Connecting [4Fe–4S] Clusters and Hemes – Towards Modeling the Active Site of Sulfite Reductase

In this paper, we present a new design for biologically inspired models for the active site of assimilatory sulfite and nitrite reductases (aSIR and aNIR), which consists of a siroheme that is directly linked to a [4Fe–4S] cubane cluster. The individual components used here to construct this model are a site‐differentiated [4Fe–4S] cluster, a bifunctional bridging ligand, and a metalloporphyrin. We have prepared two new site‐differentiated clusters, [Fe 4 S 4 (TriS)(SPy)] and [Fe 4 S 4 (TriS)(SEtIm)], which contain pyridine and imidazole linkers for the binding to a metalloporphyrin, and characterized these compounds, using UV/Vis , IR, and 1 H‐NMR spectroscopy, cyclic voltammetry (CV), and mass spectrometry. Titration experiments where then performed by using [Zn(TPP)] (TPP 2– = meso ‐tetraphenylporphyrin dianion) and corresponding fluorinated derivatives to find the best [4Fe–4S]–heme combination for an optimal binding of the two components in solution. Excitingly, our results demonstrate the formation of the desired [4Fe–4S]–heme catalytic arrays in solution with high specificity . The best combination of cubane cluster and metalloporphyrin for future catalyst development corresponds to the complex (Bu 4 N) 2 [M(T o ‐F 2 PP)–{Fe 4 S 4 (TriS)(SEtIm)}] {T o ‐F 2 PP 2– = meso ‐tetra( ortho ‐difluorophenyl)porphyrin dianion}. The binding between these components with M = Zn 2+ was further confirmed by CV. Thus, we have created a new type of biologically inspired model system for the aSIR and aNIR active site that leads to a robust attachment of the individual components in solution. The binding between fluorinated zinc porphyrins and site‐differentiated [4Fe–4S] clusters through pyridyl‐ and imidazolylthiolate bifunctional bridging ligands generates catalytic arrays inspired by the active site of sulfite reductase. The properties of the unique [4Fe–4S] clusters applied here are described, and binding constants are reported for the two components of the resulting macromolecular scaffold.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/99598/1/3883_ftp.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/99598/2/ejic_201300308_sm_miscellaneous_information.pd

Catalytic Copolymerization of CO and Ethylene with a Charge Neutral Palladium(II) Zwitterion

The synthesis of a zwitterionic Pd(II) complex supported by an anionic bis(phosphino)borate ligand, Ph_(2)B(CH_(2)PPh_(2))_(2) (abbreviated as [Ph_(2)BP_(2)]), is reported. The new complex, [Ph_(2)BP_(2)]PdMe(THF), is active for CO and ethylene copolymerization. The copolymerization activity and polyketone molecular weight for the neutral, zwitterionic system are compared with those for the cationic systems [R2E(CH2PPh2)2PdMe(THF)][B(C_(6)F_(5))_(4)] where ER_(2) = SiPh_(2) and CH_(2). Surprisingly, the more electron rich zwitterionic system is a catalyst of activity comparable to that of the more conventional cationic systems

Pseudotetrahedral Manganese Complexes Supported by the Anionic Tris(phosphino)borate Ligand [PhBP^(iPr)_3]

This paper presents aspects of the coordination chemistry of mono- and divalent manganese complexes supported by the anionic tris(phosphino)borate ligand, [PhBP^(iPr)_3] (where [PhBP^(iPr)_3] = [PhB(CH_2P^(iPr)_2)_3]-). The Mn(II) halide complexes, [PhBP^(iPr)_3]MnCl (1) and [PhBP^(iPr)_3]MnI (2), have been characterized by X-ray diffraction, SQUID magnetometry, and EPR spectroscopy. Compound 2 serves as a precursor to a series of Mn azide, alkyl, and amide species:  [PhBP^(iPr)_3]Mn(N_3) (3), [PhBP^(iPr)_3]Mn(CH_2Ph) (4), [PhBP^(iPr)_3]Mn(Me) (5), [PhBP^(iPr)_3]Mn(NH(2,6-iPr_2-C_6H_3)) (6), [PhBP^(iPr)_3]Mn(dbabh) (7), and [PhBP^(iPr)_3]Mn(1-Ph(isoindolate)) (8). The complexes 2−8 feature a divalent-metal center and are pseudotetrahedral. They collectively represent an uncommon structural motif for low-coordinate, polyphosphine-supported Mn complexes. Two Mn(I) species have also been prepared. These include the Tl−Mn adduct [PhBP^(iPr)_3]Tl−MnBr(CO)_4 (9) and the octahedral complex [PhBP^(iPr)_3]Mn(CN^tBu)_3 (10). Some of our initial synthetic efforts to generate [PhBP^(iPr)_3]Mn_≡Nx species are briefly described, as are DFT studies that probe the electronic viability of these types of multiply bonded target structures

Teachers' Perceptions and Management of Hyperactive Children in Early Childhood Proghams

Family Relations and Child Developmen

Monitoring and evaluation of a ceramic water filter and hand-washing intervention in Northern Ghana

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2012.Cataloged from PDF version of thesis.Includes bibliographical references (p. 120-124).Through a Rotary Club contract, PHW will sell Kosim filters and install Tippy-Tap hand-washing stations in 1250 households in Northern Ghana. This thesis presents the following project monitoring and evaluation components: (1) three-part evaluation framework; (2) baseline results; and (3) recommendations for an objective measure of Kosim filter use. The evaluation framework consists of a baseline survey, one-month follow-up survey, and six-month follow-up survey, and includes a staggered cross-sectional study that compares health outcome between purchasers and non-purchasers, and between purchasers from exposure households and purchasers from control households. January 2012 baseline surveys collected information on household characteristics, water source, household water management, hand-washing practices, diarrheal disease prevalence, and respiratory disease prevalence. In total 429 households were sampled from 20 villages, and the results from 10 villages are reported in this thesis. Overall, 98.6% of the survey population uses surface water as a primary dry season drinking water source, and 79.9% uses unprotected water sources in the wet season. An estimated 52.6% of households use cloth filters to treat their drinking water at home. Only 5.0% of households practice hand-washing with soap, yet 99.2% of households have soap present in the home. The prevalence rate for diarrhea was 23% (95% CI 17% to 29%) for children under the age of five and 9% (95% CI 5% to 13%) for the general population. For cough and difficulty breathing, prevalence rates were 25% (95% CI 19% to 31%) for children under the age of five and 13% (95% CI 8% to 17%) for the general population. Regarding an objective measure of Kosim filter use, the Camelbak@ Flow MeterTM accurately measures flow of at least 0.8 L/min, and is therefore sufficient to measure most flow rates through the spigot of the Kosim filter. PHW should choose one of three options obtain an objective measure of Kosim filter use: (1) adapt and develop a method to retrieve data from the Camelbak@ Flow MeterTM; (2) adopt the SWEETSenseTM monitoring and data retrieval system, or (3) develop a method to measure and retrieve data on total time that the spigot is in the "open" position.by Connie C. Lu.M.Eng

Fe(I)-Mediated Reductive Cleavage and Coupling of CO_2: An Fe^(II)(μ-O,μ-CO)Fe^(II) Core

THF solutions of a new iron(I) source, [PhBP^(CH2_Cy_3)]Fe ([PhBP^(CH_2Cy_3)] = [PhBP(CH_2P(CH_2Cy)_2)_3]-), effect the reductive cleavage of CO_2 via O-atom transfer at ambient temperature. The dominant reaction pathway is bimetallic and leads to the formation of a structurally unprecedented diiron Fe^(II)(μ-O)(μ-CO)Fe^(II) core. X-ray data are also available to suggest that bimetallic reductive CO_2 coupling to generate oxalate occurs as a minor reaction pathway. These initial observations forecast a diverse reaction landscape between CO_2 and iron(I) synthons

CO_2 reduction by Fe(I): solvent control of C-O cleavage versus C-C coupling

This manuscript explores the product distribution of the reaction of carbon dioxide with reactive iron(I) complexes supported by tris(phosphino)borate ligands, [PhBP^R_3]- ([PhBP^R_3]- =[PhB(CH_2PR_2)_3]-; R = CH_2Cy,Ph, ^iPr, mter; mter = 3,5-meta-terphenyl). Our studies reveal an interesting and unexpected role for the solvent medium with respect to the course of the CO_2 activation reaction. For instance, exposure of methylcyclohexane (MeCy) solutions of [PhBP^(CH_2Cy)_3 ]Fe(PR’_3) to CO_2 yields the partial decarbonylation product {[PhBP^(CH_2Cy)_3 ]Fe}_2(µ-O)(µ-CO). When the reaction is instead carried out in benzene or THF, reductive coupling of CO_2 occurs to give the bridging oxalate species {[PhBP^(CH_2Cy_3 ]Fe}_2(µ- κOO’: κOO’-oxalato). Reaction studies aimed at understanding this solvent effect are presented, and suggest that the product profile is ultimately determined by the ability of the solvent to coordinate the iron center. When more sterically encumbering auxiliary ligands are employed to support the iron(I) center (i.e., [PhBP^(Ph)_3]- and [PhBP^(iPr)_3 ]-), complete decarbonylation is observed to afford structurally unusual diiron(II) products of the type {[PhBP^R_3]Fe}_2(µ-O). A mechanistic hypothesis that is consistent with the collection of results described is offered, and suggests that reductive coupling of CO_2 likely occurs from an electronically saturated “Fe^(II)–CO_2-” species