Complexes yttrium

Tetradentate N,0-donor ligands have also been investigated. The yttrium complex (—)-(313) does not effect stereocontrol over the ROP of rac- or meso-LA, in contrast to related A1 initiators (262) and (263).803 Polymerization is also slower than for most lanthanide initiators with 100 equivalents meso-LA requiring 14 h at 70 °C to attain near-quantitative conversion (97%). 

Table 10 shows the results of polymerization of oc-olefins catalyzed with trivalent complexes. When a bulkiler tBuMe2Si group instead of the Me3Si group was introduced into the yttrium complex, the racemic complex was formed exclusively [74c]. However, this alkyl complex did not react with olefins, and hence it was converted to a hydride complex by reaction with H2. The complex obtained was reactive to various olefins and produced polymers at... 

The addition of trimethylsilyl (TMS) cyanide to aldehydes produces TMS-protected cyanohydrins. In a recent investigation a titanium salen-type catalyst has been employed to catalyse trimethylsilylcyanide addition to benzaldehyde at ambient temperature1118]. Several other protocols have been published which also lead to optically active products. One of the more successful has been described by Abiko et al. employing a yttrium complex derived from the chiral 1,3-diketone (41)[119] as the catalyst, while Shibasaki has used BINOL, modified so as to incorporate Lewis base units adjacent to the phenol moieties, as the chiral complexing agent11201. 

The corresponding yttrium complex is found to be isostructural with the lutetium complex. 

A homoleptic bulky a,y-diketonate yttrium complex (fod = 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedionate) was immobihzed on MCM-41.280 ( s = 1140m g, Vp = 0.93 cm g, dp = 2.7 nm) and a monopodaUy anchored surface species 6 has been proposed (Scheme 12.5). As suggested by FTIR (strong band for the Si-OH stretch vibration) as well as metal and carbon analysis (circa 3.4 wt% Y, fod/Y circa 2) only around half of the silanol population has been consumed [110]. 

Neutral lanthanide complexes are convenient models for the cationic zirconocene systems and avoid complications due to the presence of counteranions and the limited solubility of ionic compounds. Dynamic NMR studies on yttrium complexes 44-46 has allowed the determination of the alkene binding enthalpy, the activation enthalpy of alkene dissociation, and the relative rates of dissociation and alkyl site exchange (site epimerisation) (Scheme 8.20). Compared to the Zr... 

Fig. 27 Highly active amino-alkoxy-bis(phenolate)yttrium complexes, which yield syndiotactic PHB in a very controlled manner...

Lanthanide-based catalysts, despite finding a lot of application in homogeneous catalysis, can be rather problematic due to the lability of some ligand types and the versatility of their coordination chemistry in the -1-3 oxidation state this makes the controlled synthesis of single-site Ln complexes a quite ambitious goal [92]. McLain and coworkers first demonstrated the high potential of a homoleptic yttrium complex Y(OCH2CH2NMe2)3 as ROP catalyst for the preparation of PLA from rac-lactide and that it promotes a rapid and controlled polymerization... 

A series of bis(thiophosphinic amido) yttrium complexes 86a-h was promising due to their high rates and enhanced control of the L-lactide polymerization activity with high polymerization rates A app = 2.2 x lO to 1.1 x 10 s at [LA]q = 1 M, [l]o = 5 mM [104]. It is worth noting that the phosphorous substituents greatly influenced the rate of the reaction, and that the rate followed the order isopropyl > phenyl > ethoxy. 

Fig. 30 Zirconium, zinc, and yttrium complexes for heteroselective lactide ROP...

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]. 

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... 

Quinolato moieties which are linked by a butylamino group afford a powerful tetradentate ligation mode as realized in an 8-coordinate, anionic yttrium complex (Eq. 14c) [159]. Two ammonium ions and two water molecules link two complex anions to form associates of type [(NH4) YL2 H20]2 in the solid state. 

Finally, a samarium aluminate tetramer featuring /J-dikeliminalo ligands was obtained in low yield (20%) by J. Arnold et al. (Scheme 54) [224], Once again this example corroborates the feasibility of selective [amide] [alkyl] transformations in N-ligand-based postlanthanidocene chemistry, previously observed for octaethylporphyrin yttrium complexes by C.J. Schaverien [225, 226],... 

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