A series of tetraazalene radical-bridged M2 (M = CrIII, MnII, FeII, CoII) complexes with strong magnetic exchange coupling. Issue 11 (26th August 2015)

• Record Type: Journal Article
• Authors: DeGayner, Jordan A.; Jeon, Ie-Rang; Harris, T. David
• Is Part Of: Chemical science. Volume 6:Issue 11(2015:Nov.)
• Page Start: 6639
• Page End: 6648
• Publication Date: 2015-08-26
• Subjects: Chemistry -- Periodicals; 540.5
• DOI: 10.1039/c5sc02725j
• Languages: English
• ISSNs: 2041-6520

Abstract

Abstract : The ability of tetraazalene radical bridging ligands to mediate exceptionally strong magnetic exchange coupling across a range of transition metal complexes is demonstrated. Abstract : The ability of tetraazalene radical bridging ligands to mediate exceptionally strong magnetic exchange coupling across a range of transition metal complexes is demonstrated. The redox-active bridging ligand N, N ′, N ′′, N ′′′-tetra(2-methylphenyl)-2, 5-diamino-1, 4-diiminobenzoquinone ( NMePh LH2 ) was metalated to give the series of dinuclear complexes [(TPyA)2 M2 ( NMePh L 2− )] 2+ (TPyA = tris(2-pyridylmethyl)amine, M = Mn II, Fe II, Co II ). Variable-temperature dc magnetic susceptibility data for these complexes reveal the presence of weak superexchange interactions between metal centers, and fits to the data provide coupling constants of J = −1.64(1) and −2.16(2) cm −1 for M = Mn II and Fe II, respectively. One-electron reduction of the complexes affords the reduced analogues [(TPyA)2 M2 ( NMePh L 3− ˙)] + . Following a slightly different synthetic procedure, the related complex [(TPyA)2 CrIII2( NMePh L 3− ˙)] 3+ was obtained. X-ray diffraction, cyclic voltammetry, and Mössbauer spectroscopy indicate the presence of radical NMePh L 3− ˙ bridging ligands in these complexes. Variable-temperature dc magnetic susceptibility data of the radical-bridged species reveal the presence of strong magnetic interactions between metal centers and ligand radicals, with simulations to data providing exchange constants of J = −626(7), −157(7), −307(9), and −396(16) cm −1 for M = Cr III, Mn II, Fe II, and Co II, respectively. Moreover, the strength of magnetic exchange in the radical-bridged complexes increases linearly with decreasing M–L bond distance in the oxidized analogues. Finally, ac magnetic susceptibility measurements reveal that [(TPyA)2 Fe2 ( NMePh L 3− ˙)] + behaves as a single-molecule magnet with a relaxation barrier of U eff = 52(1) cm −1 . These results highlight the ability of redox-active tetraazalene bridging ligands to enable dramatic enhancement of magnetic exchange coupling upon redox chemistry and provide a rare opportunity to examine metal–radical coupling trends across a transmetallic series of complexes.