Crystal structure of benzyl triphenyl phosphonium chlorometallate: Some surface and biological properties of their metallosurfactant derivatives

The crystal structures of benzyl triphenyl phosphonium tri-chloro cupprate (1) and tetra-chloroferrate (2) have been determined by X-ray crystallography. The structures contain completely isolated elongated tetrahedral [CuCl₃]^? as dimmers with two bridging and four terminal chlorines (1), and tetrahedral [FeCl₄]^? (2). In designing a metal containing surfactant system, salts (1) and (2) were refluxed with dodecyl amine to form the corresponding metallosurfactants (1a) and (2a) or to hexadecyl pyridinium chloride to form the corresponding cationic metallosurfactants (1b) and (2b), respectively. The critical micelle concentration (CMC) values of these surfactant metal complexes in aqueous solution were recorded from surface tension measurements. They showed antibacterial activity against the growth of Gram positive and Gram negative bacteria.     

A Molecular Approach for Mitigation of Aluminum Pitting based on Films of Zinc(II) and Gallium(III) Metallosurfactants

The use of metallosurfactants to prevent pitting corrosion of aluminum surfaces is discussed based on the behavior of the metallosurfactants [Zn^II(L^N2O2)H₂O] ( 1 ) and [Ga^III(L^N2O3)] ( 2 ). These species were deposited as multilayer Langmuir–Blodgett films and characterized by IR reflection absorption spectroscopy and UV/Vis spectroscopy. Scanning electron microscopy images, potentiodynamic polarization experiments, and electrochemical impedance spectroscopy were used to assess corrosion mitigation. Both metallosurfactants demonstrate superior anticorrosion activity due to the presence of redox-inactive 3d¹⁰ metal ions that enhance the structural resistance of the ordered molecular films and limit chloride mobility and electron transfer.     

Formation of vesicles with an organometallic amphiphile bilayer by supramolecular arrangement of metal carbonyl metallosurfactants

Metallo-vesicles are formed in water medium as a result of the supramolecular arrangement of molybdenum carbonyl metallosurfactants. These new kind of surfactants contain a hydrophobic metal carbonyl fragment and are easily prepared from surfactant phosphine ligands.     

Recent metallosurfactants for sustainable catalysis in water

In the field of green chemistry, micellar catalysis plays a central role for organic solvent replacement. Micelles ensure the solubilization or dispersion of catalyst and organic substrates in water imparting unique features in terms of chemo-, regio- and stereoselectivity. For metal-catalyzed reactions, a more robust approach for catalyst recycling consists in the synthesis of so-called metallosurfactants, in which a hydrophilic metal–containing headgroup is endowed with a hydrophobic ponytail, leading in water to the formation of metallomicelles. This fast-growing field of research is critically reviewed in this contribution, describing new trends and classifying the literature since 2017 based on the nature of the newly formed bond. Particular emphasis is reported on the specific features of metallosurfactants in terms of activities, selectivities and recyclability of the self-assembling catalysts.     

High-relaxivity MRI contrast agents prepared from miniemulsion polymerization using gadolinium(III)-based metallosurfactants

Miniemulsion polymerization with amphiphilic gadolinium(III) complexes as metallosurfactants was explored as a new technique for the synthesis of high relaxivity MRI contrast agents. Well-defined metallo-colloids with up to 240% enhancement in relaxivity over their small molecular counterparts were obtained.     

The structure of metallomicelles

The morphology of micelles formed by two novel metallosurfactants has been studied by small-angle neutron scattering (SANS) and small-angle-X-ray scattering (SAXS). The two surfactants both contain a dodecyl chain as the hydrophobic moiety, but differ in the structure of the head group. The surfactants are Cu(II) complexes of monopendant alcohol derivatives of a) the face-capping macrocycle 1,4,7-triazacyclanonane (tacn), and b) an analogue based upon the tetraazamacrocycle 1,4,7,10-tetraazacyclododecane. Here, neutron scattering has been used to study the overall size and shape of the surfactant micelles, in conjunction with X-ray scattering to locate the metal ions. For the 1,4,7,10-tetraazacyclododecane-based surfactant, oblate micelles are observed, which are smaller to the prolate micelles formed by the 1,4,7-triazacyclononane analogue. The X-ray scattering analysis shows that the metal ions are distributed throughout the polar head-group region, rather than at a well-defined radius; this is in good agreement with the SANS-derived dimensions of the micelle. Indeed, the same model for micelle morphology can be used to fit both the SANS and SAXS data.     

Wormlike micelles from a cage amine metallosurfactant

We have shown that copper and cobalt metallosurfactants derived from Cu(II) and Co(III) complexes of a macrobicyclic hexamine ("cage") can form wormlike micelles in aqueous solution that may coexist with or easily interconvert with vesicle structures. The cylindrical micelle structures are unusual for triple-chain surfactants with a single headgroup and are not easily accounted for using geometrical packing arguments. The solution behavior has been characterized by cryo-TEM and SAXS measurements. Both the Cu and Co compounds display viscoelastic solutions at 1 wt %, indicating that such behavior may be anticipated for the full variety of stable metal complexes formed by the cage headgroup, auguring applications based on the incorporation of metallo aggregates into mesoporous silica structures.     

Archetypical modeling and amphiphilic behavior of cobalt(II)-containing soft-materials with asymmetric tridentate ligands

The stabilization of a bivalent oxidation state in cobalt complexes of phenolate-based asymmetric tridentate ligands with iodo and bromo substituents is studied. The complexes [CoII(LIA)2].2CH3OH (1) and [CoII(LBrA)2].CH3OH (2) were characterized by means of several spectroscopic and spectrometric techniques. The molecular structure of 1 was determined by diffractometric analysis and reveals the cobalt(II) ion in a distorted-octahedral geometry. The centrosymmetric metal ion adopts a local D2h symmetry and is surrounded by facially coordinated ligands. Equivalent donor sets in both ligands are trans to each other, and DFT calculations suggest that the fac-trans configuration is favored by a small margin when compared to the fac-cis isomers. Both DFT calculations and EPR spectroscopy agree with a high-spin S=3/2 electronic configuration given by [ag1, b1g1, ag1, b2g2, b3g2]. This oxidation state was indirectly observed by the lack of a ppiphenolate-->dsigma*cobalt(III) charge-transfer band, which is found between 430 and 470 nm for similar cobalt(III) species. On the basis of the geometrical preferences and the oxidation state of archetypical 1 and 2, two metallosurfactants [CoII(LI-ODA)2] (3) and [CoII(LI-NOBA)2].CH2Cl2 (4) were obtained. The redox chemistry of 1-4 is marked by metal- and ligand-centered activity with several follow up processes and film formation on the electrode. Both metallosurfactants exhibit amphiphilic properties and organization, as shown by compression isotherms and Brewster angle microscopy but exhibit dissimilar collapse mechanisms; whereas 3 collapses at constant pressure, 4 exhibits a constant-area collapse. Langmuir-Blodgett films are readily obtained and were characterized by equilibrium contact angle and atomic force microscopy.     

Interfacial behavior and film patterning of redox-active cationic copper(II)-containing surfactants

Herein, we describe the synthesis and characterization of a novel series of single-tail amphiphiles LPyCn (Py=pyridine, Cn=C18, C16, C14, C10) and their copper(II)-containing complexes, which are of relevance for patterned films. The N-(pyridine-2-ylmethyl)alkyl-1-amine ligands and their complexes [CuIICl2(LPyC18)] (1), [CuIICl2(LPyC16)] (2), [CuIICl2(LPyC14)] (3), [CuIIBr2(LPyC18)] (4), [CuIIBr2(LPyC16)] (5), and [CuIIBr2(LPyC10)] (6) were synthesized, isolated, and characterized by means of mass spectrometry, IR and NMR spectroscopies, and elemental analysis. Complexes 1, 2, 3, and 6 had their molecular structure solved by X-ray diffraction methods, which showed that the local geometry around the metal center is distorted square planar. With the aim of using these species as precursors for redox-responsive films, an assessment of their electrochemical properties involved cyclic voltammetry in different solvents, with different supporting electrolytes and scan rates. Density functional theory calculations of relevant species in bulk and at interfaces were used to evaluate their electronic structure and dipole moments. The morphology and order of the resulting films at the air/water interface were studied by isothermal compression and Brewster angle microscopy. Biphasic patterned Langmuir films were observed for all complexes except 3 and 6, and dependence on the chain length and the nature of the halogen coligand determine the characteristics of the isotherms and their intricate topology. Complexes 3 and 6, which have shorter chain lengths, failed to exhibit organization. These results exemplify the first comprehensive study of the behavior of single-tail metallosurfactants, which are likely to lead to high-end technological applications based on their patterned films.     

The Mechanisms of Rectification in Au|Molecule|Au Devices Based on Langmuir–Blodgett Monolayers of Iron(III) and Copper(II) Surfactants

Langmuir–Blodgett films of metallosurfactants were used in Au|molecule|Au devices to investigate the mechanisms of current rectification.     

Preparation,Surface, and Biological Activities of Some Novel Metallosurfactants

A series of novel metallosurfactants (III_a-d) was prepared and evaluated as surface active agents. The synthesis was carried out through two steps: the first was the reaction of fatty acids (I_a-d) (lauric, palmitic, myristic and steric acid) with morpholine (tetrahydro-1,4-oxazine) to give morpholin-4-yl-alkan-1-one (II_a-d), respectively. The second step is reaction of product of the first step (compounds II_a-d) with Fe (III) to give (III_a-d) metallosurfactants. The chemical structures of the prepared compounds were elucidated with elemental analysis, FTIR and ¹H NMR spectroscopic tools. The surface properties of the prepared metallosurfactants were determined at different temperature 25, 35, and 45°C. The surface tension (γ), critical micelle concentration (CMC), surface tension at CMC (γ_cmc), effectiveness (π_cmc), efficiency (Pc_2o), maximum surface excess (Γ_max), and minimum surface area (A_min) were determined. Thermodynamic data including, free energy, entropy and enthalpy changes (ΔG, ΔS, ΔH) for adsorption at the air–water interface and also for micellization in the bulk of surfactant solutions were calculated. The antimicrobial activity of the prepared compounds was determined via the inhibition zone diameter technique against Gram-positive, Gram-negative bacteria, and some fungal strains as mold and yeast. The results indicate that the prepared ferrosurfactants have a good surface properties and biological activities against the tested microorganism.     

Synthesis of amphiphilic azo-anion-radical complexes of chromium(III) and the development of ultrathin redox-active surfaces by the Langmuir-Schaefer technique

Neutral tris-chelated chromium complex [Cr(L(a))(3)] (1a), and its surfactant derivatives [Cr(L(b))(3)] (1b), [Cr(L(c))(3)] (1c), and [Cr(L(d))(3)] (1d) (where L(a)=2-(4'-methoxyphenylazo)pyridine, L(b)=2-(4'-butyloxyphenylazo)pyridine, L(c =2-(4'-octyloxyphenylazo)pyridine, and L(d)=2-(4'-dodecyloxyphenylazo)pyridine) were synthesized. The molecular structure of compound 1a, determined by X-ray diffraction, showed that the local geometry around the metal center is a distorted octahedral with meridional coordination of the ligands. The structural parameters, spectroscopic data, and density functional theory (DFT) calculations on representative complex 1a suggest that ligand L(a) is predominantly an azo-anion-radical-type, and so the complex can be represented as [Cr(III)(L(a.-))(3)]. An assessment of their physicochemical and surface properties was performed with the aim of using these triple-tailed metallosurfactants as precursors for redox-responsive films. The surface-pressure-molecular-area isotherm measurement for compound 1d shows that the complex forms a stable Langmuir film at the air/water interface. The monolayer and multilayers were successfully transferred onto the quartz substrate and the platinum working electrode at a surface pressure of 10 mN m(-1) by the Langmuir-Schaefer (LS) technique. The LS films were studied by UV/Vis spectrometry, infrared spectroscopy, field-emission scanning electron microscopy, and atomic force microscopy. A good linear relationship between the absorbance at 370 nm and the thickness of the layers against the number of deposited layers indicated the uniformity and reproducibility of this transfer process. Voltammograms for platinum-surface-bound LS film of compound 1d showed that the redox response owing to the first oxidation is stable and reproducible after many cycles (>300 cycles). Spectroscopic studies and electrochemical measurements of compound 1d on the LS films revealed that these complexes are potential candidates for molecular devices.     

Ab initio study of intriguing coordination complexes: a metal field theory picture

Two noninnocent ligands are theoretically studied using wave function based methods to demonstrate their ability to undergo singlet-triplet transition under the effect of an external charge mimicking the electrostatic role of a metal ion. It is shown that the singlet-triplet energy difference is very sensitive to the metal ion charge which tunes the HOMO-LUMO energy difference of these ligands. While the latter is reduced as the charge is enhanced in the glyoxal-bis-(2-mercaptoanil) (gma) ligand, it is increased in the bis(imino)pyridine diradical ligand. This result shows a strong analogy with the crystal field theory, interchanging the roles played by the metal ion and the ligand. As the metal ion is explicitly treated in the Fe(gma)CN complex, this analogy can be pushed further resulting in a "metal field theory" conceptualization.     

Deprotonative metalation using ate compounds: synergy, synthesis, and structure building

Historically, single-metal organometallic species such as organolithium compounds have been the reagents of choice in synthetic organic chemistry for performing deprotonation reactions. Over the past few years, a complementary new class of metalating agents has started to emerge. Owing to a variable central metal (magnesium, zinc, or aluminum), variable ligands (both in their nature and number), and a variable second metallic center (an alkali metal such as lithium or sodium), "ate" complexes are highly versatile bases that exhibit a synergic chemistry which cannot be replicated by the homometallic magnesium, zinc, or aluminum compounds on their own. Deprotonation accomplished by using these organometallic ate complexes has opened up new perspectives in organic chemistry with unprecedented reactivities and sometimes unusual and unpredictable regioselectivities.     

Metal sensor proteins: nature's metalloregulated allosteric switches

Metalloregulatory proteins control the expression of genes that allow organisms to quickly adapt to chronic toxicity or deprivation of both biologically essential metal ions and heavy metal pollutants found in their microenvironment. Emerging evidence suggests that metal ion homeostasis and resistance defines an important tug-of-war in human host-bacterial pathogen interactions. This adaptive response originates with the formation of "metal receptor" complexes of exquisite selectivity. In this perspective, we summarize consensus structural features of metal sensing coordination complexes and the evolution of distinct metal selectivities within seven characterized metal sensor protein families. In addition, we place recent efforts to understand the structural basis of metal-induced allosteric switching of these metalloregulatory proteins in a thermodynamic framework, and review the degree to which coordination chemistry drives changes in protein structure and dynamics in selected metal sensor systems. New insights into how metal sensor proteins function in the complex intracellular milieu of the cytoplasm of cells will require a more sophisticated understanding of the "metallome" and will benefit greatly from ongoing collaborative efforts in bioinorganic, biophysical and analytical chemistry, structural biology and microbiology.     

Adsorption and selectivity characteristics of several human serum proteins with immobilised hard Lewis metal ion-chelate adsorbents

In this investigation, human serum has been used as an example of a crude protein mixture to define the protein binding characteristics and selectivity of several immobilised hard Lewis metal ion affinity chromatographic (IMAC) adsorbents. Specifically, the binding properties of immobilised O-phosphoserine (im-OPS) and 8-hydroxyquinoline (im-8-HQ), with immobilised iminodiacetic acid as a control system, have been investigated in combination with the hard Lewis metal ions, Al3+, Ca2+, Fe3+, Yb3+, and the borderline metal ion, Cu2+, over the pH range pH 5.5 to pH 8.0 with buffers of 0.5 M ionic strength. The same IMAC adsorbents were also investigated for their protein binding capabilities with buffers of an ionic strength of 0.06 M at pH 5.5 and pH 8.0. The binding behaviour of four "marker" proteins, namely transferrin, alpha2-macroglobulin, gammaglobulin and human serum albumin have furthermore been employed to monitor the differences in protein selectivity exhibited by these IMAC systems. The experimental findings confirm that these hard Lewis metal ion IMAC adsorbents function in a "mixed" binding mode with both coordination and electrostatic characteristics evident, depending on the ionic strength and pH of the equilibration or elution buffers. Based on a screening protocol, several members of the im-Mn+-8-HQ and im-Mn+-OPS adsorbent series have been identified with high selectivity for transferrin and alpha2-macroglobulin. These hard Lewis metal ion IMAC adsorbents thus provide attractive alternatives for selective fractionation of human serum proteins.     

From the Metal to the Molecule-Ternary Bismuth Subhalides

Subvalent compounds, that is, metal-rich substances in which the average oxidation state of the cation is smaller than would be expected from the (8-N) rule, have proved to be a rich source of unexpected structural and physical features. The extraordinary structural chemistry generally observed in subvalent compounds is a consequence of the low and often non-integer oxidation states of the metal atoms coupled with the low concentration of valence electrons. Both factors can lead to a wide-range of bonding types within the same compound. A characteristic of these compounds is the interplay between "metallic" regions, with delocalized electrons and mainly nonpolar bonds between the metal atoms, and "saltlike" regions, which are characterized by strong localization of the electrons and heteropolar exchange between the metal and nonmetal atoms. The volumes of the different structural regions as well as the extent to which they interpenetrate can vary from compound to compound. The ternary subhalides of bismuth belong to a new class of substances which cover the whole spectrum from partially oxidized "porous" metals, through one- and two-dimensional metals, up to semiconducting ionic or molecular cluster compounds. These subvalent compounds with their unusually high chemical stabilities provide excellent vehicles for further research and their potential is described in the following article.     

Lipase active-site-directed anchoring of organometallics: metallopincer/protein hybrids

The work described herein presents a strategy for the regioselective introduction of organometallic complexes into the active site of the lipase cutinase. Nitrophenol phosphonate esters, well known for their lipase inhibitory activity, are used as anchor functionalities and were found to be ideal tools to develop a single-site-directed immobilization method. A small series of phosphonate esters, covalently attached to ECE "pincer"-type d8-metal complexes through a propyl tether (ECE=[C6H3(CH2E)(2)-2,6]-; E=NR2 or SR), were designed and synthesized. Cutinase was treated with these organometallic phosphonate esters and the new metal-complex/protein hybrids were identified as containing exactly one organometallic unit per protein. The organometallic proteins were purified by membrane dialysis and analyzed by ESI-mass spectrometry. The major advantages of this strategy are: 1) one transition metal can be introduced regioselectively and, hence, the metal environment can potentially be fine-tuned; 2) purification procedures are facile due to the use of pre-synthesized metal complexes; and, most importantly, 3) the covalent attachment of robust organometallic pincer complexes to an enzyme is achieved, which will prevent metal leaching from these hybrids. The approach presented herein can be regarded as a tool in the development of regio- and enantioselective catalyst as well as analytical probes for studying enzyme properties (e.g., structure) and, hence, is a "proof-of-principle design" study in enzyme chemistry.     

Solvent Extraction of Alkali Metal Picrates by Lipophilic Cyclodextrin Derivatives

Lipophilic cyclodextrin (CD) derivatives were prepared to extract alkali metal cations from a water phase into an organic phase. The extraction equilibrium constant, K ex, was determined by the solvent extraction method using UV absorption spectroscopy. Hydroxyl groups at the carbons in the 2,6-positions of CD molecules were dipropylated to add the hydrophobicity for dissolving into organic solvents, and furthermore hydroxyl groups at the carbons in the 3-position of these derivatives were acylated as complexing sites with the alkali metal cations. These CD derivatives formed a 1 : 1 complex with alkali metal cations, except for the case of Li+, and transported the alkali metal cations from a water phase into a benzene phase. The initial concentrations of alkali metal cation and picrate anion in the water phase and that of the CD derivatives in the organic phase strongly influenced the extraction equilibrium. Extraction of the alkali metal cation by the derivative without acyl groups was not detected. K ex values of these CD derivatives are of the same order of magnitude as or larger than those of crown ethers. The order of the K ex values in all cases is Li+ < Na+ < K+ Rb+ Cs+, although these CD derivatives have no special selectivity for the alkali metal cations. The cation extraction mechanism was interpreted by an induced-fit mechanism.     

Metrical oxidation states of 2-amidophenoxide and catecholate ligands: structural signatures of metal-ligand π bonding in potentially noninnocent ligands

Catecholates and 2-amidophenoxides are prototypical "noninnocent" ligands which can form metal complexes where the ligands are best described as being in the monoanionic (imino)semiquinone or neutral (imino)quinone oxidation state instead of their closed-shell dianionic form. Through a comprehensive analysis of structural data available for compounds with these ligands in unambiguous oxidation states (109 amidophenolates, 259 catecholates), the well-known structural changes in the ligands with oxidation state can be quantified. Using these correlations, an empirical "metrical oxidation state" (MOS) which gives a continuous measure of the apparent oxidation state of the ligand can be determined based on least-squares fitting of its C-C, C-O, and C-N bond lengths to this single parameter (a simple procedure for doing so is provided via a spreadsheet in the Supporting Information). High-valent d(0) metal complexes, particularly those of vanadium(V) and molybdenum(VI), have ligands with unexpectedly positive, and generally nonintegral, MOS values. The structural effects in these complexes are attributed not to electron transfer, but rather to amidophenoxide- or catecholate-to-metal π bonding, an interpretation supported by the systematic variation of the MOS values as a function of the degree of competition with the other π-donating groups in the structures.