Silanol complexes with group 10 transition metals

The surface of the silica may be dynamically coated with transition metals, and the selectivities observed can be attributed to the complexes between the metal ions and the analyte species [56], The use of silver-impregnated silica (adsorption of salts of transition metals on the silica surface) has been used for the analysis of saturated and unsaturated fatty acid methyl esters (FAME) and triacylglycerols (TAG) [57], The retention of the unsaturated FAME and TAG can be attributed to the stability of the complex that is formed between the K electrons of the carbon-carbon double bonds and the silver ions. The predominant interaction for saturated analytes is with the polar silanol groups. The secondary interactions are those of the silver ions with the unpaired electrons of the carbonyl oxygens of the analytes. The amount of silver adsorbed onto the silica and the pH (employment of acidic or basic modifiers) have been determined to have an effect on the retention and resolution of certain acidic and basic compounds and fatty acids [58]. 

Thus, mesoporous structures, because of their pores, make it possible to incorporate large catalytically active transition metal complexes. Covalently grafting these complexes in the hydrophobic patches provides better dispersion of the catalyst, as well as resistance to leaching. Improvement in catalytic performance is to be expected when materials with even larger pore diameters are studied. It will also be necessary to passivate the silanol groups responsible for promoting adsorption on the catalytic surface. 

F. Hoffmann und G. Roewer reported on reactions of cyclohexasilylpotassium with transition metal compounds, and C. Marschner described a Cp-fiee hafnium silyl-substituted complex. A series of four papers from the group of W. Malisch deal with the description of silanol functionalities in organometallic iron and tungsten complexes and the synthesis of perchlorinated metallo siloxanes. 

Preconcentration of transition metal cations, of obvious interest for environmental sensing and remediation, involves reaction with silanol groups via condensation processes. For instance, for mercury in aqueous solution, the prevailing species at pH values between 4 and 7 are the complexes Hg(OH)2 and Hg(OH)+. Their attachment to mesoporous silicates can be presented as (Schindler and Stumm, 1987) ... 

In contrast to groups 4 and 5 transition metal complexes supported on silica, the treatment by H2 of W surface organometallic derivatives, namely, [(=SiO)W( C Bu)(CH2 Bu)2] and [(=SiO)2W( C Bu)(CH2 Bu)], mainly lead to sintering (re-formation of silanols and detection of W particles by transmission electron microscopy (TEM)) and the hydride is obtained only in minute amounts. On the other hand, [(AlsO)W( C Bu)(CH2 Bu)2] reacts with H2 to give tungsten hydride species [(AlsO)2W(H) .] with no evidence of sintering, as evidenced by the presence of W-H and Al-H bands in the IR spectrum (Scheme 46). ... 

Metallo-silanols also proved to be synthetically valuable in the context of the generation of SiO-bridged polynuclear transition metal complexes [17]. Especially the Et3N-assisted condensation reactions with cyclopentadienyl-titanium and -zirconium chlorides result in nearly quantitative conversions due to the oxophilic nature of the group 4 metals. 

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