Ferrocenyl ligands precursors

There are several reports on the exploitation of ferrocenyl moiety in the building up of polymetallaynes. A novel heterobimetallic platinum acetylide polymer of ferrocenylfluorene, P72, was reported and spectroscopically characterized.95 The presence of significant donor-acceptor interaction leads to a narrow band-gap value of 2.1 eV for this Pt-Fe mixed material, which is much lower than for the parent polymer, P25. The polymer is electroactive, with the half-wave potential of the ferrocene moiety slightly more anodic in P72 than in the diethynyl ligand precursor, which is in line with the transfer of electron density from the ferrocenyl donor unit to the electron-accepting Pt(II) center through the acetylide linkage. 

A bidentate ferrocenyl-A-heterocyclic carbene ligand precursor in the form of the bis-imidazolinium salt was deprotonated in situ with KHMDS and reacted with [PdCl2(cod)] to provide a bis(carbene)palladium(n) dichloride complex, which was structurally characterized (eq 100). ... 

The ligands discussed so far all contained C2 symmetry. An important new class of ligands having Ci symmetry was introduced by Togni [22] (see Figure 4.19). They can be easily made from an enantiomeric amine as the precursor in a few steps. Different substituents can be introduced at the phosphorus atoms. In addition to the chiral carbon atom the molecule has planar chirality as well. The chiral carbon atom is used to introduce the planar chirality, i.e. lithiation of the ferrocenyl amine takes place at a specific side of the amine at the ferrocene moiety. 

The first iron-containing silsesquioxanes which appeared in the literature were compounds containing ferrocenyl units as side-groups.102 104 However, these are not within the scope of this review as iron is not part of the metallasilsesquioxane skeleton. Meanwhile, several ferrasilsesquioxane complexes have been synthesized. The first iron(III) compound of this type was prepared in our laboratory according to Scheme 56.105 In 161, the coordination sphere of iron is completed by TMEDA (NjNjN N -tetramethylethylenediamine) as a chelating amine ligand. Pale yellow, crystalline 161 was isolated in 80% yield and structurally characterized by X-ray diffraction. This compound was later used by Maxim et al.106 107 to prepare iron particles dispersed on microporous silica via controlled calcination of the ferrasilsesquioxane precursor as depicted in Scheme 56. 

The cationic Pd(II)-7i-allyl complexes of the formula [(Ligand)Pd(ri3-C3H5)]PF6 (Ligand = ferrocenyl imino-phosphine la-e) were isolated in the solid state by the literature procedure15,61 using [(ri3-C3H5)PdCl]2 as starting material.[7] The complexes obtained were used as catalyst precursors for the asymmetric allylic... 

Several other palladium complexes Pd(L )(Ph)(I)] and Pd(L )(trans-stil-bene)] with different chiral diphosphine ligands L were also employed as catalyst precursors. The yield and enantioselectivity of the product are strongly dependent on the diphosphine. It was found that the DuPhos series (Me/Et/f-Pr-DuPhos) produces results similar to those listed in Table 6.6. In contrast, changing to the more flexible (R,R)-Me-BPE gives a very active catalytic system but with a disappointing 18% ee. With DuXantphos a very slow system results, with the formation of by-products. Ferrocenyl phosphines (Me-FerroLANE, Et- FerroTANE, Josiphos, BoPhoz) were also employed but were found to be inferior to DuPhos. Finally, with several Ni(Me-DuPhos) catalyst precursors, reactions do not reach completion and formation of by-products is observed. 

An innovative application of this type of ligands was reported by Schmaltz and Gotov [61]. They reported the use of a chiral ferrocenyl (i ,S)-PPF-pyrrolidine system in the methoxycarbonylation of l,2-dichlorobenzene-Cr(CO)3 to introduce planar chirality in 7i-complexes (Scheme 3.4) and achieved up to 95% ee using the isolated precursor [PdCl2(P-N)] under 1 atm of CO at 60 °C in the presence of NEt3. 

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