Electronic Structure and Magnetic Anisotropy of an Unsaturated Cyclopentadienyl Iron(I) Complex with 15 Valence Electrons

The 15 valence-electron iron(I) complex [Cp^ArFe(IiPr₂Me₂)] ( 1 , Cp^Ar=C₅(C₆H₄-4-Et)₅; IiPr₂Me₂=1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene) was synthesized in high yield from the Fe^II precursor [Cp^ArFe(μ-Br)]₂. ⁵⁷Fe Mössbauer and EPR spectroscopic data, magnetic measurements, and ab initio ligand-field calculations indicate an S= 3/2 ground state with a large negative zero-field splitting. As a consequence, 1 features magnetic anisotropy with an effective spin-reversal barrier of U_eff=64 cm⁻¹. Moreover, 1 catalyzes the dehydrogenation of N,N-dimethylamine–borane, affording tetramethyl-1,3-diaza-2,4-diboretane under mild conditions.     

Synthesis,Structure, Magnetic Properties,and Catalase‐like Activity of Methoxy‐bridged Manganese(III) Coordination Polymer containing Hydrazone based (O,N,O)2‐Donor Ligand

A 1D coordination polymer of manganese(III) with a hydrazone‐based ligand, [Mn₂(L)(μ‐OCH₃)₂(OHCH₃)₂]_n ( 1 ), was synthesized and characterized by elemental analyses and spectroscopic methods {H₄L = bis[(2‐hydroxynaphthalen‐1‐yl)methylene]adipohydrazide}. The crystal structure of 1 was determined by X‐ray crystallography. The two dianionic domains of the ligand adopt trans configuration, and each coordinates in a tridentate mode via the O, N, O′‐donor atoms to a Mn^III ion forming a dinuclear compound. The methoxy ligands provide an asymmetric bridge between two central manganese atoms, which lead to the formation of a 1D coordination polymer. A 2D supramolecular structure is formed by hydrogen bonding interactions between the 1D chains. Although the methoxy ligands are labile, the polymer preserves its oligonuclearity in the solution. Temperature‐dependent magnetic susceptibility studies proved the presence of a weak antiferromagnetic interaction between manganese(III) ions with J = –3.2 cm^–1, which results from axial distortion of the manganese coordination environment. Compound 1 showed catalase‐like activity in disproportionation of H₂O₂.     

A new family of 1D exchange biased heterometal single-molecule magnets: observation of pronounced quantum tunneling steps in the hysteresis loops of quasi-linear {Mn2Ni3} clusters

First members of a new family of heterometallic Mn/Ni complexes [Mn(2)Ni(3)X(2)L(4)(LH)(2)(H(2)O)(2)] (X = Cl: 1; X = Br: 2) with the new ligand 2-{3-(2-hydroxyphenyl)-1H-pyrazol-1-yl}ethanol (H(2)L) have been synthesized, and single crystals obtained from CH(2)Cl(2) solutions have been characterized crystallographically. The molecular structures feature a quasi-linear Mn(III)-Ni(II)-Ni(II)-Ni(II)-Mn(III) core with six-coordinate metal ions, where elongated axes of all the distorted octahedral coordination polyhedra are aligned parallel and are fixed with respect to each other by intramolecular hydrogen bonds. 1 and 2 exhibit quite strong ferromagnetic exchange interactions throughout (J(Mn-Ni) ≈ 40 K (1) or 42 K (2); J(Ni-Ni) ≈ 22 K (1) or 18 K (2)) that lead to an S(tot) = 7 ground state, and a sizable uniaxial magnetoanisotropy with D(mol) values -0.55 K (1) and -0.45 K (2). These values are directly derived also from frequency- and temperature-dependent high-field EPR spectra. Slow relaxation of the magnetization at low temperatures and single-molecule magnet (SMM) behavior are evident from frequency-dependent peaks in the out-of-phase ac susceptibilities and magnetization versus dc field measurements, with significant energy barriers to spin reversal U(eff) = 27 K (1) and 22 K (2). Pronounced quantum tunnelling steps are observed in the hysteresis loops of the temperature- and scan rate-dependent magnetization data, but with the first relaxation step shifted above (1) or below (2) the zero crossing of the magnetic field, despite the very similar molecular structures. The different behavior of 1 and 2 is interpreted in terms of antiferromagnetic (1) or ferromagnetic (2) intermolecular interactions, which are discussed in view of the subtle differences of intermolecular contacts within the crystal lattice.     

Selective benzylic C-C coupling catalyzed by a bioinspired dicopper complex

A highly preorganized bioinspired dicopper complex with imidazole ligation catalyzes the selective benzylic para-C-H activation of 2,4,6-trimethylphenol under aerobic conditions, yielding either the stilbenequinone or 4-methoxymethyl-2,6-dimethylphenol depending on the solvent used.     

Dinuclear iron complexes based on parallel beta-diiminato binding sites: syntheses, structures and reaction with O2

With the background that certain natural systems utilize two Fe(II) centres in their prosthetic groups for the activation of O2, a ligand system containing two parallel beta-diiminato binding sites linked by a xanthene backbone ([RXanthdim]2- with residues R = 2,3-dimethylphenyl and 2,4-difluorophenyl at the iminato units, respectively) was investigated with respect to its Fe(II) coordination chemistry in order to study O(2) activation reactions. Hence, the corresponding lithium salts were treated with FeCl2 to yield the complexes [Me2C6H3Xanthdim]Fe2Cl3(Li(thf)3), and [F2C6H3Xanthdim]Fe2Cl3(Li(thf)3), , respectively, each of which comprises Cl-Fe(micro-Cl)Fe-Cl(Li(thf)3) units. and indeed readily react with O2 to give the oxides [RXanthdim]Fe2Cl2O containing Fe(III)-O-Fe(III) moieties. Due to the electron withdrawing F atoms reacts more slowly than . The molecular structures of , and [Me2C6H3Xanthdim]Fe2Cl2O, , as determined by single crystal X-ray diffraction are discussed, and an investigation concerning their magnetic properties revealed an antiferromagnetic coupling of the two iron centres in all complexes; naturally, the strongest coupling is observed for .     

Synthesis and structure of [2 x 2] molecular grid copper(II) and nickel(II) complexes with a new polydentate oxime-containing Schiff base ligand

A new polynucleating oxime-containing Schiff base ligand, 2-hydroxyimino- N'-[1-(2-pyridyl)ethylidene]propanohydrazone (H pop), has been synthesized and fully characterized. pH potentiometric, electrospray ionization mass spectrometric, and spectrophotometric studies of complex formation in H 2O/DMSO solution confirmed the preference for polynuclear complexes with 3d metal ions. Single-crystal X-ray diffraction analyses of [Ni 4( pop) 4(HCOO) 4].7H 2O ( 1), [Cu 4( pop-H) 4(HCOOH) 4].H 2O ( 2), and [Cu 4( pop-H) 4(H 2O) 4].9H 2O ( 3) indicated the presence of a [2 x 2] molecular grid structure in all three compounds but distinct configurations of the cores: a head-to-tail ligand arrangement with overall S 4 symmetry of the grid in the Cu (2+) complexes as opposed to a head-to-head ligand arrangement with (noncrystallographic) C 2 grid symmetry for the Ni (2+) complex. A cryomagnetic study of 3 revealed intramolecular ferromagnetic exchange between copper ions in the grid, while in 1, antiferromagnetic interactions between the metal ions were observed.     

Oligonuclear copper complexes of a bioinspired pyrazolate-bridging ligand: synthesis, structures, and equilibria in solution

The synthesis of a new bioinspired dinucleating ligand scaffold based on a bridging pyrazolate with appended bis[2-(1-methylimidazolyl)methyl]aminomethyl chelate arms is reported. This ligand forms very stable copper complexes, and a series of different species is present in solution depending on the pH. Interconversions between these solution species are tracked and characterized spectroscopically, and X-ray crystallographic structures of three distinct complexes that correspond to the species present in solution from acidic to basic pH have been determined. Overall, this provides a comprehensive picture of the copper coordination chemistry of the new ligand system. Alterations in the protonation state are accompanied by changes in nuclearity and pyrazolate binding, which cause pronounced changes in color and magnetic properties. Antiferromagnetic coupling between the copper(II) ions is switched on or off depending on the pyrazole binding mode.