Tris complex, racemization

The meso-cptn (RS-cptn) ligand chelates much more readily than ( )-cptn the tris complexes of meso-cptn also generate the stereoisomers mer and fac. The complexes of [Co(i S-cptn)3] have been separated into A and A forms (59). Very little is known about the conformation of the rings in these complexes. The reduced strain in comparison to the complexes of the racemic ligand is apparent from the relative lability of the two complexes. 

The optically active ligands, RR-ptn and SS-ptn, adopt skew boat conformations in which both methyls are equatorial (76). Thus RR-ptn adopts the X conformation, and SS-ptn forms the 8 conformation of the skew boat. The meso form of the ligand, RS-ptn, however, must adopt a chair conformation if both methyls are to be equatorially oriented. Tris complexes composed solely of RR- (or SS-) ptn are therefore restricted to the lela and 083 isomers. Molecular mechanics calculations have predicted that the lel conformation of complexes with the racemic ligands (e.g., A XXX) is significantly (4-7 kJ mol ) more stable than the 083 (A XXX) (77, 78). The tris chair form of RS-ptn was estimated to be very similar in strain energy to the RR-ptn leh complex (78). 

The cis isomer is defined as that compound in which the three methyl keto groups are on adjacent corners of the octahedral structure the trans isomer has two of the methyl keto groups on opposite apexes of the octahedron. Both the cis and trans isomers should be di-racemates, but their resolution has not been reported. The structures of the isomers are assigned on the basis of their different chromatographic adsorption (the cis isomer adheres more strongly to alumina) and the ratio of isolated products (the trans isomer predominates) by analogy with the work of Fay and Piper on tris complexes of unsymmetrical 1,3-diketones. 

In the case of tris complexes with symmetrical ligands, the process of inversion (interconversion of enantiomers) is of considerable interest. When the metal ions are of the inert type, it is often possible to resolve the complex and then the process of racemization can be followed by measurement of optical rotation as a function of time. Possible pathways for racemization fall into two broad classes (1) Those without bond rupture. (2) Those with bond rupture. 

Cobaltate, tris(l, 2-ethanediamine)-racemization solid state, 466 Cobaltate, tris(oxalato)-racemization solid state, 467 Cobaltates sepulchrates, 22 Cobalt complexes geometric isomerism, 11 hexaammine... 

Intramolecular Rearrangement of Tris-chelate Complexes.— Rate data for the dissociation and racemization of [M(phen)3] + ions (M = Cr, Fe, Co, or Ni) are collected in Table 39. Only the nickel(ii) complex racemizes at a rate comparable to the ligand... 

Optically active thiiranes have been obtained by resolution of racemic mixtures by chiral tri-o-thymotide. The dextrorotatory thymotide prefers the (5,5)-enantiomer of 2,3-dimethylthiirane which forms a 2 1 host guest complex. A 30% enantiomeric excess of (5,5)-(—)-2,3-dimethylthiirane is obtained (80JA1157). 

Iron, tris(hexafluoroacetylacetone)-structure, 1,65 Iron, tris(oxalato)-chemical actinometer, 1,409 photoreduction, 1,471 relief-image-forming systems, 6,125 Iron, tris(l,10-phenanthroline)-absorptiometry, 1,549 racemization, 1,466 solid state, 1,467 structure, 1, 64 lron(III) chloride amino acid formation prebiotic systems, 6,871 Iron complexes acetonitrile. 4,1210 acetylacetone, 2,371 amidines... 

Nickel, tris(l, 10-phenanthroline) racemization, 1,24. 466 solid state, 1, 467 structure, 1,64 Nickel complexes, 5,1-300 acetylacetone alcoholysis, 2, 380 pyridine complexes, 2, 386 solvolysis, 2,379 structure, 2,388 amidines... 

Enantioselectivities >90% were reported for a Ti-ebthi catalyst (Table 34.4 entry 4.1) and for some Rh-diphosphine complexes (entries 4.2-4.4). Interestingly, the highest ee-values were obtained using sulfonated diphosphines (bdppsuif) in an aqueous biphasic medium (entry 4.3). The degree of sulfonation strongly affected the enantioselectivity the Rh-mono-sulfonated bdpp gave 94% ee, compared to 65% ee with Rh-bdpp in MeOH, and almost racemic product with bis-or tris-sulfonated ligands. In addition, the activity of the mono-sulfonated cata-... 

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