ESTIGMA ASOCIADO AL TEA
5.4 PREGUNTAS ESPECÍFICAS PARA EVALUAR LOS OBJETIVOS DE APRENDIZAJE
3.1 Introduction
For almost 40 years phosphazenes have been coordinated to metal ions. Due to their ease of modification a range complexes with assorted properties have been formed. As a result it has been proposed that they could be used for a range of applications1 such as anti-cancer drugs2, organic light emitting diodes (OLED)3, selective metal extraction4-6, and heterogeneous catalysts7.
The cyclotrimer has been crucial in the development of these materials. This is because its reactions can be optimised, and potential side reactions determined. In addition the products can be characterised with techniques such as X-ray crystallography which is unavailable for polymers.1
As previously mentioned in Chapter 1, phosphazenes have long been associated with coordination of metal ions. They are able to do this in five typical ways:8 i) form a cationic salt with the metal ion; ii) coordinate to the ring nitrogen; iii) coordinate via exocyclic groups substituted to the phosphazene ring; iv) coordinate to the phosphorus atom via a covalent bond; and finally v) form an anionic bond with the phosphazene ring.
It is the third form of coordination that is the most pertinent to this project. Here an assortment of ligands that typically form spin crossover complexes were chosen to attach to a cyclophosphazene. While choosing substituents that may promote the coordination of the phosphazene itself may give rise to unique properties, such behaviour is often unpredictable.9-11
Substituent selection was based on six main factors: i) when coordinated to iron(II) the complexes show SCO behaviour; ii) the SCO temperature for similar complexes occurs near room temperature, making it possible to investigate any changes with the available equipment; iii) the SCO behaviour is not dependent on inter-molecular interactions, rather it is based on the ligand strength and bond strain; iv) phenol bearing analogues of the substituents must be able to be made in sufficient scales to attach to the polymer; v) the substituents must be stable with strong bases to allow phosphazene substitution; vi) ideally similar complexes display Light Induced Excited Spin State Trapping (LIESST).
Chapter 3. Introduction
A selection of suitable substituents was initially investigated to assess their reactivity with cyclophosphazenes in Chapter 2. Eight new ligands were chosen from Chapter 2 to coordinate iron, each of them contained the following coordination sites: 2,2ʹ:6ʹ,2ʺ-
terpyridine (Terpy), 2,6-di(pyridin-2-yl)-4-phenylpyridine (PhTerpy), 2,6-di(1H- benzimidazol-2-yl)pyridine (bbp) and 2,6-di(1H-pyrazol-1-yl)pyridine (bpp). Each of these groups will be described in turn.
Chapter 3. Introduction
3.1.1 2,2ʹ:6ʹ,2ʺ-terpyridines
2,2ʹ:6ʹ,2ʺ-terpyridines are a class of classic multidentate ligands which have been
extensively studied because of their high affinity for binding transition metal ions12-14 and their interesting optical properties15-18. With the recent developments by Constable19-25 and others26-31, it has become simpler to functionalise them. As a result of the large variety of substituted terpys that are now available, they have found applications in areas from actinide metal extraction to solar cell dyes (an example of a terpy-based Grätzel dye for a solar cell shown in Figure 3.1.1.1)32.
Figure 3.1.1.1 A Grätzel dye based on a terpy for use in a solar cell.32
When coordinated to iron(II), typically LS complexes are formed that cannot be excited to the HS state.33 However, it is possible to induce SCO behaviour by adding sterically bulky groups near the coordinating site.34 To reduce the steric strain the iron-nitrogen bond lengths are increased reducing the energy difference between the HS and LS states (∆EHL), making SCO thermally accessible. An example of this occurs when the ortho
and para hydrogens of one of the terpy rings are substituted by phenyl groups,34 this results in SCO occurring at room temperature (see Figure 3.1.1.2).
Chapter 3. Introduction
Figure 3.1.1.2 Left: A crystal structure of a terpy complex substituted at the ortho and
para positions reducing the SCO temperature (hydrogens, anions and solvent removed for clarity). Right: The visible spectrum of this compound as a function of temperature
(300 – 323 K).34
In addition to bulky groups near the coordination site causing changes in SCO behaviour, changes in the para position (see Figure 3.1.1.3) also have significant effects. Varying the electron withdrawing or donating ability of the substituent alters the spin state of the complex. However, the effects of the substitution cannot be predicted as they are both dependent on electronic effects and solid state interactions.35-37
Figure 3.1.1.3 Substitution of terpy at the para position, where R can be any substituent.
Not only do terpy complexes provide a path for thermal SCO, they also show light- excited SCO behaviour.38,39 They are able to do this via the charge transfer band observed when iron(II) is coordinated. The optical SCO behaviour could also be tuned via substitution, just as the thermal behaviour can be.
Chapter 3. Introduction
3.1.2 2,6-di(1H-benzimidazol-2-yl)pyridine
2,6-di(1H-benzimidazol-2-yl)pyridines has drawn a great deal of interest due to its imidazole rings. The rings are more electron poor than pyridine resulting in a stronger
σ-donor coordination than pyridine. Such a difference greatly alters the metal
complexes behaviour, e.g. lowering the energy difference between the ground and excited state. In addition to this feature the protons of the imidazole rings can be removed40-43 or substituted44 (see Figure 3.1.2.1), greatly altering the metal complexes behaviour.
Figure 3.1.2.1 From left: Parent bbp, deprotonated bbp, N-substituted bbp (where R can
be a range of substituents).
It is due to these ligands’ tuneable properties that they have been used for a range of applications such as luminescence45,46, actinide separation47, hydrogen production48,49 and high temperature SCO40-43.
The σ-donor behaviour lowers the ∆EHS-LS relative to terpy, resulting in SCO to occur
near room temperature.33 As the variation on the imidazole nitrogen changes its luminescence and metal affinity properties, the SCO behaviour is also greatly altered. The general behaviour observed when the imidazole is deprotonated is that the SCO temperature increases, this increase is due to an increase in electron-density strengthening the Fe-N bond. When the proton is substituted with an alkyl group the SCO temperature is generally increased due to the increase in electron density; however, this can be altered if the substituent causes the system additional strain (see Figure 3.1.2.2).44
Chapter 3. Introduction
Figure 3.1.2.2 Left: A Monte-Carlo simulation of a substituted iron-bbp complex. Right:
The magnetic behaviour of the complex.44
To date there have been few studies that have focused on the SCO behaviour of the para-substituted bbp ligands. The single paper that has been published observed that as the electron-withdrawing effect of the substituent is increased, ∆EHS-LS decreased
resulting in the HS state being favoured (see Figure 3.1.2.3). This is due to reduction in the electron density available for coordination to the metal ion, i.e. it produces a weaker field ligand.50
Figure 3.1.2.3 Differences in the calculated heats of formation between the HS and LS
forms (∆Etot since calculations are for single molecules) in [Fe(Xbbp)2]2+ vs. ∆δ (1H
Chapter 3. Introduction
3.1.3 2,6-di(1H-pyrazol-yl)pyridine
2,6-di(1H-pyrazol-yl)pyridines was originally introduced as a terpy analogue for ruthenium complexes because it can be easily functionalised by varying the pyrazole attached; 50 however, it is now primarily used to form SCO complexes (Figure 3.1.3.1) with brief interludes into solar cell dyes51,52 and lanthanide luminescence53-55. As with bbp the pyrazole ring is more electron poor than pyridine resulting in more σ-donor
coordination behaviour, resulting in ∆EHS-LS being smaller than that of terpy and bbp.
This in turn results in the SCO temperatures being lower than that of terpy or bbp.56
Figure 3.1.3.1 A schematic view of iron(II) coordinating to a bpp ligand, forming a
SCO complex.
The variations in SCO behaviour have been extensively investigated in relation to the pyrazole substitution; however, it is only recently that the effects of substitution at the para position of the pyridine have been investigated.57-63 It has been found that the addition of a phenyl group to the para position maintains its SCO behaviour and allows further substitution.58 It is noted that variations in crystal lattice do alter the SCO behaviour but do not prevent it (see Figure 3.1.3.2),61 this has also been found to be true for substitutions of the pyrazole ring64-66 – the SCO behaviour will be altered; however, no examples of substitution have been shown to prevent it.
Chapter 3. Introduction
Figure 3.1.3.2 Left: [Fe(Pybpp)2]3+. Right: thermal magnetic behaviour of
[Fe(Pybpp)2]3+.61
These complexes have also been studied for its photomagnetic behaviour. This, like many other SCO complexes, showed that the LS complex could be excited to the HS state through the use of laser excitation and held in the HS state for extended periods of time by keeping the complex at cryogenic temperatures. This effect is called light induced excited spin state trapping (LIESST)64,65 (see an example in Figure 3.1.3.3).
Figure 3.1.3.3 Left: [Fe(bppMe)2]2+. Right: thermal and LIESST magnetic behaviour of