Macrocyclic ligands formation

The resulting macrocyclic ligand was then metallated with nickel(II) acetate. Hydride abstraction by the strongly electrophilic trityl cation and proton elimination resulted in the formation of carbon-carbon double bonds (T.J. Truex, 1972). 

As a final example we consider noncovalent molecular complex formation with the macrocyclic ligand a-cyclodextrin, a natural product consisting of six a-D-glucose units linked 1-4 to form a torus whose cavity is capable of including molecules the size of an aromatic ring. Table 4-3 gives some rate constants for this reaction, where L represents the cyclodextrin and S is the substrate ... 

Macrocyclic effect and specific character of complex formation with rigid macrocyclic ligands such as porphyrins and phthalocyanins 97MI8. 

Problems of complex formation with macrocyclic ligands. S. L. Davydova and N. A. Plate, Coord, Chem. Rev., 1975,16, 195-225 (130). 

Complex formation between a metal ion and a macrocyclic ligand involves interaction between the ion, freed of its solvation shell, and dipoles inside the ligand cavity. The standard Gibbs energy for the formation of the complex, AGjv, is given by the difference between the standard Gibbs... 

An unusual template reaction leads to the Ni11 complex of a tetrabenzo-N2S2 macrocyclic ligand (676) via C—Cl bond cleavage and C—S bond formation according to Equation (25).1701... 

Using 1,4,8,11-tetraazacyclotetradecane, the structure of complex (800) (distorted trigonal planar Cu-Cu 6.739 A) was determined. Reactivity with 02 was investigated to demonstrate the formation of trans-l,2-peroxo species.585 As part of their work with copper(I) complexes with 02, the structure of a dicopper(I) complex ((801) distorted tetrahedral 7.04 A), supported by macrocyclic ligand environment, was reported by Comba and co-workers. Tolman and co-workers structurally characterized a three-coordinate copper(I)-phenoxide complex (802) (planar T-shaped) that models the reduced form of GO.587 The copper(I) analogue [Cu(L)][CF3-SO3]-0.43MeOI I (803) of a copper(II) complex (534) was also reported to demonstrate the role of ligand framework conformability in CV /Cu1 redox potentials.434 Wilson and co-workers... 

In contrast, electrocatalysis in a nonaqueous solvent like dichloromethane with soluble palla-dium(II) and silver(II) porphyrins produces mainly oxalate.145 However, demetallation rapidly deactivates the catalysts. In these cases the catalytic processes are interpreted in terms of reduced forms of the macrocyclic ligand, rather than by formation of Pd1 or Ag1 species following metal-centered reduction. 

A very large number of synthetic, as well as many natural, macrocycles have now been studied in considerable depth. A major thrust of many of these studies has been to investigate the unusual properties frequently associated with cyclic ligand complexes. In particular, the investigation of spectral, electrochemical, structural, kinetic, and thermodynamic aspects of macrocyclic complex formation have all received considerable attention. 

Chelate ring formation may be rate-limiting for polydentate (and especially macrocyclic) ligand complexes. Further, the rates of formation of macrocyclic complexes are sometimes somewhat slower than occur for related open-chain polydentate ligand systems. The additional steric constraints in the cyclic ligand case may restrict the mechanistic pathways available relative to the open-chain case and may even alter the location of the rate-determining step. Indeed, the rate-determining step is not necessarily restricted to the formation of the first or second metal-macrocycle bond but may occur later in the coordination sequence. 

Macrocyclic ligands have also been effective in stabilizing less-common, lower oxidation states for a variety of metal ions. In particular, a considerable number of investigations have been concerned with the formation of stable Ni(i), Cu(i), Fe(i) and Co(i) species. 

Fig. 4j6 Stepwise eomplexing of Cu(OH)4 by a tetradentate macrocyclic ligand. The first Cu(II)-N bond is formed by replacement of an axial solvent molecule (k ) followed by a Jahn-Teller inversion (Ar, ) which brings the coordinated nitrogen into an axial position. Second-bond formation follows a similar pattern (k2 and 2b)- Reproduced with permisson from J. A. Drumhiller, F. Montavon, J. M. Lehn and R. W. Taylor, Inorg. Chem. 25, 3751 (1986). (1986) American Chemical Society.

Complex formation between the macrocyclic ligand and the cation (originally hydrated) can be separated into two steps [153] ... 

The reaction of [OsCl2(PPh3)3] with P(CgH4-2-SH)3 results unexpectedly in the formation of [Os(401-P,P, 5, 5",5 ",5"")] in which the macrocyclic ligand results from oxidative coupling of two molecules of the original tertiary phosphine. A crystal structure determination of the complex confirms that the P-donors are mutually ds, and only one of each of the disulfide S atoms is involved in coordination to the metal center. ... 

As regards other coordination compounds of silver, electrochemical synthesis of metallic (e.g. Ag and Cu) complexes of bidentate thiolates containing nitrogen as an additional donor atom has been described by Garcia-Vasquez etal. [390]. Also Marquez and Anacona [391] have prepared and electrochemically studied sil-ver(I) complex of heptaaza quinquedentate macrocyclic ligand. It has been shown that the reversible one-electron oxidation wave at -1-0.75 V (versus Ag AgBF4) corresponds to the formation of a ligand-radical cation. Other applications of coordination silver compounds in electrochemistry include, for example, a reference electrode for aprotic media based on Ag(I) complex with cryptand 222, proposed by Lewandowski etal. [392]. Potential of this electrode was less sensitive to the impurities and the solvent than the conventional Ag/Ag+ electrode. 

The Ni(II) complexes l-3g with hexaaza and pentaaza macrocyclic ligands as well as the Ni(II) cyclam complex display a low-spin square-planar and high-spin octahedral complex interconversion in aqueous solution (7, 12-14). However, complexes l-3a favor formation of the high-spin form upon addition of acid, which is in contrast to the cyclam complex. The protonation of tertiary nitrogen atoms at the bridgehead position facilitates the axial coordination of anion or water because of hydrogen-bonding between them 12, 14a). 

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