Cyclidene

Cyclic silylphosphanes, see Silylphosphanes, phosphorus-rich, cyclic Cyclic sulfur-nitrogen compounds, see Sulfur-nitrogen compounds, cyclic Cyclic trithiolate ligand, 38 8-9 Cyclic voltammetry A. chroococcum Fd 1, 38 130-131 fullerene adducts, 44 19 nickel(ll) macrocyclic complexes, 44 112 Rieske proteins, 47 138, 139 Cyclidenes, as cobalt complex ligands, 44 282-284... 

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). 

A Schiff base is the common name for the (usually acyclic) imine product of the reaction of a primary aryl amine with an aldehyde or ketone. These imines are stable if there is at least one aryl group on the imino nitrogen or on the imino carbon (54). A cyclidene is generically a cyclic, multidentate imine. 

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

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... 

Cyclidenes (18) are bicyclic macrocycles which coordinate one metal ion and contain a protected void about the axial site of the metal ion. 

The lacunar cyclidenes are synthesized from a precursor tetraaza macrocycle containing a pendant vinyl carbon attached to a methoxy group, which readily undergoes addition-elimination reactions with diamines to give the vaulted bridge bicycle.36... 

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... 

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]. 

Metalloporphyrins have been used for epoxidation and hydroxylation [5.53] and a phosphine-rhodium complex for isomerization and hydrogenation [5.54]. Cytochrome P-450 model systems are represented by a porphyrin-bridged cyclophane [5.55a], macrobicyclic transition metal cyclidenes [5.55b] or /3-cyclodextrin-linked porphyrin complexes [5.55c] that may bind substrates and perform oxygenation reactions on them. A cyclodextrin connected to a coenzyme B12 unit forms a potential enzyme-coenzyme mimic [5.56]. Recognition directed, specific DNA cleavage... 

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 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. 

The cobalt(II) cyclidene product VI is reasonably stable toward autoxidation in the solid state, and a solid state IR spectrum can be obtained under aerobic conditions. The cobalt(II) complex is best identified by its ESR spectrum in an acetonitrile-1.5M M-methylimidazoIe solution, frozen to a glass at — 196 C a 2.3 and 2.00. Exposure of the thawed solution to O2 then causes replacement of this spectrum with that of the 1 1 Oj adduct % 2% 2.01 and 3% 2.08. At 20 °C in 2.5 M aqueous N-methylimidazole the complex binds dioxygen reversibly with an equilibrium constant Kq = 1.6 torr. An X-ray structure of the cobalt(II) complex confirms the presence of the commodious lacuna, which protects the coordinated dioxygen from autoxidation in polar sol vents.An X-ray structure determination of the dioxygen adduct, prepared by recrystallization of the complex from aqueous V-methylimidazole exposed to the atmosphere, confirms the existence and nature of the O, adduct. ... 

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. 

---