Thiosemiquinoid Radical-Bridged Cr^III₂ Complexes with Strong Magnetic Exchange Coupling

Abstract

Semiquinoid radical bridging ligands are capable of mediating exceptionally strong magnetic coupling between spin centers, a requirement for the design of high-temperature magnetic materials. We demonstrate the ability of sulfur donors to provide much stronger coupling relative to their oxygen congeners in a series of dinuclear complexes. Employing a series of chalcogen donor-based bis(bidentate) benzoquinoid bridging ligands, the series of complexes [(TPyA) 2 Cr 2 ( R L 4− )] 2+ ( O LH 4 = 1,2,4,5-tetrahydroxybenzene, OS LH 4 = 1,2-dithio-4,5-dihydroxybenzene, S LH 4 = 1,2,4,5-tetrathiobenzene, TPyA = tris(2pyridylmethyl)amine) was synthesized. Variable-temperature dc magnetic susceptibility data reveal the presence of weak antiferromagnetic superexchange coupling between Cr III centers in these complexes, with exchange constants of J = −2.83(3) ( O L 4− ), −2.28(5) ( OS L 4− ), and −1.80(2) ( S L 4− ) cm −1 . Guided by cyclic voltammetry and spectroelectrochemical measurements, chemical one-electron oxidation of these complexes gives the radical-bridged species [(TPyA) 2 Cr 2 ( R L 3−• )] 3+ . Variable-temperature dc susceptibility measurements in these complexes reveal the presence of strong antiferromagnetic metal− semiquinoid radical coupling, with exchange constants of J = −352(10) ( O L 3−• ), − 401(8) ( OS L 3−• ), and −487(8) ( S L 3−• ) cm −1 . These results provide the first measurement of magnetic coupling between metal ions and a thiosemiquinoid radical, and they demonstrate the value of moving from O to S donors in radical-bridged metal ions in the design of magnetic molecules and materials.

Ligand field-actuated redox-activity of acetylacetonate

“…Whilst the irregular evolution is not straightforward to explain, the magnitudes reect extremely strong antiferromagnetic interactions, which are approximately one order of magnitude larger than the values estimated experimentally or calculated for other Cr(III)radical complexes. 28,29,[37][38][39][40] The Mulliken spin-populations of Cr amounts to 2.7 for all three members of the series, 2-4. This value is close to the expected values for an S ¼ 3/2 ion of 3.0 (Fig.…”

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

“…31 Nevertheless, stronger exchange has been observed for semiquinonoid ligands with N-donors (J < À900 cm À1 with Fe 2+ and À440 cm À1 with Co 2+ ), 17,19 as expected for a more diffuse, higher-energy SOMO of N compared to O. 16,32 As such, incorporation of bridging ligands based on p-phenylenediamine radical may provide a route to complexes with even stronger coupling. Below 20 K, the magnetization data for 4, 5, and 6 were t using a giant spin Hamiltonian with values of S ¼ 7/2, 5/2, and 3/2, respectively ( Fig.…”

“…Semiquinonebased radicals are particularly well-suited toward this end, owing to their negative charge, which enables stronger interactions with metal ions relative to neutral radicals, and their chemical versatility. For instance, 2,5-disubstituted semiquinone derivatives, which can act as bis(bidentate) bridging ligands (Scheme 1, bottom), 11,12 have been employed to synthesize radical-bridged dinuclear complexes exhibiting strong magnetic exchange coupling [13][14][15][16] and in some cases single-molecule magnet behavior. [17][18][19][20] In contrast, magnetic coupling involving the bis(monodentate) benzosemiquinone molecule (Scheme 1, top) has not been reported.…”

Strong Exchange Couplings Drastically Slow Down Magnetization Relaxation in an Air‐Stable Cobalt(II)‐Radical Single‐Molecule Magnet (SMM)

“…These multiplets can function as intermediate states in an Orbach relaxation process, thus effectively short‐cutting the energy barrier. Such very strong exchange couplings can be more easily achieved in transition‐metal‐radical systems, in which coupling strengths of hundreds of cm −1 have been reported …”