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Competing Interests: Competing interests: The authors declare no competing interests.

The magnetic and magneto-optical properties of a tetrazinyl radical-bridged Er III metallocene, [(Cp* 2 Er III ) 2 (bpytz •- )][BPh 4 ] ( 1 ; Cp* = pentamethylcyclopentadienyl, bpytz = 3,6-bis(3,5-dimethyl-pyrazolyl)-1,2,4,5-tetrazine), are reported. As confirmed by these studies strong Ln-rad coupling is achieved, with 1 exhibiting slow magnetic relaxation under a 1000 Oe dc field. The optical and magneto-optical profile of 1 is completed by both near-infrared (NIR) luminescence and magnetic circularly polarized luminescence (MCPL), representing the first example of NIR MCPL with Er III .

Over the years, polynuclear cyclic or torus complexes have attracted increasing interest due to their unique metal topologies and properties. However, the isolation of polynuclear cyclic organometallic complexes is extremely challenging due to their inherent reactivity, which stems from the labile and reactive metal-carbon bonds. In this study, the pyrazine ligand undergoes a radical-radical cross-coupling reaction leading to the formation of a decanuclear [(Cp*) 20 Dy 10 (L1) 10 ] ⋅ 12(C 7 H 8 ) (1; where L1 = anion of 2-prop-2-enyl-2H-pyrazine; Cp* = pentamethylcyclopentadienyl) complex, where all Dy III metal centres are bridged by the anionic L1 ligand. Amongst the family of polynuclear Ln organometallic complexes bearing Cp R 2 Ln x units (Cp R = substituted cyclopentadienyl), 1 features the highest nuclearity obtained to date. In-depth computational studies were conducted to elucidate the proposed reaction mechanism and formation of L1, while probing of the magnetic properties of 1, revealed slow magnetic relaxation upon application of a static dc field., (© 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

The unique electronic nature of the 1,2,4,5-tetrazine or s -tetrazine (tz) ring has sparked tremendous scientific interest over the last few years. Tetrazines have found numerous applications, and their ability to coordinate to metal ions has opened the possibility of exploring their chemistry in both molecular systems and extended networks. The rich redox chemistry of s -tetrazines allows them to exchange electrons and switch between their dihydro (H 2 tz), neutral (tz), and radical (tz˙ - ) forms. Previous reports in the literature have observed electrochemically that a second electron can potentially be stored in the tetrazinyl ring and form a dianionic species. However, due to its extremely reactive nature, this has not been isolated before. Herein, the combination of strictly anhydrous and inert conditions, strong reducing agents, non-acidic solvents and most importantly blocking the accessibility of the nitrogen atoms by coordinating them to lanthanide ions allowed for the stabilization of a dianionic tetrazine in a lanthanocene complex. Three dinuclear metallocene complexes are reported, [(Cp* 2 Ln) 2 (tz˙ - )(THF) 2 ](BPh 4 ) (Ln = Y (1-Y); Cp* = pentamethylcyclopentadienyl; THF = tetrahydrofuran) and [(Cp* 2 Ln) 2 (tz 2- )(THF) 2 ]·2THF (Ln = Gd (2-Gd), or Y (2-Y)), which utilize the unsubstituted tz as the ligand. In 1-Ln, the tz ligand is reduced to the radical anion (tz˙ - ), while in 2-Ln, the tz ligand is in the -2 charge state. These complexes are the first structurally and physically characterized complexes bearing the dianion radical of an s -tetrazine. Detailed structural analysis, ab initio calculations, and physical characterization support that the tz 2- ligand is a closed-shell planar dianion with unique structural features vastly different from those of the tz, tz˙ - and H 2 tz species., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Reaction of the 1,2,4,5-tetrazine (tz˙ - ) radical and {Cp* 2 Tb} + has yielded a tetranuclear radical-bridged Tb III single-molecule magnet. The strong Ln-radical coupling and the electronic differences of the Tb III ions in [(Cp* 2 Tb) 4 (tz˙ - ) 4 ]·3C 6 H 6 (1) are probed via magnetic, magneto-optical and computational studies.

Reduction of the diamagnetic Ti(III)/Ti(III) dimer [Cl 2 Ti(μ-NIm Dipp )] 2 ( 1 ) (NIm Dipp = [1,3-bis(Dipp)imidazolin-2-iminato] - , Dipp = C 6 H 3 -2,6- i Pr 2 ) with 4 and 6 equiv of KC 8 generates the intramolecularly arene-masked, dinuclear titanium compounds [(μ-N-η 6 -Im Dipp )Ti] 2 ( 2 ) and {[(Et 2 O) 2 K](μ-N-μ -η 6 :η 6 -Im Dipp )Ti} 2 ( 3 ), respectively, in modest yields. The compounds have been structurally characterized by X-ray crystallographic analysis, and inspection of the bond metrics within the η 6 -coordinated aryl substituent of the bridging imidazolin-2-iminato ligand shows perturbation of the aromatic system most consistent with two-electron reduction of the ring. As such, 2 and 3 can be assigned respectively as possessing metal centers in formal Ti(III)/Ti(III) and Ti(II)/Ti(II) oxidation states. Exploration of their redox chemistry reveal the ability to reduce several substrate equivalents. For instance, treatment of 2 with excess C 8 H 8 N)(η Dipp N)(η 8 -COT)Ti](μ-η 2 :η 3 -COT)[Ti(η 4 -COT)(NIm Dipp )] ( 4 N)Ti(η Dipp N)Ti(η 8 -COT)(THF) ( 5 ). Addition of COT to 3 N)(η Dipp N)(η 4 -COT)Ti(μ-η 4 :η 5 -COT)K(THF)(μ-η 6 :η 4 -COT)Ti(NIm Dipp )(μ-η 4 :η 4 -COT)K(THF) 2 ] n ( 6 ). Compounds 4 and 5 are the products of the 4-electron oxidation of 2 , while 6 stands as the 8-electron oxidation product of 3 . Reduction of organozides was also explored. Low temperature reaction of 2 with 4 equiv of AdN 3 N)Ti(═NAd)](μ-NAd) Dipp N)Ti(═NAd)](μ-NAd) 2 )(N Dipp )(N 3 Ad)] ( 7 N)Ti(NHAd)](μ-NAd) Dipp N)Ti(NHAd)](μ-NAd) 2 [Ti(C 6 H 4 Me)(NIm Dipp )] ( 8-tol ). These complexes are the 6-electron oxidation products of the reaction of 2 with AdN 3 . Furthermore, treatment of 3 with 4 equiv of AdN 3 produces the thermally stable Ti(III)/Ti(III) terminal and bridged imido [K(18-crown-6)(THF) 2 N)Ti(NAd)](μ-NAd) Dipp N)Ti(NAd)](μ-NAd) 2 K[Ti(NIm Dipp )]} ( 10 ). Altogether, these reactions firmly establish 2 and 3 as unprecedented Ti(I)/Ti(I) and Ti(0)/Ti(0) synthons with the clear capacity to effect multielectron reductions ranging from 4 to 8 electrons.

Inducing magnetic coupling between 4f elements is an ongoing challenge. To overcome this formidable difficulty, we incorporate highly delocalized tetrazinyl radicals, which strongly couple with f-block metallocenes to form discrete tetranuclear complexes. Synthesis, structure, and magnetic properties of two tetranuclear [(Cp* 2 Ln) 4 (tz . ) 4 ]⋅3(C 6 H 6 ) (Cp*=pentamethylcyclopentadienyl; tz=1,2,4,5-tetrazine; Ln=Dy, Gd) complexes are reported. An in-depth examination of their magnetic properties through magnetic susceptibility measurements as well as computational studies support a highly sought-after radical-induced "giant-spin" model. Strong exchange interactions between the Ln III ions and tz . radicals lead to a strong magnet-like behaviour in this molecular magnet with a large coercive field of 30 kOe., (© 2021 Wiley-VCH GmbH.)

Tapping into the secondary coordination environment of mononuclear DyIII-complexes leads to drastic changes in luminescence and magnetism. Visualization of effects induced by stereoelectronics on the opto-magnetic properties was achieved through subtle modifications in the ligand framework.

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., (© 2020 Wiley-VCH GmbH.)