Ligand structures hexadentate ligands

Hexadentate ligands l,12-bis(2-pyridyl)-5,8-dioxa-2,ll-diazadodecane and l,12-bis(2-pyridyl)-5,8-dithia-2,ll-diazadodecane have pyridine, amine, and ether or thioether donors offering an N402 or N4S2 donor set. The X-ray structures of the zinc complexes are distorted octahedral with the zinc center encapsulated by the ligand.898... 

Give one example each (either structure or name) of a monodentate ligand and a hexadentate ligand. Explain what is meant by monodentate. 

The racemic Fe(III) complex containing a hexadentate ligand, Fe(5-Br-ehpg) , can exist in racemic RR and SS) and meso (RS) forms (Structures 7a and 7b) which have different solubilities in methanol. 

There are only trinuclear and tetranuclear manganese(III) complexes reported with hexadentate Schiff base ligands. Two such hexadentate ligands, bamen and Saltren (Figure 19), form unique core structures. 

The potentially heptadentate Schiff base, obtained from tris(2-aminoethyl)amine and three moles of SAL and shown in Figure 21a, reacts with trivalent metal ions to form neutral 1 1 compounds, and in so doing acts as a hexadentate ligand. An X-ray structural analysis of an Fem complex has shown that the tertiary nitrogen atom of the amine is not bonded to Fe.144... 

Figure 7-9. The condensation of the cobalt(m) complex of the hexadentate ligand 7.4, which contains three primary amino groups, with formaldehyde and ammonia, gives the encapsulated complex 7.5. A view of the cation 7.5 as found in the solid state structure of its perchlorate salt is also presented.

The siderophore enterobactin (enterochelin) (64) is a cyclic lactone of three N-(2,3-dihydroxybenzoyl) L-serine moieties produced by E. coli under iron stress. Enterobactin (64) was first isolated from iron-limited cultures of Salmonella typhimur-ium [83], E. coli [84], and Aerobacter aerogenes [84]. Structural analysis has confirmed that 64 chelates iron as a hexadentate ligand via the two hydroxyl groups on each catechol moiety (see Fig. 13) [85]. Of all the siderophores characterized to date, 64 has been shown to have the highest affinity for ferric iron, with a stability constant of 1052 M 1 [86, 87], which is remarkable, considering the affinity of EDTA for iron is 27 orders of magnitude lower. In mammals, serum albumin [88] and siderocalin [89, 90] bind the hydrophobic 64 which impedes siderophore-mediated transfer of iron to bacteria. Consequently, bacteria such as E. coli and... 

Except the propylenediaminetriacetic acid, recently described by Maricondi and Maricondi99, no optically active ligands are known which react only as pentadentate ones. More frequent are ligands of the three structural types F, G, and H. According to the nature of the central metal atom and the reaction conditions these compounds may react as penta- or hexadentate ligands. 

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