Pyridinium cyclopentadienide

Fragments in compounds 155—157 exhibit aromatic bond delocalization. The lowest aromaticity is calculated for Af-pyridinium cyclopentadienide 157, with the interfragmental C—N bond shorter than the corresponding one in 155 and 158. The phenolate moiety in 159 has a high NICS value (—4.6 ppm), in agreement with the one for deprotonated phenol (—6.2 ppm compared to —9.7 ppm for benzene, as cited),196 while the acceptor pyridinium counterpart has a NICS value of —5.5 ppm, showing aromatic delocalization. 

In 1966, Boyd reported the synthesis and properties of 2-(I-pyridinio)-benzimidazolate 55, a stable aza analogue of pyridinium cyclopentadienide 9 (66TL3369). With this, a novel type of heterocyclic mesomeric betaine was found. 

Ci7HpBrN2 0.5 H2O, 1-(1 -Methyl-2 -pyridinium)-3-(1 -methyl-4 -pyridinium)cyclopentadienide bromide hemihydrate, 46B, 235 C17H21NO2, Enamine 5, 44B, 198... 

Table 27. Selected examples of correlation between observed and calculated UV transitions (Del Bene and Jaffe). (Other studied compounds include benzene, pyridine, 1,2-diazine, 1,3-diazine, 1,4-diazine, cyclopentadienide ion, pyrazole, imidazole, 2-pyrrole-carboxaldehyde, furfural, benzonitrile, nitrosobenzene, phenol, phenoxide ion, pyridinium ion, 1-hydroxy-pyridinium ion, 2-cyano-pyridine, 3-cyano-pyridine, 4-cyano-pyridine, 2-amino-pyridine, 3-amino-pyridine, 4-amino-pyridine.)...

A notable reaction is that between the pyridinesulfur trioxide complex and sodium cyclopentadienide 298 which forms azulene 299 by a sequence involving opening of the pyridinium ring and subsequent closure to the seven-membered carbocyclic ring. 

Tetrakis-cyclopentadienyl cerium (CeCp4) has been reported to be formed by the reaction of bis(pyridinium) hexachlorocerate with sodium cyclopentadienide [17]. Similarly indenyl and fluorenyl derivatives were prepared. Later work showed the same reactants in tetrahydrofurane to yield, Ce(Cp3), tricyclopentadienyl cerium complex instead of Ce(Cp4) [18]. 

The reaction of the l-(diaminocyclopropenyliumyl)pyridinium dication 30 with two equivalents of potassium pentakis(methoxycarbonyl)cyclopentadienide (31) also afforded the ionic salt 32 of 1 2 composition, whose structure was determined by X-ray crystallography. On the other hand, the reaction of 30 with the lithium salt of the 7,7,8,8-tetracyanoquinodimethane adical anion 33 resulted in covalent bond formation giving the substituted bis(cyclo-propenylium) dication 34. °... 

Ring fission of pyridinium ions with acceptor substituents, e.g. 7V-2,4-dinitrophenylpyridinium salts 221 (see p 280) leads, on treatment with secondary amines, to pentamethine cyanines 222 (Konig s salts). They condense with sodium cyclopentadienide to yield vinylogous aminofulvenes 223, which on cyclization produce azulene (azulene synthesis according to Ziegler and Hafner, see p 229 [73]) ... 

Pyridinium ions with acceptor substituents, for example, N-(2,4-dinitrophenyl) pyridinium salts 231 (cf p. 356), are converted to pentamethine cyanines 232 (Konig s salts) by ring-opening with secondary amines. Cyanines 232 condense with Na-cyclopentadienide to yield vinylogous aminofulvenes 233, which on cyclization afford azulene (Ziegler/Hafner azulene synthesis, cf p. 304) [137] ... 

The synthesis of tetrakis- (or tetra-) cyclopentadienyl cerium was reported first by reaction of bis(pyridinium)hexachlorocerate and sodium cyclopentadienide (Kalsotra et al. 1971) the same reaction has been extended to other Ji-ligands such as indenyls and fluorenyls. However, in 1983 it was reported that the reaction of sodium cyclopentadienide with bis(pyridinium)hexachlorocerate in THF yields tricyclopentadienyl cerium(III) instead of tetrakiscyclopentadienyl cerium(IV) (Deacon et al. 1983). 

X=halide. Triscyclopentadienyl cerium(IV) chloride has been prepared by reaction of sodium cyclopentadienide and bis(pyridinium)hexachlorocerate in THF or benzene, or by ligand redistribution between bis(pyridinium)hexachlorocerate and tetracyclopentadienyl cerium (Kalsotra et al. 1971). These brown compounds, described as stable in water, act as starting materials for Cp CeZ (Z = alkyl, aryl, pseudohalide, mercaptide, carboxylate) (Marks and Ernst 1982). 

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