Crystallographic evidence for reversible symmetry breaking in a spin-crossover d7 cobalt(II) coordination polymer

Article abstract of DOI:10.1002/anie.201107116

All in a spin: A cobalt(II) coordination polymer has a highly cooperative spin-crossover (SCO) behavior with a small hysteresis loop (see picture; HS=high spin, LS=low spin). The correlation of the magnetic properties with the crystal structures was determined at different temperatures. This study reveals an unprecedented reversible symmetry breaking in a d⁷ SCO coordination polymer. Copyright

Full text of DOI:10.1002/anie.201107116

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Angewandte
Communications
DOI: 10.1002/anie.201107116
Spin Crossover
Crystallographic Evidence for Reversible Symmetry Breaking in
a Spin-Crossover d7 Cobalt(II) Coordination Polymer**
Kishalay Bhar, Sumitava Khan, Josꢀ Sꢁnchez Costa,* Joan Ribas, Olivier Roubeau, Partha Mitra,
and Barindra Kumar Ghosh*
In memory of Andres Goeta
An amazing example of bistability in molecular systems is
given by spin crossover (SCO) materials.[1,2] These materials
can be switched reversibly from a high-spin (HS) to a low-spin
(LS) electronic state by applying an external perturbation,
such as temperature, pressure, magnetic field, or light.[3] The
SCO phenomenon occurs in a variety of d4–d7 transition-
metal compounds. FeII[4] and FeIII[5] have been by far the most
studied metal ions in SCO, which is primarily due to their
bistability features showing abrupt spin transition with
hysteresis loops. Less attention has been paid to CoII, perhaps
owing to more gradual transitions. This d7 ion may experience
a change between S = 1/2 and S = 3/2 electronic states
involving the transfer of one electron causing an appreciable
difference in the coordination bond distances of about
0.10 ꢀ.[6,7]
An important breakthrough in the SCO field is the
application of polymeric systems to improve interactions
between metal centers.[8] 1D polymers of the general type
[Fe(4-R-1,2,4-triazole)3]2+ (R = H, NH2, and CH2CH2OH) are
one of the most remarkable examples that show bistability
with a hysteresis loop at room temperature. This polymer
SCO system was the first tested as a memory device.[9,10]
Subtle information on effective SCO systems has been
gained with the development of such characterization tech-
niques as the X-ray diffraction; indeed, using this technique it
has been illustrated that intermolecular interactions play
a major role during such transition, and also that in some
cases the transition may be accompanied with symmetry-
breaking in the crystal structure. Remarkable examples of
such phenonenon have been found in a few FeII com-
pounds[11,12] and recently in a FeIII complex.[13]
Herein we present the synthesis, structure, and magnetic
and thermal properties of a CoII 1D SCO polymer, [Co-
(enbzpy)(dca)]n(ClO4)n (1; enbzpy = N,N’-bis(2-pyridinylben-
zylidene)ethane-1,2-diamine, dca = dicyanamide), which
shows a reversible crystallographic symmetry-breaking. Com-
pound 1 was formed in an MeOH solution containing a 1:1:1
mixture of Co(ClO4)2·6H2O, enbzpy, and Na(dca). Single-
crystal X-ray structures of 1 were determined at 296 K (1RT
)
and 100 K (1LT). Compound 1RT crystallizes in the monoclinic
space group P21/c (Supporting Information, Table S1); the
coordination environment of the CoII ion is a distorted
octahedron with a CoN6 chromophore, consisting of four
equatorial N atoms of the Schiff base (enbzpy) and two axial
nitrile N atoms of two different m1,5 dca bridges that connect
ꢀ
adjacent metal centers forming an infinite 1D chain. Co N
distances lie in the range 2.023–2.136 ꢀ with an average value
II
ꢀ
dav(Co N) of 2.08 ꢀ, which is characteristic of Co HS
systems.[14] The intra-chain Co1···Co1 separation is 8.57 ꢀ.
The central nitrogen atom of the m1,5-dca bridge is disordered
ꢀ
over two positions (Figure 1), forming a m1,5-dca N6 N7-
[*] K. Bhar, S. Khan, Prof. B. K. Ghosh
Department of Chemistry, The University of Burdwan
Burdwan 713 104 (India)
E-mail: barin_1@yahoo.co.uk
Dr. J. S. Costa, Prof. J. Ribas
Departament de Quꢀmica Inorgꢁnica, Universitat de Barcelona
Diagonal, 645, 08028 Barcelona (Spain)
E-mail: josesanchezcosta@gmail.com
Dr. O. Roubeau
Instituto de Ciencia de Materiales de Aragꢂn (ICMA)
CSIC—Universidad de Zaragoza
50009 Zaragoza (Spain)
P. Mitra
Department of Inorganic Chemistry, Indian Association for the
Cultivation of Science
Kolkata 700 032 (India)
[**] This work was supported by the DST and CSIR, India. J.S.C. thanks
the Spanish MICINN through CTQ2009-06959 for the research
fellowship “Juan de la Cierva” (18-08-463B-750). J.R. also thanks the
Spanish MICINN through CTQ2009-07264.
Figure 1. An ORTEP representation of a segment of the 1D chain
along the b axis in 1RT with HS CoII. Ellipsoids set at 30% probability;
hydrogen atoms omitted for clarity. Inset: The enbzpy ligand.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201107116.
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ꢀ
(N7A) N5 bridge angle of 133.49/133.018 (Supporting Infor-
mation). The remaining crystal volume is occupied by
a disordered ClO4ꢀ anion.
In the crystal packing of 1D chains of 1RT, the aromatic
ꢀ
ꢀ
rings in enbzpy and ClO4 are connected through weak C
ꢀ
H···O hydrogen bonds and C H···p and p···p interactions
(Supporting Information, Tables S2 and S3) with the neigh-
boring enbzpy unit affording a superstructure. The adjacent
1D chains in 1RT are engaged in C H···p interactions involving
ꢀ
the H atom (H21) of a phenyl ring and the p cloud of
a pyridine ring (defined as Cg8, see the Supporting Informa-
tion) of two closest enbzpy ligands. Furthermore, they are
ꢀ
connected through cooperative C H···O hydrogen bonds
(Supporting Information, Figure S1) between ClO4ꢀ counter-
ions embedded between those chains and hydrogen atoms of
enbzpy, leading to a 3D network structure. The packing is
Figure 3. cM T versus time over four thermal cycles in the temperature
range 227–160 K. The HS!LS transition is represented in blue and
the LS!HS in red.
ꢀ
further stabilized through C H···N interactions involving the
hydrogen of a phenyl ring (H23) and nitrogen atoms (N6) of
the dca bridge along with p···p interactions of the nearest
phenyl rings (Cg9) in enbzpy ligands.
below) and reproduced over four times in the 227–160 K
temperature range, thus showing that 1 is an example of a CoII
polymer with a robust SCO behavior (Figure 3).
The magnetic susceptibilities for 1 were determined over
the temperature range 2–300 K applying a constant field of
0.5 T, in both cooling and warming modes (Figure 2). The
room-temperature cM T value of 2.43 cm3 molꢀ1 K is greater
than that expected for a d7 mononuclear cobalt(II) (S = 3/2,
cM T= 1.87 cm3 molꢀ1 K), which is presumably due to a large
first-order orbital angular momentum contribution to the
magnetic moment. On cooling, cM T decreases slightly until
In view of the magnetic behavior, we decided to resolve
the structure of 1 at low temperature. Compound 1LT
¯
crystallizes in a low-symmetry space group, the triclinic P1
(Experimental Section) with similar a and c cell lengths as for
1
RT, and a doubling of the b axis and cell volume (see crystal
data). The asymmetric unit contains four crystallographically
different CoII centers (Co1A, Co1B, Co2A, and Co2B). The
coordination environment of the metal is analogous to that
191 K and drops abruptly to reach a value of 1.65 cm3 molꢀ1
K
observed in 1RT. The average Co N distances (Supporting
ꢀ
at about 171 K. This remains almost constant down to 100 K
and again decreases to attain a value of 1.41 cm3 molꢀ1 K.
Below 10 K, the cM T value increases reaching a maximum of
1.60 cm3 molꢀ1 K. The value of cM T below the abrupt drop
around 190 K can be reasonably ascribed to two CoII HS
atoms and two CoII LS (expected = 1.4 cm3 molꢀ1 K), in
agreement with structural parameters (see below). Surpris-
ingly for this 1D CoII SCO system, the transition occurs with
a small hysteresis loop of 3 K width (Figure 2, inset). This
behavior suggests an efficient cooperative effect both among
adjacent CoII centers along the 1D supramolecular chains and
between chains through interchain interactions. Furthermore,
the hysteresis loop was confirmed by calorimetric studies (see
ꢀ
Information, Table S1) are now dav(Co1A N) 2.108 ꢀ, dav-
ꢀ
ꢀ
ꢀ
(Co2A N) 2.015 ꢀ, dav(Co1B N) 2.111 ꢀ, and dav(Co2B N)
2.016 ꢀ, thus indicating that Co1A and Co1B are in the HS
state, whereas Co2A and Co2B, are in the LS state.[14] This
spin transition in the crystal is illustrated in Figure 4. The
intrachain distances observed in chain A are Co1A···Co2A/
Co2A···Co1A 8.673/8.573 ꢀ, and in B they are Co1B···Co2B/
Co2B···Co1B 8.639/8.581 ꢀ. The angle formed by the two
crystallographically independent m1,5-dca bridges for chain A
are N5-N6-N7 127.228 and N12-N13-N14 131.638, and for
chain B they are N5-N6-N7 128.338 and N12-N13-N14 131.618
(Supporting Information, Figure S4). In both cases the angles
formed by the m1,5-dca are smaller compared to those in 1RT
.
Figure 2. cM T versus T plot for 1 illustrating the abrupt SCO transition
with a small hysteresis loop of 3 K (inset). Blue represents the
magnetic data collected in cooling mode and red in warming mode.
Figure 4. View of the 1D chains evolution during the SCO process
along the c axis. HS Co dark gray, LS light gray.
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In crystalline state, the single m1,5-dca bridged spin-active
CoII centers in 1 are organized on a global basis through
from the excess heat capacity (Supporting Information,
Figure S6), amount to 2.11 kJmolꢀ1 and 10.9 Jmolꢀ1 Kꢀ1,
respectively. Altogether, these calorimetric features are
consistent with a cooperative first-order transition, in agree-
ment with the observation of a crystallographic transition
coupled with the spin crossover. In particular, the excess
entropy is much larger than that expected from the electronic
transition of half of the CoII ions, and thus likely includes the
contribution from the crystallographic transition, and a vibra-
tional term possibly involving the ordering of the dicyana-
mide ion and intermolecular interactions.
In summary, complex 1 is the first example of a symmetry-
breaking spin transition for a CoII SCO polymer where a rare
abrupt HS$LS transition is found. It is an unprecedented X-
ray crystallographically characterized structural phase tran-
sition accompanied with sufficiently strong reorganization of
the intermolecular interactions. The spin-inactive compo-
nents, such as the ligand and the counterion, also play
a dominant role in the spin transition through supramolecular
contacts. Remarkably, the transition between both phases has
been proved to be crystallographically reversible. A huge
modification in the assemblage of the 1D chains has no
repercussions in the reversibility of the symmetry-breaking
abrupt spin crossover. DFT calculations are underway to
better understand such symmetry-breaking SCO behavior
associated with cooperativity and hysteresis in terms of
electronic.
ꢀ
different intermolecular forces, such as C H···p interactions,
p···p stacking, and hydrogen bonding. These noncovalent
interactions play a pivotal role as information transmitters
among the active metal(II) centers throughout the whole
network.
Crystal packings of 1RT and 1LT show that the pattern of
intermolecular interactions (Supporting Information,
Tables S2–S5) are similar; however, the degree of interactions
increases substantially in 1LT. In 1LT, two independent 1D
chains (Co1A···Co2A and Co1B···Co2B) are stacked alter-
nating parallel to the ab plane (Supporting Information,
ꢀ
Figure S2) through multiple C H···N hydrogen bonds involv-
ing N atoms (N7B, N13B and N14B) of the dca bridges from
chain B (Co1B/Co2B) and H atoms (H11A, H32A and
H39A) of enbzpy from chain A (Co1A/Co2A).
ꢀ
These 2D sheets are further stabilized through C H···p
interactions (Supporting Information, Figure S3, Table S4)
ꢀ
among chains A and B. Multiple C H···O hydrogen bonds to
the perchlorate counterions embedded between those sheets
and weak p···p interaction between two adjacent B chains
(Co1B···Co2B) along c axis also strengthen these supramolec-
ular interactions, affording a 3D network (Supporting Infor-
mation, Tables S4, S5). The broader array of interactions
involving the dca in 1LT is symptomatic of an internal
reorganization in the polymeric chain without loss of crystal-
linity.
ꢀ
An appealing phenomenon involving the ClO4 counter-
ions and the m1,5-dca bridged occurs during the transition
(Figure 5). The O10 (O10 and O10A) is involved in a supra-
molecular contact with one of the dicyanamide group
O10···C56A (3.235 ꢀ) and O10A···C56A (3.048 ꢀ),
O10A···N13A (3.054 ꢀ), with the adjacent dca ligand
(N12A-C55A-N13A-C56A-N14A). This contact can be con-
sidered an anion–p interaction.[15] In fact, the reorientation of
the perchlorate likely plays an important role in the occur-
rence of the small hysteresis loop[16] (inset in Figure 2).
Differential scanning calorimetry (DSC) measurements
were performed to confirm the above observations. The raw
DSC traces (Supporting Information, Figure S5) show very
sharp peaks in both cooling and warming modes at temper-
atures similar to those observed in the magnetic measure-
ments, and confirm the presence of a hysteresis of about 3 K
width. This is reproducible over various temperature cycles.
The excess enthalpy and entropy of the transition, derived
Experimental Section
Crystal data for 1RT: C28H22N7O4ClCo, Mr = 614.91, monoclinic, space
group P21/c, crystal size = 0.17 ꢁ 0.13 ꢁ 0.08 mm3, a = 15.138(14), b =
8.569(8), c = 20.963(19) ꢀ, b = 91.546(12)8, V= 2718(4) ꢀ3, Z = 4,
1calcd = 1.502 gcmꢀ1
;
T= 296(2) K, m(MoKa) = 0.779 mmꢀ1 (l =
0.71073 ꢀ); 38232 reflections measured, 8515 unique (Rint = 0.073),
refinement converged to R = 0.0664, wR(F2) [I > 2s(I)] = 0.2029, 397
parameters and 414 restraints, goodness of fit on F2 = 1.005; max./min.
residual electron density =+ 0.661/ꢀ0.889 eꢀꢀ3. Crystal data for 1LT
:
¯
C56H44N14O8Cl2Co2, Mr = 1229.81, triclinic, space group P1, crystal
size = 0.17 ꢁ 0.13 ꢁ 0.08 mm3, a = 15.294(4), b = 17.195(4), c =
22.068(6) ꢀ, a = 68.124(4), b = 89.466(4), g = 88.942(4)8, V=
5384(2) ꢀ3, Z = 4, 1calcd = 1.517 gcmꢀ1; T= 100.0(1) K, m(MoKa) =
0.786 mmꢀ1(l=0.71073 ꢀ); 59801 reflections measured, 22918
unique (Rint = 0.083), refinement converged to R = 0.1354, wR(F2) =
0.3962 [I > 2s(I)], 1486 parameters and 2464 restraints, goodness of fit
on F2 = 1.037; max./min. residual electron density =+ 4.517/
ꢀ1.791 eꢀꢀ3. CCDC 833288 (1RT) and 833289 (1LT) contain the
supplementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Synthetic details, magnetic susceptibility measurements, DSC
experiments, X-ray crystallographic analyses, crystallographic data
including bond distances, bond angles, non-covalent interaction
parameters (Tables S1–S5), and structures (Figure S1–S4) of 1RT and
1
LT are available in the supporting information.
Received: October 7, 2011
Revised: December 5, 2011
Published online: January 23, 2012
Figure 5. The supramolecular anion–p interaction in 1LT involving O10
of the disordered perchlorate unit and the p cloud of a dicyanamide
Keywords: cobalt · coordination polymers ·
magnetic properties · spin crossover · symmetry breaking
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bridge (N12A C55A N13A C56A N14A).
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