Aminoferrocenes and Aminocobaltocenes as Redox-Active Chelating Ligands: Syntheses, Structures, and Coordination Chemistry1

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 Δ(Δ E 1/2 ) = +330 mV ( 12 ·Zn 2+ ) and Δ(Δ E 1/2 ) = +720 mV ( 12 ·2Zn 2+ ). Upon complexation of Co 2+ each metal ion produces a Δ(Δ E 1/2 ) = +380 mV; thus the redox potential of 12 ·2Co 2+ is shifted by Δ(Δ 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.