Iridates from the molecular side

Investigation of Isolated IrF5 -, IrF6 - Anions and M[IrF6] (M=Na, K, Rb, Cs) Ion Pairs by Matrix-Isolation Spectroscopy and Relativistic Quantum-Chemical Calculations

The molecular IrF5 -, IrF6 - anions and M[IrF6] (M=Na, K, Rb, Cs) ion pairs were prepared by co-deposition of laser-ablated alkali metal fluorides MF with IrF6 and isolated in solid neon or argon matrices under cryogenic conditions. The free anions were obtained as well by co-deposition of IrF6 with laser-ablated metals (Ir or Pt) as electron sources. The products were characterized in a combined analysis of matrix IR spectroscopy and electronic structure calculations using two-component quasi-relativistic DFT methods accounting for spin-orbit coupling (SOC) effects as well as multi-reference configuration-interaction (MRCI) approaches with SOC. Inclusion of SOC is crucial in the prediction of spectra and properties of IrF6 - and its alkali-metal ion pairs. The observed IR bands and the computations show that the IrF6 - anion adopts an Oh structure in a nondegenerate ground state stabilized by SOC effects, and not a distorted D4h structure in a triplet ground state as suggested by scalar-relativistic calculations. The corresponding "closed-shell" M[IrF6] ion pairs with C3v symmetry are stabilized by coordination of an alkali metal ion to three F atoms, and their structural change in the series from M=Na to Cs was proven spectroscopically. There is no evidence for the formation of IrF7, IrF7 - or M[IrF7] (M=Na, K, Rb, Cs) ion pairs in our experiments.

Slow Magnetic Relaxation in Silver(II) Macrocyclic Systems

The spin-lattice relaxation time has been studied trough alternating-current susceptometry and ultralow-frequency Raman spectroscopy in a family of silver(II)-derived molecular systems with spin 1/2 and formulas [AgII(m-CTH)(NO3)2] (1) and [AgII(m-CTH)(ClO4)2] (2), where CTH = meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane. The combination of both techniques demonstrates the occurrence of slow spin magnetic relaxation induced by spin-phonon interaction. The magnetic behavior of these silver(II)-derived systems opens the door to a new cation in the scarce family of S = 1/2 systems with slow relaxation of magnetization.

Magnetic anisotropy reversal driven by structural symmetry-breaking in monolayer α-RuCl3

Layered α-RuCl₃ is a promising material to potentially realize the long-sought Kitaev quantum spin liquid with fractionalized excitations. While evidence of this state has been reported under a modest in-plane magnetic field, such behaviour is largely inconsistent with theoretical expectations of spin liquid phases emerging only in out-of-plane fields. These predicted field-induced states have been largely out of reach due to the strong easy-plane anisotropy of bulk crystals, however. We use a combination of tunnelling spectroscopy, magnetotransport, electron diffraction and ab initio calculations to study the layer-dependent magnons, magnetic anisotropy, structure and exchange coupling in atomically thin samples. Due to picoscale distortions, the sign of the average off-diagonal exchange changes in monolayer α-RuCl₃, leading to a reversal of spin anisotropy to easy-axis anisotropy, while the Kitaev interaction is concomitantly enhanced. Our work opens the door to the possible exploration of Kitaev physics in the true two-dimensional limit.

A new family of one-dimensional bromo-bridged Ir(IV)-Cu(II) complexes based on the hexabromoiridate(IV) metalloligand

By using the iridium(IV) complex (NBu₄)₂[IrBr₆] (1) as a metalloligand towards a Cu(II) metal ion, three novel Ir(IV) one-dimensional (1D) compounds of formula {IrBr₅(μ-Br)Cu(Meim)₄}_n (2), {IrBr₅(μ-Br)Cu(Viim)₄}_n (3) and {IrBr₅(μ-Br)Cu(Buim)₄}_n (4), [Meim = 1-methylimidazole; Viim = 1-vinylimidazole; Buim = 1-butylimidazole] have been prepared and structurally and magnetically characterised. Compounds 2, 3 and 4 crystallise in the triclinic, monoclinic and orthorhombic crystal systems with space groups P1̄, C2/c and Pccn, respectively. Each Ir(IV) ion in 1-4 is six-coordinate and bonded to six bromide ions in a quasi regular octahedral geometry. In compounds 2-4, the Cu^II ion shows an axially elongated octahedron with four N atoms, from four monodentate imidazole derivative ligands, that form the equatorial plane and two bromide ions that occupy the axial positions. Cu(II) and Ir(IV) ions are linked through bridging bromide anions generating Ir(IV)-Cu(II) chains [with intrachain Cu(II)⋯Ir(IV) distances covering the range of ca. 5.10-5.42 Å]. In the crystal lattice of 2 and 3 are observed significant intermolecular Ir-Br⋯Br-Ir contacts and π⋯Br interactions, which organize arrangements that contribute to stabilizing the crystal structure of these Ir(IV)-based compounds. DC magnetic susceptibility measurements reveal that 1 displays magnetic behaviour typical of noninteracting mononuclear centres with S = 1/2. Besides, antiferromagnetic behaviour (2 and 3) and ferromagnetic (4) exchange coupling occur between the Cu(II) and Ir(IV) metal ions in the one-dimensional bromo-bridged compounds 2-4. Moreover, the study of the AC magnetic susceptibility shows a field-induced slow relaxation of the magnetisation for 1, indicating the presence of the single-ion magnet (SIM) phenomenon for the magnetically isolated hexabromoiridate(IV) complex.

Unusual Slow Magnetic Relaxation in a Mononuclear Copper(II) Complex

A hexacoordinate Cu(II) complex with the {CuO4O’N} donor set shows an intermolecular π-πstacking owing to which a 1D-chain structure is formed. The DC magnetic data at low temperature is consistent...

Magnetic Weyl Semimetallic Phase in Thin Films of Eu_{2}Ir_{2}O_{7}

The interplay between electronic interactions and strong spin-orbit coupling is expected to create a plethora of fascinating correlated topological states of quantum matter. Of particular interest are magnetic Weyl semimetals originally proposed in the pyrochlore iridates, which are only expected to reveal their topological nature in thin film form. To date, however, direct experimental demonstrations of these exotic phases remain elusive, due to the lack of usable single crystals and the insufficient quality of available films. Here, we report on the discovery of signatures for the long-sought magnetic Weyl semimetallic phase in (111)-oriented Eu_{2}Ir_{2}O_{7} high-quality epitaxial thin films. We observed an intrinsic anomalous Hall effect with colossal coercivity but vanishing net magnetization, which emerges right below the onset of a peculiar magnetic phase with all-in-all-out (AIAO) antiferromagnetic ordering. The anomalous Hall conductivity obtained experimentally is consistent with the theoretical prediction, likely arising from the nonzero Berry curvature emanated by Weyl node pairs near the Fermi level that act as sources and sinks of Berry flux, activated by broken cubic crystal symmetry at the top and bottom terminations of the thin film.

Revisiting Sodium Hexafluoroiridates: Perspective Precursors for Electronic, Quantum, and Related Materials

The following salts have been synthesized and structurally characterized: Na₂[IrF₆]·2H₂O (C2/m, a = 6.6327(4), b = 10.0740(6), c = 5.9283(5) Å, β = 122.3880(10)°) and Na₃[IrF₆]·2H₂O (R-3, a = 7.5963(3), b = 7.5963(3), c = 9.8056(4) Å) (for the first time) by single-crystal X-ray diffraction; the unit cell parameters of a tetragonal phase (P4 ₂/mnm, a = 5.005(2), c = 10.074(4) Å) of the stable α-Na₂[IrF₆] were determined for the first time; and the unit cell parameters of β-Na₂[IrF₆] (P321, a = 9.332(4), c = 5.136(2) Å) and Na₃[IrF₆] (P2₁/n, a = 5.567(4), b = 5.778(4), c = 8.017(2) Å, β = 90.41(2)°) were determined using powder X-ray diffraction (PXRD). The data of the thermal stability was obtained by differential thermal analysis (DTA) for all substances. The presence of Na₃[IrF₆]·H₂O monohydrate is predicted. H₂[IrF₆] was prepared in a solution and was demonstrated to behave as a strong dibasic acid.

Density Functional Theory Study on the 193Ir Mössbauer Spectroscopic Parameters of Vaska's Complexes and Their Oxidative Adducts

In the present study, density functional theory (DFT) calculation was applied to Vaska's complexes of formula trans-[Ir^IX(CO)(PPh₃)₂] and their oxidative adducts with small molecules (YZ) including H₂, i.e., trans-[Ir^IIIClYZ(CO)(PPh₃)₂], to successfully correlate the electronic states of the complexes with the corresponding ¹⁹³Ir Mössbauer spectroscopic parameters. After confirming the reproducibility of the DFT methods for elucidating the equilibrium structures and ¹⁹³Ir Mössbauer isomer shifts of the octahedral Ir complexes, the isomer shifts and quadrupole splitting values of Vaska's complexes and their oxidative adducts were calculated. A bond critical point analysis revealed that the tendency in the isomer shifts was correlated with the strength of the covalent interaction in the coordination bonds. In an electric field gradient (EFG) analysis of the oxidative adducts, the sign of the principal axis was found to be positive for the complex with YZ = Cl₂ and negative for the complex with YZ = H₂. This reversal of the sign of the EFG principal axis was caused by the difference in the electron density distribution for the coordination bonds between Ir and YZ, according to a density of states analysis.

Slow magnetic relaxation in structurally similar mononuclear 8-coordinate Fe(II) and Fe(III) compounds

A pair of structurally-similar and stable 8-coordinate high-spin Fe(ii) and Fe(iii) compounds have been obtained. Both compounds exhibit field-induced slow magnetic relaxation behaviour. The Fe(iii) compound represents the first example of 8-coordinate Fe(iii) single-molecule magnets (SMM).

Field-Induced Single-Ion Magnet Phenomenon in Hexabromo- and Hexaiodorhenate(IV) Complexes

Two mononuclear ReIV complexes of general formula (PPh4)2[ReX6] [PPh4+ = tetraphenylphosphonium cation, X = Br (1) and I (2)] have been prepared and structurally and magnetically characterised. Both compounds crystallise in the triclinic system with space group Pī. Their structures are made up of hexahalorhenate(IV), [ReX6]2−, anions, and bulky PPh4+ cations. Each ReIV ion in 1 and 2 is six-coordinate and bonded to six halide ions in a quasi regular octahedral geometry. In their crystal packing, the [ReX6]2− anions are well separated from each other through the organic cations, generating alternated anionic and cationic layers, and no intermolecular Re−X···X−Re interactions are present. Variable-temperature dc magnetic susceptibility measurements performed on microcrystalline samples of 1 and 2 show a very similar magnetic behaviour, which is typical of noninteracting mononuclear ReIV complexes with S = 3/2. Ac magnetic susceptibility measurements reveal the slow relaxation of the magnetisation in the presence of external dc fields for 1 and 2, hence indicating the occurrence of the field-induced single-ion magnet (SIM) phenomenon in these hexabromo- and hexaiodorhenate(IV) complexes.

Access to Heteroleptic Fluorido-Cyanido Complexes with a Large Magnetic Anisotropy by Fluoride Abstraction

Silicon-mediated fluoride abstraction is demonstrated as a means of generating the first fluorido-cyanido transition metal complexes. This new synthetic approach is exemplified by the synthesis and characterization of the heteroleptic complexes, trans-[M^IV F₄ (CN)₂ ]^2- (M=Re, Os), obtained from their homoleptic [M^IV F₆ ]^2- parents. As shown by combined high-field electron paramagnetic resonance spectroscopy and magnetization measurements, the partial substitution of fluoride by cyanide ligands leads to a marked increase in the magnetic anisotropy of trans-[ReF₄ (CN)₂ ]^2- as compared to [ReF₆ ]^2- , reflecting the severe departure from an ideal octahedral (O_h point group) ligand field. This methodology paves the way toward the realization of new heteroleptic transition metal complexes that may be used as highly anisotropic building-blocks for the design of high-performance molecule-based magnetic materials.

Field-induced slow magnetic relaxation in low-spin S = 1/2 mononuclear osmium(v) complexes

Photochemical reactions of (PPh4)[OsVI(N)(L)(CN)3] (NO2-OsN) with piperidine and pyrrolidine afforded two osmium(v) hydrazido compounds, (PPh4)[OsV(L)(CN)3(NNC5H10)] ([PPh4]1) and (PPh4)[OsV(L)(CN)3(NNC4H8)] ([PPh4]2), respectively. Their structures consist of isolated, mononuclear distorted octahedral osmium anions that are well-separated from each other by PPh4+. Their low spin S = 1/2 and L = 1 ground state was confirmed by magnetometry and DFT calculations. Interestingly, both compounds exhibit slow magnetic relaxation under a bias dc-field. These osmium(v) complexes are potentially useful building-blocks for the construction of molecule-based architectures with interesting magnetic properties. In contrast, the structurally related (PPh4)[OsIII(L)(CN)3(NH3)] ([PPh4]3), which also has a low-spin S = 1/2 ground state but with a different electronic configuration (5d5), does not exhibit slow magnetic relaxation, due to the absence of any orbital moment (L = 0). Furthermore, the structurally different osmium(v) hydrazido compound reported by Meyer, [OsV(tpy)(Cl)2(NNC5H10)](PF6) (4[PF6]), also does not exhibit slow magnetic relaxation due possibly to a change in magnetic anisotropy from axial for [PPh4]1 and [PPh4]2 to planar.

Ultrafast dynamics in LMCT and intraconfigurational excited states in hexahaloiridates(iv), models for heavy transition metal complexes and building blocks of quantum correlated materials

Two heavy octahedral Ir(iv) halides in intraconfigurational and LMCT excited electronic states with ultrafast relaxation dynamics driven by the Jahn–Teller effect.

Bis(tetraphenylarsonium) hexafluoridotechnetate(IV) dihydrate: preparation, structure and spectroscopic analysis

Reports of quadrivalent transition-metal fluoride salts containing bulky organic cations are limited. In this context, we prepared the bis(tetraphenylarsonium) hexafluoridotechnetate(IV) dihydrate salt, (C₂₄H₂₀As)₂[TcF₆]·2H₂O, by a cation metathesis reaction of (NH₄)₂[TcF₆] in water. This is the first report of an arsonium salt of the hexafluoridotechnetate(IV) dianion. (AsPh₄)₂[TcF₆]·2H₂O crystallizes in the triclinic space group P-1. The [TcF₆]^2- anion adopts a slightly distorted octahedral geometry with an average Tc-F bond length of 1.933 Å. The cyclic voltammogram of (AsPh₄)₂[TcF₆]·2H₂O in CH₃CN shows a one-electron reversible oxidation wave at 1.496 V.

Ferromagnetic exchange interaction in a new Ir(iv)–Cu(ii) chain based on the hexachloroiridate(iv) anion

The new chloro-bridged heterobimetallic Ir^IVCu^II chain of formula {IrCl₅(μ-Cl)Cu(viim)₄}_n [viim = 1-vinylimidazole] is the first reported compound based on the Cu^II and Ir^IV metal ions.

Progress towards creating optically addressable molecular qubits

The emerging field of quantum information science promises to transform a diverse range of scientific fields, ranging from computation to sensing and metrology. The interdisciplinary scientific community laid the groundwork for the next generation of quantum technologies through key advances in understanding the fundamental unit of quantum information science, the qubit. Electronic spin is a promising platform for qubits, demonstrating suitably long coherence times, optical initialization, and single spin addressability. Herein, we discuss recent accomplishments and future progress from our group targeted at imbuing transition metal complexes with the aforementioned properties, creating a pathway to fusing spatial precision with long coherence times. A strong emphasis of this feature article is progressing towards single spin measurements via a chemical approach for imbuing molecular qubits with an optically-induced spin polarization mechanism.

Na2IrIVCl6: Spin-Orbital-Induced Semiconductor Showing Hydration-Dependent Structural and Magnetic Variations

Iridium(IV) oxides have gained increased attention in recent years owing to the presence of competing spin-orbit coupling and Coulomb interactions, which facilitate the emergence of novel quantum phenomena. In contrast, the electronic structure and magnetic properties of Ir^IV-based molecular materials remain largely unexplored. In this paper, we take a fresh look at an old but puzzling compound, Na₂IrCl₆, which can be hydrated to form two stable phases with formulas Na₂IrCl₆·2H₂O and Na₂IrCl₆·6H₂O. Their crystal structures are well illustrated based on X-ray powder diffraction data. Magnetic studies reveal that Na₂IrCl₆ and Na₂IrCl₆·2H₂O are canted antiferromagnets with ordering temperatures of 7.4 and 2.7 K, respectively, whereas Na₂IrCl₆·6H₂O is paramagnetic down to 1.8 K. First-principle calculations on Na₂IrCl₆ reveal a J_eff = 1/2 ground state, and the band structures show that Na₂IrCl₆ is a spin-orbital-induced semiconductor with an indirect gap of about 0.18 eV.

Relativistic DFT Calculations of Hyperfine Coupling Constants in 5d Hexafluorido Complexes: [ReF6 ]2- and [IrF6 ]2

The performance of relativistic density functional theory (DFT) methods has been investigated for the calculation of the recently measured hyperfine coupling constants of hexafluorido complexes [ReF6 ]2- and [IrF6 ]2- . Three relativistic methods were employed at the DFT level of theory: the 2-component zeroth-order regular approximation (ZORA) method, in which the spin-orbit coupling was treated either variationally (EV ZORA) or as a perturbation (LR ZORA), and the 4-component Dirac-Kohn-Sham (DKS) method. The dependence of the results on the basis set and the choice of exchange-correlation functional was studied. Furthermore, the effect of varying the amount of Hartree-Fock exchange in the hybrid functionals was investigated. The LR ZORA and DKS methods combined with DFT led to very similar deviations (about 20 %) from the experimental values for the coupling constant of complex [ReF6 ]2- by using hybrid functionals. However, none of the methods were able to reproduce the large anisotropy of the hyperfine coupling tensor of complex [ReF6 ]2- . For [IrF6 ]2- , the EV ZORA and DKS methods reproduced the experimental tensor components with deviations of ≈10 and ≈5 % for the hybrid functionals, whereas the LR ZORA method predicted the coupling constant to be around one order of magnitude too large owing to the combination of large spin-orbit coupling and very low excitation energies.

Slow magnetic relaxation influenced by change of symmetry from ideal Ci to D3d in cobalt(ii)-based single-ion magnets

Two mononuclear complexes including the same cation [Co(imidazole)₆]²⁺ and the different symmetries C_i and D_3d showed discriminatively slow magnetic relaxation.