Disubstituted Species Research Articles

Synthesis and characterization of mono- and bi-iron chalcogeno-ferrocenylcarboxylato complexes

Chalcogenocarboxylato complexes of iron bearing ferrocenyl moieties having two iron atoms in different electronic environments have been synthesized and characterized. Synthesis is accomplished by reactions of (μ-Ex)[CpFe(CO)2]2 (E = S, x = 2–4; E = Se, x = 1) with ferrocenoyl chloride (ClCOC5H4)FeCp which generated CpFe(CO)2ECOC5H4FeCp (E = S (1), Se (2)) in good isolated yields. The analogous reaction with bi-functional ferrocene dicarbonyl dichloride (ClCOC5H4)2Fe gave both the mono substituted products CpFe(CO)2ECOC5H4Fe(C5H4COOH) (E = S (3), Se (4)) or the di-substituted species [CpFe(CO)2ECOC5H4]2Fe (E = S (5), Se (6)). The authentication details of complexes 1–6 are presented along with the results from infrared, ultraviolet-visible, proton and carbon-13 nuclear magnetic resonance spectroscopies. In addition, the structures of 2, 3 and 4 were determined by single crystal X-ray diffraction, evidencing both the half-sandwich and sandwich geometries of the two iron centers.

Stability Trends in disubstituted Cobaltocenium Based on the Analysis of the Machine Learning Models.

Cobaltocenium derivatives have shown great potential as components of anion exchange membranes in fuel cells because they exhibit excellent thermal and alkaline stability under operating conditions while allowing for high anion mobility. The properties of the cobaltocenium-anion complexes can be chemically tuned through the substituent groups on the cyclopentadienyl (Cp) rings of the cation CoCp2+. However, the synthesis and characterization of the full range of possible derivatives are very challenging and time-consuming, and while the computational tools can greatly expedite this process, full screening of the electronic structure at a high level of theory is still computationally intensive. Therefore, in this work, we consider the machine learning (ML) modeling as a tool of predicting stability of disubstituted [CoCp2]OH complexes measured by their bond-dissociation energy (BDE). The relevant process here is the dissociation of the cobaltocenium-hydroxide complex into fragments [CoCpY']OH and CpY, where Y and Y' each represent one out of 42 substituent groups of experimental interest. In agreement with the previous ML study of 120 mono- and selected disubstituted species [Wetthasinghe et al. J. Chem. Phys. A (2022) 126], our analysis of the data set expanded to all possible disubstituted cobaltoceniums, points to the highest occupied and lowest unoccupied molecular orbitals, along with the Hirshfeld charge on the singly substituted benzene, to be the key features predicting the BDE of the unseen complexes. Based on the examination of the outliers, the acidity of substituents ((CO)NH2 in our case) is found to be of special significance for the cobaltocenium stability and for the model development. Moreover, we demonstrate that upon the data set refinement, the conventional ML models are capable of predicting the BDE close to 1 kcal/mol based on the properties of just the fragments, thereby greatly reducing the total number of species and of the computational time of each calculation. Such fragment-based "combinatorial" approach to the BDE modeling is noteworthy, since the geometry optimization of complexes in solution is conceptually challenging and computationally demanding, even when leveraging high-performance computing resources.

Determination of organotin compounds (OTC) at low levels in seawater by solid-phase extraction (SPE) and gas chromatography-pulsed flame photometric detection (GC-PFPD)

ABSTRACTIn this work, a simple, sensitive and affordable analytical method for the simultaneous determination of nine organotin compounds (butyltins, phenyltins and methyltins) in seawater using solid-phase extraction (SPE) and gas chromatography-pulsed flame photometric detection was developed and validated. The performance of three different SPE cartridges (Envi C18, Oasis HLB and Oasis MCX) and three elution solvents of different polarity (hexane, methanol and acetonitrile) was evaluated. The extraction parameters, such as solvent volume, presence of complexing and ion-pairing reagents, sample volume and pH and breakthrough volume, were also investigated. Tributyltin, as the organotin compound of special interest, was efficiently extracted using any of the cartridges and solvents tested. However, the simultaneous extraction of all nine organotin compounds was the most efficient using reversed-phase Envi C18 cartridge and 0.1% (w/v) tropolone in methanol as eluent. The optimised method resulted in good recovery, precision and linearity for all compounds, particularly for tri- and disubstituted species. Method detection limits ranged from 0.22 to 1.27 ng(Sn) L−1 for butyltins, 0.37 to 4.91 ng(Sn) L−1 for phenyltins and 0.45 to 1.16 ng(Sn) L−1 for methyltins. The accuracy of butyltins determination was further verified by the comparison with purchased derivatised standards. The developed method was successfully applied to the environmental samples.

Group 11 Metal Complexes with Unsymmetrical Bifunctional Ferrocene Ligands

AbstractInvited for this cover feature is the group of M. Concepción Gimeno from Instituto de Síntesis Química y Catálisis Homogénea, CSIC‐Universidad de Zaragoza, Spain. The image shows the great potential of the ferrocene molecule for custom‐made ligands, especially asymmetric disubstituted species bearing phosphine ligands and their corresponding group 11 metal complexes.

PhP=CPh2 and Related Phosphaalkenes: A Solution Equilibrium between a Phosphaalkene and a 1,2‐Diphosphetane

The synthesis of phosphaalkenes, ArP=C(R)Ph (1, a: Ar = Ph, R = Ph; b: Ar = o‐Tol, R = Ph; c: Ar = Mes, R = H), bearing sterically less hindered substituents is reported. Phosphaalkenes 1a–b were prepared by treating Ph2C=O with ArP(Li)SiMe3, whereas 1c was accessed from the AlCl3‐mediated reaction of ArP(SiMe3)2 and PhC(O)H. Both 1a and 1b dimerize to afford their respective 1,2‐diphosphetanes (2a and 2b). Compound 2a was characterized by X‐ray crystallography. Dissolution of pure 2a in THF resulted in a temperature dependent equilibrium with monomer 1a (ΔHo = –94.6 ± 14.6 kJ mol–1; ΔSo = –284 ± 48 J mol–1 K–1). Although monomer E/Z‐1c was identified in solution by its characteristic downfield 31P NMR chemical shift (δ = 247.1, 231.5), it was accompanied by the formation of what we believe are oligomers (δ = –25 to 10 ppm, br.). Attempts to trap phosphaalkenes 1a and 1b by treatment with W(CO)5(MeCN) afforded mixtures of complexes W(CO)5(1a–b) (4a–b) and W(CO)4(1a–b)2 (5a–b), the disubstituted species being subjected to X‐ray crystallographic characterization (5a).

Chemical degradation of fluorosulfonamide fuel cell membrane polymer model compounds

The durability of a polymer electrolyte fuel cell membrane, along with high proton conductivity and mechanical performance is critical to the success of these energy conversion devices. Extending our work in perfluorinated membrane stability, aromatic trifluoromethyl sulfonamide model compounds were prepared, and their oxidative degradation was examined. The chemical structures for the models were based on mono-, di- and tri-perfluorinated sulfonamide modified phenyl rings. Durability of the model compounds was evaluated by exposure to hydroxyl radicals generated using Fenton reagent and UV irradiation of hydrogen peroxide. LC–MS results for the mono-substituted model compound indicate greater stability to radical oxidation than the di-substituted species; loss of perfluorinated fonamide side chains appears to be an important pathway, along with dimerization and aromatic ring hydroxylation. The tri-substituted model compound also shows loss of side chains, with the mono-substituted compound being a major oxidation product, along with a limited amount of hydroxylation and dimerization of the starting material.

Synthesis and characterization of the diiron biomimics bearing phosphine borane for hydrogen formation

Eight new diiron carbonyl dithiolate complexes, [(μ-xdt)Fe2(CO)5(L)]; xdt=bdt, L=dppmBH3 (2), dppeBH3 (2a), dppm (2b), xdt=edt, L=dppmBH3 (4), dppeBH3 (4a), xdt=Bn-adt, L=dppmBH3 (5), dppeBH3 (5a), and [(μ-bdt)Fe2(CO)4(μ-dppm)], (3), have been prepared and characterized by FTIR, NMR spectroscopy and X-ray single crystallography. Molecular structures of complexes 2–5 reveal that the Fe2 core exhibits a butterfly conformation and each Fe center is of the pseudo-square-pyramidal coordination geometry. For the mono-substituted species, the phosphine ligand is located in the axial position. For the di-substituted species, the phosphine ligand is spanned between two Fe sites, occupying the basal position. The attached borane unit exerts electronic withdrawing influence on the electron richness about the Fe center. When the phosphine pendant is not protected by borane, coordination to the adjacent Fe site occurs under the reducing condition. It allows synthesis of 3 from chemical or electrochemical reduction of 2b. Electrocatalysis of acetic acid by complexes 2–5 is studied. The bdt derivatives are reduced at a potential 300mV more positive than the edt and adt analogs. For the latter species, electrocatalytic events occur near the first reduction. In contrast, no catalysis is observed for 2 in the available electrochemical windows. The rate constant of the catalytic reaction by the species is measured. The adt derivative reveals the highest efficiency toward hydrogen production from acetic acid.

Influential Role of Ethereal Solvent on Organolithium Compounds: The Case of Carboranyllithium

The influence of ethereal solvents (diethyl ether (Et(2)O), tetrahydrofuran (THF) or dimethoxyethane (DME)) on the formation of organolithiated compounds has been studied on the 1,2-C(2)B(10)H(12) platform. This platform is very attractive because it contains two C(c)-H adjacent units ready to be lithiated. On would expect that the closeness of both C(c)-H units would induce a higher resistance of the second C(c)-H unit being lithiated following the first lithiation. However, this is not the case, which makes 1,2-C(2)B(10)H(12) attractive to get a better understanding of the ethereal solvent influence on the lithiation process. The formation of carboranyl disubstituted species has been attributed to the existence of an equilibrium in which the carboranyl monolithiated species disproportionates into dilithium carborane and pristine carborane. The way Li(+) binds to C(c) in the carboranyl fragment and how the solvent stabilizes such a binding is paramount to drive the reaction to the generation of mono- and disubstituted carboranes. In fact, the proportion of mono- and disubstituted species is a consequence of the formation of contact ion pairs and, to a lesser extent, of separated ion pairs in ethereal solvents. All ethereal solvents generate contact ion pairs in which a large degree of covalent C(c)-Li(solvent) bonding can be assumed, according to experimental and theoretical data. Furthermore, Et(2)O tends to produce carboranyllitium ion pairs with a higher degree of contact ion pairs than THF or DME. It has been determined that for a high-yield preparation of monosubstituted 1-R-1,2-C(2)B(10)H(11), in C(c)-R (R=C, S or P) coupling reactions, the reagent type defines which is the most appropriate ethereal solvent. In reactions in which a halide is generated, as with ClPPh(2) or BrCH(2) CH=CH(2), Et(2)O appears to produce the highest degree of monosubstitution. In other situations, such as with S(8), or when no halide is generated, THF or DME facilitate the largest degree of monosubstitution. It has been shown that upon the self reaction of Li[1,2-C(2)B(10)H(11)] to produce [LiC(4)B(20)H(22)](-) the nucleophilicity of the carboranyllithium can even be further enhanced, beyond the ethereal solvent, by synergism with halide salts. The mediation of Li(+) in producing isomerizations on allyl substituents has also been demonstrated, as Et(2)O does not tend to induce isomerization, whereas THF or DME produces the propenyl isomer. The results presented here most probably can be extended to other molecular types to interpret the Li(+) mediation in C-C or other C-X coupling reactions.

Towards a monomeric structure of phenylboronic acid: The influence of ortho-alkoxy substituents on the crystal structure

The structures of three ortho-alkoxyphenylboronic acids (2-methoxy-, 2-ethoxy-, 2-isobutoxy-), and three diortho-alkoxyphenylboronic acids (2,6-dimethoxy-, 2,6-diethoxy- and 2-isobutoxy-6-methoxy) were determined by single crystal X-ray diffraction. The study was undertaken with the intention of designing a novel boronic acid having a monomeric structure, which to date has been an unavailable building block for crystal engineering. This motif can be enhanced by involving two hydroxyl groups at the boron atom in intramolecular hydrogen bonds. Although monosubstituted systems form typical dimers in the crystal lattice, disubstituted species reveal a much bigger variety of possible interactions. Among the analyzed compounds, 2,6-dimethoxyphenylboronic acid and 2,6-diethoxyphenylboronic acid crystallize in two polymorphic forms each. The unprecedented packing with monomers as the dominant structural motif has been found in the crystal structure of 2-isobutoxy-6-methoxyphenylboronic acid and one of the polymorphs of both 2,6-dimethoxyphenylboronic acid and 2,6-diethoxyphenylboronic acid. This description of the molecular packing is also supported by the analysis of fingerprint 2-D plots based on the Hirshfeld surfaces. The variety of possible types of interactions either within a single moiety or between moieties in dimers were additionally analyzed on the basis of the interaction energies, which have been estimated by ab initio calculations at the MP2/6-31+G* and B3LYP/6-311+G** level of theory.

Interference of H-bonding and substituent effects in nitro- and hydroxy-substituted salicylaldehydes

Two intramolecular interactions, i.e., (1) hydrogen bond and (2) substituent effect, were analyzed and compared. For this purpose, the geometry of 4- and 5-X-substituted salicylaldehyde derivatives (X = NO2, H or OH) was optimized by means of B3LYP/6-311 + G(d,p) and MP2/aug-cc-pVDZ methods. The results obtained allowed us to show that substituents (NO2 or OH) in the para or meta position with respect to either OH or CHO in H-bonded systems interact more strongly than in the case of di-substituted species: 4- and 3-nitrophenol or 4- and 3-hydroxybenzaldehyde by ∼31%. The substituent effect due to the intramolecular charge transfer from the para-counter substituent (NO2) to the proton-donating group (OH) is ∼35% greater than for the interaction of para-OH with the proton-accepting group (CHO). The total energy of H-bonding for salicylaldehyde, and its derivatives, is composed of two contributions: ∼80% from the energy of H-bond formation and ∼20% from the energy associated with reorganization of the electron structure of the systems in question. FigureSubstituent effect stabilization energy (SESE) estimation for the salicylaldehyde and its 4- and 5-X-substituted derivatives Electronic supplementary materialThe online version of this article (doi:10.1007/s00894-011-1044-1) contains supplementary material, which is available to authorized users.

Quadratic and Cubic Nonlinear Optical Properties of Salts of Diquat-Based Chromophores with Diphenylamino Substituents

A series of chromophoric salts has been prepared in which 4-(diphenylamino)phenyl (Dpap) electron donor groups are connected to electron-accepting diquaternized 2,2'-bipyridyl (diquat) units. The main aim is to combine large quadratic and cubic nonlinear optical (NLO) effects in potentially redox-switchable molecules with 2D structures. The chromophores have been characterized as their PF(6)(-) salts by using various techniques including electronic absorption spectroscopy and cyclic voltammetry. The visible absorption spectra are dominated by intense π → π* intramolecular charge-transfer (ICT) bands, and all of the compounds show two reversible or quasireversible diquat-based reductions and partially reversible Dpap oxidations. Single crystal X-ray structures have been obtained for one salt and for the precursor compound (E)-4-(diphenylamino)cinnamaldehyde, both of which adopt centrosymmetric space groups. First hyperpolarizabilities β have been measured by using hyper-Rayleigh scattering (HRS) with a 800 nm laser, and Stark (electroabsorption) spectroscopy of the ICT bands affords estimated static first hyperpolarizabilities β(0). The directly and indirectly derived β values are large and generally increased substantially for the bis-Dpap derivatives when compared with their monosubstituted analogues. Polarized HRS studies show that the NLO responses of the disubstituted species are dominated by "off-diagonal" β(zyy) components. Lengthening the diquaternizing alkyl unit lowers the electron-acceptor strength and therefore increases the ICT energies and decreases the E(1/2) values for diquat reduction. However, compensating increases in the ICT intensity prevent significant decreases in the Stark-based β(0) responses. Cubic NLO properties have been measured by using the Z-scan technique over a wavelength range of 520-1600 nm, revealing relatively high two-photon absorption cross-sections of up to 730 GM at 620 nm for one of the disubstituted chromophores.

Azaphenalene Phthalocyanines: Phthalocyanine Analogues with Six‐Membered‐Ring Units Instead of Five‐Membered‐Ring Units

Mixed-condensation reaction of 1,8-naphthalenedicarbonitrile and a 4,5-disubstituted phthalonitrile provided a series of phthalocyanine (Pc) analogues with azaphenalene (AP) moieties in place of the isoindole moieties. Monosubstituted species, APPc, and the two structural isomers of disubstituted species, adj-AP(2)Pc and opp-AP(2)Pc, were successfully isolated by gel-permeation chromatography on HPLC apparatus. Their structures were elucidated by (1)H NMR spectroscopy and X-ray crystallographic analysis. Replacement of the isoindole moieties with azaphenalene moieties created six-membered-ring units in the core and caused distortion of the molecular structures. The Q-band absorption shifted to the red upon an increase in the number of azaphenalene units; the shape of the absorption spectra depended on the molecular symmetries. APPc and opp-AP(2)Pc showed a large splitting of the Q band, whereas adj-AP(2)Pc exhibited a single broad Q band. These changes in the absorption spectra, as well as the unique electronic structures, are discussed in detail, based on magnetic circular dichroism spectra, electrochemical measurements, and density functional theory calculations.

Diquat Derivatives: Highly Active, Two-Dimensional Nonlinear Optical Chromophores with Potential Redox Switchability

In this article, we present a detailed study of structure-activity relationships in diquaternized 2,2'-bipyridyl (diquat) derivatives. Sixteen new chromophores have been synthesized, with variations in the amino electron donor substituents, pi-conjugated bridge, and alkyl diquaternizing unit. Our aim is to combine very large, two-dimensional (2D) quadratic nonlinear optical (NLO) responses with reversible redox chemistry. The chromophores have been characterized as their PF(6)(-) salts by using various techniques including electronic absorption spectroscopy and cyclic voltammetry. Their visible absorption spectra are dominated by intense pi --> pi* intramolecular charge-transfer (ICT) bands, and all show two reversible diquat-based reductions. First hyperpolarizabilities beta have been measured by using hyper-Rayleigh scattering with an 800 nm laser, and Stark spectroscopy of the ICT bands affords estimated static first hyperpolarizabilities beta(0). The directly and indirectly derived beta values are large and increase with the extent of pi-conjugation and electron donor strength. Extending the quaternizing alkyl linkage always increases the ICT energy and decreases the E(1/2) values for diquat reduction, but a compensating increase in the ICT intensity prevents significant decreases in Stark-based beta(0) responses. Nine single-crystal X-ray structures have also been obtained. Time-dependent density functional theory clarifies the molecular electronic/optical properties, and finite field calculations agree with polarized HRS data in that the NLO responses of the disubstituted species are dominated by 'off-diagonal' beta(zyy) components. The most significant findings of these studies are: (i) beta(0) values as much as 6 times that of the chromophore in the technologically important material (E)-4'-(dimethylamino)-N-methyl-4-stilbazolium tosylate; (ii) reversible electrochemistry that offers potential for redox-switching of optical properties over multiple states; (iii) strongly 2D NLO responses that may be exploited for novel practical applications; (iv) a new polar material, suitable for bulk NLO behavior.

Thiocyanation of closo-Dodecaborate B12H122−. A Novel Synthetic Route and Theoretical Elucidation of the Reaction Mechanism

Although vast experimental experience has been accumulated about the reactions of icosahedral B(12)H(12)(2-) borane cages, little is known about the mechanisms by which these reactions proceed. To address this issue, we have chosen the thiocyanation of B(12)H(12)(2-) and have studied this reaction using both experimental and theoretical methods. First, we present a novel and more convenient synthetic route using in situ generated thiocyanogen, (SCN)(2). The synthesized disubstituted product B(12)H(10)(SCN)(2)(2-) is exclusively the meta positional isomer, as confirmed by the X-ray crystallographic analysis. The quantum chemical calculations at the B3LYP/def2-TZVP//RI-PBE/def2-SVP level show that the free energy differences between the ortho, meta, and para disubstituted species are very small and as such cannot explain the observed positional preferences. The calculations of the kinetic aspects reveal that the reaction is best described as an electrophilic substitution. The calculated isomer preferences for the second SCN substituent are meta > para > ortho. The major outcome of this work is a clear and consistent picture of the electrophilic substitution reaction mechanism of the thiocyanation of B(12)H(12)(2-), thus contributing to our understanding of the general features of boron hydride reactivity.

Secondary Interactions or Ligand Scrambling? Subtle Steric Effects Govern the Iridium(I) Coordination Chemistry of Phosphoramidite Ligands

The like and unlike isomers of phosphoramidite (P*) ligands are found to react differently with iridium(I), which is a key to explaining the apparently inconsistent results obtained by us and other research groups in a variety of catalytic reactions. Thus, the unlike diastereoisomer (aR,S,S)-[IrCl(cod)(1 a)] (2 a; cod=1,5-cyclooctadiene, 1 a=(aR,S,S)-(1,1'-binaphthalene)-2,2'-diyl bis(1-phenylethyl)phosphoramidite) forms, upon chloride abstraction, the monosubstituted complex (aR,S,S)-[Ir(cod)(1,2-eta-1 a,kappaP)](+) (3 a), which contains a chelating P* ligand that features an eta(2) interaction between a dangling phenyl group and iridium. Under analogous conditions, the like analogue (aR,R,R)-1 a' gives the disubstituted species (aR,R,R)-[Ir(cod)(1 a',kappaP)(2)](+) (4 a') with monodentate P* ligands. The structure of 3 a was assessed by a combination of X-ray and NMR spectroscopic studies, which indicate that it is the configuration of the binaphthol moiety (and not that of the dangling benzyl N groups) that determines the configuration of the complex. The effect of the relative configuration of the P* ligand on its iridium(I) coordination chemistry is discussed in the context of our preliminary catalytic results and of apparently random results obtained by other groups in the iridium(I)-catalyzed asymmetric allylic alkylation of allylic acetates and in rhodium(I)-catalyzed asymmetric cycloaddition reactions. Further studies with the unlike ligand (aS,R,R)-(1,1'-binaphthalene)-2,2'-diyl bis{[1-(1-naphthalene-1-yl)ethyl]phosphoramidite} (1 b) showed a yet different coordination mode, that is, the eta(4)-arene-metal interaction in (aS,R,R)-[Ir(cod)(1,2,3,4-eta-1 b,kappaP)](+) (3 b).

Using Probabilistic Modeling to Evaluate Human Exposure to Organotin in Drinking Water Transported by Polyvinyl Chloride Pipe

The leaching of organotin (OT) heat stabilizers from polyvinyl chloride (PVC) pipes used in residential drinking water systems may affect the quality of drinking water. These OTs, principally mono- and di-substituted species of butyltins and methyltins, are a potential health concern because they belong to a broad class of compounds that may be immune, nervous, and reproductive system toxicants. In this article, we develop probability distributions of U.S. population exposures to mixtures of OTs encountered in drinking water transported by PVC pipes. We employed a family of mathematical models to estimate OT leaching rates from PVC pipe as a function of both surface area and time. We then integrated the distribution of estimated leaching rates into an exposure model that estimated the probability distribution of OT concentrations in tap waters and the resulting potential human OT exposures via tap water consumption. Our study results suggest that human OT exposures through tap water consumption are likely to be considerably lower than the World Health Organization (WHO) "safe" long-term concentration in drinking water (150 microg/L) for dibutyltin (DBT)--the most toxic of the OT considered in this article. The 90th percentile average daily dose (ADD) estimate of 0.034 +/- 2.92 x 10(-4)microg/kg day is approximately 120 times lower than the WHO-based ADD for DBT (4.2 microg/kg day).

Quantum Chemical Investigation of Thermal Cis-to-Trans Isomerization of Azobenzene Derivatives: Substituent Effects, Solvent Effects, and Comparison to Experimental Data

Quantum chemical calculations of various azobenzene (AB) derivatives have been carried out with the goal to describe the energetics and kinetics of their thermal cis --> trans isomerization. The effects of substituents, in particular their type, number, and positioning, on activation energies have been systematically studied with the ultimate goal to tailor the switching process. Trends observed for mono- and disubstituted species are discussed. A polarizable continuum model is used to study, in an approximate fashion, the cis --> trans isomerization of azobenzenes in solution. The nature of the transition state(s) and its dependence on substituents and the environment is discussed. In particular for push-pull azobenzenes, the reaction mechanism is found to change from inversion in nonpolar solvents to rotation in polar solvents. Concerning kinetics, calculations based on the Eyring transition state theory give usually reliable activation energies and enthalpies when compared to experimentally determined values. Also, trends in the resulting rate constants are correct. Other computed properties such as activation entropies and thus preexponential rate factors are in only moderate agreement with experiment.

Evidence for the Iron(III) Oxidation State in Bis(imino)pyridine Catalysts. A Density Functional Theory Study

We report a theoretical analysis based on density functional theory devoted to the study of the activated iron bis(imino)pyridine catalysts {[2,6-(2-C6H4(CH3))2-C5H3N]FenRx]n−x} (n = 2, 3; x = 1, 2; R = Me, Cl). The aim of this work is to obtain detailed information on the nature (oxidation state and spin multiplicity) of the iron active species, by studying the coordination and insertion of the first ethylene molecule with the 10 most reasonable activated species that can be formed out of {[2,6-(2-C6H4(CH3))2-C5H3N]FeCl2} after reaction with MAO. The relatively small exothermicity of the coordination reaction of ethylene, calculated for disubstituted species, allowed us to exclude them from further examination. The coordination and insertion reaction pathway for the two most reactive activated catalysts, i.e., monomethylated Fe(III) and Fe(II) species, were then evaluated on the B3LYP/Lacvp** potential energy surface (PES), taking into account all possible spin states and several coordination modes of the ethylene molecule. These reactions take place at the quintet PES for the Fe(II) and quartet PES for the Fe(III) species. For the latter, more favorable reaction and activation enthalpies for the insertion reaction were calculated: Fe(II), ΔH(298 K) = –14.1 kcal/mol and ΔH⧧(298 K) = +21.6 kcal/mol; Fe(III), ΔH(298 K) = −22.8 kcal/mol and ΔH⧧(298 K) = +10.0 kcal/mol. Assuming similar insertion barriers for the second insertion reaction, the β-hydrogen transfer termination reaction (ΔH⧧ = +11.9 kcal/mol) is favored over further chain growth for [FeIIMe]+, whereas for [FeIIIMe]2+ catalyst chain growth and termination reaction are clearly in competition (ΔH⧧ = +10.0 and +12.1 kcal/mol, respectively). On the basis of these results, we conclude that the most activated species has oxidation state III and is likely expected to produce oligomers in agreement with results from experimental studies.

Micellar induced regioselectivity in the two-step consecutive reaction of SO 2−3 with Br [sbnd](CH 2CH 2) n[sbnd]Br ( [formula omitted

High field (800 MHz) 1H NMR was used to monitor the two-step consecutive reaction of excess SO 2− 3 with symmetrical bifunctional α , ω -dibromoalkanes with butane (DBB), hexane (DBH), octane (DBO), and decane (DBD) chains in CTAB micelles at 25 °C. The first-order rate constant for the first substitution step for DBB and DBH is about 5 times faster than for the second, but the kinetics for DBO and DBD were not cleanly first-order. After 40 min, the solution contained about 80% of the intermediate bromoalkanesulfonate from DBB and DBH and the remainder is alkanedisulfonate and unreacted starting material. The same reactions were carried out in homogeneous MeOH/D 2O solutions at 50 °C. The rate constants for all four α , ω -dibromoalkanes were first-order throughout the time course of the reaction and the same within ±10%. However, because micellar solutions are organized on the nanoscale and bring together lipophilic and hydrophilic reactants into a small reaction volume at the micellar interface, they speed this substitution reaction considerably compared to reaction in MeOH/D 2O. The CTAB micelles also induce a significant regioselectivity in product formation by speeding the first step of the consecutive reaction more than the second. The results are consistent with the bromoalkanesulfonate intermediates having a radial orientation within the micelles with the CH 2SO − 3 group in the interfacial region and the CH 2Br group directed into the micellar core such that the concentration of CH 2Br groups in the reactive zone, i.e., the micellar interface, is significantly reduced. These results provide the first example of self-assembled surfactant system altering the relative rates of the reaction steps of a consecutive reaction and, in doing so, enhancing monosubstitution of a symmetrically disubstituted species.

N-Heterocyclic Carbene Ligands as Cyanide Mimics in Diiron Models of the All-Iron Hydrogenase Active Site

The reaction of [Fe2(μ-S(CH2)3S)(CO)6] with an excess of 1,3-dimethylimidazol-2-ylidene (LMe) affords the mono- and disubstituted species [Fe2(μ-S(CH2)3S)(CO)5(LMe)] (1) and [Fe2(μ-S(CH2)3S)(CO)4(LMe)2] (2), respectively. The structures of 1 and 2 have been determined by X-ray crystallography.