Tunable SIM properties in a family of 3D anilato-based lanthanide-MOFs

By reacting a 3,6-ditriazolyl-2,5-dihydroxybenzoquinone (H2trz2An) anilato linker with LnIII ions (LnIII = Dy, Tb, Ho), two different series of polymorphs, formulated as [Ln2(trz2An)3(H2O)4] n ·10H2O (DyIII, 1a; TbIII, 2a, HoIII, 3a) and [Ln2(trz2An)3(H2O)4] n ·7H2O (DyIII, 1b, TbIII, 2b, HoIII, 3b) have been obtained. In these series the two DyIII-coordination networks (1a and 1b) and the TbIII-coordination polymer (2b) show a Single Ion Magnet (SIM) behavior. 1-3a MOFs show reversible structural flexibility upon removal of a coordinated water molecule from a distorted hexagonal 2D framework to a distorted 3,6-brickwall rectangular 3D structure in [Ln2(trz2An)3(H2O)2] n ·2H2O (DyIII, 1a_des; TbIII, 2a_des, HoIII, 3a_des) involving shrinkage/expansion of the hexagonal-rectangular networks. Noteworthy, 2b represents the first example of a TbIII-anilate-based coordination polymer showing SIM behaviour to date and the best SIM properties within the polymorphs. Theoretical investigation via ab initio CASSCF calculations supports this behavior, since 2b shows less mixing between the m J states of the ground state among all the studied complexes.

Slow magnetic relaxation in a heteroleptic anilate-based DyIII metal-organic framework

Novel heteroleptic anilate-based lanthanide MOFs (LnIII = Tb, Dy, Ho) have been obtained under hydrothermal conditions by the ancillary ligand synthetic strategy. These structurally isomorphous species contain octacoordinated LnIII ions with coordination polyhedra approaching an ideal D2d symmetry, best described by a distorted biaugmented trigonal prismatic C2v geometry. In the whole series, only the Dy-MOF exhibits SMM behaviour.

4f-Orbital mixing increases the magnetic susceptibility of Cp'3Eu

Traditional models of lanthanide electronic structure suggest that bonding is predominantly ionic, and that covalent orbital mixing is not an important factor in determining magnetic properties. Here, 4f orbital mixing and its impact on the magnetic susceptibility of Cp'3Eu (Cp' = C5H4SiMe3) was analyzed experimentally using magnetometry and X-ray absorption spectroscopy (XAS) methods at the C K-, Eu M5,4-, and L3-edges. Pre-edge features in the experimental and TDDFT-calculated C K-edge XAS spectra provided unequivocal evidence of C 2p and Eu 4f orbital mixing in the π-antibonding orbital of a' symmetry. The charge-transfer configurations resulting from 4f orbital mixing were identified spectroscopically by using Eu M5,4-edge and L3-edge XAS. Modeling of variable-temperature magnetic susceptibility data showed excellent agreement with the XAS results and indicated that increased magnetic susceptibility of Cp'3Eu is due to removal of the degeneracy of the 7F1 excited state due to mixing between the ligand and Eu 4f orbitals.

Near-infrared circular dichroism of the ytterbium DOTMA complex: an ab initio investigation

The electronic structure and circular dichroism spectra of the ytterbium(III) complex [Yb(DOTMA)]^- are calculated using complete and restricted active space self-consistent field wavefunction methods with the spin-orbit coupling treated by the state interaction approach. The influence of the dynamical correlation effect is then included via the 2nd order perturbation method. The experimental circular dichroism spectrum is well reproduced by calculations, both in terms of relative energy excitations and in terms of rotatory strength intensities. The results allow highlighting the mechanism that drives the chiroptical properties in Yb(III) complexes and reveal the importance of taking into account the 4f¹²5d¹ electronic configurations in the calculated wavefunctions to properly describe the chiroptical properties of the 4f-4f transitions.

Spin-Orbit Natural Transition Orbitals and Spin-Forbidden Transitions

Natural transition orbitals (NTOs) are in widespread use for visualizing and analyzing electronic transitions. The present work introduces the analysis of formally spin-forbidden transitions with the help of complex-valued spin-orbit (SO) NTOs. The analysis specifically focuses on the components in such transitions that cause their intensity to be nonzero because of SO coupling. Transition properties such as transition dipole moments are partitioned into SO-NTO hole-particle pairs, such that contributions to the intensity from specific occupied and unoccupied orbitals are obtained. The method has been implemented within the restricted active space (RAS) self-consistent field wave function theory framework, with SO coupling treated by RAS state interaction. SO-NTOs have a broad range of potential applications, which is illustrated by the T₂-S₁ state mixing in pyrazine, spin-forbidden versus spin-allowed 4f-5d transitions in the Tb³⁺ ion, and the phosphorescence of tris(2-phenylpyridine) iridium [Ir(ppy)₃].

Combined Experimental/Theoretical Study on the Luminescent Properties of Homoleptic/Heteroleptic Erbium(III) Anilate-Based 2D Coordination Polymers

The synthesis, structural and photophysical characterization, and theoretical studies on homo/heteroleptic neutral 2D-layered coordination polymers (CPs), obtained by combining the Er^III ion with chlorocyananilate (ClCNAn) and/or tetrafluoroterephthalate (F₄BDC) linkers, are herein reported. The structure of the heteroleptic Er^III-based CP, formulated as [Er₂(ClCNAn)₂(F₄BDC)(DMSO)₆]_n (1) is also reported. 1 crystallizes in the triclinic P1̅ space group, and the structure consists of neutral 2D layers formed by Er^III ions linked through the two linkers oriented in such a way that the neighboring 2D layers are eclipsed along the a axis, leading to parallelogram-like cavities. Photophysical measurements highlight the prominent role of chlorocyananilate linkers as optical antennas toward lanthanide ions, while wave-function-theory analysis supports the experimental findings, providing evidence for the effect of ligand substitution on the luminescence properties of homo/heteroleptic 2D CPs.

Enhancing the Magnetic Anisotropy in Low-Symmetry Dy-Based Complexes by Tuning the Bond Length

One mononuclear complex [Dy(Htpy)(NO₃)₂(acac)] (1) and a tpy^--extended 1D chain {[Dy(CH₃OH)(NO₃)₂(tpy)]·CH₃OH}_n (2) (Htpy = 4'-(4-hydroxyphenyl)-2,2':6',2''-terpyridine, Hacac = acetylacetone) were successfully designed to investigate the effect of bond length tuning around the Dy^III cation on the magnetic dynamics of single-molecule magnets (SMMs). Interestingly, two magnetic entities possess the same local coordination sphere (N₃O₆-donor) as well as the configuration (Muffin, C_s) of dysprosium centers. Only a slight difference in structure results from purposefully substituting the acetylacetone ligand in 1 with hydroxyl oxygen from tpy^- linkage and one methanol molecule in 2. However, the remarkable differences in dynamics behavior were clearly found between them. Compound 1 possesses a thermal-activated effective energy barrier (U_eff/k_B) of 22.7 K under a 0 kOe direct current (dc) field and negligible hysteresis loop at 2.0 K, while complex 2 shows high-performance SMM behavior with the largest energy barrier of 354.36 K among the reported nine-coordinated Dy^III-based systems and the magnetic hysteresis up to 4.0 K at a sweep rate of 200 Oe s^-1. These experimental results combined with the previous reported data reveal that the shortest bond and the bond length difference around the Dy^III center synergistically determine the dynamics of SMMs. The uniaxial anisotropy increases with the decrease of the shortest bond and the increase of the bond length difference, which is confirmed by the theoretical calculations.

Magnetic properties of organolanthanide(II) complexes, from the electronic structure and the crystal field effect

The magnetic properties of a series of organometallic complexes [LnCp3]- and Ln(CNT)2, where Cp = cyclopentadienyl and CNT = cyclononatetraenyl, of the lanthanide ions in the 2+ oxidation state, are theoretically studied in terms of the electronic structure obtained via multiconfigurational wave function-based methods. Calculations are performed for two groups of ion complexes selected based on their preferred electronic configuration 4fn+1 or 4fn5d1 (n is the number of f electrons in the 3+ ion). All the properties are discussed in terms of the electron density distribution of the ground state and ligand field effects. This analysis allows giving some molecular design strategies relevant to exploit the magnetic properties in applications like Single-Molecule Magnets (SMMs) for lanthanide ions in the 2+ oxidation state.

Luminescence, chiroptical, magnetic and ab initio crystal-field characterizations of an enantiopure helicoidal Yb(iii) complex

The electronic structure of a chiral Yb(iii)-based complex is fully determined by taking advantage of experimental magnetic, luminescence, and chiroptical (NIR-ECD and CPL) characterizations in combination with ab initio wavefunction calculations.

Electronic Structure and Photoluminescence Properties of Eu(η9-C9H9)2

The electronic structure of Eu²⁺ compounds results from a complex combination of strongly correlated electrons and relativistic effects as well as weak ligand-field interaction. There is tremendous interest in calculating the electronic structure as nowadays the Eu²⁺ ion is becoming more and more crucial, for instance, in lighting technologies. Recently, interest in semiempirical methods to qualitatively evaluate the electronic structure and to model the optical spectra has gained popularity, although the theoretical methods strongly rely upon empirical inputs, hindering their prediction capabilities. Besides, ab initio multireference models are computationally heavy and demand very elaborative theoretical background. Herein, application of the ligand-field density functional theory (LFDFT) method that is recently available in the Amsterdam Modeling Suite is shown: (i) to elucidate the electronic structure properties on the basis of the multiplet energy levels of Eu configurations 4f⁷ and 4f⁶5d¹ and (ii) to model the optical spectra quite accurately if compared to the conventional time-dependent density functional theory tool. We present a theoretical study of the molecular Eu(η⁹-C₉H₉)₂ complex and its underlying photoluminescence properties with respect to the Eu 4f-5d electron transitions. We model the excitation and emission spectra with good agreement with the experiments, opening up the possibility of modeling lanthanides in complex environment like nanomaterials by means of LFDFT at much-reduced computational resources and cost.

Ligand effects on electronic structure and bonding in U(iii) coordination complexes: a combined MCD, EPR and computational study

Spectroscopy and theory enable broader insight into electronic structure and bonding in U(iii) coordination complexes, focusing on systems with Tp* ligands.

Effect of magnetic anisotropy on direct chiral discrimination in paramagnetic NMR spectroscopy

We have studied the effect of thermally populated crystal field states on room temperature chiral discrimination in NMR spectroscopy.

Role of Ab Initio Calculations in the Design and Development of Organometallic Lanthanide-Based Single-Molecule Magnets

Single-molecule magnets based on lanthanides are very attractive due to their potential applications proposed in the area of microelectronic devices. Very recent advances in this area are due to the blend of conventional lanthanide chemistry with organometallic ligands, and several breakthrough achievements are attained with this combination. Ab initio methods based on multi-reference CASSCF calculations are playing a vital role in the design and development of such molecules. In this minireview, we aim to appraise various contributions in the area of organometallic lanthanide complexes (those containing lanthanide-carbon bonds) and describe how these robust wavefunction-based methods have played a constructive role not only in rationalizing the observed magnetic properties but also proven to be a potential predictive tool with some selected examples.

Probing the structures, electronic and bonding properties of multidecker lanthanides: Neutral and anionic Lnn(COT)m (Ln = Ce, Nd, Eu, Ho and Yb; n, m = 1, 2) complexes

The ground state structures of neutral and anionic Ln_n(COT)_m (Ln = Ce, Nd, Eu, Ho and Yb; n, m = 1, 2) complexes have been identified by density functional theory. Ln(COT)_1,2^0/- and Ln₂(COT)₂^0/- complexes are found to possess sandwich ground state structures in which Ln atoms and COT molecules are alternately stacked except for Nd₂COT₂^0/-. Our calculated AEA and VDE values show good agreement with the available experimental values, which validates that our obtained ground state structures are credible. Based on the frontier molecular orbitals, we find that the bond formation between the 4f electrons of Ln atoms and the π clouds of COT molecules is weak. Then, the bond strength within these complexes is further analyzed based on the topological analysis of electron density at bond critical point. By analyzing Hirshfeld charge, we find Ln_n(COT)_m^0/- are charge-transfer complexes with weak bonding feature.

Magnetic Coupling in the Ce(III) Dimer Ce2(COT)3

The monomer [Ce(COT)₂]^- and the dimer [Ce₂(COT)₃], with Ce(III) and COT = 1,3,5,7-cyclooctatetraenide, are studied by quantum chemistry calculations. Due to the large spin-orbit coupling, the ground state of the monomer is a strong mixing of σ and π states. The experimental isotropic coupling in the dimer was evaluated by Walter et al. to be J = -7 cm^-1 (with a Heisenberg Hamiltonian [Formula: see text]) with a small anisotropic coupling of 0.02 cm^-1. The coupling between the two Ce(III) in the dimer is calculated using CI methods. The low energy part of the spectra are modeled by spin Hamiltonians. All spin Hamiltonians parameters are deduced from ab initio calculations. g factors are calculated for both the pseudodoublet of the monomer and the pseudotriplet of the dimer and their sign have been determined. The magnetic coupling in the dimer is rationalized by a model based on crystal field theory. The kinetic and exchange contributions arising from the different configurations to the isotropic and anisotropic couplings are evaluated. It is shown that the main contribution to isotropic coupling is kinetic and originates from the f_σ-f_σ interaction due to the large transfer integral between those orbitals. However, the f_π-f_π interaction plays a non-negligible role. The anisotropic coupling originates from the difference of exchange energy of states arising from the f_σf_π configuration and is, in no matter, related to the anisotropy of the local magnetic moments as already pointed by van Vleck for a fictitious s-p system. The analysis of the natural orbitals evidences a superexchange mechanism through a σ_CH^* orbital of the bridging cycle favored by a local 4f_σ/5d_σ hybridization and that the δ type orbitals, both the HOMOs of the ligands and the virtual f_δ orbitals of the cerium atoms play an important polarization role, and to a less extend the π type orbitals, the HOMOs-1 of the ligands, and the metal f_π orbitals.

High local coordination symmetry around the spin center and the alignment between magnetic and symmetric axes together play a crucial role in single-molecule magnet performance

A tetradentate 8-hydroxyquinoline-based acyl hydrazine ligand was used to construct a series of mono/di-nuclear dysprosium single-molecule magnets (SMMs) with nearly ideal pentagonal bipyramid coordination geometry (D_5h).

Similar ligand-metal bonding for transition metals and actinides? 5f1 U(C7H7)2-versus 3d n metallocenes

U(C7H7)2- is a fascinating 5f1 complex whose metal-ligand bonding was assigned in the literature as being very similar to 3d7 cobaltocene, based on a crystal-field theoretical interpretation of the experimental magnetic resonance data. The present work provides an in-depth theoretical study of the electronic structure, bonding, and magnetic properties of the 5f1 U(C7H7)2-vs. 3d metallocenes with V, Co, and Ni, performed with relativistic wavefunction and density functional methods. The ligand to metal donation bonding in U(C7H7)2- is strong and in fact similar to that in vanadocene, in the sense that the highest occupied arene orbitals donate electron density into empty metal orbitals of the same symmetry with respect to the rotational axis (3dπ for V, 5fδ for U), but selectively with α spin (↑). For Co and Ni, the dative bonding from the ligands is β spin (↓) selective into partially filled 3dπ orbitals. In all systems, this spin delocalization triggers spin polarization in the arene σ bonding framework, causing proton spin densities opposite to those of the carbons. As a consequence, the proton spin densities and hyperfine coupling constants are negative for the Co and Ni complex, but positive for vanadocene. The of U(C7H7)2- is negative and similar to that of cobaltocene, but only because of the strong spin-orbit coupling in the actinocene, which causes to be opposite to the sign of the proton spin density. The study contributes to a better understanding of actinide 5f vs. transition metal 3d covalency, and highlights potential pitfalls when interpreting experimental magnetic resonance data in terms of covalent bonding for actinide complexes.

Experimental and theoretical interpretation of the magnetic behavior of two Dy(iii) single-ion magnets constructed through β-diketonate ligands with different substituent groups (–Cl/–OCH3)

Two Dy(iii) single-ion magnets, formulated as [Dy(Phen)(Cl-tcpb)₃] (Cl-1) and [Dy(Phen)(CH₃O-tmpd)₃] (CH₃O-2) were obtained through β-diketonate ligands (Cl-tcpb = 1-(4-chlorophenyl)-4,4,4-trifluoro-1,3-butanedione and CH₃O-tmpd = 4,4,4-trifluoro-1-(4-methoxyphenyl)-1,3-butanedione) with different substituent groups (–Cl/–OCH₃) and auxiliary ligand, 1,10-phenanthroline (Phen). The Dy(iii) ions in Cl-1 and CH₃O-2 are eight-coordinate, with an approximately square antiprismatic (SAP, D_4d) and trigonal dodecahedron (D_2d) N₂O₆ coordination environment, respectively, in the first coordination sphere. Under zero direct-current (dc) field, magnetic investigations demonstrate that both Cl-1 and CH₃O-2 display dynamic magnetic relaxation of single-molecule magnet (SMM) behavior with different effective barriers (U_eff) of 105.4 cm⁻¹ (151.1 K) for Cl-1 and 132.5 cm⁻¹ (190.7 K) for CH₃O-2, respectively. As noted, compound CH₃O-2 possesses a higher effective barrier than Cl-1. From ab initio calculations, the energies of the first excited state (KD₁) are indeed close to the experimental U_eff as 126.7 cm⁻¹vs. 105.4 cm⁻¹ for Cl-1 and 152.8 cm⁻¹vs. 132.5 cm⁻¹ for CH₃O-2. The order of the calculated energies of KD₁ is same as that of the experimental U_eff. The superior SIM properties of CH₃O-2 could have originated from the larger axial electrostatic potential (ESP_(ax)) felt by the central Dy(iii) ion when compared with Cl-1. The larger ESP_(ax) of CH₃O-2 arises from synergic effects of the more negative charge and shorter Dy–O distances of the axial O atoms of the first sphere. These charges and distances could be influenced by functional groups outside the first sphere, e.g., –Cl and –OCH₃.

Further studies from the viewpoint of electrostatic potential demonstrate that the larger axial electrostatic potential (ESP) felt by the central Dy (iii) ion of CH₃O-2 is responsible for its better SIM property when compared with Cl-1.

Ligand-Field Energy Splitting in Lanthanide-Based Single-Molecule Magnets by NMR Spectroscopy

A method for the experimental determination of ligand-field (LF) energy splitting in mononuclear lanthanide complexes, based purely on variable-temperature NMR spectroscopy, was developed. The application of this method in an isostructural series of anionic lanthanide bis(cyclooctatetraenide) double-decker compounds bearing large rigid substituents is demonstrated. Using the three-nuclei plot approach devised by Reilley, the isostructurality of the compound series and the identical orientation of the magnetic main axis of all Ln³⁺ ions in the series Tb³⁺ to Tm³⁺ are demonstrated. Measurement of the ²H NMR spectra of partially deuterated analogues of the complexes permitted determination of the axial magnetic susceptibility anisotropies χ_ax for all five ions in the temperature range from 185 to 335 K. For this purpose, analysis of the hyperfine shifts was combined with structural models derived from density functional theory calculations. In a final step, the temperature dependence of the χ_ax values was used for determination of the three axial LF parameters, adapting a method employed previously for phthalocyanine-based systems. The temperature dependence dictated by the LF parameters determined by this NMR-based approach is compared to the results of recently published ab initio calculations of the system, indicating reasonable agreement of both methods. For all ions except Dy³⁺, the NMR method determines the same m_J ground state as the calculations and the order and energies of the excited states match well. However, the sign of the magnetic anisotropy of the dysprosium complex in the temperature range evaluated here is not correctly predicted by the published calculations but can be described accurately by the NMR approach. This shows that our experimental method for determination of the LF parameters is an ideal complementation to other theoretical and experimental methods.

Electronic Structure and Properties of Berkelium Iodates

The reaction of ²⁴⁹Bk(OH)₄ with iodate under hydrothermal conditions results in the formation of Bk(IO₃)₃ as the major product with trace amounts of Bk(IO₃)₄ also crystallizing from the reaction mixture. The structure of Bk(IO₃)₃ consists of nine-coordinate Bk^III cations that are bridged by iodate anions to yield layers that are isomorphous with those found for Am^III, Cf^III, and with lanthanides that possess similar ionic radii. Bk(IO₃)₄ was expected to adopt the same structure as M(IO₃)₄ (M = Ce, Np, Pu), but instead parallels the structural chemistry of the smaller Zr^IV cation. Bk^III-O and Bk^IV-O bond lengths are shorter than anticipated and provide further support for a postcurium break in the actinide series. Photoluminescence and absorption spectra collected from single crystals of Bk(IO₃)₄ show evidence for doping with Bk^III in these crystals. In addition to luminescence from Bk^III in the Bk(IO₃)₄ crystals, a broad-band absorption feature is initially present that is similar to features observed in systems with intervalence charge transfer. However, the high-specific activity of ²⁴⁹Bk (t_1/2 = 320 d) causes oxidation of Bk^III and only Bk^IV is present after a few days with concomitant loss of both the Bk^III luminescence and the broadband feature. The electronic structure of Bk(IO₃)₃ and Bk(IO₃)₄ were examined using a range of computational methods that include density functional theory both on clusters and on periodic structures, relativistic ab initio wave function calculations that incorporate spin-orbit coupling (CASSCF), and by a full-model Hamiltonian with spin-orbit coupling and Slater-Condon parameters (CONDON). Some of these methods provide evidence for an asymmetric ground state present in Bk^IV that does not strictly adhere to Russel-Saunders coupling and Hund's Rule even though it possesses a half-filled 5f ⁷ shell. Multiple factors contribute to the asymmetry that include 5f electrons being present in microstates that are not solely spin up, spin-orbit coupling induced mixing of low-lying excited states with the ground state, and covalency in the Bk^IV-O bonds that distributes the 5f electrons onto the ligands. These factors are absent or diminished in other f⁷ ions such as Gd^III or Cm^III.

Characterization of berkelium(III) dipicolinate and borate compounds in solution and the solid state

Berkelium is positioned at a crucial location in the actinide series between the inherently stable half-filled 5f(7) configuration of curium and the abrupt transition in chemical behavior created by the onset of a metastable divalent state that starts at californium. However, the mere 320-day half-life of berkelium's only available isotope, (249)Bk, has hindered in-depth studies of the element's coordination chemistry. Herein, we report the synthesis and detailed solid-state and solution-phase characterization of a berkelium coordination complex, Bk(III)tris(dipicolinate), as well as a chemically distinct Bk(III) borate material for comparison. We demonstrate that berkelium's complexation is analogous to that of californium. However, from a range of spectroscopic techniques and quantum mechanical calculations, it is clear that spin-orbit coupling contributes significantly to berkelium's multiconfigurational ground state.

Key role of higher order symmetry and electrostatic ligand field design in the magnetic relaxation of low-coordinate Er(iii) complexes

Our theoretical analysis highlights that both symmetry and a suitable ligand field is required to obtain large barrier heights in SIMs. Key role of Lanthanide–halogen covalency in enhancing U_eff is discussed.

Magnetic circular dichroism of UCl6− in the ligand-to-metal charge-transfer spectral region

We present a combined ab initio theoretical and experimental study of the magnetic circular dichroism (MCD) spectrum of the octahedral UCl₆⁻ complex ion in the UV-Vis spectral region.

NMR analysis of an Fe(i)–carbene complex with strong magnetic anisotropy

A paramagnetic, easy-plane anisotropic Fe^I complex, bearing cyclic-alkyl(amino) carbene (cAAC) ligands, is studied by means of NMR and DFT.

Modeling the electronic states and magnetic properties derived from the f1 configuration in lanthanocene and actinocene compounds

The electronic structure and magnetic properties of a series of Kramers ions with f¹ configuration in axial symmetry have been analyzed with a combination of theoretical methods: ab initio relativistic wavefunction methods as well as a crystal-field (CF) model with parameters extracted from the ab initio calculations.

Theoretical Study on the Photoelectron Spectra of Ln(COT)2-: Lanthanide Dependence of the Metal-Ligand Interaction

We have performed a theoretical analysis of the recently reported photoelectron (PE) spectra of the series of sandwich complex anions Ln(COT)₂^- (Ln = La-Lu, COT = 1,3,5,7-cyclooctatetraene), focusing on the Ln dependence of the vertical detachment energies. For most Ln, the π molecular orbitals, largely localized on the COT ligands, have the energy order of e_1g < e_1u < e_2g < e_2u as in the actinide analogues, reflecting the substantial orbital interaction with the Ln 5d and 5p orbitals. Thus, it would be expected that the lanthanide contraction would increase the orbital interaction so that the overlaps between the COT π and Ln atomic orbitals tend to increase across the series. However, the PE spectra and theoretical calculations were not consistent with this expectation, and the details have been clarified in this study. Furthermore, the energy level splitting patterns of the anion and neutral complexes have been studied by multireference ab initio methods, and the X peak splittings observed in the PE spectra only for the middle-range Ln complexes were found to be due to the specific interaction between the Ln 4f and ligand π orbitals of the neutral complexes in e_2u symmetry. Because the magnitude of this 4f-ligand interaction depends critically on the final state 4f electron configuration and the spin state, a significant Ln dependence in the PE spectra is explained.

1,2,4-Diazaphospholide complexes of lanthanum(iii), cerium(iii), neodymium(iii), praseodymium(iii), and samarium(iii): synthesis, X-ray structural characterization, and magnetic susceptibility studies

A few heteroleptic, charge-separated heterobimetallic, and polymeric alkali metalate complexes of 1,2,4-diazaphospholide lanthanum(iii), cerium(iii), neodymium(iii), praseodymium(iii), and samarium(iii) were simply prepared via the metathesis reaction of MCl3 (THF)m (m = 1-2) and K[3,5-R2dp] ([3,5-R2dp](-) = 3,5-di-substituent-1,2,4-diazaphospholide; R = tBu, Ph) in a varied ratio (1 : 3, 1 : 4, and 1 : 5, respectively) at room temperature in tetrahydrofuran. All the complexes were fully characterized by (1)H, (13)C{(1)H}, (31)P{(1)H}, IR, and X-ray single crystal diffraction analysis despite their paramagnetism (excluding La(iii) complexes). The structures of the complexes were found to feature varied coordination modes. The magnetic properties of several compounds were studied by magnetic susceptibility, and the complexes presented the magnetic moments close to or lower than the theoretical values for the free ions in the trivalent oxidation states (Pr(3+), Nd(3+)).

The roles of 4f- and 5f-orbitals in bonding: a magnetochemical, crystal field, density functional theory, and multi-reference wavefunction study

The electronic structures of 4f(3)/5f(3) Cp''3M and Cp''3M·alkylisocyanide complexes, where Cp'' is 1,3-bis-(trimethylsilyl)cyclopentadienyl, are explored with a focus on the splitting of the f-orbitals, which provides information about the strengths of the metal-ligand interactions. While the f-orbital splitting in many lanthanide complexes has been reported in detail, experimental determination of the f-orbital splitting in actinide complexes remains rare in systems other than halide and oxide compounds, since the experimental approach, crystal field analysis, is generally significantly more difficult for actinide complexes than for lanthanide complexes. In this study, a set of analogous neodymium(iii) and uranium(iii) tris-cyclopentadienyl complexes and their isocyanide adducts was characterized by electron paramagnetic resonance (EPR) spectroscopy and magnetic susceptibility. The crystal field model was parameterized by combined fitting of EPR and susceptibility data, yielding an accurate description of f-orbital splitting. The isocyanide derivatives were also studied using density functional theory, resulting in f-orbital splitting that is consistent with crystal field fitting, and by multi-reference wavefunction calculations that support the electronic structure analysis derived from the crystal-field calculations. The results highlight that the 5f-orbitals, but not the 4f-orbitals, are significantly involved in bonding to the isocyanide ligands. The main interaction between isocyanide ligand and the metal center is a σ-bond, with additional 5f to π* donation for the uranium complexes. While interaction with the isocyanide π*-orbitals lowers the energies of the 5fxz(2) and 5fyz(2)-orbitals, spin-orbit coupling greatly reduces the population of 5fxz(2) and 5fyz(2) in the ground state.

Configuration-averaged 4f orbitals in ab initio calculations of low-lying crystal field levels in lanthanide(iii) complexes

We propose an ab initio method that simplifies the CASSCF/RASSI–SO approach for crystal field levels and magnetic properties of lanthanide complexes.