Anchor species

Contrary to the observations made previously with low TON reactions, here the homogeneous catalyst was more active than the anchored species. Once the homogeneous catalyst had been used for the high TON hydrogenation, though, it appeared to have been somewhat deactivated. The anchored catalyst, however, is capable of re-use several times. In this instance there was an increase in activity after the first use, even though the catalyst had been pre-hydrogenated. 

The nature of both the anchored species and of the deposit do strongly depend upon the concentration of - OH groups on the alumina surface. The early stage of the interaction between vapours of [Mo(CO)e] and the support was identified as physisorption when using y-alumina, in which almost all of the surface aluminium atoms are hydroxylated. Indeed, in this case physisorption can be ascribed to the interaction between an oxygen atom of a surface - OH group and the carbon atom of one CO ligand, which causes... 

Figure 3 summarizes the influence of the - OH concentration on the nature of the anchored species and of the final deposit. However, these phenomena which have been fully understood for alumina cannot And a satisfactory general explanation for other oxides and for zeolites. Indeed, for silica the ab-... 

Further variations on the theme have been achieved54 by anchoring species such as [RuivO-(terpy)(py)]2+ or complexes of osmium.55 Iron complexes have also been studied for example, evaporation of a solution containing [Fe(CN)5(H20)]3 and PVP on to an electrode will immobilize the pentacyanoferrate as a pyridyl complex, one in three available pyridyl groups being used to avoid precipitation prior to evaporation of solvent. 

Alternative approaches have been proposed over the years. In none of these cases is there sufficient experimental evidence for truly heterogeneous catalysis. Frechet et al. (55) used a polyvinylpyridinium (PVP) material for supporting chlorochromate [Cr(IV)02Cl ] or dichromate [CrCVI Oy-]. Cr3+ can be immobilized by simple ion exchange on polymers such as Nation or on a Y zeolite (59, 60). However, it is doubtful whether these methods ensure complete Cr anchoring when the material is brought into contact with oxidants. Clark et al. (61) advocated the use of alumina-anchored dichromate. Particularly when a neutral alumina is used, surface-anchored species are formed ... 

The structure of the anchored species may be also deduced from in situ characterization. However, to our knowledge, only a few attempts by ESR and UV-Vis DRS have been reported [8, 19, 24]. 

When the supp-O-M bonds are hydrolyzed (Eq. 6), the released species must be removed by washing so as to keep only really anchored species in the catalyst [9, 31]. This washing step can advantageously replace step d of the preparation (Section B above) since both hydrolysis and elimination of the released metal species can take place simultaneously. This crucial step is barely mentioned in the literature. This may be the reason why some authors claim that the grafting of chloride complexes does not lead to homogeneously dispersed species on the support surface and forms three-dimensional oxide particles after calcination [29, 32, 33]. 

This example shows that, in contrast to what is observed with the metal allyls, the formation of metal particles may not involve the formation of anchored species, i.e. cationic species. 

In the cases of [Ru3(CO)i2] and [Ru4H4(CO)i2], which have very unstable anion radicals, Lewis base substitution reactions are stoichiometric rather than catalytic in the reducing agent (167) dppe and the triruthenium cluster give [Ru3(CO)io(dppe)J, [Ru3(CO)n(dppe)], or [ Ru3(CO)n 2(/u.-dppe)], depending on the relative stoichiometries of the reactants (168). Complexes such as [Ru3(CO)uL] and [Ru4H4(CO)uL] [L = PPh2CH2CH2Si(OEt)3], precursors to surface-anchored species, have also been prepared by reductively induced substitution reactions (169). 

The reactions of oxide-supported osmium clusters have been studied (108) at elevated CO and H2 pressures by in situ IR spectroscopy. Under 10 atm of a 1 4 CO -I- H2 mixture, Os3(CO)i2 is retained intact up to 573 K. However, at 523 K on AI2O3, Si02, or Ti02, it is transformed to H30s4(C0)i2 which is an active anchored species for catalyzing CO -I- H2 conversion to CH4. 

A recent paper describing the use of these anchored complexes for aldehyde hydrogenation stated that cationic complexes were required for the formation of the anchored species and that this anchored catalyst was, essentially, an ion pair comprised of a rhodium cationic species and an oxygen... 

It was considered that if these anchored species were ion pairs, flien one should be able to remove them by treatment with an excess of another anion. To test this, a sample of Rh(dppb) anchored onto PTA/A1203 was stirred for 30 hours with a large excess of LiBF4 in ethanol under an inert atmosphere. Analysis of the reaction liquid showed that, at most, 2-3% of the Rh present on the catalyst was lost. However, tungsten was also detected with a Rh W ratio of 1 12. Since there are twelve tungsten atoms in a PTA molecule, these data indicate that this procedure apparently did not remove only flie Rh complex from the PTA but, instead, somehow removed the Rh/PTA from the alumina. 

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