Oxophilic character

The catalytic cycle was devised relying on a ligand exchange between the iron complex and a chlorosilane, regenerating the iron (111) halide due to the more oxophilic character of the silicon (Scheme 34). 

The oxophilic character of the early transition metals may provide such stabilization in the form of dihapto binding (I). This unusual donor behavior was first... 

Another aspect of tin as a constituent of electrode material is shown by tin(IV)TPP complexes incorporated into PVC membrane electrodes. These increase the selectivity to salicylate over anions such as Cl-, Br- I-, I()4, Cl()4, citrate, lactate and acetate. The specificity is attributed to the oxophilic character of the Sn ion in TPP at the axial coordination sites. Indeed, carboxyl groups incorporated into the membrane polymer compete for these binding sites. The complete complex structure is important. Substitution of TPP with octaethylporphirine results in loss of salicylate selectivity231. Preparation and analytical evaluation of a lead-selective membrane electrode, containing lead diethyldithiocarbamate chelate, has also been described232. 

As a prelude to this section, intrinsic properties of the Ln-OR moiety will be passed in review. Alkoxide ligands are put into the category of hard ligands according the Pearson terminology and therefore optimally match the electropositive requirements of the hard lanthanide cations. The natural abundance of the lanthanide elements is the force behind the hard-soft relationship. The strongly electropositive and oxophilic character is expressed in high forma-... 

The oxophilic character of lanthanide is evident from the characterization of the complex (MeCp)3Yb4(/i-Cl)(,(/i3-Cl)(/i4-0)(THF)3 by the reaction of YbCb and YbOCl with MeCp in THF. The crystal structure of the complex shows an oxygen-centred tetrahedral arrangement of Yb atoms with p.-C bridging each edge and p -C 1 over the triangular face formed by the (MeCp)Yb units [137]. Yb-0(/x4-oxide) bond distance range is 2.13(1)— 2.29 (1) A. 

Formyl intermediates have been postulated as steps in the Fischer-Tropsch reduction of CO to hydrocarbons (Section 22-5). Coordinated CO can insert into the M—H bonds of metal hydrides, particularly when the reaction is driven by the oxophilic character of early transition metals. Such reactions may lead to C—C bond formation. Some examples are ... 

Niobium (formerly called columbium) and tantalum are Transition Metals having a considerable affinity for oxygen donor groups they are thus called oxophilic see Oxophilic Character). They occur as mixed-metal oxides such as columbites (Fe/Mn)(Nb/Ta)206 and pyrochlore NaCaNb206p. Their discovery in minerals extends back to the beginning of the nineteenth century, when they were believed to be identical and called tantalum. Rose showed that at least two different elements were involved in the minerals, and named the second one niobium. Their separation was resolved around 1866, especially by Marignac. These metals often display similar chemical behavior as a result of nearly identical atomic radii (1.47 A) due to the lanthanide contraction see Periodic Table Trends in the Properties of the Elements)... 

The advantage of these systems over traditional catalytic ketonization routes lies in the selectivities and conversions the two-step reaction path is very convenient from a preparative point of view. Work is continuing to control these systems better, and to investigate their mechanisms and their synthetic utility. Novel and highly efficient ways of isomerizing epoxyalcohols by careful control of the oxophilic character of the active species are sought. 

Replacing one CO group in Cr(CO)3Cp with diazomethane costs 3.6 kcal/mol in terms of free energy. In the presence of free CO, the formation of a ketene complex was observed experimentally. The structures and harmonic vibrational frequencies of the possible ketene tautomers were also computed (see Figure 10.15). Since ketenes possess C=C as well as C=0 Jt-bonds, both the r 2-(C,C) and the r 2-(C,0) coordination modes had to be considered. As chromium is of rather oxophilic character, it is not surprising that the r 2-(C,0) tautomer (Figure 10.15c) was lower in energy by 1.3 kcal/mol. 

CO has often been used as a convenient probe for investigating the electronic structures of metal oxide and zeolite supported metals. However, CO is not simply an innocent probe. For example, oxidative fragmentation occurs when CO reacts with Rh clusters on metal oxides or in zeolites. [171, 172] The breakup by CO of small clusters of metals that have some oxophilic character is consistent with the tendency of CO to fragment carbonyl clusters of these metals in solution. [31]... 

McMuriy Reaction A coupling reaction of great interest is McMurry s titanium-mediated synthesis of alkenes from two ketones (Eq. 14.69). This involves a reduced form of titanium, perhaps Ti(0), which may give the sequence of reactions shown in Eq. 14.70. These ideas are supported by the fact that 1,2-diols are also reduced to the alkene. Whatever the mechanism, the reaction shows the strongly oxophilic character of this early metal. 

The size control is essential because by decreasing the particle diameter, a higher percentage of surface atoms are exposed, and thus Pt utilization increases. In this sense, there is huge debate about particle size and catal 4ic activity (Maillard et al, 2009), in which maximum catal3hic activities are found for diameters in the range of 2-5 nm (Sun et al., 2007). A decrease in the particle size leads to an increment of low-coordinated atoms as well as that of the oxophilic character of... 

The addition of Ni as the second or third element is claimed to increase the activity of Ft electrocatalysts. The main advantage of the introduction of this metal is the reduction of the ethanol oxidation potential, coupled with the rise in current density. A literature search reveals that Sn and Ni can introduce electronic modifications into Ft [50,51], in other words, these metals decrease the energy of the chemisorption of ethanol and its oxidation intermediates, such as CO. The participation of the Sn site in a bifunctional mechanism is also claimed, in order to explain the enhanced activity of the Ft-Sn/C catalyst [50,52]. The effect that Ni introduction has on Sn-sites must also be considered. The addition of a third metal boosts the oxophilic character of the surface, thus raising the... 

In summary, the relatively electropositive, highly Lewis acidic early transition metals form O-bound enolates, as expected according to their oxophilic character. When switching to the so-called late transition metals, located to the right of group 5 in the periodic table, the clear preference for O-bound enolate structures vanishes, and the C-bound tautomers become involved but also the i/ -bound oxallyl type. A general rule on which tautomer is favored by which metal cannot be deduced, because the type of enolate structure is mainly determined by the oxidation state, the coordination number, and in particular the individual ligands at the transition metal. 

Despite the key role that has been attributed to copper enolates as postulated intermediates in additions to enones [78], structural information is very rare for enolates of copper as well as the higher elements in group 11. A recent crystal structure of a copper(I) ester enolate 46, which functions as a relevant intermediate in catalytic enolate arylation reactions, features a cationic Cu(I) center coordinated to two phenanthroline ligands and a free unligated enolate anion (Figure 3.13) [79]. A gold(I) enolate of acetophenone is a C-bound tautomer, as expected by the low oxophilic character of the noble metal and confirmed by a crystal structure analysis [80]. 

The strong oxophilic character of lanthanides has been discussed in the previous sections, and the frequent occurrence of oxoderivatives is accidental as in the tetranuclear 0x0 species [Li(THF)4]2 [(MeC5H4)]NdCl([i2-Cl)NdCl( i2.cl)NdCl2[(MeC5H4)]2(n -0) ... 

The compartmental ligands derived from 3-methoxy-salicylaldehyde and a diamine (Scheme 10c) have been specially designed to obtain binuclear 3d-4f complexes. Their exceptional versatility as building blocks is due to the presence of two metal ions that differ drastically in their chemical behaviour the rare-earth cations are hard acids, with a strong oxophilic character and high... 

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