Lacunar complexes

A number of model studies of the second type have also been investigated. In these, the iron complex provides a hydrophobic cavity so positioned that it surrounds the bound 02 molecule. These include the lacunar complexes mentioned in Chapter 3 (Goldsby, Beato Busch, 1986) particular complexes of which are quite efficient reversible carriers. A further (early) example of this type was (312) which encloses the dioxygen in a rudimentary cavity. However, this species only binds dioxygen reversibly at low temperatures (Baldwin Huff, 1973). 

The iron(II) complex XIII crystallizes as the bis(hexafluorophosphate) salt with acetonitrile as the fifth, axial, ligand. This composition appears to be dictated largely by the relative solubilities of the salts of the possible anions in the mixed solvent. Most of the iron(II) lacunar complexes crystallize as the chloride hexafluorophosphate mixed salts, in which the chloride occupies the axial site. The complex is high spin and Five coordinate in the solid state and in coordinating solvents. In N-methylimidazole at 20 °C the title complex binds dioxygen reversibly with = 0.0012 torr . In 3 1 1 acetone-N-methylimidazole-water the half-life toward irreversible auto-xidation is 24h at room temperature. This complex constitutes the First example of a nonporphyrin iron(II) complex that can act as a dioxygen carrier at room temperature. ... 

Lactonization intramolecular cobalt(III) complexes, 440 Lacunar complexes preparation... 

Busch, D.H. Alcock, N.W. Iron and cobalt "lacunar" complexes as dioxygen carriers. Chem. Rev. 1994, 94. 585-623. 

Figure 6 shows the structures of lacunar complexes having small ligands in their cavities [56-59]. The O2 adduct is easily formed because the cavity shape favors a small ligand that binds in an angular fashion (Figure 6b). In contrast, the thiocyanate ligand, that normally tends to be approximately linear when bound through nitrogen to cobalt(III), is forced into a distorted angular structure (Figure 6a).

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