The curious case of CO2 dissociation on Cu(110).

Dissociation of CO2 on copper surfaces is an important model system for understanding the elementary steps in catalytic conversion of CO2 to methanol. Using molecular beam-surface scattering methods, we measure the initial dissociation probabilities (S0) of CO2 on a flat, clean Cu(110) surface under ultrahigh vacuum conditions. The observed S0 ranges from 3.9 × 10-4 to 1.8 × 10-2 at incidence energies of 0.64-1.59 eV. By extrapolating the trend observed in the incidence energy dependence of S0, we estimate the lower limit of the dissociation barrier on terrace sites to be around 2 eV. We discuss these results in the context of what is known from previous studies on this system using different experiments and theoretical/computational methods. These findings are anticipated to be valuable for correctly understanding the elementary steps in CO2 dissociation on Cu surfaces.

Stabilizing hydroperoxyflavin intermediate formation via a peptide appendage: a neutral flavoenzyme model.

A bioinspired mimic for the stabilization of hydroperoxyflavin intermediate formation was designed and investigated for monooxygenase like catalytic properties. A suitable peptide appendage was covalently linked to the C7-position of the neutral isoalloxazine core to synthesize Fl-G, Fl-F, Fl-P, and Fl-ßA analogues. While the presence and identity of the peptide appendage were found to be crucial for catalytic efficiency, corroborative observations were made from theoretical studies as well, supporting the precise conformational and accessibility requirements for the stabilization of the key hydroperoxyflavin intermediate. A simple yet elegant flavopeptide model (Fl-G) was found to achieve almost quantitative catalytic efficiency compared to the control flavin analogue without a peptide appendage.

Unsaturated Sulfur Crown Ethers Can Extract Mercury(II) and Show Promise for Future Copernicium(II) Studies: A Combined Experimental and Computational Study.

The unsaturated hexathia-18-crown-6 (UHT18C6) molecule was investigated for the extraction of Hg(II) in HCl and HNO3 media. This extractant can be directly compared to the recently studied saturated hexathia-18-crown-6 (HT18C6). The default conformation of the S lone pairs in UHT18C6 is endodentate, where the pocket of the charge density, according to the crystal structures, is oriented toward the center of the ring, which should allow better extraction for Hg(II) compared to the exodentate HT18C6. Batch study experiments showed that Hg(II) had better extraction at low acid molarity (ca. 99% in HCl and ca. 95% in HNO3), while almost no extraction was observed above 0.4 M HCl and 4 M HNO3 (<5%). Speciation studies were conducted with the goal of delineating a plausible extraction mechanism. Density functional theory calculations including relativistic effects were carried out on both Hg(II)-encapsulated HT18C6 and UHT18C6 complexes to shed light on the binding strength and the nature of bonding. Our calculations offer insights into the extraction mechanism. In addition to Hg(II), calculations were performed on the hypothetical divalent Cn(II) ion, and showed that HT18C6 and UHT18C6 could extract Cn(II). Finally, the extraction kinetics were explored to assess whether this crown can extract the short-lived Cn(II) species in a future online experiment.

Comprehensive Studies of Magnetic Transitions and Spin-Phonon Couplings in the Tetrahedral Cobalt Complex Co(AsPh3)2I2.

A combination of inelastic neutron scattering (INS), far-IR magneto-spectroscopy (FIRMS), and Raman magneto-spectroscopy (RaMS) has been used to comprehensively probe magnetic excitations in Co(AsPh3)2I2 (1), a reported single-molecule magnet (SMM). With applied field, the magnetic zero-field splitting (ZFS) peak (2D') shifts to higher energies in each spectroscopy. INS placed the ZFS peak at 54 cm-1, as revealed by both variable-temperature (VT) and variable-magnetic-field data, giving results that agree well with those from both far-IR and Raman studies. Both FIRMS and RaMS also reveal the presence of multiple spin-phonon couplings as avoided crossings with neighboring phonons. Here, phonons refer to both intramolecular and lattice vibrations. The results constitute a rare case in which the spin-phonon couplings are observed with both Raman-active (g modes) and far-IR-active phonons (u modes; space group P21/c, no. 14, Z = 4 for 1). These couplings are fit using a simple avoided crossing model with coupling constants of ca. 1-2 cm-1. The combined spectroscopies accurately determine the magnetic excited level and the interaction of the magnetic excitation with phonon modes. Density functional theory (DFT) phonon calculations compare well with INS, allowing for the assignment of the modes and their symmetries. Electronic calculations elucidate the nature of ZFS in the complex. Features of different techniques to determine ZFS and other spin-Hamiltonian parameters in transition-metal complexes are summarized.

Correlating Electronic Structure and Magnetic Anisotropy in Actinide Complexes [An(COT)2], AnIII/IV = U, Np, and Pu.

The electronic structures and magnetic anisotropies for compounds [An(COT)2] (An = UIII/UIV, NpIII/NpIV and PuIII/PuIV, COT = cyclooctatetraene) are characterized using scalar relativistic density functional theory calculations and second-order perturbation theory based on a complete active space self-consistent field reference including spin-orbit coupling. The degree of participation of 5f orbitals in actinide-ligand bonding and the associated metal-ligand covalency is found to trend as U > Np ≥ Pu for both the tetra-positive and tripositive An complexes. A spin-Hamiltonian analysis indicates only weak single-molecule magnet (SMM) characteristics for [U(COT)2]- and [Np(COT)2] complexes and no significant SMM behavior for the other complexes. The weak SMM behavior in [U(COT)2]- and [Np(COT)2] is attributed to a subtle interplay between local symmetry and ligand-field splitting. Such a result suggests that magnetic anisotropy in 5f3 ions can be modulated in general by electrostatic ligand field design. In particular, σ-donor ligands oriented 180 degrees relative to one another will have a maximal influence on the 5f-orbital ligand field splitting, while π donors like cyclopentadiene and COT generate ligand field influences that have more acute angles associated with corresponding atoms on the individual ligands. These observations rationalize the differences in SMM characteristics for [U(BcMe)3] (BcMe- = dihydrobis(methylimidazolyl)borate) and [U(BpMe)3] (BpMe- = dihydrobis(methylpyrazolyl)borate) and indicate strategies to design new actinide-based SMMs with high magnetic relaxation barriers.

Role of (1,3) {Cu-Cu} Interaction on the Magneto-Caloric Effect of Trinuclear {CuII-GdIII-CuII} Complexes: Combined DFT and Experimental Studies.

Molecular refrigeration is found to be of great interest in the field of coordination chemistry, and GdIII ion based complexes are particularly attractive, as they exhibit a large magneto-caloric effect (MCE). As the magnetic coupling in GdIII clusters is difficult to control, other avenues to enhance the MCE values have been explored and incorporation of 3d metal ions in the cluster aggregation with GdIII yielding {3d-Gd} clusters are targeted. Among the transition-metal ions, the CuII ion is particularly attractive, as it does not possess any anisotropy, and in this regard, several di- and polynuclear {Cu-Gd} clusters are reported to yield attractive MCE values. While the role of near-neighbor {Cu-Gd} interactions in the MCE has been explored in detail, how the next-nearest-neighbor interaction influences the MCE has not been explored. To explore the importance of next-nearest-neighbor (1,3) {Cu-Cu} interaction, we have undertaken detailed density functional studies on five trinuclear {CuII-GdIII-CuII} complexes that are reported in the literature. In addition, we also report the synthesis and magnetic and EPR studies of a novel complex [(CuSALen)2Gd(NO3)3] (6; where SALen is N,N'-ethylenebis(salicylaldiminato)). Both magnetic and EPR studies reveal an S = 9/2 ground state for 6 with a very small zero-field splitting parameter (+0.01 cm-1), which aid in the achievement of a large MCE value for this molecule. Magnetization data collected for 6 yield a magnetic entropy change (-ΔSm) of 17 J kg-1 K-1 at 3.5 K by employing a 7 T magnetic field. Our calculations on all six complexes reveal that {Cu-Gd} exchange is ferromagnetic in nature, while the next-nearest-neighbor {Cu-Cu} exchange is found to vary from a weak ferromagnetic to a moderate antiferromagnetic interaction. In all of the cases studied, simulated susceptibility data are in excellent agreement with the experimental data, offering confidence in the computed J values. In addition, we have developed a mechanism of magnetic coupling for {CuII-GdIII-CuII} trinuclear complexes, where the role of formally empty 5d, 6s, and 6p orbitals of GdIII ion is established. In particular, our studies reveal that the next-nearest-neighbor {Cu-Cu} interaction is strongly correlated to Cu-Gd-Cu angle, with both smaller and larger angles yielding stronger antiferromagnetic exchange. The antiferromagnetic {Cu-Cu} interaction diminishes the gap between the ground S = 9/2 state and first excited S = 7/2 state, leading to enhancement of MCE values. In contrast to the general belief that weak interactions are desired for large MCE, our study advocates targeting a stronger antiferromagnetic {Cu-Cu} interaction to obtain larger MCE values in this class of clusters.

Role of Halide Ions in the Nature of the Magnetic Anisotropy in Tetrahedral CoII Complexes.

A series of mononuclear tetrahedral CoII complexes with a general molecular formula [CoL2 X2 ] [L=thiourea and X=Cl (1), Br (2) and I (3)] were synthesized and their structures were characterized by single-crystal X-ray diffraction. Direct-current (dc) magnetic susceptibility [χM T(T) and M(H)] and its slow relaxation of magnetization were measured for all three complexes. The experimental dc magnetic data are excellently reproduced by fitting both χM T(T) and M(H) simultaneously with the parameters D=+10.8 cm-1 , g1 =2.2, g2 =2.2, and g3 =2.4 for 1; D=-18.7 cm-1 , giso =2.21 for 2; and D=-19.3 cm-1 , giso =2.3 for 3. The replacement of chloride in 1 by bromide or iodide (in 2 and 3, respectively) was accompanied by a change in both sign and magnitude of the magnetic anisotropy D. Field-induced out-of-phase susceptibility signals observed in 10 % diluted samples of 1-3 imply slow relaxation of magnetization of molecular origin. To better understand the magnetization relaxation dynamics of complexes 1-3, detailed ab initio CASSCF/NEVPT2 calculations were performed. The computed spin Hamiltonian parameters are in good agreement with experimental data. In particular, the calculations unveil the role of halide ions in switching the sign of D on moving from Cl- to I- . The large spin-orbit coupling constant associated with the heavier halide ion and weaker π donation reduces the ground state-excited state gap, which leads to a larger contribution to negative D for complex 3 compared to complex 1. Further magnetostructural D correlations were developed to understand the role of structural distortion in the sign and magnitude of D values in this family of complexes.

Role of the Diamagnetic Zinc(II) Ion in Determining the Electronic Structure of Lanthanide Single-Ion Magnets.

Four complexes containing DyIII and PrIII ions and their LnIII -ZnII analogs have been synthesized in order to study the influence that a diamagnetic ZnII ion has on the electronic structure and hence, the magnetic properties of the DyIII and PrIII single ions. Single-crystal X-ray diffraction revealed the molecular structures as [DyIII (HL)2 (NO3 )3 ] (1), [PrIII (HL)2 (NO3 )3 ] (2), [ZnII DyIII (L)2 (CH3 CO2 )(NO3 )2 ] (3) and [ZnII2 PrIII (L)2 (CH3 CO2 )4 (NO3 )] (4) (where HL=2-methoxy-6-[(E)-phenyliminomethyl]phenol). The dc and ac magnetic data were collected for all four complexes. Compounds 1 and 3 display frequency dependent out-of-phase susceptibility signals (χM "), which is a characteristic signature for a single-molecule magnet (SMM). Although 1 and 3 are chemically similar, a fivefold increase in the anisotropic barrier (Ueff ) is observed experimentally for 3 (83 cm-1 ), compared to 1 (16 cm-1 ). To rationalize the larger anisotropic barrier (1 vs. 3), detailed ab initio calculations were performed. Although the ground state Kramer's doublet in both 1 and 3 are axial in nature (gzz =19.443 for 1 and 18.82 for 3), a significant difference in the energy gap (Ueff ) between the ground and first excited Kramer's doublet is calculated. This energy gap is governed by the electrostatic repulsion between the DyIII ion and the additional charge density found for the phenoxo bridging ligand in 3. This extra charge density was found to be a consequence of the presence of the diamagnetic ZnII ion present in the complex. To explore the influence of diamagnetic ions on the magnetic properties further, previously reported and structurally related Zn-DyIII complexes were analyzed. These structurally analogous complexes unambiguously suggest that the electrostatic repulsion is found to be maximal when the Zn-O-Dy-O dihedral angle is small, which is an ideal condition to maximize the anisotropic barrier in DyIII complexes.

[OsF6 ]x- : Molecular Models for Spin-Orbit Entangled Phenomena.

Heavy 5d elements, like osmium, feature strong spin-orbit interactions which are at the origin of exotic physical behaviors. Revealing the full potential of, for example, novel osmium oxide materials ("osmates") is however contingent upon a detailed understanding of the local single-ion properties. Herein, two molecular osmate analogues, [OsF6 ]2- and [OsF6 ]- , are reported as model systems for Os4+ and Os5+ centers found in oxides. Using X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) techniques, combined with state-of-the-art ab initio calculations, their ground state was elucidated; mirroring the osmium electronic structure in osmates. The realization of such molecular model systems provides a unique chemical playground to engineer materials exhibiting spin-orbit entangled phenomena.

Influence of the Ligand Field on the Slow Relaxation of Magnetization of Unsymmetrical Monomeric Lanthanide Complexes: Synthesis and Theoretical Studies.

A series of monomeric lanthanide Schiff base complexes with the molecular formulas [Ce(HL)3(NO3)3] (1) and [Ln(HL)2(NO3)3], where LnIII = Tb (2), Ho (3), Er (4), and Lu (5), were isolated and characterized by single-crystal X-ray diffraction (XRD). Single-crystal XRD reveals that, except for 1, all complexes possess two crystallographically distinct molecules within the unit cell. Both of these crystallographically distinct molecules possess the same molecular formula, but the orientation of the coordinating ligand distinctly differs from those in complexes 2-5. Alternating-current magnetic susceptibility measurement reveals that complexes 1-3 exhibit slow relaxation of magnetization in the presence of an optimum external magnetic field. In contrast to 1-3, complex 4 shows a blockade of magnetization in the absence of an external magnetic field, a signature characteristic of a single-ion magnet (SIM). The distinct magnetic behavior observed in 4 compared to other complexes is correlated to the suitable ligand field around a prolate ErIII ion. Although the ligand field stabilizes an easy axis of anisotropy, quantum tunnelling of magnetization (QTM) is still predominant in 4 because of the low symmetry of the complex. The combination of low symmetry and an unsuitable ligand-field environment in complexes 1-3 triggers faster magnetization relaxation; hence, these complexes exhibit field-induced SIM behavior. In order to understand the electronic structures of complexes 1-4 and the distinct magnetic behavior observed, ab initio calculations were performed. Using the crystal structure of the complexes, magnetic susceptibility data were computed for all of the complexes. The computed susceptibility and magnetization are in good agreement with the experimental magnetic data [χMT(T) and M(H)] and this offers confidence on the reliability of the extracted parameters. A tentative mechanism of magnetization relaxation observed in these complexes is also discussed in detail.

Role of Magnetic Exchange Interactions in the Magnetization Relaxation of {3d-4f} Single-Molecule Magnets: A Theoretical Perspective.

Combined density functional and ab initio calculations are performed on two isomorphous tetranuclear {Ni3 (III) Ln(III) } star-type complexes [Ln=Gd (1), Dy (2)] to shed light on the mechanism of magnetic exchange in 1 and the origin of the slow magnetization relaxation in complex 2. DFT calculations correctly reproduce the sign and magnitude of the J values compared to the experiments for complex 1. Acute ∢Ni-O-Gd bond angles present in 1 instigate a significant interaction between the 4fxyz orbital of the Gd(III) ion and 3d${{_{x{^{2}}- y{^{2}}}}}$ orbital of the Ni(II) ions, leading to rare and strong antiferromagnetic Ni⋅⋅⋅Gd interactions. Calculations reveal the presence of a strong next-nearest-neighbour Ni⋅⋅⋅Ni antiferromagnetic interaction in complex 1 leading to spin frustration behavior. CASSCF+RASSI-SO calculations performed on complex 2 suggest that the octahedral environment around the Dy(III) ion is neither strong enough to stabilize the mJ |±15/2〉 as the ground state nor able to achieve a large ground-state-first-excited-state gap. The ground-state Kramers doublet for the Dy(III) ion is found to be the mJ |±13/2〉 state with a significant transverse anisotropy, leading to very strong quantum tunneling of magnetization (QTM). Using the POLY_ANISO program, we have extracted the JNiDy interaction as -1.45 cm(-1) . The strong Ni⋅⋅⋅Dy and next-nearest-neighbour Ni⋅⋅⋅Ni interactions are found to quench the QTM to a certain extent, resulting in zero-field SMM behavior for complex 2. The absence of any ac signals at zero field for the structurally similar [Dy(AlMe4 )3 ] highlights the importance of both the Ni⋅⋅⋅Dy and the Ni⋅⋅⋅Ni interactions in the magnetization relaxation of complex 2. To the best of our knowledge, this is the first time that the roles of both the Ni⋅⋅⋅Dy and Ni⋅⋅⋅Ni interactions in magnetization relaxation of a {3d-4f} molecular magnet have been established.

Observation of Slow Relaxation and Single-Molecule Toroidal Behavior in a Family of Butterfly-Shaped Ln4 Complexes.

A family of five isostructural butterfly complexes with a tetranuclear [Ln4 ] core of the general formula [Ln4 (LH)2 (µ2 -η1 η1 Piv)(η2 -Piv)(µ3 -OH)2 ]⋅x H2 O⋅y MeOH⋅z CHCl3 (1: Ln=DyIII , x=2, y=2, z=0; 2: Ln=TbIII , x=0, y=0, z=6; 3: Ln=ErIII , x=2, y=2, z=0; 4: Ln=HoIII , x=2, y=2, z=0; 5: Ln=YbIII , x=2, y=2, z=0; LH4 =6-{[bis(2-hydroxyethyl)amino]methyl}-N'-(2-hydroxy-3-methoxybenzylidene)picolinohydrazide; PivH=pivalic acid) was isolated and characterized both structurally and magnetically. Complexes 1-5 were probed by direct and alternating current (dc and ac) magnetic susceptibility measurements and, except for 1, they did not display single-molecule magnetism (SMM) behavior. The ac magnetic susceptibility measurements show frequency-dependent out-of-phase signals with one relaxation process for complex 1 and the estimated effective energy barrier for the relaxation process was found to be 49 K. We have carried out extensive ab initio (CASSCF+RASSI-SO+SINGLE_ANISO+POLY_ANISO) calculations on all the five complexes to gain deeper insights into the nature of magnetic anisotropy and the presence and absence of slow relaxation in these complexes. Our calculations yield three different exchange coupling for these Ln4 complexes and all the extracted J values are found to be weakly ferro/antiferromagentic in nature (J1 =+2.35, J2 =-0.58, and J3 =-0.29 cm-1 for 1; J1 =+0.45, J2 =-0.68, and J3 =-0.29 cm-1 for 2; J1 =+0.03, J2 =-0.98, and J3 =-0.19 cm-1 for 3; J1 =+4.15, J2 =-0.23, and J3 =-0.54 cm-1 for 4 and J1 =+0.15, J2 =-0.28, and J3 =-1.18 cm-1 for 5). Our calculations reveal the presence of very large mixed toroidal moment in complex 1 and this is essentially due to the specific exchange topology present in this cluster. Our calculations also suggest presence of single-molecule toroics (SMTs) in complex 2. For complexes 3-5 on the other hand, the transverse anisotropy was computed to be large, leading to the absence of slow relaxation of magnetization. As the magnetic field produced by SMTs decays faster than the normal spin moments, the concept of SMTs can be exploited to build qubits in which less interference and dense packing are possible. Our systematic study on these series of Ln4 complexes suggest how the ligand design can help to bring forth such SMT characteristics in lanthanide complexes.

What Controls the Sign and Magnitude of Magnetic Anisotropy in Tetrahedral Cobalt(II) Single-Ion Magnets?

A family of mononuclear tetrahedral cobalt(II) thiourea complexes, [Co(L1)4](NO3)2 (1) and [Co(Lx)4](ClO4)2 where x = 2 (2), 3 (3), 4 (4) (where L1 = thiourea, L2 = 1,3-dibutylthiourea, L3 = 1,3-phenylethylthiourea, and L4 = 1,1,3,3-tetramethylthiourea), has been synthesized using a rationally designed synthetic approach, with the aim of stabilizing an Ising-type magnetic anisotropy (-D). On the basis of direct-current, alternating-current, and hysteresis magnetic measurements and theoretical calculations, we have identified the factors that govern the sign and magnitude of D and ultimately the ability to design a single-ion magnet for a tetrahedral cobalt(II) ion. To better understand the magnetization relaxation dynamics, particularly for complexes 1 and 2, dilution experiments were performed using their diamagnetic analogues, which are characterized by single-crystal X-ray diffraction with the general molecular formulas of [Zn(L1)4](NO3)2 (5) and [Zn(L2)4](ClO4)2 (6). Interestingly, intermolecular interactions are shown to play a role in quenching the quantum tunneling of magnetization in zero field, as evidenced in the hysteresis loop of 1. Complex 2 exhibits the largest Ueff value of 62 cm-1 and reveals open hysteresis loops below 4 K. Furthermore, the influence of the hyperfine interaction on the magnetization relaxation dynamics is witnessed in the hysteresis loops, allowing us to determine the electron/nuclear spin S(Co) = 3/2/I(Co) = 7/2 hyperfine coupling constant of 550 MHz, a method ideally suited to determine the hyperfine coupling constant of highly anisotropic metal ions stabilized with large D value, which are otherwise hard to determine by conventional methods such as electron paramagnetic resonance.

Vitamin E pretreatment prevents the immunotoxicity of dithiocarbamate pesticide mancozeb in vitro: A comparative age-related assessment in mice and chick.

Pesticides used for crop protection cause life-threatening diseases affecting the immune system of non-target organisms including birds and mammals. Functionality of immune system is age-dependent; early- as well as old-life stages are more susceptible to toxic exposures because of less competent immune system. Vitamins are so far known to reduce toxic effect of several pesticides and/or xenobiotics. The present in vitro study elucidated immunotoxicity of fungicide mancozeb through comparable stages of immune system maturation in mice (1, 3, and 12months) and chicks (4, 8, and 11weeks). In vitro splenocytes viability on exposure to mancozeb was quantitatively assessed by MTT assay and qualitatively by acridine orange and ethidium bromide (AO/EB) double fluorescence staining. Mancozeb exposure dose dependently (250, 500, 1000, 2500, 5000 and 10,000ng/ml) decreased the splenocytes viability. The in vitro preventive effect of Vitamin E has also been explored on toxicity induced by mancozeb. The increased susceptibility observed both in early and aged groups was due to less/decline competence of the immune system.

Magnetic Relaxation in Single-Electron Single-Ion Cerium(III) Magnets: Insights from Ab Initio Calculations.

Detailed ab initio calculations were performed on two structurally different cerium(III) single-molecule magnets (SMMs) to probe the origin of magnetic anisotropy and to understand the mechanism of magnetic relaxations. The complexes [Ce(III){Zn(II)(L)}2(MeOH)]BPh4 (1) and [Li(dme)3][Ce(III)(cot'')2] (1; L=N,N,O,O-tetradentate Schiff base ligand; 2; DME=dimethoxyethane, COT''=1,4-bis(trimethylsilyl)cyclooctatetraenyldianion), which are reported to be zero-field and field-induced SMMs with effective barrier heights of 21.2 and 30 K respectively, were chosen as examples. CASSCF+RASSI/SINGLE_ANISO calculations unequivocally suggest that mJ|±5/2〉 and |±1/2〉 are the ground states for complexes 1 and 2, respectively. The origin of these differences is rooted back to the nature of the ligand field and the symmetry around the cerium(III) ions. Ab initio magnetisation blockade barriers constructed for complexes 1 and 2 expose a contrasting energy-level pattern with significant quantum tunnelling of magnetisation between the ground state Kramers doublet in complex 2. Calculations performed on several model complexes stress the need for a suitable ligand environment and high symmetry around the cerium(III) ions to obtain a large effective barrier.

Probing the origin of magnetic anisotropy in a dinuclear {Mn(III)Cu(II)} single-molecule magnet: the role of exchange anisotropy.

Using ab initio calculations all the components of the magnetic anisotropy in a dinuclear [Mn(III)Cu(II)Cl(5-Br-sap)2(MeOH)] single-molecule magnet (SMM) have been computed. These calculations reveal that apart from the single-ion anisotropy, the exchange anisotropy also plays a crucial role in determining the sign as well as the magnitude of the cluster anisotropy. Developed magneto-structural correlations suggest that a large ferromagnetic exchange can in fact reduce the ground-state anisotropy, which is an integral component in the design of SMMs.

Can anisotropic exchange be reliably calculated using density functional methods? A case study on trinuclear Mn(III)-M(III)-Mn(III) (M=Fe, Ru, and Os) cyanometalate single-molecule magnets.

Density functional studies have been performed on a set of trinuclear single-molecule magnets (SMMs) of general formula [{Mn2(5-Br salen)2(MeOH)2}M(CN)6](NEt4) (M=Fe(III) (1), Ru(III) (2) and Os(III) (3); 5-Brsalen=N,N'-ethylenebis(5-bromosalicylidene)iminato anion). We have computed the orbital-dependent exchange interaction for all three complexes for the first time using DFT and complete active space self-consistent field (CASSCF) methods. DFT calculations yield the anisotropic exchange as J(ξξ)=3.5 cm(-1) for 1; J(ξξ)=12.1 cm(-1), J(ζζ)=-6.9 cm(-1) and J(ηη)=-14 cm(-1) for 2; and J(ξξ)=23.7 cm(-1) and J(ζζ) =-11.1 cm(-1) for 3. The computed values are in agreement with the experimental report, and this suggests that the established methodology can be used to compute the anisotropic exchange in larger clusters. Our calculations reiterate the fact that the exchange is described by a three-axis anisotropic exchange for complexes 2 and 3 as evidenced by the experiments. A stronger exchange coupling as we move down the periodic table from 3d to 5d is reproduced by our calculations, and the origin of this enhancement in the exchange interaction has been probed by using molecular orbital analysis. The electronic origin of different types of exchange observed in this series is found to be related to the energy difference between possible degenerate pairs and the nature of orbital interactions. By computing the exchange interaction, the single-ion anisotropy of Mn(III) and zero-field splitting of the S=9/2 ground state of complexes 1-3 using CASSCF and/or DFT methods, we have attempted to shed light on the issue of anisotropic exchange and the barrier height for the magnetisation reversal in SMMs. Comprehensive magneto-structural correlations have been developed to offer clues on how to further enhance the barrier height in this class of SMMs.

Magnetic anisotropy of mononuclear Ni(II) complexes: on the importance of structural diversity and the structural distortions.

Mononuclear Ni(II) complexes are particularly attractive in the area of single-molecule magnets as the axial zero-field splitting (D) for the Ni(II) complexes is in the range of -200 to +200 cm(-1) . Despite this advantage, very little is known on the origin of anisotropy across various coordination ligands, coordination numbers, and particularly what factors influence the D parameter in these complexes. To answer some of these questions, herein we have undertaken a detailed study of a series of mononuclear Ni(II) complexes with ab initio calculations. Our results demonstrate that three prominent spin-conserved low-lying d-d transitions contribute significantly to the D value. Variation in the sign and the magnitude of D values are found to correlate to the specific structural distortions. Apart from the metal-ligand bond lengths, two different parameters, namely, Δα and Δß, which are correlated to the cis angles present in the coordination environment, are found to significantly influence the axial D values. Developed magneto-structural D correlations suggest that the D values can be enhanced significantly by fine tuning the structural distortion in the coordination environment. Calculations performed on a series of Ni(II) models with coordination numbers two to six unfold an interesting observation-the D parameter increases significantly upon a reduction in coordination number compared with a reference octahedral coordination. Besides, if high symmetry is maintained, even larger coordination numbers yield large D values.

Magnetic anisotropy and mechanism of magnetic relaxation in Er(III) single-ion magnets.

Magnetic anisotropy is a key component in the design of single-molecule magnets (SMMs) possessing a large barrier height for magnetization reversal. Lanthanide-based SMMs are the most promising candidates in this arena as they offer a large magnetic anisotropy due to the presence of strong spin-orbit coupling. Among lanthanides, Er(III) complexes are gaining attention in the area of SMMs, because of their intriguing magnetic properties and attractive blocking temperatures. Here, we have undertaken detailed ab initio calculations on four structurally diverse Er(III) SMMs to shed light on how the magnetic anisotropy is influenced by the role of symmetry and structural distortions. The employed CASSCF+RASSI calculations have offered rationale for the observed differences in the estimated Ueff values for the studied complexes and also offered hints to the mechanism of magnetic relaxation. The differences in the mechanism of magnetic relaxations are further analyzed based on the Er-ligand interactions, which is obtained by analyzing the charges, densities, luminescent behavior and the frontier molecular orbitals. Our calculations, for the first time, have highlighted the importance of high symmetry environment and ligand donor strength in obtaining large Ueff values for the Er(III) complexes. We have examined these possibilities by modeling several structures with variable coordination numbers and point group symmetry. These results signify the need of a detailed understanding on the shape of the anisotropy and the point group symmetry in order to achieve large Ueff values in Er(III) single-ion magnets.