Encapsulating Metal Ion

Complexes containing encapsulated metal ions (clathrochelates ) with the formula [M(dioxime)3(BR)2] are known with iron(II) 135, cobalt(ll) 136, cobalt(III) 137, and ruthenium(ll) 138 (Fig. 37) [205-220]. Generally, these macrobicyclic complexes are prepared by template synthesis from a mixture of... 

Fig. 37. [M(dioxime)3(BR)2] complexes 135-138 contain encapsulated metal ions and are clathrochelates. The coordination geometry of the metal ion is described by the distortion angle 4>...

Dendrimer interior functional groups and cavities can retain guest molecules selectively, depending on the nature of the guest and the dendritic endoreceptors, the cavity size, the structure, and the chemical composition of the peripheric groups. Two main methods are known for the synthesis of metal nanoparticles inside dendrimers. The first method consists of the direct reduction of dendrimer-encapsulated metal ions (Scheme 9.4) the second method corresponds to the displacement of less-noble metal clusters with more noble elements [54]. 

Dendrimer-Encapsulated Metal Ions, Metals, and Semiconductors. 90... 

In this section, two methods used to prepare dendrimer-encapsulated metal nanoclusters are discussed direct reduction of dendrimer-encapsulated metal ions and displacement of less noble metal clusters with more noble elements. 

In the cryptate [Eu(2.2.2)(H20)2] the encapsulated metal ion is ten-coordinate and has two inner shell water molecules (165,182). The exchange rate constant is the lowest of all Eu " " chelates measured so far which is probably due to the positive charge. The value of AV is close to zero and therefore an I mechanism was assigned for the water exchange process (Table X). 

The heme moiety provides de novo designed heme proteins with an intrinsic and spectroscopically rich probe. The interaction of the amide bonds of the peptide or protein with the heme macrocycle provides for an induced circular dichroism spectrum indicative of protein-cofactor interactions. The strong optical properties of the heme macrocycle also make it suitable for resonance Raman spectroscopy. Aside from the heme macrocycle, the encapsulated metal ion itself provides a spectroscopic probe into its electronic structure via EPR spectroscopy and electrochemistry. These spectroscopic and electrochemical tools provide a strong quantitative base for the detailed evaluation of the relative successes of de novo heme proteins. 

The hexaethylenetetraamine 12 has been created as a proton cryptate and proven by its X-ray structure<99ACIE956>. Related but larger macrocyclic cages have been formed in order to encapsulate metal ions<99ACIE959>. 

Many interesting properties are exhibited by the encapsulated metal ions, but discussion of these is outside the scope of this chapter. 

Calix[ ]arenes are a family of macrocycles prepared by condensation reactions between n /v/ra-substituted phenols and n formaldehyde molecules under either base or acid catalysis. Different sizes of the macrocycles can be obtained (n = 4-20) (Stewart and Gutsche, 1999) depending on the exact experimental conditions, which were mastered in the 1960 s (Gutsche, 1998), but the most common receptors are those with n =4,6,8 (macrocycles with an odd number of phenol units are more difficult to synthesize). We use here the simplified nomenclature in which the number of phenolic units is indicated between square brackets and para substituents are listed first.4 Calixarenes, which can be easily derivatized both on the para positions of the phenolic units and on the hydroxyl groups, have been primarily developed for catalytic processes and as biomimics, but it was soon realized that they can also easily encapsulate metal ions and the first complexes with d-transition metal ions were isolated in the mid-1980 s (Olmstead et al., 1985). Jack Harrowfield characterized the first lanthanide complex with a calixarene in 1987, a bimetallic europium complex with p-terf-butylcalix[8]arene (Furphy etal., 1987). 

The types of shape selective catalysis that occur in zeolites and molecular sieves are reviewed. Specifically, primary and secondary acid catalyzed shape selectivity and encapsulated metal ion and zero valent metal particle catalyzed shape selectivity are discussed. Future trends in shape selective catalysis, such as the use of large pore zeolites and electro- and photo-chemically driven reactions, are outlined. Finally, the possibility of using zeolites as chiral shape selective catalysts is discussed. 

Distortion such as this may result in the exclusion of one or more of the crown O atoms from the coordination sphere of the metal. This situation is found in another Na+ complex of 18-crown-6 (92), again showing that subtle structural differences can occur for the same cation-crown combination. Extremely large and flexible crown ethers can completely encapsulate metal ions, as is found in the K+ complex of a 30-crown-10 ligand, and the Na+ complex of a 24-crown-8 derivative, for example. This wrapping of the cation bears structural... 

Fundamental concepts of complexes with encapsulated metal ions... 

Because the definition of metal ion encapsulation or its absence is, to a certain extent, ambiguous, we first describe the conventional criteria that we use to restrict the scope of the compounds to be considered. The major signs of complexation (formation of a complex) with an encapsulated metal ion are (a) a three-dimensional cavity (capsule, cage) produced by a macropolycyclic ligand and (b) metal ion coordinating heteroatoms in this cavity that isolate this metal ion from the environment. 

In comparison with their role in macrocyclic complexes, the role of steric factors and the correspondence of the cavity size to the metal ion size drastically increase in macrobicyclic compounds with an encapsulated metal ion. The degree of freedom in macrocyclic complexes related to the location of the metal ion outside the plane of... 

It is undoubtedly of interest that the substituents in the clathrochelate framework and in apical groups affect the structure and properties of macrobicyclic complexes. In particular, it was noted that N-methylation of the complexes must stabilize the lowest oxidation states of the encapsulated metal ion. In this case, one should take into account the steric effects of substituents whose introduction influences the dissociation kinetics of the sarcophaginates [112]. 

Free sarcophagines and their complexes were also modified by N-carboxymethylation with chloroacetate controlled by an encapsulated metal ion. 

The UV-vis spectra of the high-spin chromium(III) sarcophaginates with the electronic configuration of the encapsulated metal ion exhibit the quartet A2g- Tig, Eg and transitions, which are different from those in the spectrum of [Cr(en)3] tris-ethylenediaminate the spin-allowed quartet-quartet... 

The UV-vis spectra of sarcophaginates and sepulchrates of the platinum subgroup metal ions are much less informative. In most cases, the d-d transition bands have been observed as a shoulder at CO 4000 cm- to more intensive CTB. Only spectra of rhodium(III) complexes with electronic configuration d oi the encapsulated metal ion show the Aig—> Tigand d-d transition bands at 33 000... 

Electronic configuration (spin state) of an encapsulated metal ion. r, is physical ionic (Shannon) radius of an encapsulated metal ion [280]... 

Cu2(trom)](BF4)H20 complex). However, the X-ray data rerefinement, described in Ref. 285, showed that in the study described in Ref. 192, the space group and unit cell were determined incorrectly. This novel refinement allowed one to establish that the encapsulated metal ions, lying on the Ca axis, are equivalent. All six M-N distances in the complex are 2.17 A, and the M-0 distances are... 

The kinetics and mechanism of synthesis and decomposition of macrocyclic compounds are regarded as one of the most important aspects in the chemistry of these compounds. The majority of papers concern metal ions complexing with preliminarily synthesized macrocyclic ligands and metal ion substitutions by other metal ions in the preliminarily prepared complexes. Template synthesis, the most promising approach to the directed preparation of macrocyclic compounds with desired structures [17], plays a still more decisive role in the chemistry of macrobicyclic complexes with encapsulated metal ion. However, the literature contains only scarce data on the kinetics and the mechanism of the template synthesis of macrocyclic compounds because of the difficulties encountered in experimental determinations of kinetic and thermodynamic parameters, such as low product yields, nonaqueous media, high temperatures, and side reactions. 

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