Yttrium alkoxides

Alkoxides Yttrium and barium alkoxides can be obtained by direct reaction of the metal with alcohols (10X10- They readily... 

Aluminium alkoxides (especially aluminium isopropoxide), dialkylalumi-nium alkoxides, yttrium alkoxides, zinc alkoxides, aluminoxanes, zincoxanes, bimetallic -oxoalkoxides, aluminium porphyrins and aluminium Schiff s base complexes are the most representative coordination catalysts, containing multi-nuclear or mononuclear species, for lactone polymerisations (Table 9.5). 

The acid catalyzed hydrolysis/condensation reactions yield chain-like polyzirconoxanes, when performed with appropriate acac/Zr and HsO/Zr ratios. The stabilizer (Y, Ce, Ca or Mg) is introduced either as a salt (chloride, acetate or nitrate), a complex such as Ce (3) 2, 4-pentane dionate, or an alkoxide (yttrium triisopropoxide, Y (0 Pr)3). Furthermore, various additives [92-93] are often used to control the dehydration and the porosity of the zirconia gel, to improve its ability to form, or to act as a plasticizing additive (e.g., poly(ethyleneglycol)). 

Among the multinuclear and mononuclear active species evaluated for lactone polymerization, those effective for the polymerization of P-lactones implying an O-acyl scission (Scheme 9.16a) are aluminum and dialkylaluminum alkoxides, yttrium alkoxides, zinc alkoxides, aluminoxanes, zincoxanes, bimetallic g-oxoalkoxides and aluminum porphyrins of the (TPP)AIOR type [50]. However, aluminum porphyrins such as (TPP)AICI or (TPP)A10C(0)R and aluminum SchifFs base complexes such as (Sal)AlCl (where Sal = salcenato group) are able to promote lactone polymerization by O-alkyl scission (Scheme 9.16b) [50]. 

The alkoxides and aryloxides, particularly of yttrium have excited recent interest. This is because of their potential use in the production of electronic and ceramic materials,in particular high temperature superconductors, by the deposition of pure oxides (metallo-organic chemical vapour deposition, MOCVD). They are moisture sensitive but mostly polymeric and involatile and so attempts have been made to inhibit polymerization and produce the required volatility by using bulky alkoxide ligands. M(OR)3, R = 2,6-di-terr-butyl-4-methylphenoxide, are indeed 3-coordinate (pyramidal) monomers but still not sufficiently volatile. More success has been achieved with fluorinated alkoxides, prepared by reacting the parent alcohols with the metal tris-(bis-trimethylsilylamides) ... 

The acetylacetonates are stable in air and readily soluble in organic solvents. From this standpoint, they have the advantage over the alkyls and other alkoxides, which, with the exception of the iron alkoxides, are not as easily soluble. They can be readily synthesized in the laboratory. Many are used extensively as catalysts and are readily available. They are also used in CVD in the deposition of metals such as iridium, scandium and rhenium and of compounds, such as the yttrium-barium-copper oxide complexes, used as superconductors. 1 1 PI Commercially available acetyl-acetonates are shown in Table 4.2. 

One of the most widely studied initiators is commercially available yttrium iso-propoxide, (291), which exists as an oxo-alkoxide cluster of the formula Yf/is 0)(//2 0 Pr)4(/r3-01Pr)4(01Pr)5, usually abbreviated to Yfm 0)(0 Pr) 3.873 The ROP of rac-LA using several Lndiis 0)(0 Pr)13 complexes (Ln = La, Sm, Y and Yb) has been reported 874 in all cases polymerization occurs at room temperature with rates La Sm>Y Yb. The polymerizations... 

Non-cyclopentadienyl single-site lanthanide alkoxides mostly feature N-donor-based ancillary ligands. Examples include bulky bis(arylamidinate)-yttrium(III) alkoxides, phenoxides and amides such as complexes (307)-(309), which initiate the ROP of LA.892 However, control over molecular weight is poor and polydispersities are broad (typically >1.5), with the exception of (309) in the presence of exogenous benzyl alcohol. 

This method exclusively yields macrocyclic polyesters without any competition with linear polymers. Furthermore, the coordination-insertion ROP process can take part in a more global construction set, ultimately leading to the development of new polymeric materials with versatile and original properties. Note that other types of efficient coordination initiators, i.e., rare earth and yttrium alkoxides, are more and more studied in the framework of the controlled ROP of lactones and (di)lactones [126-129]. These polymerizations are usually characterized by very fast kinetics so as one can expect to (co)polymerize monomers known for their poor reactivity with more conventional systems. Those initiators should extend the control that chemists have already got over the structure of aliphatic polyesters and should therefore allow us to reach again new molecular architectures. It is also important to insist on the very promising enzyme-catalyzed ROP of (di)lactones which will more likely pave the way to a new kind of macromolecular control [6,130-132]. 

McLain SJ, Drysdale NE (1992) Living ring-opening polymerization of e-caprolactone by yttrium and lanthanide alkoxides. Polymer Preprints, American Chemical Society 33(1) 174-175... 

Stevels WM, Ankone MJK, Dijkstra PJ, Feijen J (1996) Kinetics and mechanism of e-caprolactone polymerization using yttrium alkoxides as initiators. Macromolecules 29 8296-8303... 

Martin E, Dubois P, Jerome R (2000) Controlled ring-opening polymerization of e-caprolactone promoted by in situ formed yttrium alkoxides. Macromolecules 33 1530-1535... 

Martin E, Dubois P, Jerome R (2003) Preparation of supported yttrium alkoxides as catalysts for the polymerization of lactones and oxirane. J Polym Sci A Polym Chem 41 569-578... 

At the same time the reaction products of La interaction with PlOH do always contain the amorphous component, being presumably the polymeric form of La(OPr )3 [953] (it was described by Mazdiyasni as the product of thermolysis or dehydrogenation of soluble yttrium alkoxides). 

Yttrium isopropoxide and yttrium 3-oxapentoxide initiators were the first lanthanide alkoxides described in the literature for the ROP of e-CL [93]. The discovery of lanthanide-based initiator systems allowed the block copolymerization of e-CL with compounds such as ethylene [94], tetrahydrofuran [95], L-LA [96], trimethylene carbonate [97], and methyl methacrylate [98]. This type of initiator has also been used to prepare poly((3-butyrolactone)s [99,100]. 

The first reports of LA ROP using yttrium complexes focused on homoleptic alkoxide complexes, such as cluster complexes of the form Ln5(p-0)(0R)i3 [27]. A patent, and preprint, published by DuPont described the application of a homoleptic yttrium alkoxide, Y(OCH2CH2NMe2)3, formed in situ by reaction of yttrium fm-Ao-propoxide with ALV-dimethy I am i noethanol. The complex showed a very high rate (kob = 0.5 s 1, [Y]0 = 3 mM) and reasonable polymerization control [28]. 

Arnold and colleagues have reported a series of chiral homoleptic yttrium and lanthanide fra(alkoxide) complexes [49, 50], These initiators (including complex 1) show high degrees of iso-selectivity and rapid rates, even at low temperatures. Thus, using the racemic mixture of the lanthanide initiator, stereoblock PLA was produced with a P, of 0.81 so far, this is the only known type of yttrium initiator able to exert such stereocontrol and a very exciting finding. Analysis of the stereoerrors indicates that an enantiomorphic site control mechanism is responsible for the iso-selectivity. 

The reduced yttrium complex was active for the polymerization of 55-lactide, 74% conversion in 3 h (0.2 M, THF, 25°C). When the complex was oxidized, using FcBArF, the polymerization was completely stopped, and it was restarted when the complex was reduced back. The authors provide good evidence that the polymerization switch operates by changing the electron density of the yttrium-alkoxide bond during oxidation/reduction cycles. Polymerizations with the indium analogue... 

Okuda and Arnold have reported a series of 5- and 6-coordinate chiral indium complexes (e.g. 12), analogous to those previously discussed with yttrium (Table 4), which show varying activities for the polymerization of rac-lactide [71]. The homoleptic species was fluxional, interconverting between a 5-coordinate and fac- and mer-isomers of a 6-coordinate complex. It resulted in controlled polymerization and some stereoselectivity (max. P, = 0.63 and max. Ps = 0.64). MALDI-ToF mass spectrometry confirmed that the chiral alkoxide ligand was an initiating group. 

The influence of steric factors was thoroughly studied in the reaction of Ln(btsa)3 with the alcohol tritox-H. While the reaction takes place with larger lanthanides like neodymium to yield the homoleptic alkoxide complexes (Eq. 18) [264], the analogous reaction does not work with smaller metals like yttrium and thulium (Eq. 19). However, variation of the reaction conditions to a stoichiometric solid reaction yielded a fully exchanged product along with an unexpected and unusual silylamine degradation [265] (Eq. 20). This degradation reaction seems to be sterically forced and points out N-Si bond disruptions and C-Si bond formations under mild conditions [114]. 

Routes to monomeric , mononuclear , monolanthanide alkoxides, enolates, siloxides and aryloxides - an expanded title which will put the scope of the article in a more concrete form. The synthesis of mononuclear alkoxides, in particularly homoleptic derivatives [1], was decisively stimulated by the discovery of high temperature superconducting ceramics based on YBa2Cu307<, where yttrium represents the lanthanide elements [2]. The support of volatile and highly soluble molecular precursors is a prerequisite for synthesizing thin films of these materials by means of MOCVD [3] and sol gel processes [4], respectively. More recently, lanthanide alkoxide reagents became established in... 

Synthetic strategies to alkoxide complexes have been covered in full by previous reviews [14]. The silylamide route proved to be an advantageous method of preparation, especially in the case of homoleptic derivatives [15]. The group (IIIA) elements - scandium, yttrium and lanthanum - are considered as lanthanides on the basis of their general chemical similarity to the true lanthanides. 

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