Precursors, catalyst

Figure Bl.25.9(a) shows the positive SIMS spectrum of a silica-supported zirconium oxide catalyst precursor, freshly prepared by a condensation reaction between zirconium ethoxide and the hydroxyl groups of the support [17]. Note the simultaneous occurrence of single ions (Ff, Si, Zr and molecular ions (SiO, SiOFf, ZrO, ZrOFf, ZrtK. Also, the isotope pattern of zirconium is clearly visible. Isotopes are important in the identification of peaks, because all peak intensity ratios must agree with the natural abundance. In addition to the peaks expected from zirconia on silica mounted on an indium foil, the spectrum in figure Bl. 25.9(a)...

Bonnemann H ef a/1996 Nanoscale colloidal metals and alloys stabilized by solvents and surfactants preparation and use as catalyst precursors J. Organometaii. Chem. 520 143... 

Promoters are sometimes added to the vanadium phosphoms oxide (VPO) catalyst during synthesis (129,130) to increase its overall activity and/or selectivity. Promoters may be added during formation of the catalyst precursor (VOHPO O.5H2O), or impregnated onto the surface of the precursor before transformation into its activated phase. They ate thought to play a twofold stmctural role in the catalyst (130). First, promoters facilitate transformation of the catalyst precursor into the desired vanadium phosphoms oxide active phase, while decreasing the amount of nonselective VPO phases in the catalyst. The second role of promoters is to participate in formation of a soHd solution which controls the activity of the catalyst. 

The red tetrathiomolybdate ion appears to be a principal participant in the biological Cu—Mo antagonism and is reactive toward other transition-metal ions to produce a wide variety of heteronuclear transition-metal sulfide complexes and clusters (13,14). For example, tetrathiomolybdate serves as a bidentate ligand for Co, forming Co(MoSTetrathiomolybdates and their mixed metal complexes are of interest as catalyst precursors for the hydrotreating of petroleum (qv) (15) and the hydroHquefaction of coal (see Coal conversion processes) (16). The intermediate forms MoOS Mo02S 2> MoO S have also been prepared (17). 

Because of its volatility, the cobalt catalyst codistills with the product aldehyde necessitating a separate catalyst separation step known as decobalting. This is typically done by contacting the product stream with an aqueous carboxyhc acid, eg, acetic acid, subsequently separating the aqueous cobalt carboxylate, and returning the cobalt to the process as active catalyst precursor (2). Alternatively, the aldehyde product stream may be decobalted by contacting it with aqueous caustic soda which converts the catalyst into the water-soluble Co(CO). This stream is decanted from the product, acidified, and recycled as active HCo(CO)4. 

In addition to rhodium(III) oxide, cobalt(II) acetylacetonate or dicobalt octacarbonyl has been used by the submitters as catalyst precursors for the hydroformylation of cyclohexene. The results are given in Table I. 

Catalyst Precursor (mole/1.) Solvent" Reaction Temperature Reaction Time (hours) Yield" (%)... 

This catalyst precursor (5 g.) in 140 ml. of heptane was heated in the autoclave at 160 with a mixture of COiH (1 1) at 150 atm. for 2 hours. The vessel was cooled, the gas released, 1 mole of cyelohexene was charged, and the reaction was carried out according to the usual procedure. 

Ionic liquids formed by treatment of a halide salt with a Lewis acid (such as chloro-aluminate or chlorostannate melts) generally act both as solvent and as co-catalyst in transition metal catalysis. The reason for this is that the Lewis acidity or basicity, which is always present (at least latently), results in strong interactions with the catalyst complex. In many cases, the Lewis acidity of an ionic liquid is used to convert the neutral catalyst precursor into the corresponding cationic active form. The activation of Cp2TiCl2 [26] and (ligand)2NiCl2 [27] in acidic chloroaluminate melts and the activation of (PR3)2PtCl2 in chlorostannate melts [28] are examples of this land of activation (Eqs. 5.2-1, 5.2-2, and 5.2-3). 

The regioselective arylation of butyl vinyl ether was carried out by the same group, using Pd(OAc)2 as catalyst precursor and l,3-bis(diphenylphosphino)-propane (dppp) as the ligand, dissolved in [BMIM][Bp4] (Scheme 5.2-17) [90]. 

With diphosphanes recently Stephan et al. reported an intriguing Al and P based macrocyclic structure [37]. A zirconium based catalyst precursor first was employed in the catalytic dehydrocoupling of the primary bidentate phosphane to give the tetraphosphane 6, (Scheme 4). The function of 6 as a molecular building block has been demonstrated by its reaction with MMe3(M = Al, Ga). Although, the gallium derivative 7 has not been... 

Catalyst Precursor Clusier Ref, (Cluster Syntheses) Support Catalyzed Reaction and/or Characterization Ref. (Catals sis Studies)... 

APPLICATIONS OF NUCLEOPHILIC CARBENES AS CATALYST PRECURSORS IN HOMOGENEOUS CATALYSIS... 

Catalyst precursor loading = 5niol /f. Calculated from h NMR spectra. 

Recently, the possibility that complexes of unsaturated "C ligands other than alkylidenes might also serve as catalyst precursors in olefin metathesis has... 

The role of complexes 23-28 as catalyst precursors in the ring closing metathesis reactions was investigated. Three different diene substrates diethyldiallyl-malonate (29), diallyltosylamine (30). and dielhyldi(2-methylallyl)malonate (31) were added to the NMR tubes containing a solution of 5 mol% of catalyst precursor in an appropriate deuterated solvent. The NMR tubes were then kept at the temperatures reported in Table X. Product formation and diene disappearance were monitored by integrating the allylic methylene peaks in the H NMR spectra and the results are presented in Table X and the catalytic transformations are depicted in Scheme 3. 

Ring Closing Min ATHi-.sis Results Using Catalyst Precursors 23-28... 

Entry Substnite Catalyst Precursor Solvent Temp. ( C) Time (min) Yield (V,)"... 

Togni s [38] approach was therefore to test the ability of sparteine to act as an ancillary ligand in Pd(II)-allyl complexes—susceptible to nucleophilic attack by stabihzed anions such as Na[CH(COOMe)2]—which could be employed as catalyst precursors. In addition he speculated that the rather rigid and bulky sparteine would be able to induce significant differentiation between the two diastereotopic sites of 1,3-disubstituted allyl hgand, thus leading to enantioselection upon nucleophilic attack. 

Some other groups have studied the opportimity to enhance the diastere-oselectivity of the transformation using the usual copper-bis(oxazohne) catalysts but modifying the carbene source. France et al. [25] observed that the use of (trimethylsilyl)diazomethane associated with a bis(oxazoline) and [Cu(CH3CN)4]PF6 as catalyst precursor allowed the formation of the trans isomer with high yield and selectivity, probably due to the steric bulk of the trimethylsilyl group. 

---