Quesada Vilar, Manuel
Citation
Quesada Vilar, M. (2007, March 29). Spin-transition frameworks based on bistetrazole and triazine ligands. Retrieved from https://hdl.handle.net/1887/11463
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5
Structural considerations on
bistetrazole-based spin-transition
compounds
Abstract
A structural comparison between spin-transition bistetrazole-based compounds reported in the literature and those described in the previous two chapters is presented. The effect of the transition on the metal-to-ligand bond length, the distortion of the octahedron, the crystal packing, including anion-π interactions, inter-molecular, intra-molecular interactions, as well as lattice parameters is evaluated. A relationship with the transition temperature of the compounds is suggested.
5.1 Introduction.
As for many other physical properties of molecular materials, the spin-transition features are closely related to the structural aspects of the compounds.1 Recently, the advances in the crystal structure determination technique has shown to be crucial to explain the structure- property relationships observed in some peculiar spin-transition behaviours.2, 3 Certain correlations have been proposed between the distortion of the octahedron and the spin- transition properties of SCO compounds.4-6 Among the various novel families of polymeric SCO materials, the bistetrazole-based series is highly valuable, since most of the synthesised [iron(II)-bistetrazole]n coordination polymers can be crystallised. Gradual as well as steep transitions are observed with bistetrazole-based SCO materials, depending on the overall rigidity of their coordination framework. [Fe(btzp)3](ClO4)2 was the first 1D polymer to be structurally characterised by means of single-crystal X-ray diffraction.7 Variations of the size of the linker between the two tetrazole rings have resulted in spin-transition coordination polymers with different dimensionality, from 1D to 3D.
In the present chapter, distinctive structural features of known bistetrazole-based spin- transition materials are reported and compared, in order to appraise the impact of each building block on the resulting coordination networks, and the consequent physical properties.
Structure-property correlations are extracted from this thorough structural inspection.
5.2 Bond Lengths.
One of the main changes that occur during the spin-transition process is the distance between the metal ion and the donor atom of the ligand.8, 9 This modification of the bond length can be explained by the different electronic distribution for the two possible stable states of FeII species. While the low-spin state has all d-electrons in the non-bonding t2g
orbitals (t2g6), in the high-spin state the anti-bonding eg* orbitals are partially occupied (t2g4eg2). The amplitude of the bond length variation is dependent on the number of electrons undergoing the transition process, which in turn is characteristic of the metal ion involved.
For FeII, the SCO process involves two electrons, and gives rise to a difference bond distance of about 0.16 Å, as estimated by theoretical calculations10 (i.e. 10% decrease with respect to the high-spin bond length).11 Although the nature of the metal ion is the main factor influencing the change in bond length, some structural features, such as the ability of the complex to adjust its coordination environment through the flexibility of the ligand or via intermolecular interactions, are also playing a significant role.4 The geometric parameters commonly used to evaluate the main structural changes due to the transition in a series of SCO compounds are the coordination bond distances Fe-L, the average iron-to-ligand bond distance r, and the interval bond length ∆r between the HS and LS states.
In Table 5.1, the Fe-L values of bistetrazole SCO complexes whose crystal structures have been determined are shown. These values are all in the expected range for HS/LS FeII species obtained from tetrazole-based ligands. However, some peculiarities are observed, which are discussed below.
In monotetrazole-based compounds, the metal-to-ligand bond length varies from 2.19 Å for the HS state to 2.01 Å for the LS state.12 The 3D [Fe(btzb)3](PF6)2 polymer shows similar values, while 1D polymeric species exhibit shorter rHS distances, most likely as a result of the conformation/coordination mode adopted by the bistetrazole ligands. This bond shortening is clearly observed for [Fe(btzmp)2(µ-btzmp)2](ClO4)2, whose asymmetric unit includes a monocoordinated ligand and two bridging-ligands (Figure 5.1). Accordingly, the Fe-L distances for the bridging ligands are shorter than those of the non-bridging ligand. A different case is that of the benzyl-based spacer compounds, [Fe(btzx)3](PF6)2·CH3OH and [Fe(btzx)3](CF3SO3)2·CH3CN(see Chapter 4). While the former shows a relatively short rHS
distance, the latter shows normal rHS distances (i.e. comparable to those of monotetrazoles coordinated to an FeII ion) for its two crystallographically independent spin-transition centres.
In this case, the different coordination modes (bridging and/or non-bridging) of the ligand can obviously not be taken into account to explain the rHS values observed for the [Fe(btzx)3]2+
cation. The location of the anions in the crystal lattice may thus play a crucial role, altering the tilting capacity of the tetrazole rings. These adjustments of the tetrazole units are known to compensate the elongation/shortening of the ND···ND’ distancesa (Figure 5.1 b) generally observed in [MII(bisazole)x]n compounds.13
a) b)
Figure 5.1 a) Representation of the polymeric structure of [Fe(btzmp)2(µ-btzmp)2](ClO4)2
showing the bridging and non-bridging ligands. b) Schematic representation of all the ligands used in the compounds discussed in this chapter: endi = 1,2-bis(tetrazol-1-yl)ethane; btzp = 1,2-bis(tetrazol-1-yl)propane; btzmp = 1,2-bis(tetrazol-1-yl)-2-methylpropane; btzb = 1,4- bis(tetrazol-1-yl)butane; btzx = 1,3-bis(tetrazol-1-yl)xylene ; btzpol = 1,3-bis(tetrazol-1-yl)-2- propanol. At the bottom a representation of the ND···ND’ distance.
The size of ∆r plays a major role in the cooperative behaviour of spin-crossover compounds. The ∆r value amounts 10% of the average HS state Fe−L bond distance for monotetrazole-based complexes. Guionneau et al.4 have observed that for [FeLn(NCS)2] complexes (L = bidentate ligand) ∆r decreases with rHS, which suggests that the major
a ND stands for “donor atom”, and represents the atom coordinated to the metal ion.
structural alterations involve a distortion of the octahedral geometry of the HS state. The same ∆r/rHS trend is observed for bistetrazoles (see Table 5.1). However, [Fe(btzx)3](CF3SO3)2·CH3CN exhibits unusual features, since it shows the smallest ∆r but the largest rHS, and thus an uncommon long rLS. These small structural particularities could explain the unusual spin transition observed for this compound. Such a drastic structural effect on the magnetic properties of a material is a well-known phenomenon. For instance, significant differences in the magnetic properties of the same material have been observed, depending on the polymorphic phase studied.14, 15
Table 5.1. Typical geometric parameters for a series of bistetrazole-based 1D polymers whose X-ray crystal structures are known. Fe-L represents the metal-to-ligand bond length distance; r is the average value of all Fe−L distances; ∆r symbolises the difference between the HS and the LS r values (the percentage of the bond distance variation, with respect to the rHS value, is reported in parentheses); Σ is the distortion parameter reflecting the deformation of the octahedral coordination environment, and ∆Σ characterises the variation caused by the spin transition.
Compound a Fe-L (Å) r (Å) ∆r (Å)
(%) Σº ∆Σº Ref.
[Fe(endi)3](BF4)2 HS 2.182 2.182 14.99 [Fe(endi)3](BF4)2 LS 2.004 2.004
0.178
(8.2) 7.68 5.03 15 [Fe(btzp)3](ClO4)2 HS 2.164 2.164 12.72
[Fe(btzp)3](ClO4)2 LS 2.038 2.038
0.126
(5.8) 9.96 2.76 5
[Fe(btzmp)2(µ-btzmp)2](ClO4)2 HS 2.181/
2.207/2.171 2.186 23.64
[Fe(btzmp)2(µ-btzmp)2](ClO4)2 LS 2.001/
2.002/1.986 1.996
0.190 (8.7)
15.60
8.04 Chapter 3 [Fe(btzb)3](PF6)2 HS 2.193 2.193 20.76
[Fe(btzb)3](PF6)2 LS 1.993 1.993
0.200
(9.0) 18.24 2.52 19 [Fe(btzx)3](PF6)2·CH3OH HS 2.16 2.160 5.82
[Fe(btzx)3](PF6)2·CH3OH LS 2.001 2.001
0.159
(7.4) 1.56 4.26 Chapter 4
[Fe(btzx)3](CF3SO3)2·CH3CN(170 K) b Chapter
4 Fe1 HS 2.178/2.208 2.193 0.002 11.73 0.84
Fe2 HS 2.192/2.196 2.194 0.134
(6.1) 21.47 11.33
Fe3 HS 2.178 2.178 -0.003 1.92 -1.32
Fe4HS 2.185 2.185 0.147
(6.7) 11.64 -0.48
[Fe(btzx)3](CF3SO3)2·CH3CN(100K)b Chapter
4
Fe1 HS 2.181/2.201 2.191 10.89
Fe2 LS 2.061/2.059 2.060 10.14
Fe3 HS 2.182 2.181 3.24
Fe4 LS 2.039 2.039 12.12
[FeII(btzpol)1.8(btzpol-OBF3)1.2]
(BF4)0.8 HS 2.175/2.182/
2.169 2.177 9.12 Chapter
3
a See Figure 5.1 (b) for the schematic representation of the ligands for corresponding complexes.
b [Fe(btzx)3](CF3SO3)2 has four crystallographically independent centres which are shown separately.
5.3 Distortion of the octahedron.
Recently, Guionneau et al. pointed out a relationship between the distortion of the octahedral coordination environment and the spin-transition properties for a number of complexes.6 Almost at the same time, Létard et al. have mentioned the probable link between the T(LIESST) and the type of dentation of the ligands used for various LIESST b coordination compounds.5 The capacity of a complex to accommodate the structural changes of its coordination sphere imposed by the occurrence of the spin transition is crucial and can be reflected in an alteration of the octahedral geometry. Two parameters have been proposed to characterise the deformation of the octahedron: Σ and θ.12, 16, 17 Σ derives from the N-Fe-N angles, and it represents the sum of the deviations from 90º of the 12 cis N-Fe-N in the coordination sphere (Figure 5.2, left). θ corresponds to the sum of the deviations from 60º of the 24 possible θ angles (see Figure 5.2 right, and its caption for the definition of θ). In this chapter, only the Σ parameter will be considered.
Figure 5.2. Structural parameters used to characterise the distortion of the FeN6 octahedron.
a) Σ symbolises the sum of the deviations from 90º of the 12 cis N-Fe-N angles. b) θ is defined as the trigonal twist angle and is calculated through the projection along the centre of two triangles formed by the N-donor ligands.
Σ is a parameter that highly depends on the intrinsic character of the ligand, i.e. its dentation (influence of the bite angle) and flexibility.4 Tetrazoles are monodentate ligands and thus do not lead to significant distortions from a perfect octahedral environment.12 In Table 5.1, the ΣHS values vary between 2 and 24º (0º defines a perfect octahedron). These values contrast with those of the [FeLn(NCS)2] series,4 (Ln = bidentate ligand) which vary between 65º and 90º. [Fe(btzmp)2(µ-btzmp)2](ClO4)2 shows the highest ΣHS value, and thus the strongest distortion of the octahedron, while [Fe(btzx)3](PF6)2·CH3OH exhibits the smallest one. The significant distortion observed in [Fe(btzmp)2(µ-btzmp)2](ClO4)2 is somehow surprising. Indeed, two of the bistetrazole ligands are not acting as bridges, and thus do not contribute to the large deviation from the perfect Oh geometry. [Fe(endi)3](BF4)2, for which the ligand endi has the same spacer as btzmp but without methyl-substituents, displays a lower distortion. These structural differences between these two related complexes suggest that the sterical hindrance of the methyl groups contribute to the distortion. For [Fe(btzp)3](ClO4)2, the HS state structure shows a lower distortion than that of
b Light-Induced Excited Spin State Trapping: process in which a spin-crossover compound by irradiation is trapped in the HS metastable state at low temperatures (see Chapter 1).
[Fe(endi)3](BF4)2, thus two methyl groups are necessary to be able to observe an effect on the geometry of the octahedron.
The structure of the free btzmp ligand shows a similar conformation, compared to the coordinated-ligand (see Figure 5.3). In contrast, the free and coordinated endi ligands reveal distinct conformations.18 Accordingly, it appears that the methyl substituents on the spacer of the btzmp ligand increase its rigidity and forces it to adopt the same spatial arrangement when coordinated to the metal ion. As a result, the ability of btzmp to accommodate the coordination changes due to the spin transition is inferior (compared to the endi ligand), which results in a greater distortion of the octahedron. An intermediate situation may be expected for the complex [Fe(btzp)3](ClO4)2, whose ligand btzp, possesses only one methyl substituent in the spacer between the two tetrazole units. The octahedral distortion observed in this complex is comparable to that found for [Fe(endi)3](BF4)2. This observation strongly suggests that two methyl groups are required to generate inflexibility of the ligand.
Figure 5.3. View along the C–C bond of the linker between the tetrazoles for all ethane-based spacers. The C–C carbons and the ND atoms of the tetrazoles are presented as spheres. Metals have been omitted for clarity.
∆Σ (Table 5.1) characterises the change of the octahedral environment observed during the spin-transition. ∆Σ shows that the contraction of the coordination sphere is mainly isotropic, as observed for monotetrazole-based compounds.12 The complex [Fe(btzx)3](CF3SO3)2·CH3CN represents a particular case since one of its two FeII spin- transition centres possesses the highest ∆Σ value of all bistetrazole-based complexes so far investigated, while the other FeII centre shows the smallest (Table 5.1). This implies that the value of ∆Σ is affected by minor differences in the crystal packing. For [Fe(btzx)3](CF3SO3)2·CH3CN, the two spin-transition centres belong to two different chains.
Although these chains do present some differences regarding the spatial arrangement adopted by the coordinated btzx ligand, the different disposition of the counterions in the planes
(formed by the iron atoms) principally explains the significantly different structural response of the material to the spin transition (see Chapter 4). Thus, for Fe2, which shows a very high ΣHS (see Table 5.1),the triflate counterions are around 0.5 Å closer (see Table 5.3) to the metal ion than for Fe4. As a result, the triflate anions are interacting with the tetrazole rings via the atom F13. These close anion-tetrazole contacts are not observed with Fe4. In addition, Fe2 is surrounded by in-plane sulfur atoms, while the sulfur atoms of the triflates enclosing Fe4 are adopting a chair-like conformation (see Figure 5.4).
The proximity of the anions influences the angle at which the tetrazole rings coordinate to the FeII metal ions, i.e. [centroid-tetrazole]–ND–Fe angle (Table 5.3). This angle considers the orientation of the whole tetrazole ring when coordinated to the FeII atom, and not just the donor atom (ND). Both in the HS state and in the LS state, [Fe(btzp)3](ClO4)2 and [Fe(endi)3](BF4)2 show similar values, while the HS and LS of [Fe(btzmp)2(µ- btzmp)2](ClO4)2 differ drastically, especially for the bridging ligand. In fact [Fe(btzmp)2(µ- btzmp)2](ClO4)2 shows a completely different value than the rest of the bistetrazole-based materials, which can be related to its peculiar packing (see below). For the [Fe(btzx)3]2+
cation, the values are slightly lower than those observed for [Fe(btzp)3](ClO4)2 and [Fe(endi)3](BF4)2. This already suggests an influence of the spacer on the orientation of the tetrazole ring. In the case of [Fe(btzx)3](CF3SO3)2·CH3CN, Fe1 and Fe3 show significantly lower values than those observed for Fe2 and Fe4, which can be related to the shorter anion- to-metal distance observed for the former two metals (see below). These values tend to increase when going from the HS state to the LS state. This indicates that, concomitant with the electronic spin-transition, there is a structural reorientation of the tetrazole rings. Most of them vary from 0.5º to 1.5º except for [Fe(btzmp)2(µ-btzmp)2](ClO4)2 (6.5º) and Fe2 in [Fe(btzx)3](CF3SO3)2·CH3CN (2.56º and 2.32º). In 1D polymers, another way to measure the orientation of the tetrazole ring with respect of the iron atom is the cone angle (described in Table 5.3), which is the angle formed by two tetrazoles (centroids) of the same side of the plane (formed by the iron(II) atoms).
Figure 5.4 Different dispositions of the sulfur atoms on the planes formed by the FeII of the two types of polymeric chains found in compound [Fe(btzx)3](CF3SO3)2·CH3CN.
5.4 Fe···Fe Intra-polymeric and inter-polymeric distances.
The effect of the distance between metal ions on the spin-crossover behaviour has long been a matter of debate. Initially, it was assumed that an effective cooperative spin transition
was closely related to the distance between metallic centres.19-21 Nowadays it is well established that the cooperativity, and thus an efficient communication between the SCO centres, is connected with the overall rigidity of the coordination network.7, 22 A close look at the structural changes (such as metal-to-ligand bond length) induced by the transition can provide information on the subtle modifications of the overall lattice architecture of polymeric bistetrazole-based materials.
In these systems, the intrapolymeric distances are obviously strongly dependent on the nature of the spacer linking the two tetrazole moieties. As clearly evidenced in Table 5.2, a short spacer length (characterised by the ND···ND’ separation, Figure 5.1, right) leads to a short distance between the metals. The ND···ND’ distance will also depend on the conformation adopted by the bistetrazole ligand. For example, the ND···ND’ distance for the free endi ligand, which adopts a trans conformation, is 7.791 Å. When the ligand is coordinated to the metal, it adopts a cis conformation with the subsequent reduction of the ND···ND’ distance to 5.073 Å.
Figure 5.5 Schematic representation of the efficiency of ∆IAP (see text). IAP stands for IntrA-Polymeric. ttz = tetrazole. ∆r symbolises the difference between the HS and the LS r values (see Table 5.1).
[Fe(endi)3](BF4)2, [Fe(btzp)3](ClO4)2 and [Fe(btzmp)2(µ-btzmp)2](ClO4)2 possess the same spacer length between the tetrazole units. The only difference is found in the number of methyl substituents on the aliphatic linker. Based on the intrapolymeric distances observed for the LS state structures of the three compounds, [Fe(endi)3](BF4)2 and [Fe(btzp)3](ClO4)2 show very similar features. On the other hand, this intrapolymeric separation is around 1 Å longer for [Fe(btzmp)2(µ-btzmp)2](ClO4)2 (see Table 5.2). Comparing the ND···ND’ separation, the ligand can only justify around 0.5 Å of the 1 Å difference (ND···ND’btzmp, 5.635 Å). The fact that the iron centres of [Fe(btzmp)2(µ-btzmp)2](ClO4)2 are only bridged by two ligands, instead of three as observed for [Fe(endi)3](BF4)2 and [Fe(btzp)3](ClO4)2, also contributes to such difference. A similar situation is found with the 2D polymeric compound [FeII(btzpol)1.8(btzpol-OBF3)1.2](BF4)0.8·(H2O)0.8(CH3CN), where both the number of linkers
and spacer conformation, account for the difference in intrapolymeric distances along each direction. (see discussion on Chapter 3).
The decrease or elongation of the metal-to-ligand bond lengths occurring during the transition affect the intrapolymeric (IAP) distances. This variation of the IAP distances, i.e
∆IAP (see Table 5.2), is specific for each compound investigated. In an ideal case, a decrease of the metal-to-ligand coordination bonds would be completely “converted” into a decrease of the intrapolymeric distances (see Figure 5.5). Consequently, the efficiency of a ligand to transmit the structural changes along the polymeric axis can be evaluated via the comparison of the theoretical maximum value, namely ∆r, with the experimental value, i.e ∆IAP, extracted from the crystal structure of the compound studied (see Figure 5.5 and Table 5.2).
For instance, for complex [Fe(btzb)3](PF6)2:
∆r = 0.200 + 0.200 = 0.400 Å (see Table 5.1), and
∆IAP (Intra-polymeric Distance) = 0.364 Å (see Table 5.2).
Thus, the ligand efficiency for this compound is: (E) = 0.364/0.400 = 91%
An efficiency of 91% hence implies that practically all the structural changes taking place at the coordination site are translated to the intrapolymeric distances. For 1D polymers, such as [Fe(btzmp)2(µ-btzmp)2](ClO4)2, [Fe(btzp)3](ClO4)2, [Fe(endi)3](BF4)2, or [Fe(btzx)3](PF6)2, the efficiency ranges from 50 to 65% (Table 5.2). 35 to 50% of the ∆r value is then directed towards other directions, or absorbed by the bending of the ligand.
From the data collected in Tables 5.1 and 5.2, it appears that for 1D polymers when the ∆r value decreases, the ligand efficiency of the corresponding coordination polymer increases.
1D coordination networks obtained from bistetrazole ligands usually exhibit typical architectures, as depicted in Figure 5.6. The distance between the polymeric chains is governed by the choice of the ligand bridging the FeII centres. The anions could only modify the interpolymeric distance by modifying the packing, which in this type of materials does not seem to be the case. Nevertheless, their nature appears to be crucial to obtain a crystalline spin-transition material (see Chapter 4). These inter-polymeric (IEP) distances vary drastically with the bulkiness of the ligands used. The IEP distance for [Fe(endi)3](BF4)2 is almost 1 Å shorter than the one observed with [Fe(btzp)3](ClO4)2 (Table 5.2). Thus, the methyl substituent present in the alkyl chain of the btzp ligandis most likely responsible for this significant IEP difference, as clearly evidenced in Figure 5.5 a) and b).7, 18 A particular case is observed with the coordination compound [Fe(btzmp)2(µ-btzmp)2](ClO4)2. Indeed, its 1D chains pack in a slightly different manner (Figure 5.5, d), due to the presence of mono- coordinated btzmp ligands which obviously increase the IEP distance. As a result, a slightly less-symmetrical crystal packing is attained, in which each counterion is now shared by two polymeric chains (Figure 5.5, d) instead of three for the previous examples (Figure 5.5, a−c).
Table 5.2. Interpolymeric (IEP) and intrapolymeric (IAP) data for a series of bistetrazoles compounds. ND···ND’ (Figure 5.1) is the distance between the two ND atoms of the tetrazoles;
∆ND···ND’ is the variation of these distances due to the spin crossover; IAP is the intrapolymeric distance observed in the solid-state structure of the material studied; ∆IAP is the variation of this parameter due to the spin crossover; IEP is the interpolymeric distance and ∆IEP its variation due to spin crossover. E (%) represents the efficiency of the ligand toward its adjustment to the structural changes occurring during the spin-transition (see text).
Compound a ND···ND’ (Å)
∆ND···ND’ (Å)
IAP (Å)
∆IAP (Å)
IEP (Å)
∆IEP (Å)
E (%) [Fe(endi)3](BF4)2 HS 5.107 7.477 10.380
[Fe(endi)3](BF4)2 LS 5.073 0.034
7.293 0.184
10.178 0.202 52 [Fe(btzp)3](ClO4)2 HS 5.038 7.422 11.098
[Fe(btzp)3](ClO4)2 LS 5.015
0.023
7.273
0.149
11.030 0.068 59 [Fe(btzmp)2(µ-
btzmp)2](ClO4)2 HS 5.687,
5.431 8.516 11.122
12.432 [Fe(btzmp)2(µ-
btzmp)2](ClO4)2 LS
5.635, 5.317
0.052, 0.124
8.304
0.212
10.950 12.311
0.172, 0.121 50 [Fe(btzb)3](PF6)2 HS 10.219 14.381 8.948
[Fe(btzb)3](PF6)2 LS 10.239 -0.02
14.017 0.364
8.702 0.246 91 [Fe(btzx)3](PF6)2·CH3OH HS 8.923 11.397 10.679 [Fe(btzx)3](PF6)2·CH3OH LS 8.899 0.024
11.205 0.192
10.543 0.136 64 [Fe(btzx)3](CF3SO3)2·CH3CN
(170 K) b 10.804 0.058 64
Fe1 HS 9.071,
9.124
0.017, 0.006
11.607 11.686
0.087 0.096
Fe2 HS 9.071,
9.124
0.017, 0.006
11.607 11.686
0.087 0.096 Fe3 HS 9.155 0.006 11.647 0.092
Fe4HS 9.155 0.006 11.647 0.092
[Fe(btzx)3](CF3SO3)2·CH3CN
(100 K)b 10.746
Fe1 HS 9.054,
9.118 11.520
11.590 Fe2 LS 9.054,
9.118
11.520 11.590
Fe3 HS 9.149 11.555
Fe4 LS 9.149 11.555
[FeII(btzpol)1.8(btzpol-
OBF3)1.2](BF4)0.8 HS 7.647 10.688 12.033
7.140
a See Figure 5.1 for the schematic representation of the ligands of the corresponding complexes.
b [Fe(btzx)3](CF3SO3)2 has four crystallographically independent centres which are shown separately.
Figure 5.6. Crystal packings of bistetrazole-based 1D polymers viewed along the corresponding polymer axis: a) [Fe(endi)3](BF4)2, b) [Fe(btzp)3](ClO4)2, c) [Fe(btzx)3](PF6)2
and d) [Fe(btzmp)2(µ-btzmp)2](ClO4)2.
5.5 Anion-tetrazole interactions.
The control over the cooperative behaviour of spin-transition compounds is still a major issue in this field. Cooperativity is in fact responsible for steep transitions observed in the solid state and, in some cases, for hysteresis loops. As mentioned earlier, strong interactions between the HS/LS metal centres, and consequently rigid structures, are required to achieve highly cooperative spin-transition systems. In this sense, supramolecular interactions, involving weak bonds,23 are paramount to create such inter/intra-connected rigid systems.
Hydrogen bonds or π−π interactions are among the most commonly used intermolecular interactions. Only recently the significance of anion−π interactions between negatively charged species and electro-deficient aromatic rings has been pointed out.24 The distance between the anion and the electron-poor π system reflects the strength of the interaction.25 For instance, for the triazine ring a thorough analysis of this distance with different anions has been evaluated in a recent paper.26
The series of bistetrazole based polymers exhibit anion−tetrazole interactions. Although their influence is difficult to estimate, these interactions may affect the spin-transition properties of the corresponding materials. As seen in Table 5.3, the 3D [Fe(btzb)3](PF6)2 lacks this type of anion–π contacts, which also seems to be a common feature observed in most of
the 1D bistetrazole-based polymers. In fact, the typical crystal packing of the 1D bistetrazole- based polymers generates a host cavity (see Figure 5.7), suitable for an anion. Indeed, the presence of metal centres in the close vicinity, as well as coordinated electron-poor tetrazole rings, creates an electrophilic pocket that can favourably interact with an electronegative ion.
As observed in Table 5.3, the anion is much closer to the metal centres in the 1D polymers than in the 2D or 3D polymers. This proximity to the metal ions may force the counterions to experience anion-π interactions. This feature is observed for [Fe(btzx)3](CF3SO3)2·CH3CN and [Fe(btzx)3](PF6)2·CH3OH, whose anion-π distances are inferior to 3 Å. By contrast, [Fe(endi)3](BF4)2 and [Fe(btzp)3](ClO4)2 show weaker anion-π interactions, reflected in longer anion-π distances (see Table 5.3). The distance between the counterion and the metal appears to be associated to the strength of the anion-π interactions. Accordingly, [Fe(btzmp)2(µ-btzmp)2](ClO4)2 shows the shortest anion-metal distance, i.e. 4.655 Å, and the strongest anion-π interaction, namely 2.882 Å.
In all 1D polymers, the anion is interacting with the three chains enclosing it (see Figure 5.7). The influence of the spin–transition on the anion–π interactions (and vice-versa) is difficult to establish because the structural changes are minor. The general trend logically suggests that the strength of the anion-π interaction increases from the HS to the LS state.
This change in distance may be due to the spin transition, but also to the change of (experimental) temperature (resulting in a higher stabilisation of the anion–π contacts).
Figure 5.7. Left, electron-deficient pocket formed by the tetrazole rings and the metal ions of three different chains in [Fe(btzx)3](PF6)2·CH3OH. Right, anion-π interactions (F1-centroid) between the tetrazole rings and the hexafluorophosphate anion.
Table 5.3. HS/LS anion-to-metal distances and anion−π separations for various FeII compounds.
Compounds a Anion- metal distance
(Å)
∆(Anion- metal distance)
(Å)
Anion-π
(Å) ∆(anion-π) (Å)
[Ttz- centroid]
–ND–Fe angle (º)
∆[Ttz- centroid]
–ND–Fe angle (º)
Cone Angle (º) [Fe(endi)3](BF4)2 HS 5.065
3.230, 3.230, 3.361 (3 ttz)
173.12 87.97
[Fe(endi)3](BF4)2 LS 4.953
0.112
3.267, 3.239, 3.267 (3 ttz)
0.009, 0.094
174.57
-1.45
88.16
[Fe(btzp)3](ClO4)2 HS 5.094 3.278 173.79 88.82 [Fe(btzp)3](ClO4)2 LS 5.048 0.046 3.265 0.013 174.09 -0.30 88.88 [Fe(btzmp)2(µ-
btzmp)2](ClO4)2 HS 4.655 3.031, 2.977
163.82,
168.27 91.23
[Fe(btzmp)2(µ-
btzmp)2](ClO4)2 LS 4.648
0.007
2.882
0.095 170.36, 170.46
-6.54, -2.19 90.99 [Fe(btzb)3](PF6)2 HS 5.536 none 173.58
[Fe(btzb)3](PF6)2 LS 5.429 0.107
none 172.93 -0.65
[Fe(btzx)3](PF6)2
·CH3OH HS
5.166,
5.146 3.137 170.22 85.67
[Fe(btzx)3](PF6)2
·CH3OH LS
5.098, 5.088
0.068, 0.058
3.089
0.048
171.84
-1.62
85.88 [Fe(btzx)3](CF3SO3)2·
CH3CN(170 K)b
Fe1 HS 5.135, 4.384
0.072, 0.065
2.998, 3.385
0.054, 0.065
172.84, 165.29
-0.02, -0.63
81.58, 86.3 Fe2 HS 4.905,
4.896
0.038, 0.002
2.968, 3.214
-0.007, 0.024
167.61, 170.81
-2.56, -2.32
83.65, 86.16 Fe3 HS 4.852 0.037 3.086 0.081 165.82 -1.04 83.13 Fe4 HS 5.369 0.026 none none 172.2 -1.19 87.39 [Fe(btzx)3](CF3SO3)2
·CH3CN(100 K)b
Fe1 HS 5.063, 4.319
2.944, 3.32
172.86, 165.92
81.55, 86.23 Fe2 LS 4.867,
4.898
2.961 3.190
170.17, 173.13
84.41, 87.12
Fe3 HS 4.815 3.005 164.78 82.77
Fe4 LS 5.343 none 173.39 88.43
[Fe(btzpol)1.8(btzpol-
OBF3)1.2](BF4)0.8 HS 5.242 3.090 173.44, 174.75
a See Figure 5.1 (b) for the schematic representation of the ligands of the corresponding complexes.
b [Fe(btzx)3](CF3SO3)2 has four crystallographically independent centres which are shown separately.
5.6 Crystallographic Unit Cells.
A thermal spin transition is accompanied by a significant modification of the coordination sphere of the iron atoms that may have a strong impact on the overall crystal lattice.
Therefore, it is possible to follow the transition via examination of certain lattice parameters.27 Moreover, these parameters are crucial to determine the interaction energy between molecules in spin-crossover materials.28-30 To perform these calculations, the change in volume caused by the spin transition and the anisotropy of the lattice deformation are required. Because a thorough crystallographic analysis at different temperatures is necessary for a reliable calculation/discussion, the purpose of the following paragraphs is just to point out the structural differences seen at the two different spin states.
The space groups for various bistetrazole-based compounds are shown in Table 5.4. In none of these materials, a crystallographic phase transition is involved in the spin transition.
In both [Fe(btzmp)2(µ-btzmp)2](ClO4)2 and [Fe(btzb)3](PF6)2, an order/disorder transition of the counterion is observed, which could be responsible for their steep spin-transition properties.
The series of bistetrazole-based polymers shows changes in the volume of the unit cell (∆VSC), from 49 to 130 Å3. It is necessary to calculate the percent-volume change with respect to the High-Spin state to be able to compare these values. In the [Fe(NCS)2(L)n] series, the
∆VSC is in all cases around 2%.4 For the present bis-tetrazole systems, ∆VSC varies from 2% to 7.5%. The highest value corresponds to the 3D polymer [Fe(btzb)3](PF6)2 and the lowest to [Fe(btzx)3](CF3SO3)2·CH3CN. All linear polymers exhibit similar % ∆VSC values, except [Fe(btzx)3](CF3SO3)2·CH3CN whose ∆VSC value is twice inferior. In the case of [Fe(btzx)3](CF3SO3)2·CH3CN this particularity can be explained by the fact that only half of the FeII centres are undergoing the transition.
As the cell parameters may face slight deviations upon decrease/increase of temperature, it is difficult to clearly ascertain the structural changes directly caused by the spin transition.
Nevertheless, the solid-state structures of [Fe(btzb)3](PF6)2 and [Fe(endi)3](BF4)2 have been determined at various temperatures, which can give some insight in the structural thermal changes. For instance, for the 3D complex [Fe(btzb)3](PF6)2, both a and V do not vary between 300 and 200 K (range during which no spin transition occurs), while c changes by about 6% (Figure 5.8).22 One would expect that the spin-transition would cause an isotropic lattice change of the 3D material, but it appears that the c axis is slightly less affected by the
“compression” arising from the HS to LS transition.
[Fe(endi)3](BF4)2 represents the only 1D polymer which has been structurally characterised at more than two temperatures.Due to the large expansion of the thermal spin transition experienced by [Fe(endi)3](BF4)2, most of the structural data are influenced by the transformations caused by the spin transition. At 200 K, i.e. temperature at which the transition has not yet started, changes with respect to the high temperature data are only observed in V and a (c remains unchanged). The non-covalent interactions present along the a axis are more affected by the temperature change than the covalent ones observed along the polymer c axis. Meanwhile, the c axis suffers a greater structural modification than the a axis
upon the spin transition. For the other linear polymers, the changes in the lattice parameters are less important, but the anisotropy follows the same trend: the polymer axis is the most affected by the structural changes resulting from the spin transition.
Table 5.4. Temperatures applied during the X-ray structure determinations, crystallographic space groups, and cell parameters with their corresponding variations owing to the spin transition.
Compounds a
Temp.
(K) Space
Group a (Å) ∆a
(Å) c (Å) ∆c
(Å) V (Å3) ∆VSC
(Å3) [Fe(endi)3](BF4)2 HS 296 10.380
(10)
14.953 (3)
1359.3 (3) [Fe(endi)3](BF4)2 LS 100
P-3c1
10.178 (16)
0.202
14.586 (4)
0.367
1308.6 (5)
50.7 (3.7%)
[Fe(btzp)3](ClO4)2 HS 200 11.098 (2)
14.844 (2)
1583.3 (5) [Fe(btzp)3](ClO4)2 LS 100
P-3c1 11.030 (18)
0.068 14.546 (18)
0.289 1532.6 (4)
50.7 (3.2%) [Fe(btzmp)2(µ-
btzmp)2](ClO4)2 HS 200 8.516 (2)
12.432 (2)
1102.8 (4) [Fe(btzmp)2(µ-
btzmp)2](ClO4)2 LS 100
P-1
8.304 (2)
0.212 12.311 (2)
0.121 1053.8 (4)
49.0 (4.5%)
[Fe(btzb)3](PF6)2 HS 200 11.259
(7) 8.887
(6) 975.6
(11) [Fe(btzb)3](PF6)2 LS 100
P3
10.989 (3)
0.268
8.702 (2)
0.185
910(4)
72.2 (7.5%)
[Fe(btzx)3](PF6)2·CH3OH
HS 200 10.679
(2)
22.793 (6)
2251.1 (8) [Fe(btzx)3](PF6)2 LS 100
P63/m
10.543 (2)
0.136
22.409 (6)
0.284
2157.2 (8)
93.9 (4.2%)
[Fe(btzx)3](CF3SO3)2
·CH3CN HS
170 P-3 18.696 (10)
23.293 (2)
7050.6 (8) [Fe(btzx)3](CF3SO3)2
·CH3CNLS
100 P-3 18.594 (12)
0.101
23.111 (15)
0.182
6919.9 (8)
130 (1.9%)
[FeII(btzpol)1.8(btzpol- OBF3)1.2](BF4)0.8 HS
150 P21/m 7.140 (4)
10.689 (7)
1831.8 (2)
a See Figure 5.1 for the schematic representation of the ligands of the corresponding complexes.
Figure 5.8 Temperature dependence of the crystal cell parameters a and c for [Fe(btzb)3](PF6)2.Taken from Weinberger et al.22
5.7 Structural vs. Magnetic Features
The search for a magneto-structural relationship is a well-developed concept in the field of molecular magnetism.31 The correlation between structure and properties is a major topic of investigations in spin-crossover (SCO) chemistry.1, 4 Important advances have been achieved in this regard; however, the main spin-transition features, such as cooperativity, LIESST and hysteresis, are not yet completely understood. Therefore, comparative studies within certain families of spin-crossover compounds are valuable and essential to appraise the relation between the structure and the physical properties of a complex. Bistetrazole-based spin- crossover compounds are attractive candidates for such detailed investigations, as they represent one of the few families of 1D polymers that can be crystallised. Unfortunately, only a limited amount of 1D bistetrazole compounds are currently reported in the literature, which means that insufficient data are available to draw strong conclusions already. Thus, only the comparison of the T1/2 (temperature at which 50% of the molecular complex is in the HS state) values of various bistetrazole-based SCO compounds has been carried out.
a) b)
Figure 5.9 a) T1/2 (temperature at which 50% of the molecular complex is in the HS state) vs.
rHS (mean metal-to-ligand bond length) and b) T1/2 vs ∆rHS (variation of the metal-to-ligand bond length upon spin-transition) plots for a series of bistetrazole-based complexes. See Table 5.5 for number/compound assignment.
The T1/2 of any spin-crossover compound is mainly dependent on the ligand-field strength and on the metal ion involved. Thus, comparable T1/2 values are expected for a series of [FeII(tetrazole-based ligand)x] complexes. However, as evidenced in Table 5.5, the T1/2
values fall in the broad 110–173 K range. Moreover, if monotetrazole ligands are included, the range becomes even wider. In all cases, the electronic influence of the substituent(s) on the tetrazole ring appears to be insignificant. Consequently, the different T1/2 observed most likely originates from structural modifications, probably within the coordination sphere of the metal centre.
Table 5.5. T1/2 values and number assigned for each compound (see Figure 5.9 and 5.10).
Compounds T1/2 (K) Number for Compound
[Fe(endi)3](BF4)2 140 1
[Fe(btzp)3](ClO4)2 130 2
[Fe(btzmp)2(µ-
btzmp)2](ClO4)2 135 3
[Fe(btzb)3](PF6)2 173 4
[Fe(btzx)3](PF6)2·CH3OH 150 5
[Fe(btzx)3](CF3SO3)2·CH3CN 110 6
[FeII(btzpol)1.8(btzpol-
OBF3)1.2](BF4)0.8 112 7
From the left plot reported in Figure 5.9, no relationship between rHS and T1/2 can be derived. Conversely, the right plot in Figure 5.9 suggests a very weak correlation, if any at all, between ∆rHS and T1/2, although more data are required to confirm it. For the series of [FeLn(NCS)2] compounds, no relationship between T1/2, and rHS has been established, and T1/2
vs. ∆rHS has not been contemplated. The inspection of ΣHS values (see Table 5.1 for its definition) with regard to the corresponding T1/2 shows no link between them (Figure 5.10 left). However, the ∆Σ vs. T1/2 plot reveals an apparent correlation (Figures 5.10 right). The uncertainty of T1/2 for [Fe(btzp)3](ClO4)2 due to its gradual transition is probably behind its deviation from the normal tendency. It appears that significant alterations of the coordination environment of the FeII centre, due to the spin transition, give rise to lower transition temperatures (Figure 5.10 right). Low transition temperatures are related to HS stable states, which in a HS-LS potential energy diagram would mean a small energy gap ∆EºHL between the two states (see Chapter 1, Figure 1.3).
a) b)
Figure 5.10 Left, plot of the octahedron distortion vs. T1/2, for the bistetrazole-based complexes, defined as ΣHS (see text); right, plot of the variation of ΣHS, i.e. ∆Σ, due to the spin-transition vs. T1/2 (the line represents the linear fit of points 1,3,4,5 and 6). See Table 4.5 for number/compound assignment.
As shown in chapter 4, there is a relation between the anion–metal distance and the distortion of the octahedron on bistetrazole-based complexes. In turn, low transition temperatures are also related to small anion–metal distances. This agrees with the observations made by Reger et al.32 in their pyrazolylborate-based ST compounds.
5.8. Concluding remarks
In the present chapter, a significant amount of structural information available in the literature for bistetrazole-based spin-transition compounds has been collected, and, based on these data, potential structure-property relationships have been indicated. The limited amount of crystallographic and physical information on bistetrazole systems implies that the structure-property correlations herein proposed may be not supported by future examples of this bistetrazole family. Nevertheless, the following conclusions may be drawn from the examples so far reported:
• The rHS (the mean metal-to-ligand bond length) values are comparable for the 1D polymers, while they differ for the 2D and 3D frameworks. This discrepancy indicates that rHS is influenced to a certain extent by the environment of the FeII centres.
• Similarly to the [FeLn(NCS)2] series, ∆r decreases with rHS.
• The values of the Σ parameters, reflecting the deformation of the octahedral coordination environment, increases in the series [Fe(endi)3](BF4)2, [Fe(btzp)3](ClO4)2, and [Fe(btzmp)2(µ-btzmp)2](ClO4)2, probably as a result of steric effects induced by the methyl substituents of the alkyl spacer linking the tetrazole moieties.
• The observed ∆Σ (variation of Σ caused by the spin-transition) shows that the contraction of the coordination sphere is mainly isotropic, as observed for monotetrazole-based compounds.
• The intrapolymeric distance (IAP) is associated with the ND···ND’ distance and related to the conformation of the bistetrazole ligand. The number of bridging ligands also has an influence on the IAP.
• ∆IAP (variation of IAP upon the spin-transition) is correlated with ∆r. Small ∆r values result in more efficient transmission of the structural changes along the polymeric axis.
• The interpolymeric distance (IEP) is dependent on the ligand chosen and, to a lesser extent, on the counterion.
• The anion-π interactions are characteristic of 1D polymers owing to their peculiar crystal packing.
• The bistetrazole-based compounds show an anisotropic change in the crystal lattice parameters. The main variation occurs along the polymer axis. The changes in volume vary from 2% to 7.5% (calculated with respect to the VHS, namely the unit cell Volume in the High-Spin state).
• ∆Σ shows a relationship with T1/2: higher ∆Σ values correspond to low T1/2
temperatures.
In summary, a number of common structural features of bistetrazole-based complexes have been pointed out and compared. This study may help in designing bistetrazole-based polymers with more specific structural requirements for the analysis of their influence on their magnetic properties. A relation between the distortion of the coordination sphere and the transition temperature has been suggested, although further examples are necessary for stronger conclusion to be made.
5.9 References.
1. Gütlich, P.; Hauser, A.; Spiering, H., Angew. Chem. Int. Ed. Engl. 1994, 33, 2024.
2. Hostettler, M.; Tornroos, K. W.; Chernyshov, D.; Vangdal, B.; Burgi, H. B., Angew.
Chem. Int. Edit. 2004, 43, 4589-4594.
3. Sorai, M., Heat capacity studies of spin crossover systems. In Spin Crossover in Transition Metal Compounds III, Topics in Current Chemistry, Gütlich, P.; Goodwin, H. A., Eds. Springer: 2004; Vol. 235, pp 153-170.
4. Guionneau, P.; Marchivie, M.; Bravic, G.; Létard, J. F.; Chasseau, D., Structural aspects of spin crossover. Example of the (FeLn)-L-II(NCS)(2) complexes. In Spin Crossover in Transition Metal Compounds II, Gütlich, P.; Goodwin, H. A., Eds. 2004;
Vol. 234, pp 97-128.
5. Létard, J. F.; Guionneau, P.; Nguyen, O.; Costa, J. S.; Marcen, S.; Chastanet, G.;
Marchivie, M.; Goux-Capes, L., Chem.-Eur. J. 2005, 11, 4582-4589.
6. Marchivie, M.; Guionneau, P.; Létard, J. F.; Chasseau, D., Acta Crystallogr., Sect. B:
Struct. Sci. 2005, 61, 25-28.
7. van Koningsbruggen, P. J.; Garcia, Y.; Kahn, O.; Fournès, L.; Kooijman, H.; Spek, A.
L.; Haasnoot, J. G.; Moscovici, J.; Provost, K.; Michalowicz, A.; Renz, F.; Gütlich, P., Inorg. Chem. 2000, 39, 1891-1900.
8. König, E.; Madeja, K., Inorg. Chem. 1967, 6, 48.
9. König, E.; Madeja, K., Chem. Commun 1966, 61.
10. Shannon, R. D.; Prewitt, C. T., Acta Cryst 1969, B 25, 925-46.
11. Gütlich, P.; Goodwin, H. A., Spin crossover - An overall perspective. In Spin
Crossover in Transition Metal Compounds I, Topics in Current Chemistry, Gütlich, P.;
Goodwin, H. A., Eds. Springer: 2004; Vol. 233, pp 1-47.
12. Alvarez, S., J. Am. Chem. Soc. 2003, 125, 6795-6802.
13. Bronisz, R., Eur. J. Inorg. Chem. 2004, 3688-3695.
14. Matouzenko, G. S.; Bousseksou, A.; Lecocq, S.; van Koningsbruggen, P. J.; Perrin, M.; Kahn, O.; Collet, A., Inorg. Chem. 1997, 36, 5869-5879.
15. Ozarowski, A.; Mcgarvey, B. R.; Sarkar, A. B.; Drake, J. E., Inorg. Chem. 1988, 27, 628-635.
16. Guionneau, P.; Marchivie, M.; Bravic, G.; Létard, J. F.; Chasseau, D., J. Mater. Chem.
2002, 12, 2546-2551.
17. Marchivie, M.; Guionneau, P.; Howard, J. A. K.; Chastanet, G.; Létard, J. F.; Goeta, A. E.; Chasseau, D., J. Am. Chem. Soc. 2002, 124, 194-195.
18. Schweifer, J.; Weinberger, P.; Mereiter, K.; Boca, M.; Reichl, C.; Wiesinger, G.;
Hilscher, G.; van Koningsbruggen, P. J.; Kooijman, H.; Grunert, M.; Linert, W., Inorg. Chim. Acta 2002, 339, 297-306.
19. Lavrenova, L. G.; Ikorskii, V. N.; Varnek, V. A.; Oglezneva, I. M.; Larionov, S. V., J.
Struct. Chem. 1993, 34, 960-965.
20. Shvedenkov, Y. G.; Ikorskii, V. N.; Lavrenova, L. G.; Drebushchak, V. A.; Yudina, N.
G., J. Struct. Chem. 1997, 38, 578-584.
21. Varnek, V. A.; Lavrenova, L. G., J. Struct. Chem. 1997, 38, 742-747.
22. Grunert, C. M.; Schweifer, J.; Weinberger, P.; Linert, W.; Mereiter, K.; Hilscher, G.;
Müller, M.; Wiesinger, G.; van Koningsbruggen, P. J., Inorg. Chem. 2004, 43, 155- 165.
23. Lehn, J.-M., Supramolecular Chemistry: Concepts and Perspectives. In Wiley-VCH:
1995.
24. Quiñonero, D.; Garau, C.; Rotger, C.; Frontera, A.; Ballester, P.; Costa, A.; Deyà, P.
M., Angew. Chem. Int. Ed. 2002, 41, 3389-3392.
25. Schottel, B. L.; Chifotides, H. T.; Shatruk, M.; Chouai, A.; Perez, L. M.; Bacsa, J.;
Dunbar, K. R., J. Am. Chem. Soc. 2006, 128, 5895-5912.
26. Mooibroek, T. J.; Gamez, P., Inorg. Chim. Acta 2007, 360, 381-404.
27. Chernyshov, D.; Hostettler, M.; Tornroos, K. W.; Burgi, H. B., Angew. Chem. Int. Ed.
2003, 42, 3825-3830.
28. Kusz, J.; Spiering, H.; Gütlich, P., J. Appl. Crystallogr. 2001, 34, 229-238.
29. Spiering, H., Elastic interaction in spin crossover compounds. In Spin Crossover in Transition Metal Compounds III, Topics in Current Chemistry, Gütlich, P.; Goodwin, H. A., Eds. Springer: 2004; Vol. 235, pp 171-195.
30. Spiering, H.; Meissner, E.; Köppen, H.; Müller, E. W.; Gütlich, P., Chem. Phys. 1982, 68, 65-71.
31. Kahn, O., Molecular Magnetism. In Molecular Magnetism, VCH: New York, 1993; p 131.
32. Reger, D. L.; Gardinier, J. R.; Smith, M. D.; Shahin, A. M.; Long, G. J.; Rebbouh, L.;
Grandjean, F., Inorg. Chem. 2005, 44, 1852-1866.
