Oxygen Atom Exchange between H₂O and Non-Heme Oxoiron(IV) Complexes: Ligand Dependence and Mechanism

Detailed studies of oxygen atom exchange (OAE) between H₂¹⁸O and synthetic non-heme oxoiron­(IV) complexes supported by tetradentate and pentadentate ligands provide evidence that they proceed by a common mechanism but within two different kinetic regimes, with OAE rates that span 2 orders of magnitude. The first kinetic regime involves initial reversible water association to the Fe^IV complex, which is evidenced by OAE rates that are linearly dependent on [H₂¹⁸O] and H₂O/D₂O KIEs of 1.6, while the second kinetic regime involves a subsequent rate determining proton-transfer step between the bound aqua and oxo ligands that is associated with saturation behavior with [H₂¹⁸O] and much larger H₂O/D₂O KIEs of 5–6. [Fe^IV(O)­(TMC)­(MeCN)]²⁺ (<b>1</b>) and [Fe^IV(O)­(MePy₂TACN)]²⁺ (<b>9</b>) are examples of complexes that exhibit kinetic behavior in the first regime, while [Fe^IV(O)­(N4Py)]²⁺ (<b>3</b>), [Fe^IV(O)­(BnTPEN)]²⁺ (<b>4</b>), [Fe^IV(O)­(1Py-BnTPEN)]²⁺ (<b>5</b>), [Fe^IV(O)­(3Py-BnTPEN)]²⁺ (<b>6</b>), and [Fe^IV(O)­(Me₂Py₂TACN)]²⁺ (<b>8</b>) represent complexes that fall in the second kinetic regime. Interestingly, [Fe^IV(O)­(PyTACN)­(MeCN)]²⁺ (<b>7</b>) exhibits a linear [H₂¹⁸O] dependence below 0.6 M and saturation above 0.6 M. Analysis of the temperature dependence of the OAE rates shows that most of these complexes exhibit large and negative activation entropies, consistent with the proposed mechanism. One exception is complex <b>9</b>, which has a near-zero activation entropy and is proposed to undergo ligand-arm dissociation during the RDS to accommodate H₂¹⁸O binding. These results show that the observed OAE kinetic behavior is highly dependent on the nature of the supporting ligand and are of relevance to studies of non-heme oxoiron­(IV) complexes in water or acetonitrile/water mixtures for applications in photocatalysis and water oxidation chemistry

Construction of a laboratory bench and evaluation of a grain yield monitor.

Com o avanço da agricultura de precisão, em que a variabilidade da produtividade entre os diversos pontos de uma determinada área é levada em consideração, é necessário aprimorar, cada vez mais, o sistema de coleta de dados, para que os resultados possam ser confiáveis. Dentre outros, faz-se necessário conhecer o desempenho dos sensores localizados nas colhedoras, para que se saiba o nível de acurácia dos dados de campo para a geração dos mapas de produtividade. O objetivo do presente trabalho foi caracterizar o desempenho, sob condições controladas, de um equipamento comercial, especialmente o sensor de fluxo de volume e suas interações com os sensores de inclinação, de velocidade de deslocamento da máquina e de grau de umidade dos grãos, destinados à mensuração da produtividade de culturas de grãos em geral. Foi montada uma bancada de ensaio constituída de um tanque alimentador com comporta de abertura variável, que escoa grãos a um condutor helicoidal, que são transportados para um elevador de taliscas de uma colhedora comercial. Os grãos transportados pelo elevador passam através dossensores de fluxo de volume e grau de umidade e, em seguida, são descarregados num tanque superior suspenso por uma célula de carga com capacidade de 2.000kg (desprezando a variação da gravidade com relação ao nível do mar) para que os dados de massa sejam comparados com os registrados pelo sensor de fluxo de volume. O monitor de produtividade foi ensaiado na bancada para simulações de fluxos constantes e variados em três diferentes posições transversais do elevador. Os resultados mostraram que a bancada de ensaio mostrou-se eficiente para os tipos de ensaios propostos. Sua estrutura é resistente e a variação da angulação do elevador é de fácil manejo. A geometria da construção do tanque de alimentação mostrou-se eficiente para fornecer vazões uniformes com o tempo, obtendo-se taxas de fluxo constantes dentro dos limites de 2,0 a 8,0kg.s-1. O sensor de velocidade apresentou erro médio relativo de 0,31% e o de grau de umidade, erro médio em módulo de 5,01% para as condições estudadas. Quanto mais afastado do fluxo médio de calibração, pior é a estimativa do fluxo pelo monitor de produtividade. O erro médio geral dos ensaios com taxas de fluxo constantes foi de -5,31%, com desvio padrão de 4,14. O monitor propiciou, em 70% dos ensaios, erros pontuais menores que 6% para ensaios com fluxo constante. Suas leituras superestimaram valores menores do que o ponto da taxa de fluxo média de calibração e subestimaram valores para taxas de fluxo maiores que este ponto. As leituras do monitor responderam imediatamente as variações impostas ao fluxo processado pelo elevador de grãos. O clinômetro ou o algoritmo que considera a inclinação é eficiente para compensar as inclinações transversais da máquina, mesmo em condições de taxas de fluxo variadas. O erro médio geral dos ensaios com taxas de fluxo variadas, calculado a partir do erro médio em módulo de cada ensaio foi, 4,84%. O erro médio global encontrado do monitor de produtividade para as leituras com taxas de fluxo constantes e variadas, foi 5,12%.With the progress of the precision agriculture where spatial yield variability is taken into account, it is necessary to improve data collection so the results can be more reliable. It is necessary to understand how yield sensor used on combines works in order to know the accuracy of the field data for generation of the yield maps. This work aims to characterize the performance, under controlled conditions, of a commercial equipment, its yield sensor and interactions with the hillside sensor, forward speed sensor and grain moisture sensor. A test bench with a tank feeder was built with a variable opening floodgate, which drains grains to the foot of the paddle elevator of a commercial combine. The grain flow transported by the elevator pass through the yield and moisture sensor and is unloaded in a superior tank hold by a load cell with capacity of 2.000kg (desconsidering the variation of the gravity) so that the mass data is compared with that registered by the monitor. The monitor was tested on simulations of constant and variable flow rates in three different transverse positions of the elevator. The results showed that the test bench was shown efficient for the types of proposed tests. Its structure is resistant and the variation of the angle of the elevator is of easy handling. The geometry of the feeding tank was shown efficient to supply uniform flow rates, between 2,0 and 8,0kg.s-1. The speed sensor showed relative mean error of 0,31% and the moisture sensor presented a module mean error of 5,01%. Flow estimation got worst increasing or decreasing the flow rate apart from calibration region. Main mean error of the test with constant flow rates was of -5,31%, with standard deviation of 4,14. On 70% of the test monitor, readings resulted in erros less than 6,00% with constant flow. Its readings overestimated flow values under the mean flow rate of calibration and underestimated flow above it. The readings of the yield monitor responded to the variations imposed to the flow through by the elevator immediately. The hillside sensor and the algorithm that considers the inclination are efficient for compensating the transverse inclinations of the machine, even in conditions of varied flow rates. The general mean error test with varied flow rates, was 4,84%. The global mean error shown by the yield monitor for the readings with varied and constant flow rates was 5,12%

Mechanism of the Selective Fe-Catalyzed Arene Carbon–Hydrogen Bond Functionalization

The complete chemoselective functionalization of aromatic C­(sp²)–H bonds of benzene and alkyl benzenes by carbene insertion from ethyl diazoacetate was unknown until the recent discovery of an iron-based catalytic system toward such transformation. A Fe­(II) complex bearing the pytacn ligand (pytacn = L1 = 1-(2-pyridylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane) transferred the CHCO₂Et unit exclusively to the C­(sp²)–H bond. The cycloheptatriene compound commonly observed through Buchner reaction or, when employing alkyl benzenes, the corresponding derivatives from C­(sp³)–H functionalization are not formed. We herein present a combined experimental and computational mechanistic study to explain this exceptional selectivity. Our computational study reveals that the key step is the formation of an enol-like substrate, which is the precursor of the final insertion products. Experimental evidences based on substrate probes and isotopic labeling experiments in favor of this mechanistic interpretation are provided

Triggering the Generation of an Iron(IV)-Oxo Compound and Its Reactivity toward Sulfides by Ru^II Photocatalysis

The preparation of [Fe^IV(O)­(MePy₂tacn)]²⁺ (<b>2</b>, MePy₂tacn = <i>N</i>-methyl-<i>N</i>,<i>N</i>-bis­(2-picolyl)-1,4,7-triazacyclononane) by reaction of [Fe^II(MePy₂tacn)­(solvent)]²⁺ (<b>1</b>) and PhIO in CH₃CN and its full characterization are described. This compound can also be prepared photochemically from its iron­(II) precursor by irradiation at 447 nm in the presence of catalytic amounts of [Ru^II(bpy)₃]²⁺ as photosensitizer and a sacrificial electron acceptor (Na₂S₂O₈). Remarkably, the rate of the reaction of the photochemically prepared compound <b>2</b> toward sulfides increases 150-fold under irradiation, and <b>2</b> is partially regenerated after the sulfide has been consumed; hence, the process can be repeated several times. The origin of this rate enhancement has been established by studying the reaction of chemically generated compound <b>2</b> with sulfides under different conditions, which demonstrated that both light and [Ru^II(bpy)₃]²⁺ are necessary for the observed increase in the reaction rate. A combination of nanosecond time-resolved absorption spectroscopy with laser pulse excitation and other mechanistic studies has led to the conclusion that an electron transfer mechanism is the most plausible explanation for the observed rate enhancement. According to this mechanism, the in-situ-generated [Ru^III(bpy)₃]³⁺ oxidizes the sulfide to form the corresponding radical cation, which is eventually oxidized by <b>2</b> to the corresponding sulfoxide