Synthesis and Catalytic Activity of Heterobimetallic Rare Earth–Zinc Ethyl BINOLate Analogues of Shibasaki’s Catalysts

Strong Circularly Polarized Luminescence at 1550 nm from Enantiopure Molecular Erbium Complexes

Circularly polarized luminescence (CPL) in two subregions of the near-infrared (NIR) has been achieved. By leveraging the rigidity and diminishing detrimental vibrations of the heterobimetallic binolate complexes of erbium [(Binol)₃ErNa₃], species exhibiting an exceptionally high dissymmetry factor (|g_lum |) of 0.47 at 1550 nm were obtained. These erbium complexes are the first reported examples of CPL observed beyond 1200 nm. Analogous complexes of ytterbium and neodymium also exhibited strong CPL (|g_lum| = 0.17, 0.05, respectively) in a higher energy NIR window (800-1200 nm). All complexes exhibit high quantum yields (Er: 0.58%, Yb: 17%, Nd: 9.3%) and high B_CPL values (Er: 57 M^-1 cm^-1, Yb: 379 M^-1 cm^-1, Nd: 29 M^-1 cm^-1). Because of their strong CPL emission in the telecom band (1550 nm), biologically relevant NIR emission window (800-1100 nm), and synthetic versatility, the complexes reported here could permit further promising developments in quantum communication technologies and biologically relevant sensors.

Expanding the Rare-Earth Metal BINOLate Catalytic Multitool beyond Enantioselective Organic Synthesis

Shibasaki's rare earth alkali metal BINOLate (REMB) framework has provided chemists with a general catalyst platform to access a range of enantioenriched small molecules from the single, commercially available pro-ligand (R)- or (S)-BINOL. A defining feature of these heterobimetallic frameworks is the high level of catalyst tunability, achieved through the simple modulation of the central rare-earth cation and peripheral alkali metal cations. While this family of multifunctional catalysts displays impressive generality and catalytic capability, detailed mechanistic understanding of these complex, multimetallic systems was lacking prior to our investigations. This backdrop served as initial inspiration for our investigations of this privileged class of complexes over the past decade, which have led to new and exciting advances in catalysis and beyond.In this Account, we describe our investigations using Shibasaki's framework focusing on the central metal-ion, the BINOLate ligands, and the secondary sphere cations. Our studies began with an investigation into the Lewis acidity of the complexes, where we demonstrated that Lewis bases readily coordinate to REMB frameworks when lithium occupies the secondary coordination sphere. This observation was contrasted by the complexes containing sodium or potassium in the secondary coordination sphere, as the rare earth cation is evidently less accessible for substrate binding. Our efforts in understanding the ligand exchange of the complexes enabled the discovery that associative processes dominate the mechanism of ligand exchange and LA/LA (Lewis acid/Lewis acid) and LA/BB (Lewis acid/Brønsted base) catalysis by the REMB frameworks. Replacing metal cations in the secondary coordination sphere with the N,N,N',N'-tetramethylguanidinium cation delivered an effective precatalyst that is air and water stable over the course of 6 months.To expand the reactivity of the REMB, we investigated the ability of U^IV cations to occupy the primary coordination sphere and ZnEt⁺ and Cu(DBU)⁺ cations to occupy the secondary coordination sphere. Synthesizing the REMB complexes using the thiol congener monothioBINOL provided an unusual anionic REMB framework, driven by the oxophilicity of the lithium cations. Using the REMB as a platform for investigating the Ce^III/Ce^IV redox couple, we demonstrated that, while oxidative cerium functionalization is observed in the case of lithium containing REMBs, salt elimination is observed in the sodium, potassium, and cesium containing REMBs. Furthermore, we found that while the rate of heterogeneous electron transfer for Ce^III was k_s(Cs^I) > k_s(K^I) > k_s(Na^I) > k_s(Li^I), the rates of reaction with the oxidant trityl chloride trended in the opposite order with k_obs(Li^I) ≫ k_obs(Na^I) > k_obs(K^I) > k_obs(Cs^I). We attribute this to the ability to form inner-sphere complexes with the oxidant, rather than differences in redox potential or reorganization energies.Applying our knowledge in ligand exchange and redox behavior of Ce containing REMB complexes, we detailed the mechanism for oxidation of the heterochiral cerium REMB frameworks, reiterating the importance of the formation of inner-sphere complexes in the oxidation chemistry of cerium. There are many different avenues for both organic and inorganic investigation of Shibasaki's REMB framework, and our works have demonstrated the richness of the structural chemistry and properties of this framework that inform mechanism and properties of these privileged catalysts.

High circularly polarized luminescence brightness from analogues of Shibasaki's lanthanide complexes

To reach the promising potential of circularly polarized luminescence (CPL) emitters, high CPL brightness must be achieved. We describe the synthesis of analogues of the C₃-symmetrical Shibasaki's lanthanide complexes (Sm, Tb, Dy) supported by enantiopure 5,5',6,6',7,7',8,8'-octahydro-1,1'-bi-2-naphthol (H₈-Binol). The complexes exhibit visible luminescence in solution with exceptionally high quantum yields for Sm (4%) and Dy (17%), and strong circularly polarized luminescence for Sm, Tb, and Dy (|g_lum| up to 0.44, 0.32, 0.33, respectively). Altogether, these complexes possess amongst the strongest CPL brightness reported to date in lanthanide molecular complexes (up to 782 M^-1 cm^-1 for Tb).

Synthesis of novel copper-rare earth BINOLate frameworks from a hydrogen bonding DBU-H rare earth BINOLate complex

The preparation of a novel H-bonding DBU-H⁺ BINOLate-lanthanide complex enabled the synthesis of the first copper-Rare Earth Metal BINOLate complex (CuDBU-REMB).

Synthesis and derivatisation of ceric tris(tert-butoxy)siloxides

Heteroleptic ceric Ce[OSi(OtBu)₃]₃Cl(thf) is straightforwardly obtained from cerous Ce[OSi(OtBu)₃]₃(thf)₂ and trityl chloride and gives access to the terminal siloxy/methoxy derivative Ce[OSi(OtBu)₃]₃(OCH₃)(thf)₂.