Compression of curium pyrrolidine-dithiocarbamate enhances covalency.

Curium is unique in the actinide series because its half-filled 5f 7 shell has lower energy than other 5f n configurations, rendering it both redox-inactive and resistant to forming chemical bonds that engage the 5f shell1-3. This is even more pronounced in gadolinium, curium's lanthanide analogue, owing to the contraction of the 4f orbitals with respect to the 5f orbitals4. However, at high pressures metallic curium undergoes a transition from localized to itinerant 5f electrons5. This transition is accompanied by a crystal structure dictated by the magnetic interactions between curium atoms5,6. Therefore, the question arises of whether the frontier metal orbitals in curium(III)-ligand interactions can also be modified by applying pressure, and thus be induced to form metal-ligand bonds with a degree of covalency. Here we report experimental and computational evidence for changes in the relative roles of the 5f/6d orbitals in curium-sulfur bonds in [Cm(pydtc)4]- (pydtc, pyrrolidinedithiocarbamate) at high pressures (up to 11 gigapascals). We compare these results to the spectra of [Nd(pydtc)4]- and of a Cm(III) mellitate that possesses only curium-oxygen bonds. Compared with the changes observed in the [Cm(pydtc)4]- spectra, we observe smaller changes in the f-f transitions in the [Nd(pydtc)4]- absorption spectrum and in the f-f emission spectrum of the Cm(III) mellitate upon pressurization, which are related to the smaller perturbation of the nature of their bonds. These results reveal that the metal orbital contributions to the curium-sulfur bonds are considerably enhanced at high pressures and that the 5f orbital involvement doubles between 0 and 11 gigapascal. Our work implies that covalency in actinides is complex even when dealing with the same ion, but it could guide the selection of ligands to study the effect of pressure on actinide compounds.

Understanding the Stabilization and Tunability of Divalent Europium 2.2.2B Cryptates.

Lanthanides such as europium with more accessible divalent states are useful for studying redox stability afforded by macrocyclic organic ligands. Substituted cryptands, such as 2.2.2B cryptand, that increase the oxidative stability of divalent europium also provide coordination environments that support synthetic alterations of Eu(II) cryptate complexes. Two single crystal structures were obtained containing nine-coordinate Eu(II) 2.2.2B cryptate complexes that differ by a single coordination site, the occupation of which is dictated by changes in reaction conditions. A crystal structure containing a [Eu(2.2.2B)Cl]+ complex is obtained from a methanol-THF solvent mixture, while a methanol-acetonitrile solvent mixture affords a [Eu(2.2.2B)(CH3OH)]2+ complex. While both crystals exhibit the typical blue emission observed in most Eu(II) containing compounds as a result of 4f65d1 to 4f7 transitions, computational results show that the substitution of a Cl- anion in the place of a methanol molecule causes mixing of the 5d excited states in the Eu(II) 2.2.2B cryptate complex. Additionally, magnetism studies reveal the identity of the capping ligand in the Eu(II) 2.2.2B cryptate complex may also lead to exchange between Eu(II) metal centers facilitated by π-stacking interactions within the structure, slightly altering the anticipated magnetic moment. The synthetic control present in these systems makes them interesting candidates for studying less stable divalent lanthanides and the effects of precise modifications of the electronic structures of low valent lanthanide elements.

CONDON 3.0: An Updated Software Package for Magnetochemical Analysis-All the Way to Polynuclear Actinide Complexes.

An update to the computational framework CONDON, introducing the ability to model and predict the magnetic and electronic properties of polynuclear exchanged-coupled actinide systems, such as homonuclear and heteronuclear coordination clusters of 5f ions, is presented. The program can intuitively fit experimental magnetic and spectroscopic data from multiple sources simultaneously, under consideration of a "full model" ligand field theory Hamiltonian. CONDON accounts simultaneously for all aspects relevant to the magnetic characteristics: interelectronic repulsion, ligand field potential, spin-orbit coupling, interatomic exchange interactions, and applied magnetic field. As exemplified by several examples, CONDON represents the first program package able to accurately describe single-ion effect in exchange-coupled actinide systems, limited only by available computational resources. © 2018 Wiley Periodicals, Inc.

Electronic Structure and Properties of Berkelium Iodates.

The reaction of 249Bk(OH)4 with iodate under hydrothermal conditions results in the formation of Bk(IO3)3 as the major product with trace amounts of Bk(IO3)4 also crystallizing from the reaction mixture. The structure of Bk(IO3)3 consists of nine-coordinate BkIII cations that are bridged by iodate anions to yield layers that are isomorphous with those found for AmIII, CfIII, and with lanthanides that possess similar ionic radii. Bk(IO3)4 was expected to adopt the same structure as M(IO3)4 (M = Ce, Np, Pu), but instead parallels the structural chemistry of the smaller ZrIV cation. BkIII-O and BkIV-O bond lengths are shorter than anticipated and provide further support for a postcurium break in the actinide series. Photoluminescence and absorption spectra collected from single crystals of Bk(IO3)4 show evidence for doping with BkIII in these crystals. In addition to luminescence from BkIII in the Bk(IO3)4 crystals, a broad-band absorption feature is initially present that is similar to features observed in systems with intervalence charge transfer. However, the high-specific activity of 249Bk (t1/2 = 320 d) causes oxidation of BkIII and only BkIV is present after a few days with concomitant loss of both the BkIII luminescence and the broadband feature. The electronic structure of Bk(IO3)3 and Bk(IO3)4 were examined using a range of computational methods that include density functional theory both on clusters and on periodic structures, relativistic ab initio wave function calculations that incorporate spin-orbit coupling (CASSCF), and by a full-model Hamiltonian with spin-orbit coupling and Slater-Condon parameters (CONDON). Some of these methods provide evidence for an asymmetric ground state present in BkIV that does not strictly adhere to Russel-Saunders coupling and Hund's Rule even though it possesses a half-filled 5f 7 shell. Multiple factors contribute to the asymmetry that include 5f electrons being present in microstates that are not solely spin up, spin-orbit coupling induced mixing of low-lying excited states with the ground state, and covalency in the BkIV-O bonds that distributes the 5f electrons onto the ligands. These factors are absent or diminished in other f7 ions such as GdIII or CmIII.

Comprehensive Characterization of the Electronic Structure of U(4+) in Uranium(IV) Phosphate Chloride.

Emerald-green single crystals of U(PO4)Cl were grown by chemical vapor transport in a temperature gradient (1000 → 900 °C). The crystal structure of U(PO4)Cl (Cmcm, Z = 4, a = 5.2289(7) Å, b = 11.709(2) Å, c = 6.9991(8) Å) consists of a three-dimensional network of [PO4] tetrahedra and bicapped octahedral [U(IV)O6Cl2] groups. Polarized absorption spectra measured for two perpendicular polarization directions show a large number of well-resolved electronic transitions. These transitions can be fully assigned on the basis of a detailed ligand-field treatment within the framework of the angular overlap model. The magnetic behavior predicted on the basis of the spectroscopic data is in agreement with an f (2) system and perfectly matched by the results of temperature-dependent susceptibility measurements.

Magnetochemical complexity of hexa- and heptanuclear wheel complexes of late-3d ions supported by N,O-donor pyridyl-methanolate ligands.

The scaffold geometries, stability and magnetic features of the (pyridine-2-yl)methanolate (L) supported wheel-shaped transition-metal complexes with compositions [M6L12] (1), [Na⊂(ML2)6](+) (2), and [M'⊂(ML2)6](2+) (3), in which M=Co(II), Ni(II), Cu(II), and Zn(II) were investigated with density functional theory (DFT). The goals of this study are manifold: 1) To advance understanding of the magnetism in the synthesized compounds [Na⊂(ML2)6](+) and [M'⊂(ML2)6](2+) that were described in Angew. Chem. Int. Ed.- 2010, 49, 4443 (I-{Na⊂Ni6}, I-{Ni'⊂Ni6}) and Dalton Trans.- 2011, 40, 10526 (II-{Na⊂Co6}, II-{Co'⊂Co6}); 2) To disclose how the structural, electronic, and magnetic characteristics of 1, 2, and 3 change upon varying M(II) from d(7) (Co(2+)) to d(10) (Zn(2+)); 3) To estimate the influence of the Na(+) and M'(2+) ions (X(Q+)) occupying the central voids of 2 and 3 on the external and internal magnetic coupling interactions in these spin structures; 4) To assess the relative structural and electrochemical stabilities of 1, 2, and 3. In particular, we focus here on the net spin polarization, the determination of the strength and the sign of the exchange coupling energies, the rationalization of the nature of the magnetic coupling, and the ground-state structures of 1, 2, and 3. Our study combines the broken symmetry DFT approach and the model Hamiltonian methodology implemented in the computational framework CONDON 2.0 for the modeling of molecular spin structures, to interpret magnetic susceptibility measurements of I-{Na⊂Ni6} and I-{Ni'⊂Ni6}. We illustrate that whereas the structures, stability and magnetism of 1, 2, and 3 are indeed influenced by the nature of 3d transition-metals in the {M6} rims, the X(Q+) ions in the inner cavities of 2 and 3 impact these properties to an even larger degree. As exemplified by I-{Ni'⊂Ni6}, such heptanuclear complexes exhibit ground-state multiplets that cannot be described by simplistic model of spin-up and spin-down metal centers. Furthermore, we assess how future low-temperature susceptibility measurements at high magnetic fields can augment the investigation of compound 3 with M=Co, Ni.

Synthesis, structure, and magnetic properties of a new family of tetra-nuclear {Mn2(III)Ln2}(Ln = Dy, Gd, Tb, Ho) clusters with an arch-type topology: single-molecule magnetism behavior in the dysprosium and terbium analogues.

Sequential reaction of Mn(II) and lanthanide(III) salts with a new multidentate ligand, 2,2'-(2-hydroxy-3-methoxy-5-methylbenzylazanediyl)diethanol (LH3), containing two flexible ethanolic arms, one phenolic oxygen, and a methoxy group afforded heterometallic tetranuclear complexes [Mn2Dy2(LH)4(µ-OAc)2](NO3)2·2CH3OH·3H2O (1), [Mn2Gd2(LH)4(µ-OAc)2](NO3)2·2CH3OH·3H2O (2), [Mn2Tb2(LH)4(µ-OAc)2](NO3)2·2H2O·2CH3OH·Et2O (3), and [Mn2Ho2(LH)4(µ-OAc)2]Cl2·5CH3OH (4). All of these dicationic complexes possess an arch-like structural topology containing a central Mn(III)-Ln-Ln-Mn(III) core. The two central lanthanide ions are connected via two phenolate oxygen atoms. The remaining ligand manifold assists in linking the central lanthanide ions with the peripheral Mn(III) ions. Four doubly deprotonated LH(2-) chelating ligands are involved in stabilizing the tetranuclear assembly. A magnetochemical analysis reveals that single-ion effects dominate the observed susceptibility data for all compounds, with comparably weak Ln···Ln and very weak Ln···Mn(III) couplings. The axial, approximately square-antiprismatic coordination environment of the Ln(3+) ions in 1-4 causes pronounced zero-field splitting for Tb(3+), Dy(3+), and Ho(3+). For 1 and 3, the onset of a slowing down of the magnetic relaxation was observed at temperatures below approximately 5 K (1) and 13 K (3) in frequency-dependent alternating current (AC) susceptibility measurements, yielding effective relaxation energy barriers of ΔE = 16.8 cm(-1) (1) and 33.8 cm(-1) (3).

Avoiding magnetochemical overparametrization, exemplified by one-dimensional chains of hexanuclear iron(III) pivalate clusters.

One-dimensional chain coordination polymers based on hexanuclear iron(III) pivalate building blocks and 1,4-dioxane (diox) or 4,4'-bipyridine (4,4'-bpy) bridging ligands, [Fe6O2(O2CH2)(O2CCMe3)12(diox)]n (1) and [Fe6O2(O2CH2)(O2CCMe3)12(4,4'-bpy)]n (2), showcase the utility of the angular overlap model, implemented in the program wxJFinder, in the predictive identification of the relative role of intra- and intercluster coupling.

Chiral hexanuclear ferric wheels.

The homochiral iron(III) wheels [Fe(6){(S)-pedea}(6)Cl(6)] and [Fe(6){(R)-pedea)}(6)Cl(6)] [(R)- and (S)-2; pedea = phenylethylaminodiethoxide] exhibit high optical activities and antiferromagnetic exchange. These homochiral products react with each other, producing the centrosymmetric, crystallographically characterized [Fe(6){(S)-pedea}(3){(R)-pedea}(3)Cl(6)] diastereomer [(RSRSRS)-2]. (1)H NMR and UV-vis studies indicate that exchange processes are slow in both homo- and heterochiral systems but that, upon combination, the reaction between (R)- and (S)-2 occurs quickly.

Cluster-based networks: 1D and 2D coordination polymers based on {MnFe2(µ3-O)}-type clusters.

A straightforward approach to heterometallic Mn-Fe cluster-based coordination polymers is presented. By employing a mixed-valent µ(3)-oxo trinuclear manganese(II/III) pivalate cluster, isolated as [Mn(II)Mn(III)(2)O(O(2)CCMe(3))(6)(hmta)(3)]·(solvent) (hmta = hexamethylenetetramine; solvent = n-propanol (1), toluene (2)) in the reaction with a µ(3)-oxo trinuclear iron(III) pivalate cluster compound, [Fe(3)O(O(2)CCMe(3))(6)(H(2)O)(3)]O(2)CCMe(3)·2Me(3)CCO(2)H, three new heterometallic {Mn(II)Fe(III)(2)} cluster-based coordination polymers were obtained: the one-dimensional polymer chain compounds {[MnFe(2)O(O(2)CCMe(3))(6)(hmta)(2)]·0.5MeCN}(n) (3) and {[MnFe(2)O(O(2)CCMe(3))(6)(hmta)(2)]·Me(3)CCO(2)H·(n-hexane)}(n) (4) and the two-dimensional layer compound {[MnFe(2)O(O(2)CCMe(3))(6)(hmta)(1.5)]·(toluene)}(n) (5). Single-crystal X-ray diffraction analysis reveals a µ(3)-oxo trinuclear pivalate cluster building block as the main constituent in all polymer compounds. Different M:hmta ratios in 1-5 are related to the different structural functions of the N-containing ligand. In clusters 1 and 2, three hmta ligands are monodentate, whereas in chains 3 and 4 two hmta ligands act as bridging ligands and one is a monodentate ligand; in 5, all hmta molecules act as bidentate bridges. Magnetic studies indicate dominant antiferromagnetic interactions between the metal centers in both homometallic {Mn(3)}-type clusters 1 and 2 and heterometallic {MnFe(2)}-type coordination polymers 3-5. Modeling of the magnetic susceptibility data to a isotropic model Hamiltonian yields least-squares fits for the following parameters: J(1)(Mn(II)-Mn(III)) = -6.6 cm(-1) and J(2)(Mn(III)-Mn(III)) = -5.4 cm(-1) for 1; J(1) = -5.5 cm(-1) and J(2)(Mn(III)-Mn(III)) = -3.9 cm(-1) for 2; J(1)(Mn(II)-Fe(III)) = -17.1 cm(-1) and J(2)(Fe(III)-Fe(III)) = -43.7 cm(-1) for 3; J(1) = -23.8 cm(-1) and J(2) = -53.4 cm(-1) for 4; J(1) = -13.3 cm(-1) and J(2) = -35.4 cm(-1) for 5. Intercluster coupling plays a significant role in all compounds 1-5.

Utilizing the adaptive polyoxometalate [As2W19O67(H2O)](14-) to support a polynuclear lanthanoid-based single-molecule magnet.

Five members of a new family of polyoxometalate (POM)-ligated tetranuclear rare earth metal complexes have been synthesized and characterized. These compounds have the general formula (HDABCO)(8)H(5)Li(8)[Ln(4)As(5)W(40)O(144)(H(2)O)(10)(gly)(2)]·25H(2)O [Ln = Gd (1), Tb (2), Dy (3), Ho (4) and Y = (5), HDABCO = monoprotonated 1,4-diazabicyclooctane, gly = glycine] and were synthesized from the preformed POM precursor [As(2)W(19)O(67)(H(2)O)](14-). The structure is comprised of two {As(2)W(19)O(68)} building blocks linked by a unit containing four rare earth ions and two additional tungsten centers, with the two glycine ligands playing a key bridging role. Two crystallographically distinct rare earth ions are present in each complex, both of which possess axially compressed, approximate square antiprismatic coordination geometry. The variable-temperature magnetic susceptibility profiles for 2-4 are dominated by population/depopulation of the M(J) sublevels of the relevant ground terms, and fitting of the data has afforded the ligand field parameters in each case, from which the energies of the M(J) sublevels can be calculated. Alternating current magnetic susceptibility data have revealed the onset of slow magnetic relaxation for 3, with the energy barrier to magnetization reversal determined to be 3.9(1) K. As for other lanthanoid complexes that display slow magnetic relaxation, this energy barrier is due to the splitting of the M(J) sublevels of the Dy(3+) ions such that the ground sublevel has a relatively large |M(J)| value, thereby affording Ising-type magnetic anisotropy. This complex is thus the first POM-supported polynuclear lanthanoid-based SMM. Simulation of the W-band EPR spectrum of 1 has afforded the spin Hamiltonian parameters for this species, while the X-band EPR spectrum of 3 indicates the presence of a non-negligible fourth-order transverse component of the anisotropy, which is responsible for the small effective energy barrier observed for 3 and the absence of slow magnetic relaxation for 4.

Magnetic coupling in enantiomerically pure trinuclear helicate-type complexes formed by hierarchical self-assembly.

Based on chiral, enantiomerically pure 7-[(S)-phenylethylurea]-8-hydroxyquinoline (1-H), trinuclear helicate-type complexes 2-5 are formed with divalent transition-metal cations. X-ray structural analyses reveal the connection of two monomeric complex units [M(1)(3)](-) (M=Zn, Mn, Co, Ni) by a central metal ion to form a "dimer". Due to the enantiopurity of the ligand, the complexes are obtained as pure enantiomers, resulting in pronounced circular dichroism (CD) spectra. Single-ion effects and intra- and intermolecular coupling are observed with dominating ferromagnetic coupling in the case of the cobalt(II) and nickel(II) and dominating antiferromagnetic coupling in the case of the manganese(II) complex.

Synthesis, characterization, and quantum-chemical studies of Ni(CN)2MX (M = Rb, Cs; X = Cl, Br).

A new series of coordination-network compounds containing Ni(CN)(2) and MX (M = Rb, Cs; X = Cl, Br) quasi two-dimensional sheets has been synthesized and structurally characterized. The tetragonal crystal structure (I4/mmm, no. 139) can be derived from the distorted perovskite type. Chemically, the Lewis-acidic Ni(CN)(2) moieties accept halide ligands, resulting both in slightly elongated [Ni(NC)(4)X(2)](4-) octahedra and strongly elongated [Ni(CN)(4)X(2)](4-) octahedra that are charge-balanced by M(+) cations. Quantum-chemical calculations of the effective Hamiltonian crystal field (EHCF) type indicate the simultaneous presence of high- and low-spin Ni(2+) in a 1:1 ratio, in agreement with GGA+U and UV studies reported here. Our magnetic susceptibility data also corroborate the theoretical findings. An analysis of the magnetic superexchange paths in the new series of compounds is performed as well as that of the tentative magnetic state of the series.

Syntheses and magnetostructural investigations on Kuratowski-type homo- and heteropentanuclear coordination compounds [MZn4Cl4(L)6] (M(II) = Zn, Fe, Co, Ni, or Cu; L = 5,6-dimethyl-1,2,3-benzotriazolate) represented by the nonplanar K(3,3) graph.

Homo- and heteropentanuclear coordination compounds [MZn(4)Cl(4)(L)(6)] (M(II) = Zn, Fe, Co, Ni, or Cu; L = 5,6-dimethyl-1,2,3-benzotriazolate) were prepared containing mu(3)-bridging N-donor ligands (1,2,3-benzotriazolate), which are structurally related to the fundamental secondary building unit of Metal-organic Framework Ulm University-4 (MFU-4). The unique topology of these T(d)-symmetrical compounds is characterized by the nonplanar K(3,3) graph, introduced into graph theory by the mathematician Casimir Kuratowski in 1930. The following "Kuratowski-type" compounds were investigated by single-crystal X-ray structure analysis: [MZn(4)Cl(4)(Me(2)bta)(6)].2DMF (M(II) = Zn, Fe, Co, and Cu; DMF = N,N'-dimethylformamide) and [MZn(4)Cl(4)(Me(2)bta)(6)].2C(6)H(5)Br (M(II) = Co and Ni; C(6)H(5)Br = bromobenzene). The mu(3)-bridging benzotriazolate ligands span the edges of an imaginary tetrahedron, in the center of which a redox-active octahedrally coordinated M(II) ion is placed. Four Zn(II) ions are located at the corners of the coordination units. Each Zn center is bound to a monodentate Cl(-) anion and three N-donor atoms stemming from different benzotriazolate ligands. The fact that open-shell redox-active M(II) ions can be introduced selectively into the central octahedral coordination sites is unambiguously proven by a combination of magnetic measurements, UV-vis spectroscopy, and energy-dispersive X-ray and inductively coupled plasma atomic emission spectrometry analysis. The phase purity of all compounds was checked by powder X-ray diffractometry, IR spectroscopy, and elemental analysis. The electronic spectra and magnetic properties of the compounds are in complete agreement with their structures determined from single-crystal data. Thermogravimetric analysis shows that all compounds possess a high thermal stability up to 673 K. The pentanuclear compounds retain their structural integrity in solution, as evidenced by time-of-flight mass spectrometry analysis and comparative solution and solid-state diffuse-reflectance spectroscopy. High stability paired with the presence of redox-active metal ions and Lewis-acidic Zn centers renders Kuratowski-type compounds structural and functional models for future MFU-4-type bi- and multifunctional heterogeneous catalysts.

Structural phase transitions in EuC(2).

Pure EuC(2), free of EuO impurities, was obtained by the reaction of elemental europium with graphite at 1673 K. By means of synchrotron powder diffraction experiments, the structural behavior was investigated in the temperature range from 10 to 1073 K. In contrast to former results, EuC(2) crystallizes in the ThC(2) type structure (C2/c, Z = 4) at room temperature. A tetragonal modification (I4/mmm, Z = 2) is only observed in a very small temperature range just below the transition to a cubic high-temperature modification (Fm3m, Z = 4) at 648 K. DTA/TG investigations confirm these results. According to Raman spectroscopy, EuC(2) contains C(2)(2-) ions (nu(C[triple bond]C) = 1837 cm(-1)). The divalent character of Eu is confirmed by the results of magnetic susceptibility measurements and (151)Eu Mossbauer spectroscopy. In these measurements a transition to a ferromagnetic state with T(C) = 15 K is observed, which is in reasonable agreement with literature data. Above T(C) EuC(2) is a semiconductor according to measurements of the electric resistivity vs temperature, again in contrast to former results. Around T(C) a sharp maximum of the electric resistivity vs temperature curve was observed, which collapses on applying external magnetic fields. The observed CMR effect (colossal magnetoresistance) is much stronger than that reported for other EuC(2) samples in the literature. These investigations explicitly show the influence of sample purity on the physical and even structural properties of EuC(2).

One-dimensional coordination polymers from hexanuclear manganese carboxylate clusters featuring a {Mn(II)4Mn(III)2(mu4-O)2} core and spacer linkers.

The bridging of hexanuclear mixed-valent carboxylate coordination clusters of the type [Mn(6)O(2)(O(2)CR)(10)] (R = CMe(3); CHMe(2)) featuring a {Mn(II)(4)Mn(III)(2)(mu(4)-O)(2)} core by geometrically rigid as well as flexible spacer ligands such as pyrazine (pyz), nicotinamide (na), or 1,2-bis(4-pyridyl)ethane (bpe) results exclusively in one-dimensional (1D) coordination polymers. The formation of {[Mn(6)O(2)(O(2)CCMe(3))(10)(Me(3)CCO(2)H)(EtOH)(na)] x EtOH x H(2)O}(n) (1), {[Mn(6)O(2)(O(2)CCHMe(2))(10)(pyz)(3)] x H(2)O}(n) (2), and {[Mn(6)O(2)(O(2)CCHMe(2))(10)(Me(2)CHCO(2)H)(EtOH)(bpe)] x Me(2)CHCO(2)H}(n) (3) illustrates a surprising preference of the interlinked {Mn(6)} units toward 1D coordination chains. In the solid-state, the observed chain propagation axes are either colinear (1 and 3) or perpendicular (2), whereby crystal packing is further influenced by solvent molecules. Magnetic properties of these network compounds can be rationalized based on that the magnetism of discrete [Mn(6)O(2)(O(2)CR)(10)]-type coordination clusters with all-antiferromagnetic intramolecular exchange and weak antiferromagnetic intercluster coupling in 1, 2, and 3 follows the expected exchange coupling strength of the employed spacer linkers.

FeNCN and Fe(NCNH)2: synthesis, structure, and magnetic properties of a nitrogen-based pseudo-oxide and -hydroxide of divalent iron.

Iron man challenge: Iron carbodiimide, FeNCN, and its precursor iron (bis)monohydrocyanamide, Fe(NCNH)(2), have been synthesized and physically characterized. Both FeNCN and Fe(NCNH)(2) exhibit nitrogen-mediated antiferromagnetic superexchange interactions with reduced magnetic moments very similar, but not identical, to the correlated 3d oxides.

Polyoxotungstate-encapsulated Gd(6) and Yb(10) complexes.

A new synthetic method for polynuclear lanthanoid complexes that are embedded in molecular polyoxotungstates has afforded the highest nuclearity polyoxometalate-encapsulated Gd complex and the first decanuclear Yb complex of any sort to be structurally characterized.

Synthesis, physicochemical characterization and MR relaxometry of aqueous ferrofluids.

The synthesis and characterization of ferrofluid based MR contrast agents, which offer R2* versatility beyond that of ferucarbotran, is described. Ferrofluids were formed after stabilizing magnetite cores with dodecanoic acid (a), oleic acid (b), dodecylamine (c), citric acid (d) or tartaric acid (e). Core sizes were deduced from TEM micrographs. Magnetic properties were determined by SQUID magnetometry. Hydrodynamic particle diameters were determined by dynamic light scattering measurements. Zeta potentials were measured by combining laser Doppler velocimetry and phase analysis light scattering. Iron contents were evaluated colorimetrically. MR relaxometry including R1 and R2* was conducted in vitro using homogeneous ferrofluid samples. The average core diameters of ferrofluids a, b and c equaled 9.4 +/- 2.8 nm and approximately 2 nm for ferrofluids d and e. Magnetization measurements at 300 K revealed superparamagnetic behaviour for the dried 9 nm diameter cores and paramagnetic-like behaviour for the dried cores of ferrofluids d and e. Iron contents were between 32-75 mg Fe/mL, reflecting the ferrofluids' high particle concentrations. Hydrodynamic particle diameters equaled 100-120 nm (a, b and c). For the ferrofluids a, b, d and e coated with anions, strong negative zeta potential values between -27.5 mV and -54.0 mV were determined and a positive zeta potential value of +33.5 mV was found for ferrofluid c, covered with cationic dodecylammonium ions. MR relaxometry yielded R1-values of 1.9 +/- 0.3 (a), 4.0 +/- 0.8 (b), 5.2 +/- 1.0 (c), 0.124 +/- 0.002 (d) and 0.092 +/- 0.005 s(-1) mM(-1) (e), and R2*-values of 856 +/- 24 (a), 729 +/- 16 (b), 922 +/- 29 (c), 1.7 +/- 0.05 (d) and 0.49 +/- 0.05 s(-1) mM(-1) (e). Thus, the synthesized ferrofluids reveal a broad spectrum of R2* relaxivities. As a result, the various MR contrast agents have a great potential to be used in studies dealing with malignant tissue targeting or molecular imaging.