Pyridines conjugative interactions

OH-pyridine and lH,2-pyridone optimized structures,122 Table 5. For the first tautomer the C4=C5 bond corresponds to an aromatic bond, while for the second, it corresponds to a formal single bond, although it undergoes a strong conjugative interaction. While changes in the SD and PSO term are... 

There have been extensive studies on the family of complexes of the general type (H3N)3Ru(pyX) with a wide range of X substituents on the pyridine. The n oAital of the conjugated system in pyridine can interact with a d, orbital from Ru(II) to produce a bonding orbital and its antibonding partner The electronic transition from the ground state... 

It would be expected that the stabilization of the adsorbed species by an extended conjugated system should increase with the number of aromatic rings in the adsorbed azahydrocarbon. However, data suitable for comparison are available only for phenanthridine, benzo-[/]quinoline, and benzo[h] quinoline. The large difference in the yields of biaryl obtained from the last two bases could be caused by steric interaction of the 7,8-benz-ring with the catalyst, which would lower the concentration of the adsorbed species relative to that with benzo[/]quinoline. The failure of phenanthridine to yield any biaryl is also noteworthy since some 5,6-dihydrophenanthridine was formed. This suggests that adsorption on the catalyst via the nitrogen atom is possible, but that steric inhibition to the combination of the activated species is involved. The same effect could be responsible for the exclusive formation of 5,5 -disubstituted 2,2 -dipyridines from 3-substi-tuted pyridines, as well as for the low yields of 3,3, 5,5 -tetramethyl-2,2 -bipyridines obtained from 3,5-lutidine and of 3,3 -dimethyl-2,2 -... 

The effect of [Pt(NH3)s ] on the [Fe(CN)g ] oxidation of parsley PCu(I) is more difficult to interpret, but merits further comment. At pH 7.5, with [Pt(NH3)6]" (and its conjugate base) present at concentrations up to 1.31 mM, rate constants increase by a factor of 2.5 [96]. In the case of the [Co(dipic)2] oxidation (dipic is pyridine-2,6-dicarboxylate) [95], a smaller 8% increase is observed, drawing attention to the importance of size of charge. There are a number of possible explanations. These include the effect of association of [Pt(NH3)e] on the net protein charge and hence its interaction with [Fe(CN)g ] . Association of [Pt(NH3)g ] at the remote acidic patch may lead to [Fe(CN)g ] " using this site to enhance the rate. A further contributing factor may be the quite separate association of [Pt(NH3)g] and [Fe(CN)6] to give an adduct which is more redox active. 

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