Lacunar cyclidenes structure

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. 

Figure 1. Structures of cydidene complexes 1) lacunar cyclidene comply 2) vaulted cyclidene...

Figure 12. Variations in the superstructures and substituents in cyclidene complexes a) general cyclidene structure b) lacunar [16]cyclidene c) retro-bridged lacunar [1 cyclidene d) vaulted [16]cyclidene e) dinuclear [16]cyclidene.

The general features of oxygenation equilibria and autoxidation involving cobalt-based lacunar cyclidene dioxygen carriers have been extensively studied by Busch and coworkers with special reference to dioxygen affinities and their correlation with structural features Lacunar systems have considerable activities for reversible dioxygen binding in their cavities under ambient conditions. 

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. 

This lacunar nickel(II) cyclidene Complex XI possesses similar physical properties to those already described. Although IR spectroscopy can be used to verify that the reaction has gone to completion, NMR spectroscopy is the most useful characterization technique. An X-ray crystal structure determination on this complex illustrates the demanding steric requirements placed upon the lacuna by the bulky peripheral substituents. NMR (CDsCN) 172.7, 168.3, 162.4, 137.8, 135.4, 134.0, 133.4, 132.4, 131.2, 130.9, 130.5, 130.2, 130.0, 128.4, 126.4, 115.5, 57.8, 56.5, 30.1, 29.4, and 21.2 relative to TMS. 

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