Dinuclear complexes with a triple N1,N2-triazole bridge that exhibit partial spin crossover and weak antiferromagnetic interactions

Article abstract of DOI:10.1002/ejic.201201126

The reaction of 4-phenylimino-1,2,4-triazole (1) with FeII, CoII, NiII and CuII thiocyanate produces a series of analogous dinuclear compounds of formula [M₂ (1)₅ (NCS)₄] (2-5) as demonstrated by single-crystal X-ray diffraction studies of the FeII (2) and CoII (3) analogues. The magnetic properties of [Fe2(1)5(NCS)4]·xMeOH (x = 3.5-5) reveal a partial and gradual spin crossover (SCO) centred at TSCO = 115 K. This is confirmed by its crystal structure solved at 100, 150 and 250 K, which exhibits a gradual decrease of the Fe-N bond lengths with temperature. However, the bulk hydrated form of 2 that is generated upon exposure to air of crystals is a high-spin compound that exhibits weak antiferromagnetic interaction. The exchange coupling among the FeII S = 2 ions within the dinuclear neutral complex was evaluated as J/kB = -1.33(3) K by using the Heisenberg Hamiltonian H = -2JS1·S2. Similarly, the magnetic properties of the NiII (4) and CuII (5) analogues are dominated by moderate and weak antiferromagnetic interactions evaluated as J/kB = -13.9(3) and -0.30(5) K, respectively. The presence of strong spin-orbit coupling of the individual CoII ions impeded the evaluation of the likely antiferromagnetic interaction that leads to a singlet ground state in 3. The reported structures of 2 and 3 are new additions to a very scarce family of dinuclear complexes bearing a unique triple N1,N2-triazole bridge. Owing to its relevance in the peculiar properties of 1D triazolebased SCO materials, which are widely studied for their various potential applications, a structural analysis of this triple N1,N2-triazole bridge in reported structures of FeII and CoII trinuclear and 1D compounds is provided.

Full text of DOI:10.1002/ejic.201201126

FULL PAPER
DOI:10.1002/ejic.201201126
Dinuclear Complexes with a Triple N1,N2-Triazole Bridge
That Exhibit Partial Spin Crossover and Weak
Antiferromagnetic Interactions
CLUSTER
ISSUE
Olivier Roubeau,*[a] Patrick Gamez,*[b,c] and Simon J. Teat[d]
Keywords: Triazole / Coordination compounds / Iron / Spin crossover / Magnetic properties
The reaction of 4-phenylimino-1,2,4-triazole (1) with FeII,
CoII, NiII and CuII thiocyanate produces a series of analogous
dinuclear compounds of formula [M2(1)5(NCS)4] (2–5) as
demonstrated by single-crystal X-ray diffraction studies of
the FeII (2) and CoII (3) analogues. The magnetic properties
of [Fe2(1)5(NCS)4]·xMeOH (x = 3.5–5) reveal a partial and
gradual spin crossover (SCO) centred at TSCO = 115 K. This
is confirmed by its crystal structure solved at 100, 150 and
250 K, which exhibits a gradual decrease of the Fe–N bond
lengths with temperature. However, the bulk hydrated form
of 2 that is generated upon exposure to air of crystals is a
high-spin compound that exhibits weak antiferromagnetic
interaction. The exchange coupling among the FeII S = 2 ions
within the dinuclear neutral complex was evaluated as J/kB
= –1.33(3) K by using the Heisenberg Hamiltonian H =
–2JS1·S2. Similarly, the magnetic properties of the NiII (4) and
CuII (5) analogues are dominated by moderate and weak
antiferromagnetic interactions evaluated as J/kB = –13.9(3)
and –0.30(5) K, respectively. The presence of strong spin–or-
bit coupling of the individual CoII ions impeded the evalu-
ation of the likely antiferromagnetic interaction that leads to
a singlet ground state in 3. The reported structures of 2 and
3 are new additions to a very scarce family of dinuclear com-
plexes bearing a unique triple N1,N2-triazole bridge. Owing
to its relevance in the peculiar properties of 1D triazole-
based SCO materials, which are widely studied for their vari-
ous potential applications, a structural analysis of this triple
N1,N2-triazole bridge in reported structures of FeII and CoII
trinuclear and 1D compounds is provided.
field strength for the occurrence of SCO, triazole and tetra-
zole ligands are known to be ideal donors.[4] In the case
of triazole, a bridging N1,N2 coordination mode is usually
observed, as exemplified by the vast family of polymeric
one-dimensional triazole-based FeII materials[5] of general
formula [Fe(Rtrz)3](A)2·xH2O (Rtrz = 4-substituted-1,2,4-
triazole), in which the FeII ions are linked through three
N1,N2-triazole bridges. These classic SCO compounds have
been at the centre of many recent developments towards
applications[6] and have allowed the design of early optical-
device prototypes,[7] the elaboration of SCO nanopar-
ticles,[8] supramolecular gels,[9] thin films,[10] dendrimers,[11]
contrast agents for magnetic resonance imaging[12] and li-
quid crystals,[13] as well as the electrical addressing of
SCO[14] or the use of plasmons to detect SCO.[15] One of
the key advantages of these materials is their chemical flexi-
bility, while maintaining the coordination chains of FeII
connected by triple N1,N2-triazole bridges. This short, ri-
gid – though strainless and thus very stable linkage (see
Scheme 1, left) – is what really makes the triazole 1D sys-
tems unique. On one hand, it represents an effective propa-
gation medium for the variation of coordination sphere vol-
ume upon SCO, as opposed to flexible linkers. On the other
hand, its stability combined with the possibility to modify
the triazole ring at the 4-position gives access to families of
compounds, as indeed has been the case with the 1D
Introduction
The spin-crossover (SCO) phenomenon is often depicted
as the paradigm of bistability at the molecular level.[1] This
is especially true for FeII compounds for which the low-
spin (LS) to high-spin (HS) transitions are associated with
diamagnetic to paramagnetic switching together with sig-
nificant optical and Fe–L bond-length changes.[2] These
changes can be triggered not only by temperature changes,
but also through other external stimuli, such as irradiation
or pressure.[3] For an FeII ion to have the adequate ligand-
[a] Instituto de Ciencia de Materiales de Aragón (ICMA), CSIC
and Universidad de Zaragoza,
Plaza San Francisco, 50009 Zaragoza, Spain
Fax: +34-976-761229
E-mail: roubeau@unizar.es
Homepage: http://fmc.unizar.es/people/roubeau
[b] Departament de Química Inorgànica, QBI, Universitat de
Barcelona,
Martí I Franquès 1–11, 08028, Barcelona, Spain
Fax: +34-934-907725
E-mail: patrick.gamez@qi.ub.es
Homepage: http://www.bio-inorganic-chemistry-icrea-ub.com
[c] Institució Catalana de Recerca i Estudis Avançats (ICREA),
Passeig Lluís Companys 23, 08010 Barcelona, Spain
[d] Advanced Light Source, Lawrence Berkeley National
Laboratory,
1 Cyclotron Road, Berkeley, CA 94720, USA
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201201126.
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[Fe(Rtrz)3](A)2 system, the SCO temperature and coopera- obtained in high yields, have been prepared with the objec-
tive character of which can be tuned by the nature of the tive of generating π–π stacking interactions that may help
4-substituent on the triazole ligand and/or the choice of the to create intermolecular contacts, and therefore potentially
counteranion.[5a,5b] The drawback of these materials is their favour more cooperative SCO behaviours. Thus, the phenyl,
usually poor crystallinity, which is probably due to their naphthyl and anthracyl derivatives have been synthesized.
polymeric nature. Indeed, the first crystal structure of an However, to date, only coordination complexes with the
[Fe(Rtrz)3](A)2 compound has only very recently been re- phenyl-containing ligand could be isolated. 4-Phenylimino-
ported.[16] Previously, structurally characterized CuII ana- 1,2,4-triazole (1, Scheme 1, right) is obtained readily from
logues[17] or trinuclear compounds with an [Fe(Rtrz)3- the reaction of 4-amino-1,2,4-triazole and benzaldehyde in
Fe(Rtz)3] core[18] were used as structural models and for ethanol. The coordination compounds 2–5 were synthesized
comparison in extended X-ray absorption fine structure by the direct addition of a methanolic solution of the phen-
(EXAFS) studies of the FeII chain compounds.[19] Surpris- yliminotriazole ligand 1 to a freshly prepared methanolic
ingly, the simpler dinuclear compounds with one central tri- solution of M(NCS)2 (M = Fe, Co, Ni, Cu), obtained from
ple N1,N2-triazole bridge are extremely scarce in the litera- MII sulfate and ammonium thiocyanate (see Experimental
ture, and most of them involve thiocyanate ions as terminal Section). In the case of 2, the synthesis was performed un-
ligands.[20] Specifically with FeII, the recently reported SCO der argon to prevent oxidation of FeII to FeIII. Although
compound
[Fe2(N-salicyliden-4-amino-1,2,4-triazole)5- rather dilute solutions were used, the precipitation of 2–5
(NCS)4]·4MeOH and the peculiar dimer–monomer com- as crystalline powders occurred within 24 h. More of the
pound [Fe2(4-p-tolyl-1,2,4-triazole)5(NCS)4][Fe(4-p-tolyl- crystalline material was obtained by slow concentration of
1,2,4-triazole)2(NCS)2(H2O)2] are the only other known the remaining solution after recovering the bulk powder by
structures with a unique triple N1,N2 bridge.[20e,20f] We re- filtration, including single crystals of 2 and 3 suitable for
port here on two additions to this short list, obtained with X-ray diffraction studies. The latter were thus found to
the 4-phenylimino-1,2,4-triazole (1) ligand, namely, [Fe2(1)5- form as methanolates of the dinuclear [M2(1)5(NCS)4] spe-
(NCS)4]·xMeOH (2·xMeOH; x = 0, 3.5, 5) and [Co2(1)5- cies (M = Fe, Co; see below). If more concentrated condi-
(NCS)4]·3.5MeOH (3·3.5MeOH). We also report the NiII tions or more than 2.5 equiv. of 1 per MII ion are used, an
(4) and CuII (5) analogues, although their structures could unidentified solid, probably formed of linear coordination
not be obtained so far. Variable-temperature X-ray diffrac- oligomer/polymers, precipitates rapidly. As previously ob-
tion and magnetic measurements reveal that the solvated served in some of the other reported similar com-
FeII compound exhibits a partial SCO and that all com- pounds,[20b,20f] the crystals disaggregate when exposed to
pounds present weak antiferromagnetic interactions. A air, which indicates that the lattice solvent molecules proba-
structural comparison of the triple N1,N2-triazole bridges bly play an important role in stabilizing these structures.
in these dinuclear compounds with those in reported struc- However, the IR spectra of crystals maintained in a small
tures of trinuclear and 1D FeII[18] and CoII[21] compounds amount of MeOH, the dry powdered crystals and the bulk
has been made.
powders are all very similar (see Experimental Section),
which indicates that they all consist of the same dinuclear
complex. In particular, the observation of two out-of-plane
ring torsion bands (at ca. 620 and 680 cm–1) indicate the
presence of both mono- and bidentate triazole ligands.[22]
Moreover, the ν(CN) band of the NCS– ion is in the range
2000–2150 cm–1, which indicates coordination through its
N donor atom, and only presents a shift ascribable to the
difference in M–N bond length. Elemental analyses of the
powder samples are in agreement with hydrates of the
[M2(1)5(NCS)4] complexes, which result from the absorp-
tion of moisture from air.
Scheme 1. Left, idealized geometry of triple N1,N2-1,2,4-triazole
coordination bridge of two metal ions (small grey spheres) to give
M–N–N angles close to the exocyclic free donor electron pair angle
of a regular five-membered ring (126°). N and C atoms are in black
and light grey respectively. Right, 4-phenylimino-1,2,4-triazole li-
gand 1.
Description of Crystal Structures
The molecular structure of 2 was determined at 100 and
250 K on a first crystal and at 150 K on a second crystal.
At these temperatures, lattice solvent loss is (partially) pre-
Results and Discussion
¯
vented, and 2 crystallizes in the triclinic space group P1.
The asymmetric unit contains one full neutral dinuclear
complex plus five lattice methanol molecules, although in
Synthesis
Triazole ligands substituted solely at their 4-position are the first crystal some of these are partially occupied re-
usually able to coordinate in the desired N1,N2-bridging sulting in a total of 3.5 methanol molecules. The two Fe
mode. Arylimino-substituted 4-amino-1,2,4-triazoles, easily sites in the dinuclear units are bridged by three N1,N2-tri-
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azole rings of 1 and separated by 3.789/3.953/3.967 Å, free electron pair angle of a regular five-membered ring.
respectively, at 100/150/250 K. Their coordination spheres The average values are 125.53, 125.9 and 126.0° at 100, 150
are each completed by one terminal triazole N1-donor and and 250 K, respectively. This clearly results in no significant
two cis-thiocyanates. An ORTEP view of the dinuclear unit
at 100 K is shown in Figure 1. The terminal C–S bond of
one of the thiocyanate ions coordinated to Fe1 is disordered
over two positions, and the component corresponding to
the eclipsed position (see Figure 2) has a relative occupancy
of 0.61/0.58/0.48. The environment and bond lengths of the
two Fe sites are very similar at the three temperatures (see
Table 1). The average Fe–N bond lengths at 250 K are typi-
cal of an FeII ion in its HS state, whereas the lower values
at 150 and 100 K clearly indicate the process of SCO. This
also agrees with the purple colour of the crystal at 100 K,
whereas it is yellow at ambient temperature, and is the ori-
gin of the shorter intramolecular Fe···Fe separation at
100 K. Nevertheless, the Fe–N bonds at 100 K are still
longer than those expected for a fully LS state with this
environment. In fact, considering the LS and HS structures
of the only other related SCO dinuclear compound as a
reference,[20f] the Fe1 and Fe2 sites would be 55 and 58%
LS FeII, respectively. This is in good agreement with the
Figure 2. Views of the structures of 2·5H20 at 150 K (left) and 2 at
magnetic measurements (see below), and points at a con-
tinuous and concomitant SCO at both Fe sites, without any
sign of HS–LS pairs. The FeN6 polyhedron remains close
to an octahedron at all temperatures for both Fe sites (see
Figure 2 and Tables 1, S2 and S3) and has only a rather
small distortion, which is smaller at the lowest temperature,
as expected for an LS state. The triazole rings are fairly
planar with their planes parallel to the molecular axis. At
100 K, their centroids form mutual angles of 114, 115.7 and
130.3° with respect to the Fe···Fe axis midpoint, and the
average value is close to 120° as in a tri-wing paddlewheel
(see Figure 2). The Fe–N–N angles range from 122.9 to
128.4°, all close to 126°, which corresponds to the exocyclic
296 K (right) perpendicular (top) and along (bottom) the Fe···Fe
axis, which show the different orientation adopted by the thiocya-
nate ions; the FeN6 coordination spheres are highlighted as brown
polyhedra. The phenylimino groups are not shown for clarity. Col-
our code: Fe brown, S yellow, N light blue, C black.
Table 1. Structural parameters describing the coordination environ-
ment of the metal sites in the dinuclear complexes of 2·3.5MeOH
(100 and 250 K), 2·5MeOH (150 K), 2 (296 K) and 3·3.5MeOH
(100 K).
2
3
T
100 K
150 K
250 K
296 K
100 K
M1–N1
M1–N5
M1–N9
M1–N13
M1–N17
M1–N18
Σ[a]
M1–NNC[bS]
M1–Ntrz
M1–N[b]
%HS[c]
2.064(2) 2.189(6) 2.195(2) 2.184(11) 2.129(2)
2.080(2) 2.188(6) 2.201(3) 2.180(11) 2.154(2)
2.088(2) 2.211(6) 2.214(2) 2.214(12) 2.162(2)
2.070(3) 2.173(7) 2.175(3) 2.170(12) 2.128(2)
2.012(3) 2.107(8) 2.099(3) 2.064(19) 2.081(2)
2.034(3) 2.125(7) 2.134(3) 2.160(13) 2.102(2)
20.21
2.023
2.075
2.058
0.55
23.30
2.116
2.190
2.166
0.02
22.83
2.116
2.196
2.170
–
23.80
2.112
2.187
2.162
–
24.60
2.091
2.143
2.126
–
[b]
M2–N2
M2–N6
M2–N10
M2–N19
M2–N23
M2–N24
Σ[a]
M2–NNC[bS]
M2–Ntrz
M2–N[b]
%LS[c]
2.055(2) 2.178(6) 2.184(2) 2.221(11) 2.123(2)
2.053(3) 2.186(7) 2.193(3) 2.198(13) 2.137(2)
2.056(2) 2.174(6) 2.190(3) 2.251(13) 2.129(2)
2.073(3) 2.178(7) 2.192(3) 2.148(14) 2.147(2)
2.012(3) 2.081(8) 2.093(3) 2.120(16) 2.081(3)
2.031(3) 2.143(7) 2.138(3) 2.162(15) 2.109(3)
16.96
2.021
2.059
2.047
0.58
27.40
2.112
2.179
2.157
0.04
22.91
2.115
2.190
2.165
–
27.70
2.141
2.204
2.183
–
19.31
2.095
2.134
2.121
–
[b]
[a] Distortion of the FeN6 polyhedron from octahedral, Σ = Σ(θ –
90), θ is the N–Fe–N angle between vertex-sharing pairs of N do-
nors.[24] [b] Averaged values. [c] Derived from the decrease in
average Fe–N bond length with respect to the fully HS case (taken
from the structure at 250 K) and compared to the decrease ob-
Figure 1. ORTEP view of the dinuclear complex in 2·3.5MeOH at
100 K; ellipsoids at 30% probability. Only the metal, sulfur and
nitrogen atoms involved in coordination bonds are labelled.
served
in
[Fe2(N-salicyliden-4-amino-1,2,4-triazole)5(NCS)4]·
4MeOH, which corresponds to a full HS–LS transition.[20f]
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strain and compares well with observed values in other di-
nuclear, trinuclear and 1D structures built on such triple triclinic P1 space group with a neutral dinuclear complex
N1,N2-triazole bridges (see below). [Co2(1)5(NCS)4] and a total of 3.5 methanol molecules
At 100 K, 3 is isomorphous to 2 and crystallizes in the
¯
The neutral dinuclear units form a dense 3D network forming the asymmetric unit. All structural parameters of
through intermolecular interactions of various types. the dinuclear complex, including the disorder shown by one
Tables S4 and S5 gather the details of all intermolecular of the thiocyanate ions and their eclipsed orientation, and
interactions in the structures described in the present re- the intermolecular interactions are very similar to those of
port. In 2·3.5MeOH at 100 K two of the bridging 1 ligands 2·3.5MeOH at 100 K (see Figure S6, Tables 1, S4, S5 and
strongly interact with one counterpart of two neighbouring S6). This supports the proposal to use CoII compounds as
dimers through parallel double π–π stacking interactions of structural (HS) references for spin-crossover FeII ana-
their respective phenyl and iminotriazole groups (red lines logues.[23]
in Figure S2, Table S4). The resulting supramolecular zig-
zag chain of dimers remains similar at 150 and 250 K. The
third bridging 1 ligand interacts in a similar manner with
the monocoordinated 1 to Fe2 of another neighbouring
Magnetic Properties
complex; however, in this case, the two phenyliminotriazole
The temperature dependence of χT (χ is the molar para-
rings are rotated with respect to each other resulting in only magnetic susceptibility) of compounds 2–5 as bulk crystal-
one single π–π stack (green line in Figure S2, Table S4). line powders were derived from magnetization measure-
This latter weaker π-stacking contact is not observed at ments in an applied field of 0.5 T (0.1 T for 3) and in the
higher temperatures. The sulfur atoms of the two thiocy- temperature range 2–300 K. Data were also obtained for
anates coordinated to Fe2 form S···H–C interactions with fresh crystals of 2·5MeOH kept in a small amount of
the phenyl and triazole rings of three different neighbouring MeOH to avoid modifications of the magnetic properties
[Fe2] complexes, as well as an S···π contact with a terminal upon possible solvent loss. The data for 2·5MeOH and dry
triazole ring (Figure S3, Table S4). The thiocyanate sulfur 2, shown in Figures 3 and 4, respectively, corroborate the
atoms also act as H-bond acceptors for the hydroxy hydro- structural observations and evidence the occurrence of a
gen atoms of the lattice MeOH molecules (Figure S4, partial thermal SCO for intact crystals of 2·5MeOH and
Table S4). The MeOH oxygen atoms in turn are acceptors the absence of such a process for dry 2. Indeed, χT of
in strong H-bonds with triazole and imine hydrogen atoms. 2·5MeOH decreases upon cooling from 6.81–
However, these H-bonds do not participate in intercomplex 6.70 cm3 mol–1 K in the range 250–170 K, in agreement with
interactions.
Warming the second crystal to 296 K resulted in poorer 2), and reaches a plateau at ca. 3.35 cm3 mol–1 K at 60–50
diffraction; however, the data collected were of sufficient K. Further decrease then sets in down to 0.50 cm3 mol–1
two FeII ions in their HS S = 2 state (6 cm3 mol–1 K for g =
K
quality and showed that the crystal had lost its lattice meth- at 2 K. Compound 2·5MeOH thus undergoes a partial and
anol molecules. Because the crystal was kept in a dry flow gradual SCO centred at ca. 115 K. In reasonable agreement
of N2, no air moisture was absorbed. Attempts to measure with the structural observations at 100 K, the plateau
crystals exposed to air failed owing to powdering. At reached at ca. 60 K has a χT value corresponding to ca.
296 K, desolvated 2 still crystallizes in the triclinic P1 space 50% of the FeII ions in their LS state. The further lowering
¯
group in a similar cell, but with the a and b axes and corre- of χT below 50 K is probably caused by a combination of
sponding angles α and β inter-exchanged. The asymmetric zero field splitting (ZFS) effects of the remaining S = 2 ions
unit now only contains the dinuclear neutral complex, and antiferromagnetic interactions within the correspond-
which is comparable in most aspects to that of the solvated ing dinuclear complexes (see below). These observations are
structures (Tables 1, S2 and S3), although no disorder of reproducible upon warming over various cycles and over
any thiocyanato ligand is observed. An important differ- several batches of crystals as long as the sample is kept in
ence is found in the orientation of the thiocyanato ligands contact with MeOH and the temperature is not brought
(Table S3); whereas they are almost perfectly eclipsed when above 250 K. On the other hand, χT for the bulk powder of
looking along the Fe1···Fe2 axis in the lower temperature 2 (most likely its hydrated form, see Experimental Section)
structures, which have S1–N17–N24–S4/S2–N18–N23–S3 remains practically constant at 6.9–6.6 cm3 mol–1 K (again
torsion angles of 1.78/–3.68° at 150 K, their orientation in agreement with the presence of two FeII ions in their HS
strongly differs at 296 K, with torsion angles of 84.57/ state) from 300 K down to 60 K; below this temperature,
43.08° (see Figure 2). This probably has an effect on the χT starts to decrease more and more abruptly to reach a
ligand field exerted by the thiocyanato ligands. However, value of 1.06 cm3 mol–1 K at 1.8 K. The magnetic properties
the structure of 2 maintains the zigzag chains of dinuclear of 2 are thus in agreement with an S = 2 HS ground state
complexes formed through π-stacking interactions of two for the FeII ion with g close to 2 throughout the whole tem-
of the bridging 1 ligands with similar geometries as that perature range considered; the decrease at low temperatures
observed in the solvated structures (Table S5). Additional is due to a combination of ZFS of the S = 2 spins and
S···H–C interactions involving three of the thiocyanate sul- weak antiferromagnetic interaction. An evaluation of the
fur atoms (Figure S5 and Table S5) result in a rather intri- antiferromagnetic interaction through the triple N1,N2-tri-
cate network of supramolecular interactions.
azole bridge can be obtained by neglecting the ZFS, which
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for octahedral HS FeII centres should be rather small. Thus, NiII ions. This is reasonable as it will likely be active only
the analytical expression of the magnetic susceptibility de- at low temperatures, whereas χT already decreases signifi-
rived[25] by applying the van Vleck equation and Kambé cantly at 150 K. Thus, the experimental data were fitted to
vector coupling method to the Heisenberg Hamiltonian H the corresponding Heisenberg expression for the magnetic
= –2JS1·S2 in the mean-field approximation was used to fit susceptibility and yielded the best parameters g = 2.25(1)
the experimental data. The resulting best fit (full line in and J = –13.9(2) K (full line in Figure 4). This moderate
Figure 4) was obtained for g = 2.18(2) and J/kB
–1.33(3) K.
=
antiferromagnetic exchange coupling should be taken as an
upper limit to the actual interaction propagated by the
N1,N2-triazole bridge in 4. For 5, the decrease at low tem-
peratures can only be due to antiferromagnetic interactions,
and the experimental data are properly reproduced by the
analytical Heisenberg expression for the magnetic suscep-
tibility. The best fit is obtained for g = 2.32(1) and J/kB
=
–0.30(5) K (full line in Figure 4), and thus confirms that
only a very weak antiferromagnetic interaction is present in
5.
The Triple N1,N2-Triazole Bridge – a Rigid Constraint
The structural parameters that describe transition metal
ion pairs bridged by three N1,N2–1,2,4-triazole ligands re-
ported in the literature for FeII and CoII are gathered in
Table 2. These include only dinuclear and trinuclear com-
pounds and one coordination polymeric chain compound.
It appears that the topology of the triple N1,N2-1,2,4-tri-
azole bridges remains very similar in these compounds and
is very close to the idealized geometry shown in Scheme 1.
This is clearly evidenced by the M–N–N angles, for which
not only the average values for the three triazoles and the
two metal ions remain very close to the ideal value of
125.26°, but also every single angle does not depart more
than 5° from it. The mutual dihedral angles of the triazole
rings are already close to 120° in most compounds, and the
average values are actually virtually equal to 120°. In fact,
deviations from this value for certain pairs of triazole rings
are all related to the presence of either external intermo-
lecular interactions, for example, the strong π–π stacking
interactions in 2·3.5MeOH and 3·3.5MeOH or the H-bond-
ing between dinuclear and mononuclear moieties in [Fe2-
(ptoltrz)5(NCS)4]·[Fe(ptoltrz)2(H2O)2(NCS)2],[20d] or disor-
der as in [Fe2(salatrz)5(NCS)4]·4MeOH.[20f] This structural
stability of the M–(N1,N2-trz)3–M bridge happens even
Figure 3. χT vs. T plot for crystals of 2·5MeOH kept in contact
with a small amount of MeOH.
Figure 4. χT vs. T plots for 2–5. Full lines are fits to the analytical
Heisenberg expression for the susceptibility of pairs of coupled S
= 2 (2), 1 (4) and 1/2 (5) spins (see text).
Similarly to that for 2, the χT vs. T data for 3, 4 and 5 though the other “side” of each metal atom of the bridged
(Figure 4) show a plateau at higher temperatures (above pair actually differs significantly; the three fac coordination
200, 150 and 10 K, respectively) in agreement with two S = sites may indeed be occupied by either solvent molecules,
3/2 CoII, S = 1 NiII and S = 1/2 CuII spins at 6.1–6.0, 2.3– terminal neutral ligands, including monocoordinated tri-
2.2 and 1.01–1.005 cm3 mol–1 K, respectively. At lower tem- azoles, anions, or another triple triazole bridge. The only
peratures, a decrease sets in down to 0.11, 0.09 and dissymmetric dinuclear complex of this family of com-
0.97 cm3 mol–1 K, respectively, at 2 K. For 3, this decrease pounds, [Co2(altrz)4(H2O)(NCS)4]·2H2O, is in this respect
already occurs from 200 K and probably involves both in- exemplar;[20c] although the coordination environment of the
tramolecular antiferromagnetic interactions that leads to a two Fe sites are completed by either an 4-allyl-1,2,4-triazole
singlet ground state and intrinsic strong spin–orbit coupling ligand or a water molecule, very similar triazole M–N bond
of the HS CoII ions, which results in the stabilization of a lengths and M–N–N angles are observed for both metal
doublet ground state, for each individual CoII ion in the sites. Similarly, in trinuclear compounds obtained with p-
dimer.[26] This makes the analysis of the magnetic properties MeOprtrz[18c] the external FeII sites have a much more dis-
difficult and impeded the modelization of the experimental torted octahedral environment, but this has only a slight
data for 3. For 4, an evaluation of the antiferromagnetic effect on the characteristics of their triple N1,N2-1,2,4-tri-
interaction can be obtained by neglecting the ZFS of the azole bridges.
Eur. J. Inorg. Chem. 2013, 934–942
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Table 2. Structural parameters describing the M–(N1,N2-trz)3–M moiety in all reported FeII and CoII coordination compounds containing
one triple N1,N2-triazole bridge.[a]
Compound[b]
T [K] M···M [Å] M–N[c][d] [Å]
Σ
N–N–M[c] [°] trz–trz angles[e] [°]
TSCO [K] Ref.
[f]
2·3.5MeOH
100
250
296
100
r.t.
3.789
3.967
3.995
3.903
3.907
3.909
3.937
3.823
3.648
3.966
2.077/2.055
2.203/2.189
2.193/2.223
2.147/2.130
2.142/2.157
2.147/2.142
2.192/2.189
2.114
20/17 125.6
23/23 126.0
24/28 126.1
25/19 126.0
18/25 125.8
21/11 124.9/126.8
23/26 125.5
119.9 (109.4–136.8) 115
119.9 (109.1–136.9)
119.7 (117.8–122.4) HS
[f]
2
[f]
3·3.5MeOH
[Co2(Phtrz)5(NCS)4]·2.7H2O
[Co2(altrz)4(H2O)(NCS)4]·2H2O
[Fe2(ptoltrz)5(NCS)4]·[Fe(ptoltrz)2(H2O)2(NCS)2] r.t.
[Co2(atrz)3(H2O)4(tp)2]·11H2O
[Fe2(salatrz)5(NCS)4]·4MeOH
120.0 (108.7–138.2)
119.5 (114.3–126.1)
119.8 (118.5–121.0)
–
–
–
[20b]
[20c]
[20d]
[20e]
[20f]
r.t.
119.7 (106.4–133.9) 111
119.7 (115.6–127.9)
130
100
200
21
19
15
125.1
125.1
125.9
–
1.962
2.165
120.0 (116.6–122.1) 150
119.6 (117.0–122.1)
[18b]
[Fe3(etrz)6(H2O)6](CF3SO3)6
105
300
r.t.
r.t.
r.t.
120
330
r.t.
100
181
202
202
3.795
3.840
3.868
3.793
3.833[c]
3.783
3.832
3.862
3.839
3.867
3.863
3.853
2.031/2.176
2.174/2.157
2.175/2.146
2.177/2.124
2.167/2.159
2.003/2.161
2.169/2.176
2.191/2.157
2.151/2.163
2.009/2.145
2.141/2.127
2.137/2.105
28/32 126.1/123.6
10/25 124.5/124.3
15/30 123.8/125.8
119.9
119.7
205
[18c]
[18c]
[18d]
[18e]
[Fe3(p-MeOptrz)8(H2O)4](BF4)6·2H2O
[Fe3(p-MeOptrz)6(H2O)6](tos)6·2MeOH·8H2O
[Fe3(iprtrz)6(H2O)6](tos)6·2H2O
120.0 (114.3–129.9) HS
119.3 (113.9–126.2) 330
120.0 (112.4–125.4) 242
120.0
120.0
119.8
119.7 (115.9–121.7) 148
119.7 (116.0–121.7)
119.8 (116.1–121.7)
119.3 (116.0–127.2)
5/35
125.4/122.1
11/22 124.7/124.5
20/20 125.7/122.7
[Fe3(hyetrz)6(H2O)6](CF3SO3)6
290
9/21
127.6/122.7
[18f]
[18g]
[Fe3(prtrz)6(H2O)6](ReO4)6·H2O
[Fe3(npt)6(EtOH)4(H2O)2](tos)2·4EtOH
21/21 123.6/124.6
17/27 127.0/125.3
185
9/19
7/15
125.5/124.6
125.1/125.6
[18g]
[18g]
[Co3(npt)6(EtOH)4(H2O)2](tos)2·4EtOH
[Co3(npt)6(MeOH)4(H2O)2](tos)2·4EtOH
–
–
14/20 126.1/124.7
[g]
[16]
[Fe(atrz)3](NO3)2·2H2O
120
3.655
1.95/1.99
15/11 123.9/127.2
119.9 (117.6–122.7)
[a] The structure of [Co3(metrz)6(H2O)6][Co3(metrz)8(H2O)4](CF3SO3)12·8H2O (metrz = 4-methyl-1,2,4-triazole) is not included owing to
its poor quality.[18h] Note that [CoII2CoIII(HL)2(L)4(H2O)6]Cl3·9H2O (HL = 3,5-diamino-1,2,4-triazole) is not included in this summary
due to its mixed-valent character and the presence of charged triazolate.[21b] [b] Phtrz = 4-phenyl-1,2,4-triazole; altrz = 4-allyl-1,2,4-
triazole; ptoltrz = 4-(p-tolyl)-1,2,4-triazole; atrz = 4-amino-1,2,4-triazole; tp = terephthalate; salatrz = N-salicyliden-4-amino-1,2,4-tri-
azole; etrz = 4-ethyl-1,2,4-triazole; p-MeOptrz = 4-(p-methoxyphenyl)-1,2,4-triazole; iprtrz = 4-isopropyl-1,2,4-triazole; tos = p-toluene-
sulfonate; hyetrz = 4-(2Ј-hydroxyethyl)-1,2,4-triazole; prtrz = 4-propyl-1,2,4-triazole; npt = 4-(4Ј-nitrophenyl)-1,2,4-triazole. [c] If there
are various values (depending on symmetry), then the average value is given. In the case of M–N bond lengths, only those corresponding
to triazole rings participating in the triple N1,N2-bridge are averaged. [d] Values are given for each M crystallographic site. For trimers,
the first value corresponds to the central metal ion that exhibits SCO for M = Fe, and the second to the outer metal ion. [e] Mutual
dihedral angles between the planar triazole rings. The first value is the average or unique value. Values in parentheses are the maximum
and minimum angles. [f] This work. [g] The structure indicates an LS state at 120 K. The dehydrated sample has an abrupt SCO with a
large hysteresis above r.t.[5b]
It is therefore evident that the geometry of the triple SCO will result in a strong elongation (or compression) of
N1,N2-triazole bridge is maintained with only very small the coordination chains.[27] Nevertheless, additional in-
variations whatever the environment of the M–(N1, terchain interactions are required to explain the very strong
N2-trz)3–M moiety and is a dominant parameter in the cooperativity of [Fe(Htrz)2(trz)](BF4), for example.[28]
structures in which it is involved. Through its stability, the
Eventually, the structurally constraining character of the
M–(N1,N2-trz)3–M bridge thus acts as a constraint on the M–(N1,N2-trz)3–M bridge probably has synthetic conse-
rest of the metal coordination sphere towards a regular oc- quences. Indeed, given the stability of the bridge, it is sur-
tahedral geometry. This is well shown by the small distor- prising that besides the coordination polymers, no longer
tion parameters Σ observed for all FeII and CoII ions in this oligomers than dinuclear and trinuclear complexes could be
family of compounds (Table 2). Such a constraint would isolated so far. These are believed to coexist in solution,[4a]
clearly become stronger in longer systems, and this is prob- possibly in the presence of larger oligomers, and this is rea-
ably what makes the 1D coordination polymers of formula sonable considering the corresponding syntheses involve
[Fe(Rtrz)3](A)2 peculiar; the repetition of triple bridges very similar M/triazole ratios, typically from 1:2 to 1:2.5 for
forces the FeII ions into a rigid and regular octahedron. dimers and trimers, and 1:3 to 1:4 for 1D compounds. Poss-
Then, the structural modifications induced by the coordina- ibly, the constrained structure created by repeating M–
tion sphere volume variation upon the SCO will thus mostly (N1,N2-trz)3–M bridges is only stabilized by a certain
be propagated along the M···M axis, so that the stable ge- number of repeating units, so that only longer oligomers
ometry of the M–(N1,N2-trz)3–M moiety is maintained. are present in solution. These will only crystallize if a
Clearly, with a variation of the coordination sphere volume homogeneous length is reached, which could explain why
of up to 20% upon SCO,[20] such a rigidity represents a only infinite coordination chains are crystallized. Shorter
strong source of intrinsic cooperativity. However, this will chains/oligomers have indeed only been reported from the
only occur within extended linear structures, in which the breaking of preformed infinite chains, either at an air/water
Eur. J. Inorg. Chem. 2013, 934–942
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interface during Langmuir–Blodgett deposition[10a] or from
a heat-set gel–sol transition induced by the change of coor-
dination number of CoII ions.[29]
tered, and the filtrate was added under Ar to a methanol (160 mL)
solution of 1 (668 mg, 3.88 mmol). The mixture was then heated to
reflux under Ar for 90 min. After cooling to room temperature, a
yellow solid formed, which was recovered by filtration, washed and
dried in air. Yield: 210 mg, 16% (based on Fe). The filtrate was left
under a weak flow of Ar to allow slow evaporation of the solvent,
which resulted in the formation of yellow crystals after about 24–
36 h. These crystals, suitable for single-crystal X-ray analysis, con-
tained variable amounts of lattice methanol, i.e., 2·3.5MeOH or
2·5MeOH. Indeed, they very rapidly lose crystallinity when ex-
posed to air, probably loosing methanol and absorbing moisture
from air, as indicated by elemental analysis of crystalline material
dried in air. The crystals were kept under Ar in a small amount
of mother solution or methanol. Prolonged storage (Ͼ 6 months)
results in damage and partial oxidation of the crystals.
C49H50Fe2N24O5S4 (2·5H2O): calcd. C 45.44, H 3.89, N 25.96;
Conclusions
A series of dinuclear compounds of formula [M2(1)5-
(NCS)4] (M = Fe, Co, Ni, Cu) are obtained from the reac-
tion of 4-phenylimino-1,2,4-triazole (1) with MII thiocy-
anates. The crystals structures of the FeII and CoII ana-
logues are new additions to a very limited number of known
structures containing one sole M–(N1,N2-trz)3–M moiety.
The structural parameters of this dinuclear unit as found in
dinuclear, trinuclear and 1D structures in the literature are
compared. The topology of the bridge is found to be main-
tained independently of its environment, which confirms its
stability and highlights its rigidity and related constraints.
The FeII analogue [Fe2(1)5(NCS)4]·xMeOH (x = 3.5–5) is
shown to present a gradual and incomplete spin crossover
centred at 115 K as confirmed by both magnetic measure-
ments and variable-temperature diffraction studies.
found C 45.4, H 3.8, N 26.1. IR: ν = 3437 (br.), 3143, 3100, 3030,
˜
2925, 2063, 1618, 1602, 1576, 1525, 1516, 1492, 1450, 1311, 1292,
1227, 1170, 1059, 1025, 955, 764, 752, 689, 623, 591, 501 cm–1. The
IR spectra of the crude yellow solid and the crystals were virtually
identical. Overall yield (crystals and powder): 545 mg, 42%.
[Co2(1)5(NCS)4] (3): CoSO4·7H2O (283 mg, 1.01 mmol) was dis-
solved in methanol (10 mL), and NH4NCS (162 mg, 2.13 mmol)
was added. The resulting mixture was stirred for 5 min and filtered.
The filtrate was added to a methanol (100 mL) solution of 1
(430 mg, 2.50 mmol) to result in a clear pinkish solution, which
was stirred at room temperature overnight. The fleshy solid formed
was recovered by filtration, washed with MeOH and diethyl ether
and dried in air. Yield: 230 mg, 18% (based on Co).
C49H50Co2N24O5S4 (3·5H2O): calcd. C 45.23, H 3.87, N 25.83;
Experimental Section
Physical Measurements: FTIR spectra were recorded with pure so-
lid samples with a Perkin–Elmer Spectrum 100 spectrometer
equipped with a universal attenuated total reflectance (ATR) sam-
pling accessory. Magnetic measurements were performed with an
MPMS-XL superconducting quantum interference device
(SQUID) magnetometer. Data were corrected for the experimen-
tally determined sample holder diamagnetism and for the intrinsic
diamagnetic susceptibility of the sample as calculated from Pascal
tables.[30] Microanalyses were carried out with a Perkin–Elmer
Series II CHNS/O Analyzer 2400 at the Servei de Microanalisi of
CSIC (Barcelona, Spain). All reagents were commercially available
and were used without purification. All manipulations were carried
out under aerobic conditions, except where specified (compound
2).
found C 45.0, H 4.0, N 25.5. IR: ν = 3426 (br.), 3151, 3100, 3030,
˜
2971, 2067, 1618, 1602, 1575, 1527, 1515, 1490, 1448, 1310, 1292,
1227, 1171, 1059, 1026, 954, 763, 752, 685, 622, 592, 502 cm–1. Sin-
gle-crystals suitable for X-ray diffraction studies were obtained by
slow concentration of the filtrate and were stored in their mother
liquor or MeOH owing to loss of crystallinity upon air exposure.
The IR spectra of the powder and crystals were virtually identical.
Overall yield (crystals and powder): 630 mg, 48%.
[Ni2(1)5(NCS)4] (4): NiSO4·5H2O (287 mg, 1.02 mmol) was dis-
solved in methanol (20 mL), and NH4NCS (165 mg, 2.17 mmol)
was added. The resulting mixture was stirred for 10 min and fil-
tered. The filtrate was added to a methanol (90 mL) solution of 1
(427 mg, 2.48 mmol) to result in a clear blue solution, which was
stirred at room temperature overnight. The fine purple solid formed
was recovered by filtration, washed with MeOH and diethyl ether
and dried in air. Yield: 655 mg, 50.3% (based on Ni). Attempts to
grow single crystals from the filtrate were unsuccessful.
C49H50Ni2N24O5S4 (4·5H2O): calcd. C 45.24, H 3.87, N 25.84;
4-Phenylimino-1,2,4-triazole (1): 4-Amino-1,2,4-triazole was pre-
pared from hydrazine monohydrate and formic acid. 4-Amino-
1,2,4-triazole (7.23 g, 0.085 mol) was dissolved in absolute ethanol
(14 mL). This solution was warmed to 50 °C, and benzaldehyde
(11.7 g, 0.11 mol) dissolved in absolute ethanol (10 mL) was added.
The resulting reaction mixture was heated to reflux for 5 h, and a
white solid corresponding to crude 1 formed upon cooling.
Recrystallization from EtOH/diethyl ether afforded a total (3 crops)
of 12.7 g (0.073 mol) of pure 1. Yield: 87%. 13C NMR (300 MHz,
acetone, 298 K): δ = 129.35, 129.95, 133.10 and 133.51 (phenyl),
158.26 (C=N) ppm. 1H NMR (300 MHz, acetone, 298 K): δ = 7.58
(t, 3 H, phenyl), 7.93 (d, 2 H, phenyl), 8.91 (s, 2 H, triazole), 9.10
(s, 1 H, C=N) ppm. C9H8N4 (172.19): calcd. C 62.78, H 4.68, N
found C 44.8, H 3.4, N 26.1. IR: ν = 3434 (br.), 3152, 3108, 3030,
˜
2080, 1614, 1603, 1575, 1527, 1522, 1491, 1450, 1334, 1312, 1294,
1228, 1174, 1060, 998, 860, 762, 753, 690, 624, 592, 512 cm–1.
[Cu2(1)5(NCS)4] (5): CuSO4·5H2O (255 mg, 1.02 mmol) was dis-
solved in methanol (10 mL), and NH4NCS (164 mg, 2.15 mmol)
was added. The resulting reaction mixture was stirred for 5 min and
filtered. The filtrate was added to a methanol (100 mL) solution of
1 (430 mg, 2.50 mmol) to result in a clear green solution, which
was stirred at room temperature overnight. The green solid formed
was recovered by filtration, washed with MeOH and diethyl ether
and dried in air. Yield: 680 mg, 53% (based on Cu). Attempts to
grow single crystals from the filtrate only resulted in the formation
of very low quantities of tiny single crystals, which did not diffract
sufficiently. C49H44Cu2N24O3S4 (5·3H2O): calcd. C 46.18, H 3.64,
32.54; found C 62.71, H 4.76, N 32.69. IR: ν = 3102, 3030, 3004,
˜
2956, 1614, 1604, 1577, 1502, 1492, 1464, 1450, 1405, 1328, 1311,
1290, 1234, 1215, 1163, 1057, 941, 865, 750, 686, 621, 589,
503 cm–1.
[Fe2(1)5(NCS)4] (2): FeSO4·7H2O (278 mg, 1.00 mmol), NH4NCS
(157 mg, 2.06 mmol) and ascorbic acid (10 mg) were mixed in
MeOH (40 mL), and the solution was stirred for 10 min to result
in a yellow solution and a white precipitate. The solution was fil-
Eur. J. Inorg. Chem. 2013, 934–942
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[5]
a) For a minireview, see: O. Roubeau, Chem. Eur. J. 2012, 18,
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diffractometer at the Advanced Light Source beamline 11.3.1 at
Lawrence Berkeley National Laboratory from a silicon 111 mono-
chromator (λ = 0.7749 Å). Data reduction and absorption correc-
tions were performed with SAINT and SADABS,[31] respectively.
Data for compound 2·5MeOH at 150 and 2 at 296 K were obtained
with an Oxford Diffraction Excalibur diffractometer with enhanced
Mo-Kα radiation (λ = 0.71073 Å) at the X-ray diffraction and
Fluorescence Analysis Service of the University of Zaragoza. Cell
refinement, data reduction and absorption corrections were per-
formed with the CrysAlisPro suite.[32] All structures were solved
with SIR92[33] and refined on F2 with the SHELXTL suite.[34] Crys-
tal data collection and refinement parameters are given in Table S1.
CCDC-901398 (for 2·3.5MeOH, 100 K), -901399 (for 2·3.5MeOH,
250 K), -901400 (for 2·5MeOH, 150 K), -901401 (for 2, 296 K)
and -901402 (for 3·3.5MeOH, 100 K) 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.
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Supporting Information (see footnote on the first page of this arti-
cle): Crystallographic data for 2·3.5MeOH at 100 and 250 K,
2·5MeOH at 150 K, 2 at 296 K and 3·3.5MeOH at 100 K
(Table S1), relevant structural parameters of the dinuclear complex
and intermolecular interactions (Tables S2–S7). IR spectra of 2–5
(Figure S1), views of the crystal packing of 2·3.5MeOH (Fig-
ures S2–S4), view of the crystal packing of 2 (Figure S5), view of
the dinuclear complex in the structure of 3·3.5MeOH (Figure S6).
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P. G. acknowledges the Institució Catalana de Recerca i Estudis
Avançats (ICREA). P. G. and O. R. are grateful to the Spanish
Ministerio de Economía y Competitividad (MINECO) for funding
(Projects CTQ2011-27929-C02-01 and MAT2011-24284). The Ad-
vanced Light Source is supported by the Director, Office of Science,
Office of Basic Energy Sciences of the U. S. Department of Energy
under contract no. DE-AC02-05CH11231.
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Received: September 21, 2012
Published Online: November 26, 2012
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