Tuning the Emission of Homometallic DyIII, TbIII, and EuIII 1-D Coordination Polymers with 2,6-Di(1H-1,2,4-triazole-1-yl-methyl)-4-R-phenoxo Ligands: Sensitization through the Singlet State.

This work reports the structural characterization and photophysical properties of DyIII, TbIII, and EuIII coordination polymers with two phenoxo-triazole-based ligands [2,6-di(1H-1,2,4-triazole-1-yl-methyl)-4-R-phenoxo, LRTr (R = CH3; Cl)]. These ligands permitted us to obtain isostructural polymers, described as a 1D double chain, with LnIII being nona-coordinated. The energies of the ligand triplet (T1) states were estimated using low-temperature time-resolved emission spectra of YIII analogues. Compounds with LClTr present higher emission intensity than those with LMeTr. The emission of TbIII compounds was not affected by the different excitation wavelengths used and was emitted in the pure green region. In contrast, DyLMeTr emits in the blue-to-white region, while the luminescence of DyLClTr remains in the white region for all excitation wavelengths. On the other hand, EuIII compounds emit in the blue (ligand) or red region (EuIII) depending on the substituent of the phenoxo moiety and excitation wavelength. Theoretical calculations were employed to determine the excited states of the ligands by using time-dependent density functional theory. These calculations aided in modeling the intramolecular energy transfer and rationalizing the optical properties and demonstrated that the sensitization of the LnIII ions is driven via S1 → LnIII, a process that is less common as compared to T1 → LnIII.

Lanthanide SMMs Based on Belt Macrocycles: Recent Advances and General Trends.

Enhancement of axial magnetic anisotropy is the central objective to push forward the performance of Single-Molecule Magnet (SMM) complexes. In the case of mononuclear lanthanide complexes, the chemical environment around the paramagnetic ion must be tuned to place strongly interacting ligands along either the axial positions or the equatorial plane, depending on the oblate or prolate preference of the selected lanthanide. One classical strategy to achieve a precise chemical environment for a metal centre is using highly structured, chelating ligands. A natural approach for axial-equatorial control is the employment of macrocycles acting in a belt conformation, providing the equatorial coordination environment, and leaving room for axial ligands. In this review, we present a survey of SMMs based on the macrocycle belt motif. Literature systems are divided in three families (crown ether, Schiff-base and metallacrown) and their general properties in terms of structural stability and SMM performance are briefly discussed.

Luminescence of Macrocyclic Mononuclear DyIII Complexes and Their Immobilization on Functionalized Silicon-Based Surfaces.

Two mononuclear DyIII complexes, [Dy(L1)(NCS)3] (Dy-EDA) and [Dy(L2)(NCS)3] (Dy-DAP), where Ln (n = 1-2) corresponds to a macrocyclic ligand derived from 2,6-pyridinedicarboxaldehyde and ethylenediamine (L1) and 1,3-diaminepropane (L2) were immobilized on functionalized silicon-based surfaces. This was achieved by the microcontact printing (µCP) technique, generating patterns on a functionalized surface via covalent bond formation through the auxiliary -NCS ligands present in the macrocyclic complex species. With this strategy, it was possible to control the position of the immobilized molecules on the surface. Water contact angle measurements, X-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectra (IRRAS), and atomic force microscopy (AFM) confirmed that the surfaces were successfully functionalized. Furthermore, the optical properties in a broad temperature range were investigated for the as-prepared compounds. At room temperature, Dy-EDA was shown to emit in the deep blue region (Commission Internationald'Eclairage (CIE): (0.175, 0.128)), while Dy-DAP in the white region (CIE: (0.252, 0.312)). The different CIE values were due to the contribution of the strong emission of the ligand in the case of Dy-EDA. Besides, surface photoluminescence measurements showed that the immobilized complexes retained their bulk emissive properties.

Structure and excited-state dynamics of dimeric copper(i) photosensitizers investigated by time-resolved X-ray and optical transient absorption spectroscopy.

Time-resolved X-ray (tr-XAS) and optical transient absorption (OTA) spectroscopy in the picosecond time scale coupled with Density Functional theory (DFT) and X-ray absorption near-edge structure (XANES) calculations are applied to study three homoleptic Cu(i) dimeric chromophores with ethyl and longer propyl spacers, denoted as [Cu2(mphenet)2]Cl2 (C1), [Cu2(mphenet)2](ClO4)2 (C2) and [Cu2(mphenpr)2](ClO4)2 (C3) (where mphenet = 1,2-bis(9-methyl-1,10-phenanthrolin-2-yl)ethane and mphenpr = 1,3-bis(9-methyl-1,10-phenanthrolin-2-yl)propane). Tr-XAS analysis after light illumination at ∼ 100 ps illustrate the formation of a flattened triplet excited state in all 3 complexes. Optical transient absorption (OTA) analysis for C1 monitored in water and C2 and C3 measured in acetonitrile reveals distinct excited-state lifetimes of 169 ps, 670 ps and 1600 ps respectively. These differences are associated to changes in the solvent (comparing C1 and C2) and the flexibility of the ligand to adapt after Cu flattening upon excitation (C2 and C3). Our results are important for the improved structural dynamics of these types of Cu-based dimeric compounds, and can guide the integration of these chromophores into more complex solar energy conversion schemes.

Room-Temperature Spin-Dependent Transport in Metalloporphyrin-Based Supramolecular Wires.

Here we present room-temperature spin-dependent charge transport measurements in single-molecule junctions made of metalloporphyrin-based supramolecular assemblies. They display large conductance switching for magnetoresistance in a single-molecule junction. The magnetoresistance depends acutely on the probed electron pathway through the supramolecular wire: those involving the metal center showed marked magnetoresistance effects as opposed to those exclusively involving the porphyrin ring which present nearly complete absence of spin-dependent charge transport. The molecular junction magnetoresistance is highly anisotropic, being observable when the magnetization of the ferromagnetic junction electrode is oriented along the main molecular junction axis, and almost suppressed when it is perpendicular. The key ingredients for the above effect to manifest are the electronic structure of the paramagnetic metalloporphyrin, and the spinterface created at the molecule-electrode contact.

Reactivity of CuIN4 Flattened Complexes: Interplay between Coordination Geometry and Ligand Flexibility.

The relation between redox activity and coordination geometry in CuIN4 complexes indicates that more flattened structures tend to be more reactive. Such a preorganization of the ligand confers to the complex geometries closer to a transition state, which has been termed the "entatic" state in metalloproteins, more recently extending this concept for copper complexes. However, many aspects of the redox chemistry of CuI complexes cannot be explained only by flattening. For instance, the role of ligand flexibility in this context is an open debate nowadays. To analyze this point, we studied oxidation properties of a series of five monometallic CuI Schiff-base complexes, [CuI(Ln)]+, which span a range of geometries from a distorted square planar (n = 3) to a distorted tetrahedron (n = 6, 7). This stepped control of the structure around the CuI atom allows us to explore the effect of the flattening distortion on both the electronic and redox properties through the series. Experimental studies were complemented by a theoretical analysis based on density functional theory calculations. As expected, oxidation was favored in the flattened structures, spanning a broad potential window of 370 mV for the complete series. This orderly behavior was tested in the reductive dehalogenation reaction of tetrachloroethane (TCE). Kinetic studies show that CuI oxidation by TCE is faster as the flattening distortion is higher and the oxidation potentials of the metal are lower. However, the most reactive complex was not the more planar, contradicting the trend expected from oxidation potentials. The origin of this irregularity is related to ligand flexibility and its connection with the atom/electron transfer reaction path, highlighting the need to consider effects beyond flattening distortion to better understand the reactivity of this important class of complexes.

Tuning Single-Molecule Conductance in Metalloporphyrin-Based Wires via Supramolecular Interactions.

Nature has developed supramolecular constructs to deliver outstanding charge-transport capabilities using metalloporphyrin-based supramolecular arrays. Herein we incorporate simple, naturally inspired supramolecular interactions via the axial complexation of metalloporphyrins into the formation of a single-molecule wire in a nanoscale gap. Small structural changes in the axial coordinating linkers result in dramatic changes in the transport properties of the metalloporphyrin-based wire. The increased flexibility of a pyridine-4-yl-methanethiol ligand due to an extra methyl group, as compared to a more rigid 4-pyridinethiol linker, allows the pyridine-4-yl-methanethiol ligand to adopt an unexpected highly conductive stacked structure between the two junction electrodes and the metalloporphyrin ring. DFT calculations reveal a molecular junction structure composed of a shifted stack of the two pyridinic linkers and the metalloporphyrin ring. In contrast, the more rigid 4-mercaptopyridine ligand presents a more classical lifted octahedral coordination of the metalloporphyrin metal center, leading to a longer electron pathway of lower conductance. This works opens to supramolecular electronics, a concept already exploited in natural organisms.

[UF6 ]2- : A Molecular Hexafluorido Actinide(IV) Complex with Compensating Spin and Orbital Magnetic Moments.

The first structurally characterized hexafluorido complex of a tetravalent actinide ion, the [UF6 ]2- anion, is reported in the (NEt4 )2 [UF6 ]⋅2 H2 O salt (1). The weak magnetic response of 1 results from both UIV spin and orbital contributions, as established by combining X-ray magnetic circular dichroism (XMCD) spectroscopy and bulk magnetization measurements. The spin and orbital moments are virtually identical in magnitude, but opposite in sign, resulting in an almost perfect cancellation, which is corroborated by ab initio calculations. This work constitutes the first experimental demonstration of a seemingly non-magnetic molecular actinide complex carrying sizable spin and orbital magnetic moments.

Steric and Electronic Factors Affecting the Conformation of Bimetallic CuI Complexes: Effect of the Aliphatic Spacer of Tetracoordinating Schiff-Base Ligands.

A family of six homoleptic [CuI (Ln )]2 (ClO4 )2 and six heteroleptic [CuI (Ln )(PPh3 )2 ]2 (ClO4 )2 bimetallic complexes, in which Ln are bis-Schiff base ligands with alkyl spacers of variable length (n=2-7 -CH2 -), were prepared to evaluate the role of the spacer on the formation of helicates or mesocates. In the homoleptic series, spectroscopic and theoretical studies indicate that preferences for a conformation are based on energetic parameters, mainly, the establishment of noncovalent interactions. The odd-even nature of the spacers preconditions the superposition of the aromatic rings to allow the juxtaposition necessary for noncovalent interactions, whereas the increase of the length reduces the strength of such interactions. Consequently, complexes with even-spacers of short length were identified as helicates in solution, [CuI (Ln )]22+ (n=2, 4). Complexes [CuI (Ln )]22+ (n=3-7) dissociate in solution to produce the monometallic complexes in equilibrium, [CuI (Ln )]+ . The stability of the bimetallic species is discussed in terms of their conformations. The set of heteroleptic complexes was prepared to evaluate the reach of the "odd-even rule" in the solid, which is based on the "zig-zag" arrangements of the spacers. Based on crystallographic information, "S-" and "C"-type conformations of Ln are related to even and odd spacers, respectively. This trend is considered in addition to other factors to explain preferences for either a mesocate or helicate conformation in the homoleptic series.

Effect of Heteroatoms on Field-Induced Slow Magnetic Relaxation of Mononuclear FeIII ( S = 5/2) Ions within Polyoxometalates.

In this paper, the synthesis and magnetic properties of mononuclear FeIII-containing polyoxometalates (POMs) with different types of heteroatoms, TBA7H10[(A-α-XW9O34)2Fe] (IIX, X = Ge, Si; TBA = tetra- n-butylammonium), are reported. In these POMs, mononuclear highly distorted six-coordinate octahedral [FeO6]9- units are sandwiched by two trivacant lacunary units [A-α-XW9O34]10- (X = Ge, Si). These POMs exhibit field-induced slow magnetic relaxation based on the single high-spin FeIII magnetic center ( S = 5/2). Combining experiment and ab initio calculations, we investigated the effect of heteroatoms of the lacunary units on the field-induced slow magnetic relaxation of these POMs. By changing the heteroatoms from Si (IISi) to Ge (IIGe), the coordination geometry around the FeIII ion is mildly changed. Concretely, the axial Fe-O bond length in IIGe is shortened compared with that in IISi, and consequently the distortion of the [FeO6]9- unit in IIGe from the ideal octahedral coordination geometry becomes larger than that in IISi. The effective demagnetization barrier of IIGe (11.4 K) is slightly larger than that of IISi (9.2 K). Multireference ab initio calculations predict zero-field splitting parameters in good agreement with experiment. Although the differences in the coordination geometries and magnetic properties of IIGe and IISi are quite small, ab initio calculations indicate subtle changes in the magnetic anisotropy which are in line with the observed magnetic relaxation properties.

Designing a Dy2 Single-Molecule Magnet with Two Well-Differentiated Relaxation Processes by Using a Nonsymmetric Bis-bidentate Bipyrimidine- N-Oxide Ligand: A Comparison with Mononuclear Counterparts.

Herein we report a dinuclear [(µ-mbpymNO){(tmh)3Dy}2] (1) single-molecule magnet (SMM) showing two nonequivalent DyIII centers, which was rationally prepared from the reaction of Dy(tmh)3 moieties (tmh = 2,2,6,6-tetramethyl-3,5-heptanedionate) and the asymmetric bis-bidentate bridging ligand 4-methylbipyrimidine (mbpymNO). Depending on whether the DyIII ions coordinate to the N^O or N^N bidentate donor sets, the DyIII sites present a NO7 ( D2 d geometry) or N2O6 ( D4 d) coordination sphere. As a consequence, two different thermally activated magnetic relaxation processes are observed with anisotropy barriers of 47.8 and 54.7 K. Ab initio calculations confirm the existence of two different relaxation phenomena and allow one to assign the 47.8 and 54.7 K energy barriers to the Dy(N2O6) and Dy(NO7) sites, respectively. Two mononuclear complexes, [Dy(tta)3(mbpymNO)] (2) and [Dy(tmh)3(phenNO)] (3), have also been prepared for comparative purposes. In both cases, the DyIII center shows a NO7 coordination sphere and SMM behavior is observed with Ueff values of 71.5 K (2) and 120.7 K (3). In all three cases, ab initio calculations indicate that relaxation of the magnetization takes place mainly via the first excited-state Kramers doublet through Orbach, Raman, and thermally assisted quantum-tunnelling mechanisms. Pulse magnetization measurements reveal that the dinuclear and mononuclear complexes exhibit hysteresis loops with double- and single-step structures, respectively, thus supporting their SMM behavior.

Metal-Controlled Magnetoresistance at Room Temperature in Single-Molecule Devices.

The appropriate choice of the transition metal complex and metal surface electronic structure opens the possibility to control the spin of the charge carriers through the resulting hybrid molecule/metal spinterface in a single-molecule electrical contact at room temperature. The single-molecule conductance of a Au/molecule/Ni junction can be switched by flipping the magnetization direction of the ferromagnetic electrode. The requirements of the molecule include not just the presence of unpaired electrons: the electronic configuration of the metal center has to provide occupied or empty orbitals that strongly interact with the junction metal electrodes and that are close in energy to their Fermi levels for one of the electronic spins only. The key ingredient for the metal surface is to provide an efficient spin texture induced by the spin-orbit coupling in the topological surface states that results in an efficient spin-dependent interaction with the orbitals of the molecule. The strong magnetoresistance effect found in this kind of single-molecule wire opens a new approach for the design of room-temperature nanoscale devices based on spin-polarized currents controlled at molecular level.

Large Conductance Switching in a Single-Molecule Device through Room Temperature Spin-Dependent Transport.

Controlling the spin of electrons in nanoscale electronic devices is one of the most promising topics aiming at developing devices with rapid and high density information storage capabilities. The interface magnetism or spinterface resulting from the interaction between a magnetic molecule and a metal surface, or vice versa, has become a key ingredient in creating nanoscale molecular devices with novel functionalities. Here, we present a single-molecule wire that displays large (>10000%) conductance switching by controlling the spin-dependent transport under ambient conditions (room temperature in a liquid cell). The molecular wire is built by trapping individual spin crossover Fe(II) complexes between one Au electrode and one ferromagnetic Ni electrode in an organic liquid medium. Large changes in the single-molecule conductance (>100-fold) are measured when the electrons flow from the Au electrode to either an α-up or a ß-down spin-polarized Ni electrode. Our calculations show that the current flowing through such an interface appears to be strongly spin-polarized, thus resulting in the observed switching of the single-molecule wire conductance. The observation of such a high spin-dependent conductance switching in a single-molecule wire opens up a new door for the design and control of spin-polarized transport in nanoscale molecular devices at room temperature.

Lanthanide Tetrazolate Complexes Combining Single-Molecule Magnet and Luminescence Properties: The Effect of the Replacement of Tetrazolate N3 by ß-Diketonate Ligands on the Anisotropy Energy Barrier.

Three new sets of mononuclear Ln(III) complexes of general formulas [LnL3 ]⋅CH3 OH [Ln(III) =Yb (1), Er (2), Dy (3), Gd (4), and Eu (5)], [LnL2 (tmh)(CH3 OH)]⋅n H2 O⋅m CH3 OH [Ln(III) =Yb (1 b), Er (2 b), Dy (3 b), Gd (4 b)], and [LnL2 (tta)(CH3 OH)]⋅CH3 OH [Ln(III) =Yb (1 c), Er (2 c), Dy (3 c), Gd (4 c)] were prepared by the reaction of Ln(CF3 SO3 )⋅n H2 O salts with the tridentate ligand 2-(tetrazol-5-yl)-1,10-phenanthroline (HL) and, for the last two sets, additionally with the ß-diketonate ligands 2,2,6,6-tetramethylheptanoate (tmh) and 2-thenoyltrifluoroacetonate (tta), respectively. In the [LnL3 ]⋅CH3 OH complexes the Ln(III) ions are coordinated to three phenanthroline tetrazolate ligands with an LnN9 coordination sphere. Dynamic ac magnetic measurements on 1-3 reveal that these complexes only exhibit single-molecule magnet (SMM) behavior when an external dc magnetic field is applied, with Ueff values of 11.7 K (1), 16.0 K (2), and 20.2 K (3). When the tridentate phenanthroline tetrazolate ligand is replaced by one molecule of methanol and the ß-diketonate ligand tmh (1 b-3 b) or tta (1 c-3 c), a significant increase in Ueff occurs and, in the case of the Dy(III) complexes 3 b and 3 c, out-of-phase χ'' signals below 15 and 10 K, respectively, are observed in zero dc magnetic field. CASSCF+RASSI ab initio calculations performed on the Dy(III) complexes support the experimental results. Thus, for 3 the ground Kramers' doublet is far from being axial and the first excited state is found to be very close in energy to the ground state, so the relaxation barrier in this case is almost negligible. Conversely, for 3 b and 3 c, the ground Kramers' doublet is axial with a small quantum tunneling of the magnetization, and the energy difference between the ground and first Kramers' doublets is much higher, which allows these compounds to behave as SMMs at zero field. Moreover, these calculations support the larger Ueff observed for 3 b compared to 3 c. Additionally, the solid-state photophysical properties of 1, 2, 4, and 5 show that the phenanthroline tetrazolate ligand can act as an effective antenna to sensitize the characteristic Yb(III) , Er(III) , and Eu(III) emissions through an energy-transfer process.

Periodic Trends in Lanthanide Compounds through the Eyes of Multireference ab Initio Theory.

Regularities among electronic configurations for common oxidation states in lanthanide complexes and the low involvement of f orbitals in bonding result in the appearance of several periodic trends along the lanthanide series. These trends can be observed on relatively different properties, such as bonding distances or ionization potentials. Well-known concepts like the lanthanide contraction, the double-double (tetrad) effect, and the similar chemistry along the lanthanide series stem from these regularities. Periodic trends on structural and spectroscopic properties are examined through complete active space self-consistent field (CASSCF) followed by second-order N-electron valence perturbation theory (NEVPT2) including both scalar relativistic and spin-orbit coupling effects. Energies and wave functions from electronic structure calculations are further analyzed in terms of ab initio ligand field theory (AILFT), which allows one to rigorously extract angular overlap model ligand field, Racah, and spin-orbit coupling parameters directly from high-level ab initio calculations. We investigated the elpasolite Cs2NaLn(III)Cl6 (Ln(III) = Ce-Nd, Sm-Eu, Tb-Yb) crystals because these compounds have been synthesized for most Ln(III) ions. Cs2NaLn(III)Cl6 elpasolites have been also thoroughly characterized with respect to their spectroscopic properties, providing an exceptionally vast and systematic experimental database allowing one to analyze the periodic trends across the lanthanide series. Particular attention was devoted to the apparent discrepancy in metal-ligand covalency trends between theory and spectroscopy described in the literature. Consistent with earlier studies, natural population analysis indicates an increase in covalency along the series, while a decrease in both the nephelauxetic (Racah) and relativistic nephelauxetic (spin-orbit coupling) reduction with increasing atomic number is calculated. These apparently conflicting results are discussed on the basis of AILFT parameters. The AILFT derived parameters faithfully reproduce the underlying multireference electronic structure calculations. The remaining discrepancies with respect to experimentally derived data are mostly due to underestimation of the ligand field splittings, while the dynamic correlation and nephelauxetic effects appears to be adequately covered by CASSCF/NEVPT2.

Single-Molecule Magnet Properties of Transition-Metal Ions Encapsulated in Lacunary Polyoxometalates: A Theoretical Study.

Single-molecule magnet (SMM) properties of transition-metal complexes coordinated to lacunary polyoxometalates (POM) are studied by means of state of the art ab initio methodology. Three [M(γ-SiW10O36)2] (M = Mn(III), Fe(III), Co(II)) complexes synthesized by Sato et al. (Chem. Commun. 2015, 51, 4081-4084) are analyzed in detail. SMM properties for the Co(II) and Mn(III) systems can be rationalized due to the presence of low-energy excitations in the case of Co(II), which are much higher in energy in the case of Mn(III). The magnetic behavior of both cases is consistent with simple d-orbital splitting considerations. The case of the Fe(III) complex is special, as it presents a sizable demagnetization barrier for a high-spin d(5) configuration, which should be magnetically isotropic. We conclude that a plausible explanation for this behavior is related to the presence of low-lying quartet and doublet states from the iron(III) center. This scenario is supported by ab initio ligand field analysis based on complete active space self-consistent field results, which picture a d-orbital splitting that resembles more a square-planar geometry than an octahedral one, stabilizing lower multiplicity states. This coordination environment is sustained by the rigidity of the POM ligand, which imposes a longer axial bond distance to the inner oxygen atom in comparison to the more external, equatorial donor atoms.

Effect of metal complexation on the conductance of single-molecular wires measured at room temperature.

The present work aims to give insight into the effect that metal coordination has on the room-temperature conductance of molecular wires. For that purpose, we have designed a family of rigid, highly conductive ligands functionalized with different terminations (acetylthiols, pyridines, and ethynyl groups), in which the conformational changes induced by metal coordination are negligible. The single-molecule conductance features of this series of molecular wires and their corresponding Cu(I) complexes have been measured in break-junction setups at room temperature. Experimental and theoretical data show that no matter the anchoring group, in all cases metal coordination leads to a shift toward lower energies of the ligand energy levels and a reduction of the HOMO-LUMO gap. However, electron-transport measurements carried out at room temperature revealed a variable metal coordination effect depending on the anchoring group: upon metal coordination, the molecular conductance of thiol and ethynyl derivatives decreased, whereas that of pyridine derivatives increased. These differences reside on the molecular levels implied in the conduction. According to quantum-mechanical calculations based on density functional theory methods, the ligand frontier orbital lying closer to the Fermi energy of the leads differs depending on the anchoring group. Thereby, the effect of metal coordination on molecular conductance observed for each anchoring could be explained in terms of the different energy alignments of the molecular orbitals within the gold Fermi level.

Rational electrostatic design of easy-axis magnetic anisotropy in a Zn(II) -Dy(III) -Zn(II) single-molecule magnet with a high energy barrier.

Two novel trinuclear complexes [ZnCl(µ-L)Ln(µ-L)ClZn][ZnCl3 (CH3 OH)]⋅3 CH3 OH (Ln(III) =Dy (1) and Er (2)) have been prepared from the compartmental ligand N,N'-dimethyl-N,N'-bis(2-hydroxy-3-formyl-5-bromo-benzyl)ethylenediamine (H2 L). X-ray studies reveal that Ln(III) ions are coordinated by two [ZnCl(L)](-) units through the phenoxo and aldehyde groups, giving rise to a LnO8 coordination sphere with square-antiprism geometry and strong easy-axis anisotropy of the ground state. Ab initio CASSCF+RASSI calculations carried out on 1 confirm that the ground state is an almost pure MJ =±15/2 Kramers doublet with a marked axial anisotropy, the magnetic moment is roughly collinear with the shortest DyO distances. This orientation of the local magnetic moment of the Dy(III) ion in 1 is adopted to reduce the electronic repulsion between the oblate electron shape of the MJ =±15/2 Kramers doublet and the phenoxo-oxygen donor atoms involved in the shortest DyO bonds. CASSCF+RASSI calculations also show that the ground and first excited states of the Dy(III) ion are separated by 129 cm(-1) . As expected for this large energy gap, compound 1 exhibits, in a zero direct-current field, thermally activated slow relaxation of the magnetization with a large Ueff =140 K. The isostructural Zn-Er-Zn species does not present significant SMM behavior as expected for the prolate electron-density distribution of the Er(III) ion leading to an easy-plane anisotropy of the ground doublet state.

Two C3 -symmetric Dy3III complexes with triple di-µ-methoxo-µ-phenoxo bridges, magnetic ground state, and single-molecule magnetic behavior.

Two series of isostructural C(3)-symmetric Ln(3) complexes Ln(3)⋅[BPh(4)] and Ln(3)⋅0.33[Ln(NO(3))(6)] (in which Ln(III) =Gd and Dy) have been prepared from an amino-bis(phenol) ligand. X-ray studies reveal that Ln(III) ions are connected by one µ(2)-phenoxo and two µ(3)-methoxo bridges, thus leading to a hexagonal bipyramidal Ln(3)O(5) bridging core in which Ln(III) ions exhibit a biaugmented trigonal-prismatic geometry. Magnetic susceptibility studies and ab initio complete active space self-consistent field (CASSCF) calculations indicate that the magnetic coupling between the Dy(III) ions, which possess a high axial anisotropy in the ground state, is very weakly antiferromagnetic and mainly dipolar in nature. To reduce the electronic repulsion from the coordinating oxygen atom with the shortest Dy-O distance, the local magnetic moments are oriented almost perpendicular to the Dy(3) plane, thus leading to a paramagnetic ground state. CASSCF plus restricted active space state interaction (RASSI) calculations also show that the ground and first excited state of the Dy(III) ions are separated by approximately 150 and 177 cm(-1), for Dy(3)⋅[BPh(4)] and Dy(3)⋅0.33[Dy(NO(3))(6)], respectively. As expected for these large energy gaps, Dy(3)⋅[BPh(4)] and Dy(3)⋅0.33[Dy(NO(3)(6)] exhibit, under zero direct-current (dc) field, thermally activated slow relaxation of the magnetization, which overlap with a quantum tunneling relaxation process. Under an applied Hdc field of 1000 Oe, Dy(3)⋅[BPh(4)] exhibits two thermally activated processes with U(eff) values of 34.7 and 19.5 cm(-1), whereas Dy(3)⋅0.33[Dy(NO(3))(6)] shows only one activated process with Ueff =19.5 cm(-1).

Guest modulation of spin-crossover transition temperature in a porous iron(II) metal-organic framework: experimental and periodic DFT studies.

The synthesis, structure, and magnetic properties of three clathrate derivatives of the spin-crossover porous coordination polymer {Fe(pyrazine)[Pt(CN)4]} (1) with five-membered aromatic molecules furan, pyrrole, and thiophene is reported. The three derivatives have a cooperative spin-crossover transition with hysteresis loops 14-29 K wide and average critical temperatures Tc =201 K (1⋅fur), 167 K (1⋅pyr), and 114.6 K (1⋅thio) well below that of the parent compound 1 (Tc =295 K), confirming stabilization of the HS state. The transition is complete and takes place in two steps for 1⋅fur, while 1⋅pyr and 1⋅thio show 50 % spin transition. For 1⋅fur the transformation between the HS and IS (middle of the plateau) phases occurs concomitantly with a crystallographic phase transition between the tetragonal space groups P4/mmm and I4/mmm, respectively. The latter space group is retained in the subsequent transformation involving the IS and the LS phases. 1⋅pyr and 1⋅thio display the tetragonal P4/mmm and orthorhombic Fmmm space groups, respectively, in both HS and IM phases. Periodic calculations using density functional methods for 1⋅fur, 1⋅pyr, 1⋅thio, and previously reported derivatives 1⋅CS2 , 1⋅I, 1⋅bz(benzene), and 1⋅pz(pyrazine) have been carried out to investigate the electronic structure and nature of the host-guest interactions as well as their relationship with the changes in the LS-HS transition temperatures of 1⋅Guest. Geometry-optimized lattice parameters and bond distances in the empty host 1 and 1⋅Guest clathrates are in general agreement with the X-ray diffraction data. The concordance between the theoretical results and the experimental data also comprises the guest molecule orientation inside the host and intermolecular distances. Furthermore, a general correlation between experimental Tc and calculated LS-HS electronic energy gap was observed. Finally, specific host-guest interactions were studied through interaction energy calculations and crystal orbital displacement (COD) curve analysis.