Rare Earth Arene-Bridged Complexes Obtained by Reduction of Organometallic Precursors

Abstract

Rare earth chemistry has witnessed remarkable advances in recent years. In particular, ancillary ligands other than cyclopentadienyl derivatives have been introduced to the organometallic chemistry and their complexes exhibit distinct reactivity and properties compared to the metallocene or half-sandwiched analogues. The present chapter reviews arene-bridged rare earth complexes with an emphasis on those compounds obtained by reduction reactions. A particular emphasis is placed on rare earth complexes supported by 1,1′-ferrocenediyl diamides since they show the most diverse chemistry with arenes: fused arenes, such as naphthalene and anthracene, formed inverse-sandwiched complexes, in which the arene is dianionic; weakly conjugated arenes, such as biphenyl, p-terphenyl, and 1,3,5-triphenylbenzene, were unexpectedly reduced by four electrons and led to 6-carbon, 10- electron aromatic systems; on the contrary, (E)-stilbene, which has a carbon-carbon double bond between the two phenyl rings, could only be reduced by two electrons, which were located on the carbon-carbon double bond instead of the phenyl rings.

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

“…47,51 Much of the work was summarized in previous reviews. 27,28,40 Herein, we are going to highlight the role of the ferrocene ligand in stabilizing unusual moieties and promoting unprecedented reactivity.…”

“…The content is organized in the following order: A. Synthesis and electronic properties of ferrocene-based ligands; B. Synthesis of rareearth metal complexes supported by ferrocene-based ligands; C. C-H activation, dearomatization, and ring-opening reaction of aromatic N-heterocycles; D. Reduction chemistry with  ligands and small molecule activation; E. Redox-switchable catalysis enabled by redox active metals and ligands. Since large parts of section C and some part of section D have been previously reviewed, 27,28,72,80 we are not going to discuss details but rather highlight the role of ferrocene-based ligands in enabling new reactivity and properties.…”

“…35 Our group has taken a different approach in order to achieve the goal of coordination flexibility and redox activity (Chart 1g, h). [27][28][29] Instead of using a purely organic ligand, we reasoned that an organometallic ligand should serve as an excellent candidate. Such a ligand needs to fulfill the following requirements: (1) the organometallic part should be chemically inert under common reaction conditions; (2) the metal should be redox active; (3) the organometallic fragment should be able to act as a weak donor.…”

“…[22][23][24][25] Besides traditional phosphine and nitrogen donors, an arene is suitable for metal-ligand cooperation as well: first, the coordination mode of the arene can vary from  6 to  2 according to the degree of delocalization. 26,27 Second, while the HOMO of an arene can act as a  or  donor, its LUMO has the appropriate symmetry and energy to engage in back-donation with electron-rich metals. 28 In some cases, a metal-toligand charge transfer can result in partial reduction of the arene and, thus, provide an electron reservoir for the metal.…”

Aromatic C–F Bond Activation by Rare-Earth-Metal Complexes

“…As part of our group's continuing efforts to explore the chemistry of rare-earth metals and actinides supported by ferrocene-derived ligands, [55][56][57][58][59][60][61][62] especially redox reactions in the presence of arenes, [63][64][65][66][67][68][69][70][71] we discovered a bimetallic cleavage of the C-H bond of benzene by (NN TBS ) . In addition, the C-H bond activation of a naphthalene dianion sandwiched between two lutetium ions in the previously isolated [(NN TBS )Lu(THF)]2(μ-C10H8) (4-Lu2) was observed upon moderate heating (Scheme 2c).…”