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.

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.

A combined ab initio and DFT study is performed in this work to explore the superhalogen properties of polynuclear structures based on the ligands of –OH, –OOH and –OF. According to high-level CCSD(T) results, all the structures here are superhalogens whose properties are superior to the corresponding mononuclear ones. Although inferior to similar structures based on F ligands, some of the superhalogens here are capable of transcending the traditional ones based on Cl atoms. Therefore the superhalogen properties of the anions here are still promising and they have an important advantage of high safety, which is crucial for practical applications. An increased degree of structural versatility is imposed by these non-halogen ligands because of the various ways in which they connect the central atoms and their multiple orientations. It is important that this increased versatility will bring new factors, e.g., the larger spatial extent of the whole cluster and the existence of intra-molecular hydrogen bonds, which should favour high VDE values. These factors are not available in traditional halogen-based systems and they may play an important role in the future search for novel superhalogens. (HF + MP2)/2, ωB97XD as well as M06-2X are capable of providing accurate VDE values, close to the CCSD(T) results, and their absolute errors are even lower than that of the OVGF. Due to the good balance between the accuracy and efficiency, these methods could provide reliable predictions on large systems which cannot be treated with CCSD(T) or even with the OVGF. Balanced distribution of the extra electron, between the terminal and bridging ligands, is also shown to be favourable to realize a high VDE value.

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.

It is crucial to understand and elucidate the self-assembly mechanism in solution systems for the construction of DyIII-based single-molecule magnets (SMMs). Herein, through fine-tuning of the anion and solvent, we prepared three nine-coordinate mononuclear dysprosium compounds, [Dy(2,3′-pcad)(NO3)2(CH3OH)2] (1), [Dy(2,3′-Hpcad)2(H2O)3]·3Cl·5H2O (2), and [Dy(2,3′-pcad)(NO3)(H2O)4]·NO3·H2O (3) [2,3′-Hpcad = N3-(2-pyridoyl)-3-pyridinecarboxamidrazone]. The reactions of formation for 1–3 are in situ thermodynamically monitored by isothermal titration calorimetry. Magnetic data analysis reveals that 2 shows SMM behavior under a zero direct-current (dc) field, whereas 1 and 3 exhibit distinct slow magnetic relaxation processes upon a 1200 Oe dc field. To deeply understand the different magnetic behaviors, the magnetic anisotropy of 1–3 has been systematically studied by ab initio calculations, which is consistent with the experimental observations. Moreover, the semiconductor behaviors of 1–3 have been investigated by experimental measurements of UV–vis spectroscopy.

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.

The regulation of the electronic properties of organic molecules induced by polynuclear superhalogens is theoretically explored here for sixteen composite structures. It is clearly indicated by the higher vertical electron detachment energy (VDE) that polynuclear superhalogens are more effective in regulating the electronic properties than mononuclear structures. However, this enhanced regulation is not only determined by superhalogens themselves but also related to the distribution of the extra electron of the final composites. The composites, in which the extra electron is mainly aggregated into the superhalogen moiety, will possess higher VDE values, as reported in the case of C1′, 7.12 eV at the CCSD(T) level. This is probably due to the fact that, compared with organic molecules, superhalogens possess stronger attraction towards the extra electron and thus should lead to lower energies of the extra electrons and to higher VDE values eventually. Compared with CCSD(T), the Outer Valence Green's Function (OVGF) method fails completely for composite structures containing Cl atoms, while MP2 results are generally consistent in terms of the relative order of VDEs. Actually if the extra electron distribution of the systems could be approximated by the HOMO, the results at the OVGF level will be consistent with the CCSD(T) results. Conversely, the difference in VDEs between OVGF and CCSD(T) is significantly large. Besides superhalogen properties, the structures, relative stabilities and thermodynamic stabilities with respect to various fragmentation channels were also investigated for all the composite structures.

The regulation of the electronic properties of organic molecules induced by polynuclear superhalogens is theoretically explored here for sixteen composite structures. It is clearly indicated by the higher vertical electron detachment energy (VDE) that polynuclear superhalogens are more effective in regulating the electronic properties than mononuclear structures. However, this enhanced regulation is not only determined by superhalogens themselves but also related to the distribution of the extra electron of the final composites. The composites, in which the extra electron is mainly aggregated into the superhalogen moiety, will possess higher VDE values, as reported in the case of C1′, 7.12 eV at the CCSD(T) level. This is probably due to the fact that, compared with organic molecules, superhalogens possess stronger attraction towards the extra electron and thus should lead to lower energies of the extra electrons and to higher VDE values eventually. Compared with CCSD(T), the Outer Valence Green's Function (OVGF) method fails completely for composite structures containing Cl atoms, while MP2 results are generally consistent in terms of the relative order of VDEs. Actually if the extra electron distribution of the systems could be approximated by the HOMO, the results at the OVGF level will be consistent with the CCSD(T) results. Conversely, the difference in VDEs between OVGF and CCSD(T) is significantly large. Besides superhalogen properties, the structures, relative stabilities and thermodynamic stabilities with respect to various fragmentation channels were also investigated for all the composite structures.

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.

The potential of 23 superhalogen anions of halogen-free structures as high-performance electrolytes of Li-ion batteries is theoretically explored here. According to high-level ab initio results at the CCSD(T) level, eight candidates, obeying the Wade–Mingos rule, should be capable of forming electrolytes, which are better than the currently used commercial products. When comparing different methods, MP2 was found to be in good agreement with CCSD(T) in the calculation of ΔELi+ and ΔEH2O, which are parameters describing the performance of potential electrolytes. Thus, MP2 represents a good choice for such calculations, particularly for large potential electrolyte systems wherein CCSD(T) calculations are actually impractical. The five functionals selected here (ωB97XD, B2GP-PLYP, B2K-PLYP, B2T-PLYP and B3LYP) are also capable of reproducing the variational trends of the relative values of different structures at the CCSD(T) level. However, the actual DFT values of ΔELi+ are usually different from those of CCSD(T) by more than 1 eV. These significant deviations may be a problem when accurate ΔELi+ values are required.

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.

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.