Metallacrown ethers

Metallacrown ethers are a class of molecular receptors with both crown ether- and transition metal-receptor sites. The metallacrown ether [PdCl2[ PPh2-(CI I2CI l20)4CH2CH2PPh2-P,P, ]m.608 exhibits a unique mode of molecular recognition in which the metallacrown ether ring is expanded by the incorporation of a PdCl2 moiety. The product of this reaction is the first reported example of a dimetallacrown ether, see Scheme 11,609... 

By far more interesting are polyether functionalised imidazolium salts, especially those that lead to metallacrown ether functionalised carbene ligands. Development of this particular ligand class follows the by now familiar pattern of adapting an existing functionalised phosphane to the corresponding carbene. Here, the favourable properties of metallacrown ether functionalised phosphanes in catalytic processes [202-205] leads to the development of similarly functionalised NHC ligands. 

On the contrary, encapsulation of the larger K" " or NH4 ions should lead to the formation of metallacrown ether sandwich complexes (M+ MC= 1 2). Reaction of diethyl ketipinate H2L 13 with copper(ll) acetate in the presence of calcium nitrate affords green microcrystals 14 after crystallization from tetrahydro-furan/diethyl ether (Scheme 1). 

Crown ethers selectively complex alkaline ions [6,41], and the complexation of different sized cations leads to coronates [5,42] of various structures. The structural analogy between crown ethers and their topologically equivalent metallacrown ethers... 

A8 Development of metallacrown ethers a new class of metal clusters... 

Fan developed tunable heterobimetallic potassium(i)-crown ether rho-dium(i) catalysts based on a,o)-bis(chiral phosphite) or a,co-(achiral phosphine-chiral phosphite) polyether ligands (Scheme 2.46). For the asymmetric hydrogenation of enamides, catalysts 79 and 80 that showed coordination to the respective metallacrown ethers with the potassium salt... 

PPh2(CH2CH2O) CH2CH2PPh2) complexes have been prepared and the X-ray structure of the metallacrown ether for n = 3 has been reported.The new ligand... 

FIGURE 8-6 A Crown Ether, a Cryptand, a Metal Ion Encased in a Cryptand, and a Metallacrown. 

The metallacrown complex Eu(N03)2[15-MCcu(ii)N(pic)-15] N03 is synthesized by dissolving Cu(OAc)2 H2O (0.40 g, 2 mmol) in 50 mL of DMF in a 500 mL Erlenmeyer flask. To this solution is added H2picHA (0.28g, 2 mmol) dissolved in 50 mL of DMF. The solution is allowed to stir for one hour, at which point solid Eu(N03)3 6H2O (0.18 g, 0.4 mmol) is added, and the solution is stirred for several hours. Slow evaporation of the solvent results in a 79% yield of dark green, rhombic crystals. (The checkers found it necessary to reduce the volume of the solution to a third of the original volume before crystals formed.) These crystals, obtained in 71% yield, were washed with ether, were dried, and gave satisfactory elemental analysis. 

Several other polymetallic structures formed by self-assembly have been reported in the literature. For instance the class of metallacrowns [48] has received increasing attention over the last few years. These inorganic analogues of crown ethers arc self-assembled from simple building blocks and transition metal ions, mainly Cu " and Fe " (Fig. 9.8). The lanthanide ion, situated in the central cavity, acts as a template in the formation of the heterometallic structure. The first lanthanide-containing stmctures reported were made of a hexanuclear cluster in which the lanthanide ion is surrounded by five transition metals... 

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