Silica with metal-coordinating sites

Another type of metal-support site may be created by the reaction of acetylacetone with the mono-amine-bound derivatized silica gels. This type has imino-acac ligands attached to the surface. There are Cu11 species with only one iminoacac chelate in its coordination sphere, and other sites coordinated in a square-planar array by two iminoacac ligands from adjacent surface locations.105,106 These types of supported IMCOS sites also have been used to separate hydrocarbons, ethers, and thioethers. 

Broensted and Lewis acid sites in crystalline phosphates, silicates and in gels with moleculeu sieve properties were studied by IR-spectroscopy. Two types of bridged hydroxyls were fovmd in SAPO-5 which were accessible to adsorbed molecules and were able to interact with ethylene. Lewis sites in metallophosphates, zirconosilicate and in Ti-containing silica gel were observed which did not interact with weak bases (CO, hydrogen) but formed strong complexes with acetonitrile. They were supposed to be framework metal ions in tetrahedral coordination. 

There is another interesting system that exploits the same FRET mechanism, to obtain in this case fluorescence amplification, but based on the addition of a non-bound second fluorophore. Bin Liu and Yusong Wang [127] have immobilized a DNA strand on monodispersed silica nanoparticles (1(X) nm in diameter) and then hybridized it with a fluorescein labelled complementary DNA engineered to induce in the final double strand three pairs of T-T mismatches that are specific coordination sites for mercury ions. After incubation with metal ions and thermal washing, a cationic conjugated polymer was added. This last component was selected in order... 

The data in Table 2.8 show, as expected, the higher the silica calcination temperature, the lower the surface hydroxyl concentration and the Mg or Al content of the sihca/metal alkyl reaction product. But the data also indicate that the amount of DBM or TEAL that can react with sihca greatly exceeds the siuTace hydroxyl concentration on each of the silicas. Hence, additional sites on the silica smface are available for reaction with metal alkyls other than siuface hydroxyl sites. Most likely the metal alkyl compounds coordinate to the silica surface through an electron pair on the oxygen atom of a siloxane (Si-O-Si) linkage. In addition, DBM reacts with each silica to a greater extent than does the TEAL. 

However, interaction of complexes with functionalised oxides does not always occur without involvement of surface sites in the metal coordination chemistry. For example, the interaction of Rh5(CO)] 5 with phosphinated silica at ambient temperatures can result in the tethering of hexanuclear units [109]. Indeed, the complex Rh6(CO)i5L (L = PPh2C2H4Si(OEt)3) can be specifically bound to a silica surface [110]. But in the presence of oxygen the cluster fragments to form Rh(I)(C0)2 sites in which there may be a direct Rh-0 surface link [111]. 

While attempting to prepare an T71-(vinylcarbene)iron complex121 by the alkylation or acylation of an a,/3-unsaturated acylferrate, Mitsudo and Wa-tanabe found122 that the major isolated product was in fact an -vinylketene complex (178), formed presumably by the carbonylation of an intermediate V-vinylcarbene, which may then undergo olefin coordination to the vacant metal site. All attempts to isolate such intermediates, or to observe them by 13C NMR spectroscopy, failed. Only in the reaction between potassium tetracarbonyl -cinnamoy ferrate (179.a) and pivaloyl chloride (180.b) was a side product (181) isolated in appreciable yield. In other reactions, only a trace (<1%) of such a compound was detected by spectroscopy. The bis(triphenylphosphine)iminium(l + ) (PPN) salts of 179.a and 179.b also reacted with 2 equiv of ethyl fluorosulfonate to give 178.g and 178.h in 21 and 37% yield, respectively. All products were somewhat unstable to silica gel, hence the low isolated yields in some cases. 

Because Mo and W are usually more stable and more reactive than Re, the reactivity of the corresponding isoelectronic complexes of Mo, [( iO)Mo( NAr) (=CH Bu)(CH 2Bu)[ [70], and W, [( iO)W(=NAr)(=CH Bu)(CH)Bu)] [71], have been studied. Overall, these systems are more stable than the corresponding silica-supported Re complex, and display reachvities better than those of the well-known corresponding bis-aUcoxide homogeneous derivahves. The better performances of these systems compared to their homogeneous analogues are probably due to the optimized coordination of the metal center in combinahon with a site isolation of the... 

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