Donor transition metal-crown ether complexes

Polyoxometalates also play an important role in the selection of a metal ion for its complete encapsulation in the cavity of a crown ether to form an unusual supramolecular cation structure. For example, the crown ethers (macrocyclic polyethers), generally, do not readily form complexes with first-row transition metals in their low oxidation states because such metal ions provide only soft coordination (acceptor) sites and crown ethers have hard donor atoms. Naturally, only a few first-row transition metal rown ether complexes had been structurally characterized in which a direct bond formation between a transition metal and the crown ether oxygen atoms became possible rare examples of this kind are offered by the smaller ring crown ethers (e.g., 15-crown-5 and... 

The development of CTOwn ether functionalised imidazolinm salts starts from the consideration that it is possible to link one polyether chain with two imidazolinm nnits at the end points. Since a transition metal can coordinate two imidazoUnm salts in trans fashion [131,162,208], two of these (poly)ether fnnctionaUsed bis-carbenes can form a macrocychc crown ether type hgand system with two transition metal carbene linkages. In favonrable cases, a pincer type C,0,C ms-coordination to the same transition metal is conceivable, bnt may not be very likely when the great affinity of late transition metals to NHC ligands and the aversion of these same late transition metals to ether donor Ugands is taken into account. However, hanilabile stabilisation of transition metal complexes in catalytic processes can certainly be hoped for. 

The second ligand type consists of a large group of cyclic compounds incorporating numbers of ether functions as donors. Structure (22) illustrates a typical example. Such crown polyethers usually show strong complexing ability towards alkali and alkaline earth ions but their tendency to coordinate to transition metal ions is less than for the above... 

These results clearly indicate that the chelate ligation is driven primarily by the enthalpic factor and the entropy plays merely a trivial role in determining the complex stability. This is quite reasonable since the structures of these chelate complexes are strictly defined by the number and direction of the coordination sites of given heavy/transition metal ions, and therefore there is little room for the entropic term to adjust flexibly the complex structure and stability. On the contrary, alkali and alkaline earth metal ions also have the formal coordination numbers, but the actual number and direction of ligand coordination are highly flexible in the weak interaction-driven ligation by hard donors like glyme and crown ether. 

Wozniak and coworkers described recently the first heterodinuclear bismacrocyclic transition metal complex 34 + (Fig. 14.5) that exhibits potential-driven intramolecular motion of the interlocked crown ether unit.25 26 Although the system contains transition metals, the main interaction between the various subunits, which also allowed to construct catenane 34+, is an acceptor-donor interaction of the charge transfer type. 

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