Polynuclear compounds, oxidation

Any substance capable of reacting with free radicals to form products that do not reinitiate the oxidation reaction could be considered to function as free-radical traps. The quinones are known to scavenge alkyl free radicals. Many polynuclear hydrocarbons show activity as inhibitors of oxidation and are thought to function by trapping free radicals [25]. Addition of R to quinone or to a polynuclear compound on either the oxygen or nitrogen atoms produces adduct radicals that can undergo subsequent dimerization, disproportionation, or reaction with a second R to form stable products. 

The interactions of dimethyl- and diethylzinc with bulky tris(hydroxyphenyl)methanes, Scheme 86, yielded, depending on the reaction conditions, a variety of alkylzinc alkoxides, featuring two-, three-, and four-coordinate zinc centers. These polynuclear compounds (Figure 63 shows the trinuclear ethylzinc derivative 136) are relatively poor catalysts for the co-polymerization of cyclohexene oxide and carbon dioxide.197... 

Oxidation Processes. Because of the presence of several metal ions, each capable of undergoing an oxidation process, our polynuclear compounds show complex, very interesting oxidation patterns. For tetranuclear complexes like and 4B (Scheme 1 and Table 1) a 3-1 oxidation pattern is expected, i.e., a three-electron process related to the oxidation at the same potential of the three peripheral, equivalent, and noninteracting RuL2(p-2,3-dpp) units followed by a one-electron process related to the oxidation of the central Ru(p-2,3-dpp)3 component. The experimental results, however, support only the first process (Table 2) because the presence of the three contiguous, already oxidized peripheral components displaces the oxidation of the central metal ion at potentials more positive, practically outside the accessible potential window. 

In conclusion, the electrochemical data offer a fingerprint of the chemical and topological structure of the polynuclear compounds. Furthermore, made-to-OTder synthetic control of the number of electrons exchanged at a certain potential can be achieved. The presence of multielectron processes makes such polynuclear complexes very attractive in view of their possible application as multielectron-transfer catalysts. Examination over a more extended oxidation potential window (in a solvent like liquid SOj) should permit one to obtain an even larger variety of oxidation patterns. 

A material suitable for studies of the fundamental reactions of dinuclear complexes has been sought as a companion to Vaska s complex [Ir(CO)(PA3)2X]. This newly described carbonyl compound allows synthesis of several series of polynuclear compounds of iridium in formal oxidation states 4-1, +11, +III and plays the role of a typical dinuclear species. 

The first reaction, hydrogenation of the alkylanthraquinone, is catalyzed by Pd. The second, the epoxidation of propene by the HP generated by air oxidation of the RAHQ, is catalyzed by TS-1. This is possible because TS-1 activity is not affected by the polynuclear compounds forming the redox couple, since they do not enter the zeolite cages due to steric hindrance (the average diameter of the channel system of TS-1 and TS-2, with ME I and MEL type structures, respectively, is 0.55 nm the cross... 

Many transition metals form carbonyl compounds where the oxidation state of the metal is zero. Polynuclear compounds are also known with metal-metal bonds, and sometimes with bridging CO groups. 

A number of examples of aromatic G—H oxidative addition to transition-metal complexes involve polynuclear compounds. In some cases, mononuclear species may be responsible for the initial attack on the G—H bond, but most mechanisms remeun unresolved. Photolysis of Ir(Gp-j )(GO)2 in benzene gives a low yield of HI ... 

The Bronsted acidity of transition metal hydrides is a well known phenomenon which has been extensively studied in both mononuclear and polynuclear compounds. [50] The Bronsted acidity is a likely property of the dinuclear products resulting from H2 additions, especially for cationic species, since they often contain metal centers in relatively high formal oxidation states. In this respect, it... 

Perfluorobenzenium salt can be conveniently prepared from the reaction of 1,4-perfluorocyclohexadiene with SbFs. " In the presence of SbFs, this salt is able to react with three equivalents of pentafluorobenzene to yield perfluoro-1,3,5-triphenylbenzene (eq 3). " When 2,2 -di-Ff-octafluorobiphenyl is used as the substrate, perfluorotriphenylene is obtained in 50% yield (eq 4). Per-fluoronaphthlenenium ion also reacts with polyfluorinated arenes in a similar fashion. " The above reaction offers a facile approach for the preparation of these perfluorinated polynuclear aromatic compounds. Oxidation of perchlorobenzene with SbFs in the presence of pentafluorobenzene leads to the formation of coupling products. ... 

Organometallic Compounds. Ruthenium, predominately in the oxidation states 0 and +2, forms numerous mononuclear and polynuclear organometaUic compounds. A few examples of compounds in both higher and lower oxidation states also exist. The chemistry of polynuclear mthenium complexes is extensive and has been reviewed (53—59). 

Organometallic Compounds. Osmium forms numerous mononuclear and polynuclear organometaUic complexes, primarily iu lower oxidation states. There are many complexes of carbon monoxide, such as [Os(CO)3] [16406-49-8], [Os(CO) H2] [22372-70-9], [Os3(CO)2 H2] [56398-24-4],... 

Organometallic Compounds. The predominant oxidation states of indium in organometalUcs are +1 and +3. Iridium forms mononuclear and polynuclear carbonyl complexes including [IrCl(P(C3H3)3)2(CO)2] [14871-41-1], [Ir2014(00)2] [12703-90-1], [Ir4(CO)22] [18827-81 -1], and the conducting, polymeric [IrCl(CO)3] [32594-40-4]. Isonitnle and carbene complexes are also known. 

Aerobic, Anaerobic, and Combined Systems. The vast majority of in situ bioremediations ate conducted under aerobic conditions because most organics can be degraded aerobically and more rapidly than under anaerobic conditions. Some synthetic chemicals are highly resistant to aerobic biodegradation, such as highly oxidized, chlorinated hydrocarbons and polynuclear aromatic hydrocarbons (PAHs). Examples of such compounds are tetrachloroethylene, TCE, benzo(a)pyrene [50-32-8] PCBs, and pesticides. 

The +3 oxidation state is exhibited by bismuth in the vast majority of its compounds. A few inorganic and a variety of organic compounds, however, contain the element in the +5 state. Other rarer oxidation states reported for bismuth include +2, +1, and —3. Bismuth also forms polynuclear ionic species with oxidation states that ate usually fractional and range from —1 to +1. 

The oxide (p. 1209), chalcogenides (p. 1210) and halides (p. 1211) have already been described. Of them, the only ionic compound is HgF2 but other compounds in which there is appreciable charge separation are the hydrated salts of strong oxoacids, e.g. the nitrate, perchlorate, and sulfate. In aqueous solution such salts are extensively hydrolysed (HgO is only very weakly basic) and they require acidification to prevent the formation of polynuclear hydroxo-bridged species or the precipitation of basic salts such as Hg(OH)(N03) which contains infinite zigzag chains ... 

This is only the beginning of a process which ultimately results in the formation of solid state hydroxides or oxides. Actually, the solution species present in neutral or alkaline solutions of transition-metal ions are relatively poorly characterized. The formation of numerous hydroxy- and oxy-bridged polynuclear species makes their investigation very difficult. However, it is clear that there is a near-continuous transition from mononuclear solution species, through polynuclear solution species to colloidal and solid state materials. By the way, the first example of a purely inorganic compound to exhibit chirality was the olated species 9.11. 

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