Direct Chill Casting and Extrusion of AA6111 Aluminum Alloy Formulated from Taint Tabor Scrap

AA6111 aluminum automotive body-sheet alloy has been formulated from 100% Taint Tabor scrap aluminum. Direct chill casting with and without high shear melt conditioning (HSMC) was used to produce the AA6111 alloy billets. Both homogenized and non-homogenized billets were extruded into sheets. The optical micrographs of the melt conditioned direct chill (MC-DC) samples showed refined equiaxed grains in comparison to direct chill (DC) cast and direct chill grain refined (DC-GR) samples. Optical metallography showed extensive peripheral coarse grain (PCG) for the DC, DC-GR and MC-DC planks extruded from the homogenized standard AA6111 billets while planks extruded from modified AA6111 billets (with recrystallization inhibitors) showed thin PCG band. The co-addition of recrystallization inhibitors Mn, Zr, and Cr with elimination of the billet homogenization step had a favorable impact on the microstructure of the AA6111 alloy following the extrusion process where a fibrous grain structure was retained across the whole section of the planks. The mechanical properties of as-cast planks extruded from non-homogenized billets were similar to those extruded from homogenized billets. Eliminating the homogenization heat treatment step prior to extrusion has important ramifications in terms of processing cost reduction.     

The Formation of 14H-LPSO in Mg–9Gd–2Y–2Zn–0.5Zr Alloy during Heat Treatment

There is a new long-period stacking ordered structure in Mg–RE–Zn magnesium alloys, namely the LPSO phase, which can effectively improve the yield strength, elongation, and corrosion resistance of Mg alloys. According to different types of Mg–RE–Zn alloy systems, two transformation modes are involved in the heat treatment transformation process. The first is the alloy without LPSO phase in the as-cast alloy, and the Mg_xRE phase changes to 14H-LPSO phase. The second is the alloy containing LPSO phase in the as-cast state, and the 14H-LPSO phase is obtained by the transformations of 6H, 18R, and 24R. The effects of different solution parameters on the second phase of Mg–9Gd–2Y–2Zn–0.5Zr alloy were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The precipitation mechanism of 14H-LPSO phase during solution treatment was further clarified. At a solution time of 13 h, the grain size increased rapidly initially and then decreased slightly with increasing solution temperature. The analysis of the volume fraction of the second phase and lattice constant showed that Gd and Y elements in the alloy precipitated from the matrix and formed 14H-LPSO phase after solution treatment at 490 °C for 13 h. At this time, the hardness of the alloy reached the maximum of 74.6 HV. After solution treatment at 500 °C for 13 h, the solid solution degree of the alloy increases, and the grain size and hardness of the alloy remain basically unchanged.     

Enhanced Fluidity of ZL205A Alloy with the Combined Addition of Al–Ti–C and La

The effects of Al–Ti–C and La on the fluidity of a ZL205A alloy after separate and combined addition were studied by conducting a fluidity test. The fluidity of the ZL205A alloy first increased and then decreased with the increasing addition of Al–Ti–C and La; it peaked at 0.3% and 0.1% for Al–Ti–C and La, respectively. The combined addition of Al–Ti–C and La led to better fluidity, which increased by 74% compared with the base alloy. The affecting mechanism was clarified through microstructure characterization and a DSC test. The heterogeneous nucleation aided by Al–Ti–C and La, the number of particles in the melt, and the evolution of the solidification range all played a role. Based on the evolution of the fluidity and grain size, the optimal levels of Al–Ti–C and La leading to both high fluidity and small grain size were identified.     

The Effect of Heat Treatment on the Tribological Properties and Room Temperature Corrosion Behavior of Fe–Cr–Al-Based OPH Alloy

The microstructure, mechanical, tribological, and corrosion properties of Fe–Cr–Al–Y-based oxide-precipitation-hardened (OPH) alloy at room temperature are presented. Two OPH alloys with a composition of 0.72Fe–0.15Cr–0.06Al–0.03Mo–0.01Ta–0.02Y₂O₃ and 0.03Y₂O₃ (wt.%) were prepared by mechanical alloying with different milling times. After consolidation by hot rolling, the alloys presented a very fine microstructure with a grain size of approximately 180 nm. Such a structure is relatively brittle, and its mechanical properties are enhanced by heat treatment. Annealing was performed at three temperatures (1000 °C, 1100 °C, and 1200 °C), with a holding time from 1 to 20 h. Tensile testing, wear testing, and corrosion testing were performed to evaluate the effect of heat treatment on the behavior and microstructural properties. The grain size increased almost 10 times by heat treatment, which influenced the mechanical properties. The ultimate tensile strength increased up to 300% more compared to the initial state. On the other hand, heat treatment has a negative effect on corrosion and wear resistance.     

Microstructure and Mechanical Properties of TiN–TiB2–hBN Composites Fabricated by Reactive Hot Pressing Using TiN–B Mixture

In this study, TiN–TiB₂–hBN composite ceramics were prepared via reactive hot pressing using TiN and amorphous B powders as raw materials. Different sintering temperatures and composition ratios were studied. The results show that the 70 vol% TiN–17.6 vol% TiB₂–12.4 vol% hBN ceramic composites obtained ideal comprehensive properties at 1600 °C. The relative density, Vickers hardness, bending strength, and fracture toughness were 99%, 11 GPa, 521 MPa, and 4.22 MPa·m^1/2, respectively. Densification was promoted by the highly active reaction product TiB₂, and the structural defects formed in the grains. Meanwhile, the good interfacial bonding between TiN and TiB₂ grains and the uniform dispersion of ultrafine hBN in the matrix contributed to the excellent bending strength. Moreover, the toughening mechanism of crack deflection and grain pull-out improved the fracture toughness.     

Effect of Ga on the Oxide Film Structure and Oxidation Resistance of Sn–Bi–Zn Alloys as Heat Transfer Fluids

The effect of gallium on the oxide film structure and overall oxidation resistance of low melting point Sn–Bi–Zn alloys was investigated under air atmosphere using thermogravimetric analyses. The liquid alloys studied had a Ga content of 1–7 wt.%. The results showed that the growth rates of the surface scale formed on the Sn–Bi–Zn–Ga alloys conformed to the parabolic law. The oxidation resistance of Sn–Bi–Zn alloys was improved by Ga addition and the activation energies increased from 12.05 kJ∙mol⁻¹ to 22.20 kJ∙mol⁻¹. The structure and elemental distribution of the oxide film surface and cross-section were found to become more complicated and denser with Ga addition. Further, the results of X-ray photoelectron spectroscopy and X-ray diffraction show that Ga elements accumulate on the surface of the liquid metal to form oxides, which significantly slowed the oxidation of the surface of the liquid alloy.     

In Situ Observation of the Tensile Deformation and Fracture Behavior of Ti–5Al–5Mo–5V–1Cr–1Fe Alloy with Different Microstructures

The plastic deformation processes and fracture behavior of a Ti–5Al–5Mo–5V–1Cr–1Fe alloy with bimodal and lamellar microstructures were studied by room-temperature tensile tests with in situ scanning electron microscopy (SEM) observations. The results indicate that a bimodal microstructure has a lower strength but higher ductility than a lamellar microstructure. For the bimodal microstructure, parallel, deep slip bands (SBs) are first noticed in the primary α (α_p) phase lying at an angle of about 45° to the direction of the applied tension, while they are first observed in the coarse lath α (α_L) phase or its interface at grain boundaries (GBs) for the lamellar microstructure. The β matrix undergoes larger plastic deformation than the α_L phase in the bimodal microstructure before fracture. Microcracks are prone to nucleate at the α_p/β interface and interconnect, finally causing the fracture of the bimodal microstructure. The plastic deformation is mainly restricted to within the coarse α_L phase at GBs, which promotes the formation of microcracks and the intergranular fracture of the lamellar microstructure.     

Properties of WC–10%Co–4%Cr Detonation Spray Coating Deposited on the Al–4%Cu–1%Mg Alloy

One of the methods of local improvement of the wear resistance of aluminum alloy parts is the deposition of hard tungsten carbide-based coatings on the surfaces subjected to intense external influence. This paper is devoted to the characterization of the WC–10Co–4Cr (wt.%) coating deposited on an Al–4Cu–1Mg (wt.%) alloy by the detonation spray method. In comparison with the common thermal spray techniques like High Velocity Oxygen Fuel (HVOF) or Atmospheric Plasma Spraying (APS), the heat input delivered to the substrate during detonation spray is significantly lower, that is especially important in case of coating deposition on aluminum alloys. The paper presents the results of morphology investigation, microstructure, phase composition, microhardness, and cohesive strength of deposited carbide-based detonation spray coating. Results showed that the coating has a porosity less than 0.5% and the carbide grain refinement down to the submicron size during coating deposition was detected. According to the investigation, the variation of spraying distance from 270 to 230 mm does not influence on the coating microstructure and composition.     

Extraction of Al from Coarse Al–Si Alloy by The Selective Liquation Method

A selective liquation process to extract Al from a coarse Al–Si alloy, produced by carbothermal reduction, was investigated on the laboratory scale. The products obtained by selective liquation–vacuum distillation were analyzed by X-ray diffraction, inductively coupled plasma optical emission spectrometry and scanning electron microscopy. During the selective liquation process with the use of zinc as the solvent, the pure aluminum in the coarse Al–Si alloy dissolved in the zinc melt to form an α-solid solution with zinc, and most of the silicon and iron-rich phases and Al–Si–Fe intermetallics precipitated and grew into massive grains that entered into the slag and separated with the Zn–Al alloy melt. However, some fine silicon particles remained in the Zn–Al alloy. Thus, Al–Si alloys conforming to industrial application standards were obtained when the Zn–Al alloys were separated by a distillation process.     

Synthesis and Characterization of Ti–Nb Alloy Films Obtained by Magnetron Sputtering and Low-Energy High-Current Electron Beam Treatment

The aim of the present work is to investigate the synthesis of Ti–Nb alloy films obtained by the physical vapor deposition (PVD) magnetron sputtering of Nb films on Ti substrates, followed by low-energy high-current electron beam (LEHCEB) alloying treatment. Ti–Nb alloys were synthetized under two different regimes, one by varying the deposited amount of Nb (from 25 to 150 nm) and treating samples with low applied voltages and a number of pulses (three pulses at either 20 or 25 kV), the second by setting the amount of Nb (100 nm) and alloying it at a higher applied voltage with a different number of pulses (from 10 to 50 at 25 and 30 kV). The synthetized Ti–Nb alloys were characterized by XRD and GDOES for phase identification and chemical composition; SEM and optical microscopy were employed for morphology evaluation; compositional investigation was done by EDS analysis and mechanical properties were evaluated by microindentation tests. LEHCEB treatment led to the formation of metastable phases (α′, α″ and β) which, together with the grain refinement effect, was responsible for improved mechanical properties.     

Exfoliation Resistance,Microstructure, and Oxide Formation Mechanisms of the White Oxide Layer on CP Ti and Ti–Nb–Ta–Zr Alloys

We found that specific biomedical Ti and its alloys, such as CP Ti, Ti–29Nb–13Ta–4.6Zr, and Ti–36Nb–2Ta–3Zr–0.3O, form a bright white oxide layer after a particular oxidation heat treatment. In this paper, the interfacial microstructure of the oxide layer on Ti–29Nb–13Ta–4.6Zr and the exfoliation resistance of commercially pure (CP) Ti, Ti–29Nb–13Ta–4.6Zr, and Ti–36Nb–2Ta–3Zr–0.3O were investigated. The alloys investigated were oxidized at 1273 or 1323 K for 0.3–3.6 ks in an air furnace. The exfoliation stress of the oxide layer was high in Ti–29Nb–13Ta–4.6Zr and Ti–36Nb–2Ta–3Zr–0.3O, and the maximum exfoliation stress was as high as 70 MPa, which is almost the same as the stress exhibited by epoxy adhesives, whereas the exfoliation stress of the oxide layer on CP Ti was less than 7 MPa, regardless of duration time. The nanoindentation hardness and frictional coefficients of the oxide layer on Ti–29Nb–13Ta–4.6Zr suggested that the oxide layer was hard and robust enough for artificial tooth coating. The cross-sectional transmission electron microscopic observations of the microstructure of oxidized Ti–29Nb–13Ta–4.6Zr revealed that a continuous oxide layer formed on the surface of the alloys. The Au marker method revealed that both in- and out-diffusion occur during oxidation in Ti–29Nb–13Ta–4.6Zr and Ti–36Nb–2Ta–3Zr–0.3O, whereas only out-diffusion governs oxidation in CP Ti. The obtained results indicate that the high exfoliation resistance of the oxide layer on Ti–29Nb–13Ta–4.6Zr and Ti-36Nb-2Ta-3Zr-0.3O are attributed to their dense microstructures composing of fine particles, and a composition-graded interfacial microstructure. On the basis of the results of our microstructural observations, the oxide formation mechanism of the Ti–Nb–Ta–Zr alloy is discussed.     

Microstructure,Micro-Mechanical and Tribocorrosion Behavior of Oxygen Hardened Ti–13Nb–13Zr Alloy

In the present work, an oxygen hardening of near-β phase Ti–13Nb–13Zr alloy in plasma glow discharge at 700–1000 °C was studied. The influence of the surface treatment on the alloy microstructure, tribological and micromechanical properties, and corrosion resistance is presented. A strong influence of the treatment on the hardened zone thickness, refinement of the α’ laths and grain size of the bulk alloy were found. The outer hardened zone contained mainly an oxygen-rich Ti α’ (O) solid solution. The microhardness and elastic modulus of the hardened zone decreased with increasing hardening temperature. The hardened zone thickness, size of the α’ laths, and grain size of the bulk alloy increased with increasing treatment temperature. The wear resistance of the alloy oxygen-hardened at 1000 °C was about two hundred times, and at 700 °C, even five hundred times greater than that of the base alloy. Oxygen hardening also slightly improved the corrosion resistance. Tribocorrosion tests revealed that the alloy hardened at 700 °C was wear-resistant in a corrosive environment, and when the friction process was completed, the passive film was quickly restored. The results show that glow discharge plasma oxidation is a simple and effective method to enhance the micromechanical and tribological performance of the Ti–13Nb–13Zr alloy.     

Novel Polyaniline–Silver–Sulfur Nanotube Composite as Cathode Material for Lithium–Sulfur Battery

The preparation and characterization of a polyaniline–silver–sulfur nanotube composite were reported in this paper. The polyaniline–silver nanotube composite was synthesized via an oxidation-reduction method in the sodium dodecyl sulfate (SDS) solution. After being vulcanized, the polyaniline–silver–sulfur (Poly (AN–Ag–S)) nanotube composite was prepared as active cathode material and assembled into lithium–sulfur (Li–S) batteries with electrolyte and negative electrode materials. When the feed ratio of raw materials (aniline and AgNO3) was 2:1, the initial specific capacity of poly (AN–Ag–S) composite cells reached 1114 mAh/g. The specific capacity was kept at 573 mAh/g, and the capacity retention rate stayed above 51% after 100 cycles. The introduction of Ag into the composite cathode material can effectively solve the poor conductivity of sulfur and improve the Li–S battery performance.     

Effect of Al2Ca Addition and Heat Treatment on the Microstructure Modification and Tensile Properties of Hypo-Eutectic Al–Mg–Si Alloys

The current study investigated the microstructure modification in Al–6Mg–5Si–0.15Ti alloy (in mass %) through the minor addition of Ca using Mg + Al₂Ca master alloy and heat treatment to see their impact on mechanical properties. The microstructure of unmodified alloy (without Ca) consisted of primary Al, primary Mg₂Si, binary eutectic Al–Mg₂Si, ternary eutectic Al–Mg₂Si–Si, and iron-bearing phases. The addition of 0.05 wt% Ca resulted in significant microstructure refinement. In addition to refinement, lamellar to fibrous-type modification of binary eutectic Al–Mg₂Si phases was also achieved in Ca-added (modified) alloy. This modification was related to increasing Ca-based intermetallics/compounds in the modified alloy that acted as nucleation sites for binary eutectic Al–Mg₂Si phases. The dendritic refinement with Ca addition was related to the fact that it improves the efficacy of Ti-based particles (TiAl₃ and TiB₂) in the melt to act as nucleation sites. In contrast, the occupation of oxide bifilms by Ca-based phases is expected to force the iron-bearing phases (as iron-bearing phases nucleate at oxide films) to solidify at lower temperatures, thus reducing their size. The as-cast microstructure of these alloys was further modified by subjecting them to solution treatment at 540 °C for 6 h, which broke the eutectic structure and redistributed Mg₂Si and Si phases in Al-matrix. Subsequent aging treatment caused a dramatic increase in the tensile strength of these alloys, and tensile strength of 291 MPa (with El% of 0.45%) and 327 MPa (with El% of 0.76%) was achieved for the unmodified alloy and modified alloy, respectively. Higher tensile strength and elongation of the modified alloy than unmodified alloy was attributed to refined dendritic structure and modified second phases.     

Experimental Study of Tensile Properties of Styrene–Butadiene–Styrene Modified Asphalt Binders

The requirements imposed on road pavements are ever increasing nowadays, necessitating the improvement of the properties of paving materials. The most commonly used paving materials include bituminous mixtures that are composed of aggregate grains bound by a bituminous binder. The properties of bitumens can be improved by modification with polymers. Among the copolymers used for modifying bitumens, styrene–butadiene–styrene, a thermoplastic elastomer, is the most commonly used. This article presents the results of tests conducted on bitumens modified with two types of styrene–butadiene–styrene copolymer (linear and radial). Two bitumen types of different penetration grades (35/50 and 160/220) were used in the experiments. The content of styrene–butadiene–styrene added to the bitumen varied between 1% and 6%. The results of the force ductility test showed that cohesion energy can be used for qualitative evaluation of the efficiency of modification of bitumen with styrene–butadiene–styrene copolymer. The determined values of the cohesion energy were subjected to the original analysis taking into account the three characteristic elongation zones of the tested binders. The performed analyses made it possible to find a parameter whose values correlate significantly with the content of styrene–butadiene–styrene copolymer in the modified bitumen. With smaller amounts of added modifier (approximately 2%), slightly better effects were obtained in the case of linear copolymer styrene–butadiene–styrene and for larger amounts of modifier (5–6%) radial copolymer styrene–butadiene–styrene was found to be more effective. This is confirmed by the changes in the binder structure, as indicated by the penetration index (PI).     

Precipitation Hardening at Elevated Temperatures above 400 °C and Subsequent Natural Age Hardening of Commercial Al–Si–Cu Alloy

The precipitation of intermetallic phases and the associated hardening by artificial aging treatments at elevated temperatures above 400 °C were systematically investigated in the commercially available AC2B alloy with a nominal composition of Al–6Si–3Cu (mass%). The natural age hardening of the artificially aged samples at various temperatures was also examined. A slight increase in hardness (approximately 5 HV) of the AC2B alloy was observed at an elevated temperature of 480 °C. The hardness change is attributed to the precipitation of metastable phases associated with the α-Al₁₅(Fe, Mn)₃Si₂ phase containing a large amount of impurity elements (Fe and Mn). At a lower temperature of 400 °C, a slight artificial-age hardening appeared. Subsequently, the hardness decreased moderately. This phenomenon was attributed to the precipitation of stable θ-Al₂Cu and Q-Al₄Cu₂Mg₈Si₆ phases and their coarsening after a long duration. The precipitation sequence was rationalized by thermodynamic calculations for the Al–Si–Cu–Fe–Mn–Mg system. The natural age-hardening behavior significantly varied depending on the prior artificial aging temperatures ranging from 400 °C to 500 °C. The natural age-hardening was found to strongly depend on the solute contents of Cu and Si in the Al matrix. This study provides fundamental insights into controlling the strength level of commercial Al–Si–Cu cast alloys with impurity elements using the cooling process after solution treatment at elevated temperatures above 400 °C.     

Rules of formation of H–C–N–O compounds at high pressure and the fates of planetary ices

The solar system’s outer planets, and many of their moons, are dominated by matter from the H–C–N–O chemical space, based on solar system abundances of hydrogen and the planetary ices H2O, CH4, and NH3. In the planetary interiors, these ices will experience extreme pressure conditions, around 5 Mbar at the Neptune mantle–core boundary, and it is expected that they undergo phase transitions, decompose, and form entirely new compounds. While temperature will dictate the formation of compounds, ground-state density functional theory allows us to probe the chemical effects resulting from pressure alone. These structural developments in turn determine the planets’ interior structures, thermal evolution, and magnetic field generation, among others. Despite its importance, the H–C–N–O system has not been surveyed systematically to explore which compounds emerge at high-pressure conditions, and what governs their stability. Here, we report on and analyze an unbiased crystal structure search among H–C–N–O compounds between 1 and 5 Mbar. We demonstrate that simple chemical rules drive stability in this composition space, which explains why the simplest possible quaternary mixture HCNO—isoelectronic to diamond—emerges as a stable compound and discuss dominant decomposition products of planetary ice mixtures.

Crystal structure prediction coupled to electronic structure calculations has emerged as a powerful tool in computational materials science, in particular in the area of high-pressure science, where it can overcome the chemical imagination attuned to ambient conditions: The predictions—and subsequent experimental confirmations—of unusual compounds such as Na2He, H3S, or LaH10 attest to the predictive power of these approaches (1–6). The planetary ices H2O, CH4, and NH3 may dominate the interiors of icy planets, but under extreme conditions (7–10). High-pressure phases of these ices have been explored computationally, and some predictions of exotic phases of individual ices have also been confirmed by experiments (11–15). Arguably, computational predictions in this field are of crucial importance because of the challenges for laboratory experiments and the indirect nature of astronomical observations. However, with increasing number of constituents the structure searches become computationally much more demanding. Hence, while structure predictions for elemental and binary systems are routine there are far fewer extensive searches of ternary systems, and none for quaternary systems.The situation in the H–C–N–O quaternary system reflects this. A vast number of publications exist on the high-pressure evolution of the individual constituents H, C, N, and O (16–19). Binary systems have also been looked at in great detail (20–24). The ternary systems are much less investigated: While H–C–O, H–N–O, and C–N–O phase diagrams have been reported (25–27), the H–C–N ternary has not, for example. The PubChem database (28) lists just under 4.8 million molecular H–C–N compounds. Overall, it contains 44.6 million H–C–N–O compounds at ambient pressure, 78% of which are of true quaternary composition—this highlights both the complexity and the relevance of this quaternary system for organic chemistry. Back in the high-pressure area, some binary mixtures of planetary ices, for instance H2O–NH3 or N2–CH4 mixtures, which form a subset of the H–N–O and H–C–N ternaries, have been studied in simulations into the megabar pressure range (29–32). However, despite the overall importance of H–C–N–O both to planetary science and Earth-bound chemical and life sciences, to our knowledge there are no computational high-pressure studies of this system, or even subsets that include all four elements, for example binary or ternary molecular mixtures such as CO2–NH3 or NH3–H2O–CH4. Here, we explore the full H–C–N–O chemical space via crystal structure searches performed at 500 GPa. This resembles the pressure—if not the temperature—at Neptune’s core–mantle boundary and therefore helps illuminate potential pressure-induced chemical reactions in the deep interiors of the outer planets and giant icy exoplanets. It also helps us understand more generally what rules govern a familiar composition space at highly unfamiliar external conditions.     

Seismic Performance of Steel Fiber Reinforced High–Strength Concrete Beam–Column Joints

In high–strength concrete, the reinforcement concentration will cause some problems in the beam–column joints (BCJs) due to a large amount of transverse reinforcement. Hence, the main object of this paper is to prevent the reinforcement concentration and reduce the amount of transverse reinforcement in the BCJs through the ideal usage of steel fibers and reinforced high–strength concrete. Pseudo–static tests on seven specimens were carried out to investigate and evaluate the seismic performance of beam–column joints in steel fiber reinforced high–strength concrete (SFRHC). Test variables were steel fiber volume ratio, concrete strength, the stirrup ratio in the core area, and an axial compression ratio of the column end. During the test, the hysteresis curves and failure mode were recorded. The seismic indicators, such as energy dissipation, ductility, strength, and stiffness degradation, were determined. The experimental results indicated that the failure modes of SFRHC beam–column joints mainly included the core area failure and the beam end bending failure. With the increase in stirrup ratio, volume ratio of steel fiber, and axial compression ratio in the core area, both the ductility and energy consumption of beam–column joints increased, while the opposite was true for concrete strength.     

Semiempirical method for examining asynchronicity in metal–oxido-mediated C–H bond activation

The oxidation of substrates via the cleavage of thermodynamically strong C–H bonds is an essential part of mammalian metabolism. These reactions are predominantly carried out by enzymes that produce high-valent metal–oxido species, which are directly responsible for cleaving the C–H bonds. While much is known about the identity of these transient intermediates, the mechanistic factors that enable metal–oxido species to accomplish such difficult reactions are still incomplete. For synthetic metal–oxido species, C–H bond cleavage is often mechanistically described as synchronous, proton-coupled electron transfer (PCET). However, data have emerged that suggest that the basicity of the M–oxido unit is the key determinant in achieving enzymatic function, thus requiring alternative mechanisms whereby proton transfer (PT) has a more dominant role than electron transfer (ET). To bridge this knowledge gap, the reactivity of a monomeric Mn^IV–oxido complex with a series of external substrates was studied, resulting in a spread of over 10⁴ in their second-order rate constants that tracked with the acidity of the C–H bonds. Mechanisms that included either synchronous PCET or rate-limiting PT, followed by ET, did not explain our results, which led to a proposed PCET mechanism with asynchronous transition states that are dominated by PT. To support this premise, we report a semiempirical free energy analysis that can predict the relative contributions of PT and ET for a given set of substrates. These findings underscore why the basicity of M–oxido units needs to be considered in C–H functionalization.

The functionalization of C–H bonds is one of the most challenging synthetic transformations in chemistry, in part because of the inherent large bond dissociation-free energies (BDFEs) associated with C–H bonds (BDFE_C–H) (1, 2). Monomeric metal–oxido species can overcome these barriers and cleave C–H bonds in a diverse set of substrates. The utility of metal–oxido species is exemplified within the active sites of metalloenzymes, such as heme and nonheme Fe monooxygenases (3–6) and in related, synthetic Mn– (7–15), Co– (16), and Fe–oxido complexes (17–25). Even with the advances made with these natural and synthetic metal–oxido species, key mechanistic questions about which properties of the metal complexes contribute to productive C–H cleavage persist (26–30). Much of our current understanding of metal–oxido-mediated C–H bond activation is predicated on a relationship between ground-state thermodynamics and the height of the activation barrier. Reactions of metal–oxido species with organic substrates often show a correlation between variations in substrate C–H bond strength, (BDFE_C–H, Eq. 1, where C_G is a constant that is dependent on the reference electrode and solvent) and the log of the rate constant for C–H bond cleavage. Systems for which these correlations are highly linear are said to follow a linear free energy relationship or display Bell–Evans–Polanyi (BEP)-like correlations (31, 32).BDFEC–H= 23.06(E˚) + 1.37(pKa) + CG.[1]The ground-state thermodynamics for C–H bond cleavage are determined by comparing the BDFE_C–H of the C–H bond to be cleaved to that of the O–H bond formed in the resulting M^n-1–OH species; this BDFE_O–H value is obtained from the reduction potential of the M = O species and its basicity, in a manner analogous to Eq. 1 (1, 33). The relationship between BDFE_O–H, reduction potential and basicity can be conveniently represented using a thermodynamic square scheme that contains three limiting, mechanistic paths (Fig. 1) (26): 1) proton transfer/electron transfer (PT-ET); 2) electron transfer/proton transfer (ET-PT); and 3) synchronous, proton-coupled electron transfer (PCET). An important outcome of the approach of comparing BDFE values is the recognition that the basicity of the M–oxido unit can be a key contributor to C–H bond cleavage. This concept is exemplified by the high-valent Fe–oxido intermediate in cytochrome P450s (compound I). The reduced intermediate is highly basic, promoting the abstraction of H atoms from strong C–H bonds at relatively low, one-electron reduction potentials (34, 35).Open in a separate windowFig. 1.Thermodynamic square scheme for M–oxido complexes involved in C–H bond cleavage. For 1, E_1/2'' = −1.0 V; E_1/2 = −0.18 V; pK_a = 15; pK_a'' = 28.1(2); and BDFE_O–H = 87(2) kcal/mol (SI Appendix, Eq. S1). Values measured in DMSO at room temperature. Potentials are referenced to [Fe^III/IICp₂]^+/0.We have also shown that the basicity of the oxido ligand drives the reactivity of a low-valent Mn^III–oxido complex that is competent at cleaving C–H bonds, even though its reduction potential is less than −2.0 V versus [Fe^III/IICp₂]^+/0 (7). Our initial mechanistic suggestion for this Mn^III–oxido complex was a stepwise PT-ET pathway with proton transfer (PT) being rate limiting. However, subsequent kinetic studies on a related series of Mn^III–oxido complexes showed that electron transfer (ET) must also be involved in the rate-determining step (8). To reconcile these results, we proposed a mechanism with an imbalanced transition state, in which PT precedes ET (8). This type of asynchronous PCET mechanism (36) was examined computationally (37) and experimentally to explain the cleavage of C–H bonds with Co^III–oxido (16), Ru^IV–oxido (38), and Cu^III−O₂CAr complexes (39). The involvement of asynchronous PCET processes in the cleavage of C–H bonds by high-valent M–oxido complexes, such as Fe^IV/Mn^IV–oxido species, is still uncertain. In fact, most reactions have synchronous PCET mechanisms that follow the BEP principle (31, 32) and exhibit the correlations between the BDFE_C–H values of the substrates and the log of the second-order rate constants [log (k)]. However, C–H bond cleavage via an asynchronous PCET mechanism that is driven by the basicity of the M–oxido unit would be beneficial because reactivity could occur at lower redox potentials, while still achieving the efficiency that is often associated with PCET processes. We reasoned that this type of mechanism could occur if the high-valent M–oxido complex was relatively basic and could therefore favor mechanisms that involve PT-driven, asynchronous transition states. The high-valent Mn^IV–oxido complex [Mn^IVH₃buea(O)]⁻ (1) is a suitable candidate for testing this premise because it has a relatively high basicity, as gauged by its conjugate acid, [Mn^IVH₃buea(OH)], which has an estimated pK_a of ∼15 in DMSO (7).We report here the kinetic studies of 1 with a variety of substrates, and the results of these studies support the involvement of PT-dominated, asynchronous transition states and show the applicability of this type of mechanism in the cleavage of C–H bonds by high-valent M–oxido complexes. To further evaluate our findings, we developed a semiempirical free energy analysis to estimate the relative contributions from the free energies of PT and ET within an asymmetric transition state. This analysis supports the assertion that the observed reactivity of 1 is driven by the pK_a, leading to PT-controlled, asynchronous transition states for the substrates examined. Moreover, we demonstrate that this analysis is useful in predicting whether a PCET reaction will be synchronous or asynchronous based on reactivity and thermodynamic parameters. Our method deviates from traditional, BEP-like analyses, in that it examines the behavior of log (k) versus a linear combination of PT and ET terms, rather than bond strengths. The appropriate combination of these free energy terms is determined by the linearity of plots versus log (k).