Synthesis and Characterization of Cerium and Yttrium Alkoxide Complexes Supported by Ferrocene-Based Chelating Ligands

Abstract

Two series of Schiff base metal complexes were investigated, where each series was supported by an ancillary ligand incorporating a ferrocene backbone and different N=X functionalities. One ligand is based on an imine, while the other is based on an iminophosphorane group. Cerium(IV), cerium(III), and yttrium(III) alkoxide complexes supported by the two ligands were synthesized. All metal complexes were characterized by cyclic voltammetry. Additionally, NMR, Mössbauer, X-ray absorption near-edge structure (XANES), and absorption spectroscopies were used. The experimental data indicate that iron remains in the +2 oxidation state and that cerium(IV) does not engage in a redox behavior with the ancillary ligand.

Reactivity and Properties of Metal Complexes Enabled by Flexible and Redox-Active Ligands with a Ferrocene Backbone

“…59 H 2 (thiolfan) (1,1′-di(2,4-di-tert-butyl-6-thiomethylenephenol)ferrocene) and H 2 (thiolfan*) (1,1′-di(2,4-di-tert-butyl-6-thiophenol)ferrocene) were prepared from 1,1'-dilithioferrocene. 43 The synthesis of 1,1'-fc(N 3 ) 2 (featured in red) and 1,1'-fc(NH 2 ) 2 (featured in blue) can be traced back to 1961 to Pauson and coworkers, who published it shortly after the discovery of ferrocene.…”

“…43,[59][60][61] We first introduced a Schiff base derivative, salfen, to cerium and yttrium 78 and later developed the phosfen ligand that could be cleanly oxidized by a ferrocenium oxidant to enable redox controlled catalysis. 61 The salfan, thiolfan and thiolfan* ligands were found to fit group 4 metals well and have been exploited in redox-switchable copolymerization processes.…”

“…43,59,78 (NN fc ): Initially, we sought out to employ salt metathesis between K 2 (NN fc ) and MX 3 (THF) y (M = rare-earth metal, X = Cl or Br, y = 3-4) and found that the reaction was poorly reproducible and usually accompanied by a side product, the bis-ligand-adduct K[(NN fc ) 2 M], which was difficult to remove. 81 At the same time, we and other groups independently published protocols to prepare key metal benzyl synthetic intermediates, M(CH 2 Ar) 3 (M = Sc, Y, La, and Lu, Ar = Ph, 3,5-C 6 H 3 Me 2 ), from MX 3 (X = Cl, Br), [95][96][97][98] which, in turn were obtained directly from metal oxides.…”

“…[59][60][61]78 Due to the high acidity of the phenolic proton of H 2 (OEEO fc ), the installation of the metal fragment was facile; the choice of synthetic routes was mainly based on the desired composition of the pre-catalysts used for polymerization and the ease of availability of a metal precursor. Group 4 metal complexes supported by salfan, thiolfan, and thiolfan* ligands were prepared in a similar way using Zr(OtBu) 4 or Ti(OiPr) 4 as metal precursors.…”

“…The cyclic voltammetry of H 2 (phosfen) and the corresponding Y(III) and Ce(IV) complexes (phosfen)Y(OtBu) and (phosfen)Ce(OtBu) 2 showed a reversible redox event at a more negative potential compared to the corresponding salfen analogues. 59 Therefore, we achieved control of lactide polymerization by chemically oxidizing or reducing the catalyst at a remote site of catalysis, the ferrocene backbone of the phosfen ligand (Figure 2). 61 The difference in reactivity between the oxidized and reduced forms of the catalyst was attributed to changes in their electronic profiles.…”