Dimerization of Fe(III) Ion in an Aqueous Medium: Mechanistic Modelling and Effects of Ligands

Aqueous iron solutions generally undergo spontaneous hydrolysis followed by aggregation resulting in the precipitation of nanocrystalline oxyhydroxide minerals. The mechanism of nucleation of such multinuclear oxyhydroxide clusters are unclear due to limited experimental evidence. Here, we investigate the mechanistic pathway of dimerization of Fe(III) ions using density functional theory (DFT) in aqueous medium considering effects of other ligands. Two hydrolyzed monomeric Fe(III) ions in aqueous medium may react to form two closely related binuclear products, the μ-oxo and the dihydroxo Fe2 dimer. Our studies indicate that the water molecules in the second coordination sphere and those co-ordinated to the Fe(III) ion, both participate in the dimerization process. The proposed mechanism effectively explains the formation of dihydroxo and μ-oxo Fe2 dimers with interconversion possibilities, for the first time. Results show, with only water molecules present in the second co-ordination sphere, dihydroxo Fe2 dimer is the thermodynamically and kinetically favored product with a low activation free energy. We calculated the step-wise reaction free energies of dimerization in the presence of nitrate ions in the first and second coordination sphere of Fe(III) ion separately, which shows that with nitrate ions in the second co-ordination sphere, the μ-oxo Fe2 dimer is the kinetically favored product.

Enzyme Control Over Ferric Iron Magnetostructural Properties

Fe³⁺ complexes in aqueous solution can exist as discrete mononuclear species or multinuclear magnetically coupled species. Stimuli-driven change to Fe³⁺ speciation represents a powerful mechanistic basis for magnetic resonance sensor technology, but ligand design strategies to exert precision control of aqueous Fe³⁺ magnetostructural properties are entirely underexplored. In pursuit of this objective, we rationally designed a ligand to strongly favor a dinuclear μ-oxo-bridged and antiferromagnetically coupled complex, but which undergoes carboxylesterase mediated transformation to a mononuclear high-spin Fe³⁺ chelate resulting in substantial T₁ -relaxivity increase. The data communicated demonstrate proof of concept for a novel and effective strategy to exert biochemical control over aqueous Fe³⁺ magnetic, structural, and relaxometric properties.

On the singularity of scandium

Hydrogen-bonding interactions of the [Sc2(OH)2(OH2)10]4+ dimer containing 7-coordinate Sc(iii).

Impacts of Oxo Interactions on Np(V) Crown Ether Complexes

Intermolecular interactions between the oxo group of an actinyl cation and other metal cations (i.e., cation-cation interactions) are dependent on the strength of the actinyl bond. These cation-cation interactions are prominently observed for the neptunyl cation [Np(V)O₂]⁺ and are sufficiently stable enough to explore using a variety of chemical techniques. Herein, we investigate these intermolecular interactions in the neptunyl 18-crown-6 system, because this macrocyclic ligand provides both stable coordination and the proper sterics to engage the oxo group in bonding with both low-valent metal cations and neighboring neptunyl units. We report the structural and spectroscopic characterization of five neptunyl, [Np(V,VI)O₂]^+,2+, compounds: Np1a ([NpO₂(18-crown-6)]ClO₄), Np1b ([NpO₂(18-crown-6)]AuCl₄), Na-Np ([Np(V)O₂(18-crown-6)(Na(H₂O)(18-crown-6)][Np(VI)O₂Cl₄], Np-Np ([NpO₂(18-crown-6)](NpO₂Cl₂NO₃)], and Np-Cl (NpO₂Cl(H₂O)_1.75). Each of these compounds were prepared from the ambient reactions of Np(V) in HX (where X = Cl, NO₃) with the 18-crown-6 ether molecule. Structural information obtained from single-crystal X-ray diffraction data was paired with solid-state and solution Raman spectroscopy to provide information on the interaction of the neptunyl oxo atom with neighboring cations. Neptunyl (Np═O) bond lengths are not perturbed upon interaction with the Na⁺ cation (Na-Np), but elongation is observed upon formation of a neptunyl-neptunyl interaction (Np-Np). This is also the first structurally characterized isolated, molecular complex that contains a simple T-shaped neptunyl-neptunyl interaction. Raman spectroscopy indicates little perturbation to the neptunyl bond until the formation of the neptunyl-neptunyl motif, which also results in activation of the ν₃ asymmetric stretch. Additional spectroscopic studies indicated that the neptunyl 18-crown-6 inclusion complexes form in solution and persist in the presence of other low-valence cations.

A high temperature reversible phase transition in a supramolecular complex of 15-crown-5 with tetraphenylboron sodium

High temperature (391 K) reversible phase transition and large thermal hysteresis (19 K) was observed in 15-crown-5 clathrates: [Na(15-crown-5)][BPh₄] (15-crown-5 = 1,4,7,10,13-pentaoxacyclopentadecane, NaBPh₄ = sodium tetraphenylboron).

Positions of amino groups on ammonium salts tunes the conformations of crown ethers: crystal structures, Hirshfeld surfaces and spectroscopic studies

The position of the amino group on the ammonium salt plays a key role in the supramolecular structure and conformation of the crown ether.

Organic-inorganic hybrid materials based on iron(III)-polyoxotungstates and 1-butyl-3-methylimidazolium cations

The iron(III) μ-oxo bridged dimeric polyoxometalate [(PW(11)O(39)Fe)(2)O](10-) was isolated by reacting the transition metal monosubstituted Keggin anion [PW(11)O(39)Fe(H(2)O)](4-) and the ionic liquid 1-butyl-3-methylimidazolium bromide, (Bmim)Br, at pH 5.5. The crystal structure of (Bmim)(10)[(PW(11)O(39)Fe)(2)O]·0.5H(2)O (1) (monoclinic, space group P2(1)/n, Z = 4) was determined by single crystal X-ray diffraction. By changing the reaction conditions, (Bmim)(4)[PW(11)O(39)Fe(H(2)O)]·H(2)O (2) was obtained, whilst the reaction between the Bmim(+) cation and the heteropolyanion [SiW(11)O(39)Fe(H(2)O](5-), in the pH conditions used for 1, afforded (Bmim)(5)[SiW(11)O(39)Fe(H(2)O)]·4H(2)O (3). The compounds were characterized by spectroscopic techniques, thermal analysis, cyclic voltammetry, magnetic measurements and mass spectrometry. This study contributes to the understanding of iron μ-oxo dimer formation in polyoxometalate chemistry and calls attention to the influence of the counter-cations on the stability and formation of compound 1. The combination of the cationic part of ionic liquids and iron-substituted polyoxotungstates is predicted to lead to new materials with interest to catalysis, electrocatalysis and ionic liquid based nanocomposites.

Accessibility and selective stabilization of the principal spin states of iron by pyridyl versus phenolic ketimines: modulation of the 6A1 ↔ 2T2 ground-state transformation of the [FeN4O2]+ chromophore

Several potentially tridentate pyridyl and phenolic Schiff bases (apRen and HhapRen, respectively) were derived from the condensation reactions of 2-acetylpyridine (ap) and 2'-hydroxyacetophenone (Hhap), respectively, with N-R-ethylenediamine (RNHCH(2)CH(2)NH(2), Ren; R = H, Me or Et) and complexed in situ with iron(II) or iron(III), as dictated by the nature of the ligand donor set, to generate the six-coordinate iron compounds [Fe(II)(apRen)(2)]X(2) (R = H, Me; X(-) = ClO(4)(-), BPh(4)(-), PF(6)(-)) and [Fe(III)(hapRen)(2)]X (R = Me, Et; X(-) = ClO(4)(-), BPh(4)(-)). Single-crystal X-ray analyses of [Fe(II)(apRen)(2)](ClO(4))(2) (R = H, Me) revealed a pseudo-octahedral geometry about the ferrous ion with the Fe(II)-N bond distances (1.896-2.041 Å) pointing to the (1)A(1) (d(π)(6)) ground state; the existence of this spin state was corroborated by magnetic susceptibility measurements and Mössbauer spectroscopy. In contrast, the X-ray structure of the phenolate complex [Fe(III)(hapMen)(2)]ClO(4), determined at 100 K, demonstrated stabilization of the ferric state; the compression of the coordinate bonds at the metal center is in accord with the (2)T(2) (d(π)(5)) ground state. Magnetic susceptibility measurements along with EPR and Mössbauer spectroscopic techniques have shown that the iron(III) complexes are spin-crossover (SCO) materials. The spin transition within the [Fe(III)N(4)O(2)](+) chromophore was modulated with alkyl substituents to afford two-step and one-step (6)A(1) ↔ (2)T(2) transformations in [Fe(III)(hapMen)(2)]ClO(4) and [Fe(III)(hapEen)(2)]ClO(4), respectively. Previously, none of the X-salRen- and X-sal(2)trien-based ferric spin-crossover compounds exhibited a stepwise transition. The optical spectra of the LS iron(II) and SCO iron(III) complexes display intense d(π) → p(π)* and p(π) → d(π) CT visible absorptions, respectively, which account for the spectacular color differences. All the complexes are redox-active; as expected, the one-electron oxidative process in the divalent compounds occurs at higher redox potentials than does the reverse process in the trivalent compounds. The cyclic voltammograms of the latter compounds reveal irreversible electrochemical generation of the phenoxyl radical. Finally, the H(2)salen-type quadridentate ketimine H(2)hapen complexed with an equivalent amount of iron(III) to afford the μ-oxo-monobridged dinuclear complex [{Fe(III)(hapen)}(2)(μ-O)] exhibiting a distorted square-pyramidal geometry at the metal centers and considerable antiferromagnetic coupling of spins (J ≈ -99 cm(-1)).

Vapour-Phase Polymerization of Pyrrole and 3,4-Ethylenedioxythiophene Using Iron(III) 2,4,6-Trimethylbenzenesulfonate

A new iron(iii) 2,4,6-trimethylbenzenesulfonate (MSA) with composition [Fe(OH2)5(MSA)3] has been prepared from the reaction of Fe(OH)3·xH2O and three molar equivalents of 2,4,6-trimethylbenzenesulfonic acid and used as oxidant in the preparation of highly conducting polypyrrole (PPy) and poly(3,4-ethylenedioxythiophene) (PEDOT) films for the first time. PPy and PEDOT films grown on non-conducting overhead transparency (polyethylene terephthalate films) using a vapour phase polymerization technique exhibited very high conductivities; 200 ± 20 S cm–1 for PPy and 1000 ± 80 S cm–1 for PEDOT.

Solution and Solid-State Preparation of 18-Crown[6] Complexes with M[HSO4]n Salts (M = NH4+, K+, Sr2+ andn = 1, 2) and an Investigation of Solvation/Desolvation Processes and Crystal Polymorphism

18-Crown[6] ether has been used to prepare a new class of organic-inorganic complexes of general formula 18-crown[6]M[HSO(4)](n) (where M = NH(4) (+), K(+), Sr(2+) and n = 1, 2) by reacting directly in solution or in the solid state the crown ether 18-crown[6] with inorganic salts such as [NH(4)][HSO(4)], K[HSO(4)], and Sr[HSO(4)](2). The structures of 18-crown[6][NH(4)][HSO(4)]2 H(2)O (12 H(2)O), 18-crown[6][NH(4)][HSO(4)] (1), 18-crown[6]K[HSO(4)]2 H(2)O (22 H(2)O), 18-crown[6]K[HSO(4)] (2), and 18-crown[6]Sr[HSO(4)](2) (3) have been characterized by single-crystal X-ray diffraction. The reversible water loss in compounds 12 H(2)O and 22 H(2)O leads to formation of the corresponding anhydrous phases 18-crown[6][NH(4)][HSO(4)] (1), and 18-crown[6]K[HSO(4)] (2), which undergo, on further heating, enantiotropic solid-solid transitions very likely associated with the on-set of a solid state dynamical process. Similar high-temperature behavior is shown by 18-crown[6]Sr[HSO(4)](2) (3). The dehydration and phase-transition processes have been investigated by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and variable temperature X-ray powder diffraction.

Reversible solid-state reaction between 18-Crown[6] and M[H2PO4](M = K, Rb, Cs) and an investigation of the decomplexation process

On heating the hydrated complexes 18-Crown[6].M[H(2)PO(4)].xH(2)O (x= 2 for M = K, Rb; x= 1.5 for M = Cs), quantitatively prepared by mechanical mixing of crystalline 18-Crown[6] and M[H(2)PO(4)], to ca. 60 degrees C, water loss takes place, accompanied by the extrusion of the crown ether from the crystalline complex and followed by reconstruction of the inorganic phase M[H(2)PO(4)](M = K, Rb, Cs); the resulting solid mixture reverts to 18-Crown[6].M[H(2)PO(4)].xH(2)O (x= 2 for M = K, Rb; x= 1.5 for M = Cs) upon grinding in air.

Synthesis, Reactivities, and Magnetostructural Properties of FeIII, FeIII−O−FeIII, and ZnIIFeIII−O−FeIIIZnII Complexes of a Tetraiminodiphenolate Macrocycle

The mononuclear iron(III) complexes [Fe(LH2)(H2O)Cl](ClO4)2.2H2O (1) and [Fe(LH2)(H2O)2](ClO4)3.H2O (2) have been prepared by reacting [Pb(LH(2))](ClO4)2 with FeCl3.6H2O and Fe(ClO(4))(3).6H(2)O, respectively. Complex 2 upon treatment with 1 equiv of alkali produces the oxo-bridged dimer [{Fe(LH2)(H2O)}2(mu-O)](ClO4)4.2H2O (3). In these compounds, LH2 refers to the tetraiminodiphenol macrocycle in the zwitterionic form whose two uncoordinated imine nitrogens are protonated and hydrogen-bonded to the metal-bound phenolate oxygens. The aqua ligands of complexes 1-3 get exchanged in acetonitrile. Reaction equilibria involving binding and exchange of the terminal ligands (Cl-/H2O/CH3CN) in these complexes have been studied spectrophotometrically. The equilibrium constant for the aquation reaction (K(aq)) [1]2+ + H2O <==> [2]3+ + Cl- in acetonitrile is 8.65(5) M, and the binding constant (K(Cl)-) for the reaction [1]2+ + Cl- [1Cl]+ + CH3CN is 4.75(5) M. The pK(D) value for the dimerization reaction 2[2]3+ + 2OH- <==> [3]4+ + 3H(2)O in 1:1 acetonitrile-water is 9.38(10). Complexes 1-3 upon reaction with Zn(ClO4)(2).6H(2)O and sodium acetate (OAc), pivalate (OPiv), or bis(4-nitrophenyl)phosphate (BNPP) produce the heterobimetallic complexes [{FeLZn(mu-X)}2(mu-O)](ClO4)2, where X = OAc (4), OPiv (5), and BNPP (6). The pseudo-first-order rate constant (k(obs)) for the formation of 4 at 25 degrees C from either 1 or 3 with an excess of Zn(OAc)2.2H2O in 1:1 acetonitrile-water at pH 6.6 is found to be the same with k(obs) = 1.6(2) x 10(-4) s(-1). The X-ray crystal structures of 3, 4, and 6 have been determined, although the structure determination of 3 was severely affected because of heavy disordering. In 3, the Fe-O-Fe angle is 168.6(6) degrees, while it is exactly 180.0 degrees in 4 and 6. Cyclic and square-wave voltammetric (CV and SWV) measurements have been carried out for complexes 1-4 in acetonitrile. The variation of the solvent composition (acetonitrile-water) has a profound effect on the E(1/2) and DeltaE(p) values. The binding of an additional chloride ion to an iron(III) center in 1-3 is accompanied by a remarkable shift of E(1/2) to more negative values. The observation of quasi-reversible CV for complexes containing a Fe(III)-O-Fe(III) unit (3 and 4) indicates that in the electrochemical time scale unusual Fe(III)-O-Fe(II) is produced. The 1H NMR spectra of complexes 3-6 exhibit hyperfine-shifted signals in the range 0-90 ppm with similar features. The metal-hydrogen distances obtained from T(1) measurements are in good agreement with the crystallographic data. Variable-temperature (2-300 K) magnetic susceptibility measurements carried out for 3 and 4 indicate strong antiferromagnetic exchange interaction (H = -2JS1.S2) between the high-spin iron(III) centers in the Fe-O-Fe unit with J = -114 cm(-1) (3) and -107 cm(-1) (4).

Mechanochemical preparation of molecular and supramolecular organometallic materials and coordination networks

This Dalton Perspective deals with solvent-free reactions taking place within solids or between solids or involving a solid and a vapour. The focus is on reactions involving organometallic and coordination compounds and occurring via reassembling of non-covalent bonding, e.g. hydrogen bonds, and/or formation of ligand-metal coordination bonds. It is argued that reactions activated by mechanical mixing of solid reactants as well as those obtained by exposing a crystalline solid to a vapour can be exploited to "make crystals", which is the quintessence of crystal engineering. It is demonstrated through a number of examples that solvent-free methods, such as co-grinding, kneading, milling of molecular solids, or reactions of solid with vapours represent viable alternative, when not unique, routes for the preparation of novel molecular and supramolecular solids as well as for the preparation of polymorphic or solvate modifications of a same species. The structural characterization of the products requires the preparation of single crystals suitable for X-ray diffraction, a goal often achieved by seeding.

Synthesis, complexation, and supramolecular assembly of 21,30-dithia-17,25-dimethyl-1,4,7,10,13- pentaoxa[13.3.3](1,2,6)cyclophane

[structure: see text] A crownophane consisting of a dithia[3.3]metacyclophane moiety was synthesized by employing a high dilution method. The crystal structure and complexation behavior of this crownophane was studied. A hydrogen-bonded linear supramolecular assembly using anion-Na(+)-crownophane-solvent as a synthon was observed in the solid state. This supramolecular assembly was also stabilized via an intermolecular S.H-C interaction.