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.