The enamine reaction can be applied to secondary amines and 3,4-diphenylcyclopentenone to produce the corresponding aminodiphenylcyclopentadienes in yields between 42 and 52%. Deprotonation of these cyclopentadienes and reaction with Fe(II) and Co(II) salts gave the respective diaminotetraphenylferrocenes and diaminotetraphenylcobaltocenes in yields of around 50%. In the amines chosen for our study the nitrogen atoms are part of a chelating ligand, which is able to form complexes with metal ions. Hence, the formation of stable complexes of Zn 2+ and Co 2+ salts with 1,1′-bis(di-2-picolylamino)-3,3′,4,4′-tetraphenylferrocene (12) was investigated. In the X-ray crystal structure of 12‚2ZnBr 2 it is apparent that the nitrogen atoms directly bonded to the ferrocene 12 take part in the coordination of Zn 2+ . This nitrogen donor thus acts as a relay, enhancing the electronic communication between the redox-active ferrocene center and the metal ion (Zn 2+ or Co 2+ ); consequently in the cyclic voltammograms of 12 addition of metal salts leads to very large changes in the redox potential of the ferrocene unit. The relative anodic shift upon addition of Zn(CF 3 SO 3 ) 2 is ∆(∆E 1/2 ) ) +330 mV (12‚Zn 2+ ) and ∆(∆E 1/2 ) ) +720 mV (12‚2Zn 2+ ). Upon complexation of Co 2+ each metal ion produces a ∆(∆E 1/2 ) ) +380 mV; thus the redox potential of 12‚2Co 2+ is shifted by ∆(∆E 1/2 ) ) +760 mV relative to that of 12. This corresponds to a reduction in the stability constant for the complexation of Co 2+ by approximately 3.4 × 10 6 per Co 2+ . It is apparent therefore that our concept of attaching donor atoms directly to the redox-active ferrocene unit leads to very large redox-switching effects.

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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)3Br2 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 LiClO4 or NaClO4 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.

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