Magnesium clusters stability

What are the processes in magnesium-organic halide films that lead to stabilization of tetranuclear magnesium clusters ... 

Reviews covering the chemistry of group 2 metal complexes with phosphorus-stabilized carbanions,279 and of molecular clusters of magnesium dimetallated primary phosphanes, are available.2 u Magnesium phosphanes remain rare compounds.281 Lithiation of bromide 98 with BuLi in the presence of tmeda in pentane produces a lithium phosphine dimer subsequent treatment with MgCl2 in EtzO gives the phosphane 99 in 69% overall yield (Equation (19)). The centrosymmetric 99 has Mg-C = 2.217 A Mg-P = 2.77 A (av.).282... 

Carboxamidation of RLi or RMgX.1 This reaction can be effected by reaction of RLi (or RMgX) with DMF to give a hemiaminal a followed by an Oppenauer oxidation. The second step requires the presence of a magnesium alkoxide such as magnesium 2-ethoxyethoxide 1, Mg(OCH2CH2OC2H5)2, either as a catalyst for the oxidation or for stabilization of a, possibly as a mixed cluster. 

Thus, early theoretical and experimental studies of low-temperature reactions in the magnesium-organic halide films allowed one to formulate the problem of obtaining cluster Grignard reagents inaccessible for classical organometallic synthesis but provided no actual evidence of their formation and stabilization. 

The system was very sensitive to the transition metal ion added to the platinum. When it was nickel, hydrocin-namaldehyde was obtained in 97% selectivity.75 The intermediates in these reductions are probably metal hydride clusters. When the colloidal platinum is supported on magnesium oxide, without another transition metal, the reduction produces the unsaturated alcohol with 97% selectivity.76 A rhodium colloid stabilized by the same polymer was used with a water-soluble phosphine in the hydro-formylation of propylene to produce 1 1 mixture of /r-bu-tyraldehyde and isobutyraldehyde in 99% yield.77 It could be used at least seven times, as long as it was not exposed to air. 

Some addilional slahili/ing factor is necessary for Ihe formation of measurable quantities of hydride clusters. It could be the formation of complexes with molecules in the matrix formed on condensation at low temperature. Alkanes cannot coordinate to magnesium, and therefore should not aid hydride cluster formation. The presence of molecules such as alkenes or aromatics that can form -complexes with magnesium, however, could exert a favourable inilu-ence on the cluster hydride yield. In solutions, there are possibilities for stabilization by association of magnesium with solvent molecules. 

Figure 15.13 Structure of an NHC-stabilized magnesium-hydride cluster [49].

The veiy small copper ciystals that form after careful diying, calcination and reduction are veiy st le and the catalyst remains active for several years of plant operation. It has been suggested that the reduced copper is stabilized as clusters of copper atoms at the oxygen rich (001) surface of the zinc oxide. In turn, the zinc oxide is stabilized by small zinc aluminate particles or other refractory oxides. The addition of small amounts of magnesium oxide enhances the stabilizing effect of zinc oxide. Zinc oxide can also absoib any traces of sulfur or chlorine in synthesis gas. 

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