Complex halides Subject

In comparison with classic Lewis acids derived from main group halides (e.g., B, Al, Sn), f-elements, and early transition metal halides, late transition metal Lewis acids often are more inert to ubiquitous impurities such as water, offer higher stability, tunable properties by ligand modification, and a well-defined structure and coordination chemistry, thus allowing detailed studies of reaction mechanisms, and a rational basis for catalyst optimization. Among this new class of late transition metal Lewis acids, ruthenium complexes - the subject of this chapter - display remarkable properties... 

Perfluoroalkyl or -aryl halides undergo oxidative addition with metal vapors to form nonsolvated fluonnated organometallic halides and this topic has been die subject of a review [289] Pentafluorophenyl halides react with Rieke nickel, cobalt, and iron to give bispentafluorophenylmetal compounds, which can be isolated in good yields as liquid complexes [290] Rieke nickel can also be used to promote the reaction of pentafluorophenyl halides with acid halides [297] (equation 193)... 

Lower oxidation states are rather sparsely represented for Zr and Hf. Even for Ti they are readily oxidized to +4 but they are undoubtedly well defined and, whatever arguments may be advanced against applying the description to Sc, there is no doubt that Ti is a transition metal . In aqueous solution Ti can be prepared by reduction of Ti, either with Zn and dilute acid or electrolytically, and it exists in dilute acids as the violet, octahedral [Ti(H20)6] + ion (p. 970). Although this is subject to a certain amount of hydrolysis, normal salts such as halides and sulfates can be separated. Zr and are known mainly as the trihalides or their derivatives and have no aqueous chemistry since they reduce water. Table 21.2 (p. 960) gives the oxidation states and stereochemistries found in the complexes of Ti, Zr and Hf along with illustrative examples. (See also pp. 1281-2.)... 

Before getting to the main subject of this chapter—the reactions of alkenes— let s take a brief look at how alkenes are prepared. The subject is a bit complex, though, so we ll return in Chapter 11 for a more detailed study. For the present, it s enough to realize that alkenes are readily available from simple precursors— usually alcohols in biological systems and either alcohols or alkyl halides in the laboratory. 

Dithiocarbamate complexes of copper have been sythesized at a high rate. Reports of new complexes include the morpholine-4- (44), thio-morpholine, AT-methylpiperazine-4-, and piperidine- (291) dithiocarba-mates. Novel, polymeric complexes of the type Cu(pipdtc)2 (CuBr) in = 4, or 6) and Cu(pipdtc)2 (CuCl)4 have been prepared by reactions of[Cu(pipdtc)2] with the respective copper halide in CHCla-EtOH (418). The crystal structures of the polymers are known to consist of sheets of individual [Cu(pipdtc)2] molecules linked to polymeric CuBr chains via Cu-S bonds. A series of copper(I) dtc complexes have been the subject of a Cu and Cu NQR-spectral study (440). 

Asymmetric Allylation. One of the recent new developments on this subject is the asymmetric allylation reaction. It was found that native and trimethylated cyclodextrins (CDs) promote enantiose-lective allylation of 2-cyclohexenone and aldehydes using Zn dust and alkyl halides in 5 1 H2O-THF. Moderately optically active products with ee up to 50% were obtained.188 The results can be rationalized in terms of the formation of inclusion complexes between the substrates and the CDs and of their interaction with the surface of the metal. 

Considerable progress has been made on C02 fixation in photochemical reduction. The use of Re complexes as photosensitizers gave the best results the reduction product was CO or HCOOH. The catalysts developed in this field are applicable to both the electrochemical and photoelectrochemical reduction of C02. Basic concepts developed in the gas phase reduction of C02 with H2 can also be used. Furthermore, electrochemical carboxyla-tion of organic molecules such as olefins, aromatic hydrocarbons, and alkyl halides in the presence of C02 is also an attractive research subject. Photoinduced and thermal insertion of C02 using organometallic complexes has also been extensively examined in recent years. 

The distinction between systems in which a complex of initiator and co-initiator reacts with the olefin (scheme (1)) and those in which the coinitiator reacts with a complex of monomer and metal halide (scheme (2)) was made very early in the history of this subject [9]. 

Torsional barriers for trimethylphosphine derivatives (63) have been obtained from Raman spectra.56 Vibrational spectra for the uranyl nitrate complex of (63a) have been published.66 Complexes of triarylphosphine derivatives (64) with iodine,67 and of (64b) with metal halides,68 have been the subject of thermodynamic67 and spectroscopic67 68 study. 

Organic electroreductive coupling reactions using transition-metal complexes as catalysts have been widely investigated. Reviews on the subject have been published [89, 90]. The method involving the most common transition-metal complexes (nickel, cobalt, palladium) appears to be a useful tool to synthetize heterocycles from organic halides via radical intermediates. Nickel catalyst precursors are nickel(II) salts that are cathodically reduced either to nickel(I) or to nickel(O) and cobalt catalyst... 

Recently, chloro-, bromo-, and iodoben-zenes have been subjected to electroreduction using Ni(0) complex mediators to yield biphenyl. NiCl2L2 and NiBr2L2 [L= P(Ph)3, (Ph)2PCH2CH2P(Ph)2] have been used as catalysts [259-265]. Pro-tic media such as alcohols, that is, methanol, ethanol or alcohol-water mixtures are found to be suitable solvents for achieving the electrosynthesis of biaryls from aryl halides according to a procedure that involves a catalytic process by nickel-2,2 -bipyridine complexes [266]. Electrochemical cross-coupling between... 

In addition to phosphate and halide anion binding, carboxylate chelation by sapphyrin macrocycles has been the subject of recent investigation. To date, two crystal structures have been solved. A 2 1 complex formed between diprotonated sapphyrin 3 and trifluoroacetic acid shows that the oxyanions are chelated above and below the sapphyrin plane (Figure 10). ° Greater complexity of organi-... 

The formation of molecular complexes between aluminum trihalides and pyridine or alkylpyridines has been the subject of systematic studies.35,36 Calorimetric data yield bond dissociation energies D(X3Al—py) of 323, 308 and 264 kJ mol-1 for X = C1, Br and I, respectively, and this same order is found for alkylpyridine adducts, although the A1—N bond is weakened in the case of lutidine by the effects of steric hindrance. For gallium halides the values of D(X3M—py) are smaller 248, 237 and 195 kJ mol-1 for chloride, bromide and iodide adducts, respectively. 

Many complexes contain the [Cr2X9]3 ion.1073-1076 This unit consists of two distorted octahedra which share a face there are three bridging and three terminal halides (226). The distortion of the octahedron is such that the two metal ions are displaced from each other. The magnetic properties of these complexes have been the subject of extensive study summarized in Table 93. 

Complexes of platinum having cyanide and non-stoichiometric quantities of halide ligand have been prepared. A review by Miller gives a broad coverage of this subject,285 and a book series Extended Linear Chain Compounds has a number of articles which are of direct interest and relevance to workers in this field.286 This section will only briefly cover the topic, and will emphasize the more recent work. 

The cis/trans isomerization of platinum(II) complexes is a subject which will be discussed in some detail when the halide (Group VII) complexes are covered. Nevertheless the importance of reductive elimination reactions of platinum(II) alkyl and aryl complexes makes it imperative that this reaction be discussed here for alkyl and aryl platinum(II) compounds. 

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