Enhancing the Magnetic Anisotropy of Linear Cr(II) Chain Compounds Using Heavy Metal Substitutions

Abstract

Magnetic properties of the series of three linear, trimetallic chain compounds Cr2Cr(dpa)4Cl2, 1, Mo2Cr(dpa)4Cl2, 2, and W2Cr(dpa)4Cl2, 3 (dpa = 2,2'-dipyridylamido), have been studied using variable-temperature dc and ac magnetometry and high-frequency EPR spectroscopy. All three compounds possess an S = 2 electronic ground state arising from the terminal Cr(2+) ion, which exhibits slow magnetic relaxation under an applied magnetic field, as evidenced by ac magnetic susceptibility and magnetization measurements. The slow relaxation stems from the existence of an easy-axis magnetic anisotropy, which is bolstered by the axial symmetry of the compounds and has been quantified through rigorous high-frequency EPR measurements. The magnitude of D in these compounds increases when heavier ions are substituted into the trimetallic chain; thus D = -1.640, -2.187, and -3.617 cm(-1) for Cr2Cr(dpa)4Cl2, Mo2Cr(dpa)4Cl2, and W2Cr(dpa)4Cl2, respectively. Additionally, the D value measured for W2Cr(dpa)4Cl2 is the largest yet reported for a high-spin Cr(2+) system. While earlier studies have demonstrated that ligands containing heavy atoms can enhance magnetic anisotropy, this is the first report of this phenomenon using heavy metal atoms as "ligands".

Extraordinarily Large Ferromagnetic Coupling (J≥150 cm−1) by Electron Delocalization in a Heterometallic Mo≣Mo−Ni Chain Complex

“…Over the past decade, we and others have studied heterometallic extended metal atom chains (HEMACs), which contain a linear string of three or more metal centers supported by four triply bridging ligands. Of specific interest to this work are trimetallic HEMACs of the form M A ≣M A −M B in which a quadruply bonded pair of group VI metals is appended to a high‐spin divalent first row transition metal.…”

Magnetic Anisotropy in Heterobimetallic Complexes

“…It is worth noting that several groups have begun exploring this approach using pnictogen and transition metal-based donors. 48,49 Though to maximize the SOC of first-row transition metals, heavier ligands with more covalent interactions are necessary. Specifically, systematic analyses of bonding between heavy main group elements and high-spin paramagnetic transition metals remain scarce in the literature.…”

“…In addition to the ligand field effects, prior theoretical research supports the chemically intuitive concept that metal–ligand covalency impacts SOC transfer. , Many experiments on the heavy-atom effect thus far have relied on using electronegative donors to change D . − An intuitive way to engender a more covalent metal–ligand interaction is to use relatively electropositive donors. It is worth noting that several groups have begun exploring this approach using pnictogen and transition metal-based donors. , Though to maximize the SOC of first-row transition metals, heavier ligands with more covalent interactions are necessary. Specifically, systematic analyses of bonding between heavy main group elements and high-spin paramagnetic transition metals remain scarce in the literature.…”

Solution structure of a pentachromium(ii) single molecule magnet from DFT calculations, isotopic labelling and multinuclear NMR spectroscopy

“…Especially controversial is the interpretation of crystallographic data for tri-and pentachromium(II) derivatives [Cr 3 (dpa) 4 Cl 2 ] (1) and [Cr 5 (tpda) 4 Cl 2 ] (2) (see Scheme 1), which both behave as single molecule magnets at low temperature (Hdpa = N-( pyridin-2-yl)pyridin-2-amine, 13 H 2 tpda = N 2 ,N 6 -bis( pyridin-2-yl)pyridine-2,6-diamine, see Scheme 1). 14,15 The solid-state structures of these EMACs show metal atoms with abnormally elongated ( prolate) displacement ellipsoids along the chain axis, which can be interpreted as due to either large thermal vibrations or to positional disorder effects. The first model results in approximately equal metalmetal distances along the chain (Scheme 1a), which hint to electronic delocalization.…”