Rare earth metal complexes reactivity

This chapter is intended to cover major aspects of the deposition of metals and metal oxides and the growth of nanosized materials from metal enolate precursors. Included are most types of materials which have been deposited by gas-phase processes, such as chemical vapor deposition (CVD) and atomic layer deposition(ALD), or liquid-phase processes, such as spin-coating, electrochemical deposition and sol-gel techniques. Mononuclear main group, transition metal and rare earth metal complexes with diverse /3-diketonate or /3-ketoiminate ligands were used mainly as metal enolate precursors. The controlled decomposition of these compounds lead to a high variety of metal and metal oxide materials such as dense or porous thin films and nanoparticles. Based on special properties (reactivity, transparency, conductivity, magnetism etc.) a large number of applications are mentioned and discussed. Where appropriate, similarities and difference in file decomposition mechanism that are common for certain precursors will be pointed out. 

Only a few tris-P-diketiminato rare-earth metal complexes (74,75, and 77) were prepared [64,71,72] and no catalytic activities were reported until Shen and coworkers very recent discovery on synthesis and catalytic applications of such complexes [86], To compare the electronic effects of ligands on the reactivity of their lanthanide complexes, L21 and its derivatives L28 (a methyl electron-donating group at the para-position on the phenyl) and L29 (a chloro electron-withdrawing group at the para-position on the phenyl) were used in the study. LnCb (Ln = Pr, Nd, Sm) and lithium salt of L29 via salt elimination led to [Ln(L29)3] (Ln = Pr (130), Nd (131), Sm (132)). [Nd(L21)3] (133) and [Nd(L28)3] (134) were prepared by the same method shown in Scheme 45. 

This chapter summarized recent advances in the reduction chemistry of rare earth metals and described our own efforts in synthesizing inverse sandwiches of rare earth arene complexes using ferrocene-based diamide ligands. Unprecedented molecules were synthesized and their unusual electronic structures were studied. Highlights included the synthesis of the first scandium naphthalene complex and its reactivity toward P4 activation and the isolation and characterization of a 6-carbon, lOTi-electron aromatic system stabilized by coordination to rare earth metals. The reactivity of those complexes was also discussed. 

Although early transition and rare earth metal complexes with N-heterocyclic ligands are no longer laboratory curiosities, studies on their reactivity are still extremely rare. 

A review article entitled "Bulky amido ligands in rare-earth chemistry Syntheses, structures, and catalysis" has been published by Roesky. Benzamidinate ligands are briefly mentioned in this contexD The use of bulky benzamidinate ligands in organolanthanide chemistry was also briefly mentioned in a review article by Okuda et al. devoted to "Cationic alkyl complexes of the rare-earth metals S mthesis, structure, and reactivity." Particularly mentioned in this article are reactions of neutral bis(alkyl) lanthanide benzamidinates with [NMe2HPh][BPh4] which result in the formation of thermally robust ion pairs (Scheme 55). ... 

The lanthanoid and group 3 metals, the so-called rare earth elements, are generally regarded as a group of 17 elements with similar properties, especially with respect to their chemical reactivity. However, in the above-mentioned catalytic asymmetric nitroaldol reactions, pronounced differences were observed both in the reactivity and in the enantioselectivity of the various rare earth metals used.29 For example, when benzaldehyde (54) and nitromethane (12) were used as starting materials, the EuLB complex gave 55 in 72% ee (91% yield) compared to 37% ee (81% yield) in the case of LLB (-40 °C, 40 h). The unique relationship... 

Liu, B., Ctii, D.M., Ma, 1. et al. (2007) Synthesis and reactivity of rare earth metal alkyl complexes stabilized by aniUdo phosphinimine and amino phosphine ligands. Chemistry —A European Journal, 13, 834. 

Arndt, S. and Okuda, J. (2005) Cationic alkyl complexes of the rare-earth metals synthesis, structure, and reactivity. Advanced Synthesis and Catalysis, 347, 339. 

Cui, D.M., Tardif, O., and Hou, Z.M. (2004) Tetranuclear rare earth metal polyhydrido complexes composed of (C5Me4SiMe3)LnH2 units. Unique reactivities toward unsaturated C-C, C-N, and C-O bonds. Journal of the American Chemical Society, 126, 1312. 

Interestingly, the reactivity pattern in rare-earth metal-catalyzed hydroamination/cyclization reactions of aminoalkynes with respect to ionic radius size and steric demand of the ancillary ligand follows the opposite trend to that observed for aminoalkenes, namely decreasing rates of cyclization with increasing ionic radius of the rare-earth metal and more open coordination sphere around the metal. This phenomenon can be explained by a negligible sterical sensitivity of a sterically less encumbered triple bond, as sterically less open complexes and smaller metal ions provide more efficient reagent approach distances and charge buildup patterns in the transition state [110]. 

The amino derivatives of rare-earth metals follow a distinct mechanism involving direct insertion of the allene or alkene into the metal-nitrogen bond (Scheme 6.73). With the less-reactive lanthanum complex 6.215, amine 6.214 underwent insertion of only the allene, giving a pyrrolidine product 6.216 as the trans isomer. On the other hand, using the more-reactive samarium complex 6.217, tandem allene and alkene insertion occurred giving, after hydrogenation, the pyrrolizidine alkaloid, xenovenine 6.189, A closely related synthesis of this alkaloid is in Scheme 6.59. Other syntheses may be found in Scheme 6.65, and Scheme 9.46. 

Ternary and quaternary a-hydroxy-phosphonates, an important class of biologically active compounds, are commonly obtained by addition of dialkylphosphites onto aldehydes or ketones [30]. Well-defined mono- or bimetallic complexes of rare-earth metals, titanium, or aluminum have emerged over the past two decades as effective catalysts for this so-called hydrophosphonylation of aldehydes [31] and, with more difficulty, that of ketones [31c,d, 32], which are far less reactive because of their lower electrophilicity. In some cases, good enantioselectivities could be achieved thanks to the use of chiral metal-based precatalysts [31, 32], Despite their several similarities with rare-earth elements, we were surprised to see that discrete complexes of the large Ae metals had never been utilized to catalyze hydrophosphonylation reactions. 

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