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

Abstract

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.

Radical‐Bridged Ln4 Metallocene Complexes with Strong Magnetic Coupling and a Large Coercive Field

“…Attempts to reduce the crystallization times were unsuccessful. Due to the relatively low half-wave reduction potential of the tz ligand (À0.78 V), [28] analogous reactions without the presence of areducing agent were performed. Although, upon mixing of the tz ligand (pink solution) with the [Cp* 2 Ln][(m-Ph 2 )BPh 2 ] (Ln = Dy or Gd) (yellow slurry) ad esirable colour change (dark red) was observed, no products were isolated via this synthetic route.…”

“…Similar N=Nbond elongation was previously reported for tzC À -derived radical anions. [28,31] Moreover,t he shortest intramolecular Dy•••Dy distance is 7.341 (10) ,w hile the longest distance separating the diagonals is found to be 10.715(18) (Figure S3). It is noteworthy that, in this centrosymmetric Dy 4 complex, all Dy ions lie in the same plane (Figure S4), likely imposed by the planarity of the tzC À radical anion.…”

“…It has been shown that the very lowlying p*LUMO of the tz ring easily favours the reduction and formation of the tzC À radical anion. [28][29][30][31][32][33][34][35] Thed iffuse spin orbitals of tzC À -based radicals are ideally suited to penetrate the core-like electron density of the lanthanide ion and reach the shielded 4f orbitals to promote stronger coupling.M oreover, its compact size and its four easily accessible nitrogen binding sites offer ag reater possibility for the isolation of polynuclear complexes and polymeric species.…”

Radical‐Bridged Ln4 Metallocene Complexes with Strong Magnetic Coupling and a Large Coercive Field

“…To overcome these issues, we turned our attention towards using the relatively unexplored unsubstituted 1,2,4,5‐tetrazine (tz) as a bridging ligand. It has been shown that the very low‐lying π* LUMO of the tz ring easily favours the reduction and formation of the tz .− radical anion [28–35] . The diffuse spin orbitals of tz .− ‐based radicals are ideally suited to penetrate the core‐like electron density of the lanthanide ion and reach the shielded 4 f orbitals to promote stronger coupling.…”

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

“…Copper(II)-and nickel(II)-radical complexes having a direct radical coordination bond are the best documented in the 3d-2p heterospin systems [9,[60][61][62][63][64][65][66][67][68][69][70]. There have been an increasing number of compounds showing a ferromagnetic 3d-2p coupling [75][76][77][78][79][80][81]. It is believed for a long time that axial coordination in copper(II)-nitroxide complexes displays relatively weak ferromagnetic interaction whilst equatorial coordination strong antiferromagnetic interaction [9].…”