A rare ligand bridged ferromagnetically coupled MnIV3 complex with a ground spin state of S = 9/2

Article abstract of DOI:10.1039/b802279h

An exclusively chelating ligand bridged high-valent [Mn^IV ₃] complex has been synthesized, in which all Mn^IV ions are ferromagnetically-coupled to exhibit an S_T = 9/2 spin ground state. The Royal Society of Chem

Full text of DOI:10.1039/b802279h

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IV
A rare ligand bridged ferromagnetically coupled Mn
ground spin state of S = 9/2w
complex with a
3
Thushan Pathmalingam, Serge I. Gorelsky, Tara J. Burchell, Anne-Catherine Be
´
dard,ab
a
b
a
ab
c
a
Andre M. Beauchemin,* Rodolphe Clerac and Muralee Murugesu*
´ ´
Received (in Cambridge, UK) 11th February 2008, Accepted 26th March 2008
First published as an Advance Article on the web 16th April 2008
DOI: 10.1039/b802279h
IV
An exclusively chelating ligand bridged high-valent [Mn
3
]
ence the nature and strength of the magnetic interactions
between the metal ions.
IV
complex has been synthesized, in which all Mn ions are
ferromagnetically-coupled to exhibit an ST = 9/2 spin
ground state.
Herein, we report the synthesis, structure, DFT calculations
IV
and magnetic properties of the first ferromagnetic {Mn 3}
linear complex obtained using the chelating ligand, 1-hydroxy-
In recent years, high-valent manganese cluster compounds
have been the focus of considerable research efforts due to
1,1-diphenylpropan-2-one oxime (H
The H dpo chelating ligand was obtained upon heating
alkynol A with aqueous NH OH overnight (eqn (1)).
2
dpo).
2
1
their potential for use in catalysis, in modelling photosystem
2
2
II and in magnetochemistry. In most cases, the coordination
3
II
chemistry of manganese is dominated by low-valent Mn ,
III
Mn , or mixed-valent Mn complexes. To date, only a handful
ð1Þ
IV
of Mn clusters have been obtained, all of which contain O
2ꢀ
/
ꢀ
OH bridging ligands. To our knowledge, no examples of
IV
exclusively chelating ligand bridged Mn polynuclear (n 4 2)
complexes have been reported. This is presumably due to the
2c
The reaction proceeds with high ‘‘Markovnikov’’ regioselec-
tivity and can routinely be performed on the gram-scale to
afford the desired oxime.
IV
difficulty in stabilizing Mn ions in the absence of metal
oxide cores.
In order to obtain such a unique complex, we have devel-
Reaction of Mn(ClO
4
)
2
ꢁ4H
2 2
O (1 equiv.) with H dpo
oped an alternative approach that is contrary to the common
ꢀ
(2 equiv.) and triethylamine (1 equiv.) in MeCN (20 ml) gave
a dark-brown opaque solution. After 4 days, black rectangular
practice of utilizing strong oxidizing agents, such as MnO in
4
2
acidic conditions which, in general leads to O /OH bridged
ꢀ
ꢀ
crystals of [Mn
3
(dpo)
6
]ꢁ2CH
3
CN (1ꢁ2CH
3
CN) were isolated in
4
complexes. By employing aerial oxidation of Mn ions in
45% yield. The structure of the centrosymmetric complex 1
IV
2ꢀ
core capped by six dpo
3
mildly basic conditions and aprotic solvents, we avoid the
formation of undesirable metal–oxide side products and ligand
protonation, and promote high-valent Mn ions and the
deprotonation of ligand molecules, as required for chelation.
Currently, with this synthetic strategy, we are investigating
the use of tunable, tridentate, oximate-based chelating ligands.
To date, the majority of previously reported oximate-based
metal complexes have been synthesized using either commer-
cially available or Schiff base reaction generated oximates. In
order to gain the controllability and tunability of the oximate
ligands and in turn of the resulting metal compounds, we have
developed a new synthetic approach to bulky oximate ligands
consists of a trinuclear Mn
ligands (Fig. 1).z The three Mn centers are linked solely via
the oximate groups in linear fashion with 1801
[Mn1–Mn2–Mn1] angle. The octahedral coordination en-
vironments of the metal centers are completed by O and
N atoms from the ligands.
a
a
In contrast to previously reported trinuclear Mn complexes,
1 is the first example of a high-valent linear trinuclear complex.
In addition, 1 exhibits higher torsion angles than other tri-
6
nuclear oximate bridged complexes (0–201). For 1, the aver-
age torsion angle between the terminal and the central
manganese ions [Mn1–N–O–Mn2] was determined to be
51.31. This increase in torsion angles is due to the presence
of bulky substituents on the oximate ligand (see below) and is
also likely to be the origin of the ferromagnetic superexchange
interactions observed. Furthermore, in comparable N–N
diazine bridged systems, similar torsion angles generally lead
5
involving direct hydroamination of alkynes. The chosen
oximate chelate has been carefully designed to incorporate
bulky steric groups that induce large torsion angles between
spin carriers via bridging oximate groups, which should influ-
a
7
Department of Chemistry, University of Ottawa, 10 Marie-Curie,
to ferromagnetic interactions between the metal centers.
Ottawa, ON, Canada K1N6N5. E-mail: m.murugesu@uottawa.ca.
E-mail: abeauche@uottawa.ca; Tel: +1 613 562 5800-2733
Centre for Catalysis Research and Innovation, University of Ottawa,
30 Marie-Curie, Ottawa, ON, Canada K1N6N5
Universite´ Bordeaux 1, Centre de Recherche Paul Pascal, UPR-
To establish the electronic structure of 1 and the influence of
b
the ligand’s bulky groups on the torsion angles, DFT calcula-
8
tions at the BPE/TZVP level were performed. The calculated
c
6
optimized structure of 1 has S symmetry and is in good
CNRS N18641, 115 Av. A. Schweitzer, 33600 Pessac, France
agreement with the X-ray structure [calculated (X-ray)
Mn1–N: 2.02 (1.99), Mn1–O: 1.86 (1.82) and Mn2–O: 1.95
w Electronic supplementary information (ESI) available: LUMO of 1,
computational details and crystallographic details. CCDC 676451. See
DOI: 10.1039/b802279h
˚
(1.91) A]. The ground spin state was calculated to be S = 9/2
T
2
782 | Chem. Commun., 2008, 2782–2784
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Fig. 2 Spin density distribution for the simplified complex 1 (R = H,
not Ph) for clarity. NPA-derived atomic spin densities are shown for
Mn, N, and O atoms.
calculations, the 12 Ph groups and 6 Me groups were replaced
by H atoms (Fig. 2). It was found that removal of the Me
groups does not affect the Mn2–O–N–Mn1 torsion angles
significantly. On the other hand, removal of the Ph groups
causes the torsion angles to decrease from an average of 511 to
4
31. Thus, the steric hindrance caused by the Ph groups on the
Fig. 1 Top: the molecular structure of [Mn
3
(dpo)
6
], 1. The central
oximate ligand plays a key role in imposing the higher torsion
angles in 1 which, in turn, help to stabilize the structure.
The dc magnetic susceptibility of 1 was measured in an
applied magnetic field of 1000 Oe and can be seen plotted as
wT vs. T in Fig. 3. As the temperature decreases from 300 to
Mn atom (Mn2) lies on an inversion centre. Symmetry equivalent
position (1 ꢀ x, 2 ꢀ y, 1 ꢀ z) denoted by an additional ‘‘a’’ in the label
(Mn1a). Color code: purple (Mn), red (O), blue (N). Bottom: the linear
central core of 1, revealing the (Mn–N–O–Mn) superexchange path-
way between the metal centres.
3
ꢀ1
7
K, the wT product gradually increases from 6.7 cm K mol
then slightly
3
to reach a maximum of 13.1 cm K mol
ꢀ1
with three unpaired electrons on each Mn ion. The calculated
Mn oxidation states are consistent with those determined by
IV
3
decreases to 12.4 cm K mol at 1.8 K. The increase in the
ꢀ1
wT product above 7 K is consistent with dominant ferromag-
netic interactions present within the trinuclear Mn complex
charge considerations, short bond Mn –L bond lengths (avg.
IV
9
of 1.90 A) and bond valence sum (BVS) calculations. The
˚
and the observed maximum value of wT is in agreement with
an S = 9/2 ground state (expected value: 12.375 cm K mol
NPA-derived atomic charges of the Mn ions are +1.03 and
3
ꢀ1
+
1.24 au for Mn1 and Mn2, respectively. Such low charges
T
IV
with g = 2). The final decrease at low temperature is pre-
sumably due to very weak magnetic anisotropy and/or inter-
molecular antiferromagnetic interactions. As seen in Fig. 1,
the molecular structure of 1 reveals that the main magnetic
interactions between the metal centers are mediated by the
for the Mn atoms indicate significant covalency in the Mn–N
and Mn–O bonds in the complex. Indeed, the Mayer bond
orders for Mn1–N, Mn1–O, and Mn2–O are 0.58, 1.03, and
0
.78, respectively. This covalency helps to stabilize the high
oxidation state of the Mn ions via ligand-to-metal charge
ꢀ
ꢀ
ꢀ
donation (2.65 e to the 3d orbitals, 0.26 e to the 4s orbital
ꢀ
of Mn1, 2.46 e to the 3d orbitals and 0.26 e to the 4s orbital
of Mn2) of the complex 1.
The cyclic voltammogram of 1 in CH CN contains one
3
irreversible one-electron reduction wave at ꢀ0.60 V (vs.
+
Fc/Fc ) indicating that only one Mn ion is reduced. The
higher positive charge of Mn2 (+1.24), relative to Mn1
(
+1.03), makes this ion a better electron acceptor. In agree-
ment with this, the a-spin LUMO of the complex (Fig. S1,
ESIw) has a 45% Mn2 3d orbital contribution while the two
Mn1 ions contribute only 6%. As a result, the DFT calcula-
IV
III
IV
tion on the reduced complex produces a Mn Mn Mn
species (S
4
T
= 5) with the central Mn ion in a high-spin d
electron configuration. Such an electron configuration under-
goes Jahn–Teller distortion at Mn2 and the MnO octahedral
6
˚
environment becomes axially elongated with 1.98 A (eq.) and
Fig. 3 Plot of wT vs. T for 1 at 1000 Oe. The solid line is the best fit
obtained with the Heisenberg trinuclear model described in the text.
Inset: M vs. H data at various temperatures (1.8, 3, 5 and 8 K) are
shown on a single M vs. H/T plot. The solid line is the best fit obtained
with an S = 9/2 Brillouin function considering all the data between
1.8 and 8 K.
˚
.19 A (axial) Mn2–O bond distances.
2
In order to probe the effects of ligand substituents on the
geometry and electronic structure of 1, a series of DFT
calculations were performed for simplified ligands. In these
This journal is ꢂc The Royal Society of Chemistry 2008
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2
NO groups from the dpo ligands. In order to quantify the
ꢀ
–
We thank the University of Ottawa (start-up), the Canada
Foundation for Innovation (CFI), FFCR, and NSERC
(Discovery and RTI grants) for their support. We thank Prof.
Tom K. Woo for use of the computing facilities funded by the
CFI and the Ontario Research Fund.
magnitude of the magnetic interactions, an isotropic Heisen-
berg trinuclear model of S = 3/2 spins was employed using the
following spin Hamiltonian: H = ꢀ2J(S S + S S ) where J
1
2
2 3
IV
is the exchange interactions between Mn ions in 1 and S the
i
IV
spin vector for each metal ion (S = 3/2 for Mn with
1
i = 1–3). Application of the van Vleck equation to Kambe’s
i
0
Notes and references
1
1
vector coupling scheme gives an excellent fit of the experi-
mental data from 300 K to 10 K. In order to fit the low
temperature wT product below 10 K, intermolecular interac-
tions (J ) were introduced in the frame of the mean field
1
theory. This approach leads to an excellent least-squares fit
of the experimental data down to 1.8 K and affords the
z Crystal data for 1: C98
3 10
H90Mn N O12, M = 1764.6, dark brown
ꢀ
rectangle, triclinic, P1, a = 11.583(5), b = 12.119(6), c = 17.112(8) A,
˚
3
˚
a = 84.547(10), b= 85.258(9), g = 67.058(10)1, V = 2199.3(18) A ,
Z = 1, T = 201(2) K, 13 737 reflections collected of which 6248 were
independent (Rint = 0.1183), 587 parameters and 54 restraints, R1 =
0
2
0.0868 [based on I 4 2s(I)], wR2 = 0.2347 (based on F and all data).
CCDC 676451.
following parameters: J/kB = +11.5(1) K, g = 2.06(1) and
0
1
2
3
. R. Hage, J. E. Iburg, J. Kerschner, J. H. Koek, E. L. M. Lempers,
R. J. Martens, U. S. Racherla, S. W. Russel, J. Swarthoff, M. R.
P. Van Vliet, J. B. Warnaar, L. van der Wolf and B. Krijnen,
Nature, 1994, 369, 637.
J /k = ꢀ0.06(1) K. This magnetic behavior is consistent with
B
a ground state of S = 9/2 with the first (S = 7/2) and second
T
(S = 5/2) excited states being 34.2 and 69.7 K higher in
. (a) G. Renger, Angew. Chem., Int. Ed. Engl., 1987, 26, 643;
(
energy, respectively.
b) K. Wieghardt, Angew. Chem., Int. Ed. Engl., 1994, 33, 725;
(c) S. Mukhopadhyay, S. K. Mandal, S. Bhaduri and
W. H. Armstrong, Chem. Rev., 2004, 104, 3981.
The field dependence (up to 7 T) of the magnetization at
different temperatures (1.8 to 8 K) has also been measured for
. (a) C. J. Milios, A. Vinslava, W. Wernsdorfer, S. Moggach,
S. Parsons, S. P. Perlepes, G. Christou and E. K. Brechin,
J. Am. Chem. Soc., 2007, 129, 2754; (b) A. M. Ako, I. J. Hewitt,
V. Mereacre, R. Clerac, W. Wernsdorfer, C. E. Anson and
A. K. Powell, Angew. Chem., Int. Ed., 2006, 45, 4926;
1
(inset Fig. 3). The magnetization at 1.8 K saturates above 4 T
at 9.6 m in good agreement with the S = 9/2 ground state.
B
T
The clear saturation of the magnetization suggests the absence
of a significant magnetic anisotropy and also the presence of a
(
c) S. Koizumi, M. Nihei, M. Nakano and H. Oshio, Inorg.
well defined S = 9/2 ground state with well-separated excited
T
Chem., 2005, 44, 1208; (d) C. Dendrinou-Samara, M. Alexiou,
C. M. Zaleski, Jeff. W. Kampf, M. L. Kirk, D. P. Kessissoglou
and V. L. Pecoraro, Angew. Chem., Int. Ed., 2003, 42, 3763.
. R. Sessoli, H.-L. Tsai, A. R. Schake, S. Wang, J. B. Vincent,
K. Folting, D. Gatteschi, G. Christou and D. N. Hendrickson,
J. Am. Chem. Soc., 1993, 115, 1804.
states as shown by the analysis of the wT vs. T data. To further
confirm the S = 9/2 ground state, the M vs. H/T data have
T
4
been fitted to an S = 9/2 Brillouin function that leads to an
T
excellent theory–experiment agreement with a g factor of
2
.11(4). Furthermore, ac susceptibility measurements have
5. A. M. Beauchemin, J. Moran, M. Lebrun, C. Seguin,
E. Dimitrijevic, L. Zhang and S. I. Gorelsky, Angew. Chem., Int.
Ed., 2008, 47, 1410.
also been performed on 1; however, no out-of-phase signal
has been detected and thus, 1 does not exhibit single molecule
magnet (SMM) behavior. To our knowledge there is only one
6
. (a) D. J. Price, S. R. Batten, K. J. Berry, B. Moubaraki and
K. S. Murray, Polyhedron, 2003, 22, 165; (b) B. F. G. Johnson,
A. Sieker, A. J. Blake and R. E. P. Winpenny, J. Chem. Soc.,
Chem. Commun., 1993, 1345; (c) F. Birkelbach, U. Florke,
H. J. Haupt, C. Butzlaff, A. X. Trautwein, K. Wieghardt and
P. Chaudhuri, Inorg. Chem., 1998, 37, 2000.
1
3
linear trinuclear manganese SMM reported to date, which
III
II
comprises a central Mn ion and two peripheral Mn ions
coupled ferromagnetically with an S = 7 spin ground state.
The magnetic anisotropy of the latter SMM originates mainly
7. Z. Xu, L. K. Thompson, D. O. Miller, H. J. Clase, J. A.
K. Howard and A. E. Goeta, Inorg. Chem., 1998, 37, 3620 and
references therein.
III
from the central Mn ion. With this in mind, the DFT
calculations and electrochemical studies on 1 clearly show
III
8
9
. DFT details and references are given in the ESIw.
. (a) BVS calculations for 1: 4.02 and 3.87 for Mn1 and Mn2,
respectively; (b) I. D. Brown and D. Altermatt, Acta Crystallogr.,
Sect. B: Struct. Sci., 1985, B41, 244.
IV
that the central Mn ion is likely to be reduced to a Mn
ion. We therefore believe a reduction of the central Mn2 ion to
III
Mn would introduce magnetic anisotropy to 1, which could
1
0. J. H. van Vleck, The Theory of Electric and Magnetic Suscept-
ibility, Oxford University Press, Oxford, 1932.
then lead to SMM properties. Synthetic efforts to isolate the
IV
one electron reduced Mn Mn Mn complex are currently
underway.
III
IV
1
1. K. Kambe, J. Phys. Soc. Jpn., 1950, 5, 48.
12. Using the mean-field approximation to treat the inter-complex
interactions, the following definition of the susceptibility has been
used:
In conclusion, we have synthesized the first example of a
ferromagnetically coupled exclusively chelating ligand bridged
IV
high-valent trinuclear Mn complex. The presence of bulky
w
trimer
0
zJ
2
2
Ph groups on the specifically designed and synthesized oximate
ligand have a direct influence on the bridging torsion angles;
confirmed by DFT calculations. We believe this could, in turn,
induce the ferromagnetic interactions observed between the
metal centers. Further calculations investigating the influence
of the Mn–N–O–Mn torsion angle on the strength and nature
of the magnetic interactions are currently underway.
w ¼
2
1
ꢀ
wtrimer
Ng m
B
see for example: (a) B. E. Myers, L. Berger and S. J. Friedberg,
J. Appl. Phys., 1969, 40, 1149; (b) C. J. O’Connor, Prog. Inorg.
Chem., 1982, 29, 203.
1
3. R. T. W. Scott, S. Parsons, M. Murugesu, W. Wernsdorfer,
G. Christou and E. K. Brechin, Chem. Commun., 2005, 2083.
2
784 | Chem. Commun., 2008, 2782–2784
This journal is ꢂc The Royal Society of Chemistry 2008