Tuning of Cr–Cr Magnetic Exchange through Chalcogenide Linkers in Cr₂ Molecular Dimers

Abstract

A set of three Cr-dimer compounds, Cr 2 Q 2 (en) 4 X 2 (Q: S, Se; X: Br, Cl; en: ethylenediamine), with monoatomic chalcogenide bridges have been synthesized via a single-step solvothermal route. Chalcogenide linkers mediate magnetic exchange between Cr 3+ centers, while bidentate ethylenediamine ligands complete the distorted octahedral coordination of Cr centers. Unlike the compounds previously reported, none of the chalcogenide atoms are connected to extra ligands. Magnetic susceptibility studies indicate antiferromagnetic coupling between Cr 3+ centers, which are moderate in Cr 2 Se 2 (en) 4 X 2 and stronger in Cr 2 S 2 (en) 4 Cl 2 . Fitting the magnetic data requires a biquadratic exchange term. High-frequency EPR spectra showing characteristic signals due to coupled S = 1 spin states could be interpreted in terms of the "giant spin" Hamiltonian. A fourth compound, Cr 2 Se 8 (en) 4 , has a single diatomic Se bridge connecting the two Cr 3+ centers and shows weak ferromagnetic exchange interactions. This work demonstrates the tunability in strength and type of exchange interactions between metal centers by manipulating the interatomic distances and number of bridging chalcogenide linkers.

Measuring Metal–Metal Communication in a Series of Ketimide-Bridged [Fe2]6+ Complexes

“…For comparison, the magnetic coupling constants in 1 , 2 , and 4 are much smaller than those measured for other ketimide-bridged dimers, such as [Fe 2 (μ-NC t Bu 2 ) 2 (NC t Bu 2 ) 3 ] ( J = −235 cm –1 ), [Mn 2 (μ-NC t Bu 2 ) 3 (NC t Bu 2 ) 2 ] − ( J = −78 cm –1 ), and [Li]­[Cr 2 (μ-NC 10 H 14 ) 3 (NC 10 H 14 ) 4 ] ( J = −200 cm –1 ). , In the case of the [Fe 2 ] 5+ complex, [Fe 2 (μ-NC t Bu 2 ) 2 (NC t Bu 2 ) 3 ], the shorter Fe–Fe distance [2.547(1) Å] likely contributes to the larger coupling. , Similarly, the [Fe 2 ] 4+ complex, {(PhCN) 2 (Mes) 2 Fe 2 [μ-NC­(Mes)­(Ph)] 2 }, also features larger antiferromagnetic coupling ( J = −63.7 cm –1 ) than 1 , 2 , and 4 , likely for the same reason. Several chalcogenide-bridged [Fe 2 ] 6+ complexes have also been characterized by magnetometry. , These exhibit antiferromagnetic coupling constants ranging from J = −75 cm –1 (for [{Fe­(salen)} 2 S] to −105 cm –1 (for [{Fe­(bipy) 2 } 2 O] 4+ . − The μ-imido complex, [L Et Fe­(μ-NPh) 2 FeL Et ], also features a large antiferromagnetic coupling constant ( J = −123 cm –1 ) .…”

“…33,36 In the case of the [Fe 2 ] 5+ complex, [Fe 2 (μ-N�C t Bu 2 ) 2 (N�C t Bu 2 ) 3 ], the shorter Fe−Fe distance [2.547(1) Å] likely contributes to the larger coupling. 82,83 Similarly, the [Fe 2 ] 4+ complex, {(PhCN) 2 (Mes) 2 Fe 2 [μ-N�C(Mes)(Ph)] 2 }, 84 also features larger antiferromagnetic coupling (J = −63.7 cm −1 ) than 1, 2, and 4, likely for the same reason. Several chalcogenidebridged [Fe 2 ] 6+ complexes have also been characterized by magnetometry.…”

Unveiling a Family of Dimerized Quantum Magnets, Conventional Antiferromagnets, and Nonmagnets in Ternary Metal Borides

“…Experimental measurements show that YCrB 4 has a very small magnetic susceptibility, which was attributed to a small concentration of magnetic impurities . In dimerized spin-gap systems, the magnetic susceptibility is suppressed at temperatures that are small compared to the singlet–triplet energy gap. − In the mean-field approximation for the interaction between quantum spin-1/2 dimers, the susceptibility is − χ = k B C k B T ( 3 4 + 1 4 exp false( 2 β | J normalD | false) ) + 2 J F ′ where it is assumed that J D < 0, C is the Curie–Weiss constant, and J F false′ .25em = ∑ j ′ J i j with J D excluded.…”