Iron(III) Ejection from a "Beheaded" TAML Activator: Catalytically Relevant Mechanistic Insight into the Deceleration of Electrophilic Processes by Electron Donors.

Kinetic studies of the acid-induced ejection of iron(III) show that the more electron-rich tetra-amido-N macrocyclic ligand (TAML) activator [FeIII{(Me2CNCOCMe2NCO)2CMe2}OH2]- (4), which does not have a benzene ring in its head component ("beheaded" TAML), is up to 1 × 104 times more resistant than much less electron-rich [FeIII{1,2-C6H4(NCOCMe2NCO)2CMe2}OH2]- (1a) to the electrophilic attack. This counterintuitive increased resistance is seen in both the specific acid (kobs = k1[H+]/(K + [H+])) and phosphate general acid (kII = (kdiKa1 + ktri[H+])/(Ka1+[H+])) demetalation pathways. Insight into this reactivity puzzle was obtained from coupling kinetic data with theoretical density functional theory modeling. First, although 1a and related complexes are six-coordinate in water, 4 has a strong tendency to repel the second aqua ligand favoring [LFe(OH2)]- and making appropriate the comparison of monoaqua-4 with diaqua-1a in the demetalation process. Second, dearomatization exerts a strong effect on the highest occupied molecular orbital (HOMO) energy of five-coordinate monoaqua-4, the presumed target in proton-induced demetalation, stabilizing it by ca. 51 kJ mol-1 compared with monoaqua-1a. Third, the monoaqua-4 HOMO is localized over the N-pπ system of all four N donors in contrast with monoaqua-1a, where N-pπ contributions from the head amides only mix with the aromatic ring π system. Fourth, addition of a second water ligand to monoaqua-1a giving [LFe(OH2)2]- reshapes the monoaqua-1a HOMO by shifting its entire locus from the head to the tail diamido-N section-this HOMO is by 54 kJ mol-1 less stable than the monoaqua-4 HOMO. These features provide the foundations for mechanistic conclusions concerning demetalation that (i) axial water ligands enable a favored path in the six-coordinate case of 1a, where a proton "slides" toward the Fe-N bond and (ii) early and late transition states are realized for 4 and 1a, respectively, with a larger free energy of activation for the beheaded TAML activator 4.

A "Beheaded" TAML Activator: A Compromised Catalyst that Emphasizes the Linearity between Catalytic Activity and pKa.

Studies of the new tetra-amido macrocyclic ligand (TAML) activator [FeIII{(Me2CNCOCMe2NCO)2CMe2}OH2]- (4) in water in the pH range of 2-13 suggest its pseudo-octahedral geometry with two nonequivalent axial H2O ligands and revealed (i) the anticipated basic drift of the first pKa of water to 11.38 due to four electron-donating methyl groups alongside (ii) its counterintuitive enhanced resistance to acid-induced iron(III) ejection from the macrocycle. The catalytic activity of 4 in the oxidation of Orange II (S) by H2O2 in the pH range of 7-12 is significantly lower than that of previously reported TAML activators, though it follows the common rate law (v/[FeIII] = kIkII[H2O2][S]/(kI[H2O2] + kII[S]) and typical pH profiles for kI and kII. At pH 7 and 25 °C the rate constants kI and kII equal 0.63 ± 0.02 and 1.19 ± 0.03 M-1 s-1, respectively. With these new values for pKa, kI and kII establishing new high and low limits, respectively, the rate constants kI and kII were correlated with pKa values of all TAML activators. The relations log k = log k0 + α × pKa were established with log k0 = 13 ± 2 and 20 ± 4 and α = -1.1 ± 0.2 and -1.8 ± 0.4 for kI and kII, respectively. Thus, the reactivity of TAML activators across four generations of catalysts is predictable through their pKa values.

Quantifying evolving toxicity in the TAML/peroxide mineralization of propranolol.

Oxidative water purification of micropollutants (MPs) can proceed via toxic intermediates calling for procedures for connecting degrading chemical mixtures to evolving toxicity. Herein, we introduce a method for projecting evolving toxicity onto composite changing pollutant and intermediate concentrations illustrated through the TAML/H2O2 mineralization of the common drug and MP, propranolol. The approach consists of identifying the key intermediates along the decomposition pathway (UPLC/GCMS/NMR/UV-Vis), determining for each by simulation and experiment the rate constants for both catalytic and noncatalytic oxidations and converting the resulting predicted concentration versus time profiles to evolving composite toxicity exemplified using zebrafish lethality data. For propranolol, toxicity grows substantially from the outset, even after propranolol is undetectable, echoing that intermediate chemical and toxicity behaviors are key elements of the environmental safety of MP degradation processes. As TAML/H2O2 mimics mechanistically the main steps of peroxidase catalytic cycles, the findings may be relevant to propranolol degradation in environmental waters.

Additively Manufactured Continuous Cell-Size Gradient Porous Scaffolds: Pore Characteristics, Mechanical Properties and Biological Responses In Vitro.

Porous scaffolds with graded open porosity combining a morphology similar to that of bone with mechanical and biological properties are becoming an attractive candidate for bone grafts. In this work, scaffolds with a continuous cell-size gradient were studied from the aspects of pore properties, mechanical properties and bio-functional properties. Using a mathematical method named triply periodic minimal surfaces (TPMS), uniform and graded scaffolds with Gyroid and Diamond units were manufactured by selective laser melting (SLM) with Ti-6Al-4V, followed by micro-computer tomography (CT) reconstruction, mechanical testing and in vitro evaluation. It was found that gradient scaffolds were preferably replicated by SLM with continuous graded changes in surface area and pore size, but their pore size should be designed to be ≥ 450 µm to ensure good interconnectivity. Both the Gyroid and Diamond structures have superior strength compared to cancellous bones, and their elastic modulus is comparable to the bones. In comparison, Gyroid exhibits better performances than Diamond in terms of the elastic modulus, ultimate strength and ductility. In vitro cell culture experiments show that the gradients provide an ideal growth environment for osteoblast growth in which cells survive well and distribute uniformly due to biocompatibility of the Ti-6Al-4V material, interconnectivity and suitable pore size.

Study on the Geometric Design of Supports for Overhanging Structures Fabricated by Selective Laser Melting.

Additional structures are usually adopted to support the overhanging structures in order to resist the deformation of parts. Improper geometric design of the support structures may result in a sharp deterioration in the surface quality and a failure of manufacture, which affects the expansion in the use of selective laser melting (SLM) technology. In this research, cuboids were added into the conventional block support for a better heat dissipation. The Taguchi method was used to analyze the effect of the geometric design of this support on the part's deformation and surface roughness. It was found that solid pieces or cuboids as support structures can reduce the deformation. However, their effects are weaker than those of teeth structures which decrease the deformation by more reliable connections. It is interesting that narrowing the gap between the cuboids and overhang can weaken the strength of teeth structures and then increases the deformation of part. In general, the distance between every two adjacent walls of support and the gap between the cuboids and the overhang have the greatest influence on the part's deformation and surface quality respectively.

Improved Mechanical Properties and Energy Absorption of BCC Lattice Structures with Triply Periodic Minimal Surfaces Fabricated by SLM.

The triply periodic minimal surface (TPMS) method is a novel approach for lattice design in a range of fields, such as impact protection and structural lightweighting. In this paper, we used the TPMS formula to rapidly and accurately generate the most common lattice structure, named the body centered cubic (BCC) structure, with certain volume fractions. TPMS-based and computer aided design (CAD) based BCC lattice structures with volume fractions in the range of 10⁻30% were fabricated by selective laser melting (SLM) technology with Ti⁻6Al⁻4V and subjected to compressive tests. The results demonstrated that local geometric features changed the volume and stress distributions, revealing that the TPMS-based samples were superior to the CAD-based ones, with elastic modulus, yield strength and compression strength increasing in the ranges of 18.9⁻42.2%, 19.2⁻29.5%, and 2⁻36.6%, respectively. The failure mechanism of the TPMS-based samples with a high volume fraction changed to brittle failure observed by scanning electron microscope (SEM), as their struts were more affected by the axial force and fractured on struts. It was also found that the TPMS-based samples have a favorable capacity to absorb energy, particularly with a 30% volume fraction, the energy absorbed up to 50% strain was approximately three times higher than that of the CAD-based sample with an equal volume fraction. Furthermore, the theoretic Gibson⁻Ashby mode was established in order to predict and design the mechanical properties of the lattice structures. In summary, these results can be used to rapidly create BCC lattice structures with superior compressive properties for engineering applications.

Mechanical Properties of Optimized Diamond Lattice Structure for Bone Scaffolds Fabricated via Selective Laser Melting.

Developments in selective laser melting (SLM) have enabled the fabrication of periodic cellular lattice structures characterized by suitable properties matching the bone tissue well and by fluid permeability from interconnected structures. These multifunctional performances are significantly affected by cell topology and constitutive properties of applied materials. In this respect, a diamond unit cell was designed in particular volume fractions corresponding to the host bone tissue and optimized with a smooth surface at nodes leading to fewer stress concentrations. There were 33 porous titanium samples with different volume fractions, from 1.28 to 18.6%, manufactured using SLM. All of them were performed under compressive load to determine the deformation and failure mechanisms, accompanied by an in-situ approach using digital image correlation (DIC) to reveal stress-strain evolution. The results showed that lattice structures manufactured by SLM exhibited comparable properties to those of trabecular bone, avoiding the effects of stress-shielding and increasing longevity of implants. The curvature of optimized surface can play a role in regulating the relationship between density and mechanical properties. Owing to the release of stress concentration from optimized surface, the failure mechanism of porous titanium has been changed from the pattern of bottom-up collapse by layer (or cell row) to that of the diagonal (45°) shear band, resulting in the significant enhancement of the structural strength.

Finite Element Analysis of Interaction of Laser Beam with Material in Laser Metal Powder Bed Fusion Process.

A deep understanding of the laser-material interaction mechanism, characterized by laser absorption, is very important in simulating the laser metal powder bed fusion (PBF) process. This is because the laser absorption of material affects the temperature distribution, which influences the thermal stress development and the final quality of parts. In this paper, a three-dimensional finite element analysis model of heat transfer taking into account the effect of material state and phase changes on laser absorption is presented to gain insight into the absorption mechanism, and the evolution of instantaneous absorptance in the laser metal PBF process. The results showed that the instantaneous absorptance was significantly affected by the time of laser radiation, as well as process parameters, such as hatch space, scanning velocity, and laser power, which were consistent with the experiment-based findings. The applicability of this model to temperature simulation was demonstrated by a comparative study, wherein the peak temperature in fusion process was simulated in two scenarios, with and without considering the effect of material state and phase changes on laser absorption, and the simulated results in the two scenarios were then compared with experimental data respectively.

Manufacturing Feasibility and Forming Properties of Cu-4Sn in Selective Laser Melting.

Copper alloys, combined with selective laser melting (SLM) technology, have attracted increasing attention in aerospace engineering, automobile, and medical fields. However, there are some difficulties in SLM forming owing to low laser absorption and excellent thermal conductivity. It is, therefore, necessary to explore a copper alloy in SLM. In this research, manufacturing feasibility and forming properties of Cu-4Sn in SLM were investigated through a systematic experimental approach. Single-track experiments were used to narrow down processing parameter windows. A Greco-Latin square design with orthogonal parameter arrays was employed to control forming qualities of specimens. Analysis of variance was applied to establish statistical relationships, which described the effects of different processing parameters (i.e., laser power, scanning speed, and hatch space) on relative density (RD) and Vickers hardness of specimens. It was found that Cu-4Sn specimens were successfully manufactured by SLM for the first time and both its RD and Vickers hardness were mainly determined by the laser power. The maximum value of RD exceeded 93% theoretical density and the maximum value of Vickers hardness reached 118 HV 0.3/5. The best tensile strength of 316-320 MPa is inferior to that of pressure-processed Cu-4Sn and can be improved further by reducing defects.