Dipotassiumtetrachloride-bridged dysprosium metallocenes: a single-molecule magnet.

The present work describes the dynamic magnetic properties of the complex [(CpAr3)4DyIII2Cl4K2]·3.5(C7H8) (1), synthesized by employing a tri-aryl-substituted cyclopentadienyl ligand (CpAr3), [4,4'-(4-phenylcyclopenta-1,3-diene-1,2-diyl)bis(methylbenzene) = CpAr3H]. Each Dy(III)-metallocene weakly couples via K2Cl4, displaying slow relaxation of magnetization below 14.5 K under zero applied dc field via KD3 energy levels with an energy barrier of 136.9/133.7 cm-1 on the Dy sites. The single-ion axial anisotropy energy barrier is reduced by geometrical distortion due to the coordination of two chloride ions at each Dy centre.

[AuI(CN)2]-Armed [FeIII2FeII2] Square Complex Showing Unusual Spin-Crossover Behavior Due to a Symmetry-Breaking Phase Transition.

The incorporation of two different cyanide building blocks of [(TpR)FeIII(CN)3]- and [AuI(CN)2]- into one molecule afforded a novel hexanuclear [FeIII2FeII2AuI2] complex (1·2Et2O), in which the cyanide-bridged [FeIII2FeII2] square was further grafted by two [AuI(CN)2]- fragments as long arms in syn orientations. Complex 1·2Et2O undergoes a gradual spin crossover (SCO) ffrom low-spin (LS) to high-spin (HS) state for the Fe(II) centers upon desolvation. Remarkably, its desolvated phase (1) exhibits a reversible but atypical two-step (sharp-gradual) SCO behavior with considerable hysteresis (21 K). Variable-temperature single-crystal X-ray structural studies reveal that the hysteretic spin transition takes place synchronously with the concerted displacive motions of the molecules, representing another rare example including multistep and hysteretic spin transitions due to the synergetic SCO and structural phase transition.

The Role of Radical Bridges in Polynuclear Single-Molecule Magnets.

Employing radical bridges between anisotropic metal ions has been a viable route to achieve high-performance single-molecule magnets (SMMs). While the bridges have been mainly considered for their ability to promote exchange interactions, the crystal-field effect arising from them has not been taken into account explicitly. This lack of consideration may distort the understanding and limit the development of the entire family. To shed light on this aspect, herein we report a theoretical investigation of a series of N 2 3 - -radical-bridged diterbium complexes. It is found that while promoting strong exchange coupling between the terbium ions, the N 2 3 - -radical induces a crystal field that interferes destructively with that of the outer ligands, and thus reduces the overall SMM behavior. Based on the theoretical results, we conclude that the SMM behavior in this series could be further maximized if the crystal field of the outer ligands is designed to be collinear with that of the radical bridge. This conclusion can be generalized to all exchange-coupled SMMs.

Mechanisms of Luminescence in Lanthanide Complexes: A Crucial Role of Metal-Ligand Covalency.

A current understanding of the luminescence of lanthanide complexes is based on the phenomenological Judd-Ofelt (JO) theory. However, the mechanisms of electric-dipole transitions lying at its basis were never subjected to a rigorous analysis. Here, we investigate the contributions to the electric-dipole transitions in the Er3+ 4S3/2 → 4I15/2 band of an erbium trensal complex using state-of-the-art ab initio calculations. We find that the conventional JO mechanism based on the electrostatic crystal field yields only a quarter of the integral intensity of this band. Accordingly, three quarters of it is contributed by covalent binding of erbium and ligand orbitals via three major mechanisms, the 4f ligand and ligand-ligand electric-dipole transitions and covalent enhancement of the hybridization of 4f and even empty orbitals of erbium. We expect that these findings will inspire the design of efficient rare-earth luminescent materials.

Stable Triarylmethyl Radicals and Cobalt(II) Ions Based 1D/2D Coordination Polymers.

The incorporation of organic radicals into coordination polymers was considered as a promising strategy to promote metal-ligand exchange interactions, but there are only a very limited number of stable organic radical-based ligands that can serve well such a purpose. Herein, we report two new tris(2,4,6-trichlorophenyl)methyl (TTM) radical-based ligands L1 and L2 with two and three imidazole substituents, respectively. The imidazole unit serves as a coordination site and it can also stabilize the TTM radical by intramolecular donor-acceptor interaction. Coordination of L1 and L2 with cobalt(II) ions gave the corresponding one- (CoCP-1) and two-dimensional (CoCP-2) coordination polymers, the structures of which were confirmed by X-ray crystallographic analysis. Magnetic measurements and theoretical calculations suggest antiferromagnetic coupling between the paramagnetic cobalt(II) ions and the radical ligands. Our study provides a rational design for stable organic radical-based ligands and further demonstrated the feasibility of a metal-radical approach toward magnetic materials.

Manipulating the spin crossover behaviour in a series of cyanide-bridged {FeIII2FeII2} molecular squares through NCE- co-ligands.

Manipulating the transition temperature (T1/2) of spin-crossover (SCO) complexes capable of fulfilling practical criteria through different synthetic strategies is one of the main focuses in the field of molecular magnetism. The reaction of the tricyanometallate precursor [(Tp*)FeIII(CN)3]- and Fe(II) salt with the "facially" tridentate ligand tris(2-pyridyl)phosphine oxide (TPPO) and NCE- anions afforded three isostructural {FeIII2FeII2} square complexes {[(Tp*)FeIII(CN)3]2[FeII(TPPO)]2[NCE]2}·Sol (E = S, Sol = 2CH3OH·6H2O, 1; E = Se, Sol = 2MeCN·2CH2Cl2·2H2O, 2; E = BH3, Sol = 4CH3OH·2MeCN, 3). Detailed structural analysis, variable-temperature IR analysis, magnetic susceptibility measurements and DFT calculations revealed that all compounds exhibit complete and one-step SCO behaviour between the {FeIII,LS2FeII,HS2} and {FeIII,LS2FeII,LS2} electronic states. As the ligand field increases from NCS- to NCSe- to NCBH3-, T1/2 shifts dramatically from 214 to 250 to 288 K for 1, 2 and 3, respectively, demonstrating another effective way to tune the SCO properties of the [FeIII-CN-FeII] systems through the introduction of NCE- co-ligands.

Toroidal versus centripetal arrangement of the magnetic moment in a Dy4 tetrahedron.

Magnetic investigation and ab initio calculations reveal toroidal arrangement of the magnetic moment rather than centripetal anisotropies in a tetrahedral Dy4 complex.

Exploring vibronic coupling in the benzene radical cation and anion with different levels of the GW approximation.

The linear vibronic coupling constants of the benzene radical cation and anion have been obtained with different levels of the GW approximation, including G0W0, eigenvalue self-consistent GW, and quasiparticle self-consistent GW, as well as DFT with the following exchange-correlation functionals: BLYP, B3LYP, CAM-B3LYP, tuned CAM-B3LYP, and an IP-tuned CAM-B3LYP functional. The vibronic coupling constants were calculated numerically using the gradients of the eigenvalues of the degenerate HOMOs and LUMOs of the neutral benzene molecule for DFT, while the numerical gradients of the quasiparticle energies were used in the case of GW. The results were evaluated against those of high level wave function methods in the literature, and the approximate self-consistent GW methods and G0W0 with long-range corrected functionals were found to yield the best results on the whole.

Magnetization Dynamics on Isotope-Isomorphic Holmium Single-Molecule Magnets.

Here we reported the deuteration of the metal-binding equatorial water molecules in a reported HoIII single-molecule magnet (SMM) with pentagonal-bipyramidal geometry, from [Ho(CyPh2 PO)2 (H2 O)5 ]3+ to [Ho(CyPh2 PO)2 (D2 O)5 ]3+ . The hyperfine structures originating from the nuclear spin of 165 HoIII can be clearly observed. Moreover, the resulting magnetization dynamics revealed the switch of the relative relaxation rates for the two isotope-isomorphic complexes-respectively faster/slower at low/high temperature. The noticeable isotope effect arises from not only the paramagnetic metal center but also the diamagnetic ligands, which can be explained by the ab initio calculated tunnel splitting and the involvement of the super-hyperfine interaction related to the difference in the nuclear spin number of protium (1 H, I=1 /2 ) and deuterium (2 H, I=1).

Towards understanding the magnetism of Os(IV) complexes: an ab initio insight.

The magnetism of a recently synthesized trans-[OsIVCl4(κN1-Hind)2] complex (5d4-system), where Hind = 2H-indazole, was studied experimentally and theoretically. Relativistic CASSCF/CASPT2 calculations for this and model [OsIVCl6]2- complexes were employed to understand the nature of the low-lying multiplets. It is found that despite strong metal-ligand covalency they are basically characterized by the total angular pseudo-momentum J̃ originating from the spin-orbit coupling of the ground-state spin S = 1 with the orbital pseudo-momentum L̃ = 1 of the OsIV ion. The strong spin-orbit interaction also preserves the dominant J̃ = 0 character of the non-magnetic ground state in the trans-[OsIVCl4(κN1-Hind)2] complex despite significant deviation of the ligand environment of OsIV from octahedral symmetry. At the same time the spin-orbit admixture of all multiplets arising from the t2g4 strong-field electronic configuration is indispensable for the correct description of magnetic properties of OsIV complexes. Moreover, based on ab initio calculations, we argue that the charge-transfer states play an important role in the magnetism of the present and probably other 5d complexes, a situation never encountered for 3d and 4f compounds.

A Family of Lanthanide Hydroxo Carboxylates with 1D Polymeric Topology and Ln4 Butterfly Core Exhibits Switchable Supramolecular Arrangement.

The unique family of coordination polymers [Ln4(OH)2(piv)10(H2O)2]∞ of 11 lanthanides (Ln = La-Er) has been prepared by a simple solution method based on controlled hydrolysis. The ribbon-like polymeric structure consisting of connected tetranuclear clusters and supported by pivalate ligands and a framework of H-bonds has been revealed by single-crystal X-ray diffraction. While the compounds demonstrate similar PXRD patterns and unit cell parameters, the joint single-crystal XRD and pair distribution function data suggest the significant local structure change along the lanthanide series. The compounds exist as two packing polymorphs (α and ß) with similar ribbon geometry, but different supramolecular arrangement of the ribbons. Dehydration of either polymorph does not disturb the tetranuclear core but leads to a translational symmetry loss along the ribbon and a transformation of the 3D-ordered crystal into a 2D-ordered mesostructure. Rehydration of the mesostructure leads to the ß polymorph (except La and Ce), allowing the deliberate switching between the polymorphs via dehydration-rehydration evidenced by means of powder X-ray diffraction, pair distribution function analysis, and density functional theory calculations. Ab initio calculations reveal significant magnetic anisotropy of Ln3+ ions with ferro- and antiferromagnetic interactions within tetranuclear [Ln4(OH)2(piv)10(H2O)2] species. Magnetic susceptibility measurements demonstrated antiferromagnetic coupling, slow magnetic relaxation for Dy, Ho, and Er complexes, and field-induced single-chain magnetism for the Dy compound.

Understanding the magnetization blocking mechanism in N23--radical-bridged dilanthanide single-molecule magnets.

Herein, we report a theoretical investigation of the electronic structure and magnetic properties in [(Cp2Me4HLn(THF))2(µ-N2˙)]- and [(Cp2Me4HLn)2(µ-N2˙)]- (THF = tetrahydrofuran, CpMe4H = tetramethylcyclopentadienyl, Ln = Tb, Dy) complexes [as reported in Demir et al., Nat. Commun., 8, 1-9, 2144 (2017)]. By ab initio methods, their magnetic blocking behaviors are successfully characterized allowing elucidation of the origin of the two blocking barriers observed experimentally. In addition, a detailed analysis of exchange wave functions explains why the blocking barrier of the Tb complexes is roughly twice as large as that of the Dy analogues, a fact which appears to be a general trend exhibited in this family of compounds.

Modern quantum chemistry with [Open]Molcas.

MOLCAS/OpenMolcas is an ab initio electronic structure program providing a large set of computational methods from Hartree-Fock and density functional theory to various implementations of multiconfigurational theory. This article provides a comprehensive overview of the main features of the code, specifically reviewing the use of the code in previously reported chemical applications as well as more recent applications including the calculation of magnetic properties from optimized density matrix renormalization group wave functions.

Highly Oxidized States of Phthalocyaninato Terbium(III) Multiple-Decker Complexes Showing Structural Deformations, Biradical Properties and Decreases in Magnetic Anisotropy.

Presented here is a comprehensive study of highly oxidized multiple-decker complexes composed of TbIII and CdII ions and two to five phthalocyaninato ligands, which are stabilized by electron-donating n-butoxy groups. From X-ray structural analyses, all the complexes become axially compressed upon ligand oxidation, resulting in bowl-shaped distortions of the ligands. In addition, unusual coexistence of square antiprism and square prism geometries around metal ions was observed in +4e charged species. From paramagnetic 1 H NMR studies on the resulting series of triple, quadruple and quintuple-decker complexes, ligand oxidation leads to a decrease in the magnetic anisotropy, as predicted from theoretical calculations. Unusual paramagnetic shifts were observed in the spectra of the +2e charged quadruple and quintuple-decker complexes, indicating that those two species are actually unexpected triplet biradicals. Magnetic measurements revealed that the series of complexes show single-molecule magnet properties, which are controlled by the multi-step redox induced structural changes.

An Inconspicuous Six-Coordinate Neutral DyIII Single-Ion Magnet with Remarkable Magnetic Anisotropy and Stability.

It is a crucial challenge to address both magnetic anisotropy and stability for single-molecule magnets (SMMs) used in next-generation nanodevices. Highly axial lanthanide SMMs with neutral charge and moderate coordination numbers represent promising magnetic materials. Here, using iodide ions with large volume and low surface charge density as weak donors, we report a six-coordinate neutral dysprosium SMM [Dy(Cy3PO)2I3(CH3CN)] with a certain degree of stability exhibiting a huge thermal barrier of 1062 K and hysteresis loops open up to 9 K. Through the elaborate reduction of ligand field strength, an apparent strongly axial crystal field is provided which elicits prominent crystal-field splitting and high axiality with the thermally activated relaxation via the third-excited Kramers' doublet. Moreover, the profound influence of strong equatorial ligand substitution on the electronic structure and relaxation pathway is clearly explored in DyIII analogues. The result suggests the great potential of the reducing the transverse ligand field in the improvement of SMMs performance.

Magnetic Anisotropy in Divalent Lanthanide Compounds.

Complexes of trivalent lanthanides (Ln) are known to possess strong magnetic anisotropy, which enables them to be efficient single-molecule magnets. High-level ab initio calculations are reported for [LnO] (where Ln is terbium (Tb), dysprosium (Dy), or holmium (Ho)), which show that divalent lanthanides can exhibit equally strong magnetic anisotropy and magnetization blocking barriers. In particular, detailed calculations predict a multilevel magnetization blocking barrier exceeding 3000 K for a [DyO] complex deposited on a hexagonal boron nitride (h-BN) surface, bringing the expected performance of single-molecule magnets to a qualitatively new level compared to the current state-of-the art complexes.

Coexistence of Spin-Lattice Relaxation and Phonon-Bottleneck Processes in GdIII -Phthlocyaninato Triple-Decker Complexes under Highly Diluted Conditions.

Gd3+ complexes have been shown to undergo unusual slow magnetic relaxation processes similar to those of single-molecule magnets (SMMs), even though Gd3+ does not exhibit strong magnetic anisotropy. To reveal the origin of the slow magnetic relaxation of Gd3+ complexes, we have investigated the magnetic properties and heat capacities of two Gd3+ -phthalocyaninato triple-decker complexes, one of which has intramolecular Gd3+ -Gd3+ interactions and the other does not. It was found that the Gd3+ -Gd3+ interactions accelerate the magnetic relaxation processes. In addition, magnetically diluted samples, prepared by doping a small amount of the Gd3+ complexes into a large amount of diamagnetic Y3+ complexes, underwent dual magnetic relaxation processes. A detailed dynamic magnetic analysis revealed that the coexistence of spin-lattice relaxation and phonon-bottleneck processes is the origin of the dual magnetic relaxation processes.

Deriving the vibronic coupling constants of the cyclopentadienyl radical with density functional theory and G0W0.

The vibronic coupling constants of the cyclopentadienyl radical have been calculated with G0W0, HF, and density functional theory (DFT) with various exchange-correlation functionals such as PBE, PBE0, LC-ωPBE, and the non-empirically tuned LC-ωPBE*. The vibronic coupling constants for HF and DFT were derived using the gradients of the eigenvalues of the degenerate HOMOs of the closed-shell cyclopentadienyl anion, while the gradients of the corresponding quasiparticle energy levels were used in the case of G0W0. The differences between the linear vibronic constants obtained using HF and DFT were found to be small and reduced further when the G0W0 correction is applied to HF and DFT. Finally, the linear vibronic coupling constants calculated with G0W0 were found to agree well with the values obtained using high level wave function methods in the literature, which suggests that G0W0 can be a useful tool toward the study of vibronic coupling.

Comparison of two field-induced ErIII single ion magnets.

We present the synthesis, magnetic and photophysical properties of four mononuclear LnIII complexes in two isostructural lattices containing GdIII and ErIII. A heptadentate Schiff base ligand and acetate versus trifluoroacetate were used to synthesise complexes 1-4, among which the two ErIII complexes 2 and 4 exhibit field-induced SIM behaviour with almost similar Ueff values (31.6 K for 2 and 32.7 K for 4). Ab initio calculations show the structure of the low-lying energy states and highlight that there is already significant tunnelling in the ground doublet state, but the application of a weak magnetic field of 0.05 T is sufficient for ac magnetic measurements to suppress tunnelling in the ground state. The calculated main magnetic axes (gZ) of the ground Kramers doublets show small differences between the two ErIII compounds 2 and 4 due to their different ligand fields.

Correction: Determination of the electronic structure of a dinuclear dysprosium single molecule magnet without symmetry idealization.

[This corrects the article DOI: 10.1039/C8SC03170C.].