Tunability in Heterobimetallic Complexes Featuring an Acyclic "Tiara" Polyether Motif.

Both cyclic "crown" and acyclic "tiara" polyethers have been recognized as useful for the binding of metal cations and enabling the assembly of multimetallic complexes. However, the properties of heterobimetallic complexes built upon acyclic polyethers have received less attention than they deserve. Here, the synthesis and characterization of a family of eight redox-active heterobimetallic complexes that pair a nickel center with secondary redox-inactive cations (K, Na, Li, Sr, Ca, Zn, La, and Lu) bound in acyclic polyether "tiara" moieties are reported.

Activation and Functionalization of the Uranyl Ion by Electrochemical Reduction.

Interconversion of the oxidation states of uranium enables separations and reactivity schemes involving this element and contributes to technologies for recycling of spent nuclear fuels. The redox behaviors of uranium species impact these processes, but use of electrochemical methods to drive reactions of molecular uranium complexes and to obtain molecular insights into the outcomes of electrode-driven reactions has received far less attention than it deserves. Here, we show that electro-reduction of the uranyl ion (UO) can be used to promote stepwise functionalization of the typically unreactive oxo groups with exogenous triphenylborane (BPh) serving as a moderate electrophile, avoiding the conventional requirement for a chemical reductant.

Redox properties of [Cp*Rh] complexes supported by mono-substituted 2,2'-bipyridyl ligands.

The redox properties of half-sandwich rhodium complexes supported by 2,2'-bipyridyl (bpy) ligands can be readily tuned by selection of an appropriately substituted derivative of bpy, but the influences of single substituents on the properties of such complexes are not well documented, as disubstituted bpy variants are much more common. Here, the synthesis, characterization, and redox properties of two new [Cp*Rh] complexes (where Cp* is η-1,2,3,4,5-pentamethylcyclopentadienyl) supported by the uncommon mono-substituted ligands 4-chloro-2,2'-bipyridyl (mcbpy) and 4-nitro-2,2'-bipyridyl (mnbpy) are reported. Single-crystal X-ray diffraction studies and related spectroscopic experiments confirm installation of the single substituents (-Cl and -NO, respectively) on the bipyridyl ligands; the precursor monosubstituted ligands were prepared a divergent route from unsubstituted bpy.

Preparation of Neptunyl and Plutonyl Acetates To Access Nonaqueous Transuranium Coordination Chemistry.

Uranyl diacetate dihydrate is a useful reagent for the preparation of uranyl (UO) coordination complexes, as it is a well-defined stoichiometric compound featuring moderately basic acetates that can facilitate protonolysis reactivity, unlike other anions commonly used in synthetic actinide chemistry such as halides or nitrate. Despite these attractive features, analogous neptunium (Np) and plutonium (Pu) compounds are unknown to date. Here, a modular synthetic route is reported for accessing stoichiometric neptunyl(VI) and plutonyl(VI) diacetate compounds that can serve as starting materials for transuranic coordination chemistry.

Entropy Drives Accelerated Ion Diffusion upon Carbon Dioxide Expansion of Electrolytes.

Carbon dioxide-expanded liquids, organic solvents with high concentrations of soluble carbon dioxide (CO) at mild pressures, have gained attention as green catalytic media due to their improved properties over traditional solvents. More recently, carbon dioxide-expanded electrolytes (CXEs) have demonstrated improved reaction rates in the electrochemical reduction of CO, by increasing the rate of delivery of CO to the electrode while maintaining facile charge transport. However, recent studies indicate that the limiting behavior of CXEs at higher CO pressures is a decline in solution conductivity due to reduced polarity, leading to poorer charge screening and greater ion pairing.

Trivalent Cations Slow Electron Transfer to Macrocyclic Heterobimetallic Complexes.

Incorporation of secondary redox-inactive cations into heterobimetallic complexes is an attractive strategy for modulation of metal-centered redox chemistry, but quantification of the consequences of incorporating strongly Lewis acidic trivalent cations has received little attention. Here, a family of seven heterobimetallic complexes that pair a redox-active nickel center with La, Y, Lu, Sr, Ca, K, and Na (in the form of their triflate salts) have been prepared on a heteroditopic ligand platform to understand how chemical behavior varies across the comprehensive series. Structural data from X-ray diffraction analysis demonstrate that the positions adopted by the secondary cations in the crown-ether-like site of the ligand relative to nickel are dependent primarily on the secondary cations' ionic radii and that the triflate counteranions are bound to the cations in all cases.

Synthesis, Isolation, and Study of Heterobimetallic Uranyl Crown Ether Complexes.

Although crown ethers can selectively bind many metal cations, little is known regarding the solution properties of crown ether complexes of the uranyl dication, UO. Here, the synthesis and characterization of isolable complexes in which the uranyl dication is bound in an 18-crown-6-like moiety are reported. A tailored macrocyclic ligand, templated with a Pt(II) center, captures UO in the crown moiety, as demonstrated by results from single-crystal X-ray diffraction analysis.

Mechanistic Basis of Conductivity in Carbon Dioxide-Expanded Electrolytes: A Joint Experimental-Theoretical Study.

Electrolyte conductivity contributes to the efficiency of devices for electrochemical conversion of carbon dioxide (CO) into useful chemicals, but the effect of the dissolution of CO gas on conductivity has received little attention. Here, we report a joint experimental-theoretical study of the properties of acetonitrile-based CO-expanded electrolytes (CXEs) that contain high concentrations of CO (up to 12 M), achieved by CO pressurization. Cyclic voltammetry data and paired simulations show that high concentrations of dissolved CO do not impede the kinetics of outer-sphere electron transfer but decrease the solution conductivity at higher pressures.

Evidence for Uranium(VI/V) Redox Supported by 2,2'-Bipyridyl-6,6'-dicarboxylate.

The 2,2'-bipyridyl-6,6'-dicarboxylate ligand () has been shown in prior work to effectively capture the uranyl(VI) ion, UO, from aqueous solutions. However, the redox properties of the uranyl complex of this ligand have not been addressed despite the relevance of uranium-centered reduction to the nuclear fuel cycle and the presence of a bipyridyl core in , a motif long recognized for its ability to support redox chemistry. Here, the complex of UO () has been synthetically prepared and isolated under nonaqueous conditions for the study of its reductive chemical and electrochemical behavior.

Computational Insights into the Influence of Ligands on Hydrogen Generation with [Cp*Rh] Hydrides.

This work reports a computational investigation of the effect of ancillary ligands on the activity of an Rh catalyst for hydrogen evolution based on the [Cp*Rh] motif (Cp* = η-pentamethylcyclopentadienyl). Specifically, we investigate why a bipyridyl (bpy) ligand leads to H generation but diphenylphosphino-based (dpp) ligands do not. We compare the full ligands to simplified models and systematically vary structural features to ascertain their effect on the reaction energy of each catalytic step.

Distinguishing the mechanism of electrochemical carboxylation in CO eXpanded Electrolytes.

We shed light on the mechanism and rate-determining steps of the electrochemical carboxylation of acetophenone as a function of CO concentration by using a robust finite element analysis model that incorporates each reaction step. Specifically, we show that the first electrochemical reduction of acetophenone is followed by the homogeneous chemical addition of CO. The electrochemical reduction of the acetophenone-CO adduct is more facile than that of acetophenone, resulting in an Electrochemical-Chemical-Electrochemical (ECE) reaction pathway that appears as a single voltammetric wave.

Molecular Influences on the Quantification of Lewis Acidity with Phosphine Oxide Probes.

Gutmann-Beckett-type measurements with phosphine oxide probes can be used to estimate effective Lewis acidity with P nuclear magnetic resonance spectroscopy, but the influence of the molecular structure of a given probe on the quantification of Lewis acidity remains poorly documented in experimental work. Here, a quantitative comparison of triethyl (), trioctyl (), and triphenyl () phosphine oxides as molecular probes of Lewis acidity has been carried out titration studies in MeCN with a test set of six mono- and divalent metal triflate salts. In comparison to , the bulkier displays a similar range of chemical shift values and binding affinities for the various test metal ions.

Revealing the Influence of Diverse Secondary Metal Cations on Redox-Active Palladium Complexes.

Incorporation of redox-inactive metals into redox-active 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 salen-derived [N ,O ] binding site for Pd and a crown-ether-derived [O ] site has been used to prepare isolable adducts of the Lewis acidic redox-inactive metal ions (M ).

Remote Oxidative Activation of a [Cp*Rh] Monohydride.

Half-sandwich rhodium monohydrides are often proposed as intermediates in catalysis, but little is known regarding the redox-induced reactivity accessible to these species. Herein, the bis(diphenylphosphino)ferrocene (dppf) ligand has been used to explore the reactivity that can be induced when a [Cp*Rh] monohydride undergoes remote (dppf-centered) oxidation by 1e . Chemical and electrochemical studies show that one-electron redox chemistry is accessible to Cp*Rh(dppf), including a unique quasi-reversible Rh process at -0.

Evidence for Charge Delocalization in Diazafluorene Ligands Supporting Low-Valent [Cp*Rh] Complexes.

Ligands based upon the 4,5-diazafluorene core are an important class of emerging ligands in organometallic chemistry, but the structure and electronic properties of these ligands have received less attention than they deserve. Here, we show that 9,9'-dimethyl-4,5-diazafluorene (Me daf) can stabilize low-valent complexes through charge delocalization into its conjugated π-system. Using a new platform of [Cp*Rh] complexes with three accessible formal oxidation states (+III, +II, and +I), we show that the methylation in Me daf is protective, blocking Brønsted acid-base chemistry commonly encountered with other daf-based ligands.

Design of robust 2,2'-bipyridine ligand linkers for the stable immobilization of molecular catalysts on silicon(111) surfaces.

The attachment of the 2,2'-bipyridine (bpy) moieties to the surface of planar silicon(111) (photo)electrodes was investigated using ab initio simulations performed on a new cluster model for methyl-terminated silicon. Density functional theory (B3LYP) with implicit solvation techniques indicated that adventitious chlorine atoms, when present in the organic linker backbone, led to instability at very negative potentials of the surface-modified electrode. In prior experimental work, chlorine atoms were present as a trace surface impurity due to required surface processing chemistry, and thus could plausibly result in the observed surface instability of the linker.

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

Manganese ([Mn(CO)]) and rhenium tricarbonyl ([Re(CO)]) 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)] and [Re(CO)] complexes are explored. In particular, a novel heterobimetallic complex featuring both [Mn(CO)] and [Re(CO)] units supported by 2,2'-bipyrimidine (bpm) has been synthesized, structurally characterized, and compared to the analogous monomeric and homobimetallic complexes.

On the Use of Aqueous Metal-Aqua p Values as a Descriptor of Lewis Acidity.

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 nonaqueous 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 P nuclear magnetic resonance (NMR) probe to quantify the Lewis acidity of a library of metal triflate salts using the Gutmann-Beckett method.

Synthesis and Reactivity Studies of a [Cp*Rh] Complex Supported by a Methylene-Bridged Hybrid Phosphine-Imine Ligand.

[Cp*Rh] complexes (Cp* = -pentamethylcyclopentadienyl) supported by bidentate chelating ligands are useful in studies of redox chemistry and catalysis, but little information is available for derivatives bearing "hybrid" [] chelates. Here, the preparation, structural characterization, and chemical and electrochemical properties of a [Cp*Rh] complex bearing the κ-[,]-2-[(diphenylphosphino)methyl]pyridine ligand (PN) are reported. Cyclic voltammetry data reveal that [Cp*Rh(PN)Cl]PF () undergoes a chemically reversible, net two-electron reduction at -1.

4,5-Diazafluorene and 9,9'-Dimethyl-4,5-Diazafluorene as Ligands Supporting Redox-Active Mn and Ru Complexes.

4,5-diazafluorene (daf) and 9,9'-dimethyl-4,5-diazafluorene (Medaf) are structurally similar to the important ligand 2,2'-bipyridine (bpy), but significantly less is known about the redox and spectroscopic properties of metal complexes containing Medaf as a ligand than those containing bpy. New complexes Mn(CO)Br(daf) (), Mn(CO)Br(Medaf) (), and [Ru(Medaf)](PF) () have been prepared and fully characterized to understand the influence of the Medaf framework on their chemical and electrochemical properties. Structural data for , , and from single-crystal X-ray diffraction analysis reveal a distinctive widening of the daf and Medaf chelate angles in comparison to the analogous Mn(CO)(bpy)Br () and [Ru(bpy)] () complexes.