Aluminum pentacoordinate

Mole (11) found that the reaction of AlEt3 with AlMe3 to form mixed alkylaluminums was slower in the coordinating solvent pyridine than in benzene. He hypothesized that at least one of the two aluminums participating in the exchange must be pentacoordinated in the transition state. He stated that in the absence of a coordinating solvent the inter-... 

Table 5.2 The distribution of AI(VI) (octahedral), AI(IV) (tetrahedral) and AI(V) (pentacoordinate) aluminum in alumina polymorphs and precursors as determined by literature Al NMR studies.

As already discussed in this chapter, aluminum, in addition to its well-known high oxygenophilicity (Al-O = 511 3 kJ mol ), has exceedingly high affinity toward fluorine this is evident from the bond strengths in several metal-fluorine diatomic molecules Al-F, 663.6 6.3 kJ moFh Li-F, 577 + 21 kJ mol" Ti-F, 569 + 34 kJ moF Si-F, 552.7 + 2.1 kJ moF Sn-F, 466.5 + 13 kJ moF and Mg-F, 461.9 + 5.0 kJ moF [76]. Organoaluminum reagents seem, therefore, quite suitable for fluorine-assisted selective alkylation of fluoro epoxides, which also represents the experimental demonstration of the intervention of pentacoordinate chelate complexes of trialkyl-aluminums as plausible intermediates [63]. 

A mechanism for the formation of the hexacoordinate species 434 is presented in Sch. 60 [89]. Association of metal bases with the ALB catalyst 394 gives species 431 which can undergo disproportionation to give tricoordinate aluminum species 432 and the bis-alkoxide of BINOL (433). Addition of this bis-alkoxide of BINOL to ALB would then produce the hexacoordinate aluminum species 434. If this scheme is correct, it is certainly possible that the three-coordinate aluminum species 432 is the active catalyst. To test for this possibility, this species was prepared by the reaction of BINOL with trimethylaluminum and was crystallized to give crystals which were characterized by X-ray diffraction as the dimeric pentacoordinate THF adduct 435. This aluminum compound has been used previously for Mukaiyama type aldol reactions... 

In a similar [4+2] reaction of a, -unsaturated esters, the aluminum catalyst complexed with the ligand S-VAPOL resulted in autoinduction , because of cooperative interaction of the product with the catalyst to generate a more selective catalytic species (Scheme 6.48) [68]. The ee% gradually increased as the reaction time lengthened. In the proposed intermediate, penta-coordinated aluminum complex 77, the cycloadduct is recognized as a complementary ligand, leading to substantial asymmetric induction. The acrylate is activated effectively within this hybridized complex which adopts pentacoordination [87]. 

It has been shown that steam ammonia treatment of an AlaOs-SiOa hydrocracking support promotes the formation of pentacoordinated aluminum species. The mechanism is associated to a dealumination and aluminum migration from tetrahedral coordination into more distorted Al-Si environment. This generates a larger proportion of strong Lewis acid centers and a broad distribution of acid strength. 

Besides the already mentioned acidic aluminum chloride catalysts, alternative Friedd-Crafts catalysts such as supported acidic tin catalysts have also been developed. The tin-based catalysts were prepared by a method which closely resembled the already mentioned two-step grafting method devised for the aluminum chloride catalyst. Here, SnCU was anchored on silica materials modified with tetraalkylammonium chloride moieties obtained for example, from reaction with [3-(trimethoxysrlyl)propyl]octadecyldimethylammonium chloride, thereafter, reaction of the Lewis acid with the chloride moieties leads to formation of pentacoordinated anionic tin species forming catalytically active complexes (i.e. [R4N][SnCl5] species), associated with the surface. The supported tin catalysts were employed for condensation reactions of olefins with aldehydes forming unsaturated alcohols (Prins condensation. Scheme 5.6-2) [76]. 

A more recent model was introduced by Digne et al. (93) on the basis of DFT calculations. According to this model, the A- and B-bands also correspond to type I hydroxyls, but the A-band is attributed to OH on tetragonal aluminum sites on the (110) plane, and the B-band is associated with octahedral aluminum on the (100) plane. In this model, the D-band is proposed to characterize type I hydroxyls that are attached to pentacoordinated aluminum sites on the (110) plane. The doubly and triply bridged hydroxyls are associated with the E- and F-bands, respectively. 

The chemistry of the precipitation of aluminum hydroxides and oxides is very complex (40). When the solubility is exceeded, gelatinous precipitates, which are found to be amorphous by X-ray diffraction, usually form initially. Al MAS NMR shows the predominance of octahedraUy coordinated Al ions in these amorphous hydroxides, as are present in the crystalline trihydroxides and oxyhydroxides. However, in the amorphous materials some pentacoordinated and tetracoordinated Al ions are also found (15). As discussed above, there are many different crystalline hydroxides or oxyhydroxides, and which of them will be formed depends on the conditions (4f). Primary factors are temperature and pH, as well as aging time however, the nature of the anions present and the possible presence of organic components (42,43) also play a role. At low temperature in an excess of water, the hydroxides are preferentially formed, specifically bayerite at pH values between 5.8 and 9 or gibbsite for pH values smaller than 5.8 or larger than 9. 

Ferreira etal. (161) performed DFT calculations to consider a spinel-type model with 37.5% tetrahedraUy coordinated and 62% octahedrally coordinated Al ions. The authors compared this model with the nonspinel-type model of Krokidis et at. (119) and concluded that the best agreement with experimental data was obtained for the spinel-type model. The same authon performed ah initio calculations of Al MAS NMR spectra for different structural models of Y-AI2O3 (162) and concluded that the best fit is obtained using a spinel-type structure with space group Fd3 m and Z equal to 32, and with the aluminum ions distributed as follows 64% octahedral coordination, 34.4% tetrahedral coordination, and 1.6% pentacoordinated. 

In several recent publications, Peden and coworkers inferred that pentacoordinated aluminum ions, which are clearly visible in Al NMR spectra ofY-Al203, determine most of the surface and bulk chemistry of this material. The authors proposed pentacoordinated aluminum ions to act as structural promoters for phase transitions (150), for anchoring of platinum oxide (332), as well as for the sintering of supported platinum metal particles (333). The authors supported their conclusion with calculations based on the nonspinel structural model of Y-AI2O3 by Digne et al. (158) the results indicated that pentacoordinated aluminum adsorption sites may be located on the (100) face. The same group used the (100) and (110) faces of this nonspinel-type structure to model the adsorption of NO (334). 

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