Enhancing Magnetic Communication between Metal Centres: The Role of s-Tetrazine Based Radicals as Ligands

Although 1,2,4,5-tetrazines or s-tetrazines have been known in the literature for more than a century, their coordination chemistry has become increasingly popular in recent years due to their unique redox activity, multiple binding sites and their various applications. The electron-poor character of the ring and stabilization of the radical anion through all four nitrogen atoms in their metal complexes provide new aspects in molecular magnetism towards the synthesis of new high performing Single Molecule Magnets (SMMs). The scope of this review is to examine the role of s-tetrazine radical ligands in transition metal and lanthanide based SMMs and provide a critical overview of the progress thus far in this field. As well, general synthetic routes and new insights for the preparation of s-tetrazines are discussed, along with their redox activity and applications in various fields. Concluding remarks along with the limitations and perspectives of these ligands are discussed.     

How to Switch Spin‐Crossover Metal Complexes at Constant Room Temperature

Spin‐crossover metal complexes represent a highly promising class of molecular switches, the diverse physicochemical properties of which can be reversibly changed by different physical and chemical stimuli. One of the most interesting and examined features of these materials is the change of magnetic properties by changing the temperature or by irradiation with light at low temperatures. However, most prospective applications of such complexes require functioning at room temperature. This Concept article provides an overview about how the switching of spin‐crossover metal complexes can be achieved at constant room temperature. The principles of switching by different physical and chemical methods in solution and in the solid state are presented in an easy‐to‐read form for nonspecialists. These are further supported and clarified by examples from the literature. The overview might also be interesting for experts that target spin‐crossover systems functioning at ambient conditions.     

Solvent Dependent Spin‐Crossover and Photomagnetic Properties in an Imidazolylimine FeII Complex

The synthesis and physico‐chemical characterization of an Fe^II complex [Fe( L1 )₃](ClO₄)₂?CH₃CN?0.5H₂O, 1 , incorporating a bidentate imidazolylimine‐based ligand are reported. Complex 1 crystallises as the mer‐isomer and the crystal lattice is replete with hydrogen bonding interactions between ClO₄^? anions, solvent molecules and imidazole N‐H groups. Variable‐temperature structural, magnetic, photomagnetic and optical reflectivity techniques have been deployed to fully characterise the spin‐crossover (SCO) behaviour in 1 along with its desolvated phase, 1?desolv . Variable‐temperature (1.8–300 K) magnetic‐susceptibility measurements reveal a broad two‐step full SCO for 1 (T_1/2=158 and 184 K) and photomagnetic experiments at 10 K under white‐light irradiation revealed complete photo‐induced SCO. 1?desolv displays considerably different magnetic behaviour with sharp single‐step SCO accompanied by a thermal hysteresis (T_1/2↑=105 K, T_1/2↓=95 K) in addition to full photo‐induced SCO at lower temperatures.     

Combining Open-Shell Verdazyl Environment and Co(II) Spin-Crossover: Spinmerism in Cobalt Oxoverdazyl Compound

The spin states of a Co(II) oxoverdazyl compound are investigated by means of wavefunction-based calculations. Within a ca. 233 K energy window, the ground state and excited states display a structure-sensitive admixture of low-spin S_M=1/2 in a dominant high-spin S_M=3/2 Co(II) ion as indicated by the localized molecular orbitals. The puzzling spin zoology that results from the coupling between open-shell radical ligands and a spin-crossover metal ion gives rise to this unusual scenario, which extends the views in molecular magnetism. In agreement with experimental observation, the low-energy spectroscopy is very sensitive to deformations of the coordination sphere, and a growing admixture of Co(II) low-spin is evidenced from the calculations. In analogy with mesomerism that accounts for charge delocalization, entanglement combines different local spin states to generate a given total spin multiplicity, a spinmerism phenomenon.     

Activation of Molecular Oxygen by a Cobalt(II) Tetra-NHC Complex**

The first dicobalt(III) μ₂-peroxo N-heterocyclic carbene (NHC) complex is reported. It can be quantitatively generated from a cobalt(II) compound bearing a 16-membered macrocyclic tetra-NHC ligand via facile activation of dioxygen from air at ambient conditions. The reaction proceeds via an end-on superoxo intermediate as demonstrated by EPR studies and DFT. The peroxo moiety can be cleaved upon addition of acetic acid, yielding the corresponding Co^III acetate complex going along with H₂O₂ formation. In contrast, both Co^II and Co^III complexes are also studied as catalysts to utilize air for olefin and alkane oxidation reactions; however, not resulting in product formation. The observations are rationalized by DFT-calculations, suggesting a nucleophilic nature of the dicobalt(III) μ₂-peroxo complex. All isolated compounds are characterized by NMR, ESI-MS, elemental analysis, EPR and SC-XRD.     

Distinct Helical Molecular Orbitals through Conformational Lock**

Several theoretical studies have proposed strategies to generate helical molecular orbitals (Hel-MOs) in [n]cumulenes and oligoynes. While chiral even-[n] cumulenes feature Hel-MOs, odd-[n] cumulenes may also present them if the terminal groups lie in different planes. However, the proposed systems have been either experimentally unfeasible or resulted in opposite pseudo-degenerated Hel-MOs. We hereby demonstrate the introduction of a remarkable energy difference between helical orbitals of opposite twist by fixing the torsion angle between the terminal groups in butadiyne fragments. To experimentally lock the conformation of the terminal groups, we designed and synthesized cyclic architectures by combining acetylenes with chiral spirobifluorenes. The high stability of these systems with distinct helical orbitals allowed their isolation and full characterization. In our view, these results constitute a step further in the development of real systems presenting helical molecular orbitals.     

Self-organization in an Open Reaction Network for Developing High-function Organized Molecular Systems**

Self-organized molecular systems such as liposomes and supramolecules have attracted considerable attention due to their characteristic properties. An open reaction network (ORN) is another interesting candidate for such systems; however, no stabilization mechanism has been clarified. This work reveals, by computer simulation and experiments, that a network of irreversible processes such as an ORN can be stabilized by self-organization through a full balance between all the involved irreversible processes, thus forming a steady state. The formation of a steady state indicates that a large spontaneous order is formed; specifically, self-organization occurs. Computer simulations also reveal that such a steady state characteristically evolves toward a high-efficiency state through the development of highly ordered structures. These findings indicate that ORN provides a new method for developing high-function organized molecular systems, such as an efficient catalytic system in a composite of ORN and equilibrium molecular structures such as supramolecules and polymers.     

Discovery of Homobivalent Bitopic Ligands of the Cannabinoid CB2 Receptor**

Single chemical entities with potential to simultaneously interact with two binding sites are emerging strategies in medicinal chemistry. We have designed, synthesized and functionally characterized the first bitopic ligands for the CB₂ receptor. These compounds selectively target CB₂ versus CB₁ receptors. Their binding mode was studied by molecular dynamic simulations and site-directed mutagenesis.     

SIMPRE1.2: Considering the hyperfine and quadrupolar couplings and the nuclear spin bath decoherence

SIMPRE is a fortran77 code which uses an effective electrostatic model of point charges to predict the magnetic behavior of rare‐earth‐based mononuclear complexes. In this article, we present SIMPRE1.2, which now takes into account two further phenomena. First, SIMPRE now considers the hyperfine and quadrupolar interactions within the rare‐earth ion, resulting in a more complete and realistic set of energy levels and wave functions. Second, and to widen SIMPRE's predictive capabilities regarding potential molecular spin qubits, it now includes a routine that calculates an upper‐bound estimate of the decoherence time considering only the dipolar coupling between the electron spin and the surrounding nuclear spin bath. Additionally, SIMPRE now allows the user to introduce the crystal field parameters manually. Thus, we are able to demonstrate the new features using as examples (i) a Gd‐based mononuclear complex known for its properties both as a single ion magnet and as a coherent qubit and (ii) an Er‐based mononuclear complex. © 2016 Wiley Periodicals, Inc.     

Rotating Magnetocaloric Effect in an Anisotropic Molecular Dimer

In contrast to the mainstream research on molecular refrigerants that seeks magnetically isotropic molecules, we show that the magnetic anisotropy of dysprosium acetate tetrahydrate, [{Dy(OAc)₃(H₂O)₂}₂]?4 H₂O ( 1 ), can be efficiently used for cooling below liquid‐helium temperature. This is attained by rotating aligned single‐crystal samples in a constant applied magnetic field. The envisioned advantages are fast cooling cycles and potentially compact refrigerators.     

Star-Shaped Ruthenium Complexes as Prototypes of Molecular Gears

The design and synthesis of two families of molecular-gear prototypes is reported, with the aim of assembling them into trains of gears on a surface and ultimately achieving controlled intermolecular gearing motion. These piano-stool ruthenium complexes incorporate a hydrotris(indazolyl)borate moiety as tripodal rotation axle and a pentaarylcyclopentadienyl ligand as star-shaped cogwheel, equipped with five teeth ranging from pseudo-1D aryl groups to large planar 2D paddles. A divergent synthetic approach was followed, starting from a pentakis(p-bromophenyl)cyclopentadienyl ruthenium(II) complex as key precursor or from its iodinated counterpart, obtained by copper-catalyzed aromatic Br/I exchange. Subsequent fivefold cross-coupling reactions with various partners allowed high structural diversity to be reached and yielded molecular-gear prototypes with aryl-, carbazole-, BODIPY- and porphyrin-derived teeth of increasing size and length.     

Generation of reactive species by one-electron reduction of Fischer-type carbene complexes of group 6 metals and their use for carbon-carbon bond formation

Carbon-carbon bond-forming reactions mediated by one-electron reduction of Fischer-type carbene complexes of Group 6 metals were investigated. In the case of aryl- or silylcarbene complexes of tungsten, the anion radical species generated by one-electron reduction smoothly underwent addition reaction to ethyl acrylate. One-electron reduction of alpha,beta-unsaturated carbene complexes afforded biscarbene complexes by dimerization of the corresponding anion radical species at the position gamma to the metal center. In contrast, one-electron reduction of chromium phenyl- or alkylcarbene complexes gave, via carbonyl insertion, alpha-methoxyacylchromate complexes, which further underwent conjugate addition to various electron-poor olefins to give the corresponding alpha-methoxyketones.     

Molecular Encryption and Reconfiguration for Remodeling of Dynamic Hydrogels

Dynamic materials have been widely studied for regulation of cell adhesion that is important to a variety of biological and biomedical applications. These materials can undergo changes mainly through one of the two mechanisms: ligand release in response to chemical, physical, or biological stimuli, and ligand burial in response to mechanical stretching or the change of electrical potential. This study demonstrates an encrypted ligand and a new hydrogel that are capable of inducing and inhibiting cell adhesion, which is controlled by molecular reconfiguration. The ligand initially exhibits an inert state; it can be reconfigured into active and inert states by using unblocking and recovering molecules in physiological conditions. Since molecular reconfiguration does not require the release of the ligand from the hydrogels, inhibiting and inducing cell adhesion on the hydrogels can be repeated for multiple cycles.     

Structures and magnetic properties of tetranuclear nickel(II) complexes with unusual mu3-1,1,3 azido bridges

Pyrazolate-based dinucleating ligands with thioether-containing chelate arms have been used for the synthesis of a family of novel tetranuclear nickel(II) complexes [L2Ni4(N3)3(O2CR)](ClO4)2 that incorporate three azido bridges and one carboxylate (R = Me, Ph). Molecular structures have been elucidated by X-ray crystallography in four cases, revealing Ni4 cores with a unique topology in which two of the azido ligands adopt an unusual mu3-1,1,3 bridging mode. The compounds were further characterized by mass spectrometry, IR spectroscopy, and variable-temperature magnetic susceptibility measurements. Magnetic data analyses indicate a combination of significant intramolecular ferromagnetic and antiferromagnetic exchange interactions that give rise to an overall S(T) = 0 ground state. The sign and the magnitude of the individual couplings have been rationalized in the framework of the common magnetostructural correlations for end-to-end and end-on azido linkages, suggesting that these correlations also remain valid for the respective fragments of multiply bridging mu3-1,1,3 azido ligands.     

Janus Microspheres for Visual Assessment of Molecular Interconnects

A rigid S‐functionalized metalloligand is used to pair Janus Au‐coated silica microspheres and the resulting assemblies are assessed with optical microscopy. New Pd complexes provide stable molecular interconnects, and the metal centre controls the structure of the linker and provides the desired rigidity, by virtue of its well‐established coordination chemistry.     

光驱动的金属富勒烯分子磁开关※

由于碳笼的保护, 从外部操控内嵌富勒烯笼内分子的特性一直是一个挑战. 通过在顺磁性金属富勒烯Sc₃C₂@C₈₀碳笼外修饰具有光活性的偶氮苯-氮氧自由基, 成功设计出基于金属富勒烯-氮氧自由基的分子开关, 实现了原位可逆地光驱动远程控制金属富勒烯的顺磁特性. 在不同光照条件下, 利用偶氮苯的光异构化特性改变双自旋中心的相对位置, 调整自旋-自旋、自旋-晶格相互作用, 进而影响金属富勒烯的电子顺磁特性. 研究发现, 紫外光照下, 氮氧自由基使金属富勒烯Sc₃C₂@C₈₀的顺磁信号逐渐减弱, 可见光照下Sc₃C₂@C₈₀的顺磁信号又增强, 由此实现了氮氧自由基作为顺磁开关的功能.     

Snowballing Radical Generation Leads to Ultrahigh Molecular Weight Polymers

Styrene is the classical monomer obeying zero‐one kinetics in radical emulsion polymerization. Accordingly, particles that are less than 100 nm in diameter contain either one or no growing radical(s). We describe a unique photoinitiated polymerization reaction accelerated by snowballing radical generation in a continuous flow reactor. Even in comparison to classical emulsion polymerization, these unprecedented snowballing reactions are rapid and high‐yielding, with each particle simultaneously containing more than one growing radical. This is a consequence of photoinitiator incorporation into the nascent polymer backbone and repeated radical generation upon photo‐irradiation.     

Structural Transformations and Spin-Crossover in [FeL2]2+ Salts (L=4-{tert-Butylsulfanyl}-2,6-di{pyrazol-1-yl}pyridine): The Influence of Bulky Ligand Substituents

4-(tert-Butylsulfanyl)-2,6-di(pyrazol-1-yl)pyridine (L) was obtained in low yield from a one-pot reaction of 2,4,6-trifluoropyridine with 2-methylpropane-2-thiolate and sodium pyrazolate in a 1:1:2 ratio. The materials [FeL₂][BF₄]₂⋅solv ( 1[BF₄]₂ ⋅solv) and [FeL₂][ClO₄]₂⋅solv ( 1[ClO₄]₂ ⋅solv; solv=MeNO₂, MeCN or Me₂CO) exhibit a variety of structures and spin-state behaviors including thermal spin-crossover (SCO). Solvent loss on heating 1[BF₄]₂ ⋅x MeNO₂ (x≈2.3) occurs in two steps. The intermediate phase exhibits hysteretic SCO around 250 K, involving a “reverse-SCO” step in its warming cycle at a scan rate of 5 K min⁻¹. The reverse-SCO is not observed in a slower 1 K min⁻¹ measurement, however, confirming its kinetic nature. The final product [FeL₂][BF₄]₂⋅0.75 MeNO₂ was crystallographically characterized, and shows abrupt but incomplete SCO at 172 K which correlates with disorder of an L ligand. The asymmetric unit of 1[BF₄]₂ ⋅y Me₂CO (y≈1.6) contains five unique complex molecules, four of which undergo gradual SCO in at least two discrete steps. Low-spin 1[ClO₄]₂ ⋅0.5 Me₂CO is not isostructural with its BF₄⁻ congener, and undergoes single-crystal-to-single-crystal solvent loss with a tripling of the crystallographic unit cell volume, while retaining the P space group. Three other solvate salts undergo gradual thermal SCO. Two of these are isomorphous at room temperature, but transform to different low-temperature phases when the materials are fully low-spin.