Platinum Metals Rev., 1987, 31, (1), 20
Pressure Sensitive Palladium Complex
The complex bis(1,2-cyclohexanedionedioximato)palladium(II) has been prepared and its absorption spectra examined. The preliminary results, which are reported here, show that the colours of thin films of this complex are a function of the applied pressure, a property which may find application as an indicator of pressure and pressure distributions up to perhaps seventy thousand atmospheres.
The development of diamond-anvil pressure cells enables the effect of pressure on various materials to be observed, in situ, visually. This is especially important for the study of polymorphs and crystal growth. Quantitative pressure measurement in a diamond-anvil cell has been carried out by Barnett, Block and Piermarini using ruby as a pressure sensor (1). The pressure is determined by measuring the pressure shift in the sharp R-line fluorescence spectrum of ruby. The pressure shift of an optical absorption band in bis(dimethylglyoximato)nickel(II), Ni(dmg)2, had already been studied as one approach to pressure measurement in the cell (2).
Various d8 square complexes are formed when Ni2+, Pd2+ and Pt2+ react with 1,2-dionedioximes such as dimethylglyoxime. The complexes are chemically stable, and have various colours at atmospheric pressure. The effect of pressure on the absorption spectra of these complexes has already been studied (3, 4, 5). The absorption bands show a large red shift with increasing pressure. The colours of the complexes change with pressure shifts of spectra. For example, the colour of Ni(dmg)2 which is initially red turns to green at pressures around 1.7 GPa. It would be convenient if a semiquantitative value of pressure could be determined visually from a change in the colour of a material with change in pressure.
Bis (1, 2-cyclohexanedionedioximato) Pd(II), Pd(niox)2, may be an excellent pressure indicator in the range 0 to 8 GPa. The complex turns from yellow to orange and then to successive colours in the visible spectrum with increasing pressure. In this article we report preliminary results for colours and absorption spectra in this complex at high pressures.
The molecular structure of Pd(niox)2 is illustrated in Figure 1. A palladium(II) ion is coordinated to the four nitrogen atoms of two 1,2-cyclohexanedionedioxime(nioxime, niox) anions. The square planar complex molecules stack face to face in parallel columns, linking the metal ions in extended linear chains throughout the crystal lattice.
Pd(niox)2 was prepared from a hot aqueous solution of K2PdCl4 and a hot alcoholic solution of nioxime(1,2-cyclohexanedionedioxime). The diamond-anvil pressure cell was used for the study of optical properties in the complex at high pressures; the optical system consisted of a standard microscope and a monochrometer with an associated photodetection system. The pressure was measured by means of the pressure shift of the sharp R-line fluorescence spectrum of ruby. Water was used as the pressure transmitting medium.
The absorption spectrum of an evaporated thin film of Pd(niox)2 at atmospheric pressure is shown in Figure 2. This spectrum is similar to that of bis(dimethylglyoximato)Pd(II), Pd(dmg)2, which has already been determined by Ohashi, Hanazaki and Nagakura (6). The band at 294 nm in Pd(niox)2 may be ascribed to a metal to ligand charge transfer transition; on the other hand the band at 477 nm is assigned to the 4dz2-5pz transition. Figure 3 exhibits the absorption spectra of Pd(niox)2 in the visible region at high pressures. The absorption band always showed a large red shift with increasing pressure. Visual observation of the sample in the diamond-anvil cell indicated that with increasing pressure the colour of Pd(niox)2 turned from yellow to orange (at 0.8 to 1.4 GPa), to red (at 1.4 to 2.3 GPa), to purple (at 2 to 3 GPa), to blue (at 2.7 to 5.5 GPa), to yellow green (at 5.5 to 6.5 GPa), and to pale yellow (above 7 GPa) with increasing pressure. The variation in the colour of Pd(niox)2 with pressure can be interpreted to show that the absorption band which is ascribed to 4dz2-5pz shifted toward longer wavelength at high pressures.
The relationship between the colour and the pressure is only the preliminary results of our work. The pressure ranges of the colours have not yet been determined in detail. The thickness of our sample is about 1000Å. It should be noted that the tone of a colour depends on a number of factors including thickness of the sample, the pressure transmitting medium and the pressure distribution. If the relationship between colour and pressure is studied in detail, a semiquantitative value of pressure could be obtained from the visual observation of the change in colour with pressure by a colorimetric method similar to pH testing paper. Furthermore, the pressure gradient in a high pressure cell can be directly observed in situ.
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