Five-coordinate species rearrangements

The changeover reaction converting 1(4)2 + to the stable five-coordinate species is quantitative. It is easily monitored by visible absorption spectroscopy since the product of the rearrangement reaction is only slightly colored (pale olive green ... 

The molecular axis contains a thin 2,2 -bipyridine motif, which is less bulky than a 1,10-phenanthroline fragment and thus is expected to spin more readily within the cavity of the ring. In addition, the bipy chelate does not bear substituents in -position to the nitrogen atoms. 5(4) + rearranges to the five-coordinate species 5(5)2+ after oxidation and vice versa. The electrochemically driven motions were studied by cyclic voltammetry. A lower limit for the rate constant of the... 

Fig. 13 Principle of the electrochemically induced molecular motion in a rotaxane copper complex. The stable, four-coordinate monovalent complex is oxidized to an intermediate tetrahedral divalent species. This compound undergoes a rearrangement to afford the stable, five-coordinate copper(u) complex.

Similar kinetic behaviour is found for substitution of H20 by CN" in tra s-[Co(CN)4-(SOjXHjO)]4-.66 However, a reactive intermediate [Co(CN)4(S03)]3- species generated by loss of H20 from the coordination sphere need not, in this case, be five coordinate since the possibility exists for rearrangement to a configuration where SO " is bidentate (Section 47.7.4.3). 

The electrochemically triggered molecular rearrangement involves oxidation from a four-coordinate Cu to an intermediate four-coordinate Cu species which has a distinctly different ESR spectrum (higher g, lower a( Cu) measured in frozen solution) than the more stable five-coordinate Cu alternative. The latter is formed by fairly slow rotation of interlocking macrocyclic rings it reverts to the four-coordinate arrangement on re-reduction [82],... 

Figure 33. Principle of operation for inducing ring rotation in a nonsymmetrical [2]catenate. Horizontal arrows represent redox processes, vertical arrows represent rearrangements. Cu" is a solid circle, whereas Cu is an open circle. The stable four-coordinate monovalent complex Cu (4) (top left) is oxidized to the intermediate tetrahedral divalent species Cu"(4j (top right) which rearranges to the stable five-coordinate Cu (5) complex, bottom right. Upon reduction, the five-coordinated monovalent Cu (s) is formed (bottom left) which finally undergoes conformational changes to restore the starting Cu (4) (top left).

Fig. 7. Principle of the electrochemically induced molecular motions in a copper(I) complex pseudorotaxane. The stable four-coordinate monovalent complex is oxidized to an intermediate tetrahedral divalent species. This compound undergoes a rearrangement to afford the stable five-coordinate copper(II) complex. Upon reduction, the five-coordinate monovalent state is formed as transient. Finally, the latter undergoes the reorganization process that regenerates the starting complex [the black circle represents Cu(I) and the white circle represents Cu(II)]...

Three-c(H)rdinate carbenium ion and five-coordinate carbonium ion intermediates satisfactorily account for many of the acid-catalyzed reactions of hydrocarbons at high temperatures. Yannoni et al. have characterized the structure and dynamics of several carbenium ions trapped in (noncatalytic) solids at low temperatures [32,94,95), but lifetimes of such ions on active surfaces at higher temperatures would preclude NMR observation in all but special cases. Maciel observed triphenyl carbenium ion on alumina 196). The alkyl-substituted cyclopentenyl ions discussed earlier are also special ions they are commonly observed products in conjunct polymerization reactions of olefins in acidic solutions. The five member ring cannot easily rearrange to an aromatic structure, and ions like I and II are apparently too hindered to be captured by the framework to form alkoxy species. 

The Michael-Arbuzov rearrangement is a basic reaction for the preparation of 4-coordinate species including phosphine oxides from 3-coordinate phosphorus esters, such as phosphinous esters. In most cases, the reaction requires a prolonged heating above 100 °C. Odinets et al. have now succeeded in carr5nng out the Arbuzov-reaction of ethyl diphenylphosphinite with a variety of alkyl halides in ionic liquids at or below 110 °C in short reaction times, mostly within half an hour. The best ionic liquid was l-hexyl-3-methylimidazolium bromide (Scheme 2). The recovered [hmim]Br could be recycled at least five times without a decrease in activity. 

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