Extra Ligand Research Articles

Can a linear metal–metal bonded array of tetravanadium be stabilized between two dicyclopenta[a,e]pentalene ligands? A theoretical investigation

The 14-π cross-linked annulene dicyclopenta[a,e]pentalene (dcpp) is suggested for the first time to function as a sandwich ligand. According to density functional theory (DFT) calculations, upon being sandwiched between two dcpp ligands, the tetravanadium chain, without the support of auxiliary ligands, has two unpaired electrons and has a tendency for deformation to gain extra stability through multicenter bondings in the ground state. The one-electron ligand chlorine can lead to two types of structures, one with two chlorine atoms connecting to the two central vanadium atoms and the other one with two chlorine atoms connecting to the two terminal vanadium atoms, both of which are energetically more favorable in the triplet state. In the carbonyl end-capping complex, the tetravanadium is stable as a linear metal–metal bonded array with alternating single and double bonds and all electrons paired for bonding, although a triplet state conformer exists with two extra carbonyl ligands bound to the two central vanadium atoms which is more energy favorable by 15.1 kcal mol−1. Bonding features within the tetravanadium moiety were obtained based on electron density analyses. We hope these discussions are helpful for the design of new extended metal atom chain (EMAC) systems and for the pursuit of effective catalysts based on the dcpp sandwich complexes.

Trifluoromethylation of Aryl and Heteroaryl Halides with Fluoroform-Derived CuCF3: Scope, Limitations, and Mechanistic Features

Fluoroform-derived CuCF3 recently discovered in our group exhibits remarkably high reactivity toward aryl and heteroaryl halides, performing best in the absence of extra ligands. A broad variety of iodoarenes undergo smooth trifluoromethylation with the "ligandless" CuCF3 at 23-50 °C to give the corresponding benzotrifluorides in nearly quantitative yield. A number of much less reactive aromatic bromides also have been trifluoromethylated, including pyridine, pyrimidine, pyrazine, and thiazole derivatives as well as aryl bromides bearing electron-withdrawing groups and/or ortho substituents. Only the most electrophilic chloroarenes can be trifluoromethylated, e.g., 2-chloronicotinic acid. Exceptionally high chemoselectivity of the reactions (no side-formation of arenes, biaryls, and C2F5 derivatives) has allowed for the isolation of a large number of trifluoromethylated products in high yield on a gram scale (up to 20 mmol). The CuCF3 reagent is destabilized by CuX coproduced in the reaction, the magnitude of the effect paralleling the Lewis acidity of CuX: CuCl > CuBr > CuI. While S(N)Ar and S(RN)1 mechanisms are not operational, there is a well-pronounced ortho effect, i.e., the enhanced reactivity of ortho-substituted aryl halides 2-RC6H4X toward CuCF3. Intriguingly, this ortho-effect is observed for R = NO2, COOH, CHO, COOEt, COCH3, OCH3, and even CH3, but not for R = CN. The fluoroform-derived CuCF3 reagent and its reactions with haloarenes provide an unmatched combination of reactivity, selectivity, and low cost.

High drug loading pH-sensitive pullulan-DOX conjugate nanoparticles for hepatic targeting

pH-sensitive pullulan-doxorubicin (DOX) conjugates were synthesized by attaching DOX onto pullulan derivate through hydrazone bond that was stable under neutral environment but readily cleaved under mildly acidic condition. By changing the feed ratio of DOX to the pullulan derivate, conjugates with drug-loading content up to 30 wt % were obtained. In aqueous solution, the conjugates spontaneously formed uniform core-shell structured nanoparticles with DOX as core and pullulan as shell. The diameters of the nanoparticles ranged from 50 to 110 nm according to the drug-loading content. In vitro releasing experiments showed that more than 75% DOX released within 2 h at pH 5.0, while less than 15% DOX released after 12 h at pH 7.4. This pH-responsive manner of DOX release might assist the quick diffusion of DOX from the acidic endosome/lysosome and the intracellular transfer into the nucleus. Pullulan on the nanoparticles surface provided the nanoparticles with active targeting property to hepatic cells through specific interaction with asialoglycoprotein receptors on the membrane of hepatic cells, without the necessity of introducing any extra ligand. These pullulan-DOX conjugate nanoparticles were expected to be promising drug delivery system for liver targeting antitumor chemotherapy.

A new manganese(III) complex anchored onto SBA-15 as efficient catalyst for selective oxidation of cycloalkanes and cyclohexene with hydrogen peroxide

A new manganese(III) complex, [Mn(saldien)(N3)], has been anchored on mesoporous SBA-15 to produce a stable, active, and selective heterogeneous catalyst SBA15-[Mn(saldien)(N3)] (saldien=N,N′-bis(salicylidene)diethylenetriamine). Complex [Mn(saldien)(N3)] was characterized by spectroscopic methods and its structure determined by single crystal X-ray diffraction. SBA15-[Mn(saldien)(N3)] was characterized by small angle X-ray diffraction (SAX), scanning electron microscopy (SEM), nitrogen sorption measurement and Fourier transform infrared spectroscopy. Both [Mn(saldien)(N3)] anchored on mesoporous SBA-15 through a covalent bond and the “free” complex catalyze the oxidation of cycloalkanes and cyclohexene in acetonitrile, with H2O2 as terminal oxidant. A remarkable result for these systems is their capability of catalyzing efficient oxidation of the hydrocarbons with 1.2equiv. of hydrogen peroxide. Compared to homogeneous catalyst, the heterogenized catalyst is more stable and can be recycled four times without any significant loss of activity. Immobilization of complex [Mn(saldien)(N3)] onto SBA-15 increased the selectivity toward cyclohexanone in the oxidation of cyclohexane. In general, metallosalen-catalyzed reactions have been carried out in the presence of an excess of a donor ligand; however, the present reactions do not need the addition of any extra donor ligand.

2-Thiopheneacetic Acid Directed Synthesis of Au Nanorosette as an SERS-Active Substrate

Rosette-like nanoscale Au materials were simply prepared through one-pot reduction of the AuCl₄⁻ precursor by 2-thiopheneacetic acid (2-TAA) without extra surface capping ligands at room temperature. 2-TAA underwent polymerization into polythiophene derivatives while the AuCl₄⁻ precursor was simultaneously reduced into various Au nanostructures. In situ generated polythiophene derivatives played a significant role of surface passivation in guiding the shape of Au nanostructures. The morphology of Au nanostructures was strongly dependent on the molar ratio of 2-TAA to AuCl₄⁻. At lower [2-TAA]/[AuCl₄⁻] ratios, uniform rosette-like Au microparticles consisting of 30-nm-thick Au nanoplates were observed. The Au nanoplates had either triangular prismatic or hexagonal geometry with many defects. Uniform Au nanorosettes could be easily deposited on the Si substrate by drop-casting and were subsequently used as highly active SERS substrates for the detection of methylene blue and crystal violet by Raman spectroscopy. Upon increasing the ratio of [2-TAA]/[AuCl₄⁻], Au nanoparticles or nanorods heavily surrounded with polythiophene polymers were obtained.

How does overcoordination create ion selectivity?

Some biological molecules can distinguish between ions of similar nature, which may be achieved by enforcing specific coordination numbers on ions in the binding site. It is suggested that when this number is favourable for one ion type, but too large for another, this creates ion selectivity through the proposed mechanism of ‘overcoordination’. Much debate has occurred about the role overcoordination plays, and suggestions made as to how molecules can enforce particular coordination numbers, but there has not been an examination of the microscopic underpinning of ion selectivity by overcoordination. Here we use molecular-dynamics to systematically investigate how the number of ligands affects the ion–ligand and ligand–ligand interaction energies, and thus the thermodynamic ion selectivity, of a combination of model systems: three ions (Li+/Na+/K+) with three different ligands (water/formaldehyde/formamide). We find that the ligand–ligand repulsion controls the changes in geometry of each system with changing ligand number. Ion selectivity by overcoordination is achieved as smaller ions exhibit anomalous geometrical changes with the addition of extra ligands, whilst larger ions do not.

Mechanism of “Turn-on” Fluorescent Sensors for Mercury(II) in Solution and Its Implications for Ligand Design

The tendency of a Hg(II) ion to strongly quench fluorescence of potential fluorescent sensors is explored. Fluorescence measurements show the expected order of the chelation-enhanced fluorescence (CHEF) effect of Zn(II) > Cd(II) >> Hg(II) ~ Cu(II), which is interpreted as (1) unpaired electrons causing the weak CHEF effect for Cu(II) and (2) the order Zn(II) > Cd(II) >> Hg(II) reflecting the "heavy atom" effect, which may be due to increasing spin-orbit coupling constants (ζ) for Zn(II) < Cd(II) << Hg(II). The structures of mercury(II) complexes of N-(9-anthracenylmethyl)-N-(2-pyridinylmethyl)-2-pyridinemethanamine (ADPA) are reported. [Hg(ADPA)Cl(2)HgCl(2)] (1) has one Hg(II) held by two bridging chlorides, while the other Hg(II) is coordinated to the ADPA ligand. The latter Hg(II) has a nearest π contact of 3.215 Å with a C atom from the anthracenyl group, which falls in the range of reported Hg-C π contacts with aromatic groups. This contact may be important in quenching the fluorescence of the Hg(II)/ADPA complex. A density functional theory study shows that the Hg-C interaction is strong enough to prevent a simple HOMO → LUMO transition of the fluorophore. In fact, the S(0) → S(1) and S(2) transitions in the Hg(II)/ADPA complex have significant charge-transfer character to mercury. An important aspect of the coordination geometry of Hg(II) is illustrated by 1, where Hg(II) tends to form a few (often only two) short bonds to the more covalently binding donor atoms present, with much longer bonds to other donor atoms. The Hg-N bonds to the two pyridyl N-donor atoms of ADPA in 1 are relatively short at 2.212(8) and 2.224(8) Å, while that to the central saturated N-donor atom of ADPA is long at 2.603(8) Å. The latter long Hg-N bond may allow a photoinduced electron-transfer (PET) effect, quenching the fluorescence of the anthracenyl fluorophore. The structure of [Hg(ADPA)Br(2)] (2) reflects the more covalent binding of the two bromine ligands compared to the clorine ligands of 1, with much longer Hg-C contacts with the anthracenyl fluorophore and a Hg-N contact with the saturated N atom of ADPA of 2.917 Å. The latter long Hg-N contact is related to the nearly negligible fluorescence of the ADPA complex in the presence of added Br(-). The addition of extra ligands to the Hg(II)/ADPA complex produces a weak increase in the fluorescence intensity for OH(-) ~ Cl(-) >> Br(-) > I(-), which is discussed in terms of an increasing PET effect, and to collisional quenching. The ligand design principles for generating turn-on sensors for mercury suggested by this work are discussed.

ChemInform Abstract: Palladium‐Catalyzed Desulfitative C—H Arylation of Heteroarenes with Sodium Sulfinates.

AbstractInexpensive and air‐stable sodium sulfinates are used as coupling partners to construct aryl—heteroaryl bonds without need for any extra ligands.

Palladium‐Catalyzed Desulfitative CH Arylation of Heteroarenes with Sodium Sulfinates

Inexpensive and air-stable sodium sulfinates are used as coupling partners to construct aryl—heteroaryl bonds without need for any extra ligands.

ChemInform Abstract: Cu(OAc)2·H2O‐Catalyzed N‐Arylation of Nitrogen‐Containing Heterocycles.

AbstractThe N‐arylation of a number of N‐containing heterocycles with aryl halides is catalyzed by Cu(OAc)2·H2O without extra ligand.

(Invited) Reaction Mechanisms in ALD of Ternary Oxides

Reaction mechanisms underlying the atomic layer deposition (ALD) of ternary oxide films are investigated via the dependence of film stoichiometry on the sequence of ALD pulses. Data on film composition are brought together from experiments on five ternary oxide systems containing La, Mn, Ca, Fe, Sr, or Co, all using beta-diketonate ligands (thd) in the metal precursor and ozone as the oxygen source. These data are compared with the predictions from two possible reaction models: one where all ligands are combusted by ozone, the other where extra ligands are eliminated during the metal precursor pulse due to the availability of surface hydroxyl. The latter reaction is seen to be strongly dependent on the strength of the metal-ligand bond. Differences in cation charge also affect the stoichiometry. In this way, factors dictating the composition of ternary oxides are elucidated, opening the way to improved control of ALD processes and material properties.

A Simple Reducing Approach Using Amine To Give Dual Functional EuSe Nanocrystals and Morphological Tuning

Rare-earth nano-materials with controlled structures are of great interest owing to possible luminescent and magnetic properties resulted from lanthanide cations, and thus promise many applications, such as in solar cells and biological labeling and imaging. Host matrices used in these materials usually were Ln ions, but it was problematic to prepare low-valent lanthanide materials on nano-meter scales. Low-valent lanthanide mono-chalcogenide nanocrystals (NCs), and especially stable europium mono-chalcogenide EuX (X=O, S, Se, and Te) NCs started to become available with appealing optical and magnetic properties. The EuX NCs have degenerate 4f orbitals which locate between the conduction band (5d orbitals of Eu) and the valence band (p orbitals of O , S , Se , and Te ). Their 4f–5d electronic transitions and spin configuration lead to unique magneto-optical properties. During the past a few years, several research groups reported various routes to prepare the EuXNCs, such as with high temperature reduction, thermal or optical decomposition of single-source precursors, and a liquid-phase synthesis. However these methods usually required special instruments and extra ligands for Eu, which were time-consuming with the product; meanwhile, the resulting EuX NCs exhibited poor optical and magnetic properties, with little potential in applications. Moreover, these previous studies focused on, mostly, the magnetic property of the EuX NCs, especially EuS NCs. The optical properties of the EuX NCs were rarely reported. Though it has been acknowledged that bulk EuX does not exhibit luminescence at room temperature, while surfacemodified EuO and EuS exhibited characteristic luminescence under UV excitation. No luminescent EuSe NCs have been documented. It has been an enduring challenge to prepare high-quality EuSe NCs with good luminescent and magnetic properties as well as with controlled morphologies. Herein, we report a one-step synthesis of blue luminescent, magnetic EuSe NCs. Also, we present our data on the experimental parameters affecting the change of the morphologies. We propose that the formation of the EuSe NCs requires the amine to act as a reducing agent. Our approach is simple and effective, using EuCl3 (H2O)6 and elemental selenium as Eu and Se sources, respectively. The overall synthetic procedure for EuSe NCs is depicted in Scheme 1. This method demonstrates several advantages,

Nickel‐Catalyzed Cross‐Coupling of Phenols and Arylboronic Acids Through an In Situ Phenol Activation Mediated by PyBroP

A new method for the Suzuki-Miyaura cross-coupling of phenols and arylboronic acids through in situ phenol activation mediated by PyBroP is presented. The reaction proceeds efficiently by using cost-effective, markedly stable [NiCl(2)(dppp)] (dppp=1,3-bis(diphenylphosphino)propane) as the catalyst in only 5 mol % loading, as well as in the absence of extra ligands. The method exhibits broad applicability and high efficiency towards a wide range of both phenols and boronic acids, including activated, nonactivated, deactivated, and heteroaromatic coupling partners. In addition, various functional groups, such as ether, amino, cyano, ester, and ketone groups, are compatible with this transformation. Notably, arylboronic acids containing an unprotected NH(2) group and 2-heterocyclic boronic acids, which are generally problematic for coupling under conventional conditions, are also viable substrates, although moderate yields were obtained for sterically hindered substrates. Consequently, the in situ cross-coupling methodology coupled with the use of an inexpensive and stable nickel catalyst provides a rapid and efficient pathway for the assembly of biaryls and heterobiaryls with structural diversity from readily available phenol compounds.

Effect of extra liganding on spectral and kinetic characteristics and on quenching of excited states of multiporphyrin complexes by molecular oxygen

Based on studies of spectral and kinetic parameters of dimers of Zn porphyrins and of multiporphyrin self-assembling complexes formed on their basis, we find that extra liganding of dimers by pyridine inappreciably lowers the energy of the triplet level E(T 1). We show that, in this case, the nonradiative deactivation T 1 ↝ S 0 of the electronic excitation energy of dimers Zn porphyrins increases not only due to increasing Franck-Condon factor. We discuss mechanisms of the quenching action of an extra ligand related to an accepting role played by high-frequency overtones of vibrations of extra ligand molecules, to an enhancement of the spin-orbit interaction due to energy lowering of σπ* states, and to out-of-plane distortion of dimers caused by the displacement of the Zn2+ ion out of the plane of the tetrapyrrole macrocycle. Quenching of triplet states of extra liganded dimers of Zn porphyrins by molecular oxygen in liquid solutions at 295 K depends on the character of donor-acceptor interactions with pyridine and rigidity of a linking molecular fragment. We find that the rate constants of oxygen quenching of the excited electronic states S 1 and T 1 of multiporphyrin complexes depend on their structure and composition, as well as steric hindrances, created by dimers (screening effects) for contact interactions of a π-conjugated system of the free base (extra ligand) with molecular oxygen. Screening effects of extra ligands by dimer molecules of Zn porphyrins, which reduce the oxygen quenching rate constants k S and k T , are found to barely affect the singlet-oxygen generation quantum yield γΔ.

Alternative split sites for fragment complementation, and glyphosate function as extra ligand and stabilizer for the AroA enzyme complexes

Protein reconstitution analysis can be useful for studies on protein evolution, protein folding and macromolecular assembly. AroA is a key enzyme in the pathway toward the synthesis of aromatic amino acids in microorganisms and plants, and is the target of the herbicide glyphosate. Our previous study showed that functional AroA enzyme from Escherichia coli could be reconstituted from two ∼220-amino acid fragments of the protein. In this study, we explored this fragment complementation of AroA. Through a systematic study of fragment complementation, we show that successful fragment complementation can be achieved in vivo, when the split sites are within secondary structure elements as well as at loops between structure elements. In addition, we provide evidence, for the first time, that extra ligand, such as glyphosate, can function as a stabilizer of the reconstituted complexes in vivo. Therefore, our results may provide important implications for protein evolution and complex assemblies.

Cationic intermediates in oxidative addition reactions of Cl2 to [PtCl2(cis-1,4-DACH)

Oxidative addition and reductive elimination are fundamental processes in transition-metal chemistry. New interest in this field has been generated by the exploitation of platinum(IV) complexes as antitumor drugs. The two extra ligands can be used to render these species more resistant to attack by biological nucleophiles compared to their platinum(II) counterparts, to anchor additional pharmacologically active moieties, or, finally, to target the drug to specific sites by conferring responsiveness to some type of chemotaxis. On the other hand, platinum(IV) species are considered to be prodrugs and to require reduction to Pt(II) to become active. Thus, reductive elimination promoted by biological reducing agents becomes an important issue and it too could be exploited for targeting purposes. In this paper, we investigated the oxidation step in more detail and shown that, independent of the solvent used, a solvent molecule assists the reaction by entering in a trans position with respect to the attacking oxidant. In the case of bifunctional solvent molecules, such as dimethylsulfoxide, both S- and O-coordinated species are formed, the latter being thermodynamically favored. The substitution of the axially coordinated solvent molecule by a free chloride ion is found to be quite slow in organic solvents, as well as in water. It is also shown that the intermediate solvato-species can be exploited for binding just one molecule of another substrate in the axial position.

Dihydrogen Reduction of Nitroarmatics, Alkenes, Alkynes, Benzaldehyde, and Benzil by Newly Synthesized Rh(I) Complexes in Homogeneous Phase

Rh(I) complexes of S-triazines and azobenzene were found to be very efficient towards the catalytic hydrogenation of alkenes, alkynes at normal pressure, and at room temperature. Reduction of aromatic nitrocompounds, benzaldehyde, and benzil were observed in appreciable amounts only at high pressure and at high temperature. No diminished catalytic activity was observed even after 6–8 repeated catalytic runs. Profound effect was observed on the rate of reduction in presence of extra ligand without affecting the nature of yield, both under normal and high pressure conditions. Kinetics was studied. Mechanism has been proposed and rate equation has been derived.

Detection of Volatile Organic Compounds Using Porphyrin Derivatives

Seven different porphyrin compounds have been investigated as colorimetric gas sensors for a wide range of volatile organic compounds. The porphyrins examined were the free base and Mg, Sn, Zn, Au, Co, and Mn derivatives of 5,10,15,20-tetrakis[3,4-bis(2-ethylhexyloxy)phenyl]-21H,23H-porphine. Chloroform solutions of these materials were prepared and changes in their absorption spectra induced by exposure to various organic compounds measured. The porphyrins that showed strong responses in solution were selected, and Langmuir-Blodgett films were prepared and exposed to the corresponding analytes. This was done to determine whether they are useful materials for solid state thin film colorimetric vapor sensors. Porphyrins that readily coordinate extra ligands are shown to be suitable materials for colorimetric volatile organic compound detectors. However, porphyrins that already have bound axial ligands when synthesized only show a sensor response to those analytes that can substitute these axial ligands. The Co porphyrin displays a considerably larger response than the other porphyrins investigated which is attributed to a switch between Co(II) and Co(III) resulting in a large spectral change.

Self-assembling effects and mechanisms of interchromophore interactions in porphyrin pentads

The directional self-assembly of nanosized, structurally organized pentads that include five tetrapyrrole macrocycles and are based on the two-point coordination interaction of two covalently bound dimers of Zn porphyrins (homo-and heterodimers) with molecules of either the free base or the Cu complex of tetrametapyridyl-substituted porphyrin extra ligand is implemented in methylcyclohexane at 295 K. Using the method of the density functional theory (DFT) in the B3LYP/6-31g(d) approximation, the geometry of the pentad is fully optimized and the main factors that determine its redox properties are determined. The energies of the lowest excited states of the pentad are calculated by the ZINDO/S method, and it is shown that the occurrence of identical molecules in the system facilitates the formation of excitonic states with different contributions from the charge-transfer component. The directional energy transfer and the photoinduced electron transfer, which leads to the formation of a low-lying charge-transfer state (CT state), are studied and the rate constants of these two processes are determined (kET ∼ 1011 s−1 and kPET ≈ (1.8−6.0) × 109 s−1, respectively). The strong effect of the temperature of the solvent on the efficiency of relaxation processes in pentad complexes under study is revealed and studied. Roles played by the low-lying CT state and d-π exchange effects (the Cu-contained pentad) in fluorescence quenching of pentad complexes are determined.

Relaxation processes in self-assembling triads based on porphyrin Zn-heterodimers

The directional self-assembly of nanosized, structurally organized triads is implemented in methylcyclohexane at 295 K, which is based on the two-point extra coordination of nonsymmetric, covalently bound heterodimers of Zn porphyrins to bipyridyl-substituted porphyrin free-base extra ligands. Based on experimental data and theoretical calculations, the structural organization is determined and information on the energetics of electronic interactions of components is obtained and the rate constants of the directional energy transfer (kET ∼ 1011 s−1) and photoinduced electron transfer (kPET ∼ 2.7 × 109 s−1) are determined. The effects of the orientation of interacting macrocycles, the intercenter distances and the solvent temperature on the efficiency of relaxation processes in the triad is investigated.