Bidentate chelation also

Mg11 complexes are also effective for controlling asymmetric radical reactions.33,34 Moreover, enantioselective radical reactions using chiral Mg11 complexes have been studied, and high enantioselectivities have been realized in the presence of stoichiometric or catalytic amounts of chiral auxiliaries such as bis-oxazolines (Scheme 8).35-39 In most cases, substrates having bidentate chelating moieties are required. 

Although diphosphine disulfides of formula R2P(S)P(S)R2 take up a trans conformation in the solid state (see Section 5.2.3), upon coordination to a metal centre they can rearrange to adopt a cis conformation, hence facilitating bidentate chelation of the metal (e.g., Equation 78).117 In addition, a cis-bridging mode has also been proposed for these ligands in bi-metallic complexes.117... 

BLM transport systems for ferrioxamine B were also devised based on first coordination shell recognition via ternary complex formation utilizing vacant coordination sites on the Fe(III) center (Fig. 29) (199). The tetra-coordinated substrate complex selectively transported was partially dechelated diaqua-ferrioxamine B and coordinately unsaturated di-hydroxamato iron(III) complexes, which utilized a hydrophobic membrane bound bidentate chelator as a carrier for selective transport. Active transport for these systems was accomplished using a pH gradient (199). 

Similar success was also achieved by Willoughby and Buchwald100a with a chiral titanocene catalyst. The high ee obtained by Burk and Feaster101a in the asymmetric hydrogenation of 98 was also consistent with the preferred coordination of one isomer forced by the bidentate chelation of the hydrazones. 

N-Diazeniumdiolates can also undergo some reactions other than those directly associated with NO release. They act as bidentate chelators and form complexes with a variety of transition metal ions. Suitable choice of the ligands can lead to the formation of complexes in which the diazeniumdiolates act as a monodentate ligand coordinated to Cu(II) at 02 only [223]. 

The reaction to form the palladium complex is similar to that reported for amine salts, although here, because a bidentate chelating ligand is used, no chlorine atoms are retained in the complex, and the system is easy to strip. Also, as both reactions involve initial ion pair extraction, fast kinetics are observed with 3-5 min contact time to reach equilibrium at ambient temperature. The extraction conditions can be easily adjusted in terms of acidity to suit any relative metal concentrations and, because the reagent is used in the protonated form, good selectivity over base metals, such as iron and copper,... 

A limited amount of work was also conducted by the group of Schultz to investigate the potential of N-formyl hydroxylamine head group to inhibit HDAC. This bidentate chelating moiety was used successfully in inhibitors of angiotensin converting enzyme and neural endopeptidase, however, in the case of HDAC a 50-fold loss in activity was observed vdth this group (17a-c) [33]. 

Significant deviations toward decreased stabilities of Li " complexes from a linear relationship between LCB and GB values also appear for several fluorine-substituted compounds while reasonably good correlation is observed for the S=0 (sulfoxides and sulfones) and P=0 bases. Within the class of oxygen bases the ethers give the worst correlation between basicities. This is because the lithium adducts are much more prone to bidentate chelate ethers as is evident by the enhanced stabilities of Li complexes of methoxy- and fluorine-substituted ethers. Their LCBs are therefore significantly higher than predicted from the linear relationship between LCB and GB values for unsubstituted... 

Many flavoproteins contain, besides the flavocoenzyme, also metal ions as prosthetic groups The study of the interaction between free flavoquinone and d-metal ions is, therefore, of biological interest. In aprotic polar solvents weak complexes are formed Bidentate chelates are formed involving N(5) and the carbonyl... 

In addition to chelate complexes, the cyclic amine 1,4,7-triazacyclononane will complex to platinum(II) and (IV). The hexacoordinate platinum(IV) complexes are bonded to two molecules of the tridentate ligand, but platinum(II) complexes with the ligand monodentate and bidentate.988 Also the formation of platinum(II) ammine complexes from chloride complexes is a reversible process. The rate constants decrease as the basicity of the leaving amine increases.989... 

The very interesting Sb(EtXant)4 and Bi(EtXant)4 complexes also have been reported (635). The presence of multiple C-S vibrations between 1020 and 1050 cm-1 suggests that these molecules may contain monodentate EtXant ligands. Six-coordinate central atoms with two monodentate and two bidentate chelates are very likely to be found in these compounds. 

---