Other Macrocyclic Ligands

Invariably hydrolytic instability of lanthanide Por complexes, particularly that observed for the larger lanthanide elements, negatively influences their prospective application in terms of biomedicine (Sect. 7.4). As a response to this problem, larger porphyrin-like or expanded porphyrins , the so-called texaphyrins (Tx), have been examined by Sessler et al. [246]. The motivation, that expanded systems better accommodate larger ions, was previously demonstrated in a uranyl superphthalocyanine (SPc) complex [247], This SPc-complex contains an expanded, cyclic five-subunit pentakis(2-iminoisoindoline) which is formed by a template reaction of o-dicyanobenzene with anhydrous uranyl chloride. The uranium is displaced by only 0.02 A from the mean N5-plane. 

The central hole or binding core in texaphyrins is roughly 20% larger than that of the porphyrins and accomplishes pentadentate binding [247]. In the course of the studies to exploit coordination behavior with lanthanide cations a serious of monoanionic texaphyrine complexes have been synthesized and fully characterized (Eq. 17, Table 20) [248], 

Meso-porphyrinogens formally represent the structural transition between pyrroles and porphyrins. Stimulated by the work of Floriani et al. [251], porphyrinogens were considered as alternative ligand sets in lanthanide chemistry by Jubb and Teuben [68,252]. The tetraamide ligand offers a coordination environment which is obviously sterically and electronically more flexible than that provided by the standard Cp2Ln- fragment. The conformational flexibility 

Distance from the mean plane of five coordinating nitrogens 

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Other macrocyclic ligands 218 with a single pendant arm have been prepared from l,4,7-triazacyclio[5.2.1.04 10]-decane 40 <1996ICA(246)343> by reacting with appropriate alkyl bromide/alkenyl bromide followed by base hydrolysis (Scheme 34). 

AC and AEC complexation is also effected efficiently by other macrocyclic ligands such as the spherands 13, cryptospherands 14 [2.9, 2.10], calixarenes [2.38, A.6, A.23], torands [2.39], etc., some of them, for instance the spherands displaying particularly high stabilities. A special case is represented by the endohedral complexes of fullerenes in which the cation (Sr2, Ba2+, lanthanides) is locked inside the closed carbon framework [2.40],... 

Since the discovery of crown ethers, cryptands, and other macrocyclic ligands by Cram, Lehn, and Pedersen, who were awarded the 1987 Nobel Prize in chemistry for their development and use of molecules with structure-specific interactions of high selectivity [1], a completely new research field was opened supramolecular chemistry [2-4-]. Since then, this research field has been extended in many fields such as molecular recognition, organic sensing, and liquid crystals. 

Iron and Ru porphyrins and phthalocyanines , which are models of biological systems, react with CO at an axial position. Solvent molecules can be involved in the exchange see also ref. 23. Other macrocyclic ligands, e.g., tetradyidines, behave similarly. 

Compounds of porphyrins and other macrocyclic ligands Compounds with P- and N, P-donor ligands... 

In contrast to the large number of studies concerned with porphyrins and phthalocyanines, other macrocyclic ligand compounds of Mn" have received somewhat less attention. In part, this... 

In contrast to the large number of studies concerned with porphyrins and phthalocyanines, other macrocyclic ligand compounds of Mn have received somewhat less attention. - In part, this appears to reflect the fact that many compounds of this type tend to be of only moderate thermodynamic stability and in some instances are air and/or moisture sensitive. In addition, Mn has been shown to be ineffective as a template metal for certain cyclization reactions which occur in the presence of other ions. - This effect may be a consequence of the relatively large size of the Mn" ion not enabling it to fit in the cavity of the target macrocycle. Alternatively, the often low affinity of Mn" for the macrocycle precursors may contribute to the absence of a template effect for particular systems. 

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