Inorganic supports covalent anchoring

One such organic-inorganic hybrid support has been synthesized by covalently anchoring a N-octyldihydroimidazolium cation fragment onto Si02 (denoted as... 

Many heterogeneous catalysts have been reported in the past to be prepared by anchoring or grafting, processes whereby stable, covalent bonds are formed between an homogeneous transition metal complex and an inert polymer or inorganic support [1-4] The aim was to combine the potential versatility and selectivity of homogeneous catalysts with the practical advantages of a solid material [5]... 

Figure 1. Ordered array of covalently anchored organic groups on a stable inorganic support with a layered structure.

Metal complexes have been immobilised in many different ways, e.g. covalent anchoring (by grafting or tethering) to inorganic supports, immobilisation by occlusion in zeolitic micro- or mesopores (ship-in-a-bottle concept), or as supported liquid-phase catalysts [2], In recent reviews the potential of (chiral) metal complexes immobilised by these different methods has been evaluated [3,4,5]. 

The covalent anchoring of hydroformylation catalysts onto inorganic silica surfaces was already studied in the late seventies [43]. Chloromethyl silicone polymers were converted to the iodo-analog and subsequently reacted to the diphenylphosphinemethyl polysiloxanes. After treatment with RhCl(CO)(PPh3)2 these materials were used as catalyst in the hydroformylation of 1-hexene (68 bar CO/H2 (1 1), 100 °C, benzene). Under these conditions, however, the metal dissociated from the support. The resulting rhodium carbonyl species was responsible for the activity leading to low selectivities. 

Even if polymers are easily functionalized, they exhibit poor heat transfer ability and poor mechanical properties and are often temperature sensitive. The choice of a reaction solvent is also important since some solvents cause the polymers to swell. Another issue is the leaching of metal from the polymer, resulting in the deactivation of the catalyst. These drawbacks can be partially eliminated if the catalyst precursors are covalently anchored on inorganic supports. 

Another deposition method is that involving the creation of a strong bond, for example, a covalent bond, between the support and the active element [7,8], This process is typically called grafting or anchoring. This procedure is carried out by a chemical reaction between functional hydroxyl groups on the surface of the support and a properly chosen inorganic or organometallic compound of the active element. 

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