Tridentate N-heterocyclic

Strongly Electron Donating Tridentate N-Heterocyclic Biscarbene Ligands for Rhodium and Iridium Catalysts... 

More recently, we have developed aromatic heterocyclic pyrazol-5-one-based tridentate N,N,0-donor ketiminate-supported magnesium and zinc benzyl-alkoxide systems [L2M2(p-OBn)2] where M = Mg 45 or Zn 46, and L = 4-... 

Rh and Ir complexes bearing at least two N-heterocyclic carbene (NHC) moieties within a tridentate ligand framework, mostly in form of a pincer ligand, are the subject of this chapter. Special properties of each ligand type, its complexes, and catalytic examples will be discussed. 

Another N-heterocyclic carbene pincer ligand was introduced to ETM chemistry by Hollis and co-workers." This tridentate ligand had two NHCs connected via a phenyl bridge, and it could easily be deprotonated by the amido ligands in [Zr(NMe2)4] acting as internal base to form the tridentate Zr complex 38. Then, treatment with methyl iodide yielded the expected triiodide [(NHQZrH] 39 at room temperature (Scheme 6.5). 

Recent studies have been directed towards the synthesis of heterocyclic hydrazones which have lower toxicities than thiosemicarbazones [44], It has been proposed that the hydrazinic N-H group is essential for activity since it might be involved in a crucial radical formation step important in the mechanism of RDR. This is supported by the loss of antileprotic activity for this series of compounds when the hydrazinic hydrogen is replaced by a methyl group [44]. The heterocyclic hydrazones, like thiosemicarbazones, behave as tridentate ligands. 

The tridentate nature of the oximate-anion, i.e., its ability to act as O-, N-, and C-nucleophile, complicates analysis of the mechanism of the superbase-catalyzed heterocyclization of ketoximes with acetylene. 

As was shown first by Rais and Tachimori,31 a synergistic effect was also observed for mixtures of CDD with heterocyclic compounds. This direction was also actively investigated in the last years. Mixtures of CCD with N-tridentate heterocyclic compounds,32 diamides of dipicolinic acid (DPA),33-35 and calixarenes with heterocyclic reaction centers36-38 were studied for extraction. For the most part, extraction of MAs and separation of MAs from lanthanides were the center of attention in these works. But, it should be mentioned that all solvents with CCD also extract cesium. 

Krejzler, J., Narbutt, J., Foreman, M.R.St.J., Hudson, M.J., Casensky, B., Madic, C. Solvent extraction of Am(III) and Eu(III) from nitrate solution using synergistic mixtures of N-tridentate heterocycles and chlorinated cobalt dicarbollide. Czech. J. Phys. (2006), 56 (Suppl. D), D459-D467. 

This metal-directed ring-opening of thiazolines, oxazolines and related compounds is a widespread reaction. In many cases, equilibria are set up between the heterocyclic compound and the imine. The position of these equilibria are metal-ion dependent. A typical example is seen in Fig. 5-81. The reaction of 2-(2-pyridyl)benzothiazoline (5.36) with copper(n) salts leads to the formation of complexes of the ring-opened tridentate form. 

Tridentate pincer bis-carbene [114] and N-carbamoyl-substituted heterocyclic carbene complexes of Pd(II) [115] have also been used to couple aryl bromides and iodides with aromatic or aliphatic alkynes. Surprisingly, the latter catalytic system requires the use of 1 mol % of PPh3. Its role in the catalytic cycle is still unclear but it might facilitate the initial generation of Pd(0) species. 

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