Of pyridine complexes

Complexes of other N-donors A range of pyridine complexes can be made [98]... 

Electrophilic iodine reagents are extensively employed in iodocyclization (see Section 4.2.1). Several salts of pyridine complexes with 1+ such as bis-(pyridinium)iodonium tetrafluoroborate and b/.s-(collidine)iodonium hexafluorophos-... 

Electrophilic iodine reagents have also been employed in iodocyclization. Several salts of pyridine complexes with I+ such as bis(pyiidinium)iodonium tetrafluoroborate and bis(collidine)iodonium hexafluorophosphate have proven especially effective.61 y-Hydroxy- and d-hydroxyalkcncs can be cyclized to tetrahydrofuran and tetrahydropyran derivatives, respectively, by positive halogen reagents.62 (see entries 6 and 8 in Scheme... 

The specific proposal is that the hydroxide attacks the 2- or the 4-position of the co-ordinated pyridine to form a hydroxy-substituted 1,2- or 1,4-dihydropyridine (Fig. 8-17). These hydroxy species are known as pseudo-bases. This behaviour is fully in accord with the known behaviour of A-alkylpyridinium cations. Although the pseudo-bases of simple A-alkylpyridinium cations are not dominant solution species under aqueous conditions, those derived from a variety of other nitrogen heterocycles are readily formed and are well-known. The suggestions had the advantage of linking the apparently divergent fields of heterocyclic and co-ordination chemistry by explaining some well-documented anomalies in the reactivity of pyridine complexes. 

X-Ray studies of crystals of pyridine complexes with trimethylsilyl bromide and iodide (264) and of N-methylimidazole adducts to trimethyl-chlorosilane (265) were performed. The tetracoordinate silicon structure of these complexes was proved. The distances between the halogen and silicon atoms in pyridine adducts are 4.359 and 4.559 A for bromine and iodine, respectively, which is approximately 2 A greater than the sum of the covalent radii and significantly longer (0.5 A) than the sum of the van der Waals radii. This result is consistent with the ionic structures of the complexes in the solid state. The distance from silicon to nitrogen (1.86 A) is evidently larger than the length of typical Si—N bonds (1.75 A), which... 

A different behavior is found when strong nucleophiles are added to a polymerization of isobutene coinitiated with Lewis acids. When the concentration of nucleophile reaches that of Lewis acid, no polymerization is observed [5,91,268]. Quite often a precipitate, identified as a complex between Lewis acid and the nucleophile, is detected. This indicates that the complexed Lewis acid is no longer capable of ionization of the covalent species. A fractional negative order (-0.3) was reported in the polymerization of isobutene initiated by alkyl chloride/TiCL with added pyridines [268]. It is possible that a small amount of pyridine complexes with the Lewis acid, reduces its concentration, and thereby additionally shifts the equilibria from the more reactive dimeric to the less reactive monomeric TiCI4. Nevertheless, a small amount of nucleophile apparently has a beneficial effect, because polymers with lower polydispersities are formed. The plausible explanation of the role of nucleophiles in these systems will be offered in Section VILE.4. 

However, there are few studies which predict such large chemical enhancements while several similar TDDFT studies on different molecules predict much lower enhancements [72-74]. For example, a study of pyridine complexed to a 20 atom Ag cluster by the same group predicted much lower enhancement factors. Factors on the order of 10 for the static chemical enhancement, 10 for charge transfer effects, and 10 for the electromagnetic contribution emphasized that the chemical enhancement is a much smaller contribution in general [72]. 

Recently, the synthesis and X-ray crystal structure of [Cu(dcmpz)2(py)2], 229, were reported (40). Complex 229 was obtained either by reacting [Cu(dcmpz)], 12, (27) with dioxygen in pyridine or by the direct reaction of CuCl2 with Hdcmpz in the presence of pyridine. Complexes having two... 

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