The search for new catalysts for more efficient, selective chemical transformations and for the synthesis of new functional materials has been a long-standing research subject in both academia and industry. To develop new generations of catalysts that are superior or complementary to the existing ones, exploring the potential of untapped elements is an important strategy. Rare-earth elements, including scandium, yttrium, and the lanthanides (La-Lu), constitute one important frontier in the periodic table. Rare-earth elements possess unique chemical and physical properties that are different from those of main-group and late-transition metals. The development of rare-earth-based catalysts by taking the advantage of these unique properties is of great interest and importance. The most stable oxidation state of rare-earth metals is 3+, which is difficult to change under many reaction conditions. The oxidative addition and reductive elimination processes often observed in catalytic cycles involving late transition metals are generally difficult in the case of rare-earth complexes. The 18-electron rule that is applicable to late-transition-metal complexes does not fit rare-earth complexes, whose structures are mainly governed by the sterics (rather than the electron numbers) of the ligands. In the lanthanide series (La-Lu), the ionic radius gradually decreases with increasing atomic number because of the influence of the 4f electrons, which show poor shielding of nuclear charge. Rare-earth metal ions generally show strong Lewis acidity and oxophilicity. Rare-earth metal alkyl and hydride species are highly reactive, showing both nucleophilicity and basicity. The combination of these features, such as the strong nucleophilicity and moderate basicity of the alkyl and hydride species and the high stability, strong Lewis acidity, and unsaturated C-C bond affinity of the 3+ metal ions, can make rare-earth metals unique candidates for the formation of excellent single-site catalysts. This Account is intended to give an overview of our recent studies on organo rare-earth catalysis, in particular the synthesis and application of half-sandwich rare-earth alkyl complexes bearing monocyclopentadienyl ligands for olefin polymerization, carbometalation, and hydroarylation. Treatment of half-sandwich rare-earth dialkyl complexes having the general formula CpMR2 with an equimolar amount of an appropriate borate compound such as [Ph3C][B(C6F5)4] can generate the corresponding cationic monoalkyl species, which serve as excellent single-site catalysts for the polymerization and copolymerization of a wide range of olefin monomers such as ethylene, 1-hexene, styrene, conjugated and nonconjugated dienes, and cyclic olefins. The cationic half-sandwich rare-earth alkyl complexes can also catalyze the regio- and stereoselective alkylative alumination of alkenes and alkynes through insertion of the unsaturated C-C bond into the metal-alkyl bond followed by transmetalation between the resulting new alkyl or alkenyl species and an alkylaluminum...

The regioregular synthesis of the first azaborine oligomers and a corresponding conjugated polymer was accomplished by Suzuki-Miyaura coupling methods. An almost perfectly coplanar syn arrangement of the heterocycles was deduced from an X-ray crystal structure of the dimer, which also suggested that NH⋅⋅⋅π interactions play an important role. Computational studies further supported these experimental observations and indicated that the electronic structure of the longer azaborine oligomers and polymer resembles that of poly(cyclohexadiene) more than poly(p-phenylene). A comparison of the absorption and emission properties of the polymer with those of the oligomers revealed dramatic bathochromic shifts upon chain elongation, thus suggesting highly effective extension of conjugation.

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A series of polymers (PBnT, n = 2–5) with boron atoms incorporated into the conjugated polythiophene main chain have been prepared via Pd-catalyzed coupling of stannylated thienylborane monomers. The polymers exhibit excellent long-term chemical stability to air and moisture and remarkable thermal stability with decomposition temperatures reaching over 300 °C. The high stability is achieved by placing very bulky pendant groups, 2,4,6-tri-tert-butylphenyl (Mes*) and 2,4,6-tris­(trifluoro­methyl)­phenyl (FMes), on boron that prevent attack by nucleophiles. All these polymers display strong absorptions in the visible region and intense fluorescence in both solution and the solid state with quantum yields of up to 38% and fast radiative decay constants (k r) of up to 3.3 × 108 s–1. Density functional theory (DFT) studies on diborylated oligothiophene model compounds suggest that the strong absorption of the polymers results from π–π* transitions on the oligothiophene borane main chain with significant charge transfer to boron. The unusually intense luminescence in the solid state is favored by the rigid planar skeleton and steric shielding of the bulky pendent groups. The emission color can be tuned from blue to deep orange by varying the length of the π-conjugated oligothiophene spacer between the boron atoms. Spectroelectrochemical studies on a dimeric model compound in THF solution reveal reversible two-step reductions to give highly colored species, while the corresponding polymeric material precipitates at higher potentials after undergoing an initial reversible reduction. The LUMO energy levels of the polymers can be effectively lowered by introduction of electron-withdrawing pendent groups on boron, affording a versatile approach for development of electron-deficient boron-containing polymers with controllable electronic structures and photophysical properties. The facile modular synthetic approach combined with the exceptional stability opens the door to broad adoption of electron-deficient organoboranes in conjugated materials design and development.