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Chapter 1 Introduction 1.6.3 Oxidation catalysis Compared to the use of heteropoly acids in acid catalysis, the use of POMs in oxidation catalysis is a much more diverse area. It is noteworthy that complex anions such as [SiW 11 O 39 ] n− have been used as inorganic analogues of metalloporphylins, 24 but with some differences and advantages: 1) The inorganic POM ligand is not susceptible to oxidative degradation and is thermally more robust than porphin or other macrocyclic ligands. 2) The acceptor properties of the POMs ligand are dependent on its reducibility and can be modified by metal replacement or by partial reduction. 3) The electron population on metal and ligand is considerably more variable than for the metalloporphyrins. POMs as catalysts can be used, for instance, in the formation of carboxylic acids from the corresponding aldehydes, as well as in the dehydrogenation of alcohols, aldehydes, and carboxylic acids to form C=C and C=O bonds; [POM] ox + nH + + S H n [POM] red + n\4 O 2 H n [POM] red + S ox [POM] ox + n\2 H 2 O Heterogeneous catalytic oxidation processes involving well-characterized POMs have been reviewed. 25 Homogeneous oxidative catalytic activity has been demonstrated for thermal dehydrogenation. 26 Polymolybdovanadates have been used as reoxidants in Wacker chemistry (e.g. oxidation of alkenes) and may themselves be reoxidized with oxygen; 27 Pd o + [POM] ox + 2H + H 2 [POM] red + ½ O 2 Pd II + H 2 [POM] red [POM] ox + H 2 O Use of the heteropolyanion instead of the normal CuCl 2 oxidant avoids corrosion by HCl. 25
Chapter 1 Introduction 1.7 Lanthanide‐containing polyoxotungstates Lanthanide-containing POMs have been investigated less than those containing 3d-transition metals. Because of the larger size of the lanthanide ions compared to 3d metals, they are not fully incorporated into the lacunary site(s) of vacant POM precursors. Due to their higher coordination numbers, each lanthanide ion can be used as linkers to one or more other lacunary POM units. Lanthanide-containing POMs can be of interest due to photoluminescence as well as catalytic, electrochemical, and magnetic properties. 28-29 The area of lanthanide-containing POMs is dominated by tungsten based POMs, as a large number of vacant (lacunary) POM precursors is known (as opposed to vanadium or molybdenum based POMs). The two main subclasses in this field are lanthanide-containing iso- and hetero-polyoxotungstates. The number of the reported lanthanide-containing POMs in the latter class is significantly larger than the former, probably because more lacunary heterothan iso-polytungstates are known. In 2007, Pope reported a review presenting the structural chemistry and properties, and applications of POMs that incorporate one or more lanthanide ions. 30 Furthermore, another subclass in this field is mixed lanthanide and d-transition metal containing heteropolyoxotungstates. 1.7.1 Lanthanide‐containing isopolyoxotungstates Until very recently, there has been only one family of lanthanide-containing isopolyanions reported so far. Peacock and Weakley first reported the decatungstate, sandwich-type family of the general formula [Ln(W 5 O 18 ) 2 ] n− . 31a This was synthesized for Ln 3+ = La, Ce, Pr, Nd, Sm, Ho, Yb and Y, and for Ce 4+ by reaction of the lanthanide ions with Na 2 WO 4 in hot aqueous (pH 6.5−7.5) solution. 31 The structure of the [Ln(W 5 O 18 ) 2 ] n− polyanions (D 4d ) consist of two monolacunary, Lindqvist based fragments [W 5 O 18 ] 6− encapsulating a central metal ion exhibiting a square antiprismatic coordination, as depicted in Figure 1.11. 26
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