Donor systems

Williams R M, Zwier J M and Verhoeven J W 1995 Photoinduced intramolecular electron transfer in a bridged Cgg (acceptor)-aniline (donor) system. Photophysical properties of the first active fullerene diad J. Am. Chem. See. 117 4093-9... 

The high acidity of superacids makes them extremely effective pro-tonating agents and catalysts. They also can activate a wide variety of extremely weakly basic compounds (nucleophiles) that previously could not be considered reactive in any practical way. Superacids such as fluoroantimonic or magic acid are capable of protonating not only TT-donor systems (aromatics, olefins, and acetylenes) but also what are called (T-donors, such as saturated hydrocarbons, including methane (CH4), the simplest parent saturated hydrocarbon. 

Alkali metal derivatives of 2-(trimethylsilyl)aminopyridines can be further derivatized by insertion of 1,3-dicyclohexylcarbodiimide. Functionalized guani-dinates are formed in this reaction via a 1,3-silyl shift. Scheme 170 illustrates the reaction sequence as well as the preparation of an aluminum complex of the modified ligand, which exhibits pseudo jS-diketiminate binding of the metal center, thus exemplifying the coordinative versatility of this new multi-N-donor system. ... 

A sulfur donor system in NR (DTDM, 1.5 CBS, 1.5 TMTD, 0.5) yields a stable network stmcture with a contribution of 80% mono- and disulfidic cross-links at the optimum cure at 143°C or 183°C curing [13]. 

As operating requirements for NBR become more stringent, improved aging and set properties become important. In order to address those requirements, formulations with sulfur donor systems (as a partial or total replacement of rhombic sulfur) are proposed. They are summarized in... 

Curatives (sulfur, accelerators, peroxides, sulfur-donor systems, etc.)... 

The reactivity of the initial halogen bonded complex has also received considerable attention. Husebye and coworkers have suggested a process whereby the dihalogen bond is cleaved to give a key cation intermediate and a halide anion (Eq. 4) [182]. This mechanism is consistent with that often proposed for nitrogen electron donor systems. 

The monovalent Co chemistry of amines is sparse. No structurally characterized example of low-valent Co complexed exclusively to amines is known. At low potentials and in non-aqueous solutions, Co1 amines have been identified electrochemically, but usually in the presence of co-ligands that stabilize the reduced complex. At low potential, the putative monovalent [Co(cyclam)]+ (cyclam = 1,4,8,11-tetraazacyclotetradecane) in NaOH solution catalyzes the reduction of both nitrate and nitrite to give mixtures of hydroxylamine and ammonia.100 Mixed N-donor systems bearing 7r-acceptor imine ligands in addition to amines are well known, but these examples are discussed separately in Section 6.1.2.1.3. 

Crown ethers may also be used to functionalize salicylideneimine complexes.1179 The crown ether-functionalized tetra-t-butyl-salicylideneimines are useful in studies on the selective com-plexation and transport of cations (e.g., Na+ and K+). 4,5-Bis(3,5-di-/-butylsalicylideneimino) benzo-18-crown-6 coordinates Ni11 via the Schiff base N202 donor system and K+ or Cs+ at the crown site.1180 These Ni-alkali metal complexes are able to transport amino acids from acidic aqueous solution to pure water. 

In aqueous solution, depending on pH, Q-aminohydroxamic acids having no competing donor system in a side chain (like glycinehydroxamic acid or alaninehydroxamic acid, H2L) form different complex species with Ni11 ions. The major complexes are the NiL, and [NiL( 11L)]... 

In addition to the backbone donor system the amino acid residues can contain a variety of donor centers (679E). The most important of these are the imidazole N atoms of His, the S atom of Cys, and to some extent the, 3-carboxylate O atom of Asp. Other side chain donors like the O atoms of Ser and phenolic O atoms of Tyr or amino N atoms of Lys are of minor importance for coordination of Ni11 ions. Also, thioether S-donors of Met play only a minor role in the interactions with Ni11 ions.1730... 

Determination of the level of total sulfur in a rubber can give information on the type of cure system used, for example, elemental sulfur plus accelerator or sulfur donor system, etc. The ISO 6528-1 1992 method — Rubber — Determination of total sulfur content — Part 1 Oxygen combustion flask method is often employed. The principle of the method is oxidation by ignition in an atmosphere of oxygen in the presence of hydrogen peroxide, conversion of sulfur to sulfuric acid and determination of the sulfate by titration with barium perchlorate. The method is... 

NR can be crosslinked by the use of sulphur, sulphur donor systems, peroxides, isocyanate cures and radiation, although the use of sulphur is the most common method. 

Polybutadiene based compounds can be cured by sulphur, sulphur donor systems and peroxides. Less sulphur and a higher level of accelerators are required when compared to NR. The cure of polybutadiene by peroxides is highly efficient in that a large number of crosslinks are produced per free radical, the resultant highly crosslinked rubber exhibiting high resilience this factor is utilised in the manufacture of superballs . 

Nitrile rubber can be cured by sulphur, sulphur donor systems and peroxides. However, the solubility of sulphur in nitrile rubber is much lower than in NR, and a magnesium carbonate coated grade (sulphur MC) is normally used this is added as early in the mixing cycle as possible. Less sulphur and more accelerator than is commonly used for curing natural rubber is required. A cadmium oxide/magnesium oxide cure system gives improved heat resistance, but the use of cadmium, a heavy metal, will increasingly be restricted. 

All N-donor systems. The N3-donor macrocycle (23) has ten atoms in its macrocyclic ring and is too small to completely encircle a metal ion... 

Other donor systems. Although not as common as all-nitrogen donor rings, macrocycles incorporating sulfur donor atoms have been widely reported. Structures (42)-(45) illustrate four representative examples of this type. 

Studies involving 2,6-diacetylpyridine derivatives. Detailed studies by Nelson and coworkers supported by X-ray structural work by Drew and coworkers have elucidated many aspects of the template synthesis of metal complexes of a Schiff-base ligand series which includes the N4-donor system (83) and the N5-systems (84)-(86) (Nelson, 1980). Since... 

Sulfur-containing ligands. Macrocyclic effects have also been documented in mixed donor systems. The Cu(ii) complex of the 14-mem-bered (cis) N2S2-donor analogue of cyclam exhibits a substantial macrocyclic effect with a log K difference of 4.6 relative to the corresponding open-chain species (Figure 6.2) (Micheloni, Paoletti, Siegfried-Hertli Kaden, 1985). The effect in this case is mainly due to a favourable... 

Crown polyethers. Macrocyclic effects involving complexes of crown polyethers have been well-recognized. As for the all-sulfur donor systems, the study of the macrocyclic effect tends to be more straightforward for complexes of cyclic polyethers especially when simple alkali and alkaline earth cations are involved (Haymore, Lamb, Izatt Christensen, 1982). The advantages include (i) the cyclic polyethers are weak, uncharged bases and metal complexation is not pH dependent (ii) these ligands readily form complexes with the alkali and alkaline earth cations... 

The dissociation kinetics of the nickel chloride complex of the 15-mem-bered, S2N2-macrocycle of type (279) has also been investigated in the presence of hydrochloric acid (Lindoy Smith, 1981). This complex has a similar trans-octahedral structure to that of the 02N2-donor systems just discussed. For the sulfur-containing complex, two consecutive (acid-independent) first-order steps were observed, with the second being slower than the first. The data are in accordance with the scheme ... 

In this chapter, representatives of the natural macrocycles are presented and selected model studies are also described. Emphasis will be given in the following discussion to a few of the better understood areas involving natural macrocyclic systems. Initially, the nature and function of the cyclic antibiotic category of macrocycles are discussed. Subsequently, aspects of the roles of the natural N4-donor systems are presented. 

Compared with the conducting anion radical salts of metal complexes, the number of molecular conductors based on cationic metal complexes is still limited. Donor type complexes M(dddt)2 (M = Ni, Pd, Pt Fig. 1) are the most studied system. The M(dddt)2 molecule is a metal complex analogue of the organic donor BEDTTTF. Formally, the central C=C bond of BEDT-TTF is substituted by a metal ion. The HOMO and LUMO of the M(dddt)2 molecule are very similar in orbital character to those of the M(dmit)2 molecule. In addition, the HOMO of the M(dddt)2 molecule is also very similar to that of BEDT-TTF. More than ten cation radical salts of M(dddt)2 with a cation (monovalent) anion ratio of 2 1 or 3 2 are reported [7]. A few of them exhibit metallic behavior down to low temperatures. The HOMO-LUMO band inversion can also occur in the donor system depending on the degree of dimerization. In contrast to the acceptor system, however, the HOMO-LUMO band inversion in the donor system leads a LUMO band with the one-dimensional character to the conduction band. 

We proceed now to the detailed treatment of the theory of acceptor and donor systems, summarizing experiments as necessary to make contact with the theory and as appropriate to construct complete pictures of the defects. We collect and compare the results of many theoretical treatments, not all of which are in agreement. 

These differences have been attributed to the "benzyne group behaving as a four- or two-electron donor system. The fluxionality in these molecules has been investigated by NMR studies over a temperature range. These have established that the benzyne ring rotates about the osmium triangle (see Section IH,D). 

In a search of jr-donor systems for the preparation of compounds having a metallic conductivity, the bis-thioxanthene cumulene 56 was obtained. It was oxidized by cone. H2SO4 to the acetylenic dication 57 rather than undergoing the expected protonation of the multiple bonds (equation 22)36. 

The choice of the R group leads to materials exhibiting properties which can find applications in diverse fields long alkyl chains help solubilization in organic solvents, whereas triethylene glycols have been used as spacers to attach water-solubilizing moieties, or even electron-donor systems such as ferrocene, which may find applications in photovoltaics, or as bio- and electrochemical sensors [33]. 

Further investigations on the acceptor-donor system arene-amine have been made by Sugimoto et al. They have reported that direct irradiation of 9-(cu-anilinoalkyl)phenanthrenes give spirocyclic pyrroline derivatives, invoked by... 

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