Crown thioethers, coordination chemistry

Crown thioethers, coordination chemistry, 35 3 (CIO4), 43 226 Crude oil, vanadium in, 35 99 Crustaceans, arsenic in, 44 150, 167, 168, 170 Cryoscopic measurements, sulfuric acid and, 1 390-391... 

Fruitful exploration of crown thioether coordination chemistry also had to await the routine availability of X-ray diffraction facilities. The paramagnetism of many crown thioether complexes vitiates the utility of NMR, while uninformative charge transfer bands dominate their optical spectra. Hence X-ray diffraction has proven indispensable to the development of crown thioether chemistry it provides one of the few ways of determining the ligand denticity, as well as the coordination geometry and stereochemistry at the metal. More fundamentally, however, the issues raised by these complexes often focus on metrical features and ligand... 

If most of the research on crown and cage thioethers has concerned their synthesis and exploratory work on their coordination chemistry, their complexing properties and capacity to stabilize low oxidation states of transition metals should be expected to lead to new applications, as for their oxygen and nitrogen analogues. 

Another impetus for the study of the coordination chemistry of crown thioethers stems from the role of thioether binding in biological systems such as d-biotin (involving tetrahydrothiophene) (145, 208) and blue copper proteins such as plastocyanin and azurin (involving methionine) (4,13, 73,109,124,185). The binding of Cu(II) and Cu(I) centers to macrocyclic thioethers has led to a greater understanding of Cu-S(thioether) interactions and the stereochemical preferences of these metal centers (91, 95, 99,121,180,181). 

Crown thioethers have found a number of uses as ligands, related particularly to their affinity for late- and post-transition elements. Attention has focused on the coordination chemistry of silver, with relevance to photography, silver-selective electrodes, and ligands for chromogenic analysis and recovery of silver. Biomedical applications include the removal of toxic heavy metals such as Cd, Hg, Pb, and T1 and delivery of radioisotopes such as ""Tc, Re, and Re to specific sites in the body. Finally, some crown thioethers have been found to promote novel reactivity in their transition metal complexes, including activation of small molecules such as N2, CO and C2H4. 

For some years we and others have investigated the chemistry of crown thioethers such as 9S3 (1,4,7-trithiacyclononane) (Figure 3). This work has resulted in development of facile synthetic routes to these previously precious compounds, understanding of their unusual confonnational properties, and providing extensive infonnation on their coordination chemistiy. From this scientific infrastructure we turned our attention to how these... 

Despite this interest in crown thioethers, arduous synthetic routes to the ligands impeded extensive investigation of their chemistry until recently. However, advances in synthetic methodology in the last five years has opened the door to work on the coordination chemistry of these ligands. This is particularly true of 9S3, the first synthesis of which proceeded in such low yield (0.04%) as to preclude further study [11]. 

A recurrent theme in the coordination chemistry of crown thioethers concerns the interplay between conformational preferences of the ligands and their coordinative behavior. In particular, the structures of complexes result from a compromise between the conformational preferences of the ligands and the electronic requirements of the metal ion. Crown thioethers such as 12S4 show a diminished propensity for chelation because of the exodentate orientation of the S atoms in the free ligand. Exodentate structures reflect the antipathy of most crown thioethers to chelation. As a consequence, complexes with incomplete chelation by the ligand form a substantial fraction of this review. 

The first group includes the compression of metal coordination spheres observed in, e.g. [Ni(9S3)2] and [Ni(18S6)] ". The second is in many respects more interesting. Use of crown thioethers can afford complexes in which thioethers dominate the coordination sphere. This may induce unusual chemistry not directly because of the crown, but because of the number of thioether groups it imposes on the metal ion. Nevertheless, because such coordination spheres are often unattainable without use of crown thioethers, it is appropriate to attribute the unusual behavior to use of these ligands. 

In addition to stabilizing lower oxidation states, crown thioethers can also be used to manipulate the coordination geometry of a metal ion. The elegant work of Rorabacher, Ochrymowycz, and coworkers demonstrates the use of closely related crown thioethers to study how coordinative plasticity affects the thermodynamics and kinetics of electron transfer [149,170], The same approach could be used with equal profit on fundamental studies on the interrelation of ligand conformation and binding affinity. The importance of such studies transcends crown thioether chemistry, which merely provides ideal systems in which to work out the requisite concepts. 

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