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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1
Introduction
1.1 Spin transition and Molecular Electronics
“… there is no question that there is enough room on the head of a pin to put all of the Encyclopaedia Britannica.” 1 This still astonishing statement was made in the 1960’s by Richard P. Feynman, who predicted the trend technology was about to take. Since then many efforts have been made, and paid-off, to reduce technological devices to the scale Feynman predicted. Far from reaching its limits, the increase in all types of communication has made this process even more important, and research in the last forty years has been able to reduce the bit size from 250 µm to 1 µm.2 The so-called top-down approach (namely, the miniaturisation of the already known materials) has been the basis of this successful development, but it is now reaching certain limits of miniaturisation in some phases of the production process; e.g photolithography.3 Moreover, the production and manipulation of such small systems make it more and more expensive to manufacture. The bottom-up approach comes up then as an alternative, based on the idea that the miniaturisation limit of an electronic function is the molecule. The size of technological devices would be enormously reduced (from µm to nm) and the processing speed increased (from nanoseconds to femtoseconds) by building them from molecules. This multidisciplinary field has already resulted in exceptional examples of molecules acting as molecular-based functional units such as memories, modulators, rectifiers, switches, transistors or wires. 4-7
Spin-crossover (SCO) systems belong to this category and could work as molecular switches or memory storage devices.8 These systems are made up of transition metal complexes with a d4 to d7 electron configuration and the proper crystal field splitting (see section 1.2). SCO compounds possess two stable states, the so-called high spin (HS) and low spin (LS) state, and are able to interconvert from one to the other by means of temperature, pressure and light. Molecular switches are systems that have an ON and OFF position, and which can interconvert from one to the other by means of an external stimulus. In a logic operation language, this would be considered as a NOT gate, the other two basic operations being AND and OR. The external stimuli by which the transition is induced for the case of SCO compounds can be temperature, pressure, magnetic field or light. Current technology is principally based on optical input/output transduction mechanisms and underlines the importance of the light switching process in SCO systems, recently discovered by Decurtins et al.9 Concomitant to the switching process, there must be a change of the output signal which reveals the state of the system. Magnetic or optical properties can be used as output signals. Conceiving applications for these systems is not difficult, but other requirements must first be fulfilled. For example, the compound must be chemically stable when embedded in a matrix or deposited on a surface; it must not suffer from fatigue and it must be isolated to assure correct addressing.10 Supposing that the system complies with these conditions, it must also be economically competitive, if any commercialisation is to be accomplished. These systems may work as temperature or pressure sensors and their use in prepaid phone cards has been envisaged 11. Another idea is to exploit the different output signals. In some of these materials, one state is magnetic and the other is non-magnetic, which can be used as sensors in magnetic resonance image to avoid exceeding a certain temperature, as the magnetic state would alter the resonance image.12
Apart from molecular switches, these systems may work as memory storage devices.
Molecular switches are not stable, meaning that once the external perturbation is turned off, the system returns to its original state. In contrast, memory devices must be stable since the information must be retained. Systems that meet this requirement need to possess what is called a thermally induced hysteresis behaviour (Figure 1.1 a). Under the same external conditions two possible stable states of the material can be achieved and the status will depend on the “history”. Nowadays, there are several systems that are used as memory devices. The hard drive of a computer is based on the alignment of magnetic particles on a surface. Compact discs are based on the different reflection of a light source on an elevated area (pits) with respect to a flat area (land), which is detected by the opto-electronic sensors.
Finally, magneto-optical systems use the slight shift in the reflected beam of polarised light caused by a magnetised surface. If the magnetisation of the surface is reversed, the angle of polarisation will shift, creating two different responses i.e. a binary system.10
a) b)
Figure 1.1. a) Plot of γHSa vs. temperature showing a hysteresis curve with its two possible paths for the temperature induced transition. Inside the hysteresis loop, two equally stable states are present, constituting a binary system. b) 2-coloured screen made of a spin-transition material before and after thermal input (taken from Letard et al.10)
The presence of a hysteresis in the spin transition is sometimes associated to elastic interactions between the coordination SCO centres occurring during the switching process.13,
14 Kahn et al. reported the requisites for a SCO material to be applied in a data display device.2 The transition needs to be abrupt (within a few Kelvins), the hysteresis loop must have a width of at least 40 K and if possible located in temperature range close to room temperature. The transition must be complete and accompanied by an easily detectable response. Finally, the hysteresis loop must not suffer from fatigue over successive thermal cycles. Up to date, of all compounds reported, [Fe(Rtrz)3]A2 materials (R= H, NH2, alkyl chain; trz =1,2,4-triazole; A = counterion) are those who best fulfil these requirements.2, 15-19
a γHS is the fraction of iron centres that are in the high spin state. It can be calculated using: γHS = χmT(T)/
χmT(HS).
They show complete and steep transitions, with wide hysteresis loops, and undergo a drastic thermochromic change during the switching process. The T1/2, i.e. temperature at which 50%
of the transiting centres are in the high spin state and 50 % are in the low spin state, can be tuned by means of e.g. the counterion size (A).20 Thus, the use of a mixture of counterions or of R-triazole ligands has yielded “alloys” that show ST (spin transition) centred at room temperature.2 These “alloys” have been encapsulated (to avoid oxidation) and applied in the thermal writing process, which however show difficulties in the localised erasure (see Figure 1.1 b).2
Although many years of intensive research on spin-transition compounds have followed the pioneering work of Haasnoot et al.21 on these 1,2,4-triazole-based polymers, it has not yet been possible to improve the properties needed for potential applications.8 Nevertheless, new prospects have come in sight. Indeed, systems with light-induced metastable states detectable up to 100 K have been developed,22 setting the goal of room temperature light-induced bistable systems closer.23 Moreover, the possibility to switch states by means of pulsed-laser light acting on a material presenting hysteresis behaviour at room temperature has already been proven.24
1.2. Ligand Field Considerations
Coordination compounds of transition metal ions with a 3d4 to 3d7 electronic configuration in a ligand field of octahedral symmetry can have two possible spin states.
Based on the ligand field theory it can be consistently explained how the degeneracy of the d orbitals of a single transition metal ion is broken when a complex is formed.25 The approach of the ligands towards the metal ion alters the energy of the orbitals; some are stabilised and others destabilised. In the case of an octahedral FeII metal ion, the d orbitals split into two subsets, namely the eg (dz2 and dx2
-y2) and t2g (dxy, dzy and dzx) irreducible representations.25 The energy separation between these subsets is the so-called ligand field splitting (∆) and its size is determined by the 10Dq parameter.25 This experimentally-determined parameter is a function of the particular set of ligands, the metal ion and the metal-ligand distance (r). In case of an FeII metal ion, the distribution of the d electrons in the corresponding orbitals will generate the two possible spin states, 1A1 and 5T2g (see Figure 1.2 a). Whether the ground state will be 1A1 and 5T2g depends on the ligand field strength, as shown in the so-called Tanabe- Sugano diagram, which gives the dependence of the d-orbital levels as a function of 10Dq (Figure 1.2 b).
Qualitatively it may be understood as follows. For weak ligand-field strengths, the ligand field splitting ∆ will be so small that the electrons will fill up the orbitals following Hund’s rule, favouring ferromagnetic spin alignment. Thus, the 5T2g will be the high-spin paramagnetic ground state. For higher values of 10Dq, the splitting becomes so large that it is more favourable for the electrons to pair up in the lower orbitals, and in the case of a d6 configuration, the low-spin (diamagnetic) 1A1 ground state then results. For compounds with a crystal field splitting value close to the critical value (∆crit), a spin transition may be induced by provoking a change in the 10Dq parameter by means of an external perturbation such as
light, temperature, magnetic field or pressure. In the Tanabe-Sugano diagram,26 the critical point corresponds to the value where both states are degenerate (Figure 1.2 b).
Experimentally, the 10Dq parameter varies discontinuously due to the change in bond length.
The difference in metal-ligand distance of the two states is due to the antibonding property of the eg orbitals. In the case of the high-spin state, two electrons are occupying these orbitals, while for the low-spin state the electrons are only occupying the t2g nonbonding orbitals. A better representation is thus achieved by considering the potential wells of the two states, with the metal-ligand distance (r) on the x axis (Figure 1.3 a). The critical point in the Tanabe- Sugano diagram where the ground state changes, corresponds to the intercept of the two potential wells in the so-called configurational coordinate diagram. This nuclear configuration, where the HS and LS states are accidentally degenerate, will never be an equilibrium configuration of the ground state.
a) b)
Figure 1.2. a) Representation of the two possible states for an octahedral FeII complex. b) Tanabe-Sugano diagram for a transition metal complex with d6 configuration.
In the diagram in Figure 1.3 a, the conditions required for a thermal spin transition to occur are evident: the zero-point energy difference, ∆EºHL, has to be of the order of thermally accessible energies and ∆EºHL> 0. The latter condition is imposed since the entropy of the HS state is larger than that of the LS state.27 The reason for this larger entropy is due to the spin degeneracy of the HS state and to its higher density of vibrational states.27 When these conditions are fulfilled, a population of the HS state can occur at elevated temperatures.
For neutral ligands, the dependence of 10Dq on the metal to ligand distance can be expressed as 10Dq ~ µ/r6, where µ is the dipole moment (in the point dipole model). On basis of the conditions mentioned before, and considering from X-ray crystal structure determination that for an FeII ion the ∆r = rHS-rLS ~ 0.2 Å is found, the range of 10Dq values for which a HS complex, a LS complex or a spin-transition compound is expected, can be defined as follows:28
10DqHS<11000 cm-1 10DqHS~11500-12500 cm-1 10DqLS~19000-21000 cm-1 10DqLS>21500 cm-1
It is clear that the range in 10Dq over which a spin crossover may occur is rather narrow (Figure 1.3 b). This explains why minor changes in coordination environment or even in the crystal lattice, allow or hinder the spin transition.28 Apart from FeII, other transition metal ions of the first row are also to form spin-transition complexes. Examples for CoIII,29 CoII,30 FeIII 31 and NiII 32 are reported in the literature. In most of the cases, their stronger ligand field and their weaker spin pairing energy makes them rare. The cases for NiII are sometimes referred to as geometrical rearrangements, which however are related to a change in spin multiplicity.
a) b)
Figure 1.3. a) Configuration coordinate diagram for an FeII compound expressed along the totally symmetric metal-ligand stretch vibration (r(Fe-L)). b) Representation of the regions where each of the states (HS, LS or Spin Transition) is stable. The shaded area is the region for the low spin state. Taken from Hauser.33
For isolated molecules, such as highly diluted systems, either in solution34 or in the solid state, a Boltzmann-type population of the HS state then determines the spin-transition behaviours (Figure 1.4 a).28 In cases where the spin-transition centres are interacting elastically with each other, the structural changes associated to the spin-transition phenomenon can induce deviations from the Boltzmann-type population of the HS state (Figure 1.4 b). The concomitant change in volume of the ligand coordination sphere around the metal ion during the spin transition can be elastically propagated through out the lattice,
LS complex
Spin-Crossover complex HS complex
leading to different types of spin-transition curves (Figure 1.4).28 This communication between the SCO centres is known as cooperativity.
a) b)
Figure 1.4. Transition curves for spin-transition compounds as a function of temperature. a) Gradual (Boltzmann-type), incomplete (dashed) and steep transitions. b) Two-step transition (dashed) and a transition with hysteresis (solid curves).
1.3. Light-Induced Excited Spin State Trapping (LIESST) and the Relaxation Process During the study of the spin-transition complex [Fe(ptz)6](BF4)2 (ptz = 1-n-propyl- tetrazole) , Decurtins et al. discovered that it was possible to populate the high spin state at low temperatures by irradiating the sample with green light, the so-called LIESST process.9 Soon after this discovery, Hauser reported the reverse process, in which irradiation with red light at low temperatures would bring back the low spin state.35 LIESST and “reverse LIESST” have given an impulse to the already popular field of spin-transition compounds, as they have opened the possibility of using spin-crossover materials as optical data storage or processing devices. However, progress has to be made to generate a light-induced metastable state stable at room temperature. Recently, using a nanosecond pulse laser, it has been proven that a light-induced transition can be achieved even at room temperature.24
Figure 1.5 a, shows the potential wells of the two spin-states and their possible excited singlet (1T1, 1T2), triplet (3T1), and quintet (5E) excited states. A possible low-lying metal-to- ligand charge transfer (MLCT) state is also shown. The 3T1 state plays a key role in the interconversion process as it lies lower in energy than the 5E and the 1T1 state. Thus, the interconversion from LS to HS state, or vice versa, at low temperatures occurs via the 3T1
state.36, 37The change in metal-ligand bond length that occurs during the transition is responsible for the trapping of the HS state at low temperatures.38 This metastable state relaxes through a tunnelling process39 if the temperature is kept below a certain temperature (~ 50-80 K). At higher temperatures, in the so-called thermally activated region, the compound relaxes to the LS state from excited states, where a more effective tunnelling process occurs.37 A correlation between the life time of the metastable state and the T1/2, or temperature at which there are equal amounts of HS and LS centres, was established by Hauser, and has become known as the inverse energy gap law.37 The energy gap between the high-spin and the low-spin state is represented by ∆EHLº or n=∆EHLº/hω. The calculated
relaxation rate constants (KHL) depends on the temperature and on the energy gap n (n =
∆EHLº/hω) , the latter in a fair estimate corresponds to approximately 0.02 · T1/2 (Figure 1.5 b).38
a) b)
Figure 1.5. a) Electronic states for an FeII spin transition complex. The LIESST and reverse LIESST process are both indicated. b) Dependence of the relaxation rate constants (KHL) on the energy gap (n) and the inverse of temperature (1/T, in K-1). Taken from Hauser.38
Cooperative phenomena have an influence on the relaxation process behaviour.36 For compounds which do not show any type of cooperative phenomen a, the relaxation curves follow first order kinetics, whereas those who do show cooperativity, the relaxation curves exhibit a sigmoidal behaviour (see Figure 1.6 a).36 The latter behaviour is characteristic of a self-accelerating process. This is mainly caused by the greater internal pressure induced by the LS state as compared to the HS state, which stabilises the state with a smaller volume.
Thus as γLS (γLS = 1– γHS)grows,the stability of the LS state increases and the process is accelerated (Figure 1.6 b).
b)
Figure 1.6. a) Relaxation kinetics for the metastable high-spin state. Dashed line shows a sigmoidal curve while the full line shows an exponential decay. b) Potential wells for the HS and LS states and their relative energies depending on the γLS value.
1.4. Cooperativity: an important concept
Diluted spin-crossover systems can be attained both in solution and in the solid state.34,
36 In all cases, a Boltzmann-type distribution is found for the temperature dependence of the high spin fraction (γHS). Some spin-crossover materials in the solid state behave differently, mainly due to cooperativity. This effect derives from the electron-phonon coupling between the molecules undergoing SCO, which leads to long-range interactions throughout the crystal lattice, and thereby to new features to the transition behaviour.36 Abrupt, incomplete, discontinuous or two-step transitions may be observed for compounds in the solid state (see section 1.2, Figure 1.4). The differences in volume and molecular vibrations between states are known to play an important role in cooperativity.40, 41 However, the different spin- transition curves observed experimentally could only be satisfactorily simulated, when these differences in volume were considered to create “communication” between the SCO metal centres. The effect of this elastic interaction on the peculiar features of the transition was experimentally proven through experimental studies based on spin-transition compounds diluted in a host lattice28 and by calorimetric experiments.27
Several theoretical models27, 42-47 have been developed to relate the experimental behaviour with theoretical thermodynamic parameters. In this sense, the simplest case which can be encountered is that of a diluted SCO system, in which the spin-transition centres (x) are isolated in a homogeneous host lattice of metal centres, which are represented by M (1-x).
The relative stability of the HS and LS states will only depend on the difference in Gibbs free energy between the two states (∆G). The natural order parameter for the phase transition is γHS. At the point where the two states are in equal amounts (γHS = 0.5), one can obtain the transition temperature (T½) the process as:
∆G(GHS – GLS ) = ∆H–T∆S (Eq 1.1)
GHS =GLS T1/2 = ∆H/∆S (for the process) (Eq 1.2) Following regular solution theory, the Gibbs free energy can be defined as:
G/N = x γHS GºHS + x(1– γHS) GºLS + (1–x) GºM
+ W(x, γHS, T) –TSmix –xTSνi (x, γHS) (Eq 1.3)
where x is the amount of spin-transition centres in the lattice, N is the number of metal centres in the system and GºHS, GºLS and GºM, are the standard Gibbs free energy per complex molecule of NA[FeII]HS, NA[FeII]LS and NA[M], respectively. Smix and Siν are the entropy terms. The latter accounts for the differences in vibrational entropies caused by the presence of M groups in the lattice. W is the interaction term, which takes into account all possible interactions between the three different entities present in the lattice. Slichter and Drickamer 44 were the first to introduce this function for a pure FeII compound:
W (γHS) = γHS Г (1–γHS) (Eq 1.4)
where Г is a parameter of enthalpic origin accounting for the interactions between the spin- transition metal centres. As in the present case the spin-transition molecules are distributed in a matrix of non-spin-transition metal centres (M), then W and Г will depend on x;
W (x, γHS) = x2 γHS (1–γHS)Г + x γHS (1– γHS)ГHS/M + x(1–x)(1–γHS) ГLS/M (Eq 1.5)
Experimental results are well described by considering a Bragg-Williams approximation 48 for a binary mixture:
W (γHS) = γHS ∆(x) – γHS2 Г(x) (Eq 1.6) With
ГM = ГHS/M – ГLS/M,
∆Sx(x) = SνHS(x) – SνLS(x)
and considering ГM as an enthalpic term and ∆Sx as an entropic term, which can be added to the corresponding standard enthalpy and entropy, one obtains the following expression by applying the equilibrium condition:
T = [∆H (x) + x (1–2 γHS) Г] / [∆S(x) + R ln((1– γHS)/ γHS)] (Eq 1.7) This equation accounts for any spin-crossover behaviour observed in the solid state. In solution, the term W and Г will be zero. The determination of the thermodynamic parameters (experimentally) gives the possibility of knowing Г, which describes the cooperative character of the transition.
Other important models for the description of the spin-transition phenomena have been reported, including the one developed by Spiering.13, 14, 49 The major consideration made in this model is that the interaction parameter has an elastic origin. The change in volume due to the transition causes strain in the crystal lattice which results in elastic energy that adds to the interaction energy. The model developed assumes an elastic homogeneous lattice made of a non-spin-transition metal centres (M), in which the misfits caused by the SCO molecules are considered as point defects.50 This means that a stress-free volume vo can be defined, and when occupied by vLS, vHS or vM (volume occupied by a LS, HS or lattice metal centre (M), respectively) it creates a stress field that has an elastic energy. Information on lattice deformations obtained from X-ray diffraction data and elastic constants calculated from Brillouin scattering51 experiments interpreted in terms of the mean field approximation, resulted in interaction constants that were in agreement with the experimental data. On basis of the considerations outlined above, one can estimate the transition temperatures and the hysteresis width by measuring the deformations caused by the transition (X-ray) and the range of elastic constants for a spin-crossover compound.51
Of course, all theoretical models have limitations. Drawbacks are observed for two- step transitions. A long-range interaction model is not sufficient, although necessary, to pattern two-step transitions.28 Short-range interactions, antiferromagnetic or ferromagnetic in nature, compete with the long-range (elastic) interactions resulting in a correlation of the distribution of the HS and LS states.50 Short-range interactions are considered to be steric in nature. This shows up as a step in the transition.52 Similarly, [Fe(Htrz)2(trz)](BF4) (Htrz =
1,2,4-triazole; trz = 1,2,4-triazolato), exhibits a very wide hysteresis loop that cannot be explained with an interaction scheme which only considers long-range interactions.53 An interplay between long and short-range interactions is necessary due to the one-dimensional character of the compound. It is now accepted that theoretical models to describe the spin- crossover phenomenon have to consider both types of interactions. Other models have also been developed, as discussed in a recent review,54 to which the reader is referred.
1.5. Tetrazole-based ligands and their bridging analogues.
The chemistry of tetrazoles has been extensively studied.55, 56 The tetrazole rings are interesting, both from a theoretical and a practical point of view. Their theoretical interest is based upon the aromatic character of the five-membered rings (see Figure 1.7), their capacity of acting as Brönsted acid or base, or their special binding properties.57 Practical uses of tetrazoles spread among many fields, such as medicine, biology, agriculture, fabrication of polymeric materials or as propellants and explosives.55 Tetrazole rings bear two nearly equivalent binding sites. The binding of the tetrazole ligand via its N1 or N2 donor atom is driven by sterical and not electronic factors.58 For instance, 1-substituted tetrazoles bind through the N4 atom of the tetrazole ring. 1-substituted tetrazoles have an exceptional property for the field of coordination chemistry and material science. When coordinated to FeII centres, the proper crystal field splitting is achieved to give rise to the spin-transition process.56, 59, 60
Figure 1.7. Schematic representation of the tetrazole ring.
The preparation of R-tetrazoles is described in Fujisawa’s patent61 and is based on the reaction of a primary amine with sodium azide in the presence of triethylorthoformate and an excess of acetic acid (see Scheme 1.1). The resulting solution is then refluxed for a few hours, depending on the reactivity, in terms of nucleophility, of the amine.
CH OCH2CH3 OCH2CH3
OCH2CH3
R-NH2
-N
N+ N-
CH3COOH +
+ +
Scheme 1.1. Reaction scheme of R-tetrazoles synthesis based on Fujisawa’s patent.61
Research on spin-crossover materials is nowadays focused on the control and understanding of the above discussed cooperativity. The increase of intermolecular interactions or the design of polymeric structures, are two of the strategies followed. The majority of the spin-crossover compounds known until now are mononuclear species and relatively very few polymeric species have been reported. The targeted search for rigid polymeric structures that would bring stability to the material and enhance the desired cooperativity has been a subject of investigation, but the development of this type of compounds is still limited. Nevertheless, spin-transition compounds based on 1,2,4-triazole ligands have yielded one dimensional polymers and have been extensively studied.11, 15-18, 20, 62-65 Another family of polymeric materials which has been thoroughly developed is represented by the cyanide compounds.66-75 A bistriazole-based ligand yielded the first 2D system, namely ([Fe(btr)2(NCX)2], btr = 4,4'-bis-1,2,4-triazole; X = S, 76, 77 or Se 78), and 3D system, namely ([Fe(btr)3](ClO4)2).79 Hence the family of bistetrazoles, also known as ditetrazoles, emerged. Variations of the spacer bridging the two heterocycles were applied to investigate the potentially different spin-transition behaviours of the resulting coordination polymers. The first reported spin-transition compound based on a bistetrazole ligand80 is [Fe(btzp)3](ClO4)2 (btzp = 1,2-bis(tetrazol-1-yl)propane). Its crystal structure is composed of FeII centers triply-bridged by the btzp ligand (Figure 1.8). This compound exhibits a gradual and incomplete spin transition with T½. = 130 K. The lack of cooperative behaviour in this material is explained by the capacity of the spacer (Figure 1.8) between the tetrazoles to rearrange and absorb the structural distortions induced by the spin transition.81 This lack of cooperativity and the low transition temperature favour the observation of the LIESST effect.
a) b)
Figure 1.8. a) and b) are two different views of [Fe(btzp)3](ClO4)2. Taken from van Koningsbruggen et al.80
A very similar behaviour is observed for [Fe(endi)3](BF4)2 (endi = 1,2-bis(tetrazol-1- yl)ethane) in which the spacer lacks a methyl group with respect to [Fe(btzp)3](ClO4)2.82 Increasing the length of the spacer to 4 carbons yields a 3D polymer, [Fe(btzb)3](ClO4)2] (btzb = 1,4-bis(tetrazole-1-yl)butane), that shows a completely different magnetic behaviour.83 This compound gives rise to a cooperative transition for only 16% of the iron centers, the other 84% remaining HS throughout the whole range of temperatures. The transition presents a hysteresis (T½↓ = 150 K, T½↑ = 170 K), which is a surprising result for such a diluted system, where the spin-transition centers should not behave cooperatively. It seems that this cooperative behaviour is related to a biphasic character of the material.
[Fe(btzb)3](PF6)2, a 3D polymer,84 shows a cooperative two-step transition (T½ (first step) = 174 K) with the second step displaying a hysteresis loop of about 4 K (T½↓ = 165 K T½↑ = 169 K). In this case, the compound reveals a threefold interpenetrated 3D structure (Figure 1.9), in which each btzb acts as bridge between the FeII metal centres. PF6- anions fill the cavities left by the cationic network and leave no space for the ligand to rearrange once the transition has occurred. Interestingly, it seems that the driving force of the crystallisation process is due to the counterion which acts as template from which suitable single crystals can grow.84
Figure 1.9. X-ray structure of [Fe(btzb)3](PF6)2. Taken from Grunert et al.84
Surprisingly, the increase in length of the spacer connecting the tetrazole rings, results in an increase of the cooperativity. This observation appears to disagree with the fact that the shock absorbing properties of the spacer increase with the number of carbons in the aliphatic chain. This leads to the conclusion that cooperativity depends on the overall rigidity of the material and not specifically on the rigidity of the spacer.84
Recently, in a systematic study on alkane-based spacers ((1,X-bis(tetrazol-1-yl)- alkane, alkane = butane, pentane, hexane, heptane, octane and nonane), Absmeier et al.
observed a relationship between the parity and length of the spacer and the properties of the transition.85 In all cases the structures are isostructural to the [Fe(btzb)3](PF6)2 previously published,84 as determined by means of X-ray powder diffraction measurements. For chains with an even number of carbons, T1/2 is higher than for odd carbon-numbered chains. The even-numbered carbon spacers also show higher degrees of excitation to the metastable high- spin state and lower T(LIESST) b temperatures, following the relation with the T1/2.
1.6. Triazines and its applications.
Triazines are six-membered heterocyclic aromatic rings consisting of three carbon atoms and three nitrogen atoms. It is a common moiety, and many of its derivatives are used as pharmaceutical products, herbicides or building blocks in the field of supramolecular and coordination chemistry.86, 87 1,3,5-triazine (c), also known as s-triazine, is one of three possible isomers, the other two being 1,2,3-triazine (a) and 1,2,4-triazine (b) (Figure 1.10).
Figure 1.10. Representation of the different triazine isomers, 1,2,3-triazine (a), 1,2,4-triazine (b) and 1,3,5-triazine (c).
From all 1,3,5-triazine-based compounds, melamine (2,4,6-triamino-1,3,5-triazine) is the most well known. It is used to form melamine resins through the reaction with formaldehyde. Another important derivative of s-triazine is cyanuric chloride or 2,4,6- trichloro-1,3,5-triazine (Figure 1.11) which is widely used in the field of synthetic chemistry as a reactant for the synthesis of more intricate organic molecules. Indeed, the selective gradual substitution of one, two or three of the chloride atoms at the 2, 4, 6 positions, can easily yield compounds with a wide variety of complexity.88-91
The possibility to control the substitution by means of temperature confers to these molecules a special interest, especially for the preparation of multifunctional materials, where, for instance, the synthesis of multitopic ligands is crucial.92 The first chloride can already be substituted at 0 ºC, the second at room temperature and the third under solvent reflux (see Figure 1.11 a). Moreover, any nucleophile can be used to attack the electrophilic carbon of the triazine ring, such as amines, alcohols and thiols.90, 91 Microwave reaction synthesis can also be used in some cases when the nucleophicity of the reactant is sterically hindered, as will be shown in this thesis (see Chapter 9).
b T(LIESST) is the temperature at which the metastable high spin state created by light irradiation, relaxes to the LS state. It may be calculated with: δχmT/δT.
N N N
N N N
N N
N
a) b) c)
a) b)
Figure 1.11. a) Schematic representation of cyanuric chloride and the reactivity dependence of its chloride atom with temperature. b) Hydrogen-bond formation of melamine with formaldehyde.93 Both pictures constructed after Gamez et al.87
In the field of supramolecular chemistry triazine derivatives are commonly used since they are known to form hydrogen bonding interactions (Figure 1.11 and 1.12) and to favour π- π interactions.93, 94 This field, described by Lehn as the “chemistry of molecular assemblies and of the intermolecular bond” 95 has been very active for the last few decades. The importance of intermolecular interactions is well known in chemistry and in nature. H2O owes to these intermolecular interactions its liquid state at ambient conditions and the two strands of the DNA are maintained by means of hydrogen bonds. The rational design of organic molecules has been essential in the control of these interactions. In the case of triazine-based molecules, these forces have been used to create aggregates of high complexity such as tubular fibres96 or three-dimensional porous networks,97 the latter being of great importance in the field of gas storage in fuel cell applications.97
Figure 1.12. a) Hydrogen-bonded network for the tetraphenylphosphonium salt of tetraphenylborate showing the stability of these interactions in triazine-based derivatives b) Channels formed due to this hydrogen bonded network.97 Both pictures taken from Gamez et al. 87
The introduction of metal components in these materials can result in the combination of the structural properties intrinsic to the network and the electronic properties of the metal.
This current line of investigation has resulted in the publication of various papers reporting materials with disparate topologies combined with various electronic properties, i.e. magnetic, luminescent and conducting.98-100 s-Triazine has been highly beneficial to develop this field since the organic ligands involved can be easily achieved starting from cyanuric chloride.
Robson and co-workers first used this concept to produce an interpenetrated network with copper(I) atoms.101 Fujita reported the case of coordination network cages to catalyse reactions (Figure 1.13 a).102
a) b)
Figure 1.13. a) Fujita’s M6L4 cage (L = 2,4,6-tri(4-pyridyl)-1,3,5-triazine ligand (tpt))
with 4 encapsulated o-carboranes.102 b) Anion-binding properties of the complex 103 [CuCl2- azadentriz] (azadentriz = N,N-{2,4-di[(dipyridin-2-yl)amine]-1,3,5-triazine}ethylenediamine).
Both pictures taken from Gamez et al. 87
Systems that would combine two or more phase transitions of different physical nature are currently under investigation by material scientists.104-106 This hybrid approach may yield compounds in which disparate properties coexist or even couple. The latter would give the remarkable possibility of acting upon one property in order to modify the other, and could lead to the enhancement of a magnetic signal or the on/off switching of the property.
Following this idea, Kepert and co-workers have synthesised a network which combines both spin transition and host-guest properties.107 In this case, the presence of the guest influences the properties of the spin transition to such an extent that in the absence of a guest molecule there is no spin transition. The material is composed of only one entity bearing two properties.
Another strategy to search for hybrid materials is the combination of two entities in the same system, each possessing a different property.104, 105, 108 The result of the combination could be the suppression of one or both properties, or the synergetic or simple coexistence of both properties. Successful examples along this line are the ferromagnetic superconductors,109 which have once more proved the capacity of synthetic research to create materials which combine two properties that cannot be found in nature.
In this laboratory, s-triazine-based ligands have been used for many purposes;
catalytic,110 anti-cancer111 and material research.88, 89 Others, such as anion-binding
properties103 (Figure 1.13 b) or spin transition,112 have only recently been found. These results prove that s-triazine-based ligands are successful in a number of topical fields. This multidisciplinarity is greatly due to the straightforward modification of the ligand and holds great prospect for the near future. A combination of these properties could lead to the so- desired multifunctional molecules, many of which are in development at the moment.
Particularly in material chemistry, rationally designed molecular building blocks enable chemists to engineer materials having a predictable order and useful solid-state properties.
The reaction of cyanuric chloride with three dipyridyl amino moieties yields the well-known dpyatriz ligand (see Figure 1.14 a).113 This ligand tends to bind through the nitrogen donor atoms of the pyridyl groups. Copper, zinc and nickel have shown to form complexes with disparate morphologies when reacting with this ligand (see Figure 1.14 b).
a) b)
Figure 1.14. a) Schematic representation of the dpyatriz ligand. b) X-ray structure of the coordination compound 91 formed by the reaction of dpyatriz with Cu(NO3)2 (picture taken from Gamez et al.) 87
.
1.7 Aim and Scope of the thesis.
The work described in this thesis was started with the intention of chemically developing two strategies to obtain spin-crossover compounds, i.e. bistetrazole-based materials and triazine-based compounds, and through them to learn about the insights of the spin-crossover phenomenon. The former constituted an already known strategy and the idea was to increase the number of compounds in this family, and with them to study various physical properties.
The latter was based on a new ligand-type which had never been used before for spin- transition materials, and thus constituted a more chemically challenging part. This second part deals with the chemical tuning of the properties of the spin-transition complexes synthesised.
In Chapter 2, the synthesis and analysis of all materials which were developed during this thesis work and the description of the experimental techniques used are included.
The three following chapters deal with the bistetrazole-based chemistry of spin-crossover compounds. In Chapter 3, the synthesis and characterisation of two new bistetrazole-based complexes with substituted alkyl-based spacers is presented. Different techniques have been employed to study the influence of the substituent on the overall lattice arrangement and the
spin-transition properties. Chapter 4 deals with a new ligand with a benzyl-based spacer whose FeII complex shows spin-crossover properties. The study of the effect of three different counterions on the spin transition behaviour is reported. Chapter 5 contains a structural comparison of all bistetrazole-based spin-transition complexes, including those in this thesis, and a conclusion on the results of the bistetrazole-chemistry developed in the two previous chapters.
The following chapters deal with triazine-based ligands and their application in spin- transition chemistry. Chapter 6 shows six dinuclear complexes based on the dpyatriz ligand with various magnetic properties. The ferromagnetic behaviour of one of the iron-based compounds is compared with the isostructural nickel and cobalt complexes. Chapter 7 is devoted to a mononuclear FeII compound based on dpyatriz which is extensively characterised, and to two other FeII spin-transition materials, which are compared to this mononuclear species. Chapter 8 presents a 1D solvent-dependent spin-transition complex based on the dpyatriz ligand. Chapter 9 contains the conclusions drawn from the chemistry of triazine-based ligands applied to spin crossover presented in the three previous chapters, as well as some final results and comments on some possible future prospects.
Parts of this thesis have been published (Chapters 6 and 7),112, 114 are in press (Chapters 3 and 8),115, 116 or will be submitted for publication (Chapters 3, 4, 5).
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