Hexametallic manganese clusters with bulky derivatised salicylaldoximes

Article abstract of DOI:10.1039/c0dt01273d

The reaction of Mn(ClO₄)₂·6H₂O with Ph-saoH₂ (Ph-saoH₂ = 2-hydroxybenzophenone oxime) in MeCN in the presence of sodium propionate forms the complex [Mn^III ₆O₂(Ph-sao)₆(prop)₂(MeCN) ₂]·5.27MeCN (1·5.27MeCN) (prop = propionate). Repeating the same reaction in EtOH produces the complex [Mn^III ₆O₂(Ph-sao)₆(prop)₂(EtOH) ₄] (2). Complexes 1 and 2 may be considered as structural isomers, since they display the same metallic core but different coordination modes of the propionate ligands; bridging in 1 and terminal in 2. Performing similar reactions and switching from sodium propionate to sodium adamantane-carboxylate (NaO₂C-ada) and sodium pivalate (Napiv) in the presence of NEt ₄OH yields the complexes [Mn^III₆O ₂(Ph-sao)₆(O₂C-ada)₂(MeOH) ₄] (3) and [Mn^III₆O₂(Ph-sao) ₆(piv)₂(EtOH)₄]·0.5Et₂O (4·0.5Et₂O), respectively. All four complexes contain the same {Mn^III₃O(Ph-sao)₃} building block. Variable temperature magnetic susceptibility and magnetization studies show that all complexes possess an S = 4 ground-state.

Full text of DOI:10.1039/c0dt01273d

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PAPER
Hexametallic manganese clusters with bulky derivatised salicylaldoximes†
Vasiliki Kotzabasaki,a Ross Inglis,b Milosz Siczek,c Tadeusz Lis,c Euan K. Brechin*b and
Constantinos J. Milios*a
Received 22nd September 2010, Accepted 25th November 2010
DOI: 10.1039/c0dt01273d
The reaction of Mn(ClO4)2·6H2O with Ph-saoH2 (Ph-saoH2 = 2-hydroxybenzophenone oxime) in
MeCN in the presence of sodium propionate forms the complex [MnIII6O2(Ph-sao)6(prop)2-
(MeCN)2]·5.27MeCN (1·5.27MeCN) (prop = propionate). Repeating the same reaction in EtOH
produces the complex [MnIII6O2(Ph-sao)6(prop)2(EtOH)4] (2). Complexes 1 and 2 may be considered as
structural isomers, since they display the same metallic core but different coordination modes of the
propionate ligands; bridging in 1 and terminal in 2. Performing similar reactions and switching from
sodium propionate to sodium adamantane-carboxylate (NaO2C-ada) and sodium pivalate (Napiv) in
the presence of NEt4OH yields the complexes [MnIII6O2(Ph-sao)6(O2C-ada)2(MeOH)4] (3) and
[MnIII6O2(Ph-sao)6(piv)2(EtOH)4]·0.5Et2O (4·0.5Et2O), respectively. All four complexes contain the
same {MnIII3O(Ph-sao)3} building block. Variable temperature magnetic susceptibility and
magnetization studies show that all complexes possess an S = 4 ground-state.
previous generations and the manipulation of physical properties
Introduction
at the molecular level is of increasing importance. In other
The last few years have witnessed a major growth in the field
words, in order to employ molecular and/or nanoscale devices
of applied coordination chemistry. Molecules with desired and/or
for specific purposes/functions their physical properties need to
designed physical properties find numerous applications in various
be understood, controlled and manipulated accordingly.
fields of technology from MRI contrast agents1 to biomimetic
In cluster-based molecular magnetism one of the primary
molecules,2 and from metal-based drugs3 to hydrogen storage.4 In
physics, coordination compounds have led to the emerging fields of
Molecular Magnetic Refrigeration5 and Molecular Spintronics.6
conditions for observing “exciting” magnetic properties is a
(relatively) large spin ground-state, S (e.g. in the fields of Single
Molecule Magnetism and Molecular Refrigeration). In order
The former describes molecules displaying an enhanced magne-
to fulfil this condition control over the intra-cluster magnetic
tocaloric effect which may potentially be used for cooling applica-
tions, while for the latter, molecules bridging molecular electronics
and spintronics may be used for reading and manipulating the spin
exchange interactions, Jij, between neighbouring metal centres
must be achieved. This is not a trivial task, especially for metallic
clusters of high nuclearity, Mx (x > 4), since in almost all
states in nanodevices. In Molecular Magnetism, and in particular
cases the presence of multiple exchange interactions favours
in Single-Molecule Magnetism, new compounds have been syn-
antiferromagnetic exchange, as well as hindering quantitative
thesized that can now retain their magnetization above/at liquid
analysis.
He temperatures,7 and both homo- and heterometallic f-block
We recently reported a semi-quantitative magnetostructural
clusters have been reported to display the SMM phenomenon.8
It thus becomes apparent that modern coordination chemistry
has been evolving towards a more technological future than in
correlation on an extended family of ~25 hexanuclear manganese
[MnIII6] complexes featuring salicylaldoximate and derivatised sal-
icylaldoximate ligands.9 In all of these complexes—which consist
of similar [MnIII6O2(O2CR)2(R¢-sao)6(sol)x] units (R = various
alkyl/aryl groups; R¢ = H, Me, Et; sol = solvent molecules)—
the magnetic exchange interactions, Jij, and consequently the
spin ground-state S are highly dependent on the (Mn–N–O–
Mn) torsion angles (a) of the bridging oximates. In particular
we observed a “magic area” of 30.4–31.3◦ which allowed us to
predict the Jij given the Mni–N–O–Mnj torsion angle. For (a) >
31.3◦ → Jij > 0 (ferromagnetic, F), while for (a) < 30.4◦ → Jij <
0 (antiferromagnetic, AF). Furthermore, we were able to switch
the nature of the exchange interactions from AF to F by enforcing
structural distortions on the metallic core of the complexes so
aDepartment of Chemistry, University of Crete, Voutes, 71003, Herakleion,
Greece. E-mail: komil@chemistry.uoc.gr; Fax: +30-2810-545001; Tel: +30-
2810-545099
bSchool of Chemistry, The University of Edinburgh, West Mains Road, EH9
3JJ, Edinburgh, UK. E-mail: ebrechin@staffmail.ed.ac.uk; Tel: +44-131-
6507545
cFaculty of Chemistry, University of Wroclaw, Joliot-Curie 14, Wroclaw,
50-383, Poland
† Electronic supplementary information (ESI) available: Details on the
syntheses of the complexes. CCDC reference numbers 793021–793024.
For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c0dt01273d
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that the value of (a) > 31.3◦. This was achieved through the use
of derivatised salicylaldoximes, R¢-saoH2 (R¢ = Me, Et), because
the steric effects imposed by the size of oximic R¢-group distorted
the Mn–N–O–Mn torsion angles of the complexes to the desired
values (> 31.3◦).
Herein we report the synthesis and magnetic properties of
four new [MnIII6] complexes featuring the phenyl-derivatised
salicylaldoxime, Ph-saoH2 (Scheme 1). These complexes constitute
an important addition to the [MnIII6] family, since from the study
of their magnetic properties, it becomes evident that it is not only
the size (or bulkiness) of the oximic R¢-substituent but its flexibility
and directionality that dictates the magnitude of the torsion angles
and, therefore, the magnetic properties of the above mentioned
complexes.
X-Ray crystallography
Diffraction data for 1 and 2 were collected at 90 K on an Xcalibur
PX diffractometer. Diffraction data for 3 and 4 were collected at
150 K on a Bruker Smart Apex CCDC diffractometer equipped
with an Oxford Cryosystems LT device using Mo radiation. The
structures were solved by direct methods and refined by full-matrix
least-squares techniques on F2 (for 1 and 2 with SHELXL11 and
for 3 and 4 with CRYSTALS12). Data collection parameters and
structure solution and refinement details are listed in Table 1.
Full details can be found in the CIF files: CCDC 793021, 793022,
793023 and 793024 for complexes 1–4, respectively.
Results and discussion
The molecular structures of complexes 1–4 are presented in Fig. 1–
2 and selected interatomic distances and angles are listed in
Tables 2–5. Bond valence sum (BVS) calculations for the metal
ions of the four complexes are given in Table 6. All four complexes
display very similar structures and so for the sake of brevity we
provide a generic description, highlighting individual differences.
Despite the fact that the starting materials contained exclusively
MnII it is evident that the products contain only MnIII ions formed
upon base-assisted atmospheric oxidation of the former, a very
common process in manganese chemistry. This oxidation can be
easily observed by the gradual colour change of the solutions from
pale pink to dark brown-green upon stirring.
Scheme 1 The structure of Ph-saoH2.
Experimental
Materials and physical measurements
All manipulations were performed under aerobic conditions using
materials as received (reagent grade). Caution! Although we
encountered no problems, care should be taken when using the
potentially explosive perchlorate ion. Ph-saoH2 was synthesised
by the reaction of the precursor ketone with hydroxylamine and
sodium acetate in EtOH, as described in the literature.10 Elemental
analyses (C, H, N) were performed by the EaStCHEM micro-
analysis service. Variable-temperature, solid-state direct current
(dc) magnetic susceptibility data down to 1.8 K were collected
on a Quantum Design MPMS-XL SQUID magnetometer (UoC)
equipped with a 5 T dc magnet for complexes 1 and 2, and on a
7 T Quantum Design MPMS-XL SQUID magnetometer (UoE)
for 3 and 4. Diamagnetic corrections were applied to the observed
paramagnetic susceptibilities using Pascal’s constants.
Syntheses
General synthetic procedure for complexes 1–4. All complexes
were prepared from the reaction of Mn(ClO4)2·6H2O with Ph-
saoH2 and the corresponding sodium carboxylate and in the
presence of NEt4OH (for 3 and 4 only) in alcoholic solvents (EtOH
for 2 and 4 and MeOH for 3) or MeCN (1). The crystals were
obtained by slow evaporation (1 and 2) or by layering with Et2O
(3 and 4) after three days in good yields (35–45%). All crystals
were dried in vacuo and analysed as solvent free. Elemental Anal.
Calcd (found) for 1: C 56.91 (56.83), H 3.80 (3.72), N 6.03 (6.14);
2: C 56.40 (56.49), H 4.53 (4.39), N 4.29 (4.21). 3: C 59.04 (59.64),
H 4.76 (4.62), N 3.97 (4.17); 4: C 57.21 (57.34), H 4.80 (4.67), N
4.17 (4.29).
Fig. 1 The molecular structures of 1 (top) and 2 (bottom). Colour code:
Mn(III) = red, O = green, N = blue, C = yellow.
1694 | Dalton Trans., 2011, 40, 1693–1699
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Table 1 Crystallographic data for complexes 1·5.27MeCN, 2, 3 and 4·0.5Et2O
1·5.27MeCN
2
3
4·0.5Et2O
Formulaa
MW
C98.55H85.82Mn6N13.27O18
2073.68
Monoclinic
P21/n
15.608(6)
19.100(9)
16.182(4)
90.00
103.07(4
90.00
4699(3)
2
C92H88Mn6N6O22
1959.32
Monoclinic
P21/n
13.004(5)
12.887(5)
26.360(9)
90.00
101.58(3)
90.00
4328(3)
2
C104H100Mn6N6O22
2115.53
Triclinic
P-1
13.8863(4)
14.4013(4)
15.7519(5)
116.3910(10)
90.639(2)
117.4650(10)
2409.38(13)
1
150
0.71073
1.458
0.839
31740/13171 (0.049)
9837
0.1652
0.0678
0.9012
C97H99Mn6N6O22.50
2038.46
Triclinic
P-1
13.7188(4)
14.4624(5)
14.9900(5)
95.712(2)
116.428(2)
110.512(2)
2375.90(13)
1
150
0.71073
1.425
0.848
30950/9711 (0.0412)
7795
0.1308
0.0510
1.070
Crystal System
Space group
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
V/A
Z
T/K
90
0.71073
1.466
90
0.71073
1.504
b
˚
l /A
Dc/g cm-3
m(Mo-Ka)/mm-1
Meas./indep. (Rint) refl.
Obs. refl. [I > 2s(I)]
wR2c,d
0.858
0.927
34378/10218 (0.1646)
3645
0.0608
0.0484
0.652
30246/12349 (0.0636)
6849
0.0553
0.0402
0.896
R1d,e
Goodness of fit on F2
-3
˚
Drmax,min/eA
0.48/-0.52
0.44/-0.49
1.98/-0.98
0.92/-0.49
a Including solvate molecules. b Mo-Ka radiation, graphite monochromator. c wR2 = [Rw(IFo2| - |Fc2|)2/Rw|Fo2|2]1/2
RꢀFo| - |Fcꢀ/R|Fo|.
.
d For observed data. e R1 =
˚
Table 2 Selected interatomic distances (A) and angles (deg) for 1
Mn1–O1
Mn1–O11
Mn1–O23
Mn1–N11
Mn1–O21a
Mn1–N1
Mn2–O13
Mn2–O1
1.877(3)
1.880(3)
1.893(2)
2.021(3)
2.253(3)
2.397(4)
1.848(2)
1.871(2)
118.13(12)
112.66(12)
120.53(13)
Mn2–O22
Mn2–N13
Mn2–O14
Mn3–O12
Mn3–O1
Mn3–O21
Mn3–N12
Mn3–O24
1.931(3)
2.020(3)
2.086(3)
1.844(3)
1.891(2)
1.948(3)
1.986(3)
2.090(3)
-30.4(3)
1.6(3)
Mn2–O1–Mn1
Mn2–O1–Mn3
Mn1–O1–Mn3
Mn1–O23–N13–Mn2
Mn2–O22–N12–Mn3
Mn3–O21–N11–Mn1
-49.6(2)
Symmetry operations:a 1-x, 1-y, 1-z.
˚
Table 3 Selected interatomic distances (A) and angles (deg) for 2
Mn1–O11
Mn1–O1
Mn1–O23
1.8891(14)
1.8914(13)
1.9405(15)
1.9986(18)
2.1253(15)
2.3556(15)
1.8799(15)
1.8894(14)
1.9161(14)
121.03(7)
118.60(7)
119.70(7)
Mn2–N13
Mn2–O14
Mn2–O11a
Mn3–O12
Mn3–O1
Mn3–O21
Mn3–N12
Mn3–O16
1.9970(17)
2.2034(15)
2.3505(15)
1.8442(15)
1.8545(14)
1.9034(14)
2.0163(17)
2.1189(16)
Mn1–N11
Mn1–O15
Mn1–O23a
Mn2–O13
Mn2–O1
Mn2–O22
Mn3–O1–Mn2
Mn3–O1–Mn1
Mn2–O1–Mn1
Mn1–O23–N13–Mn2
Mn2–O22–N12–Mn3
Mn3–O21–N11–Mn1
48.7(2)
-7.1(2)
26.0(2)
Symmetry operations:a 1-x, 1-y, 1-z.
Fig. 2 The molecular structures of 3 (top) and 4 (bottom). Colour code
as in Fig. 1.
been found previously with other (derivatised) salicylaldoximes.13
The edges of the {MnIII3O(Ph-sao)3} unit consist of –N–O– Ph-
sao2- oximate bridges, thus forming a (–Mn–N–O–)3 ring around
the central oxo m3-O2- bridge. The hexametallic cores form from
two (–Mn–N–O–)3 rings at close proximity, so that one Ooximate
The main building block in complexes 1–4 is the oxo-centered
triangular {MnIII3O(Ph-sao)3} unit which “dimerises” [in the
particular solvent used] to form the hexametallic species, as has
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˚
Table 4 Selected interatomic distances (A) and angles (deg) for 3
the different coordination modes of the carboxylates in 1 and
1
1
2–4; h : h : m bridging in the former and terminal in the latter.
Mn1–O46¢
Mn1–O4
Mn1–O5
2.306(3)
1.890(3)
1.904(3)
1.994(3)
1.962(3)
2.098(3)
1.853(3)
1.906(3)
1.847(3)
118.95(13)
119.24(14)
121.33(14)
Mn2–N29
Mn2–O66
Mn3–O5
2.016(3)
2.147(3)
2.353(3)
1.886(3)
1.900(3)
1.864(3)
1.989(3)
2.258(3)
˚
The central oxo atom is located ~0.32 A above the plane defined
by the three metal centres in the {MnIII3O(Ph-sao)3} unit in
Mn1–N13
Mn1–O46
Mn1–O53
Mn2–O4
Mn2–O14
Mn2–O21
Mn1–O4–Mn3
Mn1–O4–Mn2
Mn3–O4–Mn2
Mn3–O4
1, while it is essentially co-planar in the remaining complexes
Mn3–O30
Mn3–O37
Mn3–N45
Mn3–O68
˚
˚
˚
(~0.09 A, ~0.07 A and ~ 0.1 A, for 2–4, respectively). Given that
all complexes display the same metallic core, the deviation of the
central oxide from the metallic plane may be attributed to the
1
1
h : h : m bridging mode of the carboxylate in 1. In all previously
reported members of the [MnIII6] family only those with m-bridging
carboxylates present large deviations of the central oxide from the
[Mn3] plane, as 1.9 This also appears to be the case for the discrete
trinuclear [MnIII3O(R-sao)3(O2C-R)(sol)x] complexes.14 The Mn–
N–O–Mn torsion angles are -30.4(3)◦, 1.6(3)◦ and -49.6(2)◦ for
1, 48.7(2), -7.1(2)◦ and 26.0(2)◦ for 2, 26.59◦, 4.36◦ and 47.79◦
for 3, and 12.09◦, 27.87◦, 49.51◦ for 4; the largest value for each
triad corresponding to the Mn–N–O–Mn edge which involves the
inter-triangular linkage with the Ooximate bonding to a neighbouring
[Mn3] unit to form the hexametallic clusters.
Mn1–O46–N45–Mn3
Mn2–O14–N13–Mn1
Mn3–O30–N29–Mn2
47.79
26.59
4.36
˚
Table 5 Selected interatomic distances (A) and angles (deg) for 4
Mn1–O123
Mn1–O1
Mn1–O2
Mn1–N22
Mn1–O12
Mn1–O2¢
Mn2–O1
Mn2–O123
Mn2–O2
Mn1–O123–Mn2
Mn3–O123–Mn2
Mn3–O123–Mn1
1.8840(19)
1.897(2)
1.942(2)
2.017(3)
2.120(2)
2.353(2)
1.875(2)
1.893(2)
1.902(2)
119.09(11)
120.85(10)
119.07(11)
Mn2–N22
Mn2–O1
Mn2–O1¢
Mn3–O1
Mn3–O123
Mn3–O2
Mn3–N22
Mn3–O1¢
1.989(3)
2.233(2)
2.348(2)
1.840(2)
1.853(2)
1.913(2)
2.020(2)
2.127(2)
A structural comparison of the four complexes reported
here with those previously reported reveals additional con-
clusions for our magneto-structural correlation. As two rep-
resentative reference compounds we consider the proto-
type complex [MnIII6O2(sao)6(OAc)2(EtOH)4] (5)15 which fea-
tures the non-derivatised salicylaldoxime, and [Mn6O2(Et-
sao)6(O2CPh)2(EtOH)4(H2O)2] (6).16 Complex 5 resembles com-
plex 1 in that both contain bridging carboxylates. The torsion
angles present in 5 are 10.70◦, 16.49◦ and 22.83◦, i.e. all below
the limiting value of 30.4◦ for antiferromagnetic interactions
(vide infra). On the other hand, the Et-substituent on the
salicylaldoximate ligands in 6 results in elevated torsion angles,
31.26◦, 38.20◦, 39.92◦; i.e. all above the limiting value of 31.3◦
for ferromagnetic interactions, thus justifying our assumptions on
the structural distortion of the metallic core upon the presence
of bulky substituents on the salicylaldoximate ligands. This is the
case for all the Et-derivatised salicylaldoximate containing [Mn6]
complexes: an increase in all the torsion angles compared to the
prototype complex 5, and the likely presence of torsion angles
> 31.3◦. We thus hoped the Ph-sao2- stabilised [Mn6] complexes
would follow the same trend, and perhaps produce even more
distorted complexes since the Ph-substituent is clearly “bulkier”
than the Et-substituent. However, this was not the case for 1–4. In
fact we discovered some of the smallest torsion angles of all [Mn6]
complexes reported to date: 1.6(3)◦ in 1, -7.1(2)◦ in 2 and 4.36◦ in
3. This mismatch may be attributed to the planarity of the rigid
Ph-group of Ph-sao2-, as opposed to the pendant flexible –CH2–
CH3 arm of the Et-sao2- ligand which is clearly able to impose a
much larger distortion on to the Mn–N–O–Mn bridge, despite its
smaller size.
Mn1–O2–N22–Mn2
Mn2–O2–N22–Mn3
Mn3–O2–N22–Mn3
49.51
12.09
27.87
Table 6 BVS calculations for 1–4
Complex 1
MnII
3.47
3.42
3.41
MnIII
3.24
3.18
3.18
MnIV
3.18
3.11
3.11
Mn1
Mn2
Mn3
Complex 2
MnII
MnIII
3.24
3.24
3.25
MnIV
3.17
3.17
3.18
Mn1
Mn2
Mn3
3.48
3.48
3.49
Complex 3
MnII
MnIII
3.25
3.21
3.27
MnIV
3.18
3.14
3.21
Mn1
Mn2
Mn3
3.49
3.45
3.51
Complex 4
MnII
MnIII
3.21
3.26
3.22
MnIV
3.15
3.19
3.16
Mn1
Mn2
Mn3
3.45
3.50
3.47
from each [Mn3] unit bonds to the neighbouring [Mn3] unit,
thus forming two inter-triangular [Mn3]–[Mn3] links. In addition,
the two [Mn3] units are held together via two Ophenolate bridges,
so that overall there are four inter-triangular connections. The
six doubly deprotonated Ph-sao2- ligands are found in three
Magnetochemistry
1
1
1
different coordination modes: two in a h : h : h : m2 mode, two in
Dc magnetic susceptibility studies. Direct current magnetic
susceptibility studies were performed on polycrystalline samples
of 1–4 in the 5–300 K range in an applied field of 0.1 T. The results
are plotted as the cMT product vs. T in Fig. 3. For 1 the cMT
value of 17.76 cm3 K mol-1 at 300 K is very close to the spin-only
value of 18 cm3 K mol-1 expected for six non-interacting high-spin
MnIII (3d4) ions. This value remains constant until ~200 K, below
a h : h : h : m3 mode with the Ooximate bridging two MnIII belonging
2
1
1
2
1
1
to different triangular units, and two in a h : h : h : m3 mode
with the Ophenolate bridging the [Mn3] units. Complexes 2–4 present
the same [MnIII6O2(Ph-sao)6(O2CR)2(sol)4] general formula (sol =
MeOH, EtOH), with 1 being slightly different [MnIII6O2(Ph-
sao)6(O2CR)2(sol)2] (sol = MeCN). This discrepancy is due to
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This afforded the parameters J1 = 1.2 cm-1, J2 = -1.6 cm-1 and
g = 1.99 for 2, J1 = 1.4 cm-1, J2 = -1.6 cm-1 and g = 1.99 for 3, and
J1 = 1.6 cm-1, J2 = -1.5 cm-1 and g = 2.00 for 4. This leads to a
ground-state of S = 4 for all three complexes with a close-lying first
excited state (S = 5) located ~9 cm-1 above the ground-state. The
model we chose in order to simulate the data for 2–4 was based
on the magneto-structural correlation we recently reported.9 The
major structural parameter that we considered was the value of
the Mni–O–N–Mnj torsion angle; for values > 31.3◦ we assumed
a ferromagnetic exchange interaction, while for values < 30.4◦ we
considered an antiferromagnetic exchange inetraction. Initially we
considered two AF interactions per complex (instead of one) due
to the large difference between the two torsion angles which are
below 30.4◦ (Da ª 20◦) on the assumption that these two AF
interactions would be of significantly different magnitudes; one
very weak for the torsion angle close to 30.4◦, and one moderate
for the torsion angle with the biggest deviation from the limiting
value. However, although it is certainly possible to do this, the data
can be equally well simulated by adopting a more “generic” model
in which the AF interactions are grouped into one J-value for all
torsion angles < 30.4◦. Thus, we believe the 2-J model to be more
suitable for complexes 2–4.
Fig. 3 Plot of cMT vs. T for complexes 1–4. The solid lines represent
simulations of the experimental data in the temperature range 5–300 K,
see text for details.
which it decreases to reach the final value of 7.8 cm3 K mol-1
at 5 K, with the low temperature cMT value suggesting a small
ground-state (S ª 4). Despite much effort we were not able to
successfully simulate the data assuming various exchange models
employing up to 3 different J-values, as is the case for most S = 4
[MnIII6] complexes containing bridging carboxylates.9 Complexes
2–4 display almost identical magnetic behaviour (to each other),
although different to that of 1. Despite having the same room
temperature cMT value as 1 (17.78, 17.81 and 17.82 cm3 K mol-1,
for 2, 3 and 4, respectively) the cMT value remains essentially
constant as the temperature is decreased until ~60 K (vs. 200 K
for 1), below which it decreases to reach a final value of 9.71,
9.74 and 9.73 cm3 K mol-1, for 2, 3 and 4, respectively. All three
low temperature values indicate the presence of an S = 4 ground-
state. We were able to successfully simulate the data for the three
complexes adopting the 2-J model shown in the spin Hamiltonian
of eqn (1) and Scheme 2 and using the program MAGPACK.17
In order to verify the ground-states of 1–4, we performed vari-
able temperature and variable field dc magnetisation experiments
in the ranges 1.8–6 K and 0.5–7 T, and these data are presented
as reduced magnetisation (M/NmB) vs. H/T in Fig. 4. We were
able to fit the 0.5–3 T data for all complexes, employing a matrix
diagonalisation method to a model that assumes only the ground
2
ˆ
state is populated, includes axial zero-field splitting (DSz ) and the
Zeeman interaction, and carries out a full powder average. The
corresponding Hamiltonian is given by eqn (2)
2
ˆ
ˆ
ˆ
(2)
H = DSz + gmBm0S·H
where D is the axial anisotropy, mB is the Bohr magneton, m0 is the
ˆ
vacuum permeability, Sz is the easy-axis spin operator and H is the
applied field. This model gave: S = 4, g = 2.00 and D = -0.70 cm-1
for 1, S = 4, g = 2.00 and D = -0.73 cm-1 for 2, S = 4, g = 2.00
and D = -0.92 cm-1 for 3, and S = 4, g = 2.00 and D = -0.92 cm-1
for 4. However, given the simplistic model employed and the likely
scenario whereby the weak exchange present in all four complexes
leads to the population of many excited states—as is the case for
most members of the [Mn6] and [Mn3] families,9—the accuracy of
the above numbers should be regarded with caution until INS or
HF-EPR spectroscopy is performed.
ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
ˆ ˆ
H = -2J1(S1·S2 + S1·S2¢ + S1·S1¢ + S2·S1¢ + S1¢·S2¢) - 2J2(S1·S3
(1)
ˆ
ˆ
ˆ
ˆ
+ S1¢·S3¢ + S2·S3 + S2¢·S3¢)
Ac magnetic susceptibility studies. Alternating current mag-
netic susceptibility measurements were performed on polycrys-
talline samples of 1–4 in the 1.8–10 K range in zero applied
dc field and a 2.5 G ac field oscillating at 100–1000 Hz. All
four complexes display essentially analogous behaviour, so we
present the data for complex 4 as a representative example. The
in-phase (cM¢, plotted as cM¢T vs. T, Fig. 5, top) signal decreases
upon decreasing temperature, indicating the presence of low-lying
excited states with larger S values than the ground-state, consistent
with conclusions drawn from the dc data. Extrapolation of the in-
phase signal to 0 K from ~ 10 K gives a cM¢T value of ~7 cm3 K
mol-1, suggesting a ground-state of S = 4. Below approximately 4 K
there is a frequency dependent decrease in the value of cM¢T and
a concomitant increase in the out-of-phase signal. Fully visible
Scheme 2 The exchange interaction model used for complexes 2–4; see
text for details (green dotted line: F; red dotted lines: AF; grey shaded
area: a < 30.4◦; green shaded area: a >31.3◦).
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Dalton Trans., 2011, 40, 1693–1699 | 1697
©

View Article Online
Fig. 5 Plots of the in-phase cM¢T (top) and out-of-phase cM¢¢ (bottom)
ac susceptibility measurements for 4; black = 50 Hz, red = 200 Hz, blue =
500 Hz, green = 750 Hz and orange = 1000 Hz.
20.2 cm-1 and t0 = 3.8 ¥ 10-9 s for 2, Ueff = 17.9 cm-1 and t0 = 5.6 ¥
10-8 s for 3 and Ueff = 17.35 cm-1 and t0 = 7.8 ¥ 10-8 s for 4.
Conclusions
In conclusion, we have reported the syntheses, structures and
initial magnetic properties of four new [MnIII6] complexes sta-
bilised by the phenyl-derivatised salicylaldoxime, Ph-saoH2. The
magnetic behaviour of this class of compounds is dominated
by the degree of oximic twisting (defined by the Mn–N–O–
Mn torsion angle, a – large twisting propagating ferromagnetic
nearest neighbour exchange and small twisting favouring an-
tiferromagnetic nearest neighbouring exchange). Complexes 1–
4 constitute an important addition to this ever-growing family
because they clearly demonstrate that it is not simply the steric
bulk of the oximic R¢-substituent that defines the degree of Mn–
N–O–Mn twisting. Indeed the introduction of a phenyl group
on the oximic C-atom results in molecules with relatively small
a-values. This can be attributed to the planarity of the rigid Ph-
group of Ph-sao2- and appears analogous to the effects observed
when substituents are introduced around the phenolic ring, i.e.
tBut-saoH2 or naphth-saoH2.13 The largest distortions to the Mn–
N–O–Mn bridge occur in complexes built with the Et-sao2- ligand
(R¢ = Et), the pendant –CH2–CH3 arm being able to impose a
much larger distortion, despite its smaller size, due to its flexibility
Fig. 4 Plot of reduced magnetization (M/NmB) vs. H/T for 1–4 in
0.5–3 T fields and 1.8–6 K temperature range.
frequency-dependent out-of-phase, cM¢¢, peaks are observed in
the 50–1000 Hz range of frequencies with the peak maximum at
1000 Hz occurring at approximately 3.5 K.
An Arrhenius analysis for 1–4 showed that all of them behave
as Single Molecule Magnets, as has been observed for all S =
4 members of the [MnIII6] family.9 The characteristic Ueff and t0
and directionality. The discovery and examination of other [MnIII
]
6
clusters with alternative flexible R¢-substituents is thus an exciting
prospect.
values for 1–4 are: Ueff = 17.2 cm-1 and t0 = 1.7 ¥ 10-8 s for 1, Ueff
=
1698 | Dalton Trans., 2011, 40, 1693–1699
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©

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8 For a recent comprehensive review see: R. Sessoli and A. K. Powell,
Coord. Chem. Rev., 2009, 253, 2328.
Acknowledgements
9 C. J. Milios, R. Inglis, A. Vinslava, R. Bagai, W. Wernsdorfer, S. Parsons,
S. P. Perlepes, G. Christou and E. K. Brechin, J. Am. Chem. Soc., 2007,
129, 12505; C. J. Milios, C. P. Raptopoulou, A. Terzis, F. Lloret, R.
Vicente, S. P. Perlepes and A. Escuer, Angew. Chem., Int. Ed., 2004, 43,
210; R. Inglis, L. F. Jones, C. J. Milios, S. Datta, A. Collins, S. Parsons,
W. Wernsdorfer, S. Hill, S. P. Perlepes, S. Piligkos and E. K. Brechin,
Dalton Trans., 2009, 3403; S. Carretta, T. Guidi, P. Santini, G. Amoretti,
O. Pieper, B. Lake, J. van Slageren, F. El Hallak, W. Wernsdorfer, H.
Mutka, M. Russina, C. J. Milios and E. K. Brechin, Phys. Rev. Lett.,
2008, 100, 157203.
CJM would like to thank The University of Crete (ELKE, research
grant KA 2933) for funding. EKB would like to thank the EPSRC
and The Leverhulme Trust for funding.
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This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 1693–1699 | 1699
©