Cations recognizing

The half-life of Fe2(OH)2" at room temperature is a few seconds. An improved model for the kinetics of dissociation of this dinuclear cation recognizes significan articipation by Fe2(OH)3 + at higher pHs, thus clearing up earlier slight anomalies in this area. Phosphate ester hydrolysis at the di-iron center of uteroferrin has now been shown to involve nucleophilic attack by bridging hydroxide (as proposed but not conclusively demonstrated for several M—OH—M-containing catalytic species) rather than by hydroxide bonded to just one Fe. ... 

The ion-pair receptor 9 was designed to bring together an anion-binding core and a cation-recognizing subunit in such a way that a large separation between the constituent ions... 

Fig. V-1. Variation of m / o and n /wo with distance for = 51.38 mV and 0.01 M uni-univalent electrolyte solution at 23°C. The areas under the full lines give an excess of 0.90 X 10 mol of anions in a column of solution of 1-cm cross section and a deficiency of 0.32 x 10 mol of cations. There is, correspondingly, a compensating positive surface charge of 1.22 x 10 " mol of electronic charge per cm. The dashed line indicates the effect of recognizing a finite ion size.

Von Baeyer (Nobel Prize, 1905) should be credited for having recognized in 1902 the saltlike character of the compounds formed. He then suggested a correlation between the appearance of color and salt formation—the so-called halochromy. Gomberg (who had just shortly before discovered the related stable triphenylmethyl radical), as well as Walden, contributed to the evolving understanding of the structure of related cationic dyes such as malachite green. 

It IS important to recognize the difference between the hydrocarbon cyclohepta triene and cycloheptatrienyl cation... 

Two general types of heterocyclic cations may be recognized those in which the charge results from an increase in the bonding number of an atom by the addition of a proton (or equivalent), and those in which the charge results from a decrease in the bonding number by removal of hydride. These two possibilities are shown for pyrrole in (167) and (168). 

Some fundamental structure-stability relationships can be employed to illustrate the use of resonance concepts. The allyl cation is known to be a particularly stable carbocation. This stability can be understood by recognizing that the positive charge is delocalized between two carbon atoms, as represented by the two equivalent resonance structures. The delocalization imposes a structural requirement. The p orbitals on the three contiguous carbon atoms must all be aligned in the same direction to permit electron delocalization. As a result, there is an energy barrier to rotation about the carbon-carbon... 

It is worth noting that in the case of the reactions of ediylene and butadiene with the allyl cation, the MO description has provided a prediction that would not have been recognized by a pictorial application of valence bond terminology. Thus, we can write an apparently satisfactory description of both reactions. 

Almost as soon as Pedersen announced his discovery of the crown ethers (see Chaps. 2 and 3) it was recognized by many that these species were similar to those prepared by Busch and coworkers for binding coinage and transition metals (see Sect. 2.1). The latter compounds contained all or a predominance of nitrogen and sulfur (see also Chap. 6) in accordance with their intended use. The crown ethers and the polyazamacrocycles represented two extremes in cation binding ability and preparation of the intermediate compounds quickly ensued. In the conceptual sense, monoazacrowns are the simplest variants of the macrocyclic polyethers and these will be discussed first. 

In his original paper, Lehn recognized the interesting possibility of exo-endo isomerism. This kind of isomerism is shown in Eq. (8.2) and is similar to that referred to by Simmons and Park as in-out isomerism. Lehn and his coworkers recognized from the very beginning that this new class of molecules had the potential for binding metal cations of many types. Since the molecules were designed to have molec-... 

Dietrich, Lehn and Sauvage recognized not only the possibility of enclosing a cation completely in a lipophilic shell, but they also recognized the potential for using such systems for activating associated anions. This is made particularly clear in a paper which appeared some years later One of the original motivations for our work on cryptates rested on their potential use for salt solubilization, anion activation and phase transfer catalysis . This particular application is discussed below in Sect. 8.3. 

Figure 12.1 shows a slice through such a solid the cations are to be thought of as a rigid lattice, and the electrons form a gas. I have deliberately drawn the cations as large objects for two reasons. First, the very early models such as that due to Drude tried to treat the electron sea as a perfect gas. It was eventually recognized that the electrons would collide with the cations and with each other an uncomfortable number of times. In any case, many of the predictions of the Dmde model turned out to be demonstrably flawed. 

Both the cycioheptatrienyl radical and the anion are reactive and difficult to prepare. The six-Tr-electron cation, however, is extraordinarily stable. In fact, the cycioheptatrienyl cation was first prepared more than a century ago by reaction of Br2 with cycloheptatriene (Figure 15.7), although its structure was not recognized at the time. 

Blomstrand s formula, which had been largely disregarded, was rediscovered practically simultaneously by Bamberger (1895a) and Hantzsch (1895), who recognized how well it meets the experimental facts. Hantzsch, one of the precursors of physical organic chemistry, also realized the cationic character of these compounds and proposed the present name diazonium ion. 

Looking at the formula, we recognize the ammonium cation,, and the sulfate anion, SOq. ... 

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