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

Reactions of VCl(3)(THF)(3), bpy, and NaO(2)CR (R = Et, Ph; bpy = 2,2'-bipyridine) in a 1:1:3 ratio in Me(2)CO give [V(4)O(2)(O(2)CR)(7)(bpy)(2)](ClO(4)) (R = Et, 1; R = Ph, 4) following addition of NBu(4)(n)ClO(4). Use of 4,4'-dimethyl- or 5,5'-dimethylbipyridine (4,4'-Me(2)bpy and: 5,5'-Me(2)bpy, respectively) and R = Et leads similarly to [V(4)O(2)(O(2)CEt)(7)(L-L)(2)](ClO(4)) (L-L = 4,4'-Me(2)bpy, 2; L-L = 5,5'-Me(2)bpy, 3). Yields are in the 38-90% range. The cation of 1 is isostructural with previously prepared [M(4)O(2)(O(2)CR)7(bpy)(2)](+) (M = Cr(III), Mn(III), Fe(III)) species and possesses a [V(4)O(2)] butterfly core. 1D and 2D COSY (1)H NMR spectra of 1 show the solid-state structure is retained on dissolution. The effective magnetic moment (mu(eff)) per V(4) for 1 gradually rises from 5.79 mu(B) at 300 K to a maximum of 6.80 mu(B) at 25.0 K and then decreases rapidly to 4.72 mu(B) at 2.00 K. The data in the 7.00-300 K range were fit to the appropriate theoretical expression (based on (H) over cap = -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 (chi(M) ") below 4.0 K and corresponding small decreases in the in-phase signals (chi(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 chi(M) " Signal to move to higher temperature and consequently leading to a much stronger chi(M) " Signal and, for 5, the observation of a peak at similar to 2.0 K. A dependence of the chi(M) " peak position of 5 on the DC field intensity and AC field oscillation frequency is found.