Antiprismatic Coordination Research Articles

Synthesis and Structural Study of Tetravalent (Zr4+, Hf4+, Ce4+, Th4+, U4+) Metal Complexes with Cyclic Hydroxamic Acids

AbstractSix‐ and seven‐membered cyclic hydroxamic acids, such as 1‐hydroxypiperidine‐2‐one (1H, 1,2‐PIPOH) and 1‐hydroxyazepan‐2‐one (2H), have recently been identified in some mixed siderophores as one of their three chelating subunits. Compared to their ubiquitous noncyclic counterparts, cyclic hydroxamates are preorganized for metal binding. Surprisingly, the coordination chemistry of these bidentate, monoanionic ligands remains virtually unknown, even in the case of iron(III). We report herein the first structural study of the complexes of 1– and of 6–, an unsaturated seven‐membered ring analog of 2–, with tetravalent cations of transition metals (zirconium and hafnium), lanthanide (cerium), and actinides (thorium and uranium). Structural characterization by means of X‐ray crystallography of the corresponding ML4 complexes evidenced distorted square antiprismatic coordination geometries with the exception of U4+, which favors a dodecahedral arrangement.

Crystal structures and characterization of two one-dimensional coordination polymers containing Ln3+ ions and anthranilate (C7H6NO 2 − ) anions

The syntheses, crystal structures and characterization (IR, TGA/DSC) of the isostructural one-dimensional coordination polymers [Ce(C7H6NO2)3]n (1) and [Pr(C7H6NO2)3]n (2) are described. The metal ions adopt distorted capped square anti-prismatic MO9 coordination geometries. The anthranilate ligands bridge the metal ions in bridging-bidentate (O, μ2-O′) mode to generate [010] chains in the crystal and each ligand features an intramolecular N-H⋯O hydrogen bond. It is notable that two very similar, but crystallographically distinct chains appear in the unit cell. Crystal data: 1, C21H18CeN3O6, Mr = 548.51, monoclinic, P21/c (No. 14), a = 25.0476(10) A, b = 7.4924(2) A, c = 24.6366(7) A, β = 119.424(1)°, V = 4027.1(2) A3, R(F) = 0.037, wR(F2) = 0.068. 2, C21H18N3O6Pr, Mr = 549.29, monoclinic, P21/c (No. 14), a = 25.0077(13) A, b = 7.4497(3) A, c = 24.6063(12) A, β = 119.392(2)°, V = 3994.1(3) A3, R(F) = 0.031, wR(F2) = 0.068.

Dinuclear Lanthanide–Carboxylate Compounds: Field-Induced Slow Relaxation of Magnetization for Dysprosium(III) Analogue

Three chiral dinuclear lanthanide compounds, Ln2(µ2-L)4(L)2(phen)2 (Ln = Dy (1), Gd (2), and Er (3); phen = 1,10-phenanthroline), have been synthesized using the (S)-(+)-2-(6-methoxy-2-naphthyl)propionic acid (HL) ligand. The two lanthanide centres in compound Ln2(µ2-L)4(L)2(phen)2 are bridged by four carboxylate groups to give a dinuclear Ln2(µ2-L)4 core. The square antiprismatic coordination environment for each lanthanide centre is further completed by a chelating carboxylate group from another L– ligand and two nitrogen atoms from the phen ligand. A weak antiferromagnetic interaction between the two GdIII ions is observed in compound 2. The Dy analogue displays field-induced slow magnetic relaxation behaviour with an effective energy barrier Ueff/k of 17.24(2) K and a pre-exponential factor τ0 of 2.7(1) × 10–6 s. However, no slow relaxation phenomenon was observed for the Er derivative even in the presence of 2 kOe applied field.

Single-molecule-magnet behavior in the family of [Ln(OETAP)2] double-decker complexes (Ln=lanthanide, OETAP=octa(ethyl)tetraazaporphyrin).

Double-decker complexes of lanthanide cations can be readily prepared with tetraazaporphyrins (porphyrazines). We have synthesized and characterized a series of neutral double-decker complexes [Ln(OETAP)2 ] (Ln=Tb(3+), Dy(3+), Gd(3+), Y(3+); OETAP=octa(ethyl)tetraazaporphyrin). Some of these complexes show analogous magnetic features to their phthalocyanine (Pc) counterparts. The Tb(3+) and Dy(3+) derivatives exhibit single-molecule magnet (SMM) behavior with high blocking temperatures over 50 and 10 K, respectively. These results confirm that, in double-decker complexes that involve Tb or Dy, the (N4)2 square antiprism coordination mode has an important role in inducing very large activation energies for magnetization reversal. In contrast with their Pc counterparts, the use of tetraazaporphyrin ligands endows the presented [Ln(OETAP)2] complexes with extraordinary chemical versatility. The double-decker complexes that exhibit SMM behavior are highly soluble in common organic solvents, and easily processable even through sublimation.

Crystal Growth and Structure Determination of the New Neodymium Germanate, Na2NdGeO4(OH)

Single crystals of Na2NdGeO4(OH) were grown out of a molten sodium hydroxide flux and characterized by single crystal X-ray diffraction. Na2NdGeO4(OH) crystallizes in the orthorhombic space group Pnma with a = 9.4373(3) A, b = 7.4448(2) A, c = 6.9236(2) A, V = 486.44(2) A3, and Z = 4. The compound exhibits a three-dimensional crystal structure consisting of corner- and edge-shared NdO8, GeO4, and NaO7 polyhedra. The Nd3+ cation is observed in a distorted square antiprismatic coordination environment. The synthesis and crystal structure of a new neodymium germanate, Na2NdGeO4(OH) is reported.

Solid-state actinide acid phosphites from phosphorous acid melts

The reaction of UO3 and H3PO3 at 100°C and subsequent reaction with dimethylformamide (DMF) produces crystals of the compound (NH2(CH3)2)[UO2(HPO2OH)(HPO3)]. This compound crystallizes in space group P21/n and consists of layers of uranyl pentagonal bipyramids that share equatorial vertices with phosphite units, separated by dimethylammonium. In contrast, the reaction of phosphorous acid and actinide oxides at 210°C produces a viscous syrup. Subsequent dilution in solvents and use of standard solution-state methods results in the crystallization of two polymorphs of the actinide acid phosphites An(HPO2OH)4 (An=U, Th) and of the mixed acid phosphite–phosphite U(HPO3)(HPO2OH)2(H2O)·2(H2O). α- and β-An(HPO2OH)4 crystallize in space groups C2/c and P21/n, respectively, and comprise a three-dimensional network of An4+ cations in square antiprismatic coordination corner-sharing with protonated phosphite units, whereas U(HPO3)(HPO2OH)2(H2O)2·(H2O) crystallizes in a layered structure in space group Pbca that is composed of An4+ cations in square antiprismatic coordination corner-sharing with protonated phosphites and water ligands. We discuss our findings in using solid inorganic reagents to produce a solution-workable precursor from which solid-state compounds can be crystallized.

Metal–organic frameworks built from alkali metal ions (Li+–Cs+) and 1,2,3,4-cyclobutanetetracarboxylic acid

Six complexes formed by alkali metal ions with 1,2,3,4-cyclobutanetetracarboxylic acid (H4cbtc) have been synthesized and crystallographically characterized. In the complexes obtained at room temperature, the original cis,trans,cis form of the ligand (c-H4cbtc) is retained, while in the complexes synthesized under hydrothermal conditions, isomerization occurs to give the trans,trans,trans form (t-H4cbtc). [Li2(t-H2cbtc)(H2O)2] (1) crystallizes as a two-dimensional (2D) assembly, with only four oxygen atoms of the ligand being involved in the bis-chelating coordination of two Li+ cations. A 2D polymer is also obtained with the aliphatic analogue 1,2,3,4-butanetetracarboxylic acid (H4btc) in the complex [Li4(btc)(H2O)4]·H2O (2), with a much larger ligand denticity of 10. All the other complexes in this series crystallize as three-dimensional (3D) frameworks, with the cation coordination number and ligand denticity increasing when going from the lighter to the heavier cations. In [Na(t-H3cbtc)] (3), the cation octahedral coordination polyhedra are isolated and the {4·65} framework contains only four-fold nodes. In the case of K+, the complexes formed with both isomers of the ligand could be isolated. The structure of [K2(c-H2cbtc)(H2O)4] (4) displays {[K(H2O)2]+}∞ planar subunits which are assembled into a 3D network by the hexacoordinated c-H2cbtc2− ligands, while the {412·63} network built in [K(t-H3cbtc)] (5) contains only six-fold nodes, K+ being chelated by two ligands, with four more donors resulting in a distorted square antiprismatic coordination polyhedron. Although [Rb(c-H3cbtc)] (6) contains the other ligand isomer, it displays a coordination mode and an overall architecture similar to those in 5, but for the quite different cation coordination polyhedron, which is a distorted dodecahedron with triangular faces. Finally, [Cs(t-H3cbtc)] (7) displays the highest coordination number and ligand denticity in the series, both equal to 10. The cation coordination polyhedron is derived from the cuboctahedron through removal of two vertices in a square face. The {424·64} network formed contains chains of tightly packed Cs+ cations with face-sharing coordination polyhedra, thus confirming the prevalence of face-sharing subunits previously noticed in the case of the heavier alkali cations. With packing indexes larger than 0.80 in the whole series, none of these 3D frameworks exhibits appreciable porosity.

Structural Feature of an Octakis-DMSO Cerium(III) Complex: Tetragonal Antiprismatic Coordination Geometry along the <i>c</i> Axis

Structural Feature of an Octakis-DMSO Cerium(III) Complex: Tetragonal Antiprismatic Coordination Geometry along the <i>c</i> Axis

Neutron powder diffraction and molecular dynamics study of superionic SrBr2

The nature of the dynamic ionic disorder within the high-temperature superionic phase of strontium bromide, β-SrBr2, has been investigated using reverse Monte Carlo (RMC) modelling of neutron powder diffraction data and complementary ab initio molecular dynamics (MD) simulations. The RMC and MD results are in good agreement and indicate the presence of extensive dynamic disorder within the Br− sublattice of the cubic fluorite structure. Rapid anion diffusion predominantly occurs as hops between nearest neighbour sites in the 〈100〉 directions, though the trajectories are markedly curved and pass through the peripheries of the octahedral voids in the cation sublattice. In addition, there are extensive correlations between the motions of individual Br−, often leading to the formation of a short-lived square antiprism co-ordination environment around the Sr2+. Such polyhedra are observed within the (ambient temperature) ordered tetragonal crystal structure of α-SrBr2. The nature of the ionic disorder in SrBr2 is of particular interest because it is the only known example of a Br−-ion superionic. Owing to the large size of this anion, a comparison with the behaviour of other superionic phases gives an insight into the role of ionic size on the conducting properties within these materials.

Preparation, crystallographic and theoretical study on a bifunctional Gd-AAZTA derivative as potential MRI contrast agent precursor

The crystal structure of the complex (αR,6R)-Na[Gd(H2O)-AAZTA-C2H4COOBn]·3H2O·EtOH was determined by single crystal X-ray diffraction; the complex represents a bifunctional derivative of Gd-AAZTA, previously reported as original scaffold for the development of efficient contrast agents for MRI. The Gd(III) ion is nine coordinated with three nitrogens, five carboxylate oxygens and one water oxygen in a monocapped distorted square antiprism coordination polyhedron. Three of the five carboxylic groups are monodentate while a fourth is bidentate bridging an adjacent Gd ion with the formation of a polymeric structure. The influence on the overall structure of the propionate side arm introduced for conjugation purposes was discussed, and a computational approach to predict the structure of its (αR,6S) isomer is presented and compared with the X-ray structure of the (αR,6R) epimer.

Tm2.22Co6Sn20and TmLi2Co6Sn20stannides as disordered derivatives of the Cr23C6structure type

A new ternary dithulium hexacobalt icosastannide, Tm2.22Co6Sn20, and a new quaternary thulium dilithium hexacobalt icosastannide, TmLi2Co6Sn20, crystallize as disordered variants of the binary cubic Cr23C6 structure type (cF116). 48 Sn atoms occupy sites of m.m2 symmetry, 32 Sn atoms sites of .3m symmetry, 24 Co atoms sites of 4m.m symmetry, eight Li (or Tm in the case of the ternary phase) atoms sites of -3 symmetry and four Tm atoms sites of m3m symmetry. The environment of one Tm atom is an 18-vertex polyhedron and that of the second Tm (or Li) atom is a 16-vertex polyhedron. Tetragonal antiprismatic coordination is observed for the Co atoms. Two Sn atoms are enclosed in a heavily deformed bicapped hexagonal prism and a monocapped hexagonal prism, respectively, and the environment of the third Sn atom is a 12-vertex polyhedron. The electronic structures of both title compounds were calculated using the tight-binding linear muffin-tin orbital method in the atomic spheres approximation (TB-LMTO-ASA). Metallic bonding is dominant in these compounds, but the presence of Sn-Sn covalent dumbbells is also observed.

The molecular structure of tris-2,2,6,6-tetramethyl-heptane-3,5-dione indium: gas-phase electron diffraction and quantum chemical calculations

The molecular structure of tris-2,2,6,6-tetramethyl-heptane-3,5-dione indium, or In(thd)3, has been determined by gas-phase electron diffraction monitored by mass spectrometry (GED/MS) and quantum chemical (DFT) calculations. Both the DFT calculations and the GED data collected at 387(8) K indicate that the molecules have D 3 symmetry with a distorted anti-prismatic InO6 coordination geometry. According to GED refinements, the twist angle θ, i.e. the angle of rotation of the upper and lower O3 triangles in opposite directions relative to their positions in a regular prism is θ = ±24.9(1.2)° and the bond distances (r h1) in the chelate ring are In–O = 2.127(4) A, C–O = 1.268(3) A and C–C = 1.411(3) A, respectively. The DFT calculations yielded structure parameters in close agreement with those found experimentally.

New polyoxometalate-based mononuclear lanthanide complexes with slow relaxation of magnetization

A series of new polyoxometalate(POM)-based mononuclear lanthanide(III)–bi pyridine-N-oxide complexes, [Ln(bpyno)4][PMo12O40]·2H2O (Ln=Dy (1), Tb (2), Er (3), Ho (4); bpyno=4,4′-dimethyl-2,2′-bipyridyl-N-N′-dioxide), have been synthesized by the reactions between LnPMo12O40 salts (Ln=Dy (1), Tb (2), Er (3), Ho (4)) and bpyno ligand. All compounds were characterized by elemental analyses, TG analyses, IR, powder X-ray diffraction and single-crystal X-ray diffraction. In 1–4, each mononuclear Ln center is coordinated by eight O atoms derived from four bpyno ligands, exhibiting an approximate square antiprismatic coordination geometry. Furthermore, the [Ln(bpyno)4]3+ units are well separated by the α-Keggin-type polyoxoanions. Magnetic measurements indicate that compound 1 exhibits frequency dependent ac magnetic susceptibilities indicative of slow relaxation of the magnetization.

‘Facial’ and ‘meridional’ coordination geometries and luminescence properties of tris(N-[(imidazol-4-yl)methylidene]-dl-alaninato)europium(III) and tris(N-[(imidazol-4-yl)methylidene]-dl-phenylalaninato)europium(III)

Two racemic tridentate ligands, N-[(imidazol-4-yl)methylidene]-dl-alanine (H2Ldl-ala) and N-[(imidazol-4-yl)methylidene]-dl-phenylalanine (H2Ldl-phe), were synthesized by the 1:1 condensation reaction of 4-formylimidazole and either of dl-alanine and dl-phenylalanine, respectively. The EuIII complexes ‘fac’-[EuIII(HLdl-ala)3]·8H2O (1) and ‘mer’-[EuIII(HLdl-phe)3]·7.75H2O (2) were synthesized by mixing the ligand and EuIII(acetate)3n-hydrate with 3:1M ratio in methanol and the crystal structures were determined. Compounds 1 and 2 with three tridentate ligands have ‘facial’ and ‘meridional’ geometries, respectively, while they have a tricapped trigonal prismatic (TTP) and a capped square antiprismatic (CSAP) coordination geometry, respectively. In the crystal lattice, 1 consists of enantiomers ‘fac’-[EuIII(HLd-ala)3] and ‘fac’-[EuIII(HLl-ala)3], while 2 consists of ‘mer’-[EuIII(HLd-phe)2(HLl-phe)] and ‘mer’-[EuIII(HLd-phe)(HLl-phe)2]. Compounds 1 and 2 displayed sharp emission bands based on the f−f transitions by excitation and showed a different emission pattern due to their ‘fac’- and ‘mer’-coordination geometries. The luminescence quantum yields and life times of 1 and 2 were estimated to be (Φ=0.25 and 1.40ms) and (Φ=0.30 and 1.23ms), respectively.

Hydrothermal synthesis, crystal structure and magnetic properties of a DyIII-FeIII heteronuclear complex

A new 3d-4f heteronuclear complex [Fe(phen)3]2[FeDy(H2O)(tiron)3]·6H2O (1, Na2H2tiron=disodium 4,5-dihydro-xybenzene-1,3-disulfonate) was synthesized by the hydrothermal reaction. The complex crystallized in the cubic system, space group P213 with the cell parameters: a=2.18786(14) nm, V=10.4727(12) nm3, Z=4, F(000)=4720, R1=0.0493, wR2=0.1165, S=1.05. In each [FeDy(H2O)(tiron)3]6- unit, it was revealed that the Fe3+ ion was in a FeO6 distorted trigonal anti-prism coordination polyhedron completed by six phenolate O atoms from three tiron4- ligands, while Dy3+ in a DyO7 distorted monocapped trigonal anti-prism coordination polyhedron completed by three phenolate μ2-O atoms and three O atoms from sulfonate groups of three ligands and one O atom from water. The magnetic properties of the complex was determined in the range of 2–300 K, indicating the antiferromagnetic interaction between the central DyIII-FeIII ions.

Structures of DMF solvated potassium and sodium salts of [Fe(CO)4]2− and [M2(CO)8]2− (M = Fe, Ru)

An electrostatic interaction was detected between an alkali metal cation and the [Fe(CO)4]2− anion in the extended two-dimensional structure of [{K(DMF)}2Fe(CO)4]∞, 1, and extended three-dimensional structure of [{Na(DMF)1.25}2Fe(CO)4]∞, 2. The existence of such an interaction is supported by observed short K–Fe and Na–Fe distances of 3.594(1) Å and 2.899(1) Å and short K–C and Na–C distances of 3.080(3) Å and 2.792(4) Å. In addition to the electrostatic interactions, typical isocarbonyl linkages were also existed between the alkali metal cation and the [Fe(CO)4]2− anion. These two interactions lead to the extended multi-dimensional structures of 1 and 2. The [Fe(CO)4]2− anion basically retains tetrahedral geometry in 1 and 2, different from that in the complexes Na2[Fe(CO)4]·1.5(C4H8O2) and K2[Fe(CO)4] in which the [Fe(CO)4]2− anion is significantly distorted from a tetrahedral arrangement. The structures of [Na(DMF)3]2[M2(CO)8] (M = Fe, 3; Ru, 4) are isomorphous. They are ion pair complexes in which the [M2(CO)8]2− anions consist of two trigonal bipyramidal M(CO)4 fragments joined at axial sites. The cations have trigonal antiprismatic coordination geometry. The structure of the [Fe2(CO)8]2− anion is similar to that in the [PPN]+ and [PPh4]+ salts. The structure of the [Ru2(CO)8]2− anion differs from that previously reported for [PPh4][Ru2(CO)8]. In that case the dianion consists of a trigonal–bipyramidal Ru(CO)4 fragment joined to a basal site of a four-sided pyramidal Ru(CO)4 fragment.

Crystal structures and thermodynamic properties of lanthanide complexes with 2-chloro-4,5-difluorobenzoate and 1,10-phenanthroline

A series of lanthanide complexes with the 2-chloro-4,5-difluorobenzoate (2-cl-4,5-dfba) and 1,10-phenanthroline (phen), have been synthesized with the formulae of [La(2-cl-4,5-dfba)3phen]n·nH2O (1), [Nd(2-cl-4,5-dfba)3phenH2O]2 (2), [Ln(2-cl-4,5-dfba)3phen]2 (Ln=Eu (3), Ho (4)). The complexes are characterized by elemental analysis, infrared and fluorescent spectra and X-ray single-crystal diffraction. The structures of the four complexes are very different. Complex 1 is an infinite 1D chain polymeric structure formed by the asymmetric units with the mirror growth pattern. Each La3+ ion is coordinated to four bridging carboxylic groups, two tridentate chelating–bridging carboxylic groups, simultaneous with one phen molecule, giving the coordination number of nine. In the molecular structures of complexes 2 and 3, two Ln3+ ions are linked by four carboxyl groups, forming two binuclear molecules. In addition, each Nd3+ ion in complex 2 is bonded to one H2O molecule and one carboxyl group by monodentate mode, one phen molecule by bidentate chelating, and each Eu3+ ion is also chelated to one phen molecule and one carboxyl group in complex 3. And in complex 4, the Ho3+ ion yields a eight-coordinated distorted square anti-prism coordination geometry. The three-dimensional IR accumulation spectra of gaseous products for complexes 1 to 4 are analyzed and further authenticated the thermal decomposition processes with TG-DTG curves. The heat capacities of complexes 2 to 4 are measured and fitted to a polynomial equation by the least squares method on the basis of the reduced temperature x (x=[T−(Tmax+Tmin)/2]/[(Tmax−Tmin)/2]). Then the smoothed molar heat capacities and thermodynamic functions of complexes 2 to 4 are calculated. The fluorescence intensity of complex 3 is markedly improved as well.

Complexes of yttrium, lanthanum, and erbium halides with carbamide: Syntheses and structures

New complexes of yttrium and lanthanum bromides and erbium chloride with carbamide (Ur), [Y(Ur)4(H2O)4]Br3 (I) and [La(Ur)4(H2O)4]Br3 (II), [La(Ur)6(H2O)2]Br3 (III), and [Er(Ur)6Cl]Cl2 (IV), are synthesized and studied by X-ray diffraction analysis. In structures I–III, the coordination of the ligands (water and carbamide) by the metal occurs through the oxygen atoms. The coordination polyhedron of the rare-earth metal atoms is a distorted square antiprism (coordination number 8), and the bromide ions are not included into the internal coordination sphere of the complexes. In compound IV, the internal coordination sphere of the complex includes six carbamide molecules and one chloride ion, and the coordination polyhedron of Er is a distorted pentagonal bipyramid (coordination number 7). Specific features of the structures of the crystalline carbamide derivatives of chlorides, bromides, and iodides of the considered rare-earth metals are compared. A distortion of the planar structure of some carbamide ligands, depending on their number in the complex cation and on the nature of the complex, is observed.

Definition of an Intramolecular Eu-to-Eu Energy Transfer within a Discrete [Eu2L] Complex in Solution

Ligand L, based on two do3a moieties linked by the methylene groups of 6,6'-dimethyl-2,2'-bipyridine, was synthesized and characterized. The addition of Ln salts to an aqueous solution of L (0.01 M Tris-HCl, pH 7.4) led to the successive formation of [LnL] and [Ln(2)L] complexes, as evidenced by UV/Vis and fluorescence titration experiments. Homodinuclear [Ln(2)L] complexes (Ln = Eu, Gd, Tb, Yb, and Lu) were prepared and characterized. The (1)H and (13)C NMR spectra of the Lu and Yb complexes in D(2)O solution (pD = 7.0) showed C(1) symmetry of these species in solution, pointing to two different chemical environments for the two lanthanide cations. The analysis of the chemical shifts of the Yb complex indicated that the two coordination sites present square antiprismatic (SAP) coordination environments around the metal ions. The spectroscopic properties of the [Tb(2)L] complex upon ligand excitation revealed conventional behavior with τ(H2O) = 2.05(1) ms and ϕ(H2O) = 51%, except for the calculation of the hydration number obtained from the luminescent lifetimes in H(2)O and D(2)O, which pointed to a non-integer value of 0.6 water molecules per Tb(III) ion. In contrast, the Eu complex revealed surprising features such as: 1) the presence of two and up to five components in the (5)D(0)→(7)F(0) and (5)D(0)→(7)F(1) emission bands, respectively; 2) marked differences between the normalized spectra obtained in H(2)O and D(2)O solutions; and 3) unconventional temporal evolution of the luminescence intensity at certain wavelengths, the intensity profile first displaying a rising step before the occurrence of the expected decay. Additional spectroscopic experiments performed on [Gd(2-x)Eu(x)L] complexes (x = 0.1 and 1.9) confirmed the presence of two distinct Eu sites with hydration numbers of 0 (site I) and 2 (site II), and showed that the unconventional temporal evolution of the emission intensity is the result of an unprecedented intramolecular Eu-to-Eu energy-transfer process. A mathematical model was developed to interpret the experimental data, leading to energy-transfer rates of 0.98 ms(-1) for the transfer from the site with q=0 to that with q=2 and vice versa. Hartree-Fock (HF) and density functional theory (DFT) calculations performed at the B3LYP level were used to investigate the conformation of the complex in solution, and to estimate the intermetallic distance, which provided Förster radii (R(0)) values of 8.1 Å for the energy transfer from site I to site II, and 6.8 Å for the reverse energy transfer. These results represent the first evidence of an intramolecular energy-transfer equilibrium between two identical lanthanide cations within a discrete molecular complex in solution.

MIIIDyIII3(M = FeIII, CoIII) Complexes: Three-Blade Propellers Exhibiting Slow Relaxation of Magnetization

[Dy(III)(HBpz(3))(2)](2+) moieties (HBpz(3)(-) = hydrotris(pyrazolyl)borate) and a 3d transition-metal ion (Fe(III) or Co(III)) have been rationally assembled using an dithiooxalato dianion ligand into 3d-4f [MDy(3)(HBpz(3))(6)(dto)(3)]·4CH(3)CN·2CH(2)Cl(2) (M = Fe (1), Co (2) complexes. Single-crystal X-ray studies reveal that three eight-coordinated Dy(III) centers in a square antiprismatic coordination environment are connecting to a central octahedral trivalent Fe or Co ion forming a propeller-type complex. The dynamics of the magnetization in the two isostructural compounds, modulated by the nature of the central M(III) metal ion, are remarkably different despite their analogous direct current (dc) magnetic properties. The slow relaxation of the magnetization observed for 2 mainly originates from isolated Dy ions, since a diamagnetic Co(III) metal ion links the magnetic Dy(III) ions. In the case of 1, the magnetic interaction between S = 1/2 Fe(III) ion and the three Dy(III) magnetic centers, although weak, generates a complex energy spectrum of magnetic states with low-lying excited states that induce a smaller energy gap than for 2 and thus a faster relaxation of the magnetization.