Single-molecule magnets: Tetranuclear vanadium(III) complexes with a butterfly structure and an S = 3 ground state

Article abstract of DOI:10.1021/ja9732439

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 NBu(n)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 = Cr(III), Mn(III), Fe(III)) 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 (μ(eff)) per V₄ for 1 gradually rises from 5.79 μ(B) at 300 K to a maximum of 6.80 μ(B) at 25.0 K and then decreases rapidly to 4.72 μ(B) at 2.00 K. The data in the 7.00-300 K range were fit to the appropriate theoretical expression (based on H = 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 = V(III), Cr(III), Mn(III), and Fe(III), 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.