165751-46-2Relevant academic research and scientific papers
Aminoferrocenes and aminocobaltocenes as redox-active chelating ligands: Syntheses, structures, and coordination chemistry
Plenio, Herbert,Burth, Dirk
, p. 4054 - 4062 (2008/10/08)
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 Zn2+ and Co2+ salts with 1,1′-bis(di-2-picolylamino)-3,3′,4,4′-tetraphenylferrocene (12) was investigated. In the X-ray crystal structure of 12·2ZnBr2 it is apparent that the nitrogen atoms directly bonded to the ferrocene 12 take part in the coordination of Zn2+. This nitrogen donor thus acts as a relay, enhancing the electronic communication between the redox-active ferrocene center and the metal ion (Zn2+ or Co2+); 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(CF3SO3)2 is Δ(ΔE1/2) = +330 mV (12·Zn2+) and Δ(ΔE1/2) = +720 mV (12·2Zn2+). Upon complexation of Co2+ each metal ion produces a Δ(ΔE1/2) = +380 mV; thus the redox potential of 12·2Co2+ is shifted by Δ(ΔE1/2) = +760 mV relative to that of 12. This corresponds to a reduction in the stability constant for the complexation of Co2+ by approximately 3.4 × 106 per Co2+. 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.
