Lacunar cobalt© cyclidene complex

Cyclodene Complexes Relationships between Macrocyclic Ring Size, Lacunar Cavity Shape, and Dioxygen Affinity of the Cobalt-Cyclidene Dioxygen Carriers. 

REVERSIBLE COBALT(II) AND IRON(II) DIOXYGEN CARRIERS OF TOTALLY SYNTHETIC LACUNAR CYCLIDENE COMPLEXES ... 

The lacunar cyclidene ligands are the first family of totally synthetic ligands to form both iron(II) and cobalt(II) Oj carriers. The molecular structure of the lacunar cyclidene complexes is most simply represented by the flat projection (la), while the stereochemistry is shown in Ib. Cyclidene refers to the parent macrocycle encircling the metal ion and the lacuna is the permanent void created by bridging the cleft arising from the saddle shape of the cyclidene macrocycle. 

The syntheses of two complexes are described here the cobalt(II) lacunar cyclidene complex in which R =—(CH2)e— and = R = —CH3, and the iron(II) complex in which R =m-xylylene, R = —CH2C6H5, and R = —CgHs- The former is the most studied example of a lacunar cyclidene oxygen carrier, and the latter represents the first nonporphyrin iron complex capable of forming a stable dioxygen adduct at room temperature. 

Busch and coworkers have synthesized a series of remarkable vaulted and lacunar cobalt(II) cyclidene complexes, which reversibly bind dioxygen and undergo autoxidation to cobalt(III) species. In some cases they exhibit catalytic properties in the oxidation of phenol derivatives. 

The totally synthetic superstmctured cobalt(II) cyclidene complexes (Figure 61) CoA (vaulted), CoB (unbridged) and CoC (lacunar), function as both oxidase and oxygenase models in oxygenation of substituted phenols . 

A picnic-basket porphyrin is reminiscent of the lacunar cyclidenes mentioned earlier, in which an alkyl chain tied the ends of the cyclidene together and formed a pocket for the complexation of dioxygen. Cobalt complexes of the picnic-basket porphyrins 31 bind dioxygen reversibly at room temperature with high oxygen affinities (84). The oxygen affinity increases as the basket size decreases (as determined by the length... 

The starting material for both of these target molecules is the neutral nickel(II) complex [3,11-diacetyl-4,10-dimethyl-1,5,9,13-tetraazacyclohexa-deca-1, 3, 9, ll-tetraenato(2 —)-K N ]nickel(II), ([Ni(Ac2Me2[16]-tetra-enatoN )]). This complex is prepared by the procedure described earlier. The lacunar cyclidene ligands are constructed on the nickel(II) ion, and the formation of the second macrocyclic ring is a template reaction. The first two reactions in the Scheme are required unless R is to be methyl. Thus, the synthesis of the cobalt complex does not require these steps, whereas... 

At this point, the ligand is a cyclidene. Several crystal structures have shown that the conformations of the 16-membered cyclidene rings, in their complexes, are saddle shaped. This facilitates the fifth reaction, the bridging process. Since the presence of the metal ion is required to produce this saddle conformation, the process is indeed a template reaction. The lacunar ligand is then removed from the nickel(II) ion (sixth reaction) and used to form a cobalt or iron complex (last reaction). The procedures for forming the iron and cobalt complexes are different the scheme exemplifies the reactions used to synthesize the iron(II) complex. 

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