Oxaferrocene cryptands as efficient molecular switches for alkali and alkaline earth metal ions

The reactions of 3,4-diphenylcyclopent-2-en-1-one, cyclopent-2-en-1-one, 3,4-dimethylcyclopent-2-en-1-one, and 2,3,4,5-tetramethylcyclopent-2-en-1-one with various trialkylsilyl triflates (SiR(3) = SiMe(3), SiEt(3), SiMe(2)(t)Bu, Si(i)Pr(3)) result in the formation of the respective cyclopentadienyl silyl ethers in virtually quantitative yields. The reactions of these cyclopentadienes with n-BuLi and FeCl(2) give the respective substituted ferrocenyl trialkylsilyl ethers in 48-57% yields, which are synthetic equivalents of hydroxyferrocenes as generated by a suitable cleavage protocol: SiMe(3)-protected ferrocene alcohols (CH(3)CN + 10% H(2)O + Na(2)CO(3)); SiEt(3) (CH(3)CN + 10% H(2)O + Na(2)CO(3) + NaF); and SiMe(2)(t)Bu protection (NBu(4)F . 3H(2)O in CH(3)CN). Carrying out the deprotection in the presence of carbon electrophiles leads to the respective ferrocenyl alkyl ethers in excellent yields. Using 1-chloro-2-tosylethane results in 1,1'-bis(2-chloroethoxy)-3,3',4,4'-tetraphenylferrocene which was converted into 1,1'-bis(2-iodoethoxy)-3,3',4,4'-tetraphenylferrocene in 89% yield, to be finally reacted with diazacrown ethers (diaza-12-C-4, diaza-15-C-5, diaza-18-C-6). In this manner the respective ferrocene cryptands 23, 24, and 25 were generated as the 1 + 1 products in yields of 67-80%. The coordination of alkali and alkaline earth metal ions within the cavities of these ferrocene cryptands results in large anodic shifts of the redox potentials of the ferrocene units (23, E = +0.290 V; 23 + Ca(2+), E = +0.670 V; 23 + Na(+), E = +0.505 V; 24, E = +0.285 V; 24 + K(+), E = +0.455 V) as determined by cyclic voltammetry. These anodic shifts are correlated with a decrease in the complex stability constant K of the respective metal ion complex (redox-switching effect) and in the case of 23 and Ca(2+) amounts to a reduction of K by 3.4 x 10(6).