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1. Observations regarding the synthesis and redox chemistry of heterobimetallic uranyl complexes containing Group 10 metals

   https://doi.org/10.1515/ract-2023-0237  

   Mikeska, Emily R; Lind, Natalie M; Ervin, Alexander C; Khalife, Celine; Karnes, Joseph P; Blakemore, James D (January 2024, Radiochimica Acta)

   Abstract Literature reports have demonstrated that Schiff-base-type ligands can serve as robust platforms for the synthesis of heterobimetallic complexes containing transition metals and the uranyl dication (UO₂²⁺). However, efforts have not advanced to include either synthesis of complexes containing second- or third-row transition metals or measurement of the redox properties of the corresponding heterobimetallic complexes, despite the significance of actinide redox in studies of nuclear fuel reprocessing and separations. Here, metalloligands denoted [Ni], [Pd], and [Pt] that contain the corresponding Group 10 metals have been prepared and a synthetic strategy to access species incorporating the uranyl ion (UO₂²⁺) has been explored, toward the goal of understanding how the secondary metals could tune uranium-centered redox chemistry. The synthesis and redox characterization of the bimetallic complex [Ni,UO₂] was achieved, and factors that appear to govern extension of the chosen synthetic strategy to complexes with Pd and Pt are reported here. Infrared and solid-state structural data from X-ray diffraction analysis of the metalloligands [Pd] and [Pt] show that the metal centers in these complexes adopt the expected square planar geometries, while the structure of the bimetallic [Ni,UO₂] reveals that the uranyl moiety influences the coordination environment of Ni(II), including inducement of a puckering of the ligand backbone of the complex in which the phenyl rings fold around the nickel-containing core in an umbrella-shaped fashion. Cyclic voltammetric data collected on the heterobimetallic complexes of both Ni(II) and Pd(II) provide evidence for uranium-centered redox cycling, as well as for the accessibility of other reductions that could be associated with Ni(II) or the organic ligand backbone. Taken together, these results highlight the unique redox behaviors that can be observed in multimetallic systems and design concepts that could be useful for accessing tunable multimetallic complexes containing the uranyl dication. 

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2. Differentiating Ligand Tailoring and Cation Incorporation as Strategies for Tuning Heterobimetallic Cerium Complexes

   https://doi.org/10.1002/chem.202500474  

   Mikeska, Emily_R; Blakemore, James_D (May 2025, Chemistry – A European Journal)

   Abstract Tuning of redox‐active complexes featuring metals with high coordination numbers by incorporation of secondary redox‐inactive cations has received far less attention than it deserves. Here, appending moderate steric bulk to a tripodal ligand framework has been tested for its influence on secondary‐cation‐driven structural and electrochemical tuning of cerium, a lanthanide that tends to adopt high coordination numbers. Aquasi‐C₃‐symmetric cerium(III) complex denoted[Ce]has been prepared that features pendant benzyloxy groups, and this work demonstrates that this species offers a site capable of binding single Na⁺or Ca²⁺ions. Electrochemical and UV‐visible spectroscopic studies reveal equilibrium binding affinity of[Ce]for Na⁺in acetonitrile solvent, contrasting with tight binding of all cations in all other previously studied systems of this type. The modulated cation binding can be attributed to the bulky benzyloxy groups, which impact the thermodynamics of cation binding but do not impede the formation of cerium centers with coordination number 8 upon binding of either Na⁺or Ca²⁺. The Ce(IV/III) reduction potential was found to be tunable under the equilibrium binding conditions, highlighting the potentially significant role that controlled structural changes can play in modulating the solution properties of heterobimetallic complexes. 

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3. Iron(II) Complexes Featuring a Redox‐Active Dihydrazonopyrrole Ligand

   https://doi.org/10.1002/zaac.202100097  

   Jesse, Kate A.; Chang, Mu‐Chieh; Filatov, Alexander S.; Anderson, John S. (June 2021, Zeitschrift für anorganische und allgemeine Chemie)

   Abstract Nature uses control of the secondary coordination sphere to facilitate an astounding variety of transformations. Similarly, synthetic chemists have found metal‐ligand cooperativity to be a powerful strategy for designing complexes that can mediate challenging reactivity. In particular, this strategy has been used to facilitate two electron reactions with first row transition metals that more typically engage in one electron redox processes. While NNN pincer ligands feature prominently in this area, examples which can potentially engage in both proton and electron transfer are less common. Dihydrazonopyrrole (DHP) ligands have been isolated in a variety of redox and protonation states when complexed to Ni. However, the redox‐state of this ligand scaffold is less obvious when complexed to metal centers with more accessible redox couples. Here, we synthesize a new series of Fe‐DHP complexes in two distinct oxidation states. Detailed characterization supports that the redox‐chemistry in this set is still primarily ligand based. Finally, these complexes exist as 5‐coordinate species with an open coordination site offering the possibility of enhanced reactivity. 

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4. Accelerating ethylene polymerization using secondary metal ions in tetrahydrofuran

   https://doi.org/10.1039/C9DT04288A  

   Xiao, Dawei; Cai, Zhongzheng; Do, Loi H. (December 2019, Dalton Transactions)

   We have prepared a new series of nickel phosphine phosphonate ester complexes that feature two metal-chelating polyethylene glycol (PEG) side arms. Metal binding and reactivity studies in polar solvents showed that they readily coordinate external cations, including alkali (Li + , Na + , K + ), alkaline (Mg 2+ , Ca 2+ ), transition (Sc 3+ , Co 2+ , Zn 2+ ), post-transition (Ga 3+ ), and lanthanide (La 3+ ) metals. Although olefin polymerization reactions are typically performed in non-polar solvents, which cannot solubilize +2 and +3 metal cations, we discovered that our nickel catalysts could promote ethylene polymerization in neat tetrahydrofuran. This advance allowed us, for the first time, to systematically investigate the effects of a wide range of M + , M 2+ , and M 3+ ions on the reactivity of nickel olefin polymerization catalysts. In ethylene homopolymerization, the addition of Co(OTf) 2 to our nickel-PEG complexes provided the largest boost in activity (up to 11-fold, 2.7 × 10 6 g mol −1 h −1 ) compared to that in the absence of external salts. The catalyst enhancing effects of secondary metals were also observed in studies of ethylene and polar olefin ( e.g. , propyl vinyl ether, allyl butyl ether, methyl-10-undecenoate, and 5-acetoxy-1-pentene) copolymerization. Notably, combining either Co 2+ or Zn 2+ with our nickel complexes increased the rates of polymerization in the presence of propyl vinyl ether by about 5.0- and 2.4-fold, respectively. Although further studies are needed to elucidate the structural and mechanistic roles of the secondary metals, this work is an important advance toward the development of cation-switchable polymerization catalysts. 

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5. Synthesis, structural studies, and redox chemistry of bimetallic [Mn(CO) ₃ ] and [Re(CO) ₃ ] complexes

   https://doi.org/10.1039/D0DT03666H  

   Henke, Wade C.; Kerr, Tyler A.; Sheridan, Thomas R.; Henling, Lawrence M.; Takase, Michael K.; Day, Victor W.; Gray, Harry B.; Blakemore, James D. (March 2021, Dalton Transactions)

   null (Ed.)

   Manganese ([Mn(CO) 3 ]) and rhenium tricarbonyl ([Re(CO) 3 ]) complexes represent a workhorse family of compounds with applications in a variety of fields. Here, the coordination, structural, and electrochemical properties of a family of mono- and bimetallic [Mn(CO) 3 ] and [Re(CO) 3 ] complexes are explored. In particular, a novel heterobimetallic complex featuring both [Mn(CO) 3 ] and [Re(CO) 3 ] units supported by 2,2′-bipyrimidine (bpm) has been synthesized, structurally characterized, and compared to the analogous monomeric and homobimetallic complexes. To enable a comprehensive structural analysis for the series of complexes, we have carried out new single crystal X-ray diffraction studies of seven compounds: Re(CO) 3 Cl(bpm), anti -[{Re(CO 3 )Cl} 2 (bpm)], Mn(CO) 3 Br(bpz) (bpz = 2,2′-bipyrazine), Mn(CO) 3 Br(bpm), syn - and anti -[{Mn(CO 3 )Br} 2 (bpm)], and syn -[Mn(CO 3 )Br(bpm)Re(CO) 3 Br]. Electrochemical studies reveal that the bimetallic complexes are reduced at much more positive potentials (Δ E ≥ 380 mV) compared to their monometallic analogues. This redox behavior is consistent with introduction of the second tricarbonyl unit which inductively withdraws electron density from the bridging, redox-active bpm ligand, resulting in more positive reduction potentials. [Re(CO 3 )Cl] 2 (bpm) was reduced with cobaltocene; the electron paramagnetic resonance spectrum of the product exhibits an isotropic signal (near g = 2) characteristic of a ligand-centered bpm radical. Our findings highlight the facile synthesis as well as the structural characteristics and unique electrochemical behavior of this family of complexes. 

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