Transition metal clusters chirality

A7 Chiral transition metal clusters. Relationships between stereochemistry, electronic structure and circular dichroism spectra... 

Some general reviews on hydrogenation using transition metal complexes that have appeared within the last five years are listed (4-7), as well as general reviews on asymmetric hydrogenation (8-10) and some dealing specifically with chiral rhodium-phosphine catalysts (11-13). The topic of catalysis by supported transition metal complexes has also been well reviewed (6, 14-29), and reviews on molecular metal cluster systems, that include aspects of catalytic hydrogenations, have appeared (30-34). 

Cluster compounds, chiral, 27 191 molybdenum-sulfur, 27 47 transition metal, 27 182... 

A number of review articles have described the applications of multinuclear NMR methods for studying structures and internal rearrangements of metal clusters. The more specific cluster types covered were transition metal carbonyl clusters/ chiral clusters/ and mixed metal clusters containing carbyne or ketenylidene bridges. ... 

Polyoxometalates (POMs) are transition metal oxygen clusters with well-defined atomic coordination structures. POMs are used as functional nano-colloidal materials and also as supports for catalysts via ion-pair interactions due to their acidic properties. Combinations of chiral diamines and POM 225 effectively catalyze enamine-based aldol reactions. Less than 1 mol% of chiral amine loading is suf-ficientto catalyze the reaction (Table 28.10, entries 1 and 2) [114]. Highly diastereo-and enantioselective cross-aldol reactions of aldehydes are accomplished using chiral diamine-POM 226 under emulsion conditions (entries 3 and 4) [115]. Sul-fonated polystyrene or fluoropolymer Nafion NR50 are also good supports for the immobilization of primary-tertiary diamines. The catalyst 227 can be recovered by filtration and reused for at least four cycles with no loss of stereoselectivity (entries 5 and 6) [116]. 

To date only one such cluster has been reported. The tetranuclear cluster 33 is unstable in solution and over a period of several days eliminates the elements of Pt(COD)2, giving the Pt bridged cluster Pt[Ru3(/i-H)(ji4- 72-C=CtBu)(CO)9]2 (75) in reasonable yields (40-50%) (152). The C2 metal core (Fig. 15) is chiral, and a variable-temperature 13C NMR study showed that the cluster undergoes enantiomerization, with AG 66 = 57 kJ mol-1. The enantiomerization presumably proceeds via an intermediate or transition state with a planar coordination at the Pt atom. 

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