Silylene complexes have been proposed to occur as interme- diates for several reactions of silyl-metal compounds. But there are currently no examples for ...
Fresenius J Anal Chem (1996) 355 : 340–342
© Springer-Verlag 1996
POSTER
C. Fickert · R. Pikl · D. Gernet · S. Möller · W. Malisch · W. Kiefer
Matrix-isolation Raman spectroscopy and photochemistry of carbonyl-metal-silyl complexes
Received: 1 February 1996 / Accepted: 13 February 1996
Abstract To study the photochemistry of metal-silyl complexes the compounds Cp(CO)2Fe-SiH2(CH3) (Cp = η5–C5H5) and Cp*(CO)2((CH3)3P)Mo-SiH3 (Cp* = η5–C5(CH3)5) have been isolated in krypton or nitrogen matrices and subsequently detected by Raman spectroscopy. Vibrational assignments for the two compounds are given. Furthermore, differences in the Raman spectra induced by UV irradiation are discussed.
Introduction Silylene complexes have been proposed to occur as intermediates for several reactions of silyl-metal compounds. But there are currently no examples for complexes with M = SiR2 units existing without base stabilization. The photochemistry of disilyl or monosilyl complexes has been studied by isolating the reactive intermediates in low-temperature frozen gas matrices for their spectroscopic characterization [1]. We describe how Raman spectroscopy can be used in combination with the matrix isolation technique in order to analyze the photochemistry of Cp(CO)2Fe-SiH2(CH3) (1) and Cp*(CO)2((CH3)3P)Mo-SiH3 (2). In a first step it is necessary to characterize the Raman spectra of the starting compounds (see also [2, 3]).
10–7 and 10–8 mbar is provided [4]. Clear and glassy matrices were prepared by using the “slow-spray-on” technique. The sample material was heated in a Knudsen cell at 32° C for 1 and at 85° C for 2. Afterwards the gas was directly mixed with the matrix gas (krypton or nitrogen) in a heated nozzel, from where it was deposited on a highly polished gold plated copper cold head at 20 K. For excitation of the Raman spectra the 647 nm line of a krypton ion laser (Spectra Physics, model 2020) was applied. The laser beam was focussed on the matrix layer in an 30° angle to the target. The scattered light was dispersed by means of a double monochromator (Spex, model 1404) and the signal was detected by a CCD camera system (Photometrics, Spectra 9000). For all spectra a resolution of 3 cm–1 was chosen. Data acquisition and spectra analyses were performed by a commercial software package (MAPS, Photometrics). For the UV irradiation either a medium-pressure Hg arc lamp (Philips, 125-W HPK) or an argon ion laser (Spectra Physics, model BeamLok 2085) was used, operating multiline with a wavelength range of 333 to 364 nm.
Results and discussion Figure 1 a shows the Raman spectra of 1 in a Kr matrix. The characteristic signals together with their vibrational
Experimental The silyl complexes have been synthesized by standard procedures [2, 3]. The matrix isolation apparatus consisted of a cryocooler (CTICryogenics, model 22C) and a turbo-pumping unit (Pfeiffer/Balzers, TPH 170). The latter takes care that a pressure in the region of
C. Fickert · R. Pikl · D. Gernet · W. Kiefer (Y) Institut für Physikalische Chemie der Universität Würzburg, Marcusstrasse 9–11, D-97070 Würzburg, Germany S. Möller · W. Malisch Institut für Anorganische Chemie der Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany
Fig. 1 a, b Raman spectra of Cp(CO)2Fe-SiH2(CH3) (1) isolated in a Kr matrix at 20 K: a before, b after UV irradiation (the asterisks mark new bands)
341 Table 1 Characteristic Raman wavenumbers (in cm–1) for Cp(CO)2Fe-SiH2(CH3) (1) and Cp*(CO)2((CH3)3P)Mo-SiH3 (2) with their assignments
Cp(CO)2Fe-SiH2(CH3) (1)a
Cp*(CO)2((CH3)3P)Mo-SiH3 (2)a, b
Assignmentc
2090 m 1997 w
2091 m/p 1905 w 1868 w 1809 w 1426 s/p
νs/as(SiH) νs(CO)
1940 m 1115 vs
734 w 720 w 682 m 659 w
608 m 595 w 529 m 469 w
678 s/p 608 w/p 591 s/p
νs(PC3) δ(MoSiH) Cp* ring breathing
543 m 527 w 493 m 464 s/p
δ(MCO) and ν(MC2)
395 m 380 m/p
a vs:
very strong, s: strong, m: medium, w: weak b p: polarized c s: symmetric, as: antisymmetric
376 vs 319 vs
assignments are listed in Table 1. After UV irradiation the ejection of one CO molecule was expected. A silylene complex can then be formed by migration of the methyl group or a hydrogen atom of the silyl unit. According to this mechanism spectra should show a decrease of the intensive (Fe-Si) stretching and of the ν(SiH2) bands, some others in the (FeC) and (FeCp) region should be shifted. Also a new (FeSi) stretching mode at higher wavenumbers and a (FeMe) or (FeH) vibration should be detectable. Spectrum b in Fig. 1 was detected after UV irradiation and new bands at 281, 348, 579, 839, and 1302 cm–1 (marked by an asterisk) occur. The signal at 348 cm–1 is attributed to a new (FeSi) bond with higher bond order. Spectrum b also reveals a decrease of the (FeSi) and (SiH2) stretching band of 1. In order to confirm a possible decomposition to [CpFe(CO)2]2 also the Raman spectra of this compound in a Kr matrix were recorded [5]. Comparing these spectra to the one displayed in Fig. 1 b we found no indication of such a decomposition product. 2 was isolated in a N2 matrix. The high quality of the layer enables to obtain polarized Raman spectra (Fig. 2 a, b) before UV irradiation. The assignment of the characteristic bands are listed in Table 1. The CO photodissociation of 2 in a N2 matrix should lead to a CO/N2 exchange in the complex. The spectra show no differences after a 15 min irradiation with an Ar ion laser operating at a 2 W power level. After 30 min with 3 W laser power the matrix had changed its colour to orange, but the layer remained transparent. The spectrum (Fig. 2 c) shows only a few signals of the starting material. Most of the metal-ligand vibrational bands disappear. A very weak band at 2153 cm–1 could be assigned to a coordinated N2 mole-
νas(CO) Cp* νs(C-Me) Cp ring breathing νas(PC3) δ(FeSiH) ν(SiC)?
338 s/p 223 w
ring tilt νs(MoCp*) νs(FeCp) ν(MSi) ν(MoP)
Fig. 2 a–c Raman spectra of Cp* (CO)2((CH3)3P)Mo-SiH3 (2) isolated in N2 matrix at 20 K: a with parallel and b with perpendicular polarization before photolyses, c after 30 min irradiation with 3 W laser power (parallel polarization)
cule. Sometimes a very intensitive band can be detected at 1302 cm–1, which could not be assigned. Due to the high laser power it is assumed that the complex has been completely decomposed. Experiments with lower UV irradiation are in progress. Acknowledgements Financial support from the Deutsche Forschungsgemeinschaft (Sonderforschungsbereich 347, Teilprojekte B2 and C2) as well as from the Fonds der Chemischen Industrie is gratefully appreciated.
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3. Malisch W, Lankat R, Schmitzer S, Pikl R, Posset U, Kiefer W (1995) Organometallics 14 : 5622–5627 4. Gernet D, Kiefer W, Kammel T, Nau W, Adam W (1995) J Mol Structure 348 : 333–336 5. Fickert C, Günther P, Scholz P, Gernet D, Pikl R, Kiefer W (1996) Inorg Chim Acta 250 (submitted)
