Clathrochelates from iron dioxime complexes

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. 

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,... 

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]. 

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... 

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 direct X-ray diffraction study is undoubtedly the most optimal and informative structural method for all compounds. Unfortunately, no one has obtained crystals of the binuclear iron(II) azineoximates, which are suitable for X-ray crystallography [193], The structure of these complexes was deduced from EXAFS data, which allow one to obtain the function of the radial atom distribution (RAD) and hence to determine coordination numbers and the distances from the encapsulated iron ions to the clathrochelate framework atoms, and from Fe Mdssbauer spectra, which make it possible to predict the distortion angle (p (see above). Macrobicyclic iron(II) tris-dioximates... 

The electrochemical properties of the clathrochelate Ca-nonsymmetric FeDnD 3-n(BX)2 and Ca-nonsymmetric FeD3(BX)(BY) tris-dioximates and their dependence on electronic characteristics of the substituents in the dioximate fragments and ones at capping atoms are discussed in Refs. 64 and 68. Table 36 lists the E1/2 and the Tomes criterion values for these complexes. As seen from this table, the oxidation process for most of the boron-capped iron(II) clathrochelates is reversible or quasi-reversible. 

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