Strong ferromagnetic exchange coupling in a {Ni} cluster mediated through an air-stable tetrazine-based radical anion

Magnetic hysteresis and large coercivity in bisbenzimidazole radical-bridged dilanthanide complexes

A judicious combination of radical ligands innate to diffuse spin orbitals with paramagnetic metal ions elicits strong magnetic exchange coupling which leads to properties important for future technologies. This metal-radical approach aids in effective magnetic communication of especially lanthanide ions as their 4f orbitals are contracted and not readily accessible. Notably, a high spin density on the donor atoms of the radical is required for strong coupling. Such molecules are extremely rare owing to high reactivity rendering their isolation challenging. Herein, we present two unprecedented series of bisbenzimidazole-based dilanthanide complexes [(Cp*2Ln)2(μ-Bbim)] (1-Ln = Gd, Tb, Dy, Bbim = 2,2'-bisbenzimidazole) and [K(crypt-222)][(Cp*2Ln)2(μ-Bbim˙)] -(2-Ln = Gd, Tb, Dy), where the latter contains the first Bbim3-˙ radical matched with any paramagnetic metal ion. The magnetic exchange constant for 2-Gd of J = -1.96(2) cm-1 suggests strong antiferromagnetic Gd-radical coupling, whereas the lanthanides in 1-Gd are essentially uncoupled. Ab initio calculations on 2-Tb and 2-Dy uncovered coupling strengths of -4.8 and -1.8 cm-1. 1-Dy features open hysteresis loops with a coercive field of Hc of 0.11 T where the single-molecule magnetism can be attributed to the single-ion effect due to lack of coupling. Excitingly, pairing the effective magnetic coupling with the strong magnetic anisotropy of Dy results in magnetic hysteresis with a blocking temperature TB of 5.5 K and coercive field HC of 0.54 T, ranking 2-Dy as the second best dinuclear single-molecule magnet containing an organic radical bridge. A Bbim4- species is formed electrochemically hinting at the accessibility of Bbim-based redox-active materials.

Recent developments in supramolecular complexes of azabenzenes containing one to four N atoms: synthetic strategies, structures, and magnetic properties

For the last couple of decades, azabenzene-based ligands have drawn much attention from inorganic chemists due to their ability to coordinate with different metal ions to form supramolecular clusters. These azabenzenes are weak σ donors and strong π acceptors and electron-deficient. Metallogrid complexes and non-grid oligomers are well-defined supramolecular clusters, formed by appropriate chelating ligands, and can show interesting optical, magnetic, and electronic properties. Self-assembly of [n × n] metallogrid complexes is dominated by the entropic factor while the formation of oligonuclear metal ion complexes is dominated by other effects like CFSE, electrostatic factors, ligand conformational characters, etc. Herein, the present article gives an overview of six-membered heterocyclic azine-based ligands and their potential for different metal ions to form polynuclear complexes. Moreover, their temperature-dependent magnetic properties and SCO phenomena are well described and tabulated.

Self-assembly of Ni(II) metallacycles (a square and a triangle) supported by tetrazine radical bridges

Two Ni(II) molecular metallacycles of [Ni₄(bpz*tz˙^-)₄(N₃)₄] (1) and [Ni₃(bpz^Phtz˙^-)₃(pz^Ph(Cl)tz˙^-)₃]·1.3CH₃OH·9.3H₂O (2) (bpz*tz = 3,6-bis(3,5-dimethyl-pyrazolyl)-1,2,4,5-tetrazine; bpz^Phtz = 3,6-bis(3-phenyl-pyrazolyl)-1,2,4,5-tetrazine; and pz^Ph(Cl)tz = 3-bis(3-phenyl-pyrazolyl)-6-Cl-1,2,4,5-tetrazine) are reported. The single-crystal X-ray diffraction study reveals that 1 displays a square structure while 2 shows a triangle structure due to the steric effect, both bearing tetrazine radical bridges. Furthermore, magnetic studies reveal that the Ni-radical interaction in 1 is strongly ferromagnetic with a coupling constant (J) of 90.8 cm^-1 in the 2J formalist, while the overall antiferromagnetic behaviour of 2 is presumably due to the compete ferromagnetic (for the Ni-radical_bridging interaction with J₁ = 95.4 cm^-1) and antiferromagnetic (for the Ni-radical_terminal interaction, J₂ = -57.5 cm^-1) couplings.

A Dicobalt(II) Single-Molecule Magnet via a Well-Designed Dual-Capping Tetrazine Radical Ligand

The recent years have witnessed the glory development for the construction of high-performance mononuclear single molecule magnets (SMMs) within a specific coordination geometry, which, however, is not well applied in cluster-based SMMs due to the synthetic challenges. Given that the monocobalt(II) complexes within a trigonal-prismatic (TPR) coordination geometry have been classified as excellent SMMs with huge axial anisotropy (D ≈ -100 cm^-1), here we designed and synthesized a new dual-capping tetrazine ligand, 3,6-bis(6-(di(1H-pyrazol-1-yl)methyl)pyridin-2-yl)-1,2,4,5-tetrazine (bpptz), and prepared a novel dicobalt(II) complex, [Cp₂Co^III][{(hfac)Co^II}₂(bpptz^•-)][hfac]₂·2Et₂O (1, hfac = hexafluoroacetylacetonate). In the structure of 1, the bpptz^•- radical ligand enwraps two Co(II) centers within quasi-TPR geometries, which are further bridged by the tetrazine radical in the trans mode. The magnetic study revealed that the interaction between the Co centers and the tetrazine radical is strongly antiferromagnetic with a coupling constant (J) of -65.8 cm^-1 (in the -2J formalism). Remarkably, 1 exhibited the typical SMM behavior with an effective energy barrier of 69 cm^-1 under a 1.5 kOe dc field, among the largest for polynuclear transition metal SMMs. In addition, DFT and ab initio calculations suggested that the presence of a strong Co(II)-radical magnetic interaction effectively quenches the QTM effect and enhances the barrier height for the magnetization reversal.

A linear metal-metal bonded tri-iron single-molecule magnet

The linear trinuclear complex cation [Fe₃(DpyF)₄]²⁺ was prepared as [Fe₃(DpyF)₄](BF₄)₂·2CH₃CN. With large Fe-Fe distances of 2.78 Å, this complex demonstrates intramolecular ferromagnetic coupling between the anisotropic Fe^II centers (J/k_B = +20.9(5) K) giving an S_T = 6 ground state and exhibits single-molecule magnet properties.

Mono- and Binuclear Copper(II) and Nickel(II) Complexes with the 3,6-Bis(picolylamino)-1,2,4,5-Tetrazine Ligand

Four new compounds of formulas [Cu(hfac)₂(L)] (1), [Ni(hfac)₂(L)] (2), [{Cu(hfac)₂}₂(µ-L)]·2CH₃OH (3) and [{Ni(hfac)₂}₂(µ-L)]·2CH₃CN (4) [Hhfac = hexafluoroacetylacetone and L = 3,6-bis(picolylamino)-1,2,4,5-tetrazine] have been prepared and their structures determined by X-ray diffraction on single crystals. Compounds 1 and 2 are isostructural mononuclear complexes where the metal ions [copper(II) (1) and nickel(II) (2)] are six-coordinated in distorted octahedral MN₂O₄ surroundings which are built by two bidentate hfac ligands plus another bidentate L molecule. This last ligand coordinates to the metal ions through the nitrogen atoms of the picolylamine fragment. Compounds 3 and 4 are centrosymmetric homodinuclear compounds where two bidentate hfac units are the bidentate capping ligands at each metal center and a bis-bidentate L molecule acts as a bridge. The values of the intramolecular metal···metal separation are 7.97 (3) and 7.82 Å (4). Static (dc) magnetic susceptibility measurements were carried out for polycrystalline samples 1–4 in the temperature range 1.9–300 K. Curie law behaviors were observed for 1 and 2, the downturn of χ_MT in the low temperature region for 2 being due to the zero-field splitting of the nickel(II) ion. Very weak [J = −0.247(2) cm⁻¹] and relatively weak intramolecular antiferromagnetic interactions [J = −4.86(2) cm⁻¹] occurred in 3 and 4, respectively (the spin Hamiltonian being defined as H = −JS₁·S₂). Simple symmetry considerations about the overlap between the magnetic orbitals across the extended bis-bidentate L bridge in 3 and 4 account for their magnetic properties.

Strong Coupling and Slow Relaxation of the Magnetization for an Air-Stable [Co4] Square with Both Tetrazine Radicals and Azido Bridges

Introducing both tetrazine radical and azido bridges afforded two air-stable square complexes [M^II₄(bpztz^•-)₄(N₃)₄] (M^II = Zn²⁺, 1; Co²⁺, 2; bpztz = 3,6-bis(3,5-dimethylpyrazolyl)-1,2,4,5-tetrazine), where the metal ions are cobridged by μ_1,1-azido bridges and tetrazine radicals. Magnetic studies revealed strong antiferromagnetic metal-radical interaction with a coupling constant of -64.7 cm^-1 in the 2J formalism in 2. Remarkably, 2 exhibits slow relaxation of magnetization with an effective barrier for spin reverse of 96 K at zero applied field.

Molecular S = 2 High-Spin, S = 0 Low-Spin and S = 0 ⇄ 2 Spin-Transition/-Crossover Nickel(II)-Bis(nitroxide) Coordination Compounds

Heterospin systems have a great advantage in frontier orbital engineering since they utilize a wide diversity of paramagnetic chromophores and almost infinite combinations and mutual geometries. Strong exchange couplings are expected in 3d–2p heterospin compounds, where the nitroxide (aminoxyl) oxygen atom has a direct coordination bond with a nickel(II) ion. Complex formation of nickel(II) salts and tert-butyl 2-pyridyl nitroxides afforded a discrete 2p–3d–2p triad. Ferromagnetic coupling is favored when the magnetic orbitals, nickel(II) dσ and radical π*, are arranged in a strictly orthogonal fashion, namely, a planar coordination structure is characterized. In contrast, a severe twist around the coordination bond gives an orbital overlap, resulting in antiferromagnetic coupling. Non-chelatable nitroxide ligands are available for highly twisted and practically diamagnetic complexes. Here, the Ni–O–N–Csp2 torsion (dihedral) angle is supposed to be a useful metric to describe the nickel ion dislocated out of the radical π* nodal plane. Spin-transition complexes exhibited a planar coordination structure in a high-temperature phase and a nonplanar structure in a low-temperature phase. The gradual spin transition is described as a spin equilibrium obeying the van’t Hoff law. Density functional theory calculation indicates that the energy level crossing of the high- and low-spin states. The optimized structures of diamagnetic and high-spin states well agreed with the experimental large and small torsions, respectively. The novel mechanism of the present spin transition lies in the ferro-/antiferromagnetic coupling switch. The entropy-driven mechanism is plausible after combining the results of the related copper(II)-nitroxide compounds. Attention must be paid to the coupling parameter J as a variable of temperature in the magnetic analysis of such spin-transition materials. For future work, the exchange coupling may be tuned by chemical modification and external stimulus, because it has been clarified that the parameter is sensitive to the coordination structure and actually varies from 2J/kB = +400 K to −1400 K.

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.

Incorporation of a nitrogen-rich energetic ligand in a {Yb} complex exhibiting slow relaxation of the magnetisation under an applied field

Though often used as the precursor in energetic materials research and organic synthesis, 3,6-dihydrazinyl-1,2,4,5-tetrazine (dhtz) has not been previously explored in coordination chemistry. Herein, we present the first crystalline coordination compound with dhtz, [Yb₂(dhtz)(fod)₆] (fod^- = 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate). This compound exhibits Single-Molecule Magnet behaviour under an applied magnetic field, and m_J states were resolved through luminescence studies.

A tetranuclear nickel cluster isolated in multiple high-valent states

We report a series of high-valent tetranuclear nickel clusters isolated from the chemical oxidation of an all Ni(ii) ([Ni₄]) neutral cluster. Electrochemical analysis of [Ni₄] reveals three reversible sequential oxidations at 0.248 V (1e^-), 0.678 V (1e^-), and 0.991 V (2e^-) vs. Fc/Fc⁺ corresponding to mono-, di-, and tetra-oxidized species, [Ni₄]⁺, [Ni₄]²⁺, [Ni₄]⁴⁺, respectively. Using spectroscopic, crystallographic, magnetometric, and computational techniques, we assign the primary loci of oxidations to the Ni centers in each case, thus resulting in the isolation of the first tetranuclear all-Ni(iii) cluster, [Ni₄]⁴⁺.

Studies on the synthesis and properties of nitramino compounds based on tetrazine backbones

In this paper, a series of novel energetic compounds based on the backbone of the tetrazine-triazole structure were successfully synthesized. N,N'-((1,2,4,5-Tetrazine-3,6-diyl)bis(1,2-dihydro-3H-1,2,4-triazole-5-yl-3-ylidene))dinitramino (4) was prepared by the nitration of 5,5'-(1,4-dihydro-1,2,4,5-tetrazine-3,6-diyl)bis(1H-1,2,4-triazol-3-amine) (3) with 100% nitric acid and its energetic salts (6-14) were also prepared. All the compounds were fully characterized. The structures of 4 and 5 were further confirmed by single crystal X-ray diffraction analysis. The results show that these compounds have high heats of formation ranging from 2.09 to 3.95 kJ g^-1, good detonation pressures and detonation velocities and acceptable sensitivities. Among them, compound 4, with low sensitivities (IS: 20 J and FS: 270 N) and excellent detonation properties (v_D = 9100 m s^-1; P = 34.1 GPa) shows potential for application in the field of highly energetic and insensitive explosives. The hydroxylammonium salt (7) exhibits promising energetic properties, which, in some cases, are superior to those of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX).

1,2,4,5-Tetrazine based ligands and complexes

One of the most intriguing nitrogen based aromatic heterocycles is 1,2,4,5-tetrazine or s-tetrazine (TTZ) thanks to its electron acceptor character and fluorescence properties and the possibilities of functionalization in the 3 and 6 positions allowing access to various ligands. In this review we focus on the two main families of TTZ based ligands, i.e. ditopic symmetric and monotopic non-symmetric, together with their metal complexes, with a special emphasis on their solid state structures and physical properties. After a description of the most representative complexes containing unsubstituted TTZ as a ligand, symmetric TTZ ligands and complexes derived thereof are discussed in the order: 3,6-bis(2-pyridyl)-tetrazine, 3,6-bis(3-pyridyl)-tetrazine, 3,6-bis(4-pyridyl)-tetrazine, 3,6-bis(2-pyrimidyl)-tetrazine, 3,6-bis(2-pyrazinyl)-tetrazine, 3,6-bis(monopicolylamine)-tetrazine, 3,6-bis(vanillin-hydrazinyl)-tetrazine and TTZ containing carboxylic acids. Remarkable results have been obtained in recent years for metal-organic frameworks and magnetic compounds in which magnetic coupling is enhanced when the tetrazine bridge is reduced to radical anions. Non-symmetric ligands, such as dipicolylamine-TTZ and monopicolylamine-TTZ, are comparatively more recent than the symmetric ones. They allow in principle the preparation of mononuclear complexes in a controlled manner, although binuclear complexes have been isolated as well. Moreover, in the monopicolylamine-TTZ-Cl ligand, deprotonation of the amine, thanks to the electron acceptor character of TTZ, afforded a negatively charged ligand equivalent of a guanidinate.

Unprecedented intramolecular pancake bonding in a {Dy2} single-molecule magnet

The first example of unique coordination induced intramolecular pancake bonding was achieved through the reduction of two bis(pyrazolyl)-tetrazine ligands.

Versatile coordination behaviour of the chloro-tetrazine-picolylamine ligand: mixed-valence binuclear Cu(i)/Cu(ii) complexes

The 3-Cl-6-amino-(2'-picolyl)-1,2,4,5-tetrazine ligand HL¹ has been synthesized and structurally characterized. Its versatile coordination behavior has been evidenced through reactions with Cu(Hfac)₂ and Cu(triflate)₂ precursors, which provided mixed-valence bimetallic Cu^1.5Cu^1.5 complexes [Cu₂(μ-Cl)(L¹)₂] 2 and [Cu₂(μ-triflate)(L¹)₂] 5. Changing the Cu(ii) precursor and the solvent leads to the formation of mononuclear octahedral Cu(ii) complexes [CuCl₂(HL¹)₂]·2CH₃CN 3 and [Cu(Hfac)₂(HL¹)] 4, in which only the amino-pyridine unit is involved in the coordination of the metal center. In contrast, in complexes 2 and 5, the ligands are deprotonated and bridge the metal centers as pyridine-amido-tetrazine fragments, while a bridging chloride or triflate ligand completes the coordination sphere of the metal ions. The Cu-Cu distances of 2.4313(4) Å in 2 and 2.5198(10) Å in 5 lie among the shorter values within mixed-valence bimetallic Cu complexes. Mixed-valence character is strongly supported by DFT calculations, showing the equal repartition of the unpaired electron between the two metal centers.

Using Redox-Active π Bridging Ligand as a Control Switch of Intramolecular Magnetic Interactions

Intramolecular magnetic interactions in the dinuclear complexes [(tpy)Ni(tphz)Ni(tpy)] ⁿ⁺ ( n = 4, 3, and 2; tpy, terpyridine; tphz, tetrapyridophenazine) were tailored by changing the oxidation state of the pyrazine-based bridging ligand. While its neutral form mediates a weak antiferromagnetic (AF) coupling between the two S = 1 Ni(II), its reduced form, tphz^•-, promotes a remarkably large ferromagnetic exchange of +214(5) K with Ni(II) spins. Reducing twice the bridging ligand affords weak Ni-Ni interactions, in marked contrast to the Co(II) analogue. Those experimental results, supported by a careful examination of the involved orbitals, provide a clear understanding of the factors which govern strength and sign of the magnetic exchange through an aromatic bridging ligand, a prerequisite for the rational design of strongly coupled molecular systems and high T_C molecule-based magnets.

Tetrazine-Based Ligand Transformation Driving Metal-Metal Bond and Mixed-Valence HgI/HgII

Understanding the self-assembly of cluster aggregates remains an important challenge in coordination chemistry. A chelating tetrazine-based ligand was employed to isolate an unprecedented tetranuclear Hg₄ complex exhibiting Hg^I/Hg^II mixed valence, which also contains metal-metal bonds. Single-crystal X-ray diffraction, X-ray photoelectron spectroscopy, solid-state nuclear magnetic resonance, and theoretical approaches (density functional theory) were employed to shed some light on the structure and self-assembly of this discrete molecule.

A nitrogen-rich ligand as a scaffold for slow magnetic relaxation in dysprosium-based 0D and 1D architectures

A recently designed nitrogen-rich ligand is successfully applied as a scaffold for lanthanide ions to show that the intricate chemistry of energetic materials can be combined with other fields of research, including that of molecular magnetism. Herein, we report the synthesis of two different types of molecular architectures using a single ligand template, in which the discrete monomer exhibits single-molecule magnet-like behaviour along with two well-isolated modes of magnetic relaxation.

A Redox-Active Bridging Ligand to Promote Spin Delocalization, High-Spin Complexes, and Magnetic Multi-Switchability

A dinuclear Co^II complex, [Co₂ (tphz)(tpy)₂ ]ⁿ⁺ (n=4, 3 or 2; tphz: tetrapyridophenazine; tpy: terpyridine), has been assembled using the redox-active and strongly complexing tphz bridging ligand. The magnetic properties of this complex can be tuned from spin-crossover with T_1/2 ≈470 K for the pristine compound (n=4) to single-molecule magnet with an S_T =5/2 spin ground state when once reduced (n=3) to finally a diamagnetic species when twice reduced (n=2). The two successive and reversible reductions are concomitant with an increase of the spin delocalization within the complex, promoting remarkably large magnetic exchange couplings and high-spin species even at room temperature.

Probing Magnetic-Exchange Coupling in Supramolecular Squares Based on Reducible Tetrazine-Derived Ligands

Reducible 3,6-bis(3,5-dimethyl-pyrazolyl)1,2,4,5-tetrazine was employed to isolate supramolecular air-stable [Co₄ ] and [Zn₄ ] squares, which were achieved via careful selection of counterions rather than the use of reducing agents. Magnetic susceptibility studies revealed a strong radical-Co^II exchange coupling (J_rad-Co /hc=-118 cm^-1 , -2J formalism) with a spin ground state of S_T =4, whereas the unreduced analogue revealed negligible coupling between the Co centers (J_Co-Co /hc=-0.64 cm^-1 ). Radical-radical coupling was also probed in the [Zn₄ ] congener, which led to J_rad-rad /hc=-15.9(5) cm^-1 . These results highlight the versatile air-stable coordination chemistry of tetrazine and the importance of exploiting easily reducible delocalized radical to promote strong exchange coupling between spin carriers.

Strong Ferromagnetic Exchange Coupling Mediated by a Bridging Tetrazine Radical in a Dinuclear Nickel Complex

The radical bridged compound [(Ni(TPMA))₂-μ-bmtz^•-](BF₄)₃·3CH₃CN (bmtz = 3,6-bis(2'-pyrimidyl)-1,2,4,5-tetrazine, TPMA = tris(2-pyridylmethyl)amine) exhibits strong ferromagnetic exchange between the S = 1 Ni^II centers and the bridging S = 1/2 bmtz radical with J = 96 ± 5 cm^-1 (-2J_Ni-radS_NiS_rad). DFT calculations support the existence of strong ferromagnetic exchange.