Disparate Reactivities Research Articles

Unusual Hydrogenation Reactivities of a Thiolate-Bridged Dicobalt μ-Nitride Featuring a Bent {CoIII-N-CoIII} Core.

Transition metal nitrides have received considerable attention owing to their crucial roles in nitrogen fixation and nitrogen atom transfer reactions. Compared to the early and middle transition metals, it is much more challenging to access late transition metal nitrides, especially cobalt in group 9. So far, only a handful of cobalt nitrides have been reported; consequently, their hydrogenation reactivity is largely unexplored. Herein, we present a structurally and spectroscopically well-characterized thiolate-bridged dicobalt μ-nitride [Cp*CoIII(μ-SAd)(μ-N)CoIIICp*] (2) featuring a bent {CoIII(μ-N)CoIII} core. Remarkably, complex 2 can realize not only direct hydrogenation of nitride to amide but also stepwise N-H bond formation from nitride to ammonia. Specifically, 2 can facilely activate dihydrogen (H2) at mild conditions to generate a dicobalt μ-amide [Cp*CoII(μ-SAd)(μ-NH2)CoIICp*] (4) via an unusual mechanism of two-electron oxidation of H2 as proposed by computational studies; in the presence of protons (H+) and electrons, nitride 2 can convert to dicobalt μ-imide [Cp*CoIII(μ-SAd)(μ-NH)CoIIICp*][BPh4] (3[BPh4]) and to CoIICoII μ-amide 4, and finally release ammonia. In contrast to 2, the only other structurally characterized dicobalt μ-nitride Na(THF)4{[(ketguan)CoIII(N3)]2(μ-N)} (ketguan = [(tBu2CN)C(NDipp)2]-, Dipp = 2,6-diisopropylphenyl) (e) that possesses a linear {CoIII(μ-N)CoIII} moiety cannot directly react with H2 or H+. Further in-depth electronic structure analyses shed light on how the varying geometries of the {CoIII(μ-N)CoIII} moieties in 2 and e, bent vs linear, impart their disparate reactivities.

Tetravalent Uranium and Thorium Complexes: Elucidating Disparate Reactivities of AnIVCl2 (An = U, Th) Supported by a Pyridine-Decorated Dianionic Ligand.

Although synthesis, reactivity, and bonding of U(IV) and Th(IV) complexes have been extensively studied, direct comparison of fully analogous compounds is rare. Herein, we report corresponding complexes 1-U and 1-Th, in which U(IV) and Th(IV) are supported by the tetradentate pyridine-decorated dianionic ligand N2NN' (1,1,1-trimethyl-N-(2-(((pyridin-2-ylmethyl)(2-((trimethylsilyl)amino)benzyl)amino)methyl)phenyl)silanamine). Although 1-U and 1-Th are structurally very similar, they display disparate reactivities with TMS3SiK (tris(trimethylsilyl)silylpotassium). The reaction of (N2NN')UCl2 (1-U) and 1 equiv of TMS3SiK in THF unexpectedly formed [Cl(N2NN')U]2O (2-U) featuring an unusual bent U-O-U moiety. In contrast, a salt elimination reaction between (N2NN')ThCl2 (1-Th) and 1 equiv of TMS3SiK led to thorium complex 2-Th, in which the pyridyl group has undergone a 1,4-addition nucleophilic attack. Complex 2-Th serves as a synthon for preparing dimetallic bis-azide complex 3-Th by reaction with NaN3. The complexes were characterized by X-ray crystal diffraction, solution NMR, FT-IR, and elemental analysis. Computations of the formation mechanism of 2-U from 1-U suggest reduced U(III) as a key intermediate for promoting the cleavage of the C-O bonds of THF. The inaccessible nature of Th(III) as an intermediate oxidation state explains the very different reactivity of 1-Th versus 1-U. Given that reactants 1-U and 1-Th and products 2-U and 2-Th all comprise tetravalent actinides, this is an unusual case of very disparate reactivity despite no net change in the oxidation state. Complexes 2-U and 3-Th provide a basis for the synthesis of other dinuclear actinide complexes with novel reactivity and properties.

Probing the All-Ferrous States of Methanogen Nitrogenase Iron Proteins.

The Fe protein of nitrogenase reduces two C1 substrates, CO2 and CO, under ambient conditions when its [Fe4S4] cluster adopts the all-ferrous [Fe4S4]0 state. Here, we show disparate reactivities of the nifH- and vnf-encoded Fe proteins from Methanosarcina acetivorans (designated MaNifH and MaVnfH) toward C1 substrates in the all-ferrous state, with the former capable of reducing both CO2 and CO to hydrocarbons, and the latter only capable of reducing CO to hydrocarbons at substantially reduced yields. EPR experiments conducted at varying solution potentials reveal that MaVnfH adopts the all-ferrous state at a more positive reduction potential than MaNifH, which could account for the weaker reactivity of the MaVnfH toward C1 substrates than MaNifH. More importantly, MaVnfH already displays the g = 16.4 parallel-mode EPR signal that is characteristic of the all-ferrous [Fe4S4]0 cluster at a reduction potential of −0.44 V, and the signal reaches 50% maximum intensity at a reduction potential of −0.59 V, suggesting the possibility of this Fe protein to access the all-ferrous [Fe4S4]0 state under physiological conditions. These results bear significant relevance to the long-lasting debate of whether the Fe protein can utilize the [Fe4S4]0/2+ redox couple to support a two-electron transfer during substrate turnover which, therefore, is crucial for expanding our knowledge of the reaction mechanism of nitrogenase and the cellular energetics of nitrogenase-based processes.

Efforts towards Rh(II)-catalyzed N-alkoxyazomethine ylide generation: Disparate reactivities of O-tethered α-diazo keto and -β-ketoester oximes

Abstract Azomethine ylide [3 + 2]-cycloaddition reactions are among the most powerful and well-studied chemical transformations to construct five-membered nitrogen-containing frameworks. However, cycloadditions of azomethine ylides bearing N-alkoxy substituents have largely been overlooked. This has been the case despite the potential for (1) accessing heterocycles with good regio- and diastereocontrol, (2) the presence of a cleavable tether which can be manipulated downstream via N–O bond cleavage, and (3) the promise of interesting structural diversity. Herein, we describe our efforts toward dirhodium-catalyzed intramolecular N-alkoxyazomethine ylide generation from α-diazoketo and -β-ketoester oximes and the disparate reactivities observed.

Gradient Polymerization-Induced Self-Assembly: A One-Step Approach.

In this work, the authors report a novel single-step, one-pot process for the synthesis of self-assembled nanoparticles using a polymerization-induced self-assembly (PISA) mechanism. In contrast to conventional approaches employing a pre-formed macromolecular stabilizer, the disparate reactivities between two monomers, oligo(ethylene glycol) methyl ether methacrylate (OEGMA) and diacetone acrylamide (DAAm), are exploited instead to synthesize a gradient copolymer directly in aqueous solution. Due to the hydrophobicity of poly(DAAm), these gradient copolymers can self-assemble in situ to form spheres and worms stabilized by the OEGMA residues. A surprisingly broad range of parameters are identified in which the worm morphology can be stabilized, which is highlighted by significant gelation of the reaction mixture in situ. This single-step gradient copolymerization approach to PISA is more efficient than conventional two-step syntheses. These results demonstrate improved reproducibility owing to the production of self-assembled nanoparticles directly in a one-pot and single-step synthesis.

Anionic Copolymerization Enables the Scalable Synthesis of Alternating (AB)n Multiblock Copolymers with High Molecular Weight in n/2 Steps.

Based on the highly disparate reactivities of isoprene (I, rI = 25.4) and 4-methylstyrene (4MS, r4MS = 0.007) in the anionic copolymerization in nonpolar media, a general strategy for the rapid and scalable synthesis of tapered multiblock copolymers with an extremely steep gradient has been developed. A repetitive addition strategy of a mixture of isoprene and 4MS leads to a tapered diblock in each case, giving access to linear alternating multiblock copolymers of the (AB)n type with up to 10 blocks. All multiblock copolymers showed narrow molecular weight distributions (dispersity Đ = 1.04-1.12). High molecular weights in the range of 80 to 400 kg mol-1 were achieved. Due to the incompatibility of PI and P4MS segments, the multiblock copolymers exhibit nanophase separation, manifested by separate glass transitions for both constituents. Stress-strain measurements revealed extraordinary toughness and elongations up to 1150% strain at break, even at a 50/50 molar ratio I/4MS (i.e., 37 wt% isoprene). Our synthesis permits access to a wide range of tapered multiblock copolymer architectures with rigid (P4MS, high glass transition, Tg) and flexible (low Tg) chains, in n/2 steps, while keeping overall dispersity low.

Iridium Catalysts for Acceptorless Dehydrogenation of Alcohols to Carboxylic Acids: Scope and Mechanism.

We introduce iridium-based conditions for the conversion of primary alcohols to potassium carboxylates (or carboxylic acids) in the presence of potassium hydroxide and either [Ir(2-PyCH2(C4H5N2))(COD)]OTf (1) or [Ir(2-PyCH2PBu2 t)(COD)]OTf (2). The method provides both aliphatic and benzylic carboxylates in high yield and with outstanding functional group tolerance. We illustrate the application of this method to a diverse variety of primary alcohols, including those involving heterocycles and even free amines. Complex 2 reacts with alcohols to form crystallographically-characterized catalytic intermediates [IrH(η 1,η 3-C8H12)(2-PyCH2PtBu2)] (2a) and [Ir2H3(CO)(2-PyCH2PtBu2){μ-(C5H3N)CH2PtBu2}] (2c). The unexpected similarities in reactivities of 1 and 2 in this reaction, along with synthetic studies on several of our iridium intermediates, enable us to form a general proposal of the mechanisms of catalyst activation that govern the disparate reactivities of 1 and 2, respectively in glycerol and formic acid dehydrogenation. Moreover, careful analysis of the organic intermediates in the oxidation sequence enable new insights into the role of Tishchenko and Cannizzaro reactions in the overall oxidation.

Renewable Formate from C-H Bond Formation with CO2: Using Iron Carbonyl Clusters as Electrocatalysts.

As a society, we are heavily dependent on nonrenewable petroleum-derived fuels and chemical feedstocks. Rapid depletion of these resources and the increasingly evident negative effects of excess atmospheric CO2 drive our efforts to discover ways of converting excess CO2 into energy dense chemical fuels through selective C-H bond formation and using renewable energy sources to supply electrons. In this way, a carbon-neutral fuel economy might be realized. To develop a molecular or heterogeneous catalyst for C-H bond formation with CO2 requires a fundamental understanding of how to generate metal hydrides that selectively donate H- to CO2, rather than recombining with H+ to liberate H2. Our work with a unique series of water-soluble and -stable, low-valent iron electrocatalysts offers mechanistic and thermochemical insights into formate production from CO2. Of particular interest are the nitride- and carbide-containing clusters: [Fe4N(CO)12]- and its derivatives and [Fe4C(CO)12]2-. In both aqueous and mixed solvent conditions, [Fe4N(CO)12]- forms a reduced hydride intermediate, [H-Fe4N(CO)12]-, through stepwise electron and proton transfers. This hydride selectively reacts with CO2 and generates formate with >95% efficiency. The mechanism for this transformation is supported by crystallographic, cyclic voltammetry, and spectroelectrochemical (SEC) evidence. Furthermore, installation of a proton shuttle onto [Fe4N(CO)12]- facilitates proton transfer to the active site, successfully intercepting the hydride intermediate before it reacts with CO2; only H2 is observed in this case. In contrast, isoelectronic [Fe4C(CO)12]2- features a concerted proton-electron transfer mechanism to form [H-Fe4C(CO)12]2-, which is selective for H2 production even in the presence of CO2, in both aqueous and mixed solvent systems. Higher nuclearity clusters were also studied, and all are proton reduction electrocatalysts, but none promote C-H bond formation. Thermochemical insights into the disparate reactivities of these clusters were achieved through hydricity measurements using SEC. We found that only [H-Fe4N(CO)12]- and its derivative [H-Fe4N(CO)11(PPh3)]- have hydricities modest enough to avoid H2 production but strong enough to make formate. [H-Fe4C(CO)12]2- is a stronger hydride donor, theoretically capable of making formate, but due to an overwhelming thermodynamic driving force and the increased electrostatic attraction between the more negative cluster and H+, only H2 is observed experimentally. This illustrates the fundamental importance of controlling thermochemistry when designing new catalysts selective for C-H bond formation and establishes a hydricity range of 15.5-24.1 or 44-49 kcal mol-1 where C-H bond formation may be favored in water or MeCN, respectively.

Kinetic Monte Carlo Modeling Extracts Information on Chain Initiation and Termination from Complete PLP-SEC Traces

Complete pulsed laser polymerization (PLP) log-molar mass distributions (log-MMDs) are accurately simulated using kinetic Monte Carlo (kMC) modeling to gain not only knowledge on the propagation but also on the chain initiation and termination reactivity. The isothermal kMC model (306–325 K) accounts for diffusional limitations and all relevant elementary reactions, considering n-butyl acrylate, 2,2-dimethoxy-2-phenylacetophenone (DMPA), and a frequency of 500 s–1. The disparate reactivities toward vinylic bonds of the DMPA fragments are essential to ensure the well-defined multimodality of the log-MMDs necessary to identify consecutive inflection points. It is also illustrated that PLP log-MMD data can be used to test the validity of models for apparent termination rate coefficients at low monomer conversions and that kMC simulations are a powerful tool to track the chain growth of the different radical types between laser pulses and to identify dominant elementary reactions.

Selective Postmodification of Copolymer Backbones Bearing Different Activated Esters with Disparate Reactivities.

In this communication, we report an easy method for introducing functional groups into polymer structures by successively reacting two different activated ester functionalities (pentafluorophenyl (PFP) ester and azlactone (AZ)) with different functional amine compounds. By exploiting the difference in reactivity of the two activated esters (PFP and AZ) toward different amino compounds, we demonstrate, for the first time, a selective modification of the different activated ester groups, thereby introducing functional groups to the polymer backbone in a controlled manner. Statistical and block copolymers of vinyl dimethyl azlactone (VDM) and pentafluorophenyl acrylate (PFPA), i.e.,(p(VDM-stat-PFPA)) and (p(VDM-block-PFPA)), were prepared using reversible addition-fragmentation transfer (RAFT) polymerization and subsequently modified using a library of amino compounds, yielding macromolecules with bespoke functionality. In additional work, the functional macromolecules were self-assembled into nanoparticles.

RAFT Polymerization of Monomers with Highly Disparate Reactivities: Use of a Single RAFT Agent and the Synthesis of Poly(styrene-block-vinyl acetate)

A single reversible addition–fragmentation chain transfer (RAFT) agent, malonate N,N-diphenyldithiocarbamate (MDP-DTC) is shown to successfully mediate the polymerization of several monomers with greatly differing reactivities in radical/RAFT polymerizations, including both vinyl acetate and styrene. The chain transfer constants (Ctr) for MDP-DTC for both these monomers were evaluated; these were found to be ~2.7 in styrene and ~26 in vinyl acetate, indicating moderate control over styrene polymerization and good control of vinyl acetate polymerization. In particular, the MDP-DTC RAFT agent allowed for the synthesis of block copolymers of these two monomers without the need for protonation/deprotonation switching, as has been previously developed with N-(4-pyridinyl)-N-methyldithiocarbamate RAFT agents, or other end-group transformations. The thermal properties of the block copolymers were studied using differential scanning calorimetry, and those with sufficiently high molecular weight and styrene composition appear to undergo phase separation. Thus, MDP-DTC may be useful for the production of other block copolymers consisting of monomers with highly dissimilar reactivities.

Recalcitrant Dissolved Organic Carbon Fractions

Marine dissolved organic carbon (DOC) exhibits a spectrum of reactivity, from very fast turnover of the most bioavailable forms in the surface ocean to long-lived materials circulating within the ocean abyss. These disparate reactivities group DOC by fractions with distinctive functions in the cycling of carbon, ranging from support of the microbial loop to involvement in the biological pump to a hypothesized major source/sink of atmospheric CO(2) driving paleoclimate variability. Here, the major fractions constituting the global ocean's recalcitrant DOC pool are quantitatively and qualitatively characterized with reference to their roles in carbon biogeochemistry. A nomenclature for the fractions is proposed based on those roles.

Complex Macromolecular Architectures by Reversible Addition Fragmentation Chain Transfer Chemistry: Theory and Practice

AbstractReversible addition–fragmentation chain transfer (RAFT) chemistry can be effectively employed to construct macromolecular architectures of varying topologies. The present article explores the principle design routes to star, block, and comb polymers in the context of theoretical design criteria for the so‐called Z‐ and R‐group approaches. The specific advantages and disadvantages of each approach are underpinned by selected examples generated in the CAMD laboratories. In particular, we demonstrate how the modeling of full molecular weight distributions can be employed to guide the synthetic effort. We further explore the theory and practice of generating amphiphilic block copolymer structures and their self‐assembly. In addition, the article foreshadows how modern synthetic techniques that combine RAFT chemistry with highly orthogonal click chemistry can be employed as a powerful tool that furthers the enhancement of macromolecular design possibilities to generate block (star) copolymers of monomers with extremely disparate reactivities. Finally, the ability of RAFT chemistry to modify the surface of well‐defined nano‐ and microspheres as devices in biomedical application is detailed.magnified image

RAFT and click chemistry: A versatile approach to well-defined block copolymers

The combination of reversible chain transfer chemistry with highly orthogonal [2 + 3] cycloadditions ('click chemistry') allows for the synthesis of well-defined block copolymers of monomers with extremely disparate reactivities.