Biferrocene

Ahn et al. [20] reported the synthesis of homochiral bis(oxazolinyl)-biferrocene ligands (structure 11 in Scheme 7), which also have both planar and central chirality. With these complexes, 2-(phenyl)cyclopropane carboxy-lates were obtained in up to 99% ee and a trans/cis ratio of 88/12. 

Fig. 2. Electronic structure of biferrocene (2) (a) energy diagram (b) illustration of large electronic interaction between dx2 >2 orbitals (e2 ) of ferrocene units through pz orbitals of Cp rings (54).

Morrison and Hendrickson have reported that the IT band of biferro-cenium monocation has two peaks at 77 K (50), whereas the analysis of IT bands of biferrocene derivatives observed at room temperature is carried out assuming one broad peak, and it has been reported that Eq. (5) is applicable to biferrocene (41). As for the IT bands of higher unsubstituted oligoCferrocene-hl diylls in the mixed-valence states, only one report by Brown et al. on the IT bands of the monocation and dication of terferrocene and the dication of quaterferrocene (tetramer) has been published (64). 

Several systems have been reported regarding the connection between oligoferrocenylene multistep redox reactions and the functions of other molecular units. Colbert et al. have reported Mn and Ru complex-conjugated biferrocene derivatives, 20, showing large electron... 

Dong et al. have reported on crown ether-bound biferrocene, 21, in which the redox potentials are shifted by the capture of metal ions within the crown ether (55). For example, the electrochemical behavior observed for the biferrocene derivative with Ba2+ ion is fundamentally different, as new redox couples are observed for 9 with Ba2+ concentrations within the range 0 < [Ba2+] < 2 eq. The peak currents for the two new redox couples increase with concentrations of the Ba2+ ion until a full equivalent is added at this point, the original redox couples disappear and the new redox couples reach full development. 

The crystal structure of the salts [l, l"-R2-l,l"-biferrocene][Ni(mnt)2] (R = isopropyl or dineopentyl) consist of segregated chains of anions and cations. For the compound with R = isopropyl the chains are based on an arrangement of slightly trimerized anions, [Ni(mnt)2]33, with close interanionic separations, while in the case of the compound with R = dineopentyl, although the structure is still based on a similar arrangement, the anionic units are quite separated from each other. In both compounds a decrease of yT with cooling is observed, indicating dominant AF interactions [70],... 

Excellent ee-values (up to 94%) have been obtained for the hydrogenation of various 2-substituted N-acetyl indoles with an in-situ prepared rhodium catalyst modified with the trans-chelating diphosphine (S,S)-(R,R)-2,2"-bis[l-(diphenyl-phosphino)ethyl]-1,1"-biferrocene (Scheme 16.23) [88]. A strong base was required as co-reagent to observe both high conversion (TOFs of 50-100) and en-... 

Fig. 8. Structures of the biferrocenylene (II, III) cation (a) and the biferrocene (II, III) cation (b). Reproduced with permission from Ref.5)...

The first derivative is biferrocene, [( 5-C5H5)Fe( 5-C5H4)( 5-C5H4) Fe(jj5-C5H5)], whose molecular structure is shown in Figure 11.21... 

Figure 11 X-Ray structure of biferrocene, together with the relevant bond lengths...

As illustrated in Figure 12, biferrocene undergoes two reversible (on the cyclic voltammetric time scale) one-electron oxidations at separated potential values ( gy +=+ 0.31 V E+,2+ = + 0.65 V).22... 

Figure 12 Cyclic voltammogram recorded at a platinum electrode in a CH2Cl2 MeCN (1 1) solution of biferrocene. Supporting electrolyte [NBu4][PF6J (0.1 mol dm 3). Scan rate 0.2 Vs ...

Figure 15 Schematic representation of the cyclic voltammetric responses expected from biferrocenes, depending upon the extent of electronic delocalization... 

Other 2, 2 -disubstitued biferrocene derivatives (ligands 2, Fig. 3) also tested using the dinuclear [Ir( r-Cl)(COD)]2 as the iridium source provided total conversion in arylimine hydrogenation, although with poor-to-moderate enantioselectivity (20-33% ee) [17]. The low enantioselectivies are assumed to be related to the conformational flexibility of this type of bifep ligands. 

A series of frani -chelating chiral biferrocene diphosphine ligands enable a rhodium(I)-catalysed aldol reaction of 2-cyanopropionates to proceed in up to 93%... 

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