Single-Molecule Magnets:  Tetranuclear Vanadium(III) Complexes with a Butterfly Structure and an S = 3 Ground State

Abstract

Reactions of VCl3(THF)3, bpy, and NaO2CR (R = Et, Ph; bpy = 2,2‘-bipyridine) in a 1:1:3 ratio in Me2CO give [V4O2(O2CR)7(bpy)2](ClO4) (R = Et, 1; R = Ph, 4) following addition of NBun 4ClO4. Use of 4,4‘-dimethyl- or 5,5‘-dimethylbipyridine (4,4‘-Me2bpy and 5,5‘-Me2bpy, respectively) and R = Et leads similarly to [V4O2(O2CEt)7(L−L)2](ClO4) (L−L = 4,4‘-Me2bpy, 2; L−L = 5,5‘-Me2bpy, 3). Yields are in the 38−90% range. The cation of 1 is isostructural with previously prepared [M4O2(O2CR)7(bpy)2]+ (M = CrIII, MnIII, FeIII) species and possesses a [V4O2] butterfly core. 1D and 2D COSY 1H NMR spectra of 1 show the solid-state structure is retained on dissolution. The effective magnetic moment (μeff) per V4 for 1 gradually rises from 5.79 μB at 300 K to a maximum of 6.80 μB at 25.0 K and then decreases rapidly to 4.72 μB at 2.00 K. The data in the 7.00−300 K range were fit to the appropriate theoretical expression (based on Ĥ = −2JS i ·S j ) to give J bb = −31.2 cm-1, J wb = +27.5 cm-1, and g = 1.82, (b = body, w = wingtip). These values indicate a S T = 3 ground state, confirmed by magnetization vs field studies. Similar results were obtained for the 2-picolinate (pic) analogue of 1 (complex 5). The S T = 3, 1, 3, and 0 ground states for the M = VIII, CrIII, MnIII, and FeIII, respectively, are rationalized using spin frustration arguments based on competition between J bb and J wb interactions. AC magnetic susceptibility studies down to 1.7 K on 1 and 5 show weak out-of-phase signals (χ‘‘M) below 4.0 K and corresponding small decreases in the in-phase signals (χ‘M), indicating that the relaxation of magnetization is unusually slow and comparable with the oscillating AC field (250−1000 Hz). This is a characteristic signature of a single-molecule magnet. Simultaneous application of AC and DC fields has the effect of increasing the barrier to magnetization relaxation, causing the χ‘‘M signal to move to higher temperature and consequently leading to a much stronger χ‘‘M signal and, for 5, the observation of a peak at ∼2.0 K. A dependence of the χ‘‘M peak position of 5 on the DC field intensity and AC field oscillation frequency is found.

Butterfly hysteresis and slow relaxation of the magnetization in (Et4N)3Fe2F9: manifestations of a single-molecule magnet

“…The prefactor τ 0 = 70 s obtained from the Arrhenius analysis of the relaxation dynamics at H z = 0 is unusually large, 7 − 11 orders of magnitude larger than in Mn 12 [1][2][3][4] and other SMMs [5,6,[8][9][10][11][12][13]. It is much larger than the timescale of ac magnetic susceptibility measurements, which are usually used to characterize and quantify the relaxation behavior in SMMs.…”

Radical ligand-containing single-molecule magnets

“…After the discovery of the first single-molecule magnet, cluster compound Mn 12 O 12 (O 2 CMe) 16 (H 2 O) 4 [31], efforts to increase the spin relaxation barrier primarily involved research aimed at synthesizing larger metal clusters with higher values of S, typically featuring oxo bridging ligands between metal centers [31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49]. Here, the magnetic interactions between the metal centers occur via superexchange coupling through the diamagnetic oxido unit, and these interactions can indeed reach appreciable magnitudes for such one-atom bridging ligands.…”

[GdNi6] and [LaNi6]: High‐Field EPR Spectroscopy and Magnetic Studies of Exchange‐Coupled Octahedral Clusters

“…[6][7][8] Indeed, the vast majority of SMMs known to date are made of manganese, [9] iron, [10] nickel, [11] cobalt, [12] and vanadium. [13] In contrast, the magnetic coupling between 3d and 4f metals is still not understood very well and has therefore not been exploited as a potential entry to a new category of SMMs. [14,15] Of all open-shell 4f ions, Gd III has been studied the most since it does not show the complicating effect of spin-orbit coupling.…”