Salts with donor-acceptor

In this paper, illustrations of some peculiar aspects of the spectroscopy of CT crystals have been given with reference to simple dimeric models. Quantitative applications of such models are only appropriate for systems with localized electron states, such as insulating ion radical salts and donor-acceptor crystals. Analysis of spectroscopic data pertaining to these systems allows an accurate determination of some fundamental interaction parameters. 

Towards a simple Lewis base, for example the proton, phosphine is a poorer electron donor than ammonia, the larger phosphorus atom being less able to form a stable covalent bond with the acceptor atom or molecule. Phosphine is, therefore, a much weaker base than ammonia and there is no series of phosphonium salts corresponding to the ammonium salts but phosphonium halides. PH4X (X = Cl, Br, I) can be prepared by the direct combination of phosphine with the appropriate hydrogen halide. These compounds are much more easily dissociated than ammonium halides, the most stable being the iodide, but even this dissociates at 333 K PH4I = PH3 -t- HI... 

Single-Stack Donor. Ion-radical salts can also be formed from electron donors such as tetrathiafulvalene (TTE) or TMPD (N,N,N N-tetramethyl- phenylene diamine) with inorganic acceptors such as halogens. The resulting stmcture of compounds such as TTE(A)... 

Xenon tetrafluoride is a much weaker fluoride ion donor and only forms stable complex salts with the strongest fluoride ion acceptors, eg,... 

Eullerene-based donor-acceptor complexes and ion-radical salts with tetrathia-fulvalenes, metalloporphyrins, and cyclic amines as donors 99UK23. 

Elegant evidence that free electrons can be transferred from an organic donor to a diazonium ion was found by Becker et al. (1975, 1977a see also Becker, 1978). These authors observed that diazonium salts quench the fluorescence of pyrene (and other arenes) at a rate k = 2.5 x 1010 m-1 s-1. The pyrene radical cation and the aryldiazenyl radical would appear to be the likely products of electron transfer. However, pyrene is a weak nucleophile the concentration of its covalent product with the diazonium ion is estimated to lie below 0.019o at equilibrium. If electron transfer were to proceed via this proposed intermediate present in such a low concentration, then the measured rate constant could not be so large. Nevertheless, dynamic fluorescence quenching in the excited state of the electron donor-acceptor complex preferred at equilibrium would fit the facts. Evidence supporting a diffusion-controlled electron transfer (k = 1.8 x 1010 to 2.5 X 1010 s-1) was provided by pulse radiolysis. 

Interestingly, the sulfanes H2S are both proton acceptors and donors. In the first case sulfonium ions H3S are formed, in the second case hydrogen polysulfide anions HS are the result. While the latter have never been isolated in salts, several salts with sulfonium cations derived from the sulfanes with n = 1, 2, and 4 have been published. However, none of these salts has been structurally characterized by a diffraction technique. Therefore, the structures of the HsSn cations and HS anions are known from theoretical calculations only. 

By contrast, the acidity of the metal salts used in these cements has a less clear origin. All of the salts dissolve quite readily in water and give rise to free ions, of which the metal ions are acids in the Lewis sense. These ions form donor-acceptor complexes with a variety of other molecules, including water, so that the species which exists in aqueous solution is a well-characterized hexaquo ion, either Mg(OH2)g or Zn(OH2)g. However, zinc chloride at least has a ternary rather than binary relationship with water and quite readily forms mixtures of Zn0-HCl-H20 (Sorrell, 1977). Hence it is quite probable that in aqueous solution the metal salts involved in forming oxysalt cements dissolve to generate a certain amount of mineral acid, which means that these aqueous solutions function as acids in the Bronsted-Lowry sense. 

In a similar way, the formation of halide complexes with other jt-acceptors in Fig. 3 are revealed by the appearance of new absorption bands in the electronic spectra to reflect the yellow to red colorations of the mixtures. The spectral data thus indicate that halide salts form well-defined electron donor/acceptor complexes with organic jt-acceptors, as typified by Eq. 2 ... 

Additional combinatorial variation sites allow the heterocyclic self-assembly units. Thus, it has been shown that heterocycles 11 and 14-17 can serve as A-analogous donor-acceptor ligands self-assembling with the T-analogous acceptor-donor ligands isoquinolone 12 and 7-azaindole 18 (Scheme 30) [92]. All combinations form the heterobidentate ligands exclusively upon simple mixing in the presence of a transition metal salt (proven by X-ray, NMR). 

Bidentate ligand transfer also occurs between di- and mononuclear complexes via donor-acceptor intermediates. Reactions of the cationic complex [Au2(/i-dppm)2]2+ with [AuX2] (X = C1, Br) salts lead to tri- or dinuclear derivatives depending on the molar ratio (1 1 or 1 2) (Scheme 36).2645... 

The quinoxaline 100, with self-contained donor-acceptor properties, has potential in optoelectronic <06JACS10992>. Electroactive dendrimeric bis-quatemary salts have been prepared by direct quatemisation of pyrazine using dendrimeric benzyl bromides <06TL4711>. 

The wide diversity of the foregoing reactions with electron-poor acceptors (which include cationic and neutral electrophiles as well as strong and weak one-electron oxidants) points to enol silyl ethers as electron donors in general. Indeed, we will show how the electron-transfer paradigm can be applied to the various reactions of enol silyl ethers listed above in which the donor/acceptor pair leads to a variety of reactive intermediates including cation radicals, anion radicals, radicals, etc. that govern the product distribution. Moreover, the modulation of ion-pair (cation radical and anion radical) dynamics by solvent and added salt allows control of the competing pathways to achieve the desired selectivity (see below). 

The BFT, PFg" and SbCl salts of cation radicals are readily prepared by oxidation of organic donors with the corresponding NO+ salts in a relatively nonpolar solvent such as dichloromethane. For example, a solution of the hydroquinone ether MA in anhydrous (deaerated) dichloromethane turns purple upon the addition of crystalline NO+BFT at low temperature ( 50°C).173 The coloration is due to formation of the donor/acceptor complex [MA, NO+] (equation 34). 

Importantly, the purple color is completely restored upon recooling the solution. Thus, the thermal electron-transfer equilibrium depicted in equation (35) is completely reversible over multiple cooling/warming cycles. On the other hand, the isolation of the pure cation-radical salt in quantitative yield is readily achieved by in vacuo removal of the gaseous nitric oxide and precipitation of the MA+ BF4 salt with diethyl ether. This methodology has been employed for the isolation of a variety of organic cation radicals from aromatic, olefinic and heteroatom-centered donors.174 However, competitive donor/acceptor complexation complicates the isolation process in some cases.175... 

Reactions of highly electron-rich organometalate salts (organocuprates, orga-noborates, Grignard reagents, etc.) and metal hydrides (trialkyltin hydride, triethylsilane, borohydrides, etc.) with cyano-substituted olefins, enones, ketones, carbocations, pyridinium cations, etc. are conventionally formulated as nucleophilic addition reactions. We illustrate the utility of donor/acceptor association and electron-transfer below. 

Figure 2.10. Example of a donor-acceptor fluoroionophore in which the electron-withdrawing character of the acceptor (carbonyl group of the coumarine) is cation-controlled. Absorption and fluorescence spectra of ClS3-crown(Oj) and its complexes with perchlorate salts in acetonitrile. (Adapted from Ref. SO.)...

Organic ion-radicals exist as salts with counterions. As seen in the preceding chapters, neutral molecules with strong acceptor/donor properties form rather stable ion-radical salts. Under certain conditions (see later), components of an ion-radical salt pack up in a crystal lattice in a special manner. Different ion-radical parts (cations and anions separately) line up one over the other in the form of endlessly long, practically linear one-dimensional (1-D), piles-chains. These piles-chains form a crystal. 

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