Cluster, aluminoxane

Also present are both four- and three-coordinate Al centers, the latter probably giving rise to high Lewis acidity. Structures of some aluminoxane clusters have also been determined in the solid state by single-crystal X-ray studies. One such structure, that of [Al706Me16] anion, is shown by 6.30. Another structure, 6.31, has been proposed on the basis of mass spectroscopic data. All the evidences taken together point to open-cage rather than dense mineral structures for the aluminoxanes in MAO solutions. 

The linear CO stretching frequency for the carbonylated platinum colloid while lower than that found for surface bound CO, is in the range reported for the platinum carbonyl clusters [Pt 3 (CO) 6 ] n / sind we find that the carbonylated colloid is easily transformed into the molecular cluster [Pt 12 (CO) 24 ] (10) reaction with water. The cluster was isolated in 50 yield based on platinum content of the precipitate by extraction with tetraethylammonium bromide in methanol from the aluminum hydroxide precipitated when water is added to the aluminoxane solution. The isolation of the platinum carbonyl cluster reveals nothing about the size or structure of the colloidal platinum particles, but merely emphasizes the high reactivity of metals in this highly dispersed state. The cluster isolated is presumably more a reflection of the stability of the [Pt3(CO)6]n family of clusters than a clue to the nuclearity of the colloidal metal particles - in a similar series of experiments with colloidal cobalt with a mean particle size of 20A carbonylation results in the direct formation of Co2(CO)8. 

The Lewis acidity of aluminoxanes can arise from functionality other than three-coordinate Al centers. Barron et al. found that, in the case of aluminoxane clusters such as 5, three-coordinate aluminum is not a prerequiste for ethylene polymerization activity when combined with zirconocene dimethyl. Coordi-natively saturated aluminoxane 5 reacts exothermically with Cp2ZrMe2 at room temperature to produce, according to the H NMR spectrum, an ethylene polymerization-active tightly ion paired species, Cp2-ZrMe ( Bu)6Al6( 3-0)6Me (74). The driving force for... 

The association is destroyed when the complexes are treated with diethyl ether. In the case of MAO, an ideal cluster structure of [Al403Me6l4 could not be isolated although such a cluster with tertiarygroups instead of methyl groups was isolated and characterized by Barron [40, 41]. It was also possible to synthesize similar aluminoxane with ethyl groups. This ethylaluminoxane has a molecular... 

AlMe] -OAlMe2 with n = 5-20 with a certain amount of residual AlMes still existing in equilibrium. Speetroscopic studies predicts a cross-linking by methyl free-oxoaluminum centers to generate a microphase with an Al O core. Aluminoxane clusters [RAl(/i,3-0)] , with R = tertbutyl and n = 4, 6, or 9 has been isolated and structurally characterized by Barron and his group [130, 131]. Although complexes with four-coordinate A1 centers seem to predominate in MAO solutions, the presence of three-coordinate A1 centers has also been deduced by Siedle and co-workers [132, 133] from A1 NMR data. Zr and NMR spectra of Cp2ZrMc2/MAO solutions [134] and solid state XPS [135] studies and NMR studies... 

Al6(t-Bu)6Me ] from the reaction with Cp2ZrMc2 in [Dg] toluene solution and this species polymerizes ethylene. This tendency of four-coordinate Al centers in these aluminoxane clusters to abstract a methyl anion is ascribed by these authors to the relief of ring strain upon formation of the methyl complex. 

---