Capture and activation of the water-soluble uranyl dication (UO2 2+) remains a challenging problem, as few rational approaches are available for modulating the reactivity of this species. Here, we report the divergent synthesis of heterobimetallic complexes in which UO2 2+ is held in close proximity to a range of redox-inactive metals by a tailored macrocyclic ligand. Crystallographic and spectroscopic studies confirm assembly of homologous UVI(μ-OAr)2M n+ cores with a range of mono-, di-, and trivalent Lewis acids (M n+). Cyclic voltammetry data demonstrate that the UVI/UV reduction potential in these complexes is modulated over a span of 600 mV, depending linearly on the Lewis acidity of the redox-inactive metal with a sensitivity of 61 ± 9 mV/pK a unit. These findings suggest that interactions with Lewis acids could be effectively leveraged for rational tuning of the electronic and thermochemical properties of the 5f elements, reminiscent of strategies more commonly employed with 3d transition metals.

Assembly of heterobimetallic complexes is synthetically challenging due to the propensity of ditopic ligands to bind metalsu nselectively.H ere, we employ an ovel divergent approachf or selectivep reparation of av ariety of bimetallic complexes within ad itopic macrocyclic ligand platform. In our approach, nickel is readily coordinated to a Schiff base cavity,a nd then ar ange of redox-inactive cations (M = Na + ,C a 2 + ,N d 3 + ,a nd Y 3 + )a re installed in ap endant crown-ether-like site. This modular strategy allows access to complexes with the highly Lewis acidic trivalent cations Nd 3 + and Y 3 + ,aclass of compounds that werepreviously inaccessible. Spectroscopic and electrochemical studies reveal wide variations in properties that are governed mosts trongly by the trivalent cations. Exposure to dimethylformamide drives loss of Nd 3 + andY 3 + from the pendant crown-ether site, suggesting solvente ffects must be carefully considered in future applications involving use of highly Lewis acidic metals.

The behavior of Lewis acidic metal ions in multimetallic systems has become a subject of intense interest in recent years. Parametrizing the behavior of these ions in non-aqueous conditions, commonly used in the field, is challenging due to the lack of direct measures of the Lewis acidity of metal ions in polar organic solvents.Here, we report the use of triphenylphosphine oxide (TPPO) as a 31P nuclear magnetic resonance (NMR) probe to quantify the Lewis acidity of a library of metal triflate salts using the Gutmann-Beckett method. A plot of the pKa values of the corresponding metal-aqua species, [M(H2O)m]n+, measured in H2O, vs. the 31P NMR shifts of TPPO in the presence of these metals in deuterated acetonitrile (d3-MeCN) and deuterated dichloromethane (CD2Cl2) displays a tightly co-linear relationship, suggesting similar behavior for these ions in water, d3-MeCN, and CD2Cl2. This collinearity reinforces the utility of the common approach of using the aqueous pKa values as a descriptor of Lewis acidity, regardless of the solvent used in the immediate experiments, and provides an insight into the usefulness of this descriptor in wide-ranging applications.Titration studies in d3-MeCNsuggest 1:1 binding of TPPO with monovalent ions, greater than 1:1 binding with divalent ions, and formation of multiple species with the highly Lewis acidic trivalent ions. Together, these data suggest that both aqueous pKa values and other single-measurement descriptors, while useful, provide only a snapshot of the influence of Lewis acidity on multimetallic chemical systems. File list (2) download file view on ChemRxiv Blakemore_LewisAcids.pdf (2.74 MiB) download file view on ChemRxiv Blakemore_LewisAcids_SI.pdf (3.26 MiB)

Incorporation of redox-inactive metals into redoxactive complexes and catalysts attracts attention for engendering new reactivity modes, but this strategy has not been extensively investigated beyond the first-row of the transition metals. Here, the isolation and characterization of the first series of heterobimetallic complexes of palladium with mono-, di-, and tri-valent redox-inactive metal ions are reported. A Reinhoudt-type heteroditopic ligand with a salenderived [N 2 ,O 2 ] binding site for Pd and a crown-ether-derived [O 6 ] site has been used to prepare isolable adducts of the Lewis acidic redox-inactive metal ions (M n + ). Comprehensive data from single-crystal X-ray diffraction analysis reveal distinctive trends in the structural properties of the heterobimetallic species, including an uncommon dependence of the Pd•••M distance on Lewis acidity. The reorganization energy associated with reduction of the heterobimetallic species is strongly modulated by Lewis acidity, with the slowest heterogeneous electron transfer kinetics associated with the strongest incorporated Lewis acids. This hitherto unexplored reorganization energy penalty for electron transfer contrasts with prior thermodynamic studies, revealing that kinetic parameters should be considered in studies of reactivity involving heterobimetallic species.

Compounds containing multiple metals attract significant interest due to the useful redox and reactivity properties of such species. Here, the electrochemical properties of a family of macrocyclic complexes that feature a zinc(II) center paired with a second redox-inactive metal cation in heterobimetallic (Na+, Ca2+, Nd3+, Y3+) motifs or a homobimetallic (Zn2+) motif have been investigated. The new complexes were prepared via a divergent strategy, isolated, and structurally characterized by single-crystal X-ray diffraction (XRD) analysis. XRD results show that the structure of the complexes is modulated by the identity of the incorporated secondary metal ions. Cyclic voltammetry data reveal that ligand-centered reduction is promoted in the bimetallic complexes and that the paired metal ions synergistically influence the redox properties of the complexes. Similar to prior work from our group and others, the bimetallic complexes containing stronger Lewis acids undergo more significant reduction potential shifts; contrasting with prior work on complexes containing redox-active metals, however, the zinc(II) complexes studied here display faster electron transfer (as judged by lower reorganization energies, λ) when incorporating di-or tri-valent Lewis acids in contrast to monovalent (and more weakly acidic) sodium. The quantified trends in these data offer insights that help distinguish metal-versus ligand-centered reduction of bimetallic complexes. File list (2) download file view on ChemRxiv ZnM_ms.pdf (1.31 MiB) download file view on ChemRxiv ZnM_SI.pdf (9.98 MiB)

Treatment of [(Ind)­(tBu3PN)­TiCl2] (1; Ind = indenyl) with AlEt3 in a range of solvents affords the emerald green heterobimetallic complex [(Ind)­(tBu3PN)­Ti­(μ2-Cl)2AlEt2] (2). Single-crystal X-ray diffraction studies, as well as nuclear magnetic and X-band electron paramagnetic resonance spectra, support formulation of 2 as a paramagnetic, formally [TiIIIAlIII] complex. Both 1 and 2 are active precatalysts for ethylene polymerization in the presence of AlEt3 and a solid superacid; the product polyethylene materials are found to have similar molecular weight distributions in each case (3.29 with 1 and 3.05 with 2), implicating involvement of similar active catalysts in both. Electrochemical studies reveal that 1 can undergo one-electron reduction to TiIII near −2.0 V versus ferrocenium/ferrocene in tetrahydrofuran electrolyte, whereas 2 undergoes one-electron oxidation at a significantly more positive potential (ca. −1.30 V). Spectroelectrochemical studies show that reduction of 1 in the presence of AlEt3 leads to clean formation of 2. These findings support key roles for AlEt3 in (1) catalyst alkylation, (2) chemical reduction of 1 by one electron, and (3) provision of the highly Lewis acidic [AlEt2] fragment that stabilizes the TiIII center in 2, as judged by the voltammetric data.

Conspectus Current projections for global mining indicate that unsustainable practices will cause supply problems for many elements, called critical raw materials, in the next 20 years. These include elements necessary for renewable technologies as well as artisanal sources. Energy critical elements (ECEs) comprise a group used for clean, renewable energy applications that are in low abundance in the Earth’s crust or require an economic premium to extract from ores. Sustainable practices of acquiring ECEs is an important problem to address through fundamental research to provide alternative energy technologies such as wind turbines and electric vehicles at cheaper costs for our global energy generation and usage. Some of these green technologies incorporate rare-earth (RE) metals (Sc, Y and the lanthanides), which are challenging to separate from mineral sources because of their similar sizes (i.e., ionic radii) and chemical properties. The current process used to provide REs at requisite purities for these applications is counter-current solvent–solvent extraction, which is scalable and works efficiently for any ore composition. However, this method produces large amounts of caustic waste that is environmentally damaging, especially to areas in China that house major separation facilities. Advancement of the selectivity of this process is challenging since exact molecular speciation that affords separations is still relatively unknown. In this context, we developed a program to investigate new RE separations systems that were aimed at minimizing solvent use, controlled by molecular speciation, and could be targeted at problems in recycling these critical metals. The first ligand system that was developed to impart solubility differences between light and heavy rare-earth ions was [{(2- t BuNO)­C6H4CH2}3N]3– (TriNOx3–) (graphic below). A differential solubility allowed for a separation of Nd and Dy of SFNd:Dy = ∼300 in a single step. In other words, a 50:50 Nd/Dy sample was enriched to give 95% pure Nd and Dy through a simple filtration, which is potentially impactful to recycling magnetic materials found in wind turbines. This separations system compares favorably to other state-of-the-art molecular extractants that are based on energetic differences of the thermodynamic parameter to affect separations for neighboring elements. This straightforward, thermodynamically driven method to separate REs primed our future research for new coordination chemistry approaches to separations. Another separations system was accomplished through the variable rate of a redox event from one arm of the TriNOx3– ligand. It was determined that the rate of this one electron oxidation, which operated through an electrochemical-chemical-electrochemical mechanism, was dependent on the identity of the RE ion. This kinetically driven separation afforded a separation factor (SF) of SFEu:Y = 75. We have also described other transformations such as ligand exchange, substituent dependent, and redox-driven chelation processes with well-define...

The behavior of Lewis acidic metal ions in multimetallic systems has become a subject of intense interest in recent years. Parametrizing the behavior of these ions in non-aqueous conditions, commonly used in the field, is challenging due to the lack of direct measures of the Lewis acidity of metal ions in polar organic solvents. Here, we report the use of triphenylphosphine oxide (TPPO) as a 31P nuclear magnetic resonance (NMR) probe to quantify the Lewis acidity of a library of metal triflate salts using the Gutmann-Beckett method. A plot of the pKa values of the corresponding metal-aqua species, [M(H2O)m]n+, measured in H2O, vs. the 31P NMR shifts of TPPO in the presence of these metals in deuterated acetonitrile (d3-MeCN) and deuterated dichloromethane (CD2Cl2) displays a tightly co-linear relationship, suggesting similar behavior for these ions in water, d3-MeCN, and CD2Cl2. This collinearity reinforces the utility of the common approach of using the aqueous pKa values as a descriptor of Lewis acidity, regardless of the solvent used in the immediate experiments, and provides an insight into the usefulness of this descriptor in wide-ranging applications. Titration studies in d3-MeCNsuggest 1:1 binding of TPPO with monovalent ions, greater than 1:1 binding with divalent ions, and formation of multiple species with the highly Lewis acidic trivalent ions. Together, these data suggest that both aqueous pKa values and other single-measurement descriptors, while useful, provide only a snapshot of the influence of Lewis acidity on multimetallic chemical systems.