Asymmetrical Platinum and Rhodium Dinuclear Complex Strongly Bound to Filled d z 2 ${{_{{\rm z}{

Heterometallic extended metal atom chains (EMACs) aligned with three types of metal were rationally synthesized by forming unbridged metal-metal bonds based on the interactions between highest occupied and lowest unoccupied molecular orbitals at the d z 2 ${{_{{\rm z}{^{2}}}}}$ orbital. These chains form pentanuclear structures aligned as Rh-Pt-M-Pt-Rh with relatively large formation constants of 5.0×1013  M-2 for M=Pt and 6.3×1011  M-2 for M=Pd, while retaining their backbones in solution. In the case of M=Cu, the original Cu(+2) atoms were reduced to Cu(+1) during the synthetic process. Cu(+1) has an unprecedented trigonal bipyramidal coordination geometry. The reported synthesis based on asymmetrical dinuclear complexes provides a guideline for the synthesis of hetero-EMACs to allow several analogs through judicious combinations realized by tuning the number of metal nuclei and metal species.

Fate of Oxidation States at Actinide Centers in Redox-Active Ligand Systems Governed by Energy Levels of 5 f Orbitals.

We report the formation of a NpIV complex from the complexation of NpVI O2 2+ with the redox-active ligand tBu-pdiop2- =2,6-bis[N-(3,5-di-tert-butyl-2-hydroxyphenyl)iminomethyl]pyridine. To the best of our knowledge, this is the first example of the direct complexation-induced chemical reduction of NpVI O2 2+ to NpIV . In contrast, the complexation of UVI O2 2+ with tBu-pdiop2- did not induce the reduction of UVI O2 2+ , not even after the two-electron electrochemical reduction of [UVI O2 (tBu-pdiop)]. This contrast between the Np and U systems may be ascribed to the decrease of the energy of the 5 f orbitals in Np compared to those in U. The present findings indicate that the redox chemistry between UVI O2 2+ and NpVI O2 2+ should be clearly differentiated in redox-active ligand systems.

Syntheses, Characterizations, Crystal Structures, and Protonation Reactions of Dinitrogen Chromium Complexes Supported with Triamidoamine Ligands.

A novel dinitrogen-dichromium complex, [{Cr(LBn)}2(µ-N2)] (1), has been prepared from reaction of CrCl3 with a lithiated triamidoamine ligand (Li3LBn) under dinitrogen. The X-ray crystal structure analysis of 1 revealed that it is composed of two independent dimeric Cr complexes bridged by N2 in the unit cell. The bridged N-N bond lengths (1.188(4) and 1.185(7) Å) were longer than the free dinitrogen molecule. The elongations of N-N bonds in 1 were also supported by the fact that the ν(N-N) stretching vibration at 1772 cm-1 observed in toluene is smaller than the free N2. Complex 1 was identified to be a 5-coordinated high spin Cr(IV) complex by Cr K-edge XANES measurement. The 1H NMR spectrum and temperature dependent magnetic susceptibility of 1 indicated that complex 1 is in the S = 1 ground state, in which two Cr(IV) ions and unpaired electron spins of the bridging N22- ligand are strongly antiferromagnetically coupled. Reaction of complex 1 with 2.3 equiv of Na or K gave chromium complexes with N2 between the Cr ion and the respective alkali metal ion, [{CrNa(LBn)(N2)(Et2O)}2] (2) and [{CrK(LBn)(N2)}4(Et2O)2] (3), respectively. Furthermore, the complexes 2 and 3 reacted with 15-crown-5 and 18-crown-6 to form the respective crown-ether adducts, [CrNa(LBn)(N2)(15-crown-5)] (4) and [CrK(LBn)(N2)(18-crown-6)] (5). The XANES measurements of complexes 2, 3, 4, and 5 revealed that they are high spin Cr(IV) complexes like complex 1. All complexes reacted with a reducing agent and a proton source to form NH3 and/or N2H4. The yields of these products in the presence of K+ were higher than those in the presence of Na+. The electronic structures and binding properties of 1, 2, 3, 4, and 5 were evaluated and discussed based on their DFT calculations.

Fully Chelating N3O2-Pentadentate Planar Ligands Designed for the Strongest and Selective Capture of Uranium from Seawater.

Based on the unique fivefold equatorial coordination of UO22+, water-compatible pentadentate planar ligands, H2saldian and its derivatives, were designed for the strong and selective capture of UO22+ in seawater. In the simulated seawater condition (0.5 M NaCl + 2.3 mM HCO3-/CO32-, pH 8), saldian2- shows the strongest complexation with UO22+ to form UO2(saldian) (log ß11 = 28.05 ± 0.07), which is more than 10 order of magnitude greater than amidoxime-based or -inspired ligand systems most commonly employed for U capture from seawater. Good selectivity for UO22+ from other metal ions coexisting in seawater was also demonstrated.

Effects of Substituents on the Molecular Structure and Redox Behavior of Uranyl(V/VI) Complexes with N3O2-Donating Schiff Base Ligands.

Uranyl(VI) complexes with pentadentate N3O2-donating Schiff base ligands having various substituents at the ortho (R1) and/or para (R2) positions on phenolate moieties, R1,R2-Mesaldien2-, were synthesized and thoroughly characterized by 1H nuclear magnetic resonance, infrared, elemental analysis, and single-crystal X-ray diffraction. Molecular structures of UO2(R1,R2-Mesaldien) are more or less affected by the electron-donating or -withdrawing nature of the substituents. The redox behavior of all UO2(R1,R2-Mesaldien) complexes was investigated to understand how substituents introduced onto the ligand affect the redox behavior of these uranyl(VI) complexes. As a result, the redox potentials of UO2(R1,R2-Mesaldien) in dimethyl sulfoxide increased from -1.590 to -1.213 V with an increase in the electron-withdrawing nature of the substituents at the R1 and R2 positions. The spectroelectrochemical measurements and theoretical calculation [density functional theory (DFT) and time-dependent DFT calculations] revealed that the center U6+ of each UO2(R1,R2-Mesaldien) complex undergoes one-electron reduction to afford the corresponding uranyl(V) complex, [UO2(R1,R2-Mesaldien)]-, regardless of the difference in the substituents. Consequently, the redox active center of uranyl(VI) complexes seems not to be governed by the redox potentials but to be determined by whether the LUMO is centered on a U 5f orbital or on one π* orbital of a surrounding ligand.

Diversity of oxidation state in copper complexes with phenolate ligands.

The phenoxyl radical binding copper complexes have been widely developed and their detailed geometric and electronic structures have been clarified. While many one-electron oxidized CuII-phenolate complexes have been reported previously, recent studies of the Cu-phenolate complexes proceed toward elucidation of the complexes with other oxidation states, such as the phenoxyl radical binding CuI complexes and CuIV-phenolate complexes in the formal oxidation state. This Perspective focuses on new aspects of the properties and reactivities of various Cu-phenolate and Cu-phenoxyl radical complexes with emphasis on the relationship between geometric and electronic structures.

Effects of coordinating heteroatoms on molecular structure, thermodynamic stability and redox behavior of uranyl(vi) complexes with pentadentate Schiff-base ligands.

Uranyl(vi) complexes with pentadentate N3O2-, N2O3- and N2O2S1-donating Schiff base ligands, tBu,MeO-saldien-X2- (X = NH, O and S), were synthesized and thoroughly characterized by 1H NMR, IR, elemental analysis, and single crystal X-ray diffraction. The crystal structures of UO2(tBu,MeO-saldien-X) showed that the U-X bond strength follows U-O ≈ U-NH > U-S. Conditional stability constants (ß X) of UO2(tBu,MeO-saldien-X) in ethanol were investigated to understand the effect of X on thermodynamic stability. The log ß X decrease in the order of UO2(tBu,MeO-saldien-NH) (log ß NH = 10) > UO2(tBu,MeO-saldien-O) (log ß O = 7.24) > UO2(tBu,MeO-saldien-S) (log ß S = 5.2). This trend cannot be explained only by Pearson's Hard and Soft Acids and Bases (HSAB) principle, but rather follows the order of basicity of X. Theoretical calculations of UO2(tBu,MeO-saldien-X) suggested that the ionic character of U-X bonds decreases in the order of U-NH > U-O > U-S, while the covalency increases in the order U-O < U-NH < U-S. Redox potentials of all UO2(tBu,MeO-saldien-X) in DMSO were similar to each other regardless of the difference in X. Spectroelectrochemical measurements and DFT calculations revealed that the center U6+ of each UO2(tBu,MeO-saldien-X) undergoes one-electron reduction to afford the corresponding uranyl(v) complex. Consequently, the difference in X of UO2(tBu,MeO-saldien-X) affects the coordination of tBu,MeO-saldien-X2- with UO2 2+. However, the HSAB principle is not always prominent, but the Lewis basicity and balance between ionic and covalent characters of the U-X interactions are more relevant to determine the bond strengths.

Synthesis and characterization of a uranyl(VI) complex with 2,6-pyridine-bis(methylaminophenolato) and its ligand-centred aerobic oxidation mechanism to a diimino derivative.

A uranyl(VI) complex with 2,6-bis(3,5-di-tert-butyl-o-phenolateaminomethyl)pyridine (UO2(tBu-pdaop), 1) was synthesized and thoroughly characterized by 1H NMR, IR, elemental analysis, and single-crystal XRD. Right after the dissolution of complex 1 in pyridine or DMSO, the solution was pale red, whereas it gradually turned to dark purple under an ambient atmosphere. 1H NMR spectra at the initial and final states suggested that both of the two aminomethyl groups in 1 were converted to azomethine ones through aerobic oxidation. Indeed, a uranyl(VI) complex with 2,6-bis(3,5-di-tert-butyl-o-phenolateiminomethyl)pyridine (UO2(tBu-pdiop), 2) was obtained from the concentrated solution once the reaction was completed, and was characterized by IR, and single-crystal XRD. Kinetic analyses as well as mechanistic studies based on quantum chemical calculations suggested that hydrogen atom transfer from one of the amino groups in complex 1 to nearby O2 initiates the stepwise oxidation processes to finally afford 2. The present findings demonstrate the novel reactivity of a uranyl(VI) complex, and provide new insights to construct thermally-driven molecular conversion systems by a UO22+ complex catalyst.