Vassilios Nastopoulos

Inducing Single-Molecule Magnetism in a Family of Loop-of-Loops Aggregates: Heterometallic Mn40Na4 Clusters and the Homometallic Mn44 Analogue

The syntheses, crystal structures, and magnetic properties of a new family of heterometallic Mn(40)Na(4) and homometallic Mn(44) loop-of-loops aggregates are reported. The reactions of [Mn(3)O(O(2)CMe)(6)(py)(3)]·py with 1,3-propanediol (pdH(2)) and 2-methyl-1,3-propanediol (mpdH(2)) in the presence of NaN(3) afforded [Mn(10)Na(μ(3)-O)(2)(O(2)CMe)(13)(pd)(6)(py)(2)](4) (1)(4) and [Mn(10)Na(μ(3)-O)(2)(O(2)CMe)(13)(mpd)(6)(py)(H(2)O)](4) (2)(4), respectively. Mn(40)Na(4) complexes (1)(4) and (2)(4) consist of four Mn(10) loops linked through Na(+) ions to give a supramolecular aggregate with a saddle-like topology. Magnetic characterization of compound (1)(4) showed that each Mn(10) loop has an S = 4 ground-state spin and displays frequency-dependent in-phase and out-of-phase ac susceptibility signals. It also exhibits hysteresis loops that, however, are not typical of single-molecule magnets (SMMs) due to the existence of interloop interactions between the neighboring Mn(10) units of (1)(4) through the diamagnetic Na(+) ions, and also intermolecular interactions between different Mn(40)Na(4) aggregates. The magnetically discrete Mn(44) analogue was targeted with high priority and finally prepared from the reaction of [Mn(3)O(O(2)CMe)(6)(py)(3)]·py with pdH(2) in the presence of Mn(ClO(4))(2)·6H(2)O. The loop-of-loops structure of [Mn(44)(μ(3)-O)(8)(O(2)CMe)(52)(pd)(24)(py)(8)](ClO(4))(OH)(3) (3) is essentially identical to those of (1)(4) and (2)(4), with the most significant difference being that the four Na(+) ions of (1)(4) and (2)(4) have been replaced with Mn(2+) ions. Compound 3 is thus best described magnetically as a Mn(44) cluster. In accord with this description and the stronger exchange coupling between the four Mn(10) loops expected through the connecting Mn(2+) ions, magnetic susceptibility measurements revealed that 3 has an S = 6 ground-state spin and displays frequency-dependent in-phase and out-of-phase ac signals. Magnetization vs dc field sweeps on single-crystals of 3 displayed scan rate- and temperature-dependent hysteresis loops confirming that complex 3 is a new SMM, and is thus the second largest Mn cluster and SMM reported to date.

A Family of Enneanuclear Iron(II) Single‐Molecule Magnets

Complexes [Fe9(X)2-(O2CMe)8{(2-py)2CO2}4] (X(-)=OH(-) (1), N3(-) (2), and NCO(-) (3)) have been prepared by a route previously employed for the synthesis of analogous Co(9) and Ni(9) complexes, involving hydroxide substitution by pseudohalides (N3(-), NCO(-)). As indicated by DC magnetic susceptibility measurements, this substitution induced higher ferromagnetic couplings in complexes 2 and 3, leading to higher ground spin states compared to that of 1. Variable-field experiments have shown that the ground state is not well isolated from excited states, as a result of which it cannot be unambiguously determined. AC susceptometry has revealed out-of-phase signals, which suggests that these complexes exhibit a slow relaxation of magnetization that follows Arrhenius behavior, as observed in single-molecule magnets, with energy barriers of 41 K for 2 (tau 0=3.4 x 10(-12) s) and 44 K for 3 (tau 0=2.0 x 10(-11) s). Slow magnetic relaxation has also been observed by zero-field 57Fe Mössbauer spectroscopy. Characteristic integer-spin electron paramagnetic resonance (EPR) signals have been observed at X-band for 1, whereas 2 and 3 were found to be EPR-silent at this frequency. 1H NMR spectrometry in CD3CN has shown that complexes 1-3 are stable in solution.

Reactions of 2,2′-bipyridine (bpy) and 1,10-phenanthroline (phen) with yttrium(III) nitrate: preparation, X-ray crystal structures and spectroscopic characterization of the bis-bpy and bis-phen complexes

Y(NO3)3 · 5H2O reacts with 2,2′-bipyridine (bpy) and 1,10-phenanthroline (phen) to yield similar complexes of 1:2 yttrium:ligand stoichiometry. The crystal structures of the two complexes, namely, [Y(NO3)3(bpy)2] (1) and [Y(NO3)3(phen)2] (2), are reported. The three nitrate groups are O,O′-bidentate and the organic ligands are also bidentate. In both structures the metal ion lies on a crystallographic two-fold axis. The stereochemistry about YIII can be viewed as a sphenocorona. The new complexes were characterized by elemental analyses and spectroscopic (i.r., 1H-n.m.r. and 13C-n.m.r.) techniques. The data are discussed in terms of the nature of bonding and known solid state structures.

Eight-coordination in nitrato manganese(II) complexes with tetradentate di-Schiff bases derived from 2-pyridyl ketones: preparation, characterization and catalytic activity for alkene epoxidation

Abstract The reactions of Mn(NO3)2·6H2O with the tetradentate Schiff bases N,N′-bis[1-(pyridin-2-yl)ethylidene]ethane-1,2-diamine (LA) and N,N′-bis[1-(pyridin-2-yl)benzylidene]ethane-1,2-diamine (LB) afforded the novel eight-coordinate complexes [Mn(NO3)2(LA)] (1) and [Mn(NO3)2(LB)] (2), which have a distorted dodecahedral coordination geometry. The catalytic activity of 1 and 2 for alkene epoxidation is reported and discussed.

Synthesis and Characterization of a Linear [Mn3(O2CMe)4(py)8]2+ Complex

Two new compounds that consist of the linear trinuclear manganese(II) cation [Mn3(O2CMe)4(py)8]2+ cocrystallizing with different counteranions (I3 −, [1]; ClO4 −, [2]) are reported. Complex 1 was prepared from the reaction of [Mn(O2CMe)2] · 4H2O with I2 in MeCO2H/py, whereas complex 2 was isolated from the reaction of [Mn3O(O2CMe)6(py)3] · py with [Mn(ClO4)2] · 6H2O in MeCN/py. The crystal structures of both compounds were determined by single crystal X-ray crystallography. Magnetic susceptibility studies that were performed in microcrystalline powder of 1 in the 2–300 K range revealed the presence of antiferromagnetic exchange interactions that resulted in an S = 5/2 ground spin state.

Depolymerization approach in Mn cluster chemistry: controlled cleavage of a 1D coordination polymer consisting of Mn(8) units in its constituent, discrete Mn(8) complex.

The cleavage of a 1D coordination polymer (Mn(II)(2)Mn(III)(6))(n) containing recognizable octanuclear units to obtain the discrete Mn(II)(2)Mn(III)(6) cluster is reported.

A MnII6MnIII6 single-strand molecular wheel with a reuleaux triangular topology: synthesis, structure, magnetism, and DFT studies.

The use of the anion of 3-methyl-1,3,5-pentanetriol (mpt(3-)) in manganese carboxylate chemistry has afforded the new Mn(II/III)12 cluster [Mn(II)6Mn(III)6(mpt)6(CH3CO2)12(py)6]·3CH3CN (1·3CH3CN). Complex 1 was isolated in moderate yield by the reaction of Mn(CH3CO2)2·4H2O and H3mpt in a 2.6:1 molar ratio in a solvent mixture of acetonitrile and pyridine. The structure of 1 consists of alternating [Mn(II)2(CH3CO2)3(py)](+) and [Mn(III)2(μ-OR)2(CH3CO2)(py)](3+) dimeric units (three of each dimer), linked at each end by two alkoxo and one acetate bridges; the mpt(3-) ligands adopt the η(2):η(2):η(2):μ4 coordination mode. The overall metal topology of this new Mn12 wheel resembles a guitar plectrum, or a Reuleaux triangle. Complex 1 displays an unprecedented structural topology, being the first example of a Mn(II)6Mn(III)6 wheel constructed from alternating homovalent dimers and the only known Mn12 loop with the trigonal symmetry of a Reuleaux triangle (all other reported loops were ellipsoids). Variable-temperature, solid-state direct- and alternating-current magnetization studies were carried out on complex 1, revealing the presence of antiferromagnetic exchange interactions between the metal ions in the molecule, which lead to a spin ground-state value S = 0; the exchange coupling parameters J were calculated using density functional theory employing a hybrid B3LYP functional.

Interactions of trivalent lanthanide cations with tetradentate Schiff bases. Synthesis and characterization of lanthanide(III) complexes with N, N ′ -bis(pyridin-2-ylmethylene)benzene-1,2-diamine

Abstract The novel lanthanide(III) complexes [La(NO3)3(H2O)L] 1, and [Ln(NO3)3L] (Ln=Pr 2, Sm 3, Gd 4) where L=N,N′-bis(pyridin-2-ylmethylene)benzene-1,2-diamine, have been obtained by direct reaction of the Schiff base ligand and the corresponding hydrated lanthanide(III) nitrates in methanol. All complexes were characterized spectroscopically and thermogravimetrically. Complex 3 was also characterized with crystallographic studies. In the molecular structure of 3, Sm(III) is surrounded by all four nitrogen atoms of the Schiff base and six oxygen atoms belonging to three bidentate chelating nitrato ligands.

[Pr(NO3)3L]: a mononuclear ten-coordinate lanthanide(III) complex with a tetradentate di-Schiff base

The novel praseodymium(III) complex [Pr(NO3)3L] (1), where L=N,N′-bis[1-(pyridin-2-yl)ethylidene]ethane-1,2-diamine, has been obtained by direct reaction of the Schiff base and the metal salt; the gadolinium(III) homologue has also been prepared and so far characterized only spectroscopically. The crystal structure resembles those reported for hexadentate macrocyclic Schiff bases.

Structure of dimeric and monomeric erabutoxin a refined at 1.5 A resolution.

Erabutoxin a has been crystallized in its monomeric and dimeric forms. The structures were refined at 1.50 and 1.49 A resolution, respectively, using synchrotron radiation data. The crystals belong to space group P212121, with cell dimensions a = 49.84, b = 46.62, c = 21.22 A for the monomer and a = 55.32, b = 53.54, c = 40.76 A for the dimer. Using starting models from earlier structure determinations, the monomeric structure refined to an R value of 16.7% (8004 unique reflections, 17.0-1.50 A resolution range), while the dimeric structure has been solved by the molecular-replacement method with a final R value of 16.9% (19 444 unique reflections, 17.4-1.49 A resolution range). The high-resolution electron-density maps clearly revealed significant discrete disorder in the proteins and allowed an accurate determination of the solvent structure. For the monomer, the side chains of six residues were modelled with alternate conformers and 106 sites for water molecules and one site for a sulfate ion were included in the final model, whereas for the dimer, 206 sites for water molecules were included and both C-terminal residues together with the side chains of 11 residues adopted alternative conformations. A comparison was made with earlier structure determinations. The features of the solvent structure of the erabutoxin molecules are discussed in detail.