Mechanism lanthanide complexes

Similar polymerization of MMA using enolate-zirconocene catalysts has also been found [223]. The mechanism of this catalytic reaction is related to the process described in Scheme XI because the cationic enolate complex is isolobal to that of the corresponding lanthanide complex. Recently, similar cationic... 

Shortly after the key mechanistic papers on rhodium-catalyzed hydroboration, Marks reported a hydroboration reaction catalyzed by lanthanide complexes that proceeds by a completely different mechanism.63 Simple lanthanide salts such as Sml3 were also shown to catalyze the hydroboration of a range of olefins.64 The mechanism for this reaction was found to be complex and unknown. As in other reactions catalyzed by lanthanides, it is proposed that the entire catalytic cycle takes place without any changes in oxidation state on the central metal. 

Various enzymes are capable of degrading the RNA polymer by splitting the bonds between ribose and phosphate. The same reaction has been carried out nonenzymatically with lanthanum and cerium (III) nitrates (2) the mechanism of such a reaction probably involves the formation of a lanthanide complex, followed by cleavage of the phosphate bond ... 

It has been stated above that for a series of isostructural lanthanide complexes the paramagnetic shifts of analogous nuclei should be proportional to the parameter A, if a purely dipolar mechanism operates. The paramagnetic shift is usually taken as the difference between the chemical shift of the nucleus in the paramagnetic complex under investigation, and the... 

A few observations of photosubstitution in lanthanide complexes have been reported. Irradiation into the f—f bands of [Pr(thd)3], [Eu(thd)3] and [Ho(thd)3] (thd is the anion of 2,2,6,6-tetramethyl-3,5-heptanedione) results in substitution of thd by solvent.153 The proposed mechanism involves intramolecular energy transfer from an f—f excited state to a reactive IL excited state which is responsible for the observed ligand loss. Photosubstitution has also been observed upon direct excitation into the ligand absorption bands of [Tb(thd)3].154... 

It is also often assumed that A is constant along a series of lanthanide complexes with the same ligand. The mechanism providing spin density delocalization from the metal to the ligands is due to a weak covalent bonding involving the 6s metal orbital, which, in turn, can transfer unpaired spin density on nearby nuclei through spin polarization from 4f orbitals [82]. 

Ans. No. Other mechanisms (radical, heterolytic, etc.) may be available with transition metal complexes. With lanthanide complexes in the highly stable 3 + oxidation state, an OA/RE-based mechanism is not possible. 

Hydrolytic catalysis by metal ions is also important in the hydrolysis of nucleic acids, especially RNA (36). Molecules of RNA that catalyze hydrolytic reactions, termed ribozymes, require divalent metal ions to effect hydrolysis efficiently. Thus, all ribozymes are metalloenzymes (6). There is speculation that ribozymes may have been the first enzymes to evolve (37), so the very first enzymes may have been metalloenzymes Recently, substitution of sulfur for the 3 -oxygen atom in a substrate of the tetrahymena ribozyme has been shown to give a 1000-fold reduction in rate of hydrolysis with Mg2+ but no attenuation of the hydrolysis rate with Mn2+ and Zn2+ (38). Because Mn2+ and Zn2+ have stronger affinities for sulfur than Mg2+ has, this feature provides strong evidence for a true catalytic role of the divalent cation in the hydrolytic mechanism, involving coordination of the metal to the 3 -oxygen atom. Other examples of metal-ion catalyzed hydrolysis of RNA involve lanthanide complexes, which are discussed in this volume. 

Several of the complexes in Figure 2 were examined further for their resistance to dissociation. The europium complexes Eu(THED)3+ and Eu(TCMC)3+ were more difficult to study quantitatively by H NMR because of their broad H resonances. Decomposition was monitored by use of a UV-vis assay. Excess Cu2+ was added to solutions containing the lanthanide macrocycles. The Cu2+ ion served the dual purpose of trapping the free macrocycle and as an indicator to monitor the amount of macrocycle that had dissociated. All Cu(II) macrocyclic complexes gave an absorbance peak in the UV-vis spectrum that was characteristic of the Cu(II) macrocycle complex. For all macrocycles, Cu2+ was an effective trap formation of the Cu(II) macrocyclic complex went to completion in the presence of 0.10 mM La3+ or 0.10 mM Eu3+, 0.10 mM ligand and excess Cu2+ (1.0 mM). The increase in the concentration of Cu(II) macrocycle complex over time is a measure of the inertness of the lanthanide complex to dissociation. For the La(THED)3+ complex, the reaction rate (51) was independent of the concentration of Cu2+, consistent with the following mechanism ... 

Gruen [80] found that the vapor of Ndl3 of D3h symmetry with a spherical harmonic k = 1 does not transform as a totally symmetric representation. This is a case that cannot be explained on the basis of symmetric representation of hypersensitivity. Since Ndl3 vapor molecules are in a relatively homogeneous dielectric, the pseudoquadrupole mechanism is not operative. A vibronic mechanism including covalency has been advanced to explain the origin of hypersensitivity in lanthanide complexes. 

The dynamic coupling mechanism predicts that hypersensitivity should be observed when the point group of the lanthanide complex contains Y3m spherical harmonics in the expansion of the point potential. The good agreement between the calculated and observed values for Tj or Q.2 parameters shows that the dynamic coupling mechanism makes a significant contribution to the intensities of the quadrupole allowed f-electron transitions in lanthanide complexes. Qualitatively, the mechanism is allowed for all lanthanide group symmetries in which the electric quadrupole component 6,a,fi and the electric dipole moment p, a transform under a common representation. 

Crystal field theory, intensities of 4f-4f transitions, Judd-Ofelt theory of electric-dipole transitions, covalency model of hypersensitivity, dynamic coupling mechanism, solution spectra, spectral data for complexes, solvent effects, fluorescence and photochemistry of lanthanide complexes are dealt with in spectroscopy of lanthanide complexes. 

The reaction is catalyzed by lanthanide complexes CpjfLnR,41 although noble metal catalysts, notably rhodium, are most widely applied, particularly in asymmetric hydroboration,42 The mechanism is likely to be similar to hydrosilylation. The products may be oxidized with H202 and converted to alcohols or amines. 

In 1990, Buono-core suggested a simplified diagram to show the three different mechanisms for intra-molecular energy transition in lanthanide complexes (Figure 1.8). 

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