Sophisticated Ligand Systems

The radical anionic character of the 1,4-diazabutadiene ligand was further proven in the heteroleptic complex CpfSm(tBuDAB), synthesized from the strong single electron transfer reagent SmCpf(THF)2 and the free ligand [173]. 

Homoleptic and heteroleptic complexes of the bischelating 2-phenylamino-4-phenylimino-2-pentene ligand [(AIP)H] were synthesized in the form of Ln(AIP)3 (Ln = Sm, Gd) and (AIP)GdBr2(THF)2 [175], The AIP ligand does not provide a stable environment for Sm(II), Instead, the homoleptic Sm(III) derivative was obtained by the putative Sm(AIP)2 synthesis. 

Sm(II) mediated formation of very sophisticated chelating ligands has been observed. The powerful reducing agent CpJSm can induce facile double bond insertion of CO into C=C and C=N double bonds (Eq. 13) [176]. Carbon monoxide also inserts into the Y-pyridyl bond of CpfY(2-pyridyl) to form a bridging /i-f/2 /2-dipyridyl ketone fragment (Eq. 14) [177]. 

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The cobalt(II) complex is labile, and self-assembles to triple-helical [Co2(5-bismbmp)3] [24] which is readily oxidized to the cobalt(III) complex [Co2(5-bismbmp)3] which is sufficiently inert for the two enantiomers to be separated by classical methods [25]. The transformation of labile species to kinetically stable ones offers the possibility of using these elements as building blocks for further supramolecular construction. On the basis of our results with sophisticated ligand systems such as L232, we suspect that complicated, multicomponent self-assembly reactions without some element of kinetic stability will be rather difficult to develop. 

Needless to say, an analysis which will finally allow one to nail down all rates, activation parameters, and equilibrium constants requires a large amount of precise and reliable kinetic data from appropriate experiments, including the determination of isotope effects and the like, as well as a rather sophisticated treatment and solution of the complete kinetic scheme. Then a comparison is necessary between various organosilanes with different types of C-H and C-Si bonds as well as the comparison between the dtbpm and the dcpm ligand systems, not to speak of model calculations in order to understand the molecular origin of the kinetic and thermodynamic numbers. We are presently in the process of solving these problems. 

Despite the remarkable activity and selectivity attainable through sophisticated ligand design in homogeneous catalysis, broad commercial application has been hampered by the need to recover and/or recycle expensive catalysts and ligands. A number of approaches, termed biphasic catalysis , have been advanced where a soluble catalyst is immobilized in one liquid phase (often aqueous) and the substrates and products are isolated in a separate immiscible phase (Baker and Tumas, 1999 Cornils and Herrmann, 1998). Several recent reports have focused on systems using only water and C02, where water-soluble catalysts and C02-soluble substrates and products can be isolated in separate phases (Table 2.4). Hydrogenation of cinnamal-... 

The readily available system Ln(OiPr)3 made the running for the others in carbon-carbon bond generations and functional group modifications. Meanwhile, the steadily growing demand for stereo- and enantioselective reagents has also affected the lanthanide elements and as a result more sophisticated ligands have been introduced (Fig. 35A-G). 

This system may be recommended for the quick exploration of new synthetic applications in research laboratories which do not specialize in organometallic techniques because the cobalt salts can be used in the hydrated form under air, and no sophisticated ligands are necessary. Cobalt(I) halide complexes of the type [XC0L3] having a moderate activity in the synthesis of 2-alkylpyridines are also easily accessible (eq. (3)) [19]. 

Like BINOL, salicylaldehyde imines have become very important in asymmetric catalysis and a variety of polydentate ligands prepared from chiral monoamines and diamines are employed in oxidation reactions, carbenoid reactions and Lewis acid catalyzed reactions. As in the previous section, this section emphasizes the effect of the phenol moiety on the asymmetric catalysis. An imine derived from a chiral 1-phenethylamine and salicylaldehyde was employed in the copper catalyzed asymmetric cyclopropanation by Nozaki, Noyori and coworkers in 1966, which is the first example of the asymmetric catalysis in a homogeneous system . Salicylaldehyde imines with ethylenediamine (salen) have been studied extensively by Jacobsen and Katsuki and their coworkers since 1990 in asymmetric catalysis. Jacobsen and coworkers employed the ligands prepared from chiral 1,2-diamines and Katsuki and coworkers sophisticated ligands possess chirality not only at the diamine moiety but also at the 3,3 -positions. 

We now know conclusively that Werner was correct, or at least nearly so. Interestingly, although his octahedral shape is supremely dominant in six-coordination, we now know that there are trigonal prismatic six-coordinate shapes also - albeit with ligand systems to which Werner had no access. What sophisticated modern physical methods tell us conclusively is that coordination complexes don t just happen upon a shape. Each different coordination number known, and these vary from 2 to 14, supports a limited number of basic shapes. The shape, or stereochemistry, depends both on the metal and its oxidation state and on the form of the ligand - each contributes in different ways. 

Also, 1-dodecene can be selectively reacted with syngas in the presence of a Rh(BIPHEPHOS) catalyst to give n-tridecanal (Table 6.1, entry 5). A related Rh catalyst based on nanoparticles (2.7-4.8 nm) could be three times recycled without loss of activity when the reaction was conducted in a thermomorphic solvent system [61]. Also, under the conditions illustrated in Scheme 6.6 ( = 4), tridecanal is formed, but in comparison to shorter chain olefins the conversion dropped and the aldehyde was obtained only in a moderate Ub ratio [54]. Other attempts have been based on more sophistic ligands such as amphiphilic sulfonated monophosphines [62] and diphosphines [55, 63], phosphine-functionalized phosphonium ionic liquids [64], and MeO-substituted phosphines [65]. Tridecanal has a slightly floral scent reminiscent of citrus and grapefruit peel. 

Very efficient and sophisticated catalyst recovery technologies paved not only the way for the application of the more expensive metal rhodium but also allowed the use of more sophisticated and expensive ligand systems that have to be recycled in the same way. 

This simple picture of bonding is convenient to use, and often completely acceptable. However, it does lack sophistication and may not be used to explain some of the subtleties of these systems. One obvious point in this regard concerns infrared spectral data. Coordination of carbon monoxide to a metal invariably leads to a lower carbonyl stretching frequency (vco). implying a lower CO bond order as predicted. However, the values for vcn may be considerably higher for metal complexes of an isocyanide than are the values for the ligand itself. The valence-bond picture cannot rationalize... 

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