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 NBun4ClO4. 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 = −2JSi·Sj) to give Jbb = −31.2 cm-1, Jwb = +27.5 cm-1, and g = 1.82, (b = body, w = wingtip). These values indicate a ST = 3 ground state, confirmed by magnetization vs field studies. Similar results were obtained for the 2-picolinate (pic) analogue of 1 (complex 5). The ST = 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 Jbb and Jwb 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.