Synthetic procedures are described that allow conversion of
[Mn4O2(OAc)6(py)2(dbm)2]
(1, dbmH = dibenzoylmethane) to
[Mn4O3X(OAc)3(dbm)3]
(X = Cl, 2; X = Br, 3). Treatment of
1 with NBu
n
4Cl in
CH2Cl2 or hot
MeCN leads to 2 in 5−8% and 35−43% yields (based on
dbm), respectively. A higher yield (∼88%) is
obtained
by treating 1 with 4 equiv of Me3SiCl in
CH2Cl2. An analogous procedure with 4
equiv of Me3SiBr in
CH2Br2
gives 3 in 55% yield. Complexes 2 and
3 are isomorphous, monoclinic space group
P21/n, T = −155 °C,
Z =
4. For 2, a = 13.900(3), b
= 22.038(5), and c = 16.518(5) Å and β =
107.80(1)°; for 3, a = 13.644(2),
b =
22.190(4), and c = 16.548(3) Å, and β =
106.64(1)°. The structures were solved by direct methods
(MULTAN78)
and refined on F to R(R
w)
values of 7.85 (7.38) and 7.37 (6.89)% using 2267 and 2809 unique
reflections with
F > 2.33σ(F) for 2 and
3, respectively. Treatment of
[Mn3O(OAc)6(py)3](ClO4)
in MeCN with Me3SiCl followed
by addition of H2O and acetic acid results in
crystallization of
(pyH)3[Mn4O3Cl7(OAc)3]·2MeCN
(4) in 75%
yield (based on Mn). Complex 4 crystallizes in
monoclinic space group C2/c with the following
cell parameters
at −157 °C: a = 37.420(5), b =
13.752(1), and c = 16.139(2) Å, β =
110.33(1), V = 7787.9 Å3, and
Z = 8.
The structure was solved by direct methods (MULTAN78) and refined
on F to R(R
w) values of
5.74 (5.78)%
using 2612 unique reflections with F >
3.0σ(F). The complexes possess a
[Mn4(μ3-O)3(μ3-X)]
distorted cubane
core and a 3MnIII,MnIV trapped-valence
oxidation-state description. Three AcO- groups
bridge each MnIIIMnIV
pair, and a chelating dbm- (2 and
3) or two Cl- ions (4) on each
MnIII complete peripheral ligation. The
pyridinium
cations of 4 are involved in hydrogen-bonding interactions
with the μ3-O2- and the
terminal Cl- ions of the
anion. Variable-temperature solid-state magnetic susceptibility
studies show that the magnetic properties of 2
and 3 are very similar: μeff values steadily
rise from ∼9 μB at room temperature to ∼10
μB at 30.0 K and then
drop rapidly to ∼9.5 μB at 5 K. Fitting of the
experimental data for the two complexes to the appropriate
theoretical
equation yield the following fitting parameters, in the format
2/3: J =
J(MnIII···MnIV) =
−28.4/−30.1 cm-1,
J‘
= J(MnIII···MnIII) =
+8.3/+7.4 cm-1, and g =
1.98/2.03. Both 2 and 3 have S
= 9/2 ground states that are
well-separated (∼180 cm-1) from an
S = 7/2 first excited state.
The ground state was confirmed by magnetization
vs magnetic field studies at several fields and temperatures; fitting
of the data allowed the zero-field splitting
parameter D to be determined for both complexes. The
magnetochemical properties of 4 are very similar
to
those of 2 and 3, and the fitting parameters were
J = −29.1 cm-1, J‘
= +10.2 cm-1, and g = 1.97,
giving an S
= 9/2 ground state and showing that the
hydrogen-bonding interactions of the
μ3-O2- ions do not cause a
significant
change to the exchange parameters or to the electronic structure of the
[Mn4O3Cl]6+ core.
1H NMR spectra of
2−4 in CDCl3 or
CD3CN so...
