Cations metallic

Supports are often prepared first and the catalyst and promoter components added later (60). Metal oxide supports are usually prepared by precipitation from aqueous solutions. Nitrates are commonly used anions alkaUes and ammonium are commonly used cations. Metal oxide supports, eg, sihca and alumina, are prepared in the form of hydrogels. Mixed oxides such as siUca—alumina are made by cogelation. Carefiil control of conditions such as pH is important to give uniform products. 

In Pauli s model, we still envisage a core of rigid cations (metal atoms that have lost electrons), surrounded by a sea of electrons. The electrons are treated as non-interacting particles just as in the Drude model, but the analysis is done according to the rules of quantum mechanics. 

John D. Corbett once said There are many wonders still to be discovered [4]. This certainly holds generally for all the different areas and niches of early transition cluster chemistry and especially for the mixed-hahde systems. The results reported above so far cover a very Hmited selection of only chloride/iodide systems and basically boron as the interstitial. Because of the very sensitive dependence of the stable stracture built in the soHd-state reaction type on parameters like optimal bonding electron counts, number of cations present, size and type of cations (bonding requirements for the cations), metal/halide ratio, and type of halide, a much larger mixed-hahde cluster chemistry can be expected. Further developments, also in mixed-hahde systems, can be expected by using solution chemistry of molecular clusters, excised from solid-state precursors. 

Scheme 12. Reversible formation of cationic metal-acetylide dendrimers 39 and 40...

FIGURE 14.1 Percentages of TLC/HPTLC publications on metal cations, metal complexes, and anions that appeared during the period 1980 to 2004. 

These examples serve to illustrate several general points about use of chiral catalysts for D-A reactions. A cationic metal center is present in nearly all of the catalysts developed to date and has several functions. It is the anchor for the chiral ligands and also serves as a Lewis acid with respect to the dienophile. The chiral ligands establish the facial selectivity of the complexed dienophile. There are several indications of the importance of the anions to catalytic activity. Anions, in general,... 

Interaction of anions with cationic metal in the solution forms the compound. The anionic species, depending upon the reagent deployed, can be Cl, S04, P04, S2, OH or CO2. ... 

The intercalated catalysts can often be regarded as biomimetic oxidation catalysts. The intercalation of cationic metal complexes in the interlamellar space of clays often leads to increased catalytic activity and selectivity, due to the limited orientations by which the molecules are forced to accommodate themselves between sheets. The clays have electrostatic fields in their interlayer therefore, the intercalated metal complexes are more positively charged. Such complexes may show different behavior. For example, cationic Rh complexes catalyze the regioselective hydrogenation of carbonyl groups, whereas neutral complexes are not active.149 Cis-Alkenes are hydrogenated preferentially on bipyridyl-Pd(II) acetate intercalated in montmorillonite.150 The same catalyst was also used for the reduction of nitrobenzene.151... 

The use of weakly coordinating and fluorinated anions such as B(C6H4F-4)4, B(C6F5)4, and MeB(C6F5)3 further enhanced the activities of Group 4 cationic complexes for the polymerization of olefins and thereby their activity reached a level comparable to those of MAO-activated metallocene catalysts. Base-free cationic metal alkyl complexes and catalytic studies on them had mainly been concerned with cationic methyl complexes, [Cp2M-Me] +. However, their thermal instability restricts the use of such systems at technically useful temperatures. The corresponding thermally more stable benzyl complexes,... 

Molecular Conductors Based on Cationic Metal Complexes. 40... 

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. 

The reactions covered in Scheme 2 are initiated by protonation but a hydride could form on the metal as intermediate. In some instances, cationic metal hydrides have been shown to be actually involved. See, for example, the addition of [HNi (POEt)3 4+] to butadiene (54) or of [HNi(Ph3P)3(7r-C3H5)] to olefins (10c, Vol. II, p. 25). Thus the reaction of olefins or dienes with acids in the presence of zero-valent nickel may be considered proton-promoted as well as hydride-promoted. 

Nesmeyanov has provided interesting examples of apparent intramolecular nucleophilic attack by amine on carbonyl ligands (37). Angelici (38,39) has demonstrated that amine attack on cationic metal carbonyl complexes is a general reaction resulting in the formation of carbamoyl complexes ... 

Why are transition metals well suited for catalysis of this process Certainly the electrophilicity of cationic metal centers is important, as is the relative weakness of transition-metal-carbon bonds. However, similar electrophilicities and bond strengths could be found among main-group cations as well. A key to the effectiveness of Ti catalysts is the presence of two metal-based acceptor orbitals. In effect, two such orbitals are needed to choreograph the reversal of net charge flow at the two alkene carbons as the intermediate alkene complex moves through the transition state toward the final product. 

The marked difference in reaction efficiency that arises when different sources of metallocene are employed in the (ebthi)Zr-catalyzed hydrogenation is illustrated in Table 6.5. The mechanism by which MAO converts the zirconium(IV) salts to the active zirconium hydride species remains unclear [122]. However, it has been proposed that a chlorine atom may form an q2-bridge between aluminum and zirconium when the dichloride salt is used, thereby preventing formation of the active cationic metal center. 

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