Thorsten Glaser

Nucleation Process in the Cavity of a 48‐Tungstophosphate Wheel Resulting in a 16‐Metal‐Centre Iron Oxide Nanocluster

The 16-Fe(III)-containing 48-tungsto-8-phosphate [P(8)W(48)O(184)Fe(16)(OH)(28)(H(2)O)(4)](20-) (1) has been synthesised and characterised by IR and ESR spectroscopy, TGA, elemental analyses, electrochemistry and susceptibility measurements. Single-crystal X-ray analyses were carried out on Li(4)K(16)[P(8)W(48)O(184)Fe(16)(OH)(28)(H(2)O)(4)]66 H(2)O2 KCl (LiK-1, orthorhombic space group Pnnm, a=36.3777(9) A, b=13.9708(3) A, c=26.9140(7) A, and Z=2) and on the corresponding mixed sodium-potassium salt Na(9)K(11)[P(8)W(48)O(184)Fe(16)(OH)(28)(H(2)O)(4)].100 H(2)O (NaK-1, monoclinic space group C2/c, a=46.552(4) A, b=20.8239(18) A, c=27.826(2) A, beta=97.141(2) degrees and Z=4). Polyanion 1 contains--in the form of a cyclic arrangement--the unprecedented {Fe(16)(OH)(28)(H(2)O)(4)}(20+) nanocluster, with 16 edge- and corner-sharing FeO(6) octahedra, grafted on the inner surface of the crown-shaped [H(7)P(8)W(48)O(184)](33-) (P(8)W(48)) precursor. The synthesis of 1 was accomplished by reaction of different iron species containing Fe(II) (in presence of O(2)) or Fe(III) ions with the P(8)W(48) anion in aqueous, acidic medium (pH approximately 4), which can be regarded as an assembly process under confined geometries. One fascinating aspect is the possibility to model the uptake and release of iron in ferritin. The electrochemical study of 1, which is stable from pH 1 through 7, offers an interesting example of a highly iron-rich cluster. The reduction wave associated with the Fe(III) centres could not be split in distinct steps independent of the potential scan rate from 2 to 1000 mV s(-1); this is in full agreement with the structure showing that all 16 iron centres are equivalent. Polyanion 1 proved to be efficient for the electrocatalytic reduction of NO(x), including nitrate. Magnetic and variable frequency EPR measurements on 1 suggest that the Fe(III) ions are strongly antiferromagnetically coupled and that the ground state is tentatively spin S=2.

Electronic structures of active sites in electron transfer metalloproteins: contributions to reactivity

Abstract Many electron transfer centers in biology involve metal complexes, that exhibit unique spectral features. These reflect highly covalent electronic structures, which contribute to the electron transfer function of the protein. The blue copper center has a highly covalent copperthiolate bond, which promotes long range electron transfer. The Cu A center is a mixed valence binuclear complex that is completely delocalized even in low symmetry protein environments. The [2Fe2S] center is valence localized in the mixed valence Fe(III)Fe(II) oxidation state, while the mixed valence [2Fe2S] sub-sites in [4Fe4S] clusters are completely valence delocalized. Factors which contribute to electron localization/delocalization in these mixed valence sites are experimentally evaluated using a variety of spectroscopic and electronic structural methods. These include the very powerful technique of ligand K-edge X-ray absorption spectroscopy for determining the covalency of ligandmetal bonds.

Exchange Interactions and Zero-Field Splittings in C3-Symmetric MnIII6FeIII: Using Molecular Recognition for the Construction of a Series of High Spin Complexes Based on the Triplesalen Ligand

The reaction of the tris(tetradentate) triplesalen ligand H6talen(t-Bu2), which provides three salen-like coordination environments bridged in a meta-phenylene arrangement by a phloroglucinol backbone, with Mn(II) salts under aerobic conditions affords, in situ, the trinuclear Mn(III) triplesalen complexes [(talen(t-Bu2)){Mn(III)(solv)n}3]3+. These can be used as molecular building blocks in the reaction with [Fe(CN)6]3- as a hexaconnector to form the heptanuclear complex [{(talen(t-Bu2)){Mn(III)(solv)n}3}2{Fe(III)(CN)6}]3+ ([Mn(III)6Fe(III)]3+). The regular ligand folding observed in the trinuclear triplesalen complexes preorganizes the three metal ions for the reaction of three facially coordinated nitrogen atoms of a hexacyanometallate and provides a driving force for the formation of the heptanuclear complexes [M(t)6M(c)]n+ (M(t), terminal metal ion of the triplesalen building block; M(c), central metal ion of the hexacyanometallate) by molecular recognition, as has already been demonstrated for the single-molecule magnet [Mn(III)6Cr(III)]3+. [{(talen(t-Bu2))(Mn(III)(MeOH))3}2{Fe(III)(CN)6}][Fe(III)(CN)6] (1) was characterized by single-crystal X-ray diffraction, FTIR, ESI- and MALDI-TOF-MS, Mössbauer spectroscopy, and magnetic measurements. The molecular structure of [Mn(III)6Fe(III)]3+ is overall identical to that of [Mn(III)6Cr(III)]3+ but exhibits a different ligand folding of the Mn(III) salen subunits with a helical distortion. The Mössbauer spectra demonstrate a stronger distortion from octahedral symmetry for the central [Fe(CN)6]3- in comparison to the ionic [Fe(CN)6]3-. At low temperatures in zero magnetic fields, the Mössbauer spectra show magnetic splittings indicative of slow relaxation of the magnetization on the Mössbauer time scale. Variable-temperature-variable-field and mu(eff) versus T magnetic data have been analyzed in detail by full-matrix diagonalization of the appropriate spin-Hamiltonian, consisting of isotropic exchange, zero-field splitting, and Zeeman interaction taking into account the relative orientation of the D tensors. Satisfactory reproduction of the experimental data has been obtained for parameters sets J(Mn-Mn) = -(0.85 +/- 0.15) cm(-1), J(Fe-Mn) = +(0.70 +/- 0.30) cm(-1), and D(Mn) = -(3.0 +/- 0.7) cm(-1). Comparing these values to those of [Mn(III)6Cr(III)]3+ provides insight into why [Mn(III)6Fe(III)]3+ is not a single-molecule magnet.

Trinuclear C3-Symmetric Extension of Jacobsen’s Catalyst: Synthesis, Characterization, and Catalytic Properties of a Chiral Trinuclear MnIII Triplesalen Complex

The synthesis of a chiral version of a triplesalen ligand has been performed in two steps starting from 2,4,6-triacetyl-1,3,5-trihydroxybenzene (1). Reaction with excess trans-(1R,2R)-1,2-cyclohexanediamine and trans-(1S,2S)-1,2-cyclohexanediamine provided the chiral triplesalen half units 2,4,6-tris[1-((1R,2R)-2-aminocyclohexylimino)ethyl]-1,3,5-trihydroxybenzene (2(RR)) and 2,4,6-tris[1-((1S,2S)-2-aminocyclohexylimino)ethyl]-1,3,5-trihydroxybenzene (2(SS)), respectively. The two enantiomeric pure triplesalen ligands H(6)chand(RR) and H(6)chand(SS) were obtained by reaction of the triplesalen half units with 3,5-di-tert-butylsalicylaldehyde. Reaction with MnCl(2) x 2 H(2)O under basic aerobic conditions afforded the chiral trinuclear triplesalen complexes 3(RR) and 3(SS). Single-crystal X-ray diffraction studies on both enantiomers showed the presence of the two ionization isomers [(chand){Mn(III)Cl(MeOH)}(3)] and [(chand){Mn(III)(MeOH)(2)}(3)]Cl(3) in the solid state, resulting in the formulation of 3 (either 3(RR) or 3(SS)) as [(chand){Mn(III)Cl(MeOH)}(3)][(chand){Mn(III)(MeOH)(2)}(3)]Cl(3). The crystal structures exhibit chiral hydrophobic channels of approximately 8 A diameter decorated with tert-butyl groups. These form left-handed helices in the 3(RR) enantiomer and right-handed helices in the 3(SS) enantiomer. Magnetic measurements are in accord with weak exchange interactions between the Mn(III) S(i) = 2 ions and strong local magnetic anisotropy as has been found in other trinuclear Mn(III)(3) triplesalen complexes. As a proof-of principal, we have investigated the catalytic ability of the two enantiomers in the enantioselective epoxidation of unfunctionalized olefins. The chiral trinuclear Mn(III)(3) triplesalen acts under non-optimized conditions as a catalyst with relatively good yields and moderate enantiomeric excess.

Probing the radialene-character in triplesalophen ligands by spectroscopic and structural analysis.

The triplesalen ligand system based on three salen-like coordination environments bridged by a common phloroglucinol ring has been designed and successfully applied for the rational synthesis of single-molecule magnets from two trinuclear triplesalen complexes and one hexacyanometallate by supramolecular recognition. In order to optimize this system with respect to magnetic anisotropy, the triplesalophen ligand system has been identified, which should allow for the synthesis of nonanuclear complexes composed of two trinuclear triple-salophen complexes and three connecting units. Herein, the convergent synthesis of the triplesalophen ligand system is described, which differs fom the divergent strategy for the triplesalen ligand system. The molecular and elecrtronic structures of the triplesalophen ligands H(6)baron(R) (R = Me, Cl, Br) have been established by single-crystal X-ray diffraction, NMR, FTIR, and UV-vis spectroscopies. These complementary methods allowed the assignment of the central compartment not to be in the O-protonated tautomer but in the N-protonated tautomer with the prevalence of a keto-enamine resonance description, which resembles a heteroradialene. Furthermore, the comparison with the mononucleating unsymmetrical salophen reference ligand H(2)carl(Cl) and with compounds from the literature provides unique signatures for the appearance of the heteroradialene motif not only in NMR spectra and structural parameters but also in IR and UV-vis spectra. These signatures form the basis for the interpretation and understanding of the electronic structures of transition metal complexes with the triplesalophen ligand system.

Environmental Influence on the Single-Molecule Magnet Behavior of [MnIII6CrIII]3+: Molecular Symmetry versus Solid-State Effects

The structural, spectroscopic, and magnetic properties of a series of [Mn(III)(6)Cr(III)](3+) (= [{(talen(t-Bu(2)))Mn(III)(3)}(2){Cr(III)(CN)(6)}](3+)) compounds have been investigated by single-crystal X-ray diffraction (XRD), Fourier transform infrared (FT-IR) and electronic absorption spectroscopy, elemental analysis, electro spray ionization-mass spectrometry (ESI-MS) and matrix-assisted laser desorption ionization-mass spectrometry (MALDI-MS), cyclic voltammetry, AC and DC magnetic measurements, as well as theoretical analysis. The crystal structures obtained with [Cr(III)(CN)(6)](3-) as a counterion exhibit (quasi-)one-dimensional (1D) chains formed by hydrogen-bonded (1) or covalently linked (2) trications and trianions. The rod-shaped anion lactate enforces a rod packing of the [Mn(III)(6)Cr(III)](3+) complexes in the highly symmetric space group R3[overline] (3) with a collinear arrangement of the molecular S(6) axes. Incorporation of the spherical anion BPh(4)(-) leads to less-symmetric crystal structures (4-6) with noncollinear orientations of the [Mn(III)(6)Cr(III)](3+) complexes, as evidenced by the angle between the approximate molecular C(3) axes taking no specific values in the range of 2°-69°. AC magnetic measurements on freshly isolated crystals (1a and 3a-6a), air-dried crystals (3b-6b), and vacuum-dried powder samples (3c-6c) indicate single-molecule magnet (SMM) behavior for all samples with U(eff) values up to 28 K. The DC magnetic data are analyzed by a full-matrix diagonalization of the appropriate spin-Hamiltonian including isotropic exchange, zero-field splitting, and Zeeman interaction, taking into account the relative orientation of the D-tensors. Simulations for 3a-6a and 3c-6c indicate a weak antiferromagnetic exchange between the Mn(III) ions in the trinuclear subunits (J(Mn-Mn) = -0.70 to -0.85 cm(-1), Ĥ(ex) = -2∑(i<j )J(ij)Ŝ(i)·Ŝ(j)) that is overcome by the stronger antiferromagnetic interaction via the Cr-C≡N-Mn pathway (J(Cr-Mn) = -3.00 to -5.00 cm(-1)), leading to an overall ferrimagnetic coupling scheme with an S(t) = (21)/(2) spin ground state. The differences in U(eff), J(Mn-Mn), and J(Cr-Mn) for the investigated samples are rationalized in terms of subtle variations in the molecular and crystal structures. In particular, a magnetostructural correlation between the Mn-N(C≡N) bond length and the J(Cr-Mn) exchange coupling is inferred from the magnetic measurements and corroborated by DFT calculations. The results of this detailed study on [Mn(III)(6)Cr(III)](3+) allow the formulation of some key recipes for a rational improvement of the SMM behavior.

Molecular and Electronic Structures of Dinuclear Iron Complexes Incorporating Strongly Electron-Donating Ligands: Implications for the Generation of the One- and Two-Electron Oxidized Forms

The ligands (L(t-Bu(2)))(2-), (L(Me(2)))(2-), and (L(Cl(2)))(2-) have been employed for the synthesis of the dinuclear Fe(III) complexes [L(t-Bu(2))Fe(μ-O)FeL(t-Bu(2))], [L(Me(2))Fe(μ-O)FeL(Me(2))], and [L(Cl(2))Fe(μ-O)FeL(Cl(2))]. The strongly electron-donating groups (tert-amines and phenolates) were chosen to increase the electron density at the coordinated ferric ions and thus to facilitate the oxidation of the complexes, with the possibility of fine-tuning the electronic structures by variation of the remote substituents. Molecular structures established in the solid (by single-crystal X-ray diffraction) and in solution (by X-ray absorption spectroscopy) show that the Fe ions are five-coordinate in a square-pyramidal coordination environment with the ligand adopting a trans-conformation. Spectroscopic and magnetic characterization establishes the highly covalent nature of the Fe(III)-O(oxo) and Fe(III)-O(Ph) bonds. The variations in the donor capabilities of the phenolates (due to changes in the remote substituents) are compensated for by a flexible electron donation of the Fe(III)-O(oxo) bonding. Spectroelectrochemical characterization demonstrates that [L(t-Bu(2))Fe(μ-O)FeL(t-Bu(2))] can be oxidized reversibly at +0.27 and +0.44 V versus Fc(+)/Fc, whereas [L(Me(2))Fe(μ-O)FeL(Me(2))] and [L(Cl(2))Fe(μ-O)FeL(Cl(2))] exhibit irreversible oxidations at +0.29 and +0.87 V versus Fc(+)/Fc, respectively. UV-vis, electron paramagnetic resonance (EPR), X-ray absorption spectroscopy (XAS), and Mössbauer spectroscopy show that the successive oxidations of [L(t-Bu(2))Fe(μ-O)FeL(t-Bu(2))] are ligand-centered leading to the monophenoxyl radical complex [(•)L(t-Bu(2))Fe(III)(μ-O)Fe(III)L(t-Bu(2))](+) (with the oxidation primarily localized on one-half of the molecule) and the diphenoxyl radical complex [(•)L(t-Bu(2))Fe(III)(μ-O)Fe(III•)L(t-Bu(2))](2+). Both products are unstable in solution and decay by cleavage of an Fe(III)-O(oxo) bond. The two-electron oxidized species is more stable because of two equally strong Fe(III)-O(oxo) bonds, whereas in the singly oxidized species the Fe(III)-O(oxo) bond of the non-oxidized half is weakened. The decay of the monocation results in the formation of [L(t-Bu(2))Fe(III)](+) and [L(t-Bu(2))Fe(IV)=O], while the decay of the dication yields [(•)L(t-Bu(2))Fe(III)](2+) and [L(t-Bu(2))Fe(IV)=O]. Follow-up reactions of the oxidized fragments with the counteranion of the oxidant, [SbCl(6)](-), leads to the formation of [Fe(III)Cl(4)](-).

Strong and Anisotropic Superexchange in the Single-Molecule Magnet (SMM) [MnIII6OsIII]3+: Promoting SMM Behavior through 3d–5d Transition Metal Substitution

The reaction of the in situ generated trinuclear triplesalen complex [(talent-Bu2)MnIII3(solv)n]3+ with (Ph4P)3[OsIII(CN)6] and NaClO4·H2O affords [MnIII6OsIII](ClO4)3 (= [{(talent-Bu2)MnIII3}2{OsIII(CN)6}](ClO4)3) in the presence of the oxidizing agent [(tacn)2NiIII](ClO4)3 (tacn =1,4,7-triazacyclononane), while the reaction of [(talent-Bu2)MnIII3(solv)n]3+ with K4[OsII(CN)6] and NaClO4·H2O yields [MnIII6OsII](ClO4)2 under an argon atmosphere. The molecular structure of [MnIII6OsIII]3+ as determined by single-crystal X-ray diffraction is closely related to the already published [MnIII6Mc]3+ complexes (Mc = CrIII, FeIII, CoIII, MnIII). The half-wave potential of the OsIII/OsII couple is E1/2 = 0.07 V vs Fc+/Fc. The FT-IR and electronic absorption spectra of [MnIII6OsII]2+ and [MnIII6OsIII]3+ exhibit distinct features of dicationic and tricationic [MnIII6Mc]n+ complexes, respectively. The dc magnetic data (μeff vs T, M vs B, and VTVH) of [MnIII6OsII]2+ are successfully simulated by a full-matrix diagonalization of a spin-Hamiltonian including isotropic exchange, zero-field splitting with full consideration of the relative orientation of the D-tensors, and Zeeman interaction, indicating antiferromagnetic MnIII–MnIII interactions within the trinuclear triplesalen subunits (JMn–Mn(1) = −(0.53 ± 0.01) cm–1, Ĥex = −2∑i<j JijŜi·Ŝj) as well as across the central OsII ion (JMn–Mn(2,cis) = −(0.06 ± 0.01) cm–1, JMn–Mn(2,trans) = −(0.15 ± 0.01) cm–1), while DMn = −(3.9 ± 0.1) cm–1. The μeff vs T data of [MnIII6OsIII]3+ are excellently reproduced assuming an anisotropic Ising-like OsIII–MnIII superexchange with a nonzero component JOs–Mn(aniso) = −(11.0 ± 1.0) cm–1 along the Os–Mn direction, while JMn–Mn = −(0.9 ± 0.1) cm–1 and DMn = −(3.0 ± 1.0) cm–1. Alternating current measurements indicate a slower relaxation of the magnetization in the SMM [MnIII6OsIII]3+ compared to the 3d analogue [MnIII6FeIII]3+ due to the stronger and anisotropic Mc–MnIII exchange interaction.

Hysteresis in the ground and excited spin state up to 10 T of a [MnIII6MnIII]3+ triplesalen single-molecule magnet

We have synthesized the triplesalen-based single-molecule magnet (SMM) [MnIII6MnIII]3+ as a variation of our SMM [MnIII6CrIII](BPh4)3. The use of the rod-shaped anion lactate (lac) was intended to enforce a rod packing and resulted in the crystallization of [MnIII6MnIII](lac)3 in the highly symmetric space group R. This entails a crystallographic S6 symmetry of the [MnIII6MnIII]3+ molecules, which in addition are all aligned with the crystallographic c axis. Moreover, the molecular environment of each [MnIII6MnIII]3+ molecule is highly symmetric. Single-crystals of [MnIII6MnIII](lac)3 exhibit a double hysteresis at 0.3 K with a hysteretic opening not only for the spin ground state up to 1.8 T, but also for an excited state becoming the ground state at ≈ 3.4 T with a hysteretic opening up to 10 T. Ab initio calculations including spin-orbit coupling establish a non-magnetic behavior of the central MnIII low-spin (l.s.) ion at low temperatures, demonstrating that predictions from ligand-field theory are corroborated in the case of MnIII l.s. by ab intio calculations. Simulations of the field- and temperature-dependent magnetization data indicate that [MnIII6MnIII]3+ is in the limit of weak exchange (J ≪ D) with antiferromagnetic interactions in the trinuclear MnIII3 triplesalen subunits resulting in intermediate S* = 2 spins. Slight ferromagnetic interactions between the two trinuclear MnIII3 subunits lead to a ground state in zero-field that is approximately described by a total spin quantum number S = 4. This ground state exhibits only a very small anisotropy barrier due to the misalignment of the local zero-field splitting tensors. At higher magnetic fields of ≈ 3.4 T, the spin configuration changes to an all-up orientation of the local MnIII spins, with the main part of the Zeeman energy needed for the spin-flip being required to overcome the local MnIII anisotropy barriers, while only minor contributions of the Zeeman energy are needed to overcome the antiferromagnetic interactions. These combined theoretical analyses provide a clear picture of the double-hysteretic behavior of the [MnIII6MnIII]3+ single-molecule magnet with hysteretic openings up to 10 T.

OO Bond Formation Mediated by a Hexanuclear Iron Complex Supported on a Stannoxane Core

In recent years, much attention has been focused on the incorporation of redox-active transition-metal complexes into the dendrimer structure owing to their potential applications in various fields. Also, the antenna-like structure of the dendrimers, in many cases, was found to provide an ideal organization for these chromophores and redox centers to work in synergistic ways in carrying out a number of important transformations. For example, an extensive cooperative effect between the Cu centers was observed during the cleavage of supercoiled DNA catalyzed by a hexanuclear Cu-porphyrin complex, supported on a stannoxane core. The above-mentioned hexaporphyrin assembly was synthesized in high yields and in a single step utilizing the orACHTUNGTRENNUNGganostannoxane approach, whereby n-butyl stannoic acid was made to react with the corresponding porphyrin carboxylic acid in 1:1 stoichiometry in benzene; the molecular structure of the ligand was established on the basis of Sn NMR and DFT calculations. In the present paper we report the synthesis of a non-heme hexanucleating ligand (1) supported on a drum-like stannoxane central core utilizing the same organostannoxane approach (Scheme 1). 1 is characterized by X-ray diffraction, Sn NMR, and infrared methods. Most importantly, we show that the Fe-metalated hexa non-heme assembly, 2, in the presence of 2-(tert-butylsulfonyl)-iodosylbenzene (PhIO), performs a rare O O bond formation reaction, thereby generating a Fe-(O2 · ) Fe superoxo unit. Such a metal mediated O O bond formation step is considered to be the most critical part of dioxygen evolution in photosystem II and hence plays a vital role in the context of attaining a clean renewable energy source. The condensation reaction (Scheme 1) of equimolar amounts of n-butyl stannoic acid and 4-(1,3-bis(2-pyridylmethyl)-2-imidazolidinyl)benzoic acid (L1) in toluene afforded a pale yellow solid 1, whose molecular structure (Scheme 1) shows a giant-wheel arrangement of the six non-heme ligand units with a drum-like stannoxane central core serving as the structural support for the hexanucleating assembly. The general features of the stannoxane framework are found to be similar to the other structurally characterized drum-shaped molecules and have a crystallographic S6 symmetry, so that six tin atoms are crystallographically and chemically equivalent. Sn NMR spectrum of 1 exhibits a sharp singlet at 482.4 ppm (Figure S1 top), which is the characteristic signature for a hexameric organostannoxane [a] S. Kundu, F. F. Pfaff, F. Heims, B. R bay, Dr. B. Braun, Dr. K. Ray Humboldt-Universit t zu Berlin, Institut f r Chemie Brook-Taylor-Strasse 2, 12489 Berlin (Germany) Fax: (+49) 3020937387 E-mail : kallol.ray@chemie.hu-berlin.de [b] Dr. E. Matito Institute of Physics, University of Szczecin Wielkopolska 15, 70451 Szczecin (Poland) [c] Dr. S. Walleck, Prof. Dr. T. Glaser Lehrstuhl f r Anorganische Chemie I Fakult t f r Chemie, Universit t Bielefeld Universit tstr. 25, 33615 Bielefeld (Germany) [d] Dr. J. M. Luis Institut de Qu mica Computacional Department de Qu mica, Facultat de Ci ncies Universitat de Girona, 17071 Girona (Spain) [e] Dr. A. Company Institute of Chemistry: Metalorganic materials Technische Universit t Berlin Strasse des 17. Juni 135, 10623 Berlin (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201102326. Scheme 1. Scheme showing the synthesis of the complexes. Hydrogen atoms and the n-butyl groups on tin have been omitted for clarity. Molecular structures of the hexanuclear ligand 1 and the complex [Fe(L2)ACHTUNGTRENNUNG(CH3CN)2]2+ are determined by X-ray crystallography. Structure of 2 is proposed based on ICP-MS, Sn-NMR, F NMR, IR, Mçssbauer and DFT methods (see the Supporting Information for a color version of Scheme 1).