A typical pressure-tuning experiment involves loading the sample and appropriate pressure calibrant in the DAC with the aid of an optical microscope and, after bolting the cell onto a plate in the IR or Raman spectrometer, gradually increasing the pressure by tightening the mechanical screw (turning clockwise) on the cell. Practice reveals just how many turns are necessary to bring about small incremental steps in pressure. At the same time as the pressure is being increased, the infrared and/or Raman spectra of the sample and the calibrant are being monitored. From the data obtained, the wavenumber () vs. pressure (P) results are then plotted for selected vibrational modes and the pressure dependences (d/dP) are determined from the slopes of the resulting lines. Stretching modes are usually more pressure sensitive (0.3–1.0 cm1/ kbar) than are bending and deformation modes (<0.1–0.3 cm1/kbar).6External (i.e., lattice) modes are particularly sensitive to pressure (1–3 cm1/kbar). The vibrational modes normally increase in energy with increasing pressure because compression results in increased force constants. In certain cases, there are spectacular changes in the pressure dependences due to major structural modifica- tions such as phase transitions, changes in intermolecular interactions (e.g., H-bonding), or changes in the-backbonding in organometallic ligands (e.g., CO, CS, CNPh, NO, alkene, etc.).1,2 A few examples of these effects for organometallic and coordination compounds will now be given.
2.9.3.1 Pentacarbonyl(methyl)manganese(I)
An excellent prototype molecule for elucidating the effects of high pressures on the molecular structures of organometallic compounds is CH3Mn(CO)5, which has a simple metal–carbon bond. Under pressure, the axial a1 (CO) mode (trans to the CH3 group), at 1,975 cm1 at ambient pressure, initially decreases in wavenumber with the application of pressure until 10 kbar (Figure 2).10 At this point, the d/dPvalue changes from being negative (0.82 cm1/ kbar) to positive (0.30 cm1/kbar).
Similar results are obtained for the axiala1(CO) modes of several other pentacarbonylmetal species, such as CH3Re(CO)5,10Cr(CO)5(CS),11and Mn(CO)5Br.12,13This interesting observation is explained by a competition between two opposing effects: (i) the effect of pressure to compress the axial metal–CO bond, thereby increasing the metal–carbon force constant and increasing the metal–CO* overlap, resulting in a decrease in wavenumber of the axiala1(CO) mode; and (ii) the effect of pressure to compress the axial CO bonds, thereby increasing the CO force constants resulting in increases in wavenumber of the axiala1(CO) modes. Initially, the -backbonding effect dominates but, eventually, the second effect comes into prominence.
2.9.3.2 cis-Dithiolatobis(triphenylphosphine)platinum(II)
The petrochemical industry continues to focus much of its research effort on hydrodesulfurization and the Claus Process for the elimination and recovery of sulfur from fossil fuels. This research will become even more important in the future with the worldwide emphasis on the emission controls projected in the Kyoto Protocol. The Claus Process results in the conversion of toxic H2S gas, emitted by hydrodesulfurization of fossil fuels, into sulfur and water. In order to better understand the mechanism of the Claus Process, some research has been directed at model intermediate compounds, such ascis-(Ph3P)2Pt(SH)2. This compound reacts with SO2to produce the catenated sulfur species, (Ph3P)2PtS3O, and water, mimicking the kind of chemistry that occurs in the Claus Process.14 While the SH stretching modes of cis-(Ph3P)2Pt(SH)2 are too weak to be detected in the infrared, they are clearly present at 2,500 cm1 in the Raman spectrum. The pressure sensitivities of the SH stretching modes of cis-(Ph3P)2Pt(SH)2 have recently been investigated.15
The wavenumber () vs. pressure (P) plots for the asym(SH) and sym(SH) vibrations of cis-(Ph3P)2Pt(SH)2, a t2,552 cm1and2,540 cm1, respectively, at ambient pressure, are shown inFigure 3. Initially, the two peaks are not particularly pressure sensitive. At15 kbar, however, theasym(SH) vibration suddenly splits into two and the pressure dependence of thesym(SH) vibration increases dramatically. Similarly, the CH stretching vibrations show a marked increase in d/dP at around 15 kbar (Figure 4). There is clear evidence of a phase transition occurring in this compound at15 kbar.
0 400 450 1,950 2,000 2,050 2,100 2,150
10
Pressure (kbar)
Wavenumber (cm–1)
20 30 40 50
Figure 2 Pressure dependence of selected Raman bands of CH3Mn(CO)5 (reproduced by permission of The American Chemical Society fromInorg. Chem.1991,30, 117–120).
0 2,540 2,545 2,550 2,555 2,560
5 10 15
Pressure (kbar)
Wavenumber (cm–1)
20 25 30
Figure 3 Wavenumber () vs. pressure (P) plots for SH stretching modes ofcis-[(Ph3P)2Pt(SH)2] (repro- duced by permission of Elsevier fromSpectrochim. Acta.2002,A58, 2581–2587).
116 High-pressure Raman Techniques
0 3,070
3,065
3,060
3,055 3,075
5 10 15 20 25 30 35 40
Pressure (kbar)
Wavenumber (cm–1)
Figure 4 Wavenumber () vs. pressure (P) plots for CH stretching modes of cis-[(Ph3P)2Pt(SH)2] (reproduced by permission of Elsevier fromSpectrochim. Acta.2002,A58, 2581–2587).
250 200 150 100 50
Wavenumber (cm–1)
Intensity (a.u.)
0.2
(a) (b) (c) (d) (e)
Figure 5 FT-Raman spectra of [(bztzdtH)I2].I2 in the 25050 cm1 region at different compression pressures: (a) 12.8, (b) 26.9, (c) 32.2, (d) 46.7 kbar (reproduced by permission of Elsevier fromSpectrochim.
Acta.2002,A58, 2725–2735).
2.9.3.3 Pressure-induced Disproportionation
Sometimes, solid-state chemistry can be initiated under pressure. One interesting example is the pressure-induced disproportionation of the solid, heterocyclic, thioamide–diiodine adduct [(bztzdtH)I2].I2 (bztzdtHẳbenzothiazole-2-thione).16 The effect of high pressures (approaching 47 kbar) on the Raman spectrum of this thioamide adduct is shown in Figure 5. The peaks at 110 cm1 and 148 cm1 are growing in relative intensity with increasing pressure and are attributed to the presence of the triiodide I3 ion being formed by disproportionation of the thioamide–diiodine adduct.
2.9.3.4 Decarbonyldimanganese(0)
Another interesting example is that of the M2(CO)10 complexes, where MẳMn, Re.17,18 In the solid state, both complexes adopt a staggeredD4d-symmetry conformation, at ambient state, with asingle M—M bond between the two M(CO)5 fragments. When pressure is applied, there are marked changes occurring in the IR spectra in the (CO) region (Figure 6), which have been attributed to rotation about the M—M bond of the two M(CO)5fragments in the staggeredD4d
conformation into the eclipsed D4hconformation under pressure.
2.9.3.5 Mercuric(II) Cyanide
Mercuric(II) cyanide, Hg(CN)2, affords an excellent example of a pressure-induced phase transi- tion.19 The CN stretching modes are detected in the Raman at2,200 cm1. As the pressure is increased, the single band becomes two and eventually becomes a single band again (Figure 7).
The pressure dependences of the CN stretching modes are plotted in Figure 8. There is clear evidence of the occurrence of a pressure-induced phase transition.
Intensity
(a)
(b)
(c)
(d)
Mn2(CO)10
Re2(CO)10
2,100 2,000 600 500 400 200 100 ν (cm–1)
0
Figure 6 Raman spectrum of M2(CO)10(MẳMn, Re) at (a) ambient and (b) 16 kbar pressure. (Reprinted with permission from Ref.19.)
118 High-pressure Raman Techniques
Intensity
2,185 2,195 2,205 2,215
Wavenumber (cm–1)
(kbar) 42.4 39.0 36.4 33.1 29.8 26.4 23.8 19.8 17.8 17.2 15.9 13.9 11.9 10.6 7.9 4.6 2.0 0.7 0.0
Figure 7 Effect of pressure on the CN stretching modes of Hg(CN)2. (reproduced by permission of The Society for Applied Spectroscopy fromAppl. Spectrosc.1987,41, 915–917.)
0 10.0 20.0 30.0 40.0
P (kbar) υ (cm–1)
2,190 2,195 2,200 2,205 2,210
Figure 8 Pressure dependences of the CN stretching modes of Hg(CN)2. (reproduced by permission of The Society for Applied Spectroscopy fromAppl. Spectrosc.1987,41, 915–917.)