Bing Yin

Are polynuclear superhalogens without halogen atoms probable? A high-level ab initio case study on triple-bridged binuclear anions with cyanide ligands

The first theoretical exploration of superhalogen properties of polynuclear structures based on pseudohalogen ligand is reported here via a case study on eight triply-bridged [Mg2(CN)5](-) clusters. From our high-level ab initio results, all these clusters are superhalogens due to their high vertical electron detachment energies (VDE), of which the largest value is 8.67 eV at coupled-cluster single double triple (CCSD(T)) level. Although outer valence Greens function results are consistent with CCSD(T) in most cases, it overestimates the VDEs of three anions dramatically by more than 1 eV. Therefore, the combined usage of several theoretical methods is important for the accuracy of purely theoretical prediction of superhalogen properties of new structures. Spatial distribution of the extra electron of high-VDE anions here indicates two features: remarkable aggregation on bridging CN units and non-negligible distribution on every CN unit. These two features lower the potential and kinetic energies of the extra electron respectively and thus lead to high VDE. Besides superhalogen properties, the structures, relative stabilities and thermodynamic stabilities with respect to detachment of CN(-1) were also investigated for these anions. The collection of these results indicates that polynuclear structures based on pseudohalogen ligand are promising candidates for new superhalogens with enhanced properties.

Uniaxial magnetic anisotropy of square-planar chromium(II) complexes revealed by magnetic and HF-EPR studies

Two mononuclear square-planar Cr(II) complexes are reported, exhibiting field-induced slow magnetic relaxation. The axial zero-field splitting parameter was unambiguously determined by both a high-frequency/field electron paramagnetic resonance (HF-EPR) technique and magnetic measurements. This result represents the first observed single-molecule-magnet behavior in the square planar coordination geometry of any metal ions.

Ligand field fine-tuning on the modulation of the magnetic properties and relaxation dynamics of dysprosium(III) single-ion magnets (SIMs): synthesis, structure, magnetism and ab initio calculations

To fine-tune the magnetic anisotropy and further modulate the magnetic properties and relaxation dynamics of dysprosium(III) single-ion magnets (SIMs), it is crucial to explore their controllable synthesis and conduct a systematic theoretical investigation. Herein, the mononuclear Dy(III) precursor, [Dy(DMF)2(tffb)3] (tffb = 4,4,4-trifluoro-1-(4-fluorophenyl)-1,3-butanedione), as a “metalloligand” towards different capping ligands, affords two new mononuclear Dy(III) complexes in different solvent systems, [Dy(bpy)(tffb)3]·(C4H8O2)1/3 (1) and [Dy(Phen)(tffb)3] (2) (bpy = 2,2′-bipyridine, Phen = 1,10-phenanthroline). Using 4,4,4-trifluoro-1-(4-methylphenyl)-1,3-butanedione (tfmb) as a ligand with the coligand bpy, [Dy(bpy)(tfmb)3] (3) is obtained. In 1,4-dioxane solution, interestingly, complex 3 undergoes a dissolution/reorganization process to transform into 4, [Dy(bpy)(tfmb)3]·0.5C4H8O2. Structural analyses indicate that Dy(III) in 1–4 adopts an approximately square-antiprismatic (SAP) coordination environment with D4d axial symmetry. The magnetic properties of 1–4 are investigated and the M versus H data exhibit evident butterfly-shaped hysteresis loops at 2 K for 1–4. Although all the Dy(III) ions in 1–4 adopt similar configurations, their magnetization dynamics are apparently different from each other, as shown by the various heights of the effective energy barrier (Ueff) of magnetization reversal. To deeply understand their different magnetic behaviours, the magnetic anisotropy of 1–4 is systematically studied by ab initio calculations. The theoretical results further indicate that the capping ligands could play an important role in the fine tuning of the SMM property via an effect on the equatorial electrostatic potential, whereas the inclusion of guest solvent molecules could significantly influence the axial electrostatic potential, leading to a strong effect on the SMM property.

On balancing the QTM and the direct relaxation processes in single-ion magnets – the importance of symmetry control

Two mononuclear trigonal-planar Co(II) complexes [Na(THF)6][Co(OAr)3] 1 and [(THF)3NaCo(OAr)3] 2 (OAr− = 2,6-di-tert-butylphenoxo, THF = tetrahydrofuran) with the same coordination number and donor atoms, as well as similar ligand fields except only for the local symmetry of Co(II), were isolated to test the symmetry-magnetization correlations. Although both complexes share similar magnetic anisotropy of the central Co(II) ions (D = −85.4 cm−1 for 1 and D = −80.6 cm−1 for 2), complex 1 exhibits single-ion magnet (SIM) behaviour while 2 displays no slow magnetic relaxation of magnetization with an applied magnetic field up to 7000 Oe. Such distinct performance is ascribed to the different effects of quantum tunneling of magnetization (QTM), which is further associated with the structural symmetry, namely, a strict C2v local symmetry for 1 and Cs local symmetry for 2. Theoretical calculations also indicate a larger value of transversal factors for 2, and hence a stronger QTM to be suppressed with a larger magnetic field at which the direct process is probably promoted, leading to the absence of SIM behavior of 2.

Large Easy‐Plane Magnetic Anisotropy in a Three‐Coordinate Cobalt(II) Complex [Li(THF)4][Co(NPh2)3]

Magnetic anisotropy is the key element in the construction of single-ion magnets, a kind of nanomagnets for high-density information storage. This works describes an unusual large easy-plane magnetic anisotropy (with a zero-field splitting parameter D of +40.2 cm-1 ), mainly arising from the second-order spin-orbit coupling effect in a trigonal-planar CoII complex [Li(THF)4 ][Co(NPh2 )3 ], revealed by combined studies of magnetism, high frequency/field electron paramagnetic resonance spectroscopy, and ab initio calculations. Meanwhile, the field-induced slow magnetic relaxation in this complex was mainly attributed to the Raman process.

A two-dimensional cobalt(II) network with a remarkable positive axial anisotropy parameter exhibiting field-induced single-ion magnet behavior

Based on the 1H-3-(3-pyridyl)-5-(3′-pyridyl)-1,2,4-triazole (3,3′-Hbpt) ligand, {[Co(3,3′-Hbpt)2(SCN)2]·2H2O}n (1) and {[Ni(3,3′-Hbpt)2(SCN)2]·2H2O}n (2) have been prepared and structurally determined by single-crystal X-ray crystallography. The Co(II)/Ni(II) ions are bridged by the curved 3,3′-Hbpt ligands to generate helix chains, further forming a two-dimensional (2D) sheet in which Co(II)/Ni(II) ions are spatially separated from each other by a long spacer 3,3′-Hbpt ligand. Alternating-current magnetic susceptibility measurements show that the individual octahedral Co(II) ions in 1 exhibit field-induced slow magnetic relaxation, dominated by a Raman-like process. Compound 1 exhibits a very large positive axial anisotropy parameter (D = +70.1 cm−1) and a small transverse anisotropy parameter (|E| = 0.7 cm−1) by analysis of direct current magnetic data, which are further confirmed by high-field electron paramagnetic resonance (HF-EPR) spectroscopy and ab initio calculations. Furthermore, the semiconducting behaviors of 1 and 2 were also studied, which could be used as wide-gap semiconductors.

Excess axial electrostatic repulsion as a criterion for pentagonal bipyramidal DyIII single-ion magnets with high Ueff and TB

In this work, a large excess of electrostatic repulsion, arising from the axial ligands, over that from the equatorial ligands is taken as the design strategy for high performance pentagonal bipyramidal (PBP) DyIII single-ion magnets (SIMs). In this strategy, two PBP DyIII-SIMs (1 and 2) [Dy(bbpen-CH3)X] (X = Cl, 1; Br, 2; H2bbpen = N,N′-bis(2-hydroxybenzyl)-N,N′-bis(2-methylpyridyl)ethylenediamine) were synthesized and structurally characterized on the basis of a highly symmetrical ligand H2bbpen-CH3 in which an electron-donating group (–CH3) was installed to favor the conformation necessary for the axial oxygen atom coordinating to DyIII. Dynamic magnetic measurements verify the value of our design strategy since complex 2 exhibits high performance with large Ueff (above 1000 K) and a high magnetic hysteresis temperature (15 K). Ab initio calculations further verified the importance of the high excess of axial interaction which eventually leads to the special electronic structure possessing the desired magnetic properties. The search for excessive axial repulsion is not incompatible with the strategy based on local symmetry around the central ion since various high-performance PBP DyIII-SIMs of both clearly distorted and nearly ideal geometries successfully acquire such a kind of excess. Apparently this study presents an alternative to the designing strategy for promising SIMs.

The Combination of Superhalogens and Brønsted Acids HX (X = F, Cl, Br): An Effective Strategy for Designing Strong Superacids

A series of 27 composite structures, consisting of superhalogen and Brønsted acid, is designed and systematically studied based on combined ab initio and DFT calculations focusing on their potentials as novel superacids. As indicated by high-level CCSD(T) results, all the composites here fulfill the theoretical criterion for superacid and the acidities of two of them are close to the strongest superacid ever reported. The influences of various factors on the superacid properties of these composites were analyzed in detail. Our results demonstrate that the acidity of these superacids is mainly determined by the superhalogen components while the effect of Brønsted acids, irrespective of their number or type, is relatively mild. Therefore, it is probable to design novel composite superacid with enhanced property through the regulation of the superhalogen component. It is encouraging that MP2 and DFT could also provide reliable results when compared with the high-level CCSD(T) method. The reliability of these low-cost methods implies the capability of theoretical calculations for future composite superacid of enlarged size, and thus it is highly probable that an effective guide to the related experimental research could be provided by the theory.

Why do higher VDEs of superhalogen not ensure improved stabilities of the noble gas hydrides promoted by them? A high-level ab initio case study

A series of 20 composite structures, consisting of superhalogen and noble gas (Ng) hydrides, was explored via high-level coupled-cluster single, double and perturbative triple excitations calculations in this work. The existence of these composites, as local minima on the potential energy surface, arises from the charge transfer from the Ng hydride part to the superhalogen moiety. Clearly, this transfer could lead to stabilizing the interaction of the ionic type between the two components. The driving force of the charge transfer should be the high vertical electron detachment energy (VDE) of the superhalogen part leading to its enough capability of extracting the electron from the Ng hydride moiety. However, except triggering the ionic attractive interaction, there is nomonotonic correlation between the VDE value and the thermodynamic stability of the whole composite. This counter-intuitive result actually originates from the fact that, irrespective of various superhalogens, only two of their F ligands interact with the Ng atoms directly. Thus, although leading to higher VDE values, the increase in the number of electronegative ligands of the superhalogen moiety does not affect the stabilizing interaction of the composites here directly. In other words, with the necessary charge transfer generated, further increase of the VDE does not ensure the improvement of the thermodynamic stabilities of the whole composite. Moreover, in the transition state of the exothermic dissociation channel, more F atoms will give rise to higher probability of additional attractions between the F and H atoms which should lower the energy barrier. That is to say, increasing VDE, i.e., having more F atoms in many cases, will probably reduce the kinetic stability. Knowing the inevitable existence of the exothermic channel, kinetic stability is crucial to the ultimate goal of experimental observation of these Ng hydrides. Thus, in some cases, only the superhalogen itself may not provide enough information for the correct prediction on the properties of the whole composites. The understanding of the superhalogen-based composites will provide valuable information on the functional properties as well as the application potential of superhalogen clusters. Thus, the corresponding researches should focus on not only the superhalogen itself but also other related aspects, especially the details of the interaction between different parts.

CCDC 1446105: Experimental Crystal Structure Determination

Related Article: Sheng Zhang, Haipeng Wu, Lin Sun, Hongshan Ke, Sanping Chen, Bing Yin, Qing Wei, Desuo Yang, Shengli Gao|2017|J.Mater.Chem.C|5|1369|doi:10.1039/C6TC05188J