Macrocyclic ligands properties

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... 

Coordination compounds composed of tetrapyrrole macrocyclic ligands encompassing a large metal ion in a sandwich-like fashion have been known since 1936 when Linstead and co-workers (67) reported the first synthesis of Sn(IV) bis(phthalocyanine). Numerous homoleptic and heteroleptic sandwich-type or double-decker metal complexes with phthalocyanines (68-70) and porphyrins (71-75) have been studied and structurally characterized. The electrochromic properties of the lanthanide pc sandwich complexes (76) have been investigated and the stable radical bis(phthalocyaninato)lutetium has been found to be the first example of an intrinsic molecular semiconductor (77). In contrast to the wealth of literature describing porphyrin and pc sandwich complexes, re-... 

Optical Properties. The double-decker complexes of porphyrazines have characteristic electronic absorption spectra (Table V). The intense Soret bands of the double-decker complexes are blue shifted with respect to the single pz ligand as a consequence of the strong n-n interactions. Another characteristic of sandwich compounds is the additional appearance of absorption bands shifted to the red (termed Q ) and to the blue (termed Q") of the normal g-band region. These new transitions are thought to result from orbitals delocalized over the two macrocyclic ligands (33, 82). 

After the separation of the actinides from the high-level waste, it is desirable to remove certain other fission products from the nuclear wastes. Some Cs and Sr are low-charged cations that react well with macro-cyclic ligands (e.g., crown ethers, calixarenes). Research to synthesize and investigate the properties of macrocyclic ligands for application in nuclear waste treatment has been an active effort internationally. Some of the results obtained are discussed in section 12.7. 

All these methods have found applications in theoretical considerations of numerous problems more or less directly related to solvent extraction. The MM calculated structures and strain energies of cobalt(III) amino acid complexes have been related to the experimental distribution of isomers, their thermodynamic stability, and some kinetic data connected with transition state energies [15]. The influence of steric strain upon chelate stability, the preference of metal ions for ligands forming five- and six-membered chelate rings, the conformational isomerism of macrocyclic ligands, and the size-match selectivity were analyzed [16] as well as the relation between ligand structures, coordination stereochemistry, and the thermodynamic properties of TM complexes [17]. 

The configuration of the macrocyclic ligand affects the electrochemical properties of Ni(II) complexes (Table I) (56a, 54). For example, the oxidation and reduction potentials of CR,S,R,S)-[Ni(14)]2+ are shifted by +0.14and +0.13 V, respectively, compared with those of the Rfi,S,S isomer. Similar trends are also observed for a series of R,Sfi,S and Rfi,S,S isomers of -methylated cyclam derivatives (61a, 61b). The anodic shift of the redox potentials for the i ,S ,S-Ni(II) complex indicates that the complex is more difficult to oxidize to Ni(III) but easier to reduce to Ni(I), compared with the RJl,S,S complex. This may be related to the reduced ligand field strength of the R,Sfi,S complex, which stabilizes the antibonding -orbitals and thus makes addition of an electron more favorable while removal of an electron is less favorable. 

Receptor and substrate are terms describing the species involved in complex formation. Throughout the chapter the receptor will refer tp the macrocyclic ligand, the substrate to other interacting species. Substrates may be metal or molecular catibns, neutral molecules, or atomic or molecular anions. The terms receptor and substrate imply that the complex formed has the well-defined structural and chemical properties of a supermolecule, as in biological receptor-substrate associations. They exclude species formed only in the solid state (clathrates). They are also easily converted and understood in many languages. 

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