Clathrochelates synthesis

Y. Z. Voloshin, N. A. Kostromina, and R. Kramer Clathrochelates Synthesis, Structure and Properties , Elsevier,... 

The cycloaddition reaction proceeds under more rigid conditions and takes more time than a direct template condensation on the iron(II) ion. This can be explained by the fact that the overall mechanism of clathrochelate synthesis involves an intermediate tris-complex formation step. It is evident that macrocyclic square-planar iron(II) bis-dioximates are relatively kinetically stable, and the... 

N.A. Kostromina, Y.Z. Voloshin and A.Y. Nazarenko, Clathrochelates Synthesis, Structure, Properties, Naukova Dumka, Kiev, 1992. 

The template synthesis represents an elegant method that uses metal ions to direct reactions of ligands and provides a useful route to macrocyclic structures. Several books159-161 describe the template processes that involve reactions on matrices used to synthesize polyazamacrocyles, crown ethers, cryptands, rotaxanes, knots,159 clathrochelates,160 phthalocyanines,161 etc. which are applied, e.g., as molecular switches, in ion exchange, electron transfer or catalysis. An example of clathrochelate synthesis is given in Chapter 1.33... 

Voloshin, Y. Z. Kostromina, N. A. Kramer, R. Clathrochelates Synthesis, Structure, and Properties, Elsevier, Amsterdam 2002. 

Voloshin YZ, Kostromina NA, Kramer R (2002) Clathrochelates synthesis, structure and properties. Elsevier, Amsterdam... 

Continuous progress in the chemistry of these classes of compounds with encapsulated species has been summarized in several reviews by J. Rebek Jr. and coworkers [1-17] and, very recently, by M. Yoshizawa [18]. The coordination capsules have been nicely covered by M. Fujita [19-25], K.N. Raymond [26-31], and others [32-40] and, more recently, by J.R. Nitschke [41 5], G.H. Clever [46], and M. Shionoya [47]. hi 2002, we published the book Clathrochelates Synthesis, Structure and Properties [48] that summarized general concepts and features of the complexes with an encapsulated 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... 

In some cases, a free clathrochelate ligand has been isolated after its template construction on the metal ion and demetallation of the resultant complexes. In particular, this has made it possible to synthesize sarcophaginates of many metals incapable of forming clathrochelates via direct template synthesis they are formed from... 

The BF4 anion in the clathrochelate [CoDma(BF)2](BF4) complex can readily be replaced by another large inorganic anion (e.g. PFe ) via an exchange reaction occurring in aqueous-acetonitrile solution in the presence of a great excess of the substituting anion salt [39]. The reduction of the [CoDma(BF)2](BF4) clathrochelate with Nal solution in acetone yielded a macrobicyclic cobalt(II) CoDma(BF)2 complex. The synthesis of the latter via a template condensation on the Co2+ ion was not yet successful. 

In the synthesis of the boron-capped cobalt(II) tris-dioximates, ferrocenylboronic acid was also used as a capping agent [43], Reaction of this Lewis acid with anhydrous C0CI2 and dioximes in oxygen-free methanol gave clathrochelate CoNx3(BFc)2 and CoDm3(BFc)2 complexes ... 

Synthesis of the clathrochelate with hydridoboron capping groups via template condensation in dry acetonitrile occurs by Reaction 12 FeBrj + 3H2NX + 2NaBH4 FeNx3(BH)2 + 2NaBr + 6H2 (12)... 

With the majority of alicyclic boron-capped iron(II) dioximates, neither template condensation nor recrystallization from organic solvents gave crystals suitable for X-ray analysis. A rate-controlled template condensation within several days yielded FeGx3(B0H)2 3H20 monocrystals, since the synthesis of this clathrochelate compound proceeds much more slowly than that of analogous complexes with nioxime and 4-methylnioxime [62, 63]. 

A direct synthesis of C2-nonsymmetric tris-dioximate iron (II) clathrochelates via the formation of semiclathrochelate complex 2 cannot be realized even with a great excess of complex 1, since compound 2 readily disproportionates to give 1 and 3 (Scheme 9). 

The synthesis of C2-nonsymmetric clathrochelate iron(II) dioximates was realized through a stepwise assembling on the sorbent surface (Scheme 11). 

The best results were obtained with aluminium hydroxide resulting from hydrolysis of aluminium(III) iso-propylate. A high sorption capacity (ca 10%) of compound 2, a high degree of desorption of complex 3, and the purity of the resulted clathrochelate 4 make aluminium(III) hydroxide the most suitable matrix for the synthesis of targeted compounds [64]. 

The introduction of lipophilic substituents is of interest for producing surface-active compounds (surfactants) and liquid-crystal systems. The complexes with allyl substituents at the apical boron atoms are precursors for the synthesis of linear and netlike polymeric clathrochelates. 

The most feasible Routes I-III for the synthesis of triribbed-functionalized a-dioximate clathrochelates (Scheme 12) were proposed in Ref. 65. The halogen-carbon bonds are reasonably active in nucleophilic substitution reactions, and the dihalogenoxime complexes are relatively stable (unlike dihalogenoximes, these complexes are available and undergo no intramolecular conversions... 

The use of Route III presented problems because the attempts to obtain hexahalogenide precursors from initial dihalogendioximes by the standard procedures of synthesis of such clathrochelates have not been successful. Nevertheless, the conditions under which the yield of these complexes was 60-90 % were selected in Ref. 65 nitromethane was as a solvent, and acetonitrile FeAN4Cl2 solvato-complex as a source of Fe + ions, and the water was removed from the reaction mixture. The three hexachloride precursors with phenylboronic, re-butylboronic, and fluoroboronic capping groups (Scheme 13) were obtained [65]. 

The reaction with an excess of methylmercaptan in the presence of potassium carbonate at room temperature led to the formation of partially substituted products only, mainly trisubstituted clathrochelates. Therefore, a more active potassium methylmercaptanate was used in the synthesis of the hexasubstituted product, and the reaction readily proceeded in a high yield [65]. 

The thioalkyl-containing macrobicyclic complexes have been dealkylated and realkylated easily under the action of potassium thiolates in aprotonic media (Scheme 14). The products of de- and realkylation reactions were detected by PD and FAB mass spectrometry. In this respect the thioalkyl-containing clathrochelates are close to the aryl alkyl sulfides. In the course of thioalkyl derivative synthesis with an excess of potassium thiolate, a mixture of dealkylated products was obtained in addition to the desired hexathioalkyl clathrochelates. The addition of corresponding alkyl iodide and potassium carbonate to the reaction mixture in the final stage of reaction led to an increase in yields by alkylation of HS groups, resulting in the side dealkylation process [65]. 

The appropriate conditions for the synthesis of tris-azamacrocyclic clathrochelates containing dioximate fragments in polyazamacrocyclic rings were not selected. Attempts to use open-chain polyamines, as well as their complexes with transition metals, primarily Ni2+, gave no desired results. 

Clathrochelate ribbed-functionalized tris-dioximates have attracted interest because they offer scope for the synthesis of polynuclear complexes with targeted structural parameters and physicochemical properties (see above). In most instances, it is not necessary to functionalize all a-dioximate fragments, and it appears to be sufficient to modify only one of the three ribs in the clathrochelate framework to alter the properties significantly. Several feasible synthetic routes to clathrochelate monoribbed-functionalized tris-dioximates have been proposed in Ref. 68. A direct template condensation of the mixture of a-dioximes with Lewis acids on a metal ion (Scheme 15, Route I) leads to the formation of a poorly separable mixture of nonsymmetric and symmetric products, in which the latter predominate. Halogenation of the initial clathrochelate... 

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