Topology-Accelerated and Selective Cascade Depolymerization of Architecturally Complex Polyesters.

Despite considerable recent advances already made in developing chemically circular polymers (CPs), the current framework predominantly focuses on CPs with linear-chain structures of different monomer types. As polymer properties are determined by not only composition but also topology, manipulating the topology of the single-monomer-based CP systems from linear-chain structures to architecturally complex polymers could potentially modulate the resulting polymer properties without changing the chemical composition, thereby advancing the concept of monomaterial product design. To that end, here, we introduce a chemically circular hyperbranched polyester (HBPE), synthesized by a mixed chain-growth and step-growth polymerization of a rationally designed bicyclic lactone with a pendent hydroxyl group (BiLOH). This HBPE exhibits full chemical recyclability despite its architectural complexity, showing quantitative selectivity for regeneration of BiLOH, via a unique cascade depolymerization mechanism. Moreover, distinct differences in materials properties and performance arising from topological variations between HBPE, hb-PBiLOH, and its linear analogue, l-PBiLOH, have been revealed where generally the branched structure led to more favorable interchain interactions, and topology-amplified optical activity has also been observed for chiral (1S, 4S, 5S)-hb-PBiLOH. More intriguingly, depolymerization of l-PBiLOH proceeds through an unexpected, initial topological transformation to the HBPE polymer, followed by the faster cascade depolymerization pathway adopted by hb-PBiLOH. Overall, these results demonstrate that CP design can go beyond typical linear polymers, and rationally redesigned, architecturally complex polymers for their unique properties may synergistically impart advantages in topology-augmented depolymerization acceleration and selectivity for exclusive monomer regeneration.

Natural diversity screening, assay development, and characterization of nylon-6 enzymatic depolymerization.

Successes in biocatalytic polyester recycling have raised the possibility of deconstructing alternative polymers enzymatically, with polyamide (PA) being a logical target due to the array of amide-cleaving enzymes present in nature. Here, we screen 40 potential natural and engineered nylon-hydrolyzing enzymes (nylonases), using mass spectrometry to quantify eight compounds resulting from enzymatic nylon-6 (PA6) hydrolysis. Comparative time-course reactions incubated at 40-70 °C showcase enzyme-dependent variations in product distributions and extent of PA6 film depolymerization, with significant nylon deconstruction activity appearing rare. The most active nylonase, a NylCK variant we rationally thermostabilized (an N-terminal nucleophile (Ntn) hydrolase, NylCK-TS, Tm = 87.4 °C, 16.4 °C higher than the wild-type), hydrolyzes 0.67 wt% of a PA6 film. Reactions fail to restart after fresh enzyme addition, indicating that substrate-based limitations, such as restricted enzyme access to hydrolysable bonds, prohibit more extensive deconstruction. Overall, this study expands our understanding of nylonase activity distribution, indicates that Ntn hydrolases may have the greatest potential for further development, and identifies key targets for progressing PA6 enzymatic depolymerization, including improving enzyme activity, product selectivity, and enhancing polymer accessibility.

Cyclic and Linear Tetrablock Copolymers Synthesized at Speed and Scale by Lewis Pair Polymerization of a One-Pot (Meth)acrylic Mixture and Characterized at Multiple Levels.

Cyclic block copolymers (cBCP) are fundamentally intriguing materials, but their synthetic challenges that demand precision in controlling both the monomer sequence and polymer topology limit access to AB and ABC block architectures. Here, we show that cyclic ABAB tetra-BCPs (cABAB) and their linear counterpart (lABAB) can be readily obtained at a speed and scale from one-pot (meth)acrylic monomer mixtures, through coupling the Lewis pair polymerization's unique compounded-sequence control with its precision in topology control. This approach achieves fast (<15 min) and quantitative (>99%) conversion to tetra-BCPs of predesignated linear or cyclic topology at scale (40 g) in a one-pot procedure, precluding the needs for repeated chain extensions, stoichiometric addition steps, dilute conditions, and postsynthetic modifications, and/or postsynthetic ring-closure steps. The resulting lABAB and cABAB have essentially identical molecular weights (Mn = 165-168 kg mol-1) and block degrees/symmetry, allowing for direct behavioral comparisons in solution (hydrodynamic volume, intrinsic viscosity, elution time, and refractive indices), bulk (thermal transitions), and film (thermomechanical and rheometric properties and X-ray scattering patterns) states. To further the morphological characterizations, allylic side-chain functionality is exploited via the thiol-ene click chemistry to install crystalline octadecane side chains and promote phase separation between the A and B blocks, allowing visualization of microdomain formation.

Elastomeric vitrimers from designer polyhydroxyalkanoates with recyclability and biodegradability.

Cross-linked elastomers are stretchable materials that typically are not recyclable or biodegradable. Medium-chain-length polyhydroxyalkanoates (mcl-PHAs) are soft and ductile, making these bio-based polymers good candidates for biodegradable elastomers. Elasticity is commonly imparted by a cross-linked network structure, and covalent adaptable networks have emerged as a solution to prepare recyclable thermosets via triggered rearrangement of dynamic covalent bonds. Here, we develop biodegradable and recyclable elastomers by chemically installing the covalent adaptable network within biologically produced mcl-PHAs. Specifically, an engineered strain of Pseudomonas putida was used to produce mcl-PHAs containing pendent terminal alkenes as chemical handles for postfunctionalization. Thiol-ene chemistry was used to incorporate boronic ester (BE) cross-links, resulting in PHA-based vitrimers. mcl-PHAs cross-linked with BE at low density (<6 mole %) affords a soft, elastomeric material that demonstrates thermal reprocessability, biodegradability, and denetworking at end of life. The mechanical properties show potential for applications including adhesives and soft, biodegradable robotics and electronics.

Lewis Acid Supported Nickel Nitrenoids.

Metalation of the polynucleating ligand F,tbs LH6 (1,3,5-C6 H9 (NC6 H3 -4-F-2-NSiMe2 t Bu)3 ) with two equivalents of Zn(N(SiMe3 )2 )2 affords the dinuclear product (F,tbs LH2 )Zn2 (1), which can be further deprotonated to yield (F,tbs L)Zn2 Li2 (OEt2 )4 (2). Transmetalation of 2 with NiCl2 (py)2 yields the heterometallic, trinuclear cluster (F,tbs L)Zn2 Ni(py) (3). Reduction of 3 with KC8 affords [KC222 ][(F,tbs L)Zn2 Ni] (4) which features a monovalent Ni centre. Addition of 1-adamantyl azide to 4 generates the bridging µ3 -nitrenoid adduct [K(THF)3 ][(F,tbs L)Zn2 Ni(µ3 -NAd)] (5). EPR spectroscopy reveals that the anionic cluster possesses a doublet ground state (S = 1 / 2 ${{ 1/2 }}$ ). Cyclic voltammetry of 5 reveals two fully reversible redox events. The dianionic nitrenoid [K2 (THF)9 ][(F,tbs L)Zn2 Ni(µ3 -NAd)] (6) was isolated and characterized while the neutral redox isomer was observed to undergo both intra- and intermolecular H-atom abstraction processes. Ni K-edge XAS studies suggest a divalent oxidation state for the Ni centres in both the monoanionic and dianionic [Zn2 Ni] nitrenoid complexes. However, DFT analysis suggests Ni-borne oxidation for 5.

Dynamic crosslinking compatibilizes immiscible mixed plastics.

The global plastics problem is a trifecta, greatly affecting environment, energy and climate1-4. Many innovative closed/open-loop plastics recycling or upcycling strategies have been proposed or developed5-16, addressing various aspects of the issues underpinning the achievement of a circular economy17-19. In this context, reusing mixed-plastics waste presents a particular challenge with no current effective closed-loop solution20. This is because such mixed plastics, especially polar/apolar polymer mixtures, are typically incompatible and phase separate, leading to materials with substantially inferior properties. To address this key barrier, here we introduce a new compatibilization strategy that installs dynamic crosslinkers into several classes of binary, ternary and postconsumer immiscible polymer mixtures in situ. Our combined experimental and modelling studies show that specifically designed classes of dynamic crosslinker can reactivate mixed-plastics chains, represented here by apolar polyolefins and polar polyesters, by compatibilizing them via dynamic formation of graft multiblock copolymers. The resulting in-situ-generated dynamic thermosets exhibit intrinsic reprocessability and enhanced tensile strength and creep resistance relative to virgin plastics. This approach avoids the need for de/reconstruction and thus potentially provides an alternative, facile route towards the recovery of the endowed energy and materials value of individual plastics.

Dual Recycling of Depolymerization Catalyst and Biodegradable Polyester that Markedly Outperforms Polyolefins.

Chemically recyclable, circular polymers continue to attract increasing attention, but rendering both catalysts for depolymerization and high-performance polymers recyclable is a more sustainable yet challenging goal. Here we introduce a dual catalyst/polymer recycling system in that recyclable inorganic phosphomolybdic acid catalyzes selective depolymerization of high-ceiling-temperature biodegradable poly(δ-valerolactone) in bulk phase, which, upon reaching suitable molecular weight, exhibits outstanding mechanical performance with a high tensile strength of ≈66.6 MPa, fracture strain of ≈904 %, and toughness of ≈308 MJ m-3 , and thus markedly outperforms commodity polyolefins, recovering its monomer in pure state and quantitative yield at only 100 °C. In sharp contrast, the uncatalyzed depolymerization not only requires a high temperature of >310 °C but is also low yielding and non-selective. Importantly, the recovered monomer can be repolymerized as is to reproduce the same polymer, thereby closing the circular loop, and the recycled catalyst can be reused repeatedly for depolymerization runs without loss of its catalytic activity and efficiency.

A circular polyester platform based on simple gem-disubstituted valerolactones.

Geminal disubstitution of cyclic monomers is an effective strategy to enhance the chemical recyclability of their polymers, but it is utilized for that purpose alone and often at the expense of performance properties. Here we present synergistic use of gem-α,α-disubstitution of available at-scale, bio-based δ-valerolactones to yield gem-dialkyl-substituted valerolactones ([Formula: see text]), which generate polymers that solve not only the poor chemical recyclability but also the low melting temperature and mechanical performance of the parent poly(δ-valerolactone); the gem-disubstituted polyesters ([Formula: see text]) therefore not only exhibit complete chemical recyclability but also thermal, mechanical and transport properties that rival or exceed those of polyethylene. Through a fundamental structure-property study that reveals intriguing impacts of the alkyl chain length on materials performance of [Formula: see text], this work establishes a simple circular, high-performance polyester platform based on [Formula: see text] and highlights the importance of synergistic utilization of gem-disubstitution for enhancing both chemical recyclability and materials performance of sustainable polyesters.

Compounded Interplay of Kinetic and Thermodynamic Control over Comonomer Sequences by Lewis Pair Polymerization.

The design of facile synthetic routes to well-defined block copolymers (BCPs) from direct polymerization of one-pot comonomer mixtures, rather than traditional sequential additions, is both fundamentally and technologically important. Such synthetic methodologies often leverage relative monomer reactivity toward propagating species exclusively and therefore are rather limited in monomer scope and control over copolymer structure. The recently developed compounded sequence control (CSC) by Lewis pair polymerization (LPP) utilizes synergistically both thermodynamic (Keq) and kinetic (kp) differentiation to precisely control BCP sequences and suppress tapering and misincorporation errors. Here, we present an in-depth study of CSC by LPP, focusing on the complex interplay of the fundamental Keq and kp parameters, which enable the unique ability of CSC-LPP to precisely control comonomer sequences across a variety of polar vinyl monomer classes. Individual Lewis acid equilibrium and polymerization rate parameters of a range of commercially relevant monomers were experimentally quantified, computationally validated, and rationalized. These values allowed for the judicious design of copolymerizations which probed multiple hypotheses regarding the constructive vs conflicting nature of the relationship between Keq and kp biases, which arise during CSC-LPP of comonomer mixtures. These relationships were thoroughly explored and directly correlated with resultant copolymer microstructures. Several examples of higher-order BCPs are presented, further demonstrating the potential for materials innovation offered by this methodology.

Chromium Nitride Umpolung Tuned by the Locus of Oxidation.

Oxidation of a series of CrV nitride salen complexes (CrVNSalR) with different para-phenolate substituents (R = CF3, tBu, NMe2) was investigated to determine how the locus of oxidation (either metal or ligand) dictates reactivity at the nitride. Para-phenolate substituents were chosen to provide maximum variation in the electron-donating ability of the tetradentate ligand at a site remote from the metal coordination sphere. We show that one-electron oxidation affords CrVI nitrides ([CrVINSalR]+; R = CF3, tBu) and a localized CrV nitride phenoxyl radical for the more electron-donating NMe2 substituent ([CrVNSalNMe2]•+). The facile nitride homocoupling observed for the MnVI analogues was significantly attenuated for the CrVI complexes due to a smaller increase in nitride character in the M≡N π* orbitals for Cr relative to Mn. Upon oxidation, both the calculated nitride natural population analysis (NPA) charge and energy of molecular orbitals associated with the {Cr≡N} unit change to a lesser extent for the CrV ligand radical derivative ([CrVNSalNMe2]•+) in comparison to the CrVI derivatives ([CrVINSalR]+; R = CF3, tBu). As a result, [CrVNSalNMe2]•+ reacts with B(C6F5)3, thus exhibiting similar nucleophilic reactivity to the neutral CrV nitride derivatives. In contrast, the CrVI derivatives ([CrVINSalR]+; R = CF3, tBu) act as electrophiles, displaying facile reactivity with PPh3 and no reaction with B(C6F5)3. Thus, while oxidation to the ligand radical does not change the reactivity profile, metal-based oxidation to CrVI results in umpolung, a switch from nucleophilic to electrophilic reactivity at the terminal nitride.

Mechanism of Spatial and Temporal Control in Precision Cyclic Vinyl Polymer Synthesis by Lewis Pair Polymerization.

In typical cyclic polymer synthesis via ring-closure, chain growth and cyclization events are competing with each other, thus affording cyclic polymers with uncontrolled molecular weight or ring size and high dispersity. Here we uncover a mechanism by which Lewis pair polymerization (LPP) operates on polar vinyl monomers that allows the control of where and when cyclization takes place, thereby achieving spatial and temporal control to afford precision cyclic vinyl polymers or block copolymers with predictable molecular weight and low dispersity (≈1.03). A combined experimental and theoretical study demonstrates that cyclization occurs only after all monomers have been consumed (when) via conjugate addition of the propagating chain end to the specific site of the initiating chain end (where), allowing the cyclic polymer formation steps to be regulated and executed with precision in space and time.

Closing the "One Monomer-Two Polymers-One Monomer" Loop via Orthogonal (De)polymerization of a Lactone/Olefin Hybrid.

Two well-known low-ceiling-temperature (LCT) monomers, γ-butyrolactone (γ-BL) toward ring-opening polymerization (ROP) to polyester and cyclohexene toward ring-opening metathesis polymerization (ROMP) to poly(cyclic olefin), are notoriously "nonpolymerizable". Here we present a strategy to render not only polymerizability of both the γ-BL and cyclohexene sites, orthogonally, but also complete and orthogonal depolymerization, through creating an LCT/LCT hybrid, bicyclic lactone/olefin (BiL=). This hybrid monomer undergoes orthogonal polymerization between ROP and ROMP, depending on the catalyst employed, affording two totally different classes of polymeric materials from this single monomer: polyester P(BiL=)ROP via ROP and functionalized poly(cyclic olefin) P(BiL=)ROMP via ROMP. Intriguingly, both P(BiL=)ROP and P(BiL=)ROMP are thermally robust but chemically recyclable under mild conditions (25-40 °C), in the presence of a catalyst, to recover cleanly the same monomer via chain unzipping and scission, respectively. In the ROP, topological and stereochemical controls have been achieved and the structures characterized. Furthermore, the intact functional group during the orthogonal polymerization (i.e., the double bond in ROP and the lactone in ROMP) is utilized for postfunctionalization for tuning materials' thermal and mechanical performances. The impressive depolymerization orthogonality further endows selective depolymerization of both the ROP/ROMP copolymer and the physical blend composites into the same starting monomer.

Synchronous Control of Chain Length/Sequence/Topology for Precision Synthesis of Cyclic Block Copolymers from Monomer Mixtures.

Precision synthesis of cyclic polymers with predictable molecular weight and low dispersity is a challenging task, particularly concerning cyclic polar vinyl polymers through a rapid chain-growth mechanism and without high dilution. Harder yet is the precision synthesis of cyclic block copolymers (cBCPs), ideally from comonomer mixtures. Here we report that Lewis pair polymerization (LPP) capable of thermodynamically and kinetically compounded sequence control successfully addressed this longstanding challenge. Thus, LPP of acrylate/methacrylate mixtures under ambient temperature and normal concentration conditions rapidly and selectively affords well-defined cBCPs with high molecular weight (Mn = 247 kg/mol) and low dispersity (D = 1.04) in one step. Such cBCPs have been characterized by multiple techniques, including direct structural observation by imaging.

Enantioselective C-H Amination Catalyzed by Nickel Iminyl Complexes Supported by Anionic Bisoxazoline (BOX) Ligands.

The trityl-substituted bisoxazoline (TrHBOX) was prepared as a chiral analogue to a previously reported nickel dipyrrin system capable of ring-closing amination catalysis. Ligand metalation with divalent NiI2(py)4 followed by potassium graphite reduction afforded the monovalent (TrHBOX)Ni(py) (4). Slow addition of 1.4 equiv of a benzene solution of 1-adamantylazide to 4 generated the tetrazido (TrHBOX)Ni(κ2-N4Ad2) (5) and terminal iminyl adduct (TrHBOX)Ni(NAd) (6). Investigation of 6 via single-crystal X-ray crystallography, NMR and EPR spectroscopies, and computations revealed a Ni(II)-iminyl radical formulation, similar to its dipyrrinato congener. Complex 4 exhibits enantioselective intramolecular C-H bond amination to afford N-heterocyclic products from 4-aryl-2-methyl-2-azidopentanes. Catalytic C-H amination occurs under mild conditions (5 mol % catalyst, 60 °C) and provides pyrrolidine products in decent yield (29%-87%) with moderate ee (up to 73%). Substrates with a 3,5-dialkyl substitution on the 4-aryl position maximized the observed enantioselectivity. Kinetic studies to probe the reaction mechanism were conducted using 1H and 19F NMR spectroscopies. A small, intermolecular kinetic isotope effect (1.35 ± 0.03) suggests an H-atom abstraction step with an asymmetric transition state while the reaction rate is measured to be first order in catalyst and zeroth order in substrate concentrations. Enantiospecific deuterium labeling studies show that the enantioselectivity is dictated by both the H-atom abstraction and radical recombination steps due to the comparable rate between radical rotation and C-N bond formation. Furthermore, the competing elements of the two-step reaction where H-removal from the pro-R configuration is preferred while the preferential radical capture occurs with the Si face of the carboradical likely lead to the diminished ee observed, as corroborated by theoretical calculations. Based on these enantio-determining steps, catalytic enantioselective synthesis of 2,5-bis-tertiary pyrrolidines is demonstrated with good yield (50-78%) and moderate ee (up to 79%).

Sequential Activation of Guide RNAs to Enable Successive CRISPR-Cas9 Activities.

Currently, either highly multiplexed genetic manipulations can be delivered to mammalian cells all at once or extensive engineering of gene regulatory sequences can be used to conditionally activate a few manipulations. Here, we provide proof of principle for a new system enabling multiple genetic manipulations to be executed as a preprogrammed cascade of events. The system leverages the programmability of the S. pyogenes Cas9 and is based on flexible arrangements of individual modules of activity. The basic module consists of an inactive single-guide RNA (sgRNA)-like component that is converted to an active state through the effects of another sgRNA. Modules can be arranged to bring about an algorithmic program of sequential genetic manipulations without the need for engineering cell-type-specific promoters or gene regulatory sequences. With the expanding diversity of available tools that use spCas9, this sgRNA-based system provides multiple levels of interfacing with mammalian cell biology.

Efficient C-H Amination Catalysis Using Nickel-Dipyrrin Complexes.

A dipyrrin-supported nickel catalyst (AdFL)Ni(py) (AdFL: 1,9-di(1-adamantyl)-5-perfluorophenyldipyrrin; py: pyridine) displays productive intramolecular C-H bond amination to afford N-heterocyclic products using aliphatic azide substrates. The catalytic amination conditions are mild, requiring 0.1-2 mol% catalyst loading and operational at room temperature. The scope of C-H bond substrates was explored and benzylic, tertiary, secondary, and primary C-H bonds are successfully aminated. The amination chemoselectivity was examined using substrates featuring multiple activatable C-H bonds. Uniformly, the catalyst showcases high chemoselectivity favoring C-H bonds with lower bond dissociation energy as well as a wide range of functional group tolerance (e.g., ethers, halides, thioetheres, esters, etc.). Sequential cyclization of substrates with ester groups could be achieved, providing facile preparation of an indolizidine framework commonly found in a variety of alkaloids. The amination cyclization reaction mechanism was examined employing nuclear magnetic resonance (NMR) spectroscopy to determine the reaction kinetic profile. A large, primary intermolecular kinetic isotope effect (KIE = 31.9 ± 1.0) suggests H-atom abstraction (HAA) is the rate-determining step, indicative of H-atom tunneling being operative. The reaction rate has first order dependence in the catalyst and zeroth order in substrate, consistent with the resting state of the catalyst as the corresponding nickel iminyl radical. The presence of the nickel iminyl was determined by multinuclear NMR spectroscopy observed during catalysis. The activation parameters (ΔH‡ = 13.4 ± 0.5 kcal/mol; ΔS‡= -24.3 ± 1.7 cal/mol·K) were measured using Eyring analysis, implying a highly ordered transition state during the HAA step. The proposed mechanism of rapid iminyl formation, rate-determining HAA, and subsequent radical recombination was corroborated by intramolecular isotope labeling experiments and theoretical calculations.

Effect of Distortions on the Geometric and Electronic Structures of One-Electron Oxidized Vanadium(IV), Copper(II), and Cobalt(II)/(III) Salen Complexes.

The ligands N,N'-bis(3-tert-butyl-5-methoxysalicylidene)-1,2-ethanediamine and N,N'-bis(3-tert-butyl-5-methoxysalicylidene)-1,3-propanediamine were chelated to V(IV)═O (1, 2), Cu(II) (3, 4), Co(II) (5), and Co(III) (6). The X-ray crystal structures of 1-6 were solved. The vanadium center in 1-2 resides in square pyramidal geometry, with an axially bound oxo ligand, whereas the metal ion displays a tetrahedrally distorted square planar geometry in 3-5. The extent of distortion is correlated to the length of the diamine spacer: The longer the linker, the larger the tetrahedral distortions. Complex 6 is octahedral with a bidentate acetate molecule that completes the coordination sphere. All the complexes were characterized by UV-vis and EPR spectroscopies, as well as DFT calculations and electrochemistry. Complexes 1-6 exhibit a reversible one-electron oxidation wave in the range -0.11-0.26 V vs Fc+/Fc. The cations 1+ and 2+ were structurally characterized, showing an octahedral V(V) ion with one oxo and one water molecule coordinated in axial positions. Their vis-NIR spectra are dominated by a band at 727 and 815 nm, respectively, which is assigned to a phenolate-to-vanadium(V) charge transfer (CT) transition. The crystal structures of 3+ and 4+ are congruent with Cu(II)-radical species, wherein the metal center remains four-coordinated. Both feature a Class II (Robin-Day classification scale) IVCT transition at around 1200 nm (ε > 1 mM cm-1), indicative of partial localization of the radical. The structure of 5+ displays a square pyramidal cobalt ion, where the fifth (axial) coordination is occupied by a water molecule. It displays a NIR feature at 1244 nm and is described as intermediate between high spin Co(III) and Co(II) radical. In the presence of acetate the dimer [(5)2(µ-OAc)]+ forms, which was structurally characterized and shows a blue shift and lowering in intensity of the NIR absorption band in comparison to 5+. Complex 6+ is a genuine Co(III) radical complex, wherein the phenoxyl moiety is localized on one side of the molecule.

Compounded Sequence Control in Polymerization of One-Pot Mixtures of Highly Reactive Acrylates by Differentiating Lewis Pairs.

The ability to synthesize well-defined block copolymers (BCPs) from one-pot comonomer mixtures has powerful chemical and practical implications. However, controlling sequences between highly reactive, homologous comonomers such as acrylates during polymerization is challenging. Here we present a Lewis pair polymerization strategy that uniquely utilizes preferential Lewis acid coordination to differentiate between comonomers, distinctive kinetics, and compounded thermodynamic and kinetic differentiation to precisely control sequences and suppress tapering and misincorporation errors, thus achieving well-defined and resolved di- or tri-BCPs of acrylates.

Synthesis and electronic structure studies of a Cr-imido redox series.

The effect of metal identity, d-electron count, and coordination geometry on the electronic structure of a metal-ligand multiple bond (MLMB) is an area of active exploration. Although high oxidation state Cr imidos have been extensively studied, very few reports on low-valent Cr imidos or the interconversion of redox isomers exist. Herein, we report the synthesis and characterization of a family of dipyrrinato Cr imido complexes in oxidation states ranging from CrIII to CrV, showcasing the influence of the weak-field dipyrromethene scaffold on the electronic structure and coordination geometries of these Cr imides.

Composition-Controlled Atomic Layer Deposition of Phase-Change Memories and Ovonic Threshold Switches with High Performance.

Chalcogenide compounds are the main characters in a revolution in electronic memories. These materials are used to produce ultrafast ovonic threshold switches (OTSs) with good selectivity and moderate leakage current and phase-change memories (PCMs) with excellent endurance and short read/write times when compared with state-of-the-art flash-NANDs. The combination of these two electrical elements is used to fabricate nonvolatile memory arrays with a write/access time orders of magnitude shorter than that of state-of-the-art flash-NANDs. These devices have a pivotal role for the advancement of fields such as artificial intelligence, machine learning, and big-data. Chalcogenide films, at the moment, are deposited by using physical vapor deposition (PVD) techniques that allow for fine control over the stoichiometry of solid solutions but fail in providing the conformality required for developing large-memory-capacity integrated 3D structures. Here we present conformal ALD chalcogenide films with control over the composition of germanium, antimony, and tellurium (GST). By developing a technique to grow elemental Te we demonstrate the ability to deposit conformal, smooth, composition-controlled GST films. We present a thorough physical and chemical characterization of the solids and an in-depth electrical test. We demonstrate the ability to produce both OTS and PCM materials. GeTe4 OTSs exhibit fast switching times of ∼13 ns. Ge2Sb2Te5 ALD PCMs exhibit a wide memory window exceeding two orders of magnitude, short write times (∼100 ns), and a reset current density as low as ∼107 A/cm2-performance matching or improving upon state-of-the-art PVD PCM devices.