Octahedral-type complexes

REDOX POTENTIAL-STRUCTURE RELATIONSHIPS AND PARAMETERIZATION 50.2 PARAMETERIZATION OF LIGANDS AND METAL CENTERS 50.2.1 Octahedral-Type Complexes... 

The application of the initial Lever model to nr-sandwich or r-half-sandwich complexes requires the assumption of the validity of Eq. 50.3 to such types of complexes and of the and /jyi values quoted for the metal redox couples in octahedral-type complexes. Although full confirmation of this hypothesis requires the consideration of wider series of complexes, it has been applied by us [32] to half (or full)-sandwich complexes bearing itt-aromatic (benzene or p-cymene) ligands or polydentate boron-based or carbon-based scorpionate-type ligands. For the estimate of the corresponding El values for such polytopal ligands (based on Eq. 50.3), the El values of the co-ligands in their complexes should be known, as well as the and 7, values of their metal redox couples. 

The cationic aqua complexes prepared from traws-chelating tridentate ligand, R,R-DBFOX/Ph, and various transition metal(II) perchlorates induce absolute enantio-selectivity in the Diels-Alder reactions of cyclopentadiene with 3-alkenoyl-2-oxazoli-dinone dienophiles. Unlike other bisoxazoline type complex catalysts [38, 43-54], the J ,J -DBFOX/Ph complex of Ni(C104)2-6H20, which has an octahedral structure with three aqua ligands, is isolable and can be stored in air for months without loss of catalytic activity. Iron(II), cobalt(II), copper(II), and zinc(II) complexes are similarly active. 

Considerable investigation of the octahedral carbonyl complexes has been carried out. To a certain degree this is because definitive evidence for associative substitution in the case of type A complexes has been conspicuously lacking whereas for the type B compounds there seem to be several well-substantiated examples. A general summary of the main types of octahedral substitutions which have been kinetically examined is given in Table 15. 

Optical activity in metal complexes may also arise either if one of the ligands bound to the metal in the first co-ordination sphere is itself optically active or if the complex as a whole lacks a centre of inversion and a plane of symmetry. Thus all octahedral cts-complexes of the tris-or bis-chelate type have two isomeric forms related by a mirror plane, the d- and /-forms. These species have circular dichroism spectra of identical intensities but opposite in sign. The bands in the circular dichroism spectrum are, of course, modified if ligand exchange occurs but they are also exceedingly sensitive to the environment beyond the first co-ordination sphere. This effect has been used to obtain association constants for ion-pair formation. There also exists the possibility that, if such compounds display anti-tumour activity, only one of the mirror isomers will be effective. 

Since we shall not obtain the comparable amount of detailed information on the mechanisms of substitution in octahedral complexes from the studies of more complicated substitutions involving chelation and macrocycle complex formation (Secs. 4.4 and 4.5) it is worthwhile summarizing the salient features of substitution in Werner-type complexes. 

Experimentally based intuitive arguments have been presented to arrive at a regional rule for optical activity of d-d transitions of conformational isomers of octahedral metal complexes. Conformational preferences for chelate rings formed by 1,3-pn in its octahedral mono, bis, and tris metal complexes have been studied by calculation of the conformational energies. In all cases, the chair conformation was found to be the most stable. The lowest energy pathway for converting from one chair configuration into another has a barrier to activation of about 7 kcal mol Conformational types of metal-edta complexes have been studied. ... 

In early studies of these reactions, the turnover efficiency was not always high, and stoichiometric amounts of the promoters were often necessary to obtain reasonable chemical yields (Scheme 105) (256). This problem was first solved by using chiral alkoxy Ti(IV) complexes and molecular sieves 4A for reaction between the structurally elaborated a,/3-unsaturated acid derivatives and 1,3-dienes (257). Use of alkylated benzenes as solvents might be helpiul. The A1 complex formed from tri-methylaluminum and a C2 chiral 1,2-bis-sulfonamide has proven to be an extremely efficient catalyst for this type of reaction (258). This cycloaddition is useful for preparing optically active prostaglandin intermediates. Cationic bis(oxazoline)-Fe(III) catalysts that form octahedral chelate complexes with dienophiles promote enantioselective reaction with cyclopentadiene (259). The Mg complexes are equally effective. 

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