Indenyl Crown Ethers:  Heterotopic Ligands with π- and σ-Faces and the Synthesis of Cymantrene and Cobaltocene Crown Ethers and Their Alkaline Metal Ion Complexes

Abstract

2-Indanone reacts with aza-12-crown-4, aza-15-crown-5, aza-18-crown-6 and diaza-18-crown-6 to give the corresponding enamine 2-indenyl crown ethers 1 − 4 in almost quantitative yields. The indene subunits of 1 − 3 can be deprotonated with n -BuLi and reacted with Co(THF) 3 Br 2 to yield after oxidation the cobaltocenium salts 5 − 7 . Four oxidation states of the cobaltocene 5 are readily accessible in the CV experiment: E 1/2 = −1.99 V (−I/0, quasireversible), −0.94 V (0/+I), and +0.97 V (+I/+II), with the +I/+II oxidation occurring at an unusually low potential. Addition of LiClO 4 or NaClO 4 to 5 results in anodic shifts of the 0/+I redox wave of up to +190 mV, whereas the +I/+II redox pair is not at all influenced. The reaction of lithiated 2 with BrMn(CO) 3 (py) 2 gives the corresponding cymantrene crown ether 8 . The X-ray crystal structure of 8 displays a very short indenyl−nitrogen bond distance of 136.2(4) pm. Complexation of Li + and Na + by 8 [ E 1/2 (0/+I) = +0.91 V] leads to anodic shifts of the redox potential of +100 and +90 mV, respectively, and represents the first example of redox-switched bonding of metal ions involving a cymantrene derivative. Complexation of Li + and Na + within the crown ether subunit of 8, or protonation of the nitrogen atom, gives rise to shifts of the CO-stretching frequencies of up to 36 cm - 1; thus it is apparent that the basicity of the metal center can be influenced reversibly by added metal ions.