Cyclic ligand

In 2008, Skarzewski and Wojaezynska studied the test reaction in the presence of chiral C2-symmetric S/S-donor five- and six-membered cyclic ligands depicted in Scheme 1.7, providing moderate activity and enantioselectivity. " The best enantioselectivity (42% ee) was observed when (lR,25)-bis(phe-nylsulfenyl)cyclopentane was involved as the ligand, whereas the corresponding... 

Scheme 1.7 Test reaction with C2-symmetric S/S-donor five- and six-membered cyclic ligands.

The binding of phenoxyl radical species to zinc has been observed. The pendent arm macro-cyclic ligand 1-ethyl-4,7-t-bis(3-butyl-5-methoxy-2-hydroxybenzyl)-l,4,7-triazacyclononane... 

The 31P and H study revealed the configuration of the central atom and the conformation of the cyclic ligand fragments, as well as the orientation of M—P bonds. The Pt—P bond is equatorial in the 1,3,5-diazaphosphorinane cycle. 

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. 

Factors influencing the macrocyclic hole size. The hole size of a macrocyclic ligand is a fundamental structural parameter which will usually influence, to a large degree, the properties of resultant metal complexes relative to those of the corresponding non-cyclic ligands. The large number of X-ray diffraction studies now complete for macrocyclic systems makes it possible to define many of the parameters which affect hole size... 

A feature of the metal-ion chemistry of these large ring macrocycles is thus the structural diversity which may occur from one system to the next. This diversity can result directly from small changes in the structure of the cyclic ligand and is also aided by the inherent flexibility of the large rings involved. It is clearly also influenced by the nature of the other ligands available for complex formation. 

The macrocycle types discussed so far tend to form very stable complexes with transition metal ions and, as mentioned previously, have properties which often resemble those of the naturally occurring porphyrins and corrins. The complexation behaviour of these macrocycles contrasts in a number of ways with that of the second major category of cyclic ligands - the crown polyethers. 

Thermodynamic aspects of the interaction of metal ions with macrocyclic ligands have been well studied. In many instances such studies have involved a comparison of the behaviour of cyclic ligand systems with that of their open-chain analogues. In this manner, information concerning the thermodynamic consequences arising from the cyclic nature of the macrocyclic ligand has been obtained. Frequently these studies have been restricted to stability constant (log K) measurements and, for such studies, a variety of techniques has been employed (Izatt etal., 1985). 

The kinetics and mechanism of formation and dissociation of macrocyc-lic complexes is an area covering a wide range of behaviour. Indeed, the mechanistic details of a particular reaction are often closely associated with both the type of metal ion present and the structural features of the cyclic ligand. As such, there are often difficulties in defining general mechanisms which have wide applicability. In this discussion, some representative reactions are considered with emphasis on those features arising from the cyclic nature of the respective systems. 

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. 

The dissociation kinetics of macrocyclic complexes have received considerable attention, especially during investigations of the nature of the macrocyclic effect. Before discussing the dissociation of cyclic ligand species, it is of benefit to consider some aspects of the dissociation of open-chain ligand complexes. 

The capacity of cyclic ligands to stabilize less-common oxidation states of a coordinated metal ion has been well-documented. For example, both the high-spin and low-spin Ni(n) complexes of cyclam are oxidized more readily to Ni(m) species than are corresponding open-chain complexes. Chemical, electrochemical, pulse radiolysis and flash photolysis techniques have all been used to effect redox changes in particular complexes (Haines McAuley, 1982) however the major emphasis has been given to electrochemical studies. 

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