Lanthanide complexes isomerism

Moreover, MPVO reactions are traditionally performed with stoichiometric amounts of Al(III) alkoxides. Some improvements came from the use of dinuclear AI(III) complexes that can be used in catalytic amount [6, 7]. This is why there has been an ever-increasing interest in catalytic MPVO reactions promoted by lanthanides and transition-metal systems [8]. In these cases, it is believed that reaction proceeds via formation of a metal hydride, in contrast with the mechanism accepted for traditional aluminum alkoxide systems, which involves direct hydrogen transfer by means of a cyclic intermediate [9]. As well as La, Sm, Rh and Ir complexes, Ru complexes have been found to be excellent hydrogen transfer catalysts. The high flexibility of these systems makes them very useful not only for MPVO-type reactions, but also for isomerization processes [10]. 

Lanthanide(III) ions have been found to catalyze the geometric isomerization of trans-bis(oxalato)diaquochromate(III) and frany-bis(malonato)diaquochromate(III) to yield the corresponding cz.v-isomers. The proposed mechanism for the uncatalyzed and catalyzed pathway for the oxalato complex is shown in Fig. 7.29. 

In Fig. 7.30 log Ln is plotted against log K for the trans-cis isomerization of the oxalato complex. Here Ln refers to catalytic rate constants and K is the complex formation constant for lanthanide propionate complexes. The linear free energy relationship holds... 

A plot of log Ln vs log K for the malonate system is shown in Fig. 7.31. The catalyzed isomerization of the malonate complex proceeds by a different mechanism from that of the oxalato analogue because (i) lack of linear free energy relationship between the catalytic rate constants and the complex formation constants, (ii) linear relationship between catalytic rate constants (log A n) and formation constants of lanthanide malonate complexes (log K ), (iii) catalytic rate constants for the malonate system are smaller than the corresponding rate constants for the oxalate system. The proposed mechanism for... 

Fig. 7.30. log-log plot of the catalytic rate constants for the lanthanide ion catalyzed trans-cis isomerization of trans-bis(oxalato)diaquochromate(III) (25°C, / = 1.5 M adjusted with NaN03) vs. the formation constants for the lanthanide/propionate complexes (25°C, I = 2.0 M) [84]. 

Geometrical isomerism is potentially possible among many of the lanthanide chelates. The 1,3-diketone chelates, the species derived from the aminepolycarboxylic acids, the tropolonates, and many other complex derivatives are asymmetric and, thus, potentially capable of exhibiting optical isomerism. That the only resolutions reported have been limited to some tris (diketone) compounds (35) may refiect more the tendency of these ionically-bonded species to racemize rapidly in polar environments than in the absence of asymmetry. The existence of more than a single modification of each of several tetrakis(di-keto)europium(III) compounds may refiect asymmetry also (1). 

The synthesis of the corresponding organolanthanide chlorides was performed by transmetallation of the dilithium salt with anhydrous lutetium or yttrium trichloride. Metallocene dichloro complexes [(/ ,Y)-Me2Si(C5H3But)((+)- -Men-Cp)]Ln(/r-Cl2)Li(OEt2)2 (Ln = Y, Lu) were synthesized by treatment of the corresponding lanthanide trichlorides with the dilithium salt of the ligand and isolated isomerically pure by crystallization from diethyl ether (Scheme 162).650... 

The easy isolation of complexes of the carbinolamine macrocycle III appears to be a feature unique to the combination of 2,6-diformylpyridine, 1,2-diaminoethane, and certain lanthanide(III) nitrates. No carbinolamine complexes were isolated from the same diamine and dicarbonyl precursors when the condensation was carried out in the presence of yttrium(III) acetate (Bombieri et al. 1989b). Similarly, the use of 1,2-or 1,3-diaminopropane in place of 1,2-diaminoethane yielded only trinitrate complexes of the tetraimine macrocycles IV and V (Abid and Fenton 1984b). Complexes of ligand IV are expected to exist in various isomeric forms, since a (2 - - 2) cyclic condensation involving an unsymmetrically substituted diamine precursor may yield... 

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