Formation of a molecular spin ladder induced by a supramolecular cation structure

Article abstract of DOI:10.1039/b110368g

A novel molecular spin ladder structure of a nickel dithiolate complex has been constructed using a supramolecular cation composed of anilinium and 18-crown-6.

Full text of DOI:10.1039/b110368g

Formation of a molecular spin ladder induced by a supramolecular
cation structure
Sadafumi Nishihara, Tomoyuki Akutagawa, Tatsuo Hasegawaab and Takayoshi Nakamura*ab
a
abc
a
Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan
Research Institute for Electronic Science, Hokkaido University, Sapporo 060-0812, Japan.
b
E-mail: tnaka@imd.es.hokudai.ac.jp
PRESTO, Japan Science and Technology Corporation (JST), Kawaguchi, Japan
c
Received (in Cambridge, UK) 13th November 2001, Accepted 23rd January 2002
First published as an Advance Article on the web 11 February 2002
A novel molecular spin ladder structure of a nickel dithiolate
complex has been constructed using a supramolecular cation
composed of anilinium and 18-crown-6.
18-crown-6 with average hydrogen-bond lengths of 2.92 and
2.94 Å. The supramolecular cations are further stacked
alternately with CH
spool array along the b-axis. Pairs of [Ni(dmit)
are embedded between the 18-crown-6 molecules, not only on
3
CN molecules forming a one-dimensional
2
2
] molecules
Even-leg ladders of S = 1/2 Heisenberg antiferromagnetic spins
1
2+
are in a resonating valence bond state at low-temperature and
[p-Ph(NH
3
)
2
]
3
but also on CH CN, and are stacked along the
2
have a finite spin-gap. Recently, open-shell organic molecules
c-axis forming a one-dimensional array as in the case of salt
1.
have been utilized for constructing two-leg spin ladders, and the
crystals with a relatively high spin-gap temperature (D) have
We evaluated the transfer integrals between [Ni(dmit)
2
]2
3
been reported. Herein, we present a supramolecular approach
using extended Hückel molecular orbital calculations to
2
22
8
to construct molecular spin ladders. The [Ni(dmit)
2
]
(dmit
estimate the magnetic exchange interactions in the crystals.
=
2-thioxo-1,3-dithiole-4,5-dithiolate) having an S = 1/2 spin
The transfer integral within the dimer of salt 1, which
corresponds to the ladder rung direction (A–AA), is tA = 45.3
meV, and that in the ladder leg direction (A–A and AA–AA) is t
= 15.9 meV. Since the magnetic exchange interaction is
proportional to the square of the transfer integral, the ratio of
exchange interaction between the ladder rung (JA) and ladder leg
(J) directions (JA/J) of salt 1 was estimated to be 8.1. On the
is an excellent building block for molecular magnetic materials.
Since the interacting spins are located in singly occupied
molecular orbitals, magnetic properties can be modified by
4
regulating intermolecular interactions in the crystals. We have
2
been studying the control of [Ni(dmit)
2
]
arrangements by
supramolecular cations composed of inorganic ions and crown
5
ethers. In the present study, we have constructed dimer chains
other hand, there are two kinds of dimer pairs in salt 2, although
2
2
of [Ni(dmit)
2
] by using newly designed supramolecular cat-
they are almost in the same [Ni(dmit)
2
]
arrangement. The
ions composed of arylammonium and 18-crown-6 aimed at
forming spin ladders. Ammonium or amine moieties are known
to be included into the 18-crown-6 cavity through N–H…O
interactions within the dimer (A–AA and B–BA) are tA = 74.2 and
76.0 meV, respectively, whereas those in the ladder leg
direction (t) were 11.5 meV in both cases (A–A, AA–AA, B–B
and BA–BA). The ratio of JA/J was estimated to be 43.
6
hydrogen bonding. We have utilized this strong interaction for
constructing spool-shaped supramolecular cations of
{
[Ph(NH
pair of [Ni(dmit)
between two adjacent 18-crown-6 moieties of a spool, and the
3
)](18-crown-6)}
n
and [p-Ph(NH
3 2 2
) ](18-crown-6) . A
2
2
]
molecules formed a dimer embedded
2
2 2
one-dimensional array of the {[Ni(dmit) ] } gave dimer chain
structures, one of which exhibited the magnetic behavior of a
spin ladder.
Single crystals of [Ph(NH
p-Ph(NH ](18-crown-6) [Ni(dmit)
prepared from (n-Bu N)[Ni(dmit) ], 18-crown-6 and the corre-
sponding arylammonium–(BF ) in acetonitrile by a diffusion
method. The crystal structures of the salts 1 and 2 are shown in
3
)](18-crown-6)[Ni(dmit)
2
] (1) and
[
3
)
2
2
2
]
2
(CH CN) (2) were
3
2
4
2
4
2
Fig. 1. The [Ni(dmit)
2
]
assemblies in crystals 1 and 2 are
similar to each other.†
+
)]+ cation was included in
The NH
3
part of [Ph(NH
3
1
8-crown-6, which stacked regularly along the c-axis forming
an ionic channel-like, one-dimensional spool array in the salt 1.
+
The [Ph(NH
3
)] cations were oriented in the same direction
within a column and were opposite to those in the neighboring
+
3
columns. The NH group is located at the center of a 18-crown-
6
cavity, 0.91 Å above the 18-crown-6 molecular plane, forming
+
NH …O hydrogen bonds with oxygen atoms. The average
hydrogen-bond length is 2.92 Å, which is slightly longer than
+
7
the standard NH …O distance (2.87 Å).
2
Each pair of [Ni(dmit)
2
]
molecules forms a dimer through
face-to-face interaction, embedded between the adjacent
8-crown-6 molecules. The dimer is located on a pair of spools
1
in neighboring columns, and is aligned along the (011) direction
guided by the ditch of the supramolecular cations giving a dimer
chain structure.
3 2
)
]2+ part in salt 2 is sandwiched by 18-crown-
The [p-Ph(NH
molecules forming a spool-shape supramolecular cation. The
Fig. 1 Crystal structure of the salts 1 (above) and 2 (below). The molecular
6
NH
arrangement of [Ni(dmit)2]
2
forming dimer chain structures (red) is
+
3
groups are located 0.91and 0.97 Å above the center of the
overlaid with spool arrays (black).
4
08
CHEM. COMMUN., 2002, 408–409
This journal is © The Royal Society of Chemistry 2002

determined by the shorter l
crown ethers in the salt 1 is l
longer than l
2
. The distance between the adjacent
1
= 8.13 Å, which is quite a bit
2
.
In conclusion, we have constructed a novel molecular spin
ladder using a spool-type supramolecular cation. According to
theory,10 hole doping to two-leg spin ladders will lead to
superconductivity due to that effective attraction between extra
holes may arise from the magnetic interactions. In fact,
superconducting transitions have been reported in hole-doped
spin ladders of calcium cuprates.11 In the case of salt 1, the hole
doping will be accomplished without significant lattice distor-
tion by replacing a fraction of the ammonium cations with the
corresponding amines. Studies on this are in progress.
The authors thank Dr K. Ichimura and Professor K. Nomura
for the use of the SQUID magnetometer. This work was partly
supported by a Grant-in-Aid for Science Research from the
Ministry of Education, Culture, Sports, Science and Technol-
ogy of Japan.
Fig. 2 Temperature dependence of the magnetic susceptibility of salts 1 (1)
and 2 (8). The solid lines are fitted curves of eqn. (1) ( < 80 K) and eqn. (3)
(
> 150 K) (see text), and the dashed line is a fit by eqn. (4).
Notes and references
Fig. 2 shows the temperature dependence of the magnetic
†
Crystal data: for 1: C24
H
32NO
6
S10Ni, M = 809.82, T = 296 K,
susceptibility (c) of the salts 1 and 2 after subtracting the Curie
¯
2
1
triclinic, space group P1, a = 14.217(1), b = 16.666(2), c = 8.1271(6) Å,
component, C = 0.0060 and 0.0063 emu K mol , for salts 1
and 2, respectively. The magnetic susceptibility was measured
on a Quantum Design MPMS-XL SQUID susceptometer in a
field of 1 T. The susceptibility of salt 1 had a maximum at 120
3
c
a = 97.120(2), b = 92.599(3), g = 112.846(2)°, V = 1751.6(3) Å , D =
23
21
1
=
7
.535 g cm , Z = 2, F(000) = 838, m(Mo-Ka) = 11.87 cm , final R, R
0.051, 0.118. I
594 unique. GOF = 1.69.
w
o
= 5677 ‘observed’ [I > 3.0s(I)] reflections out of N =
2
1
K, and reached 0 emu mol
temperature-limit equation of a two-leg spin ladder,9
Low = aT21/2exp(2D/T)
around 15 K. The low-
For 2: C46
H
64
O
12
N
4
S
20Ni
2
, M = 1623.63, T = 296 K, triclinic, space
group P 1¯ , a = 13.1321(4), b = 16.3032(5), c = 17.6903(5) Å, a =
3
73.5786(9), b = 76.2712(8), g = 82.426(1)°, V = 3520.6(2) Å , D
c
=
c
(1)
23
21
1
.531 g cm , Z = 2, F(000) = 1680, m(Mo-Ka) = 11.82 cm , final R,
reproduced the observed susceptibility below 80 K with the spin
gap D = 190 K. Since the gap is related to J and JA as,
Rw = 0.055, 0.094. Io = 8095 ‘observed’ [I > 3.0s(I)] reflections out of N
= 16043 unique. GOF = 1.11. Crystallographic data collected on a Rigaku
RAXIS-RAPID Imaging Plate. All H atoms placed in calculated posi-
tions.
1
2
DªJ' -J + J / J'
(2)
2
CCDC reference numbers 176980 and 176981. See http://www.rsc.org/
suppdata/cc/b1/b110368g/ for crystallographic data in CIF or other
electronic format.
we can estimate the magnetic exchange energies J = 20 K and
J’ = 200 K by assuming JA/J = 10. The magnetic susceptibility
at higher temperatures ( > 150 K) is well reproduced by the
high-temperature equation,9
1
P. W. Anderson, Science, 1987, 235, 1196.
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Î
-1 1 Ê
1
ˆ
-2
3
-3˘
4
1.
cHigh = C T
-
J + J¢ T
+
JJ¢T
(3)
Í
˙
Ë
¯
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2
2
16
˚
using these parameters and C = 0.394 emu K mol2 determined
1
from EPR.
In the case of salt 2, c increased with the decrease in
temperature down to 210 K, and decreased exponentially at
lower temperatures. A fitting by the singlet–triplet thermal
excitation model,
4
5
C. Rovira, Chem. Eur. J., 2000, 6, 1723.
N. Takamatsu, T. Akutagawa, T. Hasegawa, T. Nakamura, T. Inabe, W.
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C
4exp(-2J / T)
cSinglet-Triplet
=
(4)
T 1+ 3exp(-2J / T)
6
7
E. Weber, L. J. Toner, I. Goldberg, F. Vögtle, A. D. Laidler, F. J.
Stoddart, A. R. Bartsch and L. C. Liotta, Crown Ethers and Analogs, S.
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San Francisco, 1960.
using C = 0.394 emu K mol2 gave a good result, and the
magnetic exchange interaction 2J was around 340 K. The ratio
of JA/J = 43 may be too large to realize the spin-ladder state in
salt 2. It should be noted that eqn. (4) does not give a good fit in
the case of salt 1.
1
8 T. Mori, A. Kobayashi, Y. Sasaki, H. Kobayashi, G. Saito and H.
Inokuchi, Bull. Chem. Soc. Jpn., 1984, 57, 627.
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The stronger face-to-face p–p interaction within the [Ni(d-
2
mit)
2
]
dimer reflects the larger value of JA/J in salt 2. The
1
3 515.
0 T. M. Rice, S. Gopalan and M. Sigrist, Europhys. Lett., 1993, 23,
45.
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2
interaction depends on the distance between the [Ni(dmit) ]
2
1
1
units forming the dimer, which should be related to the distance
between crown ethers. The distances between the 18-crown-6
4
2+
molecular planes sandwiching the [p-Ph(NH
are l = 7.49 and l = 8.80 Å (Fig 1). Since each dimer is on
3 2 3
both of [p-Ph(NH ) ] and CH CN, the interaction should be
3
)
2
]
3
and CH CN
2
3
2+
CHEM. COMMUN., 2002, 408–409
409