Tris-bidentate metal complex

Figure 1 Chiral (enantiomeric) forms of an octahedral tris (bidentate) metal complex.

Hexa-coordinated tris-bidentate metal complexes exist as A- and A-enantiomers which may form diastereomeric host-guest complexes with native and modified CDs. This is exemplified by the A- and A-tris-(l,10-phenan-tholine)ruthenium(II) complexes A- and A-[Ru(phen)3] ", ... 

Representative example of tris-chelated metal complexes with Do, symmetry [Cr(en)3]3+. The bidentate ligand ethylenediamine is represented by N—-N. 

As with hydroxamate siderophores, simple tris(catecholato) metallate(lll) complexes have served as models for enterobactin. Unlike hydroxamate, catecholate is a symmetric, bidentate ligand. Consequently, there are no geometrical isomers of simple tris(catecholato) metal complexes, and only A and A optical isomers are possible. However all siderophore catecholates are substituted asymmetrically on the catechol ring, such that geometric isomers may in principle exist. However, in the case of enterobactin molecular models show only the more symmetric cis chelate is possible, as the A or A form. 

Organometallic compounds apart, oxidation states below - -2 are best represented by complexes with tris-bidentate nitrogen-donor ligands such as 2,2 -bipyridyl. Reduction by LiAlH4 in thf yields tris(bipyridyl) complexes in which the formal oxidation state of vanadium is -1-2 to —1. Magnetic moments are compatible with low-spin configurations of the metal but. 

There are more examples of a second type in which the chirality of the metal center is the result of the coordination of polydentate ligands. The easiest case is that of octahedral complexes with at least two achiral bidentate ligands coordinated to the metal ion. The prototype complex with chirality exclusively at the metal site is the octahedral tris-diimine ruthenium complex [Ru(diimine)3 with diimine = bipyridine or phenanthroline. As shown in Fig. 2 such a complex can exist in two enantiomeric forms named A and A [6,7]. The bidentate ligands are achiral and the stereoisomery results from the hehcal chirality of the coordination and the propeller shape of the complex. The absolute configuration is related to the handness of the hehx formed by the hgands when rotated... 

In general, metal complexes of bidentate formazans have low stability towards acids and, following the comparable work on metal complexes of azo compounds, Wizinger116 was prompted to investigate tri- and tetra-dentate formazans in an endeavour to obtain metal complexes having enhanced stability. 

Metallization. Bident ate formazans that are insoluble in water can be warmed with cobalt, nickel, and copper salts (preferably acetates) to form metal chelates in solvents such as methanol, ethanol, acetone, and dimethylformamide. Metal complexes of tri- and tetradentate formazans are much more stable. Metallization with divalent salts occurs rapidly at room temperature. On reaction with diazo-tized 2-aminophenols or 2-aminonaphthols, coupling and metallization with divalent metal salts can take place concurrently under the same conditions. When coupling is complete, the dye is usually fully metallized. 

Octahedral Six Coordinate In addition to the tris-bidentate complexes and the unidentate complexes discussed above, there are many other examples of chiral metal complexes that are based on octahedral geometry. For example, there are a number of reports of edge-sharing binuclear structures containing four bidentate chelates. The individual metal centers in these binuclear species can exist as either A or A enantiomers, and, therefore, the overall structure can be chiral AA and AA, or the achiral (meso) AA pair as illustrated in Figure 5.12a and b. Studies of complexes of this type were important in the proof of Werner s coordination theory. In these pioneering studies, Werner was able to prove by experiment the existence of the three isomers (AA, AA, and AA) of [Co2(p-NH2)( l-NC>2)(en)2]4 +. 23 For Cr(III) complexes, Werner showed that the / >.vo-AA- Cr2(p-OH)2(en)414 1 may be prepared by... 

E.s.r. spectra have been recorded for a large range of tris(bidentate sulphur) metal complexes, including [Ru(S-S)3]"- (S-S = sacsac, d.s-l,2-dicyanoethylene-l,2-dithio-late, dimethyldithiocarbamate, diphenyldithiophosphinate, or dithio-oxalate n = 0 or 3).26 Some of the complexes were prepared for the first time. The near isotropic g-values obtained indicate large low-symmetry distortion, both geometric and electronic in origin. 

Compared with the T-symmetric edge-bridged complexes described in Sect. 3, there are far fewer examples of T-symmetric complexes in which the octahedrally coordinated metal centers in the vertices of a tetrahedron are linked by tripodal tris-(bidentate) ligands that occupy the tetrahedral faces. In a one-pot reaction, the tetranuclear iron(III) chelate complex [Fe4(L7)4] (22) was generated from benzene-1,3,5-tricarboxylic acid trichloride 20 and bis-tcrt-butyl malonate 21 (R1 = /Bu). Alternatively, hexanuclear trigonal antiprismatic iron chelate complex [Fe6(L8)6] (23) was formed starting from bis-para-tolyl malonate 21 (R2 = pTol) by employing the same reaction conditions as for the synthesis of 22 (Scheme 8) [86-90],... 

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