Pyridine ligands benzimidazoles

Stabilization of a metal-substituted derivative of a minor tautomeric form of the Ugand was reported for the complexes of mixed benzothiazo-line-benzimidazole (388 —> 389) (71ZOB1370 98POL381) and benzothia-zoline-pyridine ligands (390 —> 391/392) (Scheme 144) (77JA7704). 

Rates for intramolecular A/A isomerisations have been reported for Cu(L)2+, where L = atropoisomer of di-imine benzimidazole-pyridine ligands. H, C and 31P NMR spectra suggest that MX[P(C7H7)3], where M = Cu or Ag, X = Cl, Br, are non-rigid in solution at room temperature.980 Dynamic 31P NMR spectroscopy was used to follow fluxional processes in [(3,5-di-/er/-butyl-l, 2,4-triphospholyl)Cu(PPh3)]. The preferred coordination mode at room temperature is r 5-7i coordination of copper. At low temperatures, two cr-isomers are seen.981... 

FIGURE 18 Preparation of substituted 2,6-bis(benzimidazol-2-yl)pyridine ligands L17 following (A) original intermolecular Phillips reaction, (B) intramolecular Piguet cycliza-tion, and (C) Krdhnke methodology. 

FIGURE 19 Cj-symmetrical tridentate 2,6-bis(benzimidazol-2-yl)pyridine ligands prepared for the design of lanthanide triple-helical building blocks [R(L17)3]. ... 

TABLE 14 Thermal Behavior of the Lanthanide Complexes of 2,6-Bis (1-Ethyl-Benzimidazol-2-yl)pyridines Ligands L19, L23, L24, L25, and L26 ... 

Imidazole is characterized mainly by the T) (N) coordination mode, where N is the nitrogen atom of the pyridine type. The rare coordination modes are T) - (jt-) realized in the ruthenium complexes, I-ti (C,N)- in organoruthenium and organoosmium chemistry. Imidazolium salts and stable 1,3-disubsti-tuted imidazol-2-ylidenes give a vast group of mono-, bis-, and tris-carbene complexes characterized by stability and prominent catalytic activity. Benzimidazole follows the same trends. Biimidazoles and bibenzimidazoles are ligands as the neutral molecules, mono- and dianions. A variety of the coordination situations is, therefore, broad, but there are practically no deviations from the expected classical trends for the mono-, di-, and polynuclear A -complexes. 

Most of the examples listed are pentacyanide, corrinoid, or DMG complexes. The axial ligands are not identified in the tables, but are as follows corrinoids, 5,6-dimethylbenziminazole (cobalamins), H O or none (cobinamides), (DMG)2, usually pyridine or H2O, less frequently NHj, imidazole, benzimidazole, PBuj, etc. The nature of the axial and equatorial ligands may have a striking effect on reactivity, but few direct comparisons are available these are discussed in the next section. 

Table 12 shows the cis effect of the axial ligand X or L on the optical spectrum of cobalt(III) etioporphyrin derivatives Co(Etio-I)LX [32a-h] which behave similarly to the corresponding rhodium(III) complexes (Table 4). Analogous trends are found in the Co(TTP)LX series (133). The only notable feature is the large bathochromic shift caused by benzimidazole in [32h] obviously this ligand is far more different from pyridine than imidazole, the condensed ring system acting as a stronger 7r-acceptor.

A similar chemistry to that described for (substituted) pyridine rhenium(V) oxo complexes is also observed for other heterocyclic nitrogen donor ligands such as pyrimidine, pyrazine, substituted imidazoles, benzimidazoles, or benzotriazole. ... 

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