Dimer cation neutralization

Finally, radical cations can be generated in solution by different types of pulse radiolysis225. Like PET, this is inherently a method for transient spectroscopic observations, but it has proved to be invaluable in investigations of dimer cations, e.g of polyenes, which form spontaneously upon diffusion of radical cations in the presence of an excess of the neutral parent compound, but a discussion of the electronic structure of such species is beyond the scope of this review. Pulse radiolysis is of interest in the present context because it allows the observation of large carotenoid radical cations which are difficult to create in solid-state or gas-phase experiments... 

Alike metallocomplex anion-radicals, cation-radicals of odd-electron structure exhibit enforced reactivity. Thus, the 17-electron cyclopentadienyl dicarbonyl cobalt cation-radical [CoCp(CO)2] undergoes an unusual organometallic chemical reaction with the neutral parent complex. The reaction leads to [Co2Cp2(CO)4]. This dimeric cation-radical contains a metal-metal bond unsupported by bridging ligands. The Co—Co bond happens to be robust and persists in all further transformations of the binuclear cation-radical (Nafady et al. 2006). 

The cyclodimerization depicted in Scheme 7.19 is one of the many examples concerning cation-radicals in the synthesis and reactions of cyclobutanes. An authoritative review by Bauld (2005) considers the problem in detail. Dimerization is attained through the addition of an olefin cation-radical to an olefin in its neutral form one chain ends by a one-electron reduction of the cyclic dimer cation-radical. Unreacted phenylvinyl ether acts as a one-electron donor and the transformation continues. Up to 500 units fall per one cation-radical. The reaction has an order of 0.5 and 1.5 with respect to the initiator and monomer, respectively (Bauld et al. 1987). Such orders are usual for branched-chain reactions. In this case, cyclodimerization involves the following steps ... 

Accordingly the formation and neutralization of dimer cations may be important processes in systems where molecular ions are generated. 

Most of the dimer cations and the bonded dimer cations were produced during the pulse irradiation. This can be explained by assuming the existence of the equilibrium between styrene monomers (St) and neutral dimers of styrene (St2) under the experimental conditions [24]. 

Both the styrene monomer and the neutral dimer can trap a migrating positive hole or positive charge from solvent radical-cations (solventt) or related cationic species, which leads to the formation of radical cations, dimer cations, and bonded dimer cations. 

The behavior of cationic intermediates produced in styrene and a-methyl-styrene in bulk remained a mystery for a long time. The problem was settled by Silverman et al. in 1983 by pulse radiolysis in the nanosecond time-domain [32]. On pulse radiolysis of deaerated bulk styrene, a weak, short-lived absorption due to the bonded dimer cation was observed at 450 nm, in addition to the intense radical band at 310 nm and very short-lived anion band at 400 nm (Fig. 4). (The lifetime of the anion was a few nanoseconds. The shorter lifetime of the radical anion compared with that observed previously may be due to the different purification procedures adopted in this experiment, where no special precautions were taken to remove water). The bonded dimer cation reacted with a neutral monomer with a rate constant of 106 mol-1 dm3s-1. This is in reasonable agreement with the propagation rate constant of radiation-induced cationic polymerization. 

Reduction of acridizinium ion (106) and substituted acridizinium ions in MeCN or DMF gives a dimer ( 80%) [306]. Although not confirmed experimentally, the most likely positions for coupling are indicated. The results of LSV measurements were in agreement with rate-determining dimerization of neutral radicals, and for 106 a lower limit for the rate constant of 10 M s was obtained by CV measurements [306]. The dimer could be quantitatively reoxidized to the substrate cations either electrochemically (at a potential 0.5 V anodic relative to the initial reduction peak) or by action of oxygen [306]. 

P, As) in which the aromatic molecules occur in stacks this situation results in metal-like electrical conductivity (a) (a = 0.12 0.046 ft-1 cm-1 for a polycrystalline pellet). This finding suggested the possibility of analogous behavior in the fluoroaromatic series. Materials containing dimeric cations, however, have not been isolated from reaction mixtures containing excess amounts of the neutral monomers or from controlled reduction of monocation salts. In each case, the cations are monomeric and magnetically independent of one another. 

Despite improvements in experimental techniques, the fundamental processes in radiation-induced cationic polymerizations remain largely hypothetical. Pulse-radiolysis studies - on styrene solutions have led to the conclusion that charge transfer from the solvent produces a styrene cation-radical which then dimerizes to form both associated dimer cation-radicals and bonded dimer cation-radicals. These initial steps are thought to be sev al orders of magnitude faster than the subsequrat prop tion reactions. The presence of trace impurities can dictate the course of polymerization, and rate studies provide circumstantial evidence for the theory that nucleophiles can neutralize the cations in these systems and allow free-radical polymerization to occur alone. 

Reaction (1) describes the electrochemical oxidation of the arene (Ar) at the anode to give a short lived intermediate cation radical which will be stabilized by complexation to a dimer cation radical as described by equ. 2. The conducting crystals finally grow from the supersaturated solution of the dimer-complex on the electrode surface (equ. 3). Following this principle a large number of different structures has been synthesized. A complicating factor arises when the structure allows for the inclusion of solvent. In these cases solvent will replace some of the counterions X and - in order to keep electro-neutrality-a deviation from the stoichiometray (Ar) is observed in other words, the stack of tne arenes contains more neutral than positive centers. 

The synthesis of dirhodium complexes 29 containing the P-S-chelate ligands P,A-[SC2BioHio(CH2) PPh2] was described. A mixed bidentate ferrocenyl ligand Fe(7] -C5Me4P(S)Ph2)( -C5Me4PPh2) (abbreviated P-P=S ) was synthesized and used for complexation with [Rh(/i-Cl)(CO)2]2 The neutral [4]-ferrocenophane RhCl(CO) P-P=S) and the unusual dimeric cationic species [ P,P=S Rh(CO)(jU-Cl)(CO)Rh P,P=S ] were obtained. ... 

The dilithium triimidochalcogenites [Ei2 E(N Bu)3 ]2 form dimeric structures in which two pyramidal [E(N Bu)3] dianions are bridged by four lithium cations to form distorted, hexagonal prisms of the type 10.13. A fascinating feature of these cluster systems is the formation of intensely coloured [deep blue (E = S) or green (E = Se)] solutions upon contact with air. The EPR spectra of these solutions (Section 3.4), indicate that one-electron oxidation of 10.13a or 10.13b is accompanied by removal of one Ei" ion from the cluster to give neutral radicals in which the dianion [E(N Bu)3] and the radical monoanion [E(N Bu)3] are bridged by three ions. ... 

The dimerization of skatole proceeds in an entirely analogous manner, cation (44) now being the electrophilic reagent. This is sufficiently reactive to effect substitution at the a-position of a neutral skatole molecule. Attack by the less hindered side of cation (44) will be favored, leading to the stereochemistry shown in structure (30). The failure of 2-methylindole to dimerize is paralleled by the failure of 2-methylpyrrole dimer to react with a further molecule of 2-methylpyrrole. The main reason is almost certainly again the reduction in the electrophilic character of the immonium carbon by... 

---