Ligand and Complex Synthesis

2 Crystal Structures of the Zn(II) Complexes with CH3HL4 and CH3HL5 

7 Asymmetric Zn(II) Complexes as Stractural and Functional Models for GpdQ 

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In spite of the success that Ir—PCP pincer compounds have enjoyed in the development of more efficient and promising alkane dehydrogenation systems, one caveat of using these complexes has been the sometimes difficult or tedious synthesis of the pincer ligands and complexes. An answer to this problem was... 

Scheme 10 Synthesis of 1,6-bidentate carbene ligands and complexes...

In the synthesis of precursor complex-metal or complex-ligand species, byproducts of higher nuclearity are also formed due to the presence in the target molecules of reactive sites (labile ligands or free chelating sites). In the reaction between complex-ligands and complex-metals, the by-products (precursors and partially unreacted species) are lower nuclearity species with respect to the desired... 

Various approaches have been taken to the synthesis of effective luminescent materials, using a variety of large encapsulating antenna-containing ligands, including podands, calixarenes, macrocycles, and macrobicycles (cryptands). These have been divided into acyclic (sub-section A) and cyclic (sub-section B). Representative ligands and complexes will be presented and discussed. 

The main use of ligand reactions in synthesis is in the construction of multidentate chelates. In this manner, the field of coordination chemistry has been enriched, as the development of new ligands has led to the introduction of new influences in metal ions. The various properties of coordination compounds are considered elsewhere in this book, but this chapter will focus attention on the synthesis of new ligands and complexes as a consequence of ligand reactions. 

A typical example is seen in the addition of hydrogen cyanide to an imine to yield a cyanoamine (Fig. 4-29). Many of these reactions have been used to best advantage in the synthesis of macrocyclic ligands and complexes, and as such are considered in Chapter 6. A simple example of such a reaction is seen in the addition of HCN to the cobalt(m) complex indicated in Fig. 4-30. The starting complex is also readily prepared by a metal-directed reaction. 

The synthesis of the complex is followed by the most important step of characterization of the complex. The composition and the structural features of both the ligand and complex have to be established before embarking on further studies. There exist many methods by which the composition and structural features of the complexes are studied. Some of the methods are (i) elemental analysis, (ii) X-ray crystallography, (iii) UV-Vis absorption spectra, (iv) infrared spectroscopy, (v) Raman spectroscopy, (vi) thermal methods of analysis such as thermogravimetry, differential thermal analysis, (vii) nuclear magnetic resonance spectroscopy (proton, multinuclear), (viii) electrospray mass spectrometry. Depending upon the complexity of the system, some or all the methods are used in the studies of complexes. 

The utility of the traditional approach to asymmetric catalysis, involving synthesis and screening of chiral ligands and complexes in catalytic reactions is a powerfiil method for the development and optimization of catalytic asymmetric processes. Such an approach, however, can be cumbersome if the ligands are difficult to synthesize and modify. In these cases, alternative strategies to the generation of asymmetric catalysts provide access to a large number of catalysts with minimal effort. The remainder of this section will focus on these methods. 

The encapsulated catalyst can be obtained by assembling the catalytic complex within the pores or by building the support around the isolated complex (vide infra). Both methods today are known in literature as the flexible ligand and zeolite synthesis method, respectively. In the flexible ligand method the ligand or its precursors should be able to diffuse freely through the zeolite pores exchanged with the preferred transition metal ion,... 

Chiral catalysts with heterocyclic ligands and asymmetric synthesis of heterocycles with participation of metal carbene complexes 01JOM(617-618)98. 

In comparison to the zeolite synthesis approach there are many disadvantages associated with the preparation of intrazeolite complexes by the flexible ligand and template synthesis methods. The complexes are difficult to characterize, especially if the ligand has multiple coordination modes available and some of the target metal ions may remain uncomplexed which will complicate any reactivity studies. Additionally, there are limitations to the types of metal complexes that might be encapsulated in a zeolite. The only criteria for incorporating metal complexes... 

Purification of silylated compounds can be an even more challenging task than their synthesis. Due to the sensitivity of the silylated side chains, many silylated ligands and complexes are used in immobilization procedures as obtained from the synthetic reactions without purification (179). It is no accident that most of the TM complexes bearing silylated side chains that have been successfully prepared and purified to date are neutral species, and have mostly been organometallics. These compounds can be prepared reliably from water-free precursors, and nonaqueous preparation, and purification routes are readily available. 

Synthesis of Dithiolene Ligands and Complexes from Protected Dithiolene Precursors... 

Reproduced from Ferguson M, Girl N, Huang X, Apperley D, James SL. One-pot two-step mechanochemtal synthesis ligand and complex preparation without isolating intermediates. 

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