A Multistep Oxidative Cascade Reaction from a Naphthalenediol-Based Pre-Ligand to a Tetranuclear Perylenequinone-Based Fe Complex.

We have developed a family of dinuclear complexes using 2,7-disubstituted 1,8-naphthalenediol ligands that bind by molecular recognition to two neighboring phosphate diesters of the DNA backbone with the dinuclear Cu and Ni complexes exhibiting a severe cytotoxicity for human cancer cells. To increase the binding affinity, we intended to synthesize the corresponding dinuclear Fe complex. Surprisingly, we obtained a tetranuclear Fe perylene-based complex instead of the expected dinuclear Fe naphthalene-based complex.

Increasing the electron donation in a dinucleating ligand family: molecular and electronic structures in a series of CoCo complexes.

We have developed a family of dinucleating ligands with varying terminal donors to generate dinuclear peroxo and high-valent complexes and to correlate their stabilities and reactivities with their molecular and electronic structures as a function of the terminal donors. It appears that the electron-donating ability of the terminal donors is an important handle for controlling these stabilities and reactivities. Here, we present the synthesis of a new dinucleating ligand with potentially strong donating terminal imidazole donors.

Generation and Reactivity of μ-1,2-Peroxo CuCu and Bis-μ-oxo CuCu Species and Catalytic Hydroxylation of Benzene to Phenol with Hydrogen Peroxide.

Tetradentate-N ligands stabilize dinuclear {Cu(μ-1,2-peroxo)Cu} and {Cu(μ-O)Cu} species, and Cu complexes of these ligands were reported to catalyze the oxidation of benzene with HO. Here, we report {Cu(μ-1,2-peroxo)Cu} and {Cu(μ-O)Cu} intermediates of dinucleating bis(tetradentate-N) ligands depending on the absence or presence of 6-methyl substituents on the terminal pyridine donors, respectively, generated either from {CuCu} precursors with O or from {CuCu} precursors with HO and NEt. Both intermediates are not stable even at low temperatures, but they show no electrophilic HAT reactivity with DHA.

Direct and remote control of electronic structures and redox potentials in μ-oxo diferric complexes.

Non-heme diiron enzymes activate O for the oxidation of substrates in the form of peroxo FeIII2 or high-valent FeIV2 intermediates. We have developed a dinucleating bis(tetradentate) ligand system that stabilizes peroxo and hydroperoxo FeIII2 complexes with terminal 6-methylpyridine donors, while the peroxo FeIII2 intermediate is reactive with terminal pyridine donors presumably conversion to a fluent high-valent FeIV2 intermediate. We present here a derivative with electron-donating methoxy substituents at the pyridine donors and its diferric complexes with an {FeX(μ-O)FeX} (X = Cl, OAc, and OH) or an {Fe(μ-O)(μ-OAc)Fe} core.

Reactivities and Electronic Structures of μ-1,2-Peroxo and μ-1,2-Superoxo CoCo Complexes: Electrophilic Reactivity and O Release Induced by Oxidation.

Peroxo complexes are key intermediates in water oxidation catalysis (WOC). Cobalt plays an important role in WOC, either as oxides CoO or as {Co(μ1,2-peroxo)Co} complexes, which are the oldest peroxo complexes known. The oxidation of {Co(μ1,2-peroxo)Co} complexes had usually been described to form {Co(μ1,2-superoxo)Co} complexes; however, recently the formation of {Co(μ1,2-peroxo)Co} species were suggested.

Generation of a μ-1,2-hydroperoxo FeFe and a μ-1,2-peroxo FeFe Complex.

μ-1,2-Peroxo-diferric intermediates (P) of non-heme diiron enzymes are proposed to convert upon protonation either to high-valent active species or to activated P' intermediates via hydroperoxo-diferric intermediates. Protonation of synthetic μ-1,2-peroxo model complexes occurred at the μ-oxo and not at the μ-1,2-peroxo bridge. Here we report a stable μ-1,2-peroxo complex {Fe(μ-O)(μ-1,2-O)Fe} using a dinucleating ligand and study its reactivity.

Evaluation of the binding mode of a cytotoxic dinuclear nickel complex to two neighboring phosphates of the DNA backbone.

A family of dinuclear complexes based on 2,7-disubstituted 1,8-naphthalenediol-ligands has been designed to bind covalently to two neighboring phosphate diester groups in the backbone of DNA. The dinuclear Cu and Ni complexes bind to DNA resulting in the inhibition of DNA synthesis in PCR experiments and in a cytotoxicity that is stronger for human cancer cells than for human stem cells of the same proliferation rate. These experiments support but cannot prove that the dinuclear complexes bind as intended to two neighboring phosphate ester groups of the DNA backbone.

Catalytic HO Activation by a Diiron Complex for Methanol Oxidation.

In nature, C-H bond oxidation of CH involves a peroxo intermediate that decays to the high-valent active species of either a "closed" {Fe(μ-O)Fe} core or an "open" {Fe(O)(μ-O)Fe(O)} core. To mimic and to obtain more mechanistic insight in this reaction mode, we have investigated the reactivity of the bioinspired diiron complex [(susan){Fe(OH)(μ-O)Fe(OH)}] [susan = 4,7-dimethyl-1,1,10,10-tetrakis(2-pyridylmethyl)-1,4,7,10-tetraazadecane], which catalyzes CHOH oxidation with HO to HCHO and HCOH. The kinetics is faster in the presence of a proton.

Proof of Phosphate Diester Binding Ability of Cytotoxic DNA-Binding Complexes.

We have rationally designed a family of dinuclear transition-metal complexes to bind two neighboring phosphate diester groups of DNA. The two metal ions are positioned at the distance of two neighboring phosphate diesters in DNA of 6-7 Å by a 1,8-naphthalenediol backbone. Two sterically demanding dipicolylamine pendant arms in the 2 and 7 positions stabilize coordination of the metal ions and prevent coordination to the less exposed nucleobases of DNA.

Rational Improvement of Single-Molecule Magnets by Enforcing Ferromagnetic Interactions.

The anisotropy barrier of polynuclear single-molecule magnets is expected to be higher with less tunneling the better stabilized the spin ground state is so that less M mixing in the ground state and with excited spin states occur. We have realized this experimentally in two structurally related heptanuclear SMMs: the triplesalen-based [Mn Cr ] and the triplesalalen-based *[Mn Cr ] . The ligand system triplesalen was developed to enforce ferromagnetic interactions by the spin-polarization mechanism.

Two Unsupported Terminal Hydroxido Ligands in a μ-Oxo-Bridged Ferric Dimer: Protonation and Kinetic Lability Studies.

The dinuclear complex [(susan){Fe(OH)(μ-O)Fe(OH)}](ClO) (Fe(OH)(ClO); susan = 4,7-dimethyl-1,1,10,10-tetra(2-pyridylmethyl)-1,4,7,10-tetraazadecane) with two unsupported terminal hydroxido ligands and for comparison the fluorido-substituted complex [(susan){FeF(μ-O)FeF}](ClO) (FeF(ClO)) have been synthesized and characterized in the solid state as well in acetonitrile (CHCN) and water (HO) solutions. The Fe-OH bonds are strongly modulated by intermolecular hydrogen bonds (1.85 and 1.

Reversible Carboxylate Shift in a μ-Oxo Diferric Complex in Solution by Acid-/Base-Addition.

A reversible carboxylate shift has been observed in a μ-oxo diferric complex in solution by UV-vis-NIR and FTIR spectroscopy triggered by the addition of a base or an acid. A terminal acetate decoordinates upon the addition of a proton, resulting in a shift of the remaining terminal acetato to a μ-η:η bridge. The addition of a base restores the original structure containing only terminal acetates.

Suppression of Magnetic Quantum Tunneling in a Chiral Single-Molecule Magnet by Ferromagnetic Interactions.

Single-molecule magnets (SMMs) retain a magnetization without applied magnetic field for a decent time due to an energy barrier U for spin-reversal. Despite the success to increase U, the difficult to control magnetic quantum tunneling often leads to a decreased effective barrier U and a fast relaxation. Here, we demonstrate the influence of the exchange coupling on the tunneling probability in two heptanuclear SMMs hosting the same spin-system with the same high spin ground state S = 21/2.

A Mixed-Valence Fluorido-Bridged FeFe Complex.

The reaction of the new dinucleating ligand susan with Fe(BF)·6HO results in formation of the homovalent FeFe complex [(susan){Fe(μ-F)Fe}] and the mixed-valence FeFe complex [(susan){FeF(μ-F)FeF}] depending on the absence or presence of dioxygen, respectively. Complex [(susan){FeF(μ-F)FeF}] is the first molecular mixed-valence complex with a fluorido bridge. The short Fe-μ-F bond of 1.

Design and synthesis of a dinucleating ligand system with varying terminal donor functions that provides no bridging donor and its application to the synthesis of a series of Fe(III)-μ-O-Fe(III) complexes.

Based on a rational ligand design for stabilizing high-valent {Fe(μ-O)2Fe} cores, a new family of dinucleating bis(tetradentate) ligands with varying terminal donor functions has been developed: redox-inert biomimetic carboxylates in H4julia, pyridines in susan, and phenolates in H4hilde(Me2). Based on a retrosynthetic analysis, the ligands were synthesized and used for the preparation of their diferric complexes [(julia){Fe(OH2)(μ-O)Fe(OH2)}]·6H2O, [(julia){Fe(OH2)(μ-O)Fe(OH2)}]·7H2O, [(julia){Fe(DMSO)(μ-O)Fe(DMSO)}]·3DMSO, [(hilde(Me2)){Fe(μ-O)Fe}]·CH2Cl2, [(hilde(Me2)){FeCl}2]·2CH2Cl2, [(susan){FeCl(μ-O)FeCl}]Cl2·2H2O, [(susan){FeCl(μ-O)FeCl0.75(OCH3)0.

Enhancing the ferromagnetic coupling in extended phloroglucinol complexes by increasing the metal SOMO-ligand overlap: synthesis and characterization of a trinuclear Co(II)(3) triplesalophen complex.

The triplesalophen complex [(baron(Me))Co(II)(3)] has been synthesized and characterized. The low-spin Co(II) ions possess an (2)A2 ground state with the magnetic orbitals of dyz type. These are well oriented for a strong π overlap with the bridging phloroglucinol, which results in the strongest ferromagnetic interactions by the spin-polarization mechanism for a 3d phloroglucinol complex.

Rational design of a cytotoxic dinuclear Cu2 complex that binds by molecular recognition at two neighboring phosphates of the DNA backbone.

The mechanism of the cytotoxic function of cisplatin and related anticancer drugs is based on their binding to the nucleobases of DNA. The development of new classes of anticancer drugs requires establishing other binding modes. Therefore, we performed a rational design for complexes that target two neighboring phosphates of the DNA backbone by molecular recognition resulting in a family of dinuclear complexes based on 2,7-disubstituted 1,8-naphthalenediol.

Aromatic versus heteroradialene character in extended thiophloroglucinol ligands and their trinuclear nickel(II) complexes.

Extended phloroglucinol ligands and complexes are best described as nonaromatic heteroradialenes. Herein, the electronic structures of extended thiophloroglucinol ligands and their Ni(II) 3 complexes are evaluated by comparison to their phloroglucinol analogs by means of NMR, FTIR, UV/Vis, and structural parameters. To provide a full set of compounds for this comparison of S versus O substitution, a new triplesalen ligand, its Ni(II) 3 complex, and a new thiophloroglucinol were synthesized.

Switching from antiferromagnetic to ferromagnetic coupling in heptanuclear [M(t)6M(c)](n+) complexes by going from an achiral to a chiral triplesalen ligand.

The chiral triplesalen ligand H6chand(RR) has been used to synthesize the chiral heptanuclear complexes [{(chand(RR))Mn(III)3}2{Fe(II)(CN)6}](ClO4)2 ((RR)[Mn(III)6Fe(II)](ClO4)2) and [{(chand(RR))Fe(III)3}2{Fe(II)(CN)6}](ClO4)2 ((RR)[Fe(III)6Fe(II)](ClO4)2), which have been characterized by single-crystal X-ray diffraction, mass spectrometry, elemental analysis, FT-IR, Mössbauer, and UV-vis spectroscopies, electrochemistry, as well as DC and AC magnetic susceptibility measurements. The half-wave potential of the Fe(III)/Fe(II) couple in (RR)[Mn(III)6Fe(II)](2+) and (RR)[Fe(III)6Fe(II)](2+) is E1/2 = +0.21 and +0.

A streamlined synthesis of extended thiophloroglucinol ligands and their trinuclear NiII3 complexes.

A protocol for the synthesis of trinucleating C3-symmetric ligands based on a central meta-phenylene bridging 1,3,5-trimercaptobenzene (thiophloroglucinol) backbone has been established. The key compound turned out to be the trialdehyde obtained from the triple nucleophilic attack of dimethyldithiocarbamate at 1,3,5-tribromo-2,4,6-triformylbenzene. Reacting this trialdehyde with six equivalents of a primary amine results in the simultaneous dithiocarbamate cleavage and Schiff-base formation providing the extended thiophloroglucinol ligands H3bertdien, H6bert(Me), H6bert(t-Bu2), and H6habbi.

Strong and anisotropic superexchange in the single-molecule magnet (SMM) [MnIII(6)OsIII]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.

Environmental influence on the single-molecule magnet behavior of [Mn(III)6Cr(III)]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.

Structural influences on the exchange coupling and zero-field splitting in the single-molecule magnet [Mn(III)6Mn(III)]3+.

A comprehensive synthetic, structural, mass spectrometrical, FT-IR and UV/Vis spectroscopic, electrochemical, and magnetic study on [Mn(III)(6)Mn(III)](3+) (= [{(talen(t-Bu(2)))Mn(III)(3)}(2){Mn(III)(CN)(6)}](3+)) is presented. The high stability of [Mn(III)(6)Mn(III)](3+) in solution allows the preparation of different salts and solvates: [Mn(III)(6)Mn(III)](BPh(4))(3)·3MeOH·3MeCN·3Et(2)O (), [Mn(III)(6)Mn(III)(MeOH)(4)](BPh(4))(3)·5MeOH (), [Mn(III)(6)Mn(III)(MeOH)(6)](BF(4))(3)·9MeOH (), [Mn(III)(6)Mn(III)(MeOH)(6)](PF(6))(2)(OAc)·11MeOH (), and [Mn(III)(6)Mn(III)(MeOH)(6)](lactate)(3)·5MeOH·10H(2)O (). The molecular structure of [Mn(III)(6)Mn(III)](3+) is closely related to the already published [Mn(III)(6)M(c)](3+) complexes (M(c) = Cr(III), Fe(III), Co(III)).