Rare-earth metal π-complexes of reduced arenes, alkenes, and alkynes: bonding, electronic structure, and comparison with actinides and other electropositive metals

Abstract

Rare-earth metal complexes of reduced π ligands are reviewed with an emphasis on their electronic structure and bonding interactions. This perspective discusses reduced carbocyclic and acyclic π ligands; in certain categories, when no example of a rare-earth metal complex is available, a closely related actinide analogue is discussed. In general, rare-earth metals have a lower tendency to form covalent interactions with π ligands compared to actinides, mainly uranium. Despite predominant ionic interactions in rare-earth chemistry, covalent bonds can be formed with reduced carbocyclic ligands, especially multiply reduced arenes.

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.…”

“…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. 29,30 Therefore, in theory, arenes could serve as flexible and redox non-innocent ligands.…”

“…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.…”

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).…”