**6. Raman and IR spectra of the compounds [ PI _{4] }^{+[ }MF_{6] – }(M = As, Sb) and [ PI_{4}^{+}EI_{4– ] n} (E = Al, Ga, In)**

**Christoph Aubauer,
Thomas M. Klapötke ^{*}
and Axel Schulz**

Institute of Inorganic Chemistry,

University of Munich (LMU),

Butenandtstrasse 5-13 (Haus D),

D-81377 Munich (Germany)

fax: +49-89-2180-7492

__e-mail:tmk@cup.uni-muenchen.de__

**Abstract
**

[ PI_{4}]_{ }^{+}[ AsF_{6}]_{ }^{–}_{ }, [ PI_{4}]_{ }^{+}[ SbF_{6}]^{–}_{ },

[ PI_{4}^{+}AlI_{4}^{ –} ]_{ n}, [ PI_{4}^{+}GaI_{4}^{ –} ]_{ n} and the new compound [ PI_{4}^{+}InI_{4}^{–} ]_{ n} were prepared and characterised by Raman and IR spectroscopy. The vibrational spectra were compared with related compounds. Vibrational assignments for the normal modes of the PI_{4}^{+} cation has been made on the basis of density functional calculations.

**Introduction
**

The first compound containing a PI_{4}^{+} cation was obtained by S. Pohl in 1983. The reaction of PI_{3}, I_{2} and AlI_{3} in CS_{2} led quantitatively to the polymeric compound of the composition [PI_{4}^{+}AlI_{4 }^{–}_{ }]_{n}, which was structurally characterised by X-ray diffraction. [1] I. Tornieporth-Oetting and T. M. Klapötke synthesised the first isolated binary phosphorus (V) iodine cation PI_{4}^{+} in 1989. The [ PI_{4}]_{ }^{+[ }AsF_{6}]_{ }^{–}_{ }salt was characterised by Raman spectroscopy. Because of the non-coordinating character of the AsF_{6}^{–}_{ }anion, they suggested that no significant interactions between complex cations and anions are involved for this system. [2]

Recently, M. Kaupp et al. [3] showed in a combined theoretical and experimental study, that the PI_{4}^{+} cation has an extremely large negative [31]P chemical shift in the compounds [ PI_{4}]^{+[ }AsF_{6}]_{ }^{–}_{ }(-519 ppm) and [ PI_{4}]_{ }^{+[ }SbF_{6}]_{ }^{–}_{ }(-517 ppm), which is entirely due to a spin-orbit contributions from the four heavy iodine substituents, transmitted to the phosphorus nucleus by a very effective Fermi-contact mechanism. The less negative solid-state [31]P NMR chemical shifts found in the polymeric [ PI_{4}^{+}AlI_{4}^{–}_{ }]_{n} (-305 ppm) and

[ PI_{4}^{+}GaI_{4}^{–}_{ }]_{n} (-295 ppm), suggests that the P-I bond orders are reduced due to intermolecular I × × × I interaction between PI_{4}^{+} cations and AlI_{4}^{–}_{ }anions.

**Experimental Section
**

All the compounds reported here are moisture sensitive. Consequently, strictly anaerobic and anhydrous conditions were employed for their synthesis. All manipulations were carried out in an inert gas atmosphere (dry-box).

I_{2} (Merck), PI_{3}, AlI_{3}, GaI_{3} and InI_{3} (all Aldrich) were used as received. CFCl_{3} and CS_{2} were dried over P_{4}O_{10}. [ PI_{4}]_{ }^{+[ }SbF_{6}]_{ }^{–}_{ }, [ PI_{4}^{+}AlI_{4}^{–}_{ }]_{n} and [ PI_{4}^{+}GaI_{4}^{– }]_{n} were prepared according to the literature. [1, 3]The preparation of I_{3}^{+}AsF_{6}^{–}_{ }and I_{3}^{+}SbF_{6}^{–}_{ }also followed literature procedures [4,5].

Raman spectra were obtained on powdered solid samples contained in 4 mm glass capillary tubes with a Perkin Elmer 2000 NIR spectrometer fitted with a Nd-YAG laser (1064 nm) using the 180° geometry in the range at 500 – 50 cm^{-1}. The spectra of [ PI_{4}]_{ }^{+[ }MF_{6}]_{ }^{–}_{ }(M = As, Sb) were recorded with 30 mW at -100°C using a Ventacon low temperature cell. The spectra of [ PI_{4}^{+}EI_{4}^{–}_{ }]_{n} (E = Al, Ga, In) were measured with

30 mW at room temperature.

IR spectra were recorded on Nujol mulls between CsI plates in the range at 800 – 200 cm^{-1}with a Nicolet 520 FT IR spectrometer. Nujol was dried with sodium.

For the determination of decomposition points, samples were heated in sealed glass capillaries in a Büchi B450 instrument.

**Preparation of [ PI _{4}**

**]**

^{+[ }AsF_{6}**]**

_{ }^{–}*In a typical reaction PI*

_{ }._{3}(0.72 g, 1.76 mmol) was reacted with I

_{3}

^{+}AsF

_{6}

^{–}

_{ }(1.00 g, 1.76 mmol) in CFCl

_{3}(15 mL) with stirring at room temperature in a two-bulbed glass vessel incorporating a coarse sintered-glass frit and a Young valve. An intense dark purple solution of iodine over a pale yellow solid was obtained. After stirring for 24 h the solution was filtered, and refiltered for several times, by condensing about half the solvent back and refiltering. Solvent and traces of remaining iodine were removed under dynamic vacuum, leaving a pale yellowish solid. Yield: 0.60 g (47 %), decomposition point 74°C.

**Preparation of [ PI _{4}^{+}InI_{4}^{–}_{ }**

**]**

**.**

_{ n}*In a typical reaction PI*

_{3}(0.49 g, 1.20 mmol) was reacted with I

_{2}(0.31 g, 1.20 mmol) and InI

_{3}(0.60 g, 1.20 mmol) in CS

_{2}(15 mL) with stirring at room temperature. After stirring for 24 h the solvent was removed under dynamic vacuum, leaving a black solid. Yield: 1.26 g (90 %), decomposition point 71°C.

**Computational Methods. **The structure and vibrational data for PI_{4}^{+} were calculated by using the density functional theory with the program package Gaussian 94 (optimised *d* (P-I) = 2.431 Å, calculated frequency see Table 1). [6] For phosphorus a standard 6-31G(d,p) basis set was used and for I a quasi-relativistic pseudopotential (ECP46MWB) [7] and a (5s5p1d)/[3s3p1d]-DZ+P basis set. [8] The computations were carried out at the DFT level using the hybrid method B3LYP which includes a mixture of Hartree-Fock exchange with DFT exchange-correlation. Becke’s 3 parameter functional where the non-local correlation is provided by the LYP expression (Lee, Yang, Parr correlation functional) was used which is implemented in Gaussian 94. For a concise definition of the B3LYP functional see ref. [9]

[ PI_{4}]_{ }^{+}[ ^{ }AsF_{6}]_{ }^{–} |
[ PI_{4}]_{ }^{+}[ ^{ }SbF_{6}]_{ }^{–} |
[PI_{4}^{+}AlI _{4}^{–}_{ }]_{ n} |
[ PI_{4}^{+}GaI _{4}^{–}_{ }]_{ n} |
[ PI_{4}^{+}InI _{4}^{–} ]_{ n} |
Calculation ( IR) ^{a} |
Assignment |

– | – | 380 (2) | 378 (2) | 386 (1)/376 (1) | 385 (67) | n _{3} (T_{2}), PI_{4}^{+} |

321 (0.5) | 211 (0.5) | 194 (3.5) | n _{3} (T_{2}), EI_{4}^{–} |
|||

178 (2) | 181 (3) | 152 (10) | 151 (10) | 156 (10) | 165 (0.0) | n _{1} (A_{1}), PI_{4}^{+} |

149 (4) | 147 (3) | 134 (2) | n _{1} (A_{1}), EI_{4}^{–} |
|||

82 (10) | 83 (10) | 95 (2) | 94 (2) | 99 (2) | 96 (0.1) | n _{4} (T_{2}), PI_{4}^{+} |

71 (6) | 72 (6) | 77 (0.5) | 72 (0.5) | 88 (2) | 62 (0.0) | n _{2} (E), PI_{4}^{+} |

**Table 1 **Raman frequencies of the compounds [ PI_{4}] ^{+[ }MF_{6}]_{ }^{–}(M = As, Sb) and [ PI_{4}^{+}EI_{4}^{– }]_{n} (E = Al, Ga, In) and the calculated frequencies for the PI_{4}^{+} cation (frequencies in cm^{-1})

^{a} calculated IR intensities in km/mol

^{a}

Discussion of the Raman and IR spectra

Although [ PI_{4}]_{ }^{+[ }AsF_{6}]_{ }^{–}_{ }and [ PI_{4}]_{ }^{+[ }SbF_{6}]_{ }^{–}_{ }are thermally stable compounds, however, they decompose in the IR laser beam at room temperature and at low temperature, as well.

Table 1 summarises the computed and experimentally observed Raman frequencies of the PI_{4}^{+} compounds. The IR frequencies of these salts are presented in Table 2. Figure 1 shows the Raman spectra of the compounds [ PI_{4}^{+}EI_{4}^{–}_{ }]_{n} (E = Al, Ga, In).

Figure 1 Raman spectra of the compounds [ PI_{4}^{+}EI_{4}^{–}_{ }]_{n} (E = Al, Ga, In) |

Like in the related cations PCl_{4}^{+}, PBr_{4}^{+} and the isoelectronic compound SiI_{4}, possessing T_{d}symmetry, there are four normal modes of vibrations expected for the PI_{4}^{+} cation. The totally symmetric n _{1} (A_{1}) stretching mode can be observed with low intensity at ca. 180 cm^{-1} for the compounds [ PI_{4}]_{ }^{+[ }MF_{6}]_{ }^{–}_{ }(M = As, Sb). Presumably, the low intensity of the n _{1} (A_{1}) stretching mode is associated with a progressive decomposition of the compound in the laser beam. The polymeric PI_{4}^{+} compounds seem to be more stable, which might be due to the strong cation × × × anion interactions. Moreover, the less intense n _{1} (A_{1}) vibration in the Raman spectra of [ PI_{4}]_{ }^{+[ }AsF_{6}]_{ }^{–}_{ }and [ PI_{4}]_{ }^{+[ }SbF_{6}]_{ }^{–}_{ }can be explained by fluorescence, often leading to wrong peak intensities in Raman spectra.

The Raman spectra of [ PI_{4}^{+}EI_{4}^{– }]_{n} (E = Al, Ga, In) show the most intensive peak at ca.152 cm^{-1 }for the n _{1} (A_{1}) vibration of PI_{4}^{+}. This appears consistent with the suggestion, that the vibration frequencies should be at lower wavenumbers, because the P-I order in the compounds [ PI_{4}^{+}EI_{4}^{–}_{ }]_{n} (E = Al, Ga, In) is reduced by strong I × × × I cation × × × anion interactions, [1] whereas the PI_{4}^{+} cation in [ PI_{4}]_{ }^{+[ }MF_{6}]_{ }^{–}_{ }(M = As, Sb) is almost isolated, which was shown by [31] P MAS NMR spectroscopy [3].

The sharp peaks in the Raman spectra of [ PI_{4}]_{ }^{+[ }AsF_{6}]_{ }^{–}_{ }and [ PI_{4}]_{ }^{+[ }SbF_{6}]_{ }^{–}_{ }at ca. 82 cm^{-1}and 71 cm^{-1} can be assigned to the symmetric n _{4} (A_{1}) and the asymmetric

n _{2} (E) deformation modes of PI_{4}^{+}, respectively. The Raman spectra of [ PI_{4}^{+}EI_{4}^{–}_{ }]_{n} (E = Al, Ga, In) show a broad tale below 100 cm^{-1} with two weak peaks at ca. 95 cm^{-1} and 77 cm^{-1}, which can be assigned to the deformation modes n _{4} (T_{2}) and

n _{2} (E) of PI_{4}^{+}.

The asymmetric n _{3} (T_{2}) stretching mode of the PI_{4}^{+} cation can only be observed in the IR (Table 2) and Raman spectra of [ PI_{4}^{+}AlI_{4}^{–}_{ }]_{n}, [ PI_{4}^{+}GaI_{4}^{–}_{ }]_{n} and [ PI_{4}^{+}InI_{4}^{–}_{ }]_{n} at ca. 380 cm^{-1}, which agree excellently with our theoretical calculation (B3LYP) for the PI_{4}^{+} cation. No n _{3} (T_{2}) vibration could be observed in the IR spectra of [ PI_{4}]_{ }^{+[ }AsF_{6}]_{ }^{–}_{ }and [ PI_{4}] ^{+[}SbF_{6}]_{ }^{–}_{ }, due to reaction with CsI plates.

[ PI_{4}]_{ }^{+}[ ^{ }AsF_{6}]_{ }^{–} |
[ PI_{4}] ^{+}[ ^{ }SbF_{6}]^{ –} |
[ PI_{4}^{+}AlI _{4}^{– }]_{ n} |
[ PI_{4}^{+}GaI _{4}^{–}_{ }]_{ n} |
[ PI_{4}^{+}InI _{4}^{–} ]_{ n} |
Assignment |

697 (s) | 657 (s) | n _{3} (T_{1u}), MF_{6}^{–} |
|||

392 (s) | 285 (s) | n _{4} (T_{1u}), MF_{6}^{–} |
|||

– | – | 380 (br) | 382 (m) / 375 (m) | 386 (vs) / 375 (m) | n _{3} (T_{2}), PI_{4}^{+} |

329 (br) | 234 (s) / 223 (vs) | n _{3} (T_{2}), EI_{4}^{–} |

**Table 2 **IR frequencies of the compounds [ PI_{4}]^{+[ }MF_{6}]– (M = As, Sb) and [ PI_{4}^{+}EI_{4}^{–}_{ }]_{n} (E = Al, Ga, In) (frequencies in cm^{-1})

### The presence of the anions EI_{4}^{–}_{ }(E = Al, Ga, In) is confirmed by the symmetric stretching mode, n _{1} (A_{1}), at 149 cm^{-1} ([ PI_{4}^{+}AlI_{4}^{–}_{ }]_{n}), 147 cm^{-1} ([ PI_{4}^{+}GaI_{4}^{–}_{ }]_{n}) and 134 cm^{-1}

([ PI_{4}^{+}InI_{4}^{– }]_{n}). They are consistent with literature values (n _{1} (AlI_{4}^{–}_{ }): 146 cm^{-1};[13] n _{1}(GaI_{4}^{–}_{ }): 145 cm^{-1}; n _{1} (InI_{4}^{–}_{ }): 138 cm^{-1} [14]). The weak peak at 211 cm^{-1} (Raman) and the strong peaks at 234 and 223 cm^{-1} (IR) in PI_{4}^{+}GaI_{4}^{–}_{ }can be assigned to the asymmetric stretching mode, n _{3} (T_{2}), of GaI_{4}^{–}_{ }(n _{3} (GaI_{4}^{–}_{ }): 222 cm^{-1} [14]). The n _{3} (T_{2}) vibration of AlI_{4}^{–}_{ }in [ PI_{4}^{+}AlI_{4}^{–}_{ }]_{n} was observed at 321 cm^{-1} (Raman) and 329 cm^{-1} (IR), respectively. The peak at 194 cm^{-1} in the spectra of [ PI_{4}^{+}InI_{4}^{–}_{ }]_{n} can be assigned to the asymmetric stretching vibration, n _{3} (T_{2}), of InI_{4}^{–}_{ }.

The IR spectra of [ PI_{4}]^{+}[^{ }MF_{6}]^{–} (M = As, Sb) show two expected IR active modes, n _{3} (T_{1u}) and n _{3} (T_{1u}), for an isolated MF_{6}^{–}_{ }anion with O_{h} symmetry, which are consistent with literature values. [15]

**Conclusion
**

The Raman and IR spectra of [ PI_{4}]_{ }^{+[ }AsF_{6}]_{ }^{–}_{ }, [ PI_{4}] ^{+[ }SbF_{6}]_{ }^{–}_{ }, [ PI_{4}^{+}AlI_{4}^{–} ]_{ n},

[ PI_{4}^{+}GaI_{4}^{– }] _{n} and [ PI_{4}^{+}InI_{4}^{–}_{ }]_{ n} were recorded, assigned and compared with theoretically obtained frequencies. Comparison of the spectra of the ionic with the polymeric salts show that the n _{1} (A_{1}) stretching mode is shifted to higher frequencies in the purely ionic species [PI_{4}]_{ }^{+}[^{ }AsF_{6}]_{ }^{–}_{ }and [ PI_{4}]_{ }^{+}[^{ }SbF_{6}] ^{–}_{ }. Moreover, a new assignment of the n _{1} (A_{1}) stretching mode has been made, which was reported in a previous study at 193.5 cm^{-1}. [2]

The Raman and IR experiments show that the isolated PI_{4}^{+} cation in AsF_{6}^{–}_{ }and SbF_{6}^{–}_{ }salts are less stable than the polymeric compounds [ PI_{4}^{+}EI_{4}^{–}_{ }]_{n} (E = Al, Ga, In) where the cation is stabilised by strong I × × × I interactions.

**Acknowledgments**

**
**We are indebted to and thank Mr. Gunnar Spieß for Raman spectroscopic measurements. Financial support by the University of Munich and the Fonds der Chemischen Industrie is gratefully acknowledged.

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*Received 1st March 1999, received in revised format 29th March 1999, **accepted 12th April 1999.*