Clathrochelates from iron

Clathrochelates comprise a new type of coordination compound containing a metal ion both coordinately saturated and encapsulated by a single ligand.1 5 The ligands of one class of clathrochelates are derived from dioximes and various boron compounds.1,4 Typical examples of iron and cobalt clathrochelates of this structural class are shown in Fig. 19 and Table I. These complexes are readily prepared from iron(II) or cobalt(II) salts,... 

For cobalt, iron, and ruthenium sarcophaginates, the Log kn values increase with an increase in the redox potentials. This is the case when the main factor affecting a variation in both values in the same direction is the electronic configuration. The increase in E values in a series of clathrochelates from cobalt to ruthenium is attributed to differences in spin states Is > hs for cobalt. Is > hs, Is for iron. Is > Is for ruthenium complexes. The increase in Logftii values correlates with ALFSE for +3 and +2 oxidation states. 

The oxidation of iron(II) clathrochelates to iron(III) complexes is a quasi-reversible process with Eirz from 775 to 580 mV for clathrochelate dimethylglyoximates, from 850 to 570 mV for nioxime compounds, from 1 250 to 1 040 mV for glyoximates, and from 940 to 760 mV for a-benzyldioxime complexes. In the dioxime series Nx > Dm > Bd > Gm, the Eyz values becomes higher. 

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... 

IRON(II) AND COBALT(III) CLATHROCHELATES DERIVED FROM DIOXIMES... 

Iron(II) and Cobalt(III) Clathrochelates Derived from Dioximes 141... 

The clathrochelate complexes, [M(ClgH30Ni2)] [BF4]2, are prepared by adding 5.0 g of 37% aqueous formaldehyde solution to a suspension of 5 g of the appropriate tris(2,3-butanedione dihydrazone)metal bis(tetrafluoroborate) complex in acetonitrile. The condensation reaction is catalyzed by the addition of 0.5 mL of concentrated tetrafluoroboric acid. An immediate darkening of the solutions is observed as the condensation reaction proceeds. The iron (II)-containing solution becomes a very intense purple the nickel(II) solution becomes very dark olive-brown. Precipitation of the clathrochelates requires the addition of approximately 30% by volume of diethyl ether and refrigeration. The products are filtered from the solution, washed with methanol, and air dried. The iron(II) complex is somewhat more soluble in acetonitrile than the other complexes, and increased yields can be obtained by the dropwise addition... 

Clathrochelate iron(II) complexes derived from alicyclic dioximes (nioxime, heptoxime, 4-methylnioxime, and octoxime) are of particular interest. The first clathrochelate FeNx3(BOH)2 and FeNx3(BF)2 compounds of this type, prepared by template cross-linking of three dioxime molecules on the Fe2+ ion with a boric acid in water and BF3-(C2H5)20 in re-butanol, respectively, were described in Ref. 40. The syntheses with alicyclic dioximes have been realized for a number of boron-containing cross-linking agents in many cases, in a quantitative yield, and in dilute solutions well [49, 54-61]. 

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]. 

The synthesis of clathrochelates resulting from capping with antimony(V) compounds was realized for the first time as described in Ref. 74. With antimony(V) halogenides, only polymeric complexes were isolated, but antimony(V) triorganyles, unlike tin(IV) triorganyles, readily form nioximate iron(II) clathrochelates by Reaction 19. 

An analogous zinc-containing clathrochelate resulted from capping with an initial ttnCrBrs compex [86]. The encapsulated nickel and iron(II) ions were oxidized with nitrosyl perchlorate to nickel(IV) and iron(III) ions in acetonitrile (Scheme 33) [85],... 

The condensation of butanedione-2,3-dihydrazone with formaldehyde on a metal ion (Fe-, Co-" and Ni +) matrix (Scheme 74), performed by Goedken and Peng, led to the formation of clathrochelate [M(thz)](BFi)2 complexes. Direct reaction between the three components proved to be efficient only with iron(II) ion [183]. Therefore, nickel, cobalt, and iron(II) tris-dihydrazonates were preliminarily synthesized. It was noted that even when the reaction was carried out under nitrogen and cobalt(II) tris-dihydrazonate was used as the starting material, only cobalt(III) clathrochelate could be isolated from the reaction mixture. Its reduction with anhydrous hydrazine yielded cobalt(II) clathrochelate [95, 183]. 

Homobinuclear macrobicyclic copper, cobalt, and iron(II) complexes and a heteronuclear iron(II)/cobalt(II) clathrochelate of the [MiM2(trom)] type arise from interaction of a sodium complex of trom ligand with the corresponding metal salts. 

The QS in the Fe Mdssbauer spectra is determined by the geometry of the iron(II) coordination polyhedron and has been utilized to gain information on the structure of the complexes from the spectral parameter vs structure correlation. Conversely, the data available on the structure of clathrochelates employed for working out a modern version of the partial quadrupole splitting (PQS) concept permits one to obtain absolute PQS values and to analyse the results for macrobicyclic complexes [264, 265]. 

The sTjTe Mdssbauer parameters for the tin-, germanium-, and antimony-capped clathrochelate compounds (Tables 22-24) are typical low-spin iron(II) complexes. The increase in the ligand field strength (a "macrocyclic" effect), causes the increase in s-electron density on the iron nucleus, is less pronounced for these compounds than for their boron-capped analogs experimental ISs are only slightly lower than those calculated from a PIS concept. 

The solution UV-vis spectra of the monomeric clathrochelate tin-, germanium-, and antimony-capped complexes (Tables 23 and 25) and the UV-vis reflection spectra of the germanium-capped polymeric clathrochelates (Table 22) are appreciably different from those for the FeD3(BR)2 complexes and resemble the spectra for iron(II) tris-phenanthrolinates and tris-bipyridinates [236]. In the visible region, two metal-ligand CTBs at 17 560-18 480 and 20 560-21 320 cm i... 

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