Magnetostructural Correlation for High-Nuclearity Iron(III)/Oxo Complexes and Application to Fe5, Fe6, and Fe8 Clusters

Semiempirical Magnetostructural Correlation for High-Nuclearity MnIII-Oxo Complexes: Accommodation of Different Relative Jahn-Teller Axis Orientations

The previous development of a magnetostructural correlation (MSC) for polynuclear FeIII/oxo clusters has now been extended to one for polynuclear MnIII/oxo clusters. A semiempirical model estimating each pairwise Mn2 exchange constant (Jij) from the Mn-O bond lengths and Mn-O-Mn angles has been formulated based on the angular overlap model. The extra complication, compared with the FeIII/oxo MSC, of different relative orientations of the Jahn-Teller distortion axes typical of high-spin MnIII in near-octahedral geometry was accommodated by developing a separate MSC variant for each possible situation. The final coefficients of the three MSC variants were determined by using reliable crystal structure data and experimentally determined Jij values from the literature. The estimated JMSC values from the new MnIII/oxo MSC have been employed to successfully rationalize the magnetic properties of a number of MnIII clusters in the nuclearity range Mn3-Mn10. These properties include relative spin vector alignments in the ground state, the presence of spin frustration effects, and the resulting overall ground state spin. In addition, the JMSC values can be used to simulate the direct-current magnetic susceptibility versus temperature data and provide realistic input values for fits of these data to minimize false-fit problems. A protocol for the use of the new MSC is also reported.

Substituent-Guided Cluster Nuclearity for Tetranuclear Iron(III) Compounds with Flat {Fe4(μ3-O)2} Butterfly Core

The tetranuclear iron(III) compounds [Fe4(μ3-O)2(μ-LZ)4] (1–3) were obtained by reaction of FeCl3 with the shortened salen-type N2O2 tetradentate Schiff bases N,N’-bis(salicylidene)-o-Z-phenylmethanediamine H2LZ (Z = NO2, Cl and OMe, respectively), where the one-carbon bridge between the two iminic nitrogen donor atoms guide preferentially to the formation of oligonuclear species, and the ortho position of the substituent Z on the central phenyl ring selectively drives towards Fe4 bis-oxido clusters. All compounds show a flat almost-symmetric butterfly-like conformation of the {Fe4(μ3-O)2} core, surrounded by the four Schiff base ligands, as depicted by both the X-ray molecular structures of 1 and 2 and the optimized geometries of all derivatives as obtained by UM06/6-311G(d) DFT calculations. The strength of the antiferromagnetic exchange coupling constants between the iron(III) ions varies among the three derivatives, despite their magnetic cores remain structurally almost unvaried, as well as the coordination of the metal ions, with a distorted octahedral environment for the two-body iron ions, Feb, and a pentacoordination with trigonal bipyramidal geometry for the two-wing iron ions, Few. The different magnetic behavior within the series of examined compounds may be ascribed to the influence of the electronic features of Z on the electron density distribution (EDD) of the central {Fe4(μ3-O)2} core, substantiated by a Quantum Theory of Atoms In Molecules (QTAIM) topological analysis of the EDD, as obtained by UM06 calculations 1–3.

Synthesis, Structure, and Magnetic Properties of an Fe36 Dimethylarsinate Cluster: The Largest "Ferric Wheel"

The synthesis and characterization of a high-nuclearity Fe^III/O/arsinate cluster is reported within the salt [Fe₃₆O₁₂(OH)₆(O₂AsMe₂)₆₃(O₂CH)₃(H₂O)₆](NO₃)₁₂ (1). The compound was prepared from the reaction of Fe(NO₃)₃·9H₂O, dimethylarsinic acid (Me₂AsO₂H), and triethylamine in a 1:2:4 molar ratio in acetonitrile. The Fe₃₆ cation of 1 is an unprecedented structural type consisting of nine Fe₄ butterfly units of two types, three {Fe^III₄(μ₃-O)₂} units A, and six {Fe^III₄(μ₃-O)(μ₃-OH)} units B, linked by multiple bridging Me₂AsO₂^- groups into an Fe₃₆ triangular wheel/loop with C₃ crystallographic and D₃ virtual symmetry that looks like a guitar plectrum. The unusual structure has been rationalized on the basis of the different curvatures of units A and B, the presence of intra-Fe₃₆ hydrogen bonding, and the tendency of Me₂AsO₂^- groups to favor μ₃-bridging modes. The cations stack into supramolecular nanotubes parallel to the crystallographic c axis and contain badly disordered solvent and NO₃^- anions. The cation of 1 is the highest-nuclearity "ferric wheel" to date and also the highest-nuclearity Fe/O cluster of any structural type with a single contiguous Fe/O core. Variable-temperature direct-current magnetic susceptibility data and alternating-current in-phase magnetic susceptibility data indicate that the cation of 1 possesses an S = 0 ground state and dominant antiferromagnetic interactions. The Fe₂ pairwise J_i,j couplings were estimated by the combined use of a magnetostructural correlation for high-nuclearity Fe^III/oxo clusters and density functional theory calculations using broken-symmetry methods and the Green's function approach. The three methods gave satisfyingly similar J_i,j values and allowed the identification of spin-frustration effects and the resulting relative spin-vector alignments and thus rationalization of the S = 0 ground state of the cation.

Validation of the Green's Function Approximation for the Calculation of Magnetic Exchange Couplings

In this work, we assess the potential of the Green's function approximation to predict isotropic magnetic exchange couplings and to reproduce the standard broken-symmetry energy difference approach for transition metal complexes. To this end, we have selected a variety of heterodinuclear, homodinuclear, and polynuclear systems containing 3d transition metal centers and computed the couplings using both the Green's function and energy difference methods. The Green's function approach is shown to have mixed results for the cases tested. For dinuclear complexes with large strength couplings (≳50 cm^-1), the Green's function method is unable to reliably reproduce the energy difference values. However, for weaker dinuclear couplings, the Green's function approach acceptably reproduces broken-symmetry energy difference couplings. In polynuclear cases, the Green's function approximation worked remarkably well, especially for Fe^III complexes. On the other hand, for a Ni^II polynuclear complex, qualitatively wrong couplings are predicted. Overall, the evaluation of exchange couplings from local rigid magnetization rotations offers a powerful alternative to time-consuming energy differences methods for large polynuclear transition metal complexes, but to achieve a quantitative agreement, some improvements to the method are needed.

Magnetic Properties of High-Nuclearity Fex-oxo (x = 7, 22, 24) Clusters Analyzed by a Multipronged Experimental, Computational, and Magnetostructural Correlation Approach

The synthesis, structure, and magnetic properties of three related iron(III)-oxo clusters are reported, [Fe₇O₃(O₂CPh)₉(mda)₃(H₂O)] (1), [Fe₂₂O₁₄(OH)₃(O₂CMe)₂₁(mda)₆](ClO₄)₂ (2), and [Fe₂₄O₁₅(OH)₄(OEt)(O₂CMe)₂₁(mda)₇](ClO₄)₂ (3), where mdaH₂ is N-methyldiethanolamine. 1 was prepared from the reaction of [Fe₃O(O₂CPh)₆(H₂O)₃](NO₃) with mdaH₂ in a 1:2 ratio in MeCN, whereas 2 and 3 were prepared from the reaction of FeCl₃/NaO₂CMe/mdaH₂ in a 2:∼13:2 ratio and FeCl₃/NaO₂CMe/mdaH₂/pyridine in a 2:∼13:2:25 ratio, respectively, both in EtOH. The core of 1 consists of a central octahedral Fe^III ion held within a nonplanar Fe₆ loop by three μ₃-O^2- and three μ₂-RO^- arms from the three mda^2- chelates. The cores of the cations of 2 and 3 consist of an A:B:A three-layer topology, in which a central Fe₆ (2) or Fe₈ (3) layer B is sandwiched between two Fe₈ layers A. The A layers structurally resemble 1 with the additional Fe added at the center to retain virtual C₃ symmetry. The central Fe₆ layer B of 2 consists of a {Fe₄(μ₄-O)₂(μ₃-OH)₂}⁶⁺ cubane with an Fe on either side attached to cubane O^2- ions, whereas that of 3 has the same cubane but with an {Fe₃(μ₃-O)(μ-OH)} unit attached on one side and a single Fe on the other. Variable-temperature dc and ac magnetic susceptibility studies revealed dominant antiferromagnetic coupling in all complexes leading to ground-state spins of S = ⁵/₂ for 1 and S = 0 for 2 and 3. All Fe₂ pairwise exchange parameters (J_ij) for 1-3 were estimated by two independent methods: density functional theory (DFT) calculations using broken symmetry methods and a magnetostructural correlation previously developed for high-nuclearity Fe^III/O complexes. The two approaches gave satisfyingly similar J_ij values, and the latter allowed rationalization of the experimental ground states by identification of the spin frustration effects operative and the resultant relative spin vector alignments at each Fe^III ion.

A new hexanuclear Fe(III) nanocluster: Synthesis, structure, magnetic properties, and efficient activity as a precatalyst in water oxidation

The oxo-bridged hexanuclear iron cluster formulated, [Fe6III(µ4-O)2(edteH)2(piv)4(SCN)4]∙2MeCN∙2H2O (1) (where, edteH = N,N,N′,N′-tetrakis(2-hydroxyethyl)ethylenediamine; piv = pivalic acid) is synthesized by the reaction of FeCl2∙4H2O with edteH4 and piv in the presence...

Breaking Symmetry Relaxes Structural and Magnetic Restraints, Suppressing QTM in Enantiopure Butterfly Fe2 Dy2 SMMs*

The {Fe2 Dy2 } butterfly systems can show single molecule magnet (SMM) behaviour, the nature of which depends on details of the electronic structure, as previously demonstrated for the [Fe2 Dy2 (μ3 -OH)2 (Me-teaH)2 (O2 CPh)6 ] compound, where the [N,N-bis-(2-hydroxyethyl)-amino]-2-propanol (Me-teaH3 ) ligand is usually used in its racemic form. Here, we describe the consequences for the SMM properties by using enantiopure versions of this ligand and present the first homochiral 3d/4 f SMM, which could only be obtained for the S enantiomer of the ligand for [Fe2 Dy2 (μ3 -OH)2 (Me-teaH)2 (O2 CPh)6 ] since the R enantiomer underwent significant racemisation. To investigate this further, we prepared the [Fe2 Dy2 (μ3 -OH)2 (Me-teaH)2 (O2 CPh)4 (NO3 )2 ] version, which could be obtained as the RS-, R- and S-compounds. Remarkably, the enantiopure versions show enhanced slow relaxation of magnetisation. The use of the enantiomerically pure ligand suppresses QTM, leading to the conclusion that use of enantiopure ligands is a "gamechanger" by breaking the cluster symmetry and altering the intimate details of the coordination cluster's molecular structure.

[Fe15]: a frustrated, centred tetrakis hexahedron

The combination of two different Fe^III salts in a solvothermal reaction with triethanolamine results in the formation of a high symmetry [FeIII15] cluster whose structure conforms to a centred, tetrakis hexahedron.

Exploring the Role of Intramolecular Interactions in the Suppression of Quantum Tunneling of the Magnetization in a 3d-4f Single-Molecule Magnet

Hydroxide-bridged Fe^III₄Ln^III₂ clusters having the general formula [Fe₄Ln₂(μ₃-OH)₂(mdea)₆(SCN)₂(NO₃)₂(H₂O)₂]·4H₂O·2MeCN {Ln = Y (1), Dy (2), mdea = N-methyldiethanolamine} were synthesized and magnetically characterized. The thermal relaxation of the magnetization for 2 and the diluted Fe^III₄Dy^IIIY^III complex 3 (with and without applied field) has been analyzed. The diluted sample shows a dominant QTM at low temperatures that can be removed with a 0.15 T dc field. Both 2 and 3 show moderately high U_eff barriers and exhibit hysteresis loops until 5 K.

M, Shahid M, AlDamen MA, Khalid M, Akram M. Cationic dye adsorption and separation at discrete molecular level: first example of an iron cluster with rapid and selective adsorption of methylene blue from aqueous system

A novel Fe₆ cluster was designed as a rare example of any discrete molecule as a highly efficient, selective and rapid functional material for the adsorption of cationic dyes, i.e. methylene blue (MB), from contaminated water bodies.

Iron(III)-Oxo Cluster Chemistry with Dimethylarsinate Ligands: Structures, Magnetic Properties, and Computational Studies

A program has been initiated to develop Fe^III/oxo cluster chemistry with the "pseudocarboxylate" ligand dimethylarsinate (Me₂AsO₂^-) for comparison with the well investigated Fe^III/oxo/carboxylate cluster area. The synthesis and characterization of three polynuclear Fe^III complexes are reported, [Fe₁₂O₄(O₂C^tBu)₈(O₂AsMe₂)₁₇(H₂O)₃]Cl₃ (1), Na₂[Fe₁₂Na₂O₄(O₂AsMe₂)₂₀(NO₃)₆(Me₂AsO₂H)₂(H₂O)₄](NO₃)₆ (2), and [Fe₃(O₂AsMe₂)₆(Me₂AsO₂H)₂(hqn)₂](NO₃) (3), where hqnH is 8-hydroxyquinoline. The Fe₁₂ core of 1 is a type never previously encountered in Fe^III carboxylate chemistry, consisting of two Fe₆ units each of which comprises two {Fe₃(μ₃-O^2-)} units bridged by three Me₂AsO₂^- groups and linked into an Fe₁₂ loop structure by two anti-anti η¹:η¹:μ Me₂AsO₂^- groups, a bridging mode extremely rare with carboxylates. 2 also consists of two Fe₆ units, differing in their ligation from those in 1, and this time linked together into a linear structure by a central {Na₂(NO₃)₂} bridging unit. 3 is a linear Fe₃ complex with no monatomic bridges between Fe^III ions, a very rare situation in Fe^III chemistry with any ligands and unprecedented in Fe carboxylate chemistry. The distinct differences observed in arsinate vs carboxylate ligation modes are rationalized largely based on the greater basicity of the former vs the latter. Variable-temperature dc and ac magnetic susceptibility data reveal all Fe₂ pairwise interactions to be antiferromagnetic. For 1 and 2, the different J_ij couplings were estimated by use of a magnetostructural correlation for high nuclearity Fe^III-oxo clusters and by density functional theory calculations using broken symmetry methods, allowing identification of their relative spin vector alignments and thus rationalization of their S = 0 ground states. The J_ij values were then used as input values to give excellent fits of the experimental χ_MT vs T data. For 3, the fits of the experimental χ_MT vs T data to the Van Vleck equation or with PHI gave a very weak J₁₂ = -0.8(1) cm^-1 (H = -2JŜ_i·Ŝ_j convention) between adjacent Fe^III ions and an S = ⁵/₂ ground state. These initial Fe^III arsinate complexes also provide structural parameters that help validate literature assignments of arsinate binding modes to iron oxide/hydroxide minerals as part of environmental concerns of using arsenic-containing herbicides in agriculture.

A New FeIII Substituted Arsenotungstate [FeIII2(AsIIIW6O23)2(AsIIIO3H)2]12−: Synthesis, Structure, Characterization and Magnetic Properties

The iron(III)-containing arsenotungstate [FeIII2(AsIIIW6O23)2(AsIIIO3H)2]12− (1) was prepared via a simple, one-pot reaction in aqueous basic medium. The compound was isolated as its sodium salt, and structurally-characterized by Single Crystal X-ray Diffraction (SCXRD), Powder X-ray Diffraction (PXRD), Fourier-Transform Infrared (FT-IR) spectroscopy, Thermogravimetric Analysis (TGA) and elemental analysis. Its magnetic properties are reported; the antiferromagnetic coupling between the two FeIII centers is unusually weak as a result of the bridging geometry imposed by the rigid arsenotungstate metalloligands.

Structural characterization and magnetic property studies of a mixed-valence {CoIIICo} complex with a μ4-oxo tetrahedral {Co} motif

We have synthesized and structurally characterized a new mixed valence pentanuclear Co complex, bearing a rare μ4-O-tetrahedral CoII4 unit, by employing a pyridine-like Schiff base ligand. We have performed DC magnetic susceptibility and magnetization measurements over polycrystalline samples and chemical quantum computations in order to understand the exchange interaction pattern within Co(ii) sites and ground state magnetic anisotropy. This new complex shows an overall antiferromagnetic exchange interaction whose strength strongly depends on the local symmetry of Co(ii) sites. Also, local ion magnetic anisotropy reveals a strongly axial behaviour with the lowest Kramers doublet (KD) at each Co(ii) ion sufficiently isolated from excited states at low temperatures. Two Co(ii) sites show tetrahedral symmetry and the spin only formalism including axial zero-field splitting (ZFS) term properly described them; on the other hand, the other two Co(ii) sites have distorted octahedral and square base pyramidal coordination spheres, and a strong orbital contribution leads to a failure of the spin only formalism. A model of four Seff = 1/2 exchange interacting sites is necessary in order to account for low temperature magnetization behaviour. In view of the strongly anisotropic KD states, the exchange interactions are forced to be modelled as anisotropic ones. Overall, experimental data and quantum chemical computations are in good agreement, supporting the proposed model for magnetic behaviour.

Novel cage-like MOF for gas separation, CO2 conversion and selective adsorption of an organic dye

A Zn-MOF was constructed by a F-decorated ligand and bipyridyl linkers, displaying selective adsorption for CO₂, C₂H₆, C₂H₂ over CH₄, catalysis for CO₂ conversion and selective adsorption for methyl blue.

Remote Ligand Modifications Tune Electronic Distribution and Reactivity in Site-Differentiated, High-Spin Iron Clusters: Flipping Scaling Relationships

We report the synthesis, characterization, and reactivity of [LFe₃O(^RArIm)₃Fe][OTf]₂, the first Hammett series of a site-differentiated cluster. The cluster reduction potentials and CO stretching frequencies shift as expected on the basis of the electronic properties of the ligand: electron-donating substituents result in more reducing clusters and weaker C-O bonds. However, unusual trends in the energetics of their two sequential CO binding events with the substituent σ_p parameters are observed. Specifically, introduction of electron-donating substituents suppresses the first CO binding event (ΔΔH by as much as 7.9 kcal mol^-1) but enhances the second (ΔΔH by as much as 1.9 kcal mol^-1). X-ray crystallography, including multiple-wavelength anomalous diffraction, Mössbauer spectroscopy, and SQUID magnetometry, reveal that these substituent effects result from changes in the energetic penalty associated with electronic redistribution within the cluster, which occurs during the CO binding event.

Theoretical Studies on Hexanuclear [M3(μ3-O/OH)]2 (M = Fe(III), Mn(III), and Ni(II)) Clusters: Magnetic Exchange, Magnetic Anisotropy, and Magneto-Structural Correlations

Controlling spin Hamiltonian parameters such as magnetic exchange and magnetic anisotropy of polynuclear clusters is of great interest in the area of single molecule magnets (SMMs). Among large polynuclear clusters, hexanuclear clusters offer the best compromise in terms of size as they are often rigid, solution stable, and chemically amenable. The {M₆O₂} core is one of the common architectures known for many hexanuclear clusters and generally reported to possess a diamagnetic S_T = 0 spin ground state, barring a few exceptions. In these clusters, there are several open questions that are poorly understood: (a) What controls the nature of magnetic exchange, which in turn dictates the ground state spin values? (b) For clusters possessing a nonzero spin ground state, what dictates the magnetic anisotropy? Here, using density functional methods, we have attempted to shed light on these two question by evaluating the exchange coupling constants in [Fe₆^IIIO₂(OH)₂{(C₄N₂H₂SMe)₂C(OH)O}₂( ^tBuCO₂)₁₀] (1), [Fe₆^III(O)₂(O₂CH₂)(O₂CCH₂ ^tBu)₁₂(py)₂] (2), [Fe₆^III(O₂)(O)₂(O₂CCMe₃)₁₂(py)₂] (3), [Fe^III₆O₃(O₂CMe)₉(OEt)₂(bpy)₂]ClO₄ (4), [Mn^III₆O₂(O₂CH₂)(O₂CPe ^t)₁₁(HO₂CPe ^t)₂(O₂CMe)] (5), and [Ni^II₆(OH)₄(O₂C ^tBu)₈( ^tBuCO₂H)₄] (6) complexes. We have estimated all the eight near-neighbor exchange coupling constants in these clusters. Our calculations not only agree with the experimental results but also offer insight on the origin of the spin ground state. Extensive magneto-structural correlations developed by varying M-O-M angles and M-O distances reveal that J values are extremely sensitive to small structural distortions. Correlations developed indicate that both the parameters are important for Fe(III), but for Mn(III) and Ni(II), the angles were found to play a dominant role. Quite interestingly, the computed zero-field splitting parameter D _S=5 of complex 1 reveals that the exchange contribution to the anisotropy controls the sign of the ground state D value-an observation which differs from the general perception that the ground state D is controlled by the single-ion zero-field splitting parameter.

Regulation of magnetic relaxation behavior by replacing 3d transition metal ions in [M2Dy2] complexes containing two different organic chelating ligands

Two [MIII2DyIII2] complexes (M = Fe for 1 and Co for 2) with mixed organic ligands were obtained. Complex 2 exhibits single molecule magnet behavior with U_eff = 64.0(9) K.

Decanuclear FeIII clusters with hemiacetal ligands: a new {M10(μ3-O)8} cluster core

The characterization of a Fe^III₁₀ cluster with α-methyl-2-pyridinemethanolate lead us to explore the direct syntheses of medium nuclearity Fe^III clusters with aldehydes in methanol. The complexes exhibit an unprecedented {Fe₁₀(μ₃-O)₈} cluster core.

Heterometallic CuIIFeIII and CuIIMnIII alkoxo-bridged complexes revealing a rare hexanuclear M6(μ-X)7(μ3-X)2 molecular core

The novel hexanuclear complexes [Cu4Fe2(OH)(Piv)4(tBuDea)4Cl]·0.5CH3CN (1) and [Cu4Mn2(OH)(Piv)4(tBuDea)4Cl] (2) were prepared through one-pot self-assembly reactions of copper powder and iron(ii) or manganese(ii) chloride with N-tert-butyldiethanolamine (H2tBuDea) and pivalic acid (HPiv) in acetonitrile. Crystallographic studies revealed the uncommon molecular core type M6(μ-X)7(μ3-X)2 in 1 and 2, which can be viewed as a combination of two trimetallic M3(μ-X)2(μ3-X) fragments joined by three bridging atoms. The analysis and classification of the hexanuclear complexes having a M3(μ-X)2(μ3-X) moiety as a core forming fragment using data from the Cambridge Structural Database (CSD) were performed. Variable-temperature (1.8-300 K) magnetic susceptibility measurements of 1 showed a decrease of the effective magnetic moment value at low temperature, indicative of antiferromagnetic coupling between the magnetic centres (JFe-Cu/hc = -6.9 cm-1, JCu-Cu/hc = -4.1 cm-1, JFe-Fe/hc = -24.2 cm-1). Complex 1 acts as a catalyst in the reaction of mild oxidation of cyclohexane with H2O2, showing the yields of products, cyclohexanol and cyclohexanone, up to 17% using pyrazinecarboxylic acid as a promoter. In the oxidation of cis-1,2-dimethylcyclohexane with m-chloroperoxybenzoic acid (m-CPBA), 70% of retention of stereoconfiguration was observed for tertiary alcohols. Compound 1 also catalyses the amidation of cyclohexane with benzamide. In all three catalytic reactions the by-products were investigated in detail and discussed.

Chiroptical and magnetic properties of star-shaped Fe complexes from chiral Schiff bases. Structural and magnetic correlations based on continuous shape measures

New chiral FeIII4 star-shaped complexes have been synthesized starting from enantiomerically pure Schiff bases and chiroptically and magnetically characterized. The structural and magnetic properties of the complete family of 40 Fe4 complexes reported in the literature have been analyzed in the search for synthetic and magnetostructural correlations.

New molecular heptanuclear cobalt phosphonates: synthesis, structures and magnetic properties

Synthesis, structures and magnetic properties (strong anisotropy, ferromagnetic and antiferromagnetic interactions) of novel {Co₇} homoleptic molecular cobalt phosphonates with a similar structure motif are described.

Hexanuclear iron(iii) α-aminophosphonate: synthesis, structure, and magnetic properties of a molecular wheel

A new hexanuclear molecular iron phosphonate complex, [Fe₆(HAIPA)₁₂(OH)₆]·nH₂O (1·nH₂O) (H₂AIPA = NH₂(CH₃)₂CP(O)(OH)₂, (2-aminopropan-2-yl)phosphonic acid), was synthesized from Fe²⁺ and Fe³⁺ salts in water by interaction with the ligand sodium salt.

Long-Chain 4-Aminoquinolines as Quorum Sensing Inhibitors in Serratia marcescens and Pseudomonas aeruginosa

Antibiotic resistance has become a serious global threat to public health; therefore, improved strategies and structurally novel antimicrobials are urgently needed to combat infectious diseases. Here we report a new type of highly potent 4-aminoquinoline derivatives as quorum sensing inhibitors in Serratia marcescens and Pseudomonas aeruginosa, exhibiting weak bactericidal activities (minimum inhibitory concentration (MIC) > 400 μM). Through detailed structure-activity study, we have identified 7-Cl and 7-CF₃ substituted N-dodecylamino-4-aminoquinolines (5 and 10) as biofilm formation inhibitors with 50% biofilm inhibition at 69 μM and 63 μM in S. marcescens and P. aeruginosa, respectively. These two compounds, 5 and 10, are the first quinoline derivatives with anti-biofilm formation activity reported in S. marcescens. Quantitative structure-activity relationship (QSAR) analysis identified structural descriptors such as Wiener indices, hyper-distance-path index (HDPI), mean topological charge (MTC), topological charge index (TCI), and log D(o/w)_exp as the most influential in biofilm inhibition in this bacterial species. Derivative 10 is one of the most potent quinoline type inhibitors of pyocyanin production described so far (IC₅₀ = 2.5 μM). While we have demonstrated that 5 and 10 act as Pseudomonas quinolone system (PQS) antagonists, the mechanism of inhibition of S. marcescens biofilm formation with these compounds remains open since signaling similar to P. aeruginosa PQS system has not yet been described in Serratia and activity of these compounds on acylhomoserine lactone (AHL) signaling has not been detected. Our data show that 7-Cl and 7-CF₃ substituted N-dodecylamino-4-aminoquinolines present the promising scaffolds for developing antivirulence and anti-biofilm formation agents against multidrug-resistant bacterial species.

Binding of ligands containing carbonyl and phenol groups to iron(iii): new Fe6, Fe10 and Fe12 coordination clusters

The initial use of ligands 2'-hydroxyacetophenone (HL¹), 2-hydroxybenzophenone (HL²) and 2,2'-dihydroxybenzophenone (H₂L³) in iron(iii) chemistry is described. The syntheses and crystal structures are reported for five iron(iii) clusters: [Fe₁₀O₄(OMe)₁₄(L¹)₆(MeOH)₂](NO₃)₂·3MeOH (1·3MeOH), [Fe₁₂O₄(OH)(OMe)₁₇(L¹)₈](ClO₄)₂·2H₂O (2·2H₂O), [Fe₁₀O₄(OMe)₁₄Cl₄(L²)₄(MeOH)₂] (3), [Fe₁₀O₄(OMe)₁₄(L²)₆(py)₂](ClO₄)₂·MeOH (4·MeOH), where py = pyridine, and [Fe₆O₂(OEt)₆(O₂CMe)₂(L³)₂(HL³)₂] (5). The molecular structures of the decanuclear clusters 1, 3 and 4 are organized around a {Fe₁₀(μ₄-O)₄(μ₃-OMe)₂(μ-OMe)₁₂}⁸⁺ core consisting of ten {Fe₃O₄} face-sharing defective cubane units. The core of 2 consists of a {Fe₁₂(μ₄-O)₄(μ₃-OMe)₄(μ-OH)(μ-OMe)₁₃}¹⁰⁺ unit composed of twelve {Fe₃O₄} face-sharing defective cubanes. The ligands (L¹)^- and (L²)^- in 1-4 adopt the O,O'-bidentate chelating coordination mode and their roles are to terminate the further aggregation of the Fe^III/O^2-/RO^- cores. Complex 5 contains the {Fe₆(μ₄-O)₂(μ-OEt)₆(μ-O_carbonyl)₂}⁴⁺ core, where the μ-O_carbonyl atoms are the bridging carbonyl oxygens of the two η¹:η²:η¹:μ (L³)^2- ligands; the (HL³)^- groups behave as O_phenolate, O_carbonyl-bidentate chelating ligands with the neutral hydroxyl group being unbound to the Fe^III atoms. The core is composed of four {Fe₃O₄} face-sharing defective cubanes. The Fe^III atoms in 1-5 are all six-coordinate with distorted octahedral geometries. The IR spectra of the complexes are discussed in terms of the known coordination modes of the ligands and the ionic character of nitrates and perchlorates. Variable-temperature magnetic susceptibility and variable-field magnetization measurements establish that 2, 3 and 5 have S = 3, 0 and 5 ground states, respectively. The susceptibility data for 5 were fitted using a 3-J model indicating the simultaneous presence of both antiferromagnetic and ferromagnetic Fe^IIIFe^III exchange interactions. Known magnetostructural correlations in oxido-bridged iron(iii) systems have been applied to rationalise the magnetic behaviour of the three clusters. The results of the present study demonstrate the utility of HL¹, HL² and H₂L³ in the stabilisation of robust iron(iii)/oxido/alkoxido clusters.

Valence directed binding mode of [2 × 2] iron grids of an unsymmetrical picolinic hydrazone based ligand

Valence directed binding mode of the unsymmetrical ditopic ligand was observed in tetranuclear trivalent (FeIII4) and mixed valent (FeII2FeIII2) [2 × 2] iron grid complexes.