Combining Metal-Metal Cooperativity, Metal-Ligand Cooperativity and Chemical Non-Innocence in Diiron Carbonyl Complexes

Several metalloenzymes, including [FeFe]-hydrogenase, employ cofactors wherein multiple metal atoms work together with surrounding ligands that mediate heterolytic and concerted proton-electron transfer (CPET) bond activation steps. Herein, we report a new dinucleating PNNP expanded pincer ligand, which can bind two low-valent iron atoms in close proximity to enable metal-metal cooperativity (MMC). In addition, reversible partial dearomatization of the ligand’s naphthyridine core enables both heterolytic metal-ligand cooperativity (MLC) and chemical non-innocence through CPET steps. Thermochemical and computational studies show how a change in ligand binding mode can lower the bond dissociation free energy of ligand C(sp3)–H bonds by ~25 kcal mol-1. H-atom abstraction enabled trapping of an unstable intermediate, which undergoes facile loss of two carbonyl ligands to form an unusual paramagnetic (S = 1/2) complex containing a mixed-valent iron(0)-iron(I) core bound within a partially dearomatized PNNP ligand. Finally, cyclic voltammetry experiments showed that these diiron complexes show catalytic activity for the electrochemical hydrogen evolution reaction. This work presents the first example of a ligand system that enables MMC, heterolytic MLC and chemical non-innocence, thereby providing important insights and opportunities for the development of bimetallic systems that exploit these features to enable new (catalytic) reactivity.

LayOpt: an educational web-app for truss layout optimization

A new interactive truss layout optimization web-app has been developed for educational use. This has been designed to be used on a range of devices, from mobile phones to desktop PCs. Truss designs are first generated via numerical layout optimization and then rationalized via geometry optimization. It is then shown that these designs can be simplified using a computationally inexpensive process that allows the user to control the trade-off between complexity and structural volume. The process involves the use of smooth Heaviside representations of member existence variables, with nodal slack forces employed that allow unstable intermediate truss structures. Full details of the web-app are provided in this contribution, from underlying formulation to cloud computing implementation. A range of numerical examples are used to demonstrate the efficacy of the web-app, and to show how it can potentially be used in educational and practical engineering settings.

Structure of a reaction intermediate mimic in t6A biosynthesis bound in the active site of the TsaBD heterodimer from Escherichia coli

The tRNA modification N6-threonylcarbamoyladenosine (t6A) is universally conserved in all organisms. In bacteria, the biosynthesis of t6A requires four proteins (TsaBCDE) that catalyze the formation of t6A via the unstable intermediate l-threonylcarbamoyl-adenylate (TC-AMP). While the formation and stability of this intermediate has been studied in detail, the mechanism of its transfer to A37 in tRNA is poorly understood. To investigate this step, the structure of the TsaBD heterodimer from Escherichia coli has been solved bound to a stable phosphonate isosteric mimic of TC-AMP. The phosphonate inhibits t6A synthesis in vitro with an IC50 value of 1.3 μM in the presence of millimolar ATP and L-threonine. The inhibitor binds to TsaBD by coordination to the active site Zn atom via an oxygen atom from both the phosphonate and the carboxylate moieties. The bound conformation of the inhibitor suggests that the catalysis exploits a putative oxyanion hole created by a conserved active site loop of TsaD and that the metal essentially serves as a binding scaffold for the intermediate. The phosphonate bound crystal structure should be useful for the rational design of potent, drug-like small molecule inhibitors as mechanistic probes or potentially novel antibiotics.

Air and Moisture Tolerant Synthesis of a Chelated bis(NHC) Methylpalladium(II) Complex Relevant to Alkyl Migration Processes in Catalysis

An air- and moisture-tolerant alternate synthetic pathway to the preparation of a cationic chelated bis(NHC) methylpalladium(ii) complex, [{(MesIm)2CH2}Pd(Me)(NCMe)][PF6], is described. The pathway involves the isolation of a bis(NHC) AgI complex, [{(MesIm)2CH2}2Ag2][PF6]2, via metallation of the corresponding diimidazolium salt with Ag2O followed by carbene transfer to [(COD)PdBrMe]. This new method avoids a previously reported unstable intermediate that displayed rapid decomposition at room temperature, attaining the targeted cationic methylpalladium(ii) complex in high yield. CO/ethylene copolymerisation catalysis trials are reported showing solvent dependent catalyst lifetime and copolymer yields. Preliminary ethylene insertion studies are also outlined revealing possible pathways leading towards catalyst deactivation.

Reaction Behaviors of Associated Minerals in Molten Salt Smelting of Stibnite and Kilogram-Class Trials

The molten salt metallurgy of Sb, which involves the smelting of stibnite in a binary NaCl-Na2CO3 salt with sulfur-fixing and the addition of a reductant, has been proposed as a clean method for Sb extraction. However, the reacting behaviors of the minerals associated with stibnite (Sb2S3) during the smelting are still unclear, and industrial tests have not been conducted. This study investigated the behaviors of PbS, FeS2, SiO2, and CaCO3, which are the main minerals associated with stibnite, during reducing smelting by using the NaCl-Na2CO3 molten salt. The results showed that PbS could react with Na2CO3 to generate metallic Pb at 950 °C. FeS2 and SiO2 formed stable NaFeS2 and Na2SiO3 with the molten salt at a high temperature, respectively. CaCO3 formed an unstable intermediate product of Na2Ca(CO3)2 at 675 °C and decomposed with increasing temperature. Kilogram-class trials were also performed using 50 kg of concentrate and more than 300 kg of mixture material, and the results showed that the direct recovery rate of Sb and Au reached maximum values of 93.22% and 92.06% at temperature 920 °C in eutectic Na2CO3-NaCl molten salt, respectively, while the total sulfur-fixing ratio reached 99.49%. Thus, the associated minerals consumed the molten salt, and the feasibility of molten salt smelting was verified by this kilogram-class pilot experiment.

Attempted synthesis of a meta-metalated calix[4]arene

An evidence for the formation of a rare meta-metalated inherently chiral calix[4]arene is described. Our strategy involved using a mesoionic carbene to direct C–H activation, but proved to form an unexpectedly unstable intermediate that was identified through high-resolution mass spectrometry. On route to our target, a new optimized method to mononitrocalix[4]arenes was developed, including optimized and high yielding transformations to azide and 1,2,3-triazole derivatives which may have application in other areas of research.

Comments on the recent crystal structure of TsaBDE complex of bacterial t6A biosynthesis system and its significance for understanding TC-AMP processing

ABSTRACTThe N(6)-threonylcarbamoyl adenosine (t6A) modification at position 37 of a tRNA of the anticodon loop is universal and central to the translational fidelity of all known organisms. The ternary complex of TsaBDE is the central and essential workstation for t6A biosynthesis in bacteria. The recently published crystal structure of Thermotoga maritima (T.maritima) TsaBDE complex (Missoury et al., 2018) has ~15% incorrectly-placed, misplaced/mistraced, or missing residues. These structural errors have precipitated incorrect conclusions about the disordering of the active site and inferred action of the TsaE element. In this report, we rectify the published structural model of the T.maritima TsaBDE complex. In stark contrast, a corrected structural model of TsaBDE shows that both active sites of the TsaD element are fully occupied with threonylcarbamoyladenosine (TC-AMP), an unstable intermediate chemical moiety of the t6A biosynthesis pathway. This observation has profound implications for understanding the funneling of intermediates in the t6A pathway and also in helping to elucidate tRNA binding modes. Based on the structural details described in here we propose a unifying principle for binding the tRNA to the TsaD subunit of the complex which is universally required in all known t6A modification pathways.

Investigations of the key macrolactamisation step in the synthesis of cyclic tetrapeptide pseudoxylallemycin A

During total synthesis of pseudoxylallemycin A, an unstable intermediate was observed and appeared to be reactivated by coupling reagent by-products.