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.
