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

Abstract

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 n 4 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 (μ eff ) per V 4 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 Ĥ = −2 JS 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.