Excitation energies expressed as orbital energies of Kohn-Sham density functional theory with long-range corrected functionals.

A new simple and conceptual theoretical scheme is proposed for estimating one-electron excitation energies using Kohn-Sham (KS) solutions. One-electron transitions that are dominated by the promotion from one initially occupied orbital to one unoccupied orbital of a molecular system can be expressed in a two-step process, ionization, and electron attachment. KS with long-range corrected (LC) functionals satisfies Janak's theorem and LC total energy varies almost linearly as a function of its fractional occupation number between the integer electron points. Thus, LC reproduces ionization energies (IPs) and electron affinities (EAs) with high accuracy and one-electron excitation energies are expressed as the difference between the occupied orbital energy of a neutral molecule and the corresponding unoccupied orbital energy of its cation. Two such expressions can be used, with one employing the orbital energies for the neutral and cationic systems, while the other utilizes orbital energies of just the cation. Because the EA of a molecule is the IP of its anion, if we utilize this identity, the two expressions coincide and give the same excitation energies. Reasonable results are obtained for valence and core excitations using only orbital energies.

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.

Characterization of a robust co(II) fluorescent complex deposited intact on HOPG.

The new diimine fluorescent ligand ACRI-1 based on a central acridine yellow core is reported along with its coordination complex [Co2 (ACRI-1)2 ] (1), a fluorescent weak ferromagnet. Due to the strong fluorescence of the acridine yellow fluorophore, it is not completely quenched when the ligand is coordinated to Co(II) . The magnetic properties of bulk complex 1 and its stability in solution have been studied. Complex 1 has been deposited on highly ordered pyrolitic graphite (HOGP) from solution. The thin films prepared have been characterized by AFM, time-of-flight secondary ion mass spectrometry (TOF-SIMS), grazing incidence X-ray diffraction (GIXRD), X-ray absorption spectroscopy (XAS), X-ray magnetic circular dichroism (XMCD) and theoretical calculations. The data show that the complex is robust and remains intact on the surface of graphite.

Solvate-dependent spin crossover and exchange in cobalt(II) oxazolidine nitroxide chelates.

Two oxazolidine nitroxide complexes of cobalt(II), [Co(II)(L(•))2](B(C6F5)4)2·CH2Cl2 (1) and [Co(II)(L(•))2](B(C6F5)4)2·2Et2O (2), where, L(•) is the tridentate chelator 4,4-dimethyl-2,2-bis(2-pyridyl)oxazolidine N-oxide, have been investigated by crystallographic, magnetic, reflectivity, and theoretical (DFT) methods. This work follows on from a related study on [Co(II)(L(•))2](NO3)2 (3), a multifunctional complex that simultaneously displays magnetic exchange, spin crossover, and single molecule magnetic features. Changing the anion and the nature of solvation in the present crystalline species leads to significant differences, not only between 1 and 2 but also in comparison to 3. Structural data at 123 and 273 K, in combination with magnetic data, show that at lower temperatures 1 displays low-spin Co(II)-to-radical exchange with differences in fitted J values in comparison to DFT (broken symmetry) calculated J values ascribed to the sensitive influence of a tilt angle (θ) formed between the Co(dz(2)) and the trans-oriented O atoms of the NO radical moieties in L(•). Spin crossover in 1 is evident at higher temperatures, probably influenced by the solvate molecules and crystal packing arrangement. Complex 2 remains in the high-spin Co(II) state between 2 and 350 K and undergoes antiferromagnetic exchange between Co-radical and radical-radical centers, but it is difficult to quantify. Calculations of the magnetic orbitals, eigenvalue plots, and the spin densities at the Co and radical sites in 1 and 2 have yielded satisfying details on the mechanism of metal-radical and radical-radical exchange, the radical spins being in π*NO orbitals.

Observation of ferromagnetic exchange, spin crossover, reductively induced oxidation, and field-induced slow magnetic relaxation in monomeric cobalt nitroxides.

The reaction of [Co(II)(NO3)2]·6H2O with the nitroxide radical, 4-dimethyl-2,2-di(2-pyridyl) oxazolidine-N-oxide (L(•)), produces the mononuclear transition-metal complex [Co(II)(L(•))2](NO3)2 (1), which has been investigated using temperature-dependent magnetic susceptibility, electron paramagnetic resonance (EPR) spectroscopy, electrochemistry, density functional theory (DFT) calculations, and variable-temperature X-ray structure analysis. Magnetic susceptibility measurements and X-ray diffraction (XRD) analysis reveal a central low-spin octahedral Co(2+) ion with both ligands in the neutral radical form (L(•)) forming a linear L(•)···Co(II)···L(•) arrangement. This shows a host of interesting magnetic properties including strong cobalt-radical and radical-radical intramolecular ferromagnetic interactions stabilizing a S = (3)/2 ground state, a thermally induced spin crossover transition above 200 K and field-induced slow magnetic relaxation. This is supported by variable-temperature EPR spectra, which suggest that 1 has a positive D value and nonzero E values, suggesting the possibility of a field-induced transverse anisotropy barrier. DFT calculations support the parallel alignment of the two radical π*NO orbitals with a small orbital overlap leading to radical-radical ferromagnetic interactions while the cobalt-radical interaction is computed to be strong and ferromagnetic. In the high-spin (HS) case, the DFT calculations predict a weak antiferromagnetic cobalt-radical interaction, whereas the radical-radical interaction is computed to be large and ferromagnetic. The monocationic complex [Co(III)(L(-))2](BPh4) (2) is formed by a rare, reductively induced oxidation of the Co center and has been fully characterized by X-ray structure analysis and magnetic measurements revealing a diamagnetic ground state. Electrochemical studies on 1 and 2 revealed common Co-redox intermediates and the proposed mechanism is compared and contrasted with that of the Fe analogues.

Magnetic Anisotropy in CoII X4 (X=O, S, Se) Single-Ion Magnets: Role of Structural Distortions versus Heavy Atom Effect.

Mononuclear four coordinate CoII complexes have drawn a great deal of attention as they often exhibit excellent single-ion magnet (SIM) properties. Among the reported complexes, the axial zero-field splitting parameter (D) was found to vary drastically both in terms of the sign as well as strength. There are various proposals in this respect such as structural distortions, heavier atom substitution, metal-ligand covalency, tuning secondary coordination sphere, etc. that are expected to control the D values. To assess the importance of structural distortions vs. heavier atom substitution effect, here we have undertaken detailed theoretical studies based on the ab initio CASSCF/NEVPT2 method to estimate zero-field splitting parameters for twelve complexes reported in the literature. Our test set includes the {CoII X4 } (where X=O, S, Se) core structure where the D value was found to vary from +19 to -118 cm-1 . Based on the structural variation, we have classified the complexes into three types (I-III) where type I complexes were found to exhibit the largest negative D value as desired for SIMs. The other two types (II and III) of complexes have been found to be inferior with respect to type I. The secondary coordination sphere was also found to influence D, as substitution on the secondary coordination sphere atom was found to significantly alter the magnitude of D values. Particularly, two structural parameters, namely, the dihedral angle between the two ligand planes and the ∠ X-Co-X polar angle were found to heavily influence the sign and strength of D values. Our analysis clearly reveals that these structural factors are much more important than the heavier atom substitution, or metal-ligand covalency. A large variation in the D and E/D values among these complexes despite possessing a very close structural similarity offers an exquisite playground for a chemist to design and develop new-generation CoII -based SIMs.

Thermally-Induced Spin Crossover and LIESST Effect in the Neutral [FeII(Mebik)2(NCX)2] Complexes: Variable-Temperature Structural, Magnetic, and Optical Studies (X = S, Se; Mebik = bis(1-methylimidazol-2-yl)ketone).

Two new iron(II) neutral complexes of bis(1-methylimidazol-2-yl)ketone (Mebik) with molecular formula [FeII(Mebik)2(NCS)2] (1) and [FeII(Mebik)2(NCSe)2] (2) have been synthesized and characterized by magnetic measurements, single-crystal X-ray diffraction, and solid state UV-vis spectroscopy. The temperature dependent magnetic susceptibility measurements of crystalline samples of both compound show the occurrence of a gradual spin transition centered at T1/2 = 260 K and 326 K, respectively. The crystal structures of both compounds were determined at different temperatures, below and above the transition, in order to detect the structural changes associated with the spin transition. The main structural modifications, when passing from the low-spin to the high-spin form, consist of an important lengthening of the Fe-N(Mebik) and Fe-N (C-S/Se) distances (by ca. 0.20 and 0.18 Å, respectively) and a noticeable variation of the N-Fe-N angles, leading to a more distorted [Fe-N6] octahedron. The spin-transition phenomenon also affects the optical properties, with significant decrease of the intensity of the Metal-to-Ligand charge transfer band upon increasing the temperature. Finally, both complexes exhibit a light-induced excited spin-state trapping under laser light irradiation at low temperature. DFT calculations were also carried out on these complexes in order to rationalize the theoretically predicted magnetic and optical behavior with those of the experimental one. The results clearly highlights the dramatic alteration of the magneto-structural behavior of the tris-chelate [FeII(Mebik)3]2+ spin-crossover complex upon substituting one Mebik with NCS and NCSe ligands.

A synthetic strategy for switching the single ion anisotropy in tetrahedral Co(II) complexes.

Four novel mononuclear tetrahedral cobalt(II) complexes containing exocyclic mesoionic ligands of molecular formulae [Co(II)(L1)(X)2(MeCN)] X = Cl (1) or Br (2) and [Co(II)(L2)(X)2(MeCN)], X = Cl (3) or Br (4) have been reported. It is found that simple substitution of L1 (O donor in 1 and 2) by L2 (S donor in 3 and 4) results in switching of the single ion magnetic anisotropy parameter (D) from positive to negative, with a significant change in magnitude.

Enhancing the double exchange interaction in a mixed valence {V(III)-V(II)} pair: a theoretical perspective.

Combined DFT-TD-DFT methodology has been employed to fully characterise a mixed valence {V(II)-V(III)} complex of molecular formula [(PY5Me2)2V2(m-5,6-dimethylbenzimidazolate)](4+). These calculations offer viable ways to enhance double-exchange parameters - a key ingredient in the synthesis of SMMs.