Optimization of Single-Molecule Magnets by Suppression of Quantum Tunneling of the Magnetization

Oldengott J, Schnack J, Glaser T. Optimization of Single-Molecule Magnets by Suppression of Quantum Tunneling of the Magnetization. European Journal of Inorganic Chemistry. 2020;20(34):3222-3235.The ligand system triplesalen was rationally designed following requirements for polynuclear 3d single‐molecule magnets (SMMs). The essential central part is the C3 symmetric, meta‐phenylene bridging unit phloroglucinol for ferromagnetic interactions via the spin‐polarization mechanism. The triplesalen‐based [MnIII6CrIII]3+ SMMs strongly suppress the quantum tunneling of the magnetization (QTM) but exhibit blocking temperatures not exceeding 2 K. We have analyzed the reason for this behavior and found that the triplesalen ligands are not in the anticipated aromatic phloroglucinol form but in a non‐aromatic heteroradialene form. Here we present our strategies to optimize the triplesalen ligand system to suppress the heteroradialene formation and to enforce ferromagnetic interactions. This allowed us to study in detail the influence of exchange coupling on the QTM and relaxation properties of SMMs and provides valuable insights for further rational improvements of our triplesalen ligand system and of SMMs in general

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

Finke S, Stammler A, Oldengott J, Walleck S, Glaser T. Direct and remote control of electronic structures and redox potentials in mu-oxo diferric complexes. Dalton Transactions. 2023.Non-heme diiron enzymes activate O2 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 via 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 {FeIIIX(mu-O)FeIIIX} (X- = Cl-, OAc-, and OH-) or an {FeIII(mu-O)(mu-OAc)FeIII} core. The complex-induced oxidation of EtOH with H2O2 provides mu-OAc-, and in acetone, the complex with mixed OH-/OAc- exogenous donors is obtained. Both reactivities indicate a reactive fluent peroxo FeIII2 intermediate. The coupling constant J and the LMCT transitions are insensitive to the nature of the directly bound ligands X- and reflect mainly the electronic structure of the central {FeIII(mu-O)FeIII} core, while Mossbauer spectroscopy and d-d transitions probe the local FeIII sites. The remote methoxy substituents decrease the potential for the oxidation to FeIV by 100 mV, while directly bound OH- in {FeIII(OH)(mu-O)FeIII(OH)} with a short 1.91 A FeIII-OOH bond decreases the potential by 590 mV compared to {FeIII(OAc)(mu-O)FeIII(OAc)} with a 2.01 A FeIII-OOAc bond. Interestingly, this FeIII-OH bond is even shorter (1.87 A) in the mixed OH-/OAc- complex but the potential is the mean value of the potentials of the OH-/OH- and OAc-/OAc- complexes, thus reflecting the electron density of the central {FeIII(mu-O)FeIII} core and not of the local FeIII-OH unit

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

Simon J, Stammler A, Oldengott J, Bögge H, Glaser T. Proof of Phosphate Diester Binding Ability of Cytotoxic DNA-Binding Complexes. Inorganic chemistry. 2020;59(19):14615–14619.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 A 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. Although the dinuclear NiII2 and CuII2 bind to DNA, inhibit DNA synthesis, and preferentially kill human cancer cells over fast proliferating human stem cells, the DNA binding mode was elusive. Here, we prove the principle phosphate diester binding ability of this family of dinuclear complexes by a new dinuclear NiII2 complex with dibenzimidazolamine pendant arms. The distance of the oxygen atoms of the coordinated phosphate diesters of 6.5 A confirms the initial design and binding ability to two neighboring phosphate diesters of the DNA backbone. Moreover, the facile exchange of coordinated acetates by phosphate diesters indicates a preferential binding to phosphate diesters

Zero-Field Splittings and Redox Potentials in an Isostructural Series of Dinuclear (FeTiIV)-Ti-II, (FeTiIV)-Ti-III, and (MnTiIV)-Ti-II Complexes with a Face-Sharing Bridging Motive

Liratzis I, Walleck S, Oldengott J, et al. Zero-Field Splittings and Redox Potentials in an Isostructural Series of Dinuclear (FeTiIV)-Ti-II, (FeTiIV)-Ti-III, and (MnTiIV)-Ti-II Complexes with a Face-Sharing Bridging Motive. European Journal of Inorganic Chemistry . 2022.The ligand H(6)ioan has been used to synthesize the three dinuclear complexes [(ioan)(MnTiIV)-Ti-II], [(ioan)(FeTiIV)-Ti-II], and [(ioan)(FeTiIV)-Ti-III](+). The face-sharing bridging mode of the three phenolates provides short M-Ti-IV distances of approximate to 3.0 angstrom. Mossbauer spectra of [(ioan)(FeTiIV)-Ti-III](+) show a magnetically split six-line spectrum at 3 K in zero magnetic field demonstrating a slow magnetic relaxation. Magnetic measurements provide a zero-field splitting of |D|=5 cm(-1) in [(ioan)(FeTiIV)-Ti-II]. EPR spectroscopy demonstrates sizable zero-field splittings of the S=5/2 spin systems of [(ioan)(MnTiIV)-Ti-II] (D=0.246 cm(-1)) and [(ioan)(FeTiIV)-Ti-III](+) (D<-1 cm(-1)) that can be related to enforced covalency of the M-O-ph bonds. [(ioan)(FeTiIV)-Ti-III](+) exhibits a reversible reduction at -0.26 V vs. Fc(+)/Fc demonstrating the facile accessibility of Fe-III and Fe-II. In contrast to an irreversible oxidation in [(ioan)(NiTiIV)-Ti-II] at 0.78 V vs. Fc(+)/Fc, the reversible oxidation at 0.25 V vs. Fc(+)/Fc in [(ioan)(MnTiIV)-Ti-II] indicates even the access of Mn-III. These results indicate that pentanuclear complexes [(ioan)(FeMMMFe)-M-1-M-2-Fe-1(ioan)](n+) are meaningful targets to access electron delocalization in mixed-valence systems over five ions due to the facile accessibility of both Fe-II and Fe-III in the terminal positions. This study provides important local spin-Hamiltonian and Mossbauer parameters that will be essential for the understanding of the potentially complicated electronic structure in the anticipated pentanuclear complexes

Molecular and electronic structures of a series of Dinuclear Co-II Complexes varied by exogeneous ligands: Influence of pi-bonding on redox potentials

Depenbrock F, Limpke T, Stammler A, Oldengott J, Bögge H, Glaser T. Molecular and electronic structures of a series of Dinuclear Co-II Complexes varied by exogeneous ligands: Influence of pi-bonding on redox potentials. European Journal of Inorganic Chemistry . 2022;2022(6): e202100992.Four dinuclear Co-II complexes have been synthesized and structurally characterized with the dinucleating ligand susan (susan - 4,7-di methyl-1,1,10,10-tetra(2-pyridyl methyl)-1,4,7,1 0tetraazadecane) varying in the exogeneous ligands: [(susan)- {Co-II(CH3CN)(2)}(2)](PF6)(4), [(susan){Co-II Cl)(2)](ClO4)(2), [(susan)- {(CoBr)-Br-II}(2)](ClO4)(2), and [(susan){Co-II(mu-OH)COII}](ClO4)(3). The Co-II ions are six-coordinate with CH,CN ligands and five-coordinate with anionic ligands. The electronic absorption spectra reflect the differences in the electronic structures, not only between the six- and five-coordinate complexes, but also between the fivecoordinate complexes. These variations are also reflected in the magnetic properties with the highest orbital angular momen tum contribution in six-coordinate [(susan){Co-II(CH3CN)(2)}(2)](PF6)(4). The lower symmetry in the five-coordinate complexes reduces the orbital angular momentum contribution. The hydroxobridged complex [(susan){Co-II(mu-OH)COII}](ClO4)(3) exhibits an additional antiferromagnetic interaction. The electrochemical characterization reveals that the pi-acceptor ligand CH3CN facilitates not only reduction from Co-II to COI but also oxidation to Co-III while the pi-donor ligands Br-, Cl-, and mu-OH impede both reduction to COI and oxidation to Co-III. [(susan){Co-II- (CH3CN)(2)}(2)](PF6)(4) and [(susan){(CoCl)-Cl-II}(2)](ClO4)(2) show electrocata- lytic proton reduction at a potential of approximate to - 1.9 V vs Fc(+)/Fc that is associated with a ligand-centered reduction

Modelling the Binding of Cytotoxic Dinuclear Copper Complexes to two Neighboring Phosphate Esters of DNA Using Glutarate: A Tetranuclear Metallo‐Macrocycle a Dinuclear Model

Riediger TW, Aydin Z, Stammler A, Oldengott J, Glaser T. Modelling the Binding of Cytotoxic Dinuclear Copper Complexes to two Neighboring Phosphate Esters of DNA Using Glutarate: A Tetranuclear Metallo‐Macrocycle a Dinuclear Model. Zeitschrift für anorganische und allgemeine Chemie. 2024;650(4).A family of dinuclear complexes was rationally designed to bind to two neighboring phosphate diesters of DNA by molecular recognition. Although these complexes bind strongly and irreversibly to DNA and induce apoptosis in cancer cells, a direct proof of the intended binding mode is missing. To mimic two neighboring phosphate diesters of DNA, we use here the dicarboxylate glutarate as a difunctional ligand. Starting from the acetate‐bound CuII2complex, protonation results in the de‐coordination of acetate and allows the synthesis of the glutarate‐bound complex. Instead of bridging two CuIIions of one dinuclear complex, two glutarates bridge two dinuclear complexes resulting in a tetranuclear metallo‐macrocycle. The molecular structures of two different salts differ in the square‐pyramidal coordination to the CuIIions. The carboxylate stretching modes, the d‐d and the CT transtitions are not sensitive enough to resolve these differences. The exchange coupling is weakly antiferromagnetic between the CuIIions in the dinuclear subunits and negligible between the subunitsviathe glutarates. The consequences for suited ligands to model the binding to two neighboring phosphate diesters in the DNA backbone is discussed

Increasing the pi-pi Interactions in Trinuclear Ni-3(II) Triplesalophen

Oldengott J, Roehs FLB, Stammler A, Bögge H, Glaser T. Increasing the pi-pi Interactions in Trinuclear Ni-3(II) Triplesalophen. ZEITSCHRIFT FUER ANORGANISCHE UND ALLGEMEINE CHEMIE. 2017;643(13):819-825.The new triplesalophen ligand H(6)kruse(Br) was synthesized as a variation of the triplesalophen ligands H(6)baron(R) by replacing a phenyl by a methyl group at the terminal ketimine in order to allow closer contacts of trinuclear complexes due to less steric hindrance by the smaller methyl group. The ligand H(6)kruse(Br) was used to synthesize the trinuclear complex [(kruse(Br)) Ni-3(II)], which is insoluble in organic solvents despite the coordinating solvent pyridine. Recrystallization from pyridine results in the complex [(kruse(Br)){Ni-2(Ni(py)(2))}], which was characterized by single-crystal X-ray diffraction. Two Ni-II ions are four-coordinate by the salophen-like subunits while the third Ni-II ion is six-coordinate by two additional pyridine donors. The analysis of the molecular and crystal structure in comparison to that of Ni-3(II) complexes of (baron(R))(6-) reveals that the methyl group in [(kruse(Br)){Ni-2(Ni(py)(2))}] results in less ligand folding and in closer contact distance of two Ni-3(II) complexes by pi-pi interactions of 3.2 angstrom. This indicates that trinuclear complexes of H(6)kruse(Br) are more suitable than complexes of H(6)baron(R) as molecular building blocks for the anticipated synthesis of nonanuclear single-molecule magnets

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

Depenbrock F, Limpke T, Stammler A, Oldengott J, Bögge H, Glaser T. Increasing the electron donation in a dinucleating ligand family: molecular and electronic structures in a series of CoIICoII complexes. Dalton Transactions. 2024.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. As CoII ions are sensitive to variations in donor strength in terms of coordination number, magnetism, UV-Vis-NIR spectra, redox potentials, we probe the electron donation ability of this new ligand in CoIICoII complexes in comparison to the parent CoIICoII complexes with terminal pyridine donors and we synthesize the analogous CoIICoII complexes with terminal 6-methylpyridines and methoxy-substituted pyridines. The molecular structures show indeed strong variations in coordination numbers and bond lengths. These differences in the molecular structures are reflected in the magnetic properties and in the d-d transitions demonstrating that the molecular structures remain intact upon dissolution. The redox potentials are analyzed with respect to the electron donation ability and are the only handle to observe an effect of the methoxy-substituted pyridines. All data taken together show the following order of electron donating ability for the terminal donors: 6-methylpyridines ≪ pyridines < methoxy-substituted pyridines ≪ imidazoles

Synthesis of Asymmetric Variants in a Dinucleating Ligand Family and Application for Dinuclear Copper(II) Complexes

Siebe L, Butenuth C, Köhler A, et al. Synthesis of Asymmetric Variants in a Dinucleating Ligand Family and Application for Dinuclear Copper(II) Complexes. European Journal of Inorganic Chemistry. 2024: e202400142.We developed a dinucleating ligand system with two identical tetradentate coordination compartments and varying terminal donors. Here, we report the synthesis of two differently asymmetric dinucleating ligands: the ligand selma (= 4,7,10-trimethyl-1,1,10-tris(2-pyridylmethyl)-1,4,7,10-tetraazadecane) with one N-3- and one N-4- and the ligand susan(H2Me2) (= 4,7-dimethyl-1,1-bis(6-methylpyridine-2-yl-methyl)-10,10-bis(2-pyridylmethyl)-1,4,7,10-tetraazadecane) with two different N-4-coordination compartments. The initial synthesis of the ligand selma based on extending dipicolylamine with 2-(methylamino)ethyl units in repeated reductive aminations. Synthesis of 1-trifluoroacetyl-4,7-dimethyl-1,4,7,10-tetraazadecane allowed the synthesis of both ligands selma and susan(H2Me2). To study the influence of the terminal donors, the Cu-II chloride complexes of selma and susanH(2)Me(2) and the two symmetric susan and susan(6) (Me) were synthesized. The introduction of the 6-methyl substituents on the pyridines increases the bond lengths, reduces the electron donation, and shifts the reduction potentials anodically. The large 6-methyl substituents enforce a meridional coordination of the dipicolylamine subunits, while it is mainly facial without the 6-methyl substituents. Analysis of the solid-state structures and the d-d transitions in solution demonstrate a flat potential energy surface for the 5-coordinate polyhedra. Analysis of d-d transitions and reduction potentials indicate that the properties of the (CuCuII)-Cu-II complexes are additive for but cooperative for selma reflecting the difference of these asymmetric dinucleating ligands. [GRAPHICS]

Dinuclear Diferrous Complexes of a Bis(tetradentate) Dinucleating Ligand: Influence of the Exogenous Ligands on the Molecular and Electronic Structures

Röhs F, Dammers S, Stammler A, et al. Dinuclear Diferrous Complexes of a Bis(tetradentate) Dinucleating Ligand: Influence of the Exogenous Ligands on the Molecular and Electronic Structures. European Journal of Inorganic Chemistry . 2022;2022(18): e202200177.The ligand susan(6-Me) (susan(6-Me) 4,7-dimethyl-1,1,10,10-tetra(6 methyl-2-pyridylmethyl)-1,4,7,10-tetraazadecane) allows the synthesis of a peroxo complex that is only a transient species under catalytic conditions with the closely related ligand susan (susan - 4,7-dimethyl-1,1,10,10-tetra(2-pyridylmethyl)-1,4,7,10- tetraazadecane). For the synthesis of a peroxo complex - analogous to that of susan(6-Me) we present here the three air- sensitive complexes [(susan){(FeCl2)-Cl-II}(2)], [(susan){Fe-II(OTf)(2)}(2)], and [(susan){Fe-II(mu-OH)(2)Fe-II}](ClO4)(2) as potential precursors. [(susan){Fe-II(mu-OH)(2)Fe-II}](ClO4)(2) shows weak antiferromagnetic coupling and a g -16 EPR signal. Dissolving [(susan){Fe-II(OTf)(2)}(2)] in CH3CN forms Rsusan){Fe-II(CH3CN)(2)}(2)](4+) that shows a spin transition upon cooling as evidenced by UV-Vis, H-1 NMR, and Mbssbauer spectroscopies. In the UV-Vis-NIR spectra the combination of (i) the energy of the MLCT, (ii) the energy, and (iii) the splitting of the d-d transitions provide insight into the overall electron donation and into the different sigma-donor/pi-donor/pi-acceptor capabilities of the exogenous ligands. These experimental signatures provide insight into the molecular structures in solution e.g. during reactions with O-2