Sodium complexes, ring metalation

In 1967, DuPont chemist Charles J. Pedersen (21) discovered a class of ligands capable of complexing alkaU metal cations, a discovery which led to the Nobel Prize in Chemistry in 1987. These compounds, known as crown ethers or cryptands, allow gready enhanced solubiUty of sodium and other alkaU metals in amines and ethers. About 50 crown ethers having between 9—60 membered oligoether rings were described (22). Two such stmctures, dibenzo-18-crown-6 (1) and benzo-9-crown-3 (2), are shown. 

The salt metathesis reaction of sodium fluorenone ketyl with Cp 2ZrCl2 in THF produces the corresponding zirconium fluorenone ketyl complex 856657 which was structurally characterized (Scheme 215). In the case of sodium benzophenone ketyl, further reaction between the ketyl radical and a Cp ligand takes place to give finally the ring-metallated zirconocene bis(alkoxide) complex 857. 

The crowns as model carriers. Many studies involving crown ethers and related ligands have been performed which mimic the ion-transport behaviour of the natural antibiotic carriers (Lamb, Izatt Christensen, 1981). This is not surprising, since clearly the alkali metal chemistry of the cyclic antibiotic molecules parallels in many respects that of the crown ethers towards these metals. As discussed in Chapter 4, complexation of an ion such as sodium or potassium with a crown polyether results in an increase in its lipophilicity (and a concomitant increase in its solubility in non-polar organic solvents). However, even though a ring such as 18-crown-6 binds potassium selectively, this crown is expected to be a less effective ionophore for potassium than the natural systems since the two sides of the crown complex are not as well-protected from the hydro-phobic environment existing in the membrane. 

The first heavier alkali metal phosphide species to be structurally characterized were the sodium and potassium complexes [M P(mes) (SiFBu2) (THF)2]2 (M = Na, K) (67). These isostructural complexes adopt an eight-membered (MPSiF)2 ring motif in which each alkali... 

Generally, cyclohexyne is an unstable molecule because of its ring strain. However, it can be stabilized by coordination to transition metals.35 The reduction of 1,2-dibromocyclohexene by sodium/mercury in the presence of a nickel-bromide complex afforded the Ni-alkyne complex 66 as a thermally stable and isolable compound (Scheme 22).36 Complex 66 smoothly reacted with C02 under atmospheric pressure to give nickelacycle 67 in good yield. Dimethyl acetylenedicarboxylate was inserted into the vinyl-nickel bond in 67 to give the seven-membered oxanickelacycle 68. 

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