Magnetic Anisotropy: Structural Correlation of a Series of Chromium(II)–Amidinate Complexes

Abstract

Systematic substituent variations on amidinate ligands bring delicate changes of CrN4 coordination in a family of chromium­(II) complexes with the common formula of Cr­(RNC­(CH3)­NR)2, where R = iPr (1), Cy (2), Dipp (Dipp = 2, 6-diisopropylphenyl) (3), and tBu (4). With the largest substituent group, 4 shows the largest distortion of the N4 coordination geometry from square-planar to seesaw shape, which leads to its field-induced single-molecule magnet (SMM) behavior. This is an indication that 4 has the strongest axial magnetic anisotropy and/or optimized magnetic relaxation process. Combined with high-frequency/field electron paramagnetic resonance (HF-EPR) experiments and ab initio calculations, we deduce that the smallest energy gap between ground 4Ψ0 and the first excited 4Ψ1 orbitals in 4 contributes the most to its strongest magnetic anisotropy. Moreover, the lower E value of 4 ensures its being a field-induced SMM. Specifically, the D and E values were found to be correlated to the dihedral angle between the ΔN1CrN2 and ΔN3CrN4 triangles, indicating that distortion from ideal square-planar geometry to the seesaw help increase axial magnetic anisotropy and suppress the transversal part. Thus, the study on this system not only expands the family of Cr­(II)-based SMMs but also contributes to a deeper understanding of magneto-structural correlation in four-coordinate Cr­(II) SMMs.

A C,S bonded quasi-two-coordinate chromium(ii) complex showing field-induced slow magnetic relaxation behaviour

“…S9†). 17–21,33 The two-dimensional array of resonant field ( H ) vs. frequency ( v ) is plotted in Fig. 4 (left) and the allowed (|Δ M s | = 1) transitions can be evidently distinguished from the nominally forbidden (|Δ M s | > 1) ones, which is also observed in other high-spin complexes.…”

“…The sign of D is correlated with the relative energies between low-lying excited states and the ground state. 20,34 In particular, the NEVPT2 energies of the first two low-lying excited states are 7995.1 and 11 085.5 cm −1 , and the second excited state ( 5 ψ 2 ) gives a predominant contribution towards the negative D value (−1.27 cm −1 ), while the other state ( 5 ψ 1 ) makes a positive contribution (0.30 cm −1 ), suggesting that axial magnetic anisotropy in 1 originates from the spin–orbit coupling between the ground state ( 5 ψ 0 ) and these low-lying excited states. Five active orbitals of 1 , which are mainly composed of 3d orbitals of the Cr( ii ) ion, are plotted in Fig.…”

“…Although considerable numbers of mononuclear SMMs comprising 3d transition metals have been reported in the past years, the vast majority is based on cobalt( ii ), 9–11 and iron( i , ii or iii ) 12–16 with only a few of them related to the chromium( ii ) ion, such as in some multinuclear clusters and four-coordinate mononuclear complexes. 17–21 In addition, the coordination atoms in these Cr( ii ) complexes are mainly N and O. Herein, we studied the dynamic magnetism of a previously reported C,S bonded mononuclear Cr( ii ) compound Cr(SAr*) 2 ( 1 , HSAr* = HSC 6 H 3 -2,6(C 6 H 2 -2,4,6-Pr i 3 ) 2 ). 22 Its slow magnetic relaxation behaviour under a 1500 Oe dc field was observed, with relatively large rhombic characteristics of magnetic anisotropy, which was determined by dc magnetic measurements, high-frequency/field electron paramagnetic resonance (HF-EPR) experiments and ab initio calculations.…”

“…It should be noted that both positive and negative D values can accurately reproduce dc magnetic data and its absolute value is quite close to that of our reported Cr( ii ) complexes. 18,20 …”

Heteroleptic Dysprosium(III) Single-Molecule Magnets with Amidinate and Cyclopentadienyl Ligands

“…All operations were carried out by using standard Schlenk and glovebox techniques under a dry and oxygen-free argon atmosphere. Anhydrous Dy Cl 3 , Cp*Li (Cp* = C 5 Me 5 ), and Am dipp Li were prepared according to literature procedures. − THF, toluene, and pentane were dehydrated and deoxygenated using a solvent purification system and subsequently stored under an argon atmosphere over activated 4 Å molecular sieves. Elemental analyses were performed on a Vario EL cube Elemantar.…”