Ditz ligand

Figure 14 Heterobimetallic complexes with ditz ligand.

The catalytic performance of NHC-Pd complexes in this reaction is however usually less satisfactory than that of catalytic systems based on other ligands, such as, for example, phosphane hgands, and poly-NHC Pd complexes make no exception to this general trend. Consequently, although reports on the catalysis of the Heck reactions by these complexes have been numerous, they are often hmited to the reaction of aryl iodide or electron-poor, activated aryl bromide substrates, whereas electron-neutral or electron-rich aryl bromides require high temperatures for satisfactory yields. Reactivity of aryl chlorides is confined to electron-poor substrates and is observed only under very drastic conditions (T> 150 °C with tetra-alkylammonium salts as promoters). " An exception is represented by the work of Huynh and Guo on dipalladium(II) complexes with the Janus-type ditz ligand of structure 68." In this case, Heck reactions of activated aryl chlorides were possible already at 120 °C without addition of promoters the authors ascribe this result to the cooperative effect of the two metal centers. Poly-NHC complexes of type 69 have been found to catalyze also the less common diarylation (i.e., double Heck reaction) of ethyl acrylate with aryl bromides, albeit at 120 °C and with tetrabutylammonium bromide as additive. " Simple Pd species such as Pd(OAc)2 and PdCl2 were found to be quite ineffective under these conditions. 

Nevertheless, the group of Peris reported in 2009 that dinuclear Pd complexes with the Janus-type ditz ligand were quite effective in the sequential Sonogashira/hydroxyalkoxylation coupling leading to benzofurans (Fig. 17), with a catalytic efficiency comparable to that ofmono-NHC analogues. The reaction worked well with 2-iodobenzyl alcohol, whereas only moderate yields could be obtained with the bromo derivative and the chloro derivative was quite unreactive." ... 

The separation between the two redox potentials in the majority of the cited bimetallic complexes is around 60-80 mV, indicating that between the two metals, there is a weak electronic delocahzation. Only in the case of a Ru(II)/Ru(II) with ditz ligand, the separation is 120 mV (see Ref. 43). 

The best way to limit or prevent the tendency to chelation toward a single metal center is the use ofligands similar to Janus-type poly-NHCs (see Section 2.3), in which the carbene units point to opposite directions. In this regard, several reports deal with the ligand l,2,4-triazol-3,5-diylidene (ditz. Fig. 14) already described in Section... 

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