Lacunar ligands

The lacunar ligands of Busch and co-workers (18) comprise a series of bicyclic macrocycles called cyclidenes.36 174-176 These macrocycles typically coordinate one transition metal ion and, due to their... 

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. 

Cyclidenes (6) are a subset of the polyaza macrocycles and are the lacunar ligands first synthesized and extensively... 

Ni(III) complexes often exhibit equilibrium with Ni(II) ligand radical species. For example, the Ni(III) complex of lacunar cyclidene is octahedral [NimL(CH3CN)2]3+ at low temperature, but at high temperature it transforms into the Ni(II) complex with an oxidized ligand radical [NinL+]3+, identifiable by the absorption at 590 nm. The Ni(III) complexes and Ni(II) species with an oxidized ligand radical exhibit a thermal equilibrium (Eq. 12) (105). 

As mentioned earlier, cyclidenes are cyclic multidentate imino-donor ligands. The lacunar cyclidenes developed by Busch and co-workers... 

The synthetic routes may often involve template directed condensations, a widely used reaction being the (carbonyl + amine) to imine condensation that efficiently leads to a variety of Schiff-base macrocycles [2.58-2.60, A.7, A.14], macrobicyclic cryptands [2.61-2.63] and lacunar cyclidene ligands [2.60, 2.64]. 

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 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... 

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. ... 

Electrochemical data for one unbridged and four vaulted cyclidene complexes of iron(II) appear in Table 2. The most obvious feature is the dependence of redox potential on the axial ligand. For mixed chloride/bromide derivatives, the potential is much more negative than that for complexes having neutral acetonitrile as the axial ligand. The potentials are generally consistent with values reported for lacunar iron(II) cyclidene... 

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).

D. H. Busch Totally Synthetic Iron(II) Dioxygen Carriers Based on Lacunar Cyclidene Ligands, La Transfusions del Sangue, 33, No. 1, 57 (1988). 

The main achievements in the field of macrocyclic coordination compounds with lacunar superstructure ligands have been discussed in detail [225]. 

Reactions of some lacunar and some novel binuclear iron(II) complexes with O2 are only partly reversible and occur with the formation of free O2, as shown by ESR spectroscopy. Reversible one-electron transfer occurs between O2 and the pentacoordinated Fe(II) complex, followed by the coordination of a base (pyridine or 1-methylimidazole) yielding hexacoor-dinated Fe(III). The reaction of O2 with Fe(II) complexes of bis(j8-diimine)-containing macrocyclic ligands produces diketone ligands through the intermediacy of HO2 and hydroperoxide species. " ... 

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