Unexpected diversity and novel features within a family of new azide-bridged MnII complexes of pyridyl/imine ligands

Article abstract of DOI:10.1039/b510701f

A family of tetradentate ligands, [N,N′-bis(pyridine-2-yl)]ethane-1, 2-diamine (L1), [N,N′-bis(pyridine-2-yl)]propane-1,3-diamine (L2) and [N,N′-bis(pyridine-2-yl)]butane-1,4-diamine (L3) has been prepared. These ligands differ only in the number of CH₂ groups separating two pyridyl/imine moieties, however, in reactions with Mn^II and N-3 salts, they produce structurally very diverse solids; [Mn₂(L1) ₃(N₃)]_n(ClO₄)_3n (1), [Mn₂(L2)₂(N₃)₂](PF₆) ₂ (2) and [Mn₂(L3)(N₃)₂] _n(ClO₄)_2n (3). Complexes 1, 2 and 3 consist of azido-bridged magnetically dilute Mn₂ pairs, arranged as 1-D, discrete and 2-D arrays, respectively. In these materials, the connection between Mn centers within the dinuclear entities occurs through mono-1,3-N-3, bis-1,3-N-3 and bis-1,1-N-3 bridges, respectively, for 1, 2 and 3. Bulk magnetic measurements reveal that the coupling within such units is antiferromagnetic (J = -1.8 cm^-1, 1; J = -4.8 cm^-1, 2) and ferromagnetic (J = +1.45 cm^-1, 3), depending on whether the N-3 binding mode is end-to-end (EE) or end-on (EO). The reported values are given according to the convention H = -2JS₁S₂ for the spin-Hamiltonian. the Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b510701f

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Unexpected diversity and novel features within a family of new
azide-bridged Mn complexes of pyridyl/imine ligands
II
a
Tapan K. Karmakar, Guillem Arom ´ı ,* Barindra K. Ghosh, A. Usman, Hoong-Kun Fun, Talal Mallah,
b
a
c
c
d
e
f
U. Behrens, Xavier Solans and Swapan K. Chandra*
ag
Received 27th July 2005, Accepted 17th October 2005
First published as an Advance Article on the web 4th November 2005
DOI: 10.1039/b510701f
A family of tetradentate ligands, [N,N9-bis(pyridine-2-yl)]ethane-1,2-diamine (L1),
[
N,N9-bis(pyridine-2-yl)]propane-1,3-diamine (L2) and [N,N9-bis(pyridine-2-yl)]butane-1,4-
diamine (L3) has been prepared. These ligands differ only in the number of CH groups
separating two pyridyl/imine moieties, however, in reactions with Mn and N 23 salts, they
produce structurally very diverse solids; [Mn (L1) (N )] (ClO 3n (1), [Mn (L2) (N ](PF
and [Mn (L3)(N ) ] (ClO ) (3). Complexes 1, 2 and 3 consist of azido-bridged magnetically
2
II
2
3
3
n
4
)
2
2
3
)
2
6 2
) (2)
2
3 2 n
4 2n
dilute Mn pairs, arranged as 1-D, discrete and 2-D arrays, respectively. In these materials, the
2
2
3
connection between Mn centers within the dinuclear entities occurs through mono-1,3-N ,
bis-1,3-N2 and bis-1,1-N2 bridges, respectively, for 1, 2 and 3. Bulk magnetic measurements
3
3
2
reveal that the coupling within such units is antiferromagnetic (J = 21.8 cm , 1; J = 24.8 cm , 2)
1
21
21
and ferromagnetic (J = +1.45 cm , 3), depending on whether the N 23 binding mode is end-to-end
(
1 2
EE) or end-on (EO). The reported values are given according to the convention H = 22JS S for
the spin-Hamiltonian.
the system depends primarily on the nature of the co-ligand
2
Introduction
–5
used. Thus extended architectures featuring the EO
2,6–9
or the
The use of azide as a bridging ligand and as a medium to
propagate superexchange magnetic interactions between para-
magnetic centers has been widely exploited in materials
EE
systems combining both.
coordination mode have been reported, as well as
4,10–13
Some remarkable examples are
the only homometallic ferrimagnetic 1-D extended system ever
14
prepared and characterized to date or the observation of
1
chemistry. The reasons are the versatility of this ligand to
bind more than one metal in a variety of modes and its
capacity to mediate different types of magnetic coupling. The
two most commonly observed bridging coordination modes
spontaneous resolution of a chiral 2-D crystalline network
15
containing azide and a non-chiral ligand. Surprisingly,
II
however, only very few examples of discrete Mn species
are m
2
-1,1 (end-on, EO) and m
2
-1,3 (end-to-end, EE), which,
containing N 23 have been reported. There are two reports
16,17
most of the time, facilitate ferro- and antiferromagnetic
exchange, respectively. A main goal in the area of molecular
magnetism is the preparation of new discrete or extended
coordination compounds exhibiting large spin numbers by
establishing ferromagnetic interactions between spin carriers.
This has been sought by, for example, combining within
of bis-m -1,1-azido dinuclear complexes
and only two
II 18,19
2
2
examples of m -1,3-azido bridged dimmers of Mn ,
whereas the [Mn–(m -1,3-N ) –Mn] moiety has not yet been
3 2
2
described within a molecular species. We recently reported on
the reactivity of a family of three pyridyl/imine based ligands
II
2
(
L91 to L93 in Scheme 1) with sources of both, Mn and N .
3
II
chemical reactions the ligand azide and the ion Mn , which
Interestingly, in the three cases discrete ferromagnetically
often hosts five unpaired electrons. A large number of systems
with great structural diversity have been prepared in this
manner, where the precise coordination mode of the N 23 group,
the connectivity between the metals and the dimensionality of
a
Department of Chemistry, The University of Burdwan, Burdwan,
7
13104, India. E-mail: dr_swapan@sify.com; Fax: +91 342 2557938
b
Universitat de Barcelona, Departament de Qu ´ı mica Inorg a` nica,
Diagonal 647, Barcelona, 08028, Spain. E-mail: guillem.aromi@qi.ub.es;
Fax: +34 93 4907725;
c
X-Ray Crystallography Unit, School of Physics, Universiti Sains
Malaysia, 11800 USM, Penang, Malaysia
d
Laboratoire de Chimie Inorganique, Universit e´ Paris-Sud, Bat. 420,
UMR CNRS 8613, 91405 Orsay, France
e
Institute Anorgan. und Angew. Chem, Martin Luther King Pl 6, D 2016,
Germany
f
Departament de Cristallografia i Mineralogia, Universitat de
Barcelona, Mart ´ı i Franqu e` s s/n, 08028, Barcelona, Spain
Department of Chemistry, Visva Bharati, Santiniketan, 731235, India
g
Scheme 1 Imine/pyridyl ligands.
2
78 | J. Mater. Chem., 2006, 16, 278–285
This journal is ß The Royal Society of Chemistry 2006

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2
–Mn] core
+
coupled dinuclear species with a [Mn–(m-1,1-N
3
)
2
(0.181 g, 0.5 mmol) and the stirring maintained constant for
0.5 h at room temperature. To the resulting orange solution
was slowly added an aqueous methanolic solution (5 mL
1
7
were obtained. In a previous report, it had been shown
that derivatives of these ligands with n = 0 (see Scheme 1)
and various substituents on the C-atom of the CLN group led
to a variety of 1-D and 2-D Mn –N architectures with
5
interesting magnetic and optical properties. We have now
3 2
CH OH and 1 mL H O) of sodium azide (0.016 g, 0.25 mmol).
II
2
3
The resulting orange solution was left undisturbed to slowly
evaporate. After 3 weeks, an orange crystalline compound
was obtained by filtration, washed with methanol and dried
in vacuum desiccators. The yield was ca. 54% (0.16 g).
1
systematically studied the reactivity of the related series L1 to
II
L3 (Scheme 1) towards mixtures containing Mn and N .
2
3
Surprisingly, the mere replacement of phenyl substituents by
hydrogen atoms on the CLN functionality leads, instead of
ferromagnetic dinuclear molecules, to a variety of structures
with different dimensionality. These include a 1-D polymer
Analytical data calcd for C42H N O12Cl Mn : C, 43.29; H,
42 15 3 2
5.92; N, 18.03, Mn, 9.43%. Found: C, 43.34; H, 5.86; N, 18.11;
Mn, 9.32%.
of (m
the first reported discrete bis-(m
Mn (L2) (N ](PF (2), and a unique 2-D network of bis-
m2-1,1-azido) Mn2 units, [Mn (L3)(N ) ] (ClO ) (3). The
2
-1,3-azido) Mn
2
moieties, [Mn
2
(L1)
3
(N
3
)]
n
(ClO
4
)
3n (1),
[Mn
2
(L2)
2
(N
3 2 6 2
) ](PF ) (2). To an ethanolic solution (10 mL)
II
-1,3-azido) Mn dimer,
of MnCl
2
?4H O (0.050 g, 0.25 mmol) was added ligand L2
2
2
[
2
2
3
)
2
6
)
2
(0.063 g, 0.25 mmol) in methanol (6 mL) over a period of
15 min. To the resulting yellow–orange solution, an aqueous
(
2
3 2 n
4 2n
magnetic properties of this diverse group of compounds have
been investigated.
ethanolic solution (2 mL C H OH and 2 mL H O) of sodium
2 5 2
azide (0.016 g, 0.25 mmol) was added slowly followed by
addition of an aqueous ethanolic solution (2 mL C H OH
2
5
and 2 mL H
2
O) of ammonium hexafluorophosphate (0.082 g,
Experimental
0
.50 mmol). The solution was then filtered, an insoluble
Synthesis
precipitate was discarded and the filtrate was left undisturbed
to slowly evaporate. After 6–7 d, yellowish-green needle-
shaped crystals of 2 were obtained in ca. 80% yield (0.10 g).
Analytical data calcd for C H N P F Mn : C, 36.45; H,
All manipulations were performed under aerobic conditions
using reagents and solvents as received. Ethylenediamine,
3
0
32 14
2
12
2
1
,3-diaminopropane, 1,4-diaminobutane, pyridine-2-carboxal-
3
1
.26; N, 19.84, Mn, 11.11%. Found: C, 36.34; H, 3.18; N,
9.88; Mn, 11.06%.
dehyde and sodium azide were purchased from Lancaster and
used as received. All other solvents and chemicals were of
analytical grade.
2 2 3 2 n 4
[Mn (L3) (N ) ] (ClO )2n (3). A methanolic solution (15 mL)
CAUTION: Perchlorate salts and azido compounds are
potentially explosive especially in the presence of organic
compounds. They must be prepared and handled in small
amounts, and with special care.
of ligand L3 (0.133 g, 0.50 mmol) was slowly added to a
stirred warm (ca. 40 uC) methanolic solution (30 mL) of
Mn(ClO ) ?6H O (0.181 g, 0.50 mmol). The stirring and
4
2
2
heating was maintained for 30 min and then an aqueous
methanolic solution (5 mL CH OH and 1 mL H O) of sodium
3
2
Preparation of ligands
azide (0.033 g, 0.50 mmol) was added slowly to the mixture
and the stirring maintained for another further 15 min. The
resulting solution after filtration was left to stand at room
temperature and it was again filtered after 2 d. After two
weeks, yellow needle-shaped crystals of 3 were obtained
from the filtrate in ca. 35% yield (0.080 g). Analytical
data calcd for C H N O ClMn: C, 41.53; H, 3.92; N,
[
N,N9-Bis(pyridine-2-yl)]ethane-1,2-diamine (L1), [N,N9-bis-
(
pyridine-2-yl)]propane-1,3-diamine (L2) and [N,N9-bis-
pyridine-2-yl)]butane-1,4-diamine (L3) were prepared via
(
condensation reactions similar to those previously reported
1
7
for a similar ligand. Ethylenediamine (0.60 g, 10 mmol, L1),
,3-diaminopropane (0.74 g, 10 mmol, L2) or 1,4-diaminobu-
tane (0.88 g, 10 mmol, L3) and pyridine-2-carboxaldehyde
2.14 g, 20 mmol) were refluxed in 20 mL dehydrated ethanol
1
1
6
18
7
4
21.19, Mn, 11.87%. Found: C, 41.44; H, 3.86; N, 21.11; Mn,
11.73%.
(
for 6–9 h. The respective ligands were isolated after evaporat-
ing the solvent and were recrystallized from EtOH–H O (5 : 1
2
Physical measurements. Elemental analyses for carbon,
hydrogen and nitrogen were performed using a Perkin Elmer
2400II elemental analyzer. Manganese contents were deter-
mined by titrimetric methods. Infrared spectra (4000–
of volume ratio). The products were isolated as brown
waxy materials after drying in vacuum over P O . The yields
4
10
were 2.30 g (ca. 97%), 2.40 g (ca. 95%) and 2.85 g (ca. 94%)
for L1, L2 and L3, respectively. Analytical data calcd
for C H N (L1): C, 70.57; H, 5.92; N, 23.51. Found: C,
2
1
400 cm ) were recorded on KBr pellets (at 298 K) using a
JASCO FT/IR-420 spectrometer. The magnetic studies were
carried out on polycrystalline samples using a Quantum
Design MPMS SQUID magnetometer operating in the
300–2 K temperature range and 0–5.5 Tesla. Susceptibility
measurements were performed within applied fields of 3000 Oe
and for compound 3 below 100K also at 500 Oe in order to
avoid saturation effects. Pascal’s constants were utilized to
estimate diamagnetic corrections, the value in each case being
subtracted from the experimental susceptibility data to give the
1
4
14
4
7
0.46; H, 5.81; N, 23.39%. Analytical data calcd for
C H N (L2): C, 71.40; H, 6.39; N, 22.20. Found: C, 71.46;
1
5
16
4
H, 6.31; N, 22.11%. Analytical data calcd for C16
C, 72.15; H, 6.81; N, 21.04. Found: C, 72.06; H, 6.69; N,
1.09%.
18 4
H N (L3):
2
[
Mn
of ligand L1 (0.179 g, 0.75 mmol) was added dropwise to
a stirred methanolic solution (12 mL) of Mn(ClO ?6H
2 3 3 n 4
(L1) (N )] (ClO )3n (1). A methanolic solution (5 mL)
4
)
2
2
O
molar magnetic susceptibility (x
M
).
This journal is ß The Royal Society of Chemistry 2006
J. Mater. Chem., 2006, 16, 278–285 | 279

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Table 1 Crystallographic data for complexes 1, 2 and 3
1
2
3
Formula
C
H
42 42
N
15
O
12Cl
3
Mn
2
C
H
30 32
14
N F
12Mn
2
P
2
C
16 18 7 4
H N O ClMn
Crystal size/mm
Fw/g mol
Space group
0.50 6 0.36 6 0.26
1165.12
C2/c
0.48 6 0.30 6 0.20
988.52
0.40 6 0.20 6 0.10
462.76
Pbcn
2
1
P2
1
/c
˚
a/A
19.5706(3)
20.7246(5)
14.1497(3)
90
118.309(1)
90
5052.65(18)
4
1.540
9.4938(1)
16.1284(2)
12.8164(1)
90
94.5494(2)
90
1956.26(4)
2
1.678
12.5606(6)
17.2500(8)
18.1617(9)
90
90
90
3935.1(3)
8
1.562
˚
b/A
˚
c/A
a (deg)
b (deg)
c (deg)
3
˚
V/A
Z
2
3
r
calc/g cm
m (Mo Ka)/mm
min/hmax (deg)
Ind. reflections
2
1
0.71073
0.830
0.846
h
2.56/28.31
6147 (Rint = 0.1317)
2503
0.0877
0.2214, 0.876
2 2
2.03/28.00
4680 (Rint = 0.1014)
3319
0.0453
0.0980, 0.927
2.24/27.50
4509 (Rint = 0.0592)
3109
0.0585
0.1502, 1.025
Obs. reflections [I . 2.0s(I)]
a
R
Rw , S
b
a
b
2
2 2 1/2
0 c 0 0 c 0
R = S (||F | 2 |F ||)/S|F |; wR = {S w(|F | 2 |F | ) /S w(|F | ) } .
X-Ray crystallography. Single-crystals suitable for X-ray
quantitative. We were interested in investigating the effect that
a seemingly irrelevant modification on the ligand, namely
removal of the phenyl substituent from the C atom of the CLN
function, would have on the structure of the solids resulting
crystallographic analysis{ were selected following examination
under a microscope. X-Ray data of 1 and 2 were collected at
2
93 K with a Siemens SMART CCD diffractometer. X-Ray
II
2
data of 3 were collected at 200 K on a Bruker-AXS SMART
from reaction with the Mn /N3 system. The family of ligands
APEX/CCD diffractometer. All data collections were carried
˚
out using Mo Ka radiation (l = 0.71073 A) in an v 2 2h scan
L9 had produced in all cases discrete bis-EO-azido bridged
Mn I2 I complexes. However, it has been shown in the past
that small changes on a co-ligand, L, may lead to dramatic
modifications in structure and properties of the resulting
17
mode. 14478 reflections were collected for 1, of which 6147
were independent (Rint = 0.1317); 13359 reflections for 2, of
which 4680 were independent (Rint = 0.1014) and 44314
II
2
14,25–27
Mn /L/N3 assemblies.
Reactions of L1 with Mn(ClO
water mixture using various molar ratios yielded invariably
an orange crystalline compound formulated [Mn (L1) (N )]
ClO 3n (1) on the basis of elemental analyses, infrared (IR)
reflections for 3, of which 4509 were independent (Rint
.059). The intensity data were corrected for Lorentz and
polarization effects. Empirical absorption corrections were
=
4 2 3
) and NaN in a methanol–
0
2
3
3 n
-
2
0
made based on psi-scans for 1 and 2 and using the SADABS
(
4
)
2
program for 3. The structures were solved by direct methods
1
spectroscopy and X-ray crystallographic data. Compound 1
consists of a 1D chain of mono-EE-N 23 bridged Mn 2I I pairs
linked by L1 (see below), thus, the absence of the phenyl
substituent induces the formation of a completely different
architecture when compared with the related reaction with L91.
When the Ph group is present, the compound formed is a
discrete bis-EO-N 32 bridged Mn I2 I complex. There is not an
apparent reason preventing the formation of such a molecular
complex with L1, thus, the polymeric structure must be more
stable than the discrete complex and the formation of the
former with L91 might have been prevented for steric reasons
2
and the structure solution and refinement was based on |F| .
All non-hydrogen atoms were refined with anisotropic
displacement parameters. Even when located in the Fourier
map, hydrogen atoms were placed in calculated positions and
given isotropic U values 1.2 times that of the atom to which
they are bonded. Acetonitrile hydrogen atoms were neither
located nor placed in calculated positions. The atomic
scattering factors and anomalous dispersion terms were taken
2
2
from the standard compilation. All crystallographic calcula-
24
2
tions were carried out using SHELX-97 and ORTEP-3
3
program packages. The crystal data and data collection details
are gathered in Table 1.
(
in particular at the ligand molecule that acts as link between
Mn I2 I pairs, see below). The formation of 1 was optimized by
adjusting the molar ratio of the reagents to the composition of
the final product. This reaction can be described by eqn 1.
Results and discussion
Synthesis
2
nMn(ClO ) + 3nL1 + nNaN A
4
2
3
(
1)
[Mn
2
(L1)
3
(N
3
)]
n
(ClO
4
)
3n + nNaClO
4
Ligands L1, L2, and L3 were prepared by the analogous
synthetic method to that used for the previously reported
7
series of ligands L91 to L93 (Scheme 1). As for the published
1
A reaction similar to that above was explored with L2,
which features a propylene group in between both imine
system, the yields obtained for L1 to L3 were essentially
moieties. In particular, L2 was mixed with MnCl
2 3
and NaN in
the presence of NH PF . This led to the formation of a
6
4
{ CCDC reference numbers 279584–279586. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/b510701f
yellowish-green, needle-shaped crystalline product which was
2
80 | J. Mater. Chem., 2006, 16, 278–285
This journal is ß The Royal Society of Chemistry 2006

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identified by single crystal X-ray crystallography as [Mn
2
(L2)
2
-
Table 3 Selected interatomic distances ( A˚ ) and angles (deg) for
compound 2; ‘a’ refers to symmetry operation [1 2 x, 2y, 1 2 z]
(
N
3
)
2 6 2
](PF ) (2). A similar reaction with the corresponding L9
ligand (L92, Scheme 1) had also led to a dinuclear molecular
species, however, with N 23 bound as end-on. Complex 2 is in
Mn(1)–N(1)
Mn(1)–N(3)
Mn(1)–N(5)
N(5)–N(7)a
2.309(2)
2.281(2)
2.244(2)
1.169(3)
5.569(1)
124.71(7)
83.03(8)
79.62(7)
132.43(8)
72.20(7)
82.03(7)
120.27(8)
127.2(2)
177.5(2)
Mn(1)–N(2)
Mn(1)–N(4)
Mn(1)–N(6)
N(6)–N(7)
N(1)–Mn(1)–N(2)
N(1)–Mn(1)–N(4)
N(1)–Mn(1)–N(6)
N(2)–Mn(1)–N(4)
N(2)–Mn(1)–N(6)
N(3)–Mn(1)–N(5)
N(4)–Mn(1)–N(5)
N(5)–Mn(1)–N(6)
Mn(1)–N(6)–N(7)
2.246(2)
2.289(2)
2.244(2)
1.171(3)
72.58(7)
94.57(7)
142.61(8)
134.07(7)
89.85(7)
145.60(8)
87.01(8)
85.52(8)
139.8(2)
II
fact the first reported discrete system of Mn featuring the bis-
2
3
(
m
2
-1,3-N ) bridge. This raises the intriguing question as to
…
Mn(1) Mn(1)a
what defines the preference of EO versus the EE binding
mode. In the present example, the steric argument is hardly
applicable, since the less sterically demanding mode arises
with the least encumbered ligand. Another difference in this
equation that might seem unimportant is the use of a different
N(1)–Mn(1)–N(3)
N(1)–Mn(1)–N(5)
N(2)–Mn(1)–N(3)
N(2)–Mn(1)–N(5)
N(3)–Mn(1)–N(4)
N(3)–Mn(1)–N(6)
N(4)–Mn(1)–N(6)
Mn(1)–N(5)–N(7)a
N(6)–N(7)–N(5)a
II
source of Mn together with ‘‘external’’ addition of the
2
counter ion PF
6
. These factors can dramatically modify the
nature of the isolated product, sometimes for solubility
reasons, or because they influence in particular manners the
formation of by-products that might not even be observed or
identified. Eqn 2 describes the reaction leading to 2.
˚
Table 4 Selected interatomic distances (A) and angles (deg) for
compound 3; ‘a’ refers to symmetry operation [1 2 x, y, 3/2 2 z]
Mn(1)–N(1)
Mn(1)–N(6)
Mn(1)–N(4)a
Mn(2)–N(1)
Mn(2)–N(7)
Mn(2)–N(5)a
N(1)–N(2)
2.249(2)
2.244(3)
2.280(3)
2.218(2)
2.261(3)
2.245(3)
1.210(4)
3.471(1)
157.3(1)
77.37(9)
97.42(9)
84.68(9)
97.78(9)
97.78(9)
157.3(1)
73.69(9)
94.41(9)
170.0(1)
73.99(9)
93.0(1)
Mn(1)–N(4)
Mn(1)–N(1)a
Mn(1)–N(6)a
Mn(2)–N(5)
Mn(2)–N(1)a
Mn(2)–N(7)a
N(2)–N(3)
2.280(3)
2.249(2)
2.244(3)
2.245(3)
2.218(2)
2.261(3)
1.144(4)
2
MnCl + 2 L2 + 2 NaN + 2 NH PF A
2
2
3
4
6
(
2)
[Mn
2 3 2 6 2 4
(L2) (N ) ](PF ) + 2 NaCl + 2 NH Cl
When an analogous butylene ligand, L3 is allowed to react
with Mn(ClO and NaN in methanol–water mixtures,
needle shaped crystals of a product are obtained, the identity
of which was established as [Mn (L3) (N (ClO 2n (3) by
means of single crystal X-ray diffraction. Complex 3 comprises
Mn units featuring two m
-1,1-N2 bridges such as in the
4
)
2
3
…
Mn(1) Mn(2)
N(1)–Mn(1)–N(4)
N(1)–Mn(1)–N(1)a
N(1)–Mn(1)–N(6)a
N(4)–Mn(1)–N(1)a
N(4)–Mn(1)–N(6)a
N(6)–Mn(1)–N(4)a
N(1)a–Mn(1)–N(4)a
N(4)a–Mn(1)–N(6)a
N(1)–Mn(2)–N(7)
N(1)–Mn(2)–N(5)a
N(5)–Mn(2)–N(7)
N(5)–Mn(2)–N(5)a
N(7)–Mn(2)–N(1)a
N(7)–Mn(2)–N(7)a
N(1)a–Mn(2)–N(7)a
N(1) –Mn(1)–N(6)
N(1)–Mn(1)–N(4)a
N(4)–Mn(1)–N(6)
N(4)–Mn(1)–N(4)a
N(6)–Mn(1)–N(1)a
N(6)–Mn(1)–N(6)a
N(1)a–Mn(1)–N(6)a
N(1)–Mn(2)–N(5)
N(1)–Mn(2)–N(1)a
N(1)–Mn(2)–N(7)a
N(5)–Mn(2)–N(1)a
N(5)–Mn(2)–N(7)a
N(7)–Mn(2)–N(5)a
N(1)a–Mn(2)–N(5)a
N(5)a–Mn(2)–N(7)a
94.86(9)
84.68(9)
73.69(9)
115.8(1)
97.42(9)
164.3(1)
94.86(9)
94.6(1)
78.68(9)
98.92(9)
170.0(1)
94.0(1)
2
2
3
)
2
]
n
4
)
2
2
3
compound previously prepared with L93. The recently
reported complex however is a discrete molecule, with each
of the multidentate ligands wrapped around one manganese
center. In complex 3, each Mn atom is linked by two L3
2
ligands to two other Mn centers from different Mn units,
forming a 2-D network (see below). The optimal yield was
obtained using equimolar amounts of all reagents, as
summarized in eqn 3.
98.92(9)
162.8(1)
94.41(9)
94.0(1)
94.6(1)
73.99(9)
2
Mn
nMn(ClO ) + 2nL3 + 2nNaN A
4
2
3
(
3)
[
2
(L3)
2
(N
3
)
2
]
n
(ClO
4
)
2n + 2nNaClO
4
data for all three complexes are in Table 1, whereas selected
geometric parameters are listed in Tables 2, 3 and 4.
The reactivity just described shows that it is important to
explore the diversity that might be brought by small changes
on a particular ligand system. This has allowed unveiling
structural moieties that had never been observed before.
[
2 3 3 n 4
Mn (L1) (N )] (ClO )3n (1). Complex 1 consists of an
II
infinite chain (Fig. 1) of equivalent Mn ions in which
end-to-end N 23 links alternate with a bridging form of the L1
ligand, which is attained after adopting a peculiar transoid
Description of structures
The identity of complexes 1, 2 and 3 could only be established
through the determination of their molecular structure by
means of single-crystal X-ray diffraction. Crystallographic
˚
Table 2 Selected interatomic distances (A) and angles (deg) for
compound 1; ‘b’ refers to symmetry operation [1/2 2 x ,1/2 2 y, 2z]
Mn(1)–N(1)
Mn(1)–N(3)
Mn(1)–N(5)
N(1)–Mn(1)–N(2)
N(1)–Mn(1)–N(4)
N(2)–Mn(1)–N(3)
N(2)–Mn(1)–N(5)
N(3)–Mn(1)–N(5)
Mn(1)–N(5)–N(6)
2.312(4)
2.280(5)
2.187(5)
71.1(2)
143.3(2)
110.0(2)
87.4(2)
Mn(1)–N(2)
Mn(1)–N(4)
N(5)–(6)
N(1)–Mn(1)–N(3)
N(1)–Mn(1)–N(5)
N(2)–Mn(1)–N(4)
N(3)–Mn(1)–N(4)
N(4)–Mn(1)–N(5)
N(5)–N(6)–N(5)b
2.250(6)
2.258(4)
1.120(5)
93.9(1)
98.2(2)
145.5(2)
73.5(2)
88.4(2)
180.00
2 3 3 n 4
Fig. 1 PLATON representation of [Mn (L1) (N )] (ClO )3n (1),
161.3(2)
162.9(5)
emphasizing the connectivity within the infinite chain. Hydrogen
atoms have been omitted. Unique non-carbon atoms are labelled.
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conformation and chelating two metals. The coordination
around each metal is completed by the four N-donors of
another L1 moiety, resulting in a [MnN ] chromophore. Two
7
of the Mn–N bonds however, are very long (2.609(6) and
˚
.619(4) A) and correspond to the pyridyl N-atoms of the non-
2
bridging L1. The remaining bond distances span the range
…
.187(5) to 2.312(4). The intrachain first neighbor Mn Mn
2
˚
distances are 6.547(1) and 7.650(1) A for the N and L1
2
3
Scheme 2
separated Mn pairs, respectively, whereas the shortest distance
˚
between metals of different chains is 8.952 A. Since the azide
reported example of a discrete molecular system showing the
ligands contain a crystallographic center of symmetry on their
central atom, the N5–N6–N5b angle is exactly 180u. Each
chain of 1 runs along the c direction of the unit cell and is
linked to two other such 1-D entities located on opposite sides.
This link occurs through a network of p–p interactions taking
place between the pyridyl rings of the non-bridging L1 ligands
that interdigitate with their neighboring counterparts through-
out the entire length of the chains, forming sheets of weakly
bound 1-D polymers that spread on the a–c plane. These sheets
are stacked on top of each other and separated by layers of
double bridged unit [Mn(m-1,3-N ) Mn].
2
3
[Mn (L3) (N ) ] (ClO ) (3). Complex 3 consists of a 2-D
2 2 3 2 n 4 2n
II
2
extended structure formed by bis-(m -1,1-N ) bridged [Mn ]
2
3
pairs (Fig. 3) interconnected by bridging L3 ligands (Fig. 4).
2
ClO4 ions. Compounds exhibiting only one type azide bridge
in form of non-interacting [Mn(m-1,3-N
3
)Mn] moieties have
2
been observed only once within a coordination polymer or as
8
1
8,19
discrete molecules.
[
Mn (L2) (N ) ](PF ) (2). Complex 2 (Fig. 2) is a centro-
6 2
2
2
3 2
II
symmetric dinuclear complex of two Mn centers, each
chelated by one tetradentate L2 ligand, and both mutually
linked by two end-to-end N2 ligands. The metals are thus
3
hexacoordinated and feature a coordination geometry closer to
trigonal prismatic than to octahedral, with Mn–N distances
˚
that spread over a narrow range of 2.244(2) to 2.309(2) A and
…
a Mn Mn separation of 5.569(1) A. The positive charge is
˚
2
compensated by PF
6
anions. The cycle formed by the
[
2
3 2
Mn (N ) ] core exhibits a chair conformation. The folding
of this ‘‘chair’’ can be gauged by the angle, d, formed between
the N–Mn–N plane and the idealized plane containing the six
N atoms (Scheme 2), which in complex 2 is 16.5(1)u. The
molecules of 2 interact in two directions of the space through
p-stacking interactions of the pyridyl rings from L2, so that
2 3 2 n 4
Fig. 3 PovRay representation of [Mn (L3)(N ) ] (ClO )2n (3) empha-
sizing the unique dinuclear units within the infinite sheets. Hydrogen
atoms have been omitted. Unique non-carbon atoms are labelled.
they are disposed in form of infinite sheets accross the b–c
2
plane, separated by PF
6
ions. Complex 2 represents the first
Fig. 2 PovRay representation of [Mn
2
(L2) (N
2
3
)
2
](PF
6
)
2
(2).
2 3 2 n 4
Fig. 4 PLATON representation of [Mn (L3)(N ) ] (ClO )2n (3)
Hydrogen atoms have been omitted. Unique non-carbon atoms are
labelled.
emphasizing the connectivity between dinuclear entities within the
infinite chain.
2
82 | J. Mater. Chem., 2006, 16, 278–285
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field of 500 Oe were performed below 100 K in order to avoid
the saturation effects that may appear in high-spin systems.
M
The results are shown in Fig. 5 in form of x T vs. T plots,
where xM is the molar paramagnetic susceptibility per two
manganese ions.
M
The product x T near room temperature for compound 1 is
3
.24 cm K mol , slightly below the value corresponding
21
8
II
to two uncoupled high-spin Mn ions (S = 5/2) with g =
3 21
(8.75 cm K mol ). This value decreases slowly as the
2
temperature is decreased, the drop becoming steeper at lower
temperatures. This clearly indicates the existence of antiferro-
magnetic coupling between the Mn centers within the chain.
II
The distance between adjacent Mn being too large for
considering direct overlap between metallic magnetic orbitals,
the coupling must be taking place via superexchange coupling
pathways. While end-to-end N 23 bridges are well known to
mediate efficiently magnetic interactions between metallic
centers, the coupling via an ethylenediimine bridge such as
that provided by L1 in this complex is expected to be extremely
weak, or nil. Thus, in order to describe quantitatively the
magnetic behavior, a model was used that considered the
M 2
Fig. 5 Plots of x T vs. T per mol of [Mn ] dimeric moiety of
complexes 1, 2 and 3. Solid lines are, respectively, the best fits to the
experimental data using the appropriate model in each case (see text
for details).
2
system as an ensemble of magnetically exchanged Mn pairs,
Each metal of these dinuclear entities is chelated by two L3
isolated from each other, therefore with the exchange
ligands via two of four N-donors per ligand, and bridged to
described by the Heisenberg spin-Hamiltonian H = 22JS S ,
1
2
two other Mn ions from adjacent dimers. Thus, each [Mn
2 2
(m -
1 2
where S = S = 5/2, J being the exchange coupling constant.
1
3 2
,1-N ) ] core is linked by four L3 ligands to a total of four
The van Vleck equation expressing the temperature depen-
dence of the magnetic susceptibility under such model is given
29
equivalent such units. The charge in this system is compen-
sated by two molecules of ClO2 per dimer of Mn. The metals
4
in eqn 4.
in this dimer are inequivalent, exhibit a distorted octahedral
environment and are located on a crystallographic C2 axis,
featuring Mn–N bond distances in the range of 2.218(2) to
By using this equation, a very good agreement between the
simulated curve (solid line on plot 1, Fig. 5) and the experi-
mental data was obtained. The parameters obtained from the
˚
.261(3) A. The atoms of the [Mn
…
2
2
N
2
] core are strictly
…
2
1
fitting procedure were J = 21.8 cm and g = 2.00 (goodness
2
of the fit, R = 0.999) . Similar weak antiferromagnetic
contained on a plane with Mn Mn and N N distances
˚
of 3.471(1) and 2.812(3) A, and with a Mn–N–Mn angle of
II
2
coupling has been observed for the only two isolated [Mn ]
2
1
02.0(1)u. The two N3 groups point away from opposite sides
complexes reported to date, exclusively bridged by the m -1,3-
2
of this plane and the inclination of the azide groups with
respect to the plane can be gauged by the angle N(1)–N(1a)–
N(3a), which equals 37.7(1)u. The shortest intermetallic
2
18,19
N3 moiety.
Near room temperature, compound 2 exhibits a value for
21
3
M
x T of 7.68 cm K mol , slightly lower than 1. The curve
˚
distance involving different dimers is 8.125(1) A. The 2-D
drops with lowering temperatures in a more pronounced
manner than for the previous complex, which shows that the
II
antiferromagnetic coupling between Mn ions in 2 is stronger
network formed by this compound spans the plane containing
crystallographic axes a and b and these sheets stack along the
c direction. Whereas many polymeric solids exist exhibiting
than in 1. Since this complex consists of discrete dinuclear
molecules, the magnetic coupling within the molecule can be
described by the spin-Hamiltonian H = 22JS S , and eqn 4 is
the [Mn ] fragment bridged by two end-on azide groups, it is
2
the first time that this moiety is observed within an extended
network as independent and distant pairs.
1
2
thus appropriate for fitting the experimental data and
quantitatively assessing the extent of this coupling. A fit was
thus performed (solid line on plot 2 of Fig. 5) yielding best
Magnetic properties
2
1
2
The magnetic properties of compounds 1 to 3 were examined
with a SQUID magnetometer in order to ascertain whether
the structural diversity observed in this family of materials
was followed by the magnetic behavior. The magnetization
of all compounds was measured under a constant magnetic
field of 3000 Oe in the 2–290 uC temperature range. For
compound 3 additional measurements within an applied
fit parameters of J = 24.8 cm and g = 2.01 (R = 0.997).
This interaction remaining rather weak, it is interesting
to note that it is significantly more efficient than that
observed in 1. A plausible reason for this is that complex 2
features two exchange pathways (one per bridging ligand)
as compared to only one in 1. Since complex 2 is the
first reported of its kind, it is not possible to compare
ꢀ
kT
ꢁ
ꢀ
ꢁ
ꢀ
ꢁ
ꢁ
ꢀ
ꢁ
ꢁ
{
10J
{18J
{24J
{28J
Ng2b2
kT
ꢀ
kT
kT
kT
330z180e
z84e
z30e
z6e
x ~
M
|
ꢀ
ꢁ
ꢁ
ꢀ
ꢀ
ꢀ
ꢁ
(4)
3kT
{10J
{18J
{24J
{28J
{30J
kT
kT
kT
kT
1
1z9e
z7e
z5e
z3e
ze
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J. Mater. Chem., 2006, 16, 278–285 | 283

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the magnitude of the coupling with any previously reported
II
value. However, the [Mn (m
In these equations, y = J/kT and the rest of the terms have
II
Mn ] moiety has been
2
-1,3-N
3
)
2
their usual meaning. The parameters obtained from a fit using
21
2
this model are J = +1.46 cm , g = 1.97 and zJ9 = 20.03 cm
1
often found within 1-D polymeric structures and in several
cases the magnetic within this unit has been described
quantitatively. The calculated J values have been correlated
to the dihedral angle, h, formed between the planes N–Mn–N
2
(R = 0.993). Even if the models used are approximate,
both of the above fits are rather satisfactory and provided
essentially identical values of their common parameters,
however, the latter was slightly better and was therefore
the one used in the representation for Fig. 5. These results
corroborate the established fact that the N 23 ligand mediates
(almost exclusively) ferromagnetic coupling between metals
when bound in the end-on mode. We recently reported a
series of discrete dinuclear complexes with core very similar to
the magnetic pairs present in 3. All were ferromagnetically
coupled and a correlation between J and the Mn–N–Mn
angle was observed. The coupling in 3 (with Mn–N–Mn of
101.98u) shows a similar trend as in that family, but it is
significantly off the reported correlation. Additional data
points are necessary in order to understand the subtleties of
this possible relationship.
(
N-atoms bound to Mn) and the idealized plane of the six
3
azido atoms (see Scheme 2). The reported J values fall
0
2
1
within the range of 3.0 to 7.7 cm
1 2
(in the H = 22JS S
convention) for d angles between 6.9 and 38.0u, and the
coupling decreases as the angle gets larger. The angle in
complex 2 is 16.5(1)u and J falls approximately in the region
expected from this magneto-structural study. Thus, despite
being a discrete system, complex 2 follows similar trends in
terms of magnetic coupling as 1D extended systems with the
same exchange pathways between spin carriers.
For complex 3 the product x T at room temperature is
M
3 21
.73 cm mol K , slightly higher than for the previous two
8
complexes, although still near the value expected for two
II
independent Mn ions with S = 5/2. Contrary to both systems
M
above, the value of x T increases as the temperature is
Conclusions
3
21
lowered, to reach a maximum of 13.25 cm mol K at 9 K.
II
The solid-state structural diversity of the series of Mn azide-
Upon further cooling, a sudden drop is observed, down to
3 21
2.12 cm mol K at 2 K. These results show that within this
bridged compounds synthesized and characterized with the
pyridyl/imine ligands L1, L2 and L3 is in contrast with the
monotony seen for the group of complexes previously formed
with the analogous set of ligands L91, L92 and L93. This
underscores the importance of exploring the effect that small
changes on the co-ligand, L, have on the structure and
1
complex the magnetic exchange is ferromagnetic. The drop at
lower temperatures can be due to zero field splitting (ZFS),
intermolecular antiferromagnetic interactions, or both. These
two factors have the same effect on the magnetic susceptibility
and very often are of the same order of magnitude, thus it is
not possible to model reliably both at the same time.
Approximate models contemplating the influence of either
one or the other effect were considered. First, the van Vleck
equation was constructed (eqn 5), as arising from the spin-
II
2
properties of the resulting assembly from Mn /N /L reaction
3
systems. Thus, three very different compounds have been
prepared by using three ligands differing only in the number of
2
CH groups separating two pyridyl/imine moieties. These
compounds consist of discrete ([Mn (L2) (N ) ](PF ) , 2),
6 2
2
2
3 2
Hamiltonian H = gbB(S
1 2 1 2 2 1
+S ) 22JS S (S = S = 5/2)
1
-D ([Mn
(ClO
and featuring magnetically isolated Mn
Of particular interest are compounds 2 and 3, the first
synthesized discrete Mn complex incorporating the bis-(m
2
(L1)
2n, 3) assemblies, respectively, all unprecedented
azide bridged pairs.
3 3 n 4 3n 2 2
(N )] (ClO ) , 1) and 2-D ([Mn (L3) -
where the energies of the ground state (ST = 5) had been
(
N
3
)
2
]
n
4
)
modified according to an axial ZFS term of the form HZFS
D[S2 2 S (S + 1)/3].
=
29
2
Tz
T
T
In eqn 5, x = D/kT and y = J/kT. A fit of the data using this
2
model provided the parameters J = +1.45 cm , g = 1.97 and
1
-
2
2
2
D = 0.04 cm (R = 0.992).
1
2
1,3-N ) bridging moiety and the first system featuring
3
magnetically dilute [Mn (m-EE-N ) ] units as a part of an
2 3 2
Ng2b2
extended coordiantion network. Magnetic studies corroborate
xM
~
|
(5)
3kT
(
5)
the ferromagnetic nature of the coupling mediated by EO
2
e9xz24e6xz54e z96e{6xz150e{15xz180e{10yz84e{18yz30e{24yz6e{28y
x
6
e10xz2e9xz2e6xz2e z2e{6xz2e{15xz9e{10yz7e{18yz5e{24yz3e{28yze{30y
x
N3 bridges (complex 3) and the antiferromagnetic coupling
existing between EE N 23 bridged metal pairs (complexes 1
In the second model, a term for the interaction between
and 2).
molecules of the form zJ9<S >S was included in the spin-
Tz Tz
Hamiltonian, according to the molecular field approximation
J9 being the interaction between molecules, z the number of
first neigbours and <STz> the mean value of the operator
Acknowledgements
(
This work was supported by grants from the Department
of Science and Technology (Grant No. SR/S1/IC-22/2003),
Government of India and the Council of Scientific and
Industrial Research (Grant No. 01(1836)/03/EMR-II),
New Delhi. T. K. K. thanks University Grants Commission,
New Delhi, for a teacher fellowship. H. K. F thanks the
Malaysian Government and Universiti Sains Malaysia for
research grant R&D No. 305/PFIZIK/610961. GA thanks the
Spanish Ministry of Science and Technology for a ‘‘Ram o´ n y
Cajal’’ research contract.
2
9
STz).
The expression that results for the temperature
dependence of the paramagnetic susceptibility is below
(
eqn 6 and 7).
2
4
3
5
Ng2b2
FðJ,TÞ
x ~
M
|
ꢀ
ꢁ
(6)
(7)
0
3k
zJ F(J,T)
T{
=
3k
3
30e30yz180e20yz84e12yz30e z6e
6y
2y
F(J,T)~
6y
2y
1
1e30yz9e20yz7e12yz5e z3e z1
2
84 | J. Mater. Chem., 2006, 16, 278–285
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1
1
1
1
4 M. A. M. Abu-Youssef, A. Escuer, M. A. S. Goher, F. A. Mautner,
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Soc., 2003, 125, 4984.
6 R. Cortes, J. L. Pizarro, L. Lezama, M. I. Arriortua and T. Rojo,
Inorg. Chem., 1994, 33, 2697.
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