Insight into the Electronic Structure, Optical Properties, And Redox Behavior of the Hybrid Phthalocyaninoclathrochelates from Experimental and Density Functional Theory Approaches

An insight into the electronic structure of several hafnium­(IV), zirconium­(IV), and lutetium­(III) phthalocyaninoclathrochelates has been discussed on the basis of experimental UV–vis, MCD, electro- and spectroelectrochemical data as well as density functional theory (DFT) and time-dependent DFT (TDDFT) calculations. On the basis of UV–vis and MCD spectroscopy as well as theoretical predictions, it was concluded that the electronic structure of the phthalocyninoclathrochelates can be described in the first approximation as a superposition of the weakly interacting phthalocyanine and clathrochelate substituents. Spectroelectrochemical data and DFT calculations clearly confirm that the highest occupied molecular orbital (HOMO) in all tested complexes is localized on the phthalocyanine ligand. X-ray crystallography on zirconium­(IV) and earlier reported hafnium­(IV) phthalocyaninoclathrochelate complexes revealed a slightly distorted phthalocyanine conformation with seven-coordinated metal center positioned ∼1 Å above macrocyclic cavity. The geometry of the encapsulated iron­(II) ion in the clathrochelate fragment was found to be between trigonal-prismatic and trigonal-antiprismatic

Transition Ion Strikes Back: Large Magnetic Susceptibility Anisotropy in Cobalt(II) Clathrochelates

Transition-metal complexes are rarely considered as paramagnetic tags for NMR spectroscopy due to them generally having relatively low magnetic anisotropy. Here we report cobalt­(II) cage complexes with the largest (among the transition-metal complexes) axial anisotropy of magnetic susceptibility, reaching as high as 12.6 × 10^–32 m³ at room temperature. This remarkable anisotropy, which results from an unusual trigonal prismatic geometry of the complexes and translates into large negative value of the zero-field splitting energy, is high enough to promote reliable paramagnetic pseudocontact shifts at the distance beyond 2 nm. Our finding paves the way toward the applications of cobalt­(II) clathrochelates as future paramagnetic tags. Given the incredible stability and functionalization versatility of clathrochelates, the fine-tuning of the caging ligand may lead to new chemically stable mononuclear single-molecule magnets, for which magnetic anisotropy is of importance

Very Large Magnetic Anisotropy of Cage Cobalt(II) Complexes with a Rigid Cholesteryl Substituent from Paramagnetic NMR Spectroscopy

Variable-temperature NMR spectroscopy has recently emerged as a new alternative to the magnetometry methods for studying single molecule magnets. Its use is based on an accurate determination of magnetic susceptibility tensor anisotropy Δχ, which is not always achievable due to some contact contribution to NMR chemical shifts and possible conformational dynamics. Here, we applied this approach to cholesteryl-substituted cage cobalt­(II) complexes featuring a very large magnetic anisotropy. Conformational rigidity and large size of the cholesteryl substituent with many magnetically nonequivalent nuclei resulted in an excellent convergence of experimental and calculated ¹H and ¹³C chemical shifts, thus allowing for the determination of Δχ value for all of the synthesized cobalt­(II) complexes with a very high accuracy and providing a more reliable zero-field splitting energy for further calculations

Insight into the Electronic Structure, Optical Properties, And Redox Behavior of the Hybrid Phthalocyaninoclathrochelates from Experimental and Density Functional Theory Approaches

An insight into the electronic structure of several hafnium­(IV), zirconium­(IV), and lutetium­(III) phthalocyaninoclathrochelates has been discussed on the basis of experimental UV–vis, MCD, electro- and spectroelectrochemical data as well as density functional theory (DFT) and time-dependent DFT (TDDFT) calculations. On the basis of UV–vis and MCD spectroscopy as well as theoretical predictions, it was concluded that the electronic structure of the phthalocyninoclathrochelates can be described in the first approximation as a superposition of the weakly interacting phthalocyanine and clathrochelate substituents. Spectroelectrochemical data and DFT calculations clearly confirm that the highest occupied molecular orbital (HOMO) in all tested complexes is localized on the phthalocyanine ligand. X-ray crystallography on zirconium­(IV) and earlier reported hafnium­(IV) phthalocyaninoclathrochelate complexes revealed a slightly distorted phthalocyanine conformation with seven-coordinated metal center positioned ∼1 Å above macrocyclic cavity. The geometry of the encapsulated iron­(II) ion in the clathrochelate fragment was found to be between trigonal-prismatic and trigonal-antiprismatic

A Mononuclear Mn(II) Pseudoclathrochelate Complex Studied by Multi-Frequency Electron-Paramagnetic-Resonance Spectroscopy

Knowledge of the correlation between structural and spectroscopic properties of transition-metal complexes is essential to deepen the understanding of their role in catalysis, molecular magnetism, and biological inorganic chemistry. It provides topological and, sometimes, functional insight with respect to the active site properties of metalloproteins. The electronic structure of a high-spin mononuclear Mn­(II) pseudoclathrochelate complex has been investigated by electron-paramagnetic-resonance (EPR) spectroscopy at 9.5 and 275.7 GHz. A substantial, virtually axial zero-field splitting with <i>D</i> = −9.7 GHz (−0.32 cm^–1) is found, which is the largest one reported to date for a Mn­(II) complex with six nitrogen atoms in the first coordination sphere

Spin-Crossover Anticooperativity Induced by Weak Intermolecular Interactions

As a rule, rational design of cooperative spin-crossover (SCO) molecular switches is largely based on consideration of sizes and structures of individual building blocks, whereas a meticulous analysis of crystal packing, including the weakest intermolecular interactions, is often assumed to play a secondary role or is even fully neglected. By investigating cobalt­(II) clathrochelates, which do not change the molecular volume upon SCO, we showed that even weak (1.2 kcal/mol) π···Cl intermolecular interactions can cause a pronounced anticooperativity of SCO, being more gradual in the solid state than in solution. Our results clearly demonstrate that the “chemical pressure” concept is not as general as it is thought to be, and the successful design of molecular switches requires in-depth analysis of intermolecular interactions, however weak they seem

Very Large Magnetic Anisotropy of Cage Cobalt(II) Complexes with a Rigid Cholesteryl Substituent from Paramagnetic NMR Spectroscopy

Variable-temperature NMR spectroscopy has recently emerged as a new alternative to the magnetometry methods for studying single molecule magnets. Its use is based on an accurate determination of magnetic susceptibility tensor anisotropy Δχ, which is not always achievable due to some contact contribution to NMR chemical shifts and possible conformational dynamics. Here, we applied this approach to cholesteryl-substituted cage cobalt­(II) complexes featuring a very large magnetic anisotropy. Conformational rigidity and large size of the cholesteryl substituent with many magnetically nonequivalent nuclei resulted in an excellent convergence of experimental and calculated ¹H and ¹³C chemical shifts, thus allowing for the determination of Δχ value for all of the synthesized cobalt­(II) complexes with a very high accuracy and providing a more reliable zero-field splitting energy for further calculations

A Trigonal Prismatic Mononuclear Cobalt(II) Complex Showing Single-Molecule Magnet Behavior

Single-molecule magnets (SMMs) with one transition-metal ion often rely on unusual geometry as a source of magnetically anisotropic ground state. Here we report a cobalt­(II) cage complex with a trigonal prism geometry showing single ion magnet behavior with very high Orbach relaxation barrier of 152 cm^–1. This, to our knowledge, is the largest reported relaxation barrier for a cobalt-based mononuclear SMM. The trigonal prismatic coordination provided by the macrocyclic ligand gives intrinsically more stable molecular species than previously reported SMMs, thus making this type of cage complexes more amendable to possible functionalization that will boost their magnetic anisotropy even further

Synthesis and Temperature-Induced Structural Phase and Spin Transitions in Hexadecylboron-Capped Cobalt(II) Hexachloroclathrochelate and Its Diamagnetic Iron(II)-Encapsulating Analogue

Template condensation of dichloroglyoxime with <i>n</i>-hexadecylboronic acid on the corresponding metal ion as a matrix under vigorous reaction conditions afforded <i>n</i>-hexadecylboron-capped iron and cobalt­(II) hexachloroclathrochelates. The complexes obtained were characterized using elemental analysis, MALDI-TOF mass spectrometry, IR, UV–vis, ¹H and ¹³C­{¹H} NMR, ⁵⁷Fe Mössbauer spectroscopies, SQUID magnetometry, electron paramagnetic resonance, and cyclic voltammetry (CV) and by X-ray crystallography. The multitemperature single-crystal X-ray diffraction, SQUID magnetometry, and differential scanning calorimetry experiments were performed to study the temperature-induced spin-crossover [for the paramagnetic cobalt­(II) complex] and the crystal-to-crystal phase transitions (for both of these clathrochelates) in the solid state. Analysis of their crystal packing using the molecular Voronoi polyhedra and the Hirshfeld surfaces reveals the structural rearrangements of the apical long-chain alkyl substituents resulting from such phase transitions being more pronounced for a macrobicyclic cobalt­(II) complex. Its fine-crystalline sample undergoes the gradual and fully reversible spin transition centered at approximately 225 K. The density functional theory calculated parameters for an isolated molecule of this cobalt­(II) hexachloroclathrochelate in its low- and high-spin states were found to be in excellent agreement with the experimental data and allowed to localize the spin density within a macrobicyclic framework. CV of the cobalt­(II) complex in the cathodic range contains one reversible wave assigned to the Co^2+/+ redox couple with the reduced anionic cobalt­(I)-containing species stabilized by the electronic effect of six strong electron-withdrawing chlorine substituents. The quasireversible character of the Fe^2+/+ wave suggests that the anionic iron­(I)-containing macrobicyclic species undergo substantial structural changes and side chemical reactions after such metal-centered reduction

A New Series of Cobalt and Iron Clathrochelates with Perfluorinated Ribbed Substituents

The study tackles one of the challenges in developing platinum-free molecular electrocatalysts for hydrogen evolution, which is to seek for new possibilities to ensure large turnover numbers by stabilizing electrocatalytic intermediates. These species are often much more reactive than the initial electrocatalysts, and if not properly stabilized by a suitable choice of functionalizing substituents, they have a limited long-time activity. Here, we describe new iron and cobalt­(II) cage complexes (clathrochelates) that in contrast to many previously reported complexes of this type do not act as electrocatalysts for hydrogen evolution. We argue that the most probable reason for this behavior is an excessive stabilization of the metal­(I) species by perfluoroaryl ribbed groups, resulting in an unprecedented long-term stability of the metal­(I) complexes even in acidic solutions