Crown thioethers, coordination

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... 

Irregular 6- and 7-fold coordination of Sb occurs in the complexes of SbCl3 with crown thioethers, and 8-fold coordination has been established in its complex with the j -ether... 

The ability of thioether macrocyclic complexes (and especially those of [9]aneS3) to support multi-redox behaviour at the coordinated metal centre is particularly notable. This allows a series of reversible stepwise one-electron oxidation and/or reduction processes, and stabilization of highly unusual transition metal oxidation states e.g. mononuclear [Pd([9]aneS3)2]2+/3+/4+,149 [Au([9]aneS3)2]+/2+/3+,150 [Ni([9]aneS3)2]2+/3+,151 and [Rh([9]aneS3)2]+/2+/3+.152 It appears to be the ability of the crown thioethers to readily adjust their... 

In certain systems the crown thioether ligand itself may undergo some chemical transformation when coordinated to a metal ion. For example, in mildly basic conditions the Rh(III) species [Rh([9]aneS3)2]3+ and [Rh([18]aneS6)]3+... 

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). 

Contrary to hard and soft acid-base predictions, the first example of a vanadyl-thioether complex has been made (151).693 The crown thioether 1,4,7-trithiacyclononane forms a stable 1 1 coordination complex upon reaction with VC13 the presence of adventitious water likely explains the hydrolysis/oxidation of V111 to form VlvC)21. Complexes with a variety of bi- and tetradentate S-donor functionalities have been prepared as model complexes for nitrogen-ase.498,694 These complexes have some structural similarities to the binding site of nitrogenase, however, in contrast to the cofactor they fail to convert N2 to NH3. 

Conformation and sulfiu- atom placement in crown thioethers are of paramount importance to the coordination of metals. For example, endo orientation (i.e. towards the central cavity) of all three sulfur atoms in 9S3 ideally suits facial coordination to octahedral metal centers in contrast, exo placement of the sulfur atoms in 14S4 reduces the macrocyclic effect and often leads to bridging between metal ions. 

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]. 

Crown thioethers prompt attempts to impose poly(thioether) coordination upon metal ions, even those for which few thioether complexes were previously known. As a class, simple thioethers (e.g. MejS) are not particularly good ligands for both electronic and steric reasons. Thioethers exhibit weaker a-donor and 7t-acceptor ability than, for example, phosphines. Consequently they also show less binding affinity. In addition, the appreciable bulk of their terminal alkyl groups further hinders complexation. This latter factor is partially circumvented in acyclic polythioethers, and such ligands display respectable chelate effects. 

In addition, crown-type ligands can also alter the properties of a metal ion by, e.g. constricting or dilating its coordination sphere (the macrocyclic constriction effect [16,17,18]). Both through imposition of an unusual environment - comprising predominantly or exclusively thioether coordination - and through manipulation of that environment, coordination complexes of crown thioethers often exhibit unusual properties and reactivities. Crown thioether complexes are an ideal system in which to study the effect on optical and redox properties of geometric deformations of the coordination sphere. 

Crown thioethers such as 9S3 and 18S6 commonly enforce six-coordination even in some cases where lower coordination numbers might have been expected. In addition, through use of different crown thioethers it is possible to study how compression/dilation of the MfSRj) coordination sphere affects, e.g. redox potentials and hyperfine splittings. Thus crown thioethers provide an excellent means not only of imposing a high-symmetry homoleptic thioether environment, but also... 

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