Enhancement of spin-crossover cooperativity mediated by lone pair-π interactions and halogen bonding

Article abstract of DOI:10.1039/c3cc48334g

Rational ligand design has allowed the generation of a highly cooperative spin-transition iron(ii) complex, an unprecedented result in the family of (2,2′-dipyridylamino/s-triazine)-based SCO materials. This journal is

Full text of DOI:10.1039/c3cc48334g

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Enhancement of spin-crossover cooperativity
mediated by lone pair–p interactions and
halogen bonding†
Cite this: Chem. Commun., 2014,
50, 1003
Received 30th October 2013,
Accepted 29th November 2013
Nassim Nassirinia,ab Saeid Amani,b Simon J. Teat,c Olivier Roubeau*d and
Patrick Gamez*ae
DOI: 10.1039/c3cc48334g
www.rsc.org/chemcomm
Rational ligand design has allowed the generation of a highly
Stimulated by these findings, we have now prepared a new
cooperative spin-transition iron(II) complex, an unprecedented result member of this family of ligands, i.e. 2-chloro-4-(N,N-(2-pyridyl)-
in the family of (2,20-dipyridylamino/s-triazine)-based SCO materials. amino)-6-(pentafluorophenoxy)-(1,3,5)triazine (L1, Scheme S1, ESI†),
specifically designed to favour various types of intermolecular inter-
The remarkable switching properties of spin crossover (SCO) actions (viz. p–p, lone pair–p and halogen bonding contacts), to
materials have driven their utilization in several areas, namely promote the cooperative character of its ensuing iron(II) SCO
for the development of display and memory devices,1 electrical complex. Actually, the compound [Fe(L1)2(NCS)2]ꢀ2CH3OH (1) herein
and electroluminescent devices,2 MRI contrast agents,3 switchable described shows the highest cooperative comportment achieved to
liquid crystals,4 nanoparticles,5 and thin films.6 In most applica- date with a dipyridylamino-substituted-triazine ligand.
tions, highly cooperative systems, i.e. abrupt low-spin (LS) 2
Ligand L1 is easily obtained in two steps by reaction of 2,4,6-
trichloro-1,3,5-triazine with 2,20-dipyridylamine and pentafluoro-
high-spin (HS) transitions, are required.6,7
For about seven years, our group and that of Murray have been phenol (ESI†). Reaction of two equivalents of L1 with one equivalent
concurrently developing a family of dipyridylamino-substituted- of Fe(NCS)2 in MeOH–CH2Cl2 produces [Fe(L1)2(NCS)2]ꢀ2CH3OH (1)
triazine that are designed to generate iron(II) SCO compounds.8–17 with a yield of 77% (ESI†). 1 crystallizes in the triclinic space group
%
The iron(II) systems obtained so far from such ligands solely exhibit P1, both at 100 K (LS state, purple crystal) and 200 K (HS state, yellow
gradual spin transition behaviours,18 indicative of poor cooperativity crystal) (Table S1, ESI†). A representation of the molecular structure
between the metal centres. Recently, we have discovered that the of 1 at 100 K (LS 1) is shown in Fig. 1a. Selected bond distances and
incorporation of pentafluorophenyl rings on the triazine unit allows angles for LS and HS 1 are listed in Table S2 (ESI†). The octahedral
the occurrence of very particular supramolecular interactions, viz. coordination environment of the iron(II) ion in 1 is typical for this
lone pair–p contacts and halogen bonding, which apparently type of SCO compounds,16,17 with two L1 ligands at the equatorial
enhance the cooperative properties of the corresponding SCO plane of the octahedron and two trans isothiocyanates at the apical
complex, i.e. [Fe(L1F)2(NCS)2]ꢀ2CH3CN (L1F = 2-(N,N-bis(2-pyridyl)- positions. At 100 K, the Fe–NL1 distances in the range 2.005(3)–
amino)-4,6-bis(pentafluorophenoxy)-(1,3,5)triazine).18
2.012(2) Å and Fe–NNCS bond lengths of 1.953(3) Å are typical of a LS
iron(II) ion (Table S2, ESI†). At 200 K, these coordination bond
lengths increase by ca. 0.22 Å for Fe–NL1 and ca. 0.12 Å for Fe–NNCS
(Table S2, ESI†), thus describing a full spin transition as also
observed by variable-temperature magnetometry. Indeed, the tem-
perature dependencies of the wT product for bulk 1 (Fig. 2a, w being
the molar paramagnetic susceptibility), evidence an abrupt thermal
SCO, from 3.17 cm3 molꢁ1 K above 200 K, a typical value for an Fe(II)
ion in a HS S = 2 state, to ca. 0.22–0.20 cm3 molꢁ1 K below 50 K,
which is indicative of a mostly LS S = 0 state.19 In addition to its
abruptness, the process of thermal SCO in 1 exhibits a small but
reproducible hysteretic behaviour, the transition being centred at
123 K and 124 K respectively upon cooling and warming (Fig. S1,
ESI†). Such strong cooperative behaviour is unique among the
family of (2,20-dipyridylamino/s-triazine)-based SCO compounds
reported so far. Hence the SCO properties of 1 are compared to
a
´
`
Departament de Quımica Inorganica, QBI, Universitat de Barcelona,
´
`
Martı i Franques 1-11, 08028 Barcelona, Spain. E-mail: patrick.gamez@qi.ub.es;
Web: www.bio-inorganic-chemistry-icrea-ub.com; Fax: +34 934907725
b Arak University, Faculty of Sciences, Department of Chemistry,
Arak 38156-8-8349, Iran
c Advanced Light Source (ALS), Lawrence Berkeley National Laboratory,
1 Cyclotron Road, Berkeley, CA 94720, USA
d
´
Instituto de Ciencia de Materiales de Aragon (ICMA), CSIC and Universidad de
Zaragoza, Plaza San Francisco s/n, 50009 Zaragoza, Spain.
E-mail: roubeau@unizar.es
e
´
Institucio Catalana de Recerca i Estudis Avançats (ICREA),
´
Passeig Lluıs Companys 23, 08010 Barcelona, Spain
† Electronic supplementary information (ESI) available: Experimental proce-
dures, crystallographic data and figures showing the different supramolecular
bonds present in the solid-state structure of 1. CCDC 968835 (LS 1) and 968836
(HS 1). For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c3cc48334g
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Chem. Commun., 2014, 50, 1003--1005 | 1003

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entropy is well above the purely electronic component, Rln 5,
indicating an important role played by the coupling of the electro-
nic transition with lattice phonons. This enhanced cooperative
behaviour of 1 most likely arises from packing features involving
intermolecular interactions.
The octahedral distortion parameters S and F gauge the
magnitude of the deformation of the coordination geometry
relative to a perfect octahedron (for which S = F = 0).7,22,23 For
LS 1, S = 32 and F = 42, and their values increase to respectively
39 and 64 upon LS - HS transition (Table S3, ESI†). The
corresponding DS and DF values of respectively 7 and 22 thus reflect
a distortion of the Oh geometry, although they are smaller than those
of the less cooperative compound 2 (respectively 11 and 36).18 This
observation disagrees with literature suggestions that associate a
large distortion with a high cooperativity between the transiting
centres.7 The DS and DF data observed for 1 and 2 rather indicate
that low distortion values may also be associated with a cooperative
behaviour, as in other trans-[Fe(L)2(NCS)2] compounds (where L is an
alpha-diimine ligand) with strong intermolecular interactions.24 This
can be explained by the way the local distortion of the coordination
sphere is affected by its environment; accordingly, other structural
aspects such as intra- and intermolecular interactions can play an
important role regarding cooperativity. Indeed, low DS and DF
values can indicate a lack of freedom for the iron(II) centres
to undergo the geometry distortion that takes place upon the
Fig. 1 (a) Molecular structure of 1 (LS state, determined at 100 K) with partial
atom-numbering scheme. The H atoms and the lattice methanol molecules are
not shown for clarity. Symmetry operation: a, 1 ꢁ x, 1 ꢁ y, 1 ꢁ z. (b and c)
Supramolecular framework observed in the solid-state structure of 1, generated
by p–p, lone pair–p and halogen bonding interactions (blue, green and red
dotted lines, respectively), all involving the pentafluorophenyl ring.
6
2
4
t2g (LS) - eg t2g (HS) transition, possibly as the result of steric
constraints arising from a more ‘‘rigid’’ solid-state packing of the
molecules. In fact, such a situation is expected to allow an efficient
communication of the SCO distortion to adjacent iron(II) ions, giving
rise to an abrupt transition (with possibly hysteresis).
The crystal packing of 1 corroborates the above assumptions.
Ligand L1 has been designed on the basis of the earlier results
achieved with ligand L1F,18 with the objective to favour a better
interaction between the iron(II) ions in the solid state (and thus
increase the cooperativity). Therefore, one of the two pentafluoro
units of L1F has not been introduced in L1. Consequently, L1 is
Fig. 2 (a) wT vs. T plot for 1 (empty black circles) and 2 (full grey squares).18
Lines are only guides to the eye. (b) Excess molar heat capacities and the
excess entropy associated with the SCO in 1. The full red line is a fit to the
domain model of Sorai (see ESI† and ref. 21) with n = 14.2.
those of [Fe(L1F)2(NCS)2]ꢀ2CH3CN (2) (whose ligand L1F includes two less bulky than L1F (which, in principle, will endorse a closer
pentafluoro rings instead of one for L1), which already displayed packing of the metal complexes) but still possesses a fluorinated
an improved cooperative behaviour with respect to other similar phenyl ring, which is of paramount importance for the generation
compounds;18 the significantly smaller DT80 value (the temperature of supramolecular interactions (that may enhance cooperativity).18
range within which 80% of the transition occurs) of ca. 8 K for 1 Careful analysis of the crystal structure of 1 reveals that, as
(DT80 E 50 K for 218) is a rough measure of its increased cooperative anticipated, the pentafluorophenyl unit plays a crucial role in
character (Fig. 2). This higher cooperativity is confirmed by Differ- the close association of the molecules in the lattice (as observed
ential Scanning Calorimetry (DSC). The derived molar heat capacity for 2,18 but to a lesser extent). Indeed, this fluorinated arene is
of 1 at constant pressure, i.e. Cp, exhibits a sharp anomaly between involved in an intricate network of noncovalent bonds that
117 and 145 K, culminating at 130 K and with an onset at 125 K. produces a 3D framework (Fig. 1b).
This feature is in perfect agreement with the magnetic data (Fig. S2,
First, molecules of 1 are connected via parallel-displaced p–p
ESI†), and is thus due to the SCO process in 1. Both the associated interactions characterized by short Fꢀ ꢀ ꢀC contact distances of
excess enthalpy and entropy of respectively 7.29 kJ molꢁ1 and 3.076(6) Å for LS 1 and 3.205(6) Å for HS 1 (Fig. S4, ESI†). Since
56.3 J molꢁ1 Kꢁ1 are large for a SCO at these relatively low tempera- the sum of the van der Waals radii (vdW) of F and C is 3.17 Å,
tures (Fig. 2b and Fig. S3, ESI†), and are ascribed to its cooperative the distances above are illustrative of a strong interaction. The
character.20 Fitting the excess heat capacity of 1 to Sorai’s domain pentafluorophenyl ring strongly interacts with a neighbouring
model21 actually results in a high number of interacting molecules isothiocyanate sulfur atom (Fig. S5, ESI†). This lone pair–p
per domain, i.e. n = 14.2 (red line in Fig. 2b, see ESI† for details), interaction is defined by Sꢀ ꢀ ꢀC and Sꢀ ꢀ ꢀF contact distances of
which is characteristic of a cooperative SCO. This value is more respectively 3.347(6)–3.382(5) Å and 3.269(4) Å (for LS 1), which
than twice that of 2, namely n = 6.2.18 Importantly, the excess are below the corresponding sums of vdW radii, namely 3.5 Å
1004 | Chem. Commun., 2014, 50, 1003--1005
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and 3.27 Å. An additional lone pair–p interaction is observed ions observed in the crystal packing (a direction) of both
between the pentafluorophenyl ring and the ligand oxygen complexes are strongly interacting with each other in 1 in the
atom from an adjacent complex (Fig. S6, ESI†). The consequent b direction (particularly via strong Clꢀ ꢀ ꢀF halogen bonds),
Oꢀ ꢀ ꢀF and Oꢀ ꢀ ꢀC contact distances of respectively 2.922(4) Å and in contrast to 2. Current investigation is aimed at designing
3.085(6) Å (for LS 1), describing this supramolecular bond, a new ligand that would favour the interaction of the Fe(II)
again are clearly below the corresponding sums of vdW radii chains (along a) both in the b and c directions, which would
(namely 2.99 and 3.22 Å), thus characterizing a strong interaction. further improve the strength and multidirectionality of the
Lastly, the pentafluorophenyl group is involved in a very strong supramolecular interactions.
´
halogen bond (Fig. S7, ESI†), as evidenced by the short FꢀꢀꢀCl
Support by the Ministerio de Economıa y Competitividad of
distances of 2.969(4) Å (LS 1) and 3.013(4) Å (HS 1), which are Spain (Projects CTQ2011-27929-C02-01 and MAT2011-24284) is
notably below the sum of vdW of F and Cl (i.e. 3.22 Å). This network thanked. ALS is supported by the Director, Office of Science,
of noncovalent bonds generated by the pentafluorophenyl ring Office of Basic Energy Sciences of the U. S. Department of Energy
produces a compact and rigid molecular packing, which creates under contract no. DE-AC02-05CH11231.
efficient paths for the propagation of the local distortion due to the
SCO process at the iron(II) ions, resulting in a strongly cooperative
spin crossover (abrupt transition with hysteresis loop).
Notes and references
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´
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