spectroscopy, electrospray ionization mass spectrometry, and in some cases X-ray diffraction analysis. The influence of the 15 bidentate ligands L2 on the (13)Ccarbene signals of the (i)Pr2-bimy reporter ligand in the chelate complexes was studied, on the basis of which a facile methodology for the donor strength determination of bidentate ligands was developed.
A systematic evaluation of N-heterocyclic carbene ligands in the iron-catalyzed cross-coupling reactions of aryl chlorides and arylmagnesium reagents is performed. There is no clear correlation between the donor strength of the N-heterocyclic carbene and the reaction outcome. Instead, the highest yields of the desired biaryl product are obtained with sterically demanding ligands possessing large %V bur values. Through this study, SIPrNap has been identified as an efficient and general ligand for the coupling of both aryl chlorides and tosylates.
Oxidation of easily accessible thiolato-functionalized dinuclear Pd(II) NHC complexes 1−3 by Oxone gave rise to sulfonate-NHC complexes 4−6. This represents the first templatedirected approach to NHC complexes bearing sulfonate functions, where the sulfur atoms undergo a six-electron oxidation, changing their oxidation states from −II to +IV. The catalytic activities of water-soluble 4−6 were also tested in aqueous Mizoroki−Heck reactions.
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