Silica-supported complexes

The impact of the new activation procedure on the WGS rate, using either Na2C03 or NaOH, on silica-supported complexes at 150 °C, is reported in Table 51 156,163 Coitions 1-1.4 g dried silica-supported catalyst with 1.6 wt% loading of Ru. 

Table 51 Impact of Na2C03 or NaOH activation on silica-supported complexes WGS (mol product/day). T = 150 °C 156,163 for... 

Mesoporous silica supported complexes of samarium(II), also having an amine and tetrahydrofuran ligand, were found to incandesce on exposure to air. The initial deposition was from samarium(II) bis(dimethylsilyl)amide, and bis(dimethylsilyl)-amine is itself not a compound likely to be air stable. 

The local structure of the fast solid ion-conductor Cu2P3I2, i.e. (CuI)8Pi2, was investigated by 1-D and 2-D 31P and 65Cu MAS-NMR spectroscopy.55131P NMR studies on SmFe4P12 show that the system remains in a paramagnetic state above the Curie temperature.552 The silica-supported complex =Si-ORh(P Pr3)2(H)2 was characterised by 31P MAS-NMR.553... 

The rate of expoxidation was measure in situ via the uptake of cyclohexene vapor by the catalyst. At low pressures (ca. 10 Torr), cyclohexene does not adsorb on the unmodified silica surface nor on either of the silica-supported 2-alkoxide conplexes 1 and 3. However, the addition of cyclohexene vapor to 2 resulted in a rapid, exponential loss of v(C=C) intensity in the IR spectrum of the gas phase above the silica-supported complex. Figure 2. We infer that epoxidation results in adsorption of cyclohexene oxide on the catalyst surface. 

As can be verified for the silica-supported sample, the characteristic bands observed for the free complex, such as those at (free complex) 1675/1645 cm due to the u C=0 vibration mode and 1430 cm associated with the u C=N mode. In the silica-supported complex, the C=0 bands are shifted to 1699/ 1656 cm , indicating that the metal—hgand bond order is higher in this case. So, it is possible to suppose that the sihca-supported complex is more stable than the free one. 

Previous studies by Sorokin with iron phthalocyanine catalysts made use of oxone in the oxidation of 2,3,6-trimethylphenol [134]. Here, 4 equiv. KHSO5 were necessary to achieve full conversion. Otherwise, a hexamethyl-biphenol is observed as minor side-product. Covalently supported iron phthalocyanine complexes also showed activity in the oxidation of phenols bearing functional groups (alcohols, double bonds, benzylic, and allylic positions) [135]. Besides, silica-supported iron phthalocyanine catalysts were reported in the synthesis of menadione [136]. 

An alternative that has received a great deal of attention in recent years is the immobilisation of a chiral catalyst on a nonsoluble support (polystyrene resins, silica gel, zeolites, etc.), thereby creating a chiral heterogeneous catalyst. Unlike homogeneous catalysts, these supported complexes can be recovered from the... 

A PRP -1 (Hamilton Reno, NV) reversed phase column was coated with cetylpyridinium and eluted with tetramethylammonium salicylate acetoni-trile water.89 The separation was comparable to that observed on conventional ion exchange. Coated phases were also used to separate oxalate complexes of manganese, cobalt, copper, and zinc.90 Reversed phase silica supports were coated with poly(N-ethyl-4-vinylpyridinium bromide), poly(dimethydiallylammonium chloride), poly(hexamethyleneguanidinium... 

Figure 2.20. Transformation of silica supported dinitrosyl complexes of nickel(II) leading to formation of nitrogen dioxide and its final stabilization on the support. The picture shows the molecular structure and the spin density contours calculated with BP/DNP method of the involved species, and evolution of the X-band EPR spectra of the NiN02 Si02 complex due to spillover of the ligand (adopted from [71]).

The above example outlines a general problem in immobilized molecular catalysts - multiple types of sites are often produced. To this end, we are developing techniques to prepare well-defined immobilized organometallic catalysts on silica supports with isolated catalytic sites (7). Our new strategy is demonstrated by creation of isolated titanium complexes on a mesoporous silica support. These new materials are characterized in detail and their catalytic properties in test reactions (polymerization of ethylene) indicate improved catalytic performance over supported catalysts prepared via conventional means (8). The generality of this catalyst design approach is discussed and additional immobilized metal complex catalysts are considered. 

Silica is the most commonly used solid support. Treatment with complexes bearing alkoxy- or chlorosilane functional groups is a common way to generate supported complexes on silica or any inorganic oxide containing surface silanol (Si-OH) groups (Scheme 7.2).23,24... 

Silica-supported metal (e.g., Pd/Si02) catalysts also have surface silanol groups that can react with the alkoxysilane groups of the complexes. These combination catalysts consist of a tethered complex on a supported metal. A Rh complex was tethered to the surface of a Pd/Si02 catalyst, and the tethered catalyst was more active for the hydrogenation of aromatic compounds than the free complex or the supported catalyst separately.33 It is possible that the H2 is activated on the supported metal and the hydrogen atoms migrate to the silica, where they react with the reactant molecules coordinated by the tethered complex. 

The reaction of silica-supported bis(allyl)Rh complexes with PMe3 followed by hydrogen treatment leads to the synthesis of the surface SiO-RhH2(PMe3)4+ as the first example of a cationic organometallic complex attached to the silica surface by ion pairing. The counterion is presumed to be a siloxy group SiO- on the silica surface.276 277... 

Alkylidene complexes are generally considered to be reactive intermediates but the actual surface organometallic species have never been fully characterized. However, the synthesis of silica-supported tantalum(V) carbene complexes and their characterization have been reported.332... 

The highest ethylene polymerization activity for a tetradentate salen-type group 4 complex was reported for silica supported (64) (600gmmol-1h bar ).193 Activities for a range of related zirconium and titanium complexes such as (65)-(67) are typically an order of magnitude lower.194-196... 

The more hindered alkoxide Ti(OiPr)4 was used as the precursor complex with surface silanols of an amorphous silica support this reaction is reported to lead to the same environment of Ti as in TS-1, but only when the reaction is carried in cyclohexanol as the solvent. Epoxidation of octene, cyclohexenol, and norbornene with FI202 in phenylethanol leads to 95-98% epoxide selectivity.147... 

It is evident that the silica support influences the catalytic performance and it is important to understand the details of the processes involved. For the sol-gel material it was shown by 31P NMR spectroscopy that the immobilised cationic complex completely transforms to the neutral rhodium-hydride species under a CO/H2 atmosphere (Scheme 3.3). On dried silica, however, this conversion might not be complete since the dried support is more acidic [32], It is therefore very likely that the neutral and cationic rhodium complexes co-exist on the silica support. 31P NMR measurements on homogeneous rhodium complexes have shown that a simple protonation indeed converts the neutral rhodium hydride species into the cationic complex. 

The existence of two different rhodium species co-existing on the silica support can be used as an advantage by controlling their relative amount. Under standard hydroformylation conditions, the cationic species and the neutral hydride complex are both present in significant amounts. Hence hydroformylation and hydrogenation will both proceed under a CO/H2 atmosphere. Indeed a clean one-pot reaction of 1-octene to 1-nonanol was performed, using the supported catalyst for a hydroformylation-hydrogenation cascade reaction. 98 % of the 1-octene was converted in the... 

From the data available, Hartley and Vezey (27) discussed the variable parameters. The type of support material is important. On phosphinated silica, the complex is on the outside the surface and is more accessible... 

---