Semiquinone radical-bridged M₂ (M = Fe, Co, Ni) complexes with strong magnetic exchange giving rise to slow magnetic relaxation

Abstract

The use of radical bridging ligands to facilitate strong magnetic exchange between paramagnetic metal centers represents a key step toward the realization of single-molecule magnets with high operating temperatures. Moreover,...

Molecular and Electronic Structures and Single-Molecule Magnet Behavior of Tris(thioether)–Iron Complexes Containing Redox-Active α-Diimine Ligands

“…12,21 This strategy has proven to be very effective and productive, generating diiron, 22−24 dicobalt, 25−30 and dinickel SMMs 25 with strong exchange couplings (in one example, 22 |J| > 900 cm −1 ) and U eff values up to 267(3) K 26 (U eff is the effective spin-reversal barrier). A wide range of bridging radicals have been employed to construct these dinuclear transition metal-radical SMMs, including semiquinone radical, 25 tetraoxolene radical, 24 tetraazalene radical, 22,23 nindigo radical, 30 2,2′-bipyrimidine radical, 28 tetrazine radical, 29 tetrapyridophenazine radical, 27 and 1,2,4,5-tetrakis-(methanesulfonamido)benzene) radical ligands. 26 In contrast, mononuclear transition metal-radical SMMs have received less attention despite the rapid growth in the number of transition metal-based mononuclear SMMs.…”

“…In the pursuit of radical-ligand-containing SMMs, an emerging trend is to use strongly exchange-coupled redox-active bridging radicals to generate multinuclear transition metal-based SMMs. , This strategy has proven to be very effective and productive, generating diiron, − dicobalt, − and dinickel SMMs with strong exchange couplings (in one example, | J| > 900 cm –1 ) and U eff values up to 267(3) K 26 ( U eff is the effective spin-reversal barrier). A wide range of bridging radicals have been employed to construct these dinuclear transition metal-radical SMMs, including semiquinone radical, tetraoxolene radical, tetraazalene radical, , nindigo radical, 2,2′-bipyrimidine radical, tetrazine radical, tetrapyridophenazine radical, and 1,2,4,5-tetrakis­(methanesulfonamido)­benzene) radical ligands . In contrast, mononuclear transition metal-radical SMMs have received less attention despite the rapid growth in the number of transition metal-based mononuclear SMMs. , Current examples of mononuclear transition metal-radical SMMs are limited to cobalt–nitroxide radical complexes, − cobalt–carbene radical complexes, − and an iron–dithiadiazolyl radical complex .…”

Probing the Magnetic Anisotropy of Co(II) Complexes Featuring Redox-Active Ligands

“…Changes in ligand oxidation state are expected to perturb the intrinsic anisotropy of Co­(II) ions, and the presence of ligand-based radicals generates a “ladder” of different spin states via exchange interactions. While the valence tautomerism of six-coordinate cobalt-semiquinonate complexes has been studied extensively, − efforts to develop transition-metal SMMs consisting of one or more radical ligands also show promise. − The most common approach in this direction has employed radicals as bridging ligands between paramagnetic centers to create multimetallic complexes with large total spin ( S tot ) values. A similar strategy uses radical ligands as organic linkers in metal–organic frameworks (MOFs) that combine magnetic and microporous properties. − Strong exchange coupling between a given metal and ligand radical has been shown to facilitate slow magnetic relaxation by discouraging quantum tunneling and increasing the energy gap between the ground and excited states. , In addition to these benefits, redox-noninnocent ligands capable of undergoing reversible redox events could serve as “on–off” switches for SMM behavior. , …”

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

“…Representatives can be found in Dy­(III)-based complexes within the pentagonal-bipyramidal (PBP) geometry or metallocene, ,− , giving the observed magnetic hysteresis above liquid nitrogen temperature and the energy barriers over 1500 cm –1 . On the other hand, a majority of 3d transition metal SMMs within the low-coordinate environment have also been reported to show huge magnetic anisotropy with the barriers up to several hundred wavenumbers due to the first-order or enhanced second-order spin–orbit coupling (SOC). ,, In addition to the geometry modulation of single-ion magnetic anisotropy, incorporation of strong intramolecular magnetic coupling ( J ) represents another important way to achieve true bistability and longer relaxation time, in which realization of both specific geometry and strong magnetic coupling within one molecule is extremely limited . It should be mentioned that if J is not sufficiently large, the relaxation would proceed through the low-lying excited states, leading to an undesirable suppression of the SMM property.…”