Alkyl alcohols, ligand property

Amphoteric molecules of this type, where the acidic and basic sites are relatively close to each other but cannot interact directly, can heterolytically cleave H-X and C-X bonds where X is a halide, alkoxide, amide, alcohol, thiol, trimethylsilyl, or alkyl group.18,18a The ability to effect changes in the reactivity of borollide complexes by adjusting metal oxidation states and ligands allows fine-tuning of catalytic and other properties, which in turn advances the application of these compounds in synthesis. 

Numerous complexes of nickel(II) with phosphorodithioate ligands (also called dithio-phosphates 280 R — R" = 0-alkyl, O-aryl) and dithiophosphinates (280 R = R" = alkyl, aryl) have been reported to date. A few dithiophosphonato complexes (280 R = alkyl, R" = O-alkyl) were also reported. The bis(dialkyldithiophosphato)nickel(II) complexes were obtained as purple diamagnetic compounds by means of the direct synthesis between the appropriate dithioacid (RO)2P(S)SH and a nickel(II) salt, often the acetate hydrate. The dithioacid can be conveniently prepared in situ, by reacting P4Si0 with the appropriate alcohol which sometimes acts by itself as reaction medium. Structural properties of selected nickel(II) complexes are reported in Table 90. 

The electronic properties of both alkyl [5] and aryl alcohols [6] play a clearly definable role in ester formation, with formation constants decreasing with increase in electron withdrawing ability of the ligand. For both types of ligands, the electronic influences are quite small, but the resonance effects found with the aromatic ligands indicate there are 71-electron contributions to the empty d orbitals of vanadate [6], The influences of the electronic properties of ligands on coordination mode and geometry are discussed in detail in Chapter 9. 

Re has recently come to the forefront in liquid phase oxidation catalysis, mainly as a result of the discovery of the catalytic properties of the alkyl compound CH3Re03 [methyltrioxorhenium (MTO)]. MTO forms mono-and diperoxo adducts with H2O2 these species are capable of transferring an oxygen atom to almost any nucleophile, including olefins, allylic alcohols, sulfur compounds, amides, and halide ions (9). Moreover, MTO catalysis can be accelerated by coordination of N ligands such as pyridine (379-381). An additional effect of such bases is that they buffer the strong Lewis acidity of MTO in aqueous solutions and therefore protect epoxides, for example. 

Free alkoxide and aryloxide anions are Bronsted bases with pK values of the corresponding alcohols ranging from 5 to 20 in water. The basicity is highly dependent on the electronic properties of the alkyl or aryl moieties. For example, the pK value of hexafluoro-tert-butanol, (CF3)jMeCOH, is 9.6, which is considerably lower than the pK value of tert-butanol (19.2), but roughly the same as that of phenol (9.9). Such differences in electronic, as well as steric, environments often leads to the different structures and reactivity patterns for compounds containing similar ancillary ligands, but different alkoxides or aryloxides. 

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