Linear Polymers With a Backbone of Metal Atoms
- Fig. 1: Chemical structures of examples representing different types of linear polymeric structures with a backbone of metal atoms.
- Fig. 2: Optical micrograph of a semiconducting [Pt(NH2eh)4][PtCl4] fiber placed between two crossed polarizers. The light intensity distribution in the fiber with respect to the position of polarizers indicates that the polymer backbones are oriented.
- Fig. 3: Gels of [Fe(NH2trz)3](2ns)2 and transparent thermochromic films prepared thereof.
Soluble and processible inorganic or organometallic polymeric structures comprising a linear backbone of metal atoms are uncommon. Different types of related compounds have been studied, which differ in the interactions between the metal atoms (Fig. 1). Thus, polymeric structures were established by covalent metal-metal bonds, by electrostatic attraction of oppositely charged coordination units (resulting in weak interactions between adjacent metal atoms) and by bidentate ligands connecting neighboring metal atoms (which do not interact with each other). Emphasis was put in the synthesis of the related compounds in pure form in order to investigate their materials properties and to explore processing into (oriented) films and fibers.
It seems that polystannanes are hitherto the only polymers with a backbone of covalently bound metal atoms. These compounds, of the general formula (SnR2)n (Fig. 1), have been described for aliphatic and aromatic side groups R [1-6]. They were synthesized e.g. by catalytic polymerization of dihydrostannanes, H2SnR2 (under release of H2) or by polymerization of Cl2SnR2 under the action of sodium (under release of NaCl). Typically, polystannanes are of yellow color which is caused by delocalization of σ-electrons, a rare phenomenon. This delocalization can also promote semiconductivity.
Polystannanes can be processed to films by pressing in the solid state of by casting from solution followed by evaporation of the solvent. When solvent evaporation takes place on substrates with an oriented surface structure consisting of parallel poly(tetrafluoroethylene) (PTFE, Teflon) molecules, the deposited polystannane molecules often also orient. Remarkably, the orientation direction of the polymer backbone depends on the length of the alkyl groups R: the backbone orients parallel to the PTFE molecules for R = butyl but perpendicular for R = dodecyl. Obviously, in presence of the short butyl groups the polymer backbone orients parallel to the PTFE molecules, while the rather long dodecyl groups orient parallel to the PTFE molecules, thus forcing the polystannane backbone to perpendicular orientation.
Oriented films of semiconducting polystannanes could basically be of interest for field effect transistors (FETs) in the area of plastic electronics, which targets to novel products for the production of thin flexible polymer displays (electronic paper). However, although polystannanes are frequently thermally stable up to at least 200 °C, the polystannanes synthesized so far are sensitive to light and ambient atmosphere and thus degrade in the course of hours, days or weeks at ambient conditions, depending on the substituent R. Therefore, an important objective in polystannane research is the synthesis of polystannanes which are stable at ambient conditions.
Magnus’ salt derivatives, [Pt(NH2R)4][PtCl4]
A number of compounds of the generic formula [Pt(NH2R)4][PtCl4] contain linearly arranged platinum atoms. The first compound of this type, [Pt(NH3)4][PtCl4], was synthesized by Magnus in 1828 (Magnus’ green salt). The Pt–Pt distances in this salt are relatively short (3.25 Å), which allows significant orbital overlap between adjacent platinum atoms, thus resulting in semiconductivity. In addition, this material possesses virtually unlimited stability at ambient conditions. Unfortunately, however, Magnus’ green salt cannot readily be processed because it is insoluble in common solvents and does not melt before decomposition. In order to overcome this disadvantage, we have substituted ammonia by 1-aminoalkanes, NH2R [7-11]. The synthesis of such Magnus’ salt derivatives proceeds straightforward by combination of K2[PtCl4] and [Pt(NH2R)4]Cl2, and the compounds with longer alkyl groups can indeed be dissolved in common organic solvents.
The Pt–Pt distance in [Pt(NH2R)4][PtCl4] tends to be shorter for branched than for linear alkyl groups. At first glance, this might be against the expectations as steric demands of branched groups are higher. Yet linear alkyl groups can pack more favorably, and it appears that the crystal packing of the linear alkyl groups enlarges the Pt–Pt spacing. Hence, the compounds with 1-amino-2-ethylhexane and 1-amino-3,7-dimethyloctane (NH2dmoc) show sufficiently short Pt–Pt distances (< 3.4 Å) to render the materials semiconductive. They can be processed from solution to oriented films (as mentioned above for polystannanes) and fibers by electrostatic spinning (Fig. 2). Field effect transistors of remarkable stability were prepared with [Pt(NH2dmoc)4][PtCl4] as an oriented semiconductive layer, the performance of the devices did not change noteworthy after boiling of the FETs in water over night.
Complexes of iron(II) and substituted 1,2,4-triazoles, [Fe(Rtrz)3]X2
In soluble compounds of the composition [Fe(Rtrz)3]X2, Rtrz acts as a bridging ligand which connects adjacent iron(II) ions, which thus establish a linear backbone of a coordination polymer (Fig. 1). These Fe–Fe distances, however, are too large to allow significant orbital overlap between adjacent iron ions. The charges resulting from the iron(II) ions are balanced by counter ions which do not coordinate to the metal centers. Corresponding compounds were synthesized e.g. by combination of [Fe(H2O)6](2ns)2 and Rtrz, with R = NH2 or a long alkyl group and 2ns = 2-naphthalene sulfonate [12-14]. These compounds are stable in the solid state in the atmosphere and electrically insulating, and some are liquid crystalline at room temperature.
Most remarkably, many compounds of the composition [Fe(Rtrz)3]X2 show spin crossover, i.e. they reversibly change from the low-spin to the high-spin state upon increase in temperature. Spin-crossover manifests in iron(II) compounds by a change of magnetism from paramagnetic to diamagnetic and a concomitant color change from pink or violet to colorless (thermochromism). With 2ns as a counter ion, these phenomena occur around or somewhat above room temperature. Moreover, the color change associated with spin crossover can also depend on the coordination of water molecules (solvatochromism), i.e. related materials act as humidity sensors.
As these materials form gels in organic solvents we prepared thermochromic films by solvent evaporation (Fig. 3) as well as porous solids of low density (0.02 g/cm3 – 0.03 g/cm3) by supercritical drying with CO2. Further, blends of the compound with R = octadecyl and 50% - 80% w/w polyethylene were processed to thermochromic fibers. Due to their thermochromic behavior and straightforward synthesis, Fe(II)-Rtrz complexes were proposed as over-heating sensors and document security systems.
In spite of the different nature of these types of polymeric systems described above, they are processible from solution, to yield (oriented) films or fibers which in the above described examples, depending on the system, show semiconductivity, thermochromism, solvatochromism, or change in magnetic properties.
I cordially thank P. Smith, I. Bräunlich, J. Bremi, F. Choffat, M. Fontana, M. Trummer, K. Feldman, F. Uhlig, M.-L. Lechner, R. Mezzenga, A. Sánchez-Ferrer, N. Stingelin and M. Bauer for fruitful collaboration, as well as the coworkers and collaborators who are indicated in the respective references.
Department of Materials, ETH Zürich, Zürich, Switzerland
Department of Materials
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