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

Abstract

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.

Bis(ferrocenylimine)palladium(II) and platinum(II) complexes: Synthesis, molecular structures and evaluation as antitumor agents

“…The advantages of these complexes include their ease of preparation, facile modification and convenience of handling [6]. Ferrocene derivatives containing atoms with good donor abilities have attracted research interest because coordination of a metal to these heteroatoms produces multicentre molecules with metals in close proximity, but in different environments, which may influence the mutual cooperation of the metals in a variety of processes [9][10][11]. Furthermore, some ferrocenyl complexes have also been investigated for their anti-cancer activity, and exhibited promising cytotoxic activities [12][13].…”

Synthesis, Structure and Binding Properties of Nickel and Copper [14]Cyclidene Complexes with Appended Aza Crown Ethers

“…Complex 4Cu (Figure 1) crystallizes in the triclinic P1 space group, as a salt in which two PF 6 Ϫ ions interact with one complexed Cu II cation. The complex occupies a special position, with the central Cu atom located exactly on the crystallographic symmetry center.…”

“…Therefore, the inner symmetry of the molecule becomes a crystallographic feature and, crystallographically, only half of the molecule is independent. Together with one PF 6 Ϫ ion the independent half of the 4Cu molecule defines the crystallographic asymmetric unit.…”

“…1.90 Å ). C (6) and N(3) are coplanar with the Cu II cation and the coordinating N(1) and N(2) atoms and both C(4)ϪC(6) and C(6)ϪN(3) bonds show partial double character, indicating that the lone electron pair of N(3) contributes to the electron density of the aromatic macrocycle. Important bond lengths and angles are given in Table 1.…”

“…Ferrocene crown ethers [1Ϫ6] and ferrocene cryptands [7Ϫ11] have been the most extensively studied species. However, such electro-active components as cobaltocene [6] quinone, nitrobenzene, and others have also been used. [4] Plenio et al [8,9] found that efficient electronic communication between the guest coordinated in the macrocyclic cavity and the redox active unit is crucial for the optimum performance of such molecular devices.…”