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This work has been digitalized and published in 2013 by Verlag Zeitschrift für Naturforschung in cooperation with the Max Planck Society for the Advancement of Science under a Creative Commons Attribution 4.0 International License. Dieses Werk wurde im Jahr 2013 vom Verlag Zeitschrift für Naturforschung in Zusammenarbeit mit der Max-Planck-Gesellschaft zur Förderung der Wissenschaften e.V. digitalisiert und unter folgender Lizenz veröffentlicht: Creative Commons Namensnennung 4.0 Lizenz. Photoreactivity of Titanocene Pentasulfide Horst Kunkely, Amd Vogler* Institut für Anorganische Chemie, Universität Regensburg, D-93040 Regensburg, Germany Z. Naturforsch. 53 b, 224-226 (1998); received December 12, 1997 Charge Transfer, Titanium Complexes, Sulfide Complexes, Photochemistry The electronic spectrum o f Cp2TiSs shows a long-wavelength absorption at Amax = 492 nm which is assigned to the lowest-energy S52~ ^ T i IV ligand-to-metal charge transfer (L M C T ) transition. The photolysis of the complex in CH 2CI2 leads to the formation o f Cp2 TiCl 2 and elemental sulfur. It is suggested that L M C T excitation initiates a reductive elimination with the extrusion o f S5 while the reduced titanocene is reoxidized by the solvent. 1. Introduction The electronic spectra and photochemistry of transition metal complexes which contain sulfur- coordinating ligands have been studied extensively for many years. Very simple compounds o f this type are the tetrasulfido complexes of the d° metal ions such as V v , M oVI, W VI and Revn [1, 2], Their absorption spectra are characterized by low- energy ligand-to-metal charge transfer (L M C T ) bands. L M C T excitation induces photoredox pro cesses [3]. In contrast to these thiometallates very little is known about the electronic spectra and pho toreactivity of polysulfide complexes which contain S,2" (x > 1) ligands although the general chemistry o f such compounds has attracted much attention [4]. This class o f coordination compounds includes complexes such as [PtIV(S5 ) 3 ]2- and Cp2T iIVS5 which contain pentasulfide, Ss2~, as a bidentate lig and. In order to explore the electronic spectra and photoreactivity o f compounds with M Sn moieties we selected Cp2T iIVS5 for the present study. This choice is based on several considerations. The pho tochemistry o f a variety o f titanocene derivates has been studied in quite some detail [5,6]. The spectra o f simple Cp2TiX 2 complexes show only LM C T ab sorptions [5,7], These observations should facilitate the present investigation. In this context it is quite surprising that the photochemical formation but not the light sensitivity o f Cp2TiSs were reported 20 years ago [8]. * Reprint requests to Prof. Dr. A. Vogler. 2. Experimental 2.7. Materials Cp2TiS 5 was commercially available by Aldrich. All solvents were spectrograde. 2.2. Photolyses The light source was an Osram HBO 100 W/2 or a Hanovia Xe/Hg 977 B-l (1 kW) lamp. Monochromatic light was obtained using a Schoeffel GM 250/1 high- intensity monochromator (band width 20 nm). The pho tolyses were carried out in solutions o f CH 2 CI2, CHCI 3, and C H 3CN in 1 cm spectrophotometer cells at room temperature. Solutions were air saturated since deaera tion did not affect the results. Progress of the photoly ses was monitored by UV-visible spectrophotometry. The photoproducts were identified by their absorption spectra. For quantum yield determinations the complex concen trations were such as to have essentially complete light absorption. The total amount of photolysis was limited to less than 5% to avoid light absorption by the photo product. Absorbed light intensities were determined by a Polytec pyroelectric radiometer which was calibrated by actinometry and equipped with a RkP-345 detector. 2.3. Instrumentation Absorption spectra were measured with a Hewlett Packard 8452A diode array or an Uvikon 860 absorption spectrometer. 3. Results The electronic spectrum of Cp2TiSs in CH 2CI 2 (Fig. 1) shows absorptions at Amax = 492 (e = 2300 dm3 M -1 cm“ 1), 376 (sh, 2400), 316 (11800), and 288 (sh, 10100) nm. This solution is light sensi tive. The photolysis of Cp2TiSs in CH 2CI2 is as sociated with spectral variations (Fig. 1) which 0939-5075/98/0200-0224 $ 06.00 © 1998 Verlag der Zeitschrift für Naturforschung. A ll rights reserved. K

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  • This work has been digitalized and published in 2013 by Verlag Zeitschrift für Naturforschung in cooperation with the Max Planck Society for the Advancement of Science under a Creative Commons Attribution4.0 International License.

    Dieses Werk wurde im Jahr 2013 vom Verlag Zeitschrift für Naturforschungin Zusammenarbeit mit der Max-Planck-Gesellschaft zur Förderung derWissenschaften e.V. digitalisiert und unter folgender Lizenz veröffentlicht:Creative Commons Namensnennung 4.0 Lizenz.

    Photoreactivity of Titanocene PentasulfideHorst Kunkely, Amd Vogler*

    Institut für Anorganische Chemie, Universität Regensburg, D-93040 Regensburg, Germany

    Z. Naturforsch. 53 b, 224-226 (1998); received December 12, 1997 Charge Transfer, Titanium Complexes, Sulfide Complexes, Photochemistry

    The electronic spectrum o f Cp2TiSs shows a long-wavelength absorption at Amax = 492 nm which is assigned to the lowest-energy S52~ ^ T i IV ligand-to-metal charge transfer (LM CT) transition. The photolysis o f the complex in CH2CI2 leads to the formation o f Cp2TiCl2 and elemental sulfur. It is suggested that LM CT excitation initiates a reductive elimination with the extrusion o f S5 while the reduced titanocene is reoxidized by the solvent.

    1. Introduction

    The electronic spectra and photochemistry of transition metal complexes which contain sulfur- coordinating ligands have been studied extensively for many years. Very simple compounds o f this type are the tetrasulfido complexes o f the d° metal ions such as V v , M oVI, W VI and Revn [1, 2], Their absorption spectra are characterized by low- energy ligand-to-metal charge transfer (LM CT) bands. LM CT excitation induces photoredox processes [3]. In contrast to these thiometallates very little is known about the electronic spectra and photoreactivity o f polysulfide complexes which contain S,2" (x > 1) ligands although the general chemistry o f such compounds has attracted much attention [4]. This class o f coordination compounds includes complexes such as [PtIV(S5)3 ]2- and Cp2TiIVS5 which contain pentasulfide, Ss2~, as a bidentate ligand. In order to explore the electronic spectra and photoreactivity o f compounds with MSn moieties we selected Cp2TiIVS5 for the present study. This choice is based on several considerations. The photochemistry o f a variety o f titanocene derivates has been studied in quite some detail [5,6]. The spectra o f simple Cp2T iX 2 complexes show only LM CT absorptions [5,7], These observations should facilitate the present investigation. In this context it is quite surprising that the photochemical formation but not the light sensitivity o f Cp2TiSs were reported 20 years ago [8].

    * Reprint requests to Prof. Dr. A. Vogler.

    2. Experimental

    2.7. Materials

    Cp2TiS5 was commercially available by Aldrich. A ll solvents were spectrograde.

    2.2. Photolyses

    The light source was an Osram HBO 100 W/2 or a Hanovia Xe/Hg 977 B-l (1 kW) lamp. Monochromatic light was obtained using a Schoeffel GM 250/1 high- intensity monochromator (band width 20 nm). The photolyses were carried out in solutions o f CH2CI2, CHCI3, and CH3CN in 1 cm spectrophotometer cells at room temperature. Solutions were air saturated since deaeration did not affect the results. Progress o f the photolyses was monitored by UV-visible spectrophotometry. The photoproducts were identified by their absorption spectra. For quantum yield determinations the complex concentrations were such as to have essentially complete light absorption. The total amount o f photolysis was limited to less than 5% to avoid light absorption by the photoproduct. Absorbed light intensities were determined by a Polytec pyroelectric radiometer which was calibrated by actinometry and equipped with a RkP-345 detector.

    2.3. Instrumentation

    Absorption spectra were measured with a Hewlett Packard 8452A diode array or an Uvikon 860 absorption spectrometer.

    3. Results

    The electronic spectrum of Cp2TiSs in CH2CI2 (Fig. 1) shows absorptions at Amax = 492 (e = 2300 dm3 M - 1 cm“ 1), 376 (sh, 2400), 316 (11800), and 288 (sh, 10100) nm. This solution is light sensitive. The photolysis o f Cp2TiSs in CH2CI2 is associated with spectral variations (Fig. 1) which

    0939-5075/98/0200-0224 $ 06.00 © 1998 Verlag der Zeitschrift für Naturforschung. A ll rights reserved. K

  • H. Kunkely - A . Vogler • Photoreactivity o f Titanocene Pentasulfide 225

    Fig. 1. Spectral changes during the photolysis o f 1.68 x 10-4 M CpaTiSs in CFbCb after 0 (a), 2,4 and 6 (d) min irradiation time with Ain- = 366 nm, 1-cm cell.

    include isosbestic points at A = 432, 380 and 287 nm. The spectral changes are indicative o f the formation o f Cp2TiCl2 which displays absorptions at Amax = 522 (e = 240), 393 (2400) and 312 (5800) nm [9,10]. The formation of Cp2TiCl2 is also confirmed by its reaction with [Ru(CN)6]4~ which generates the trinuclear complex ion {Cp2Tilv[/z- NCRun(C N )5]2 } 6~ (Amax = 630 nm) [6 ], However, the spectral changes (Fig. 1) could not be simulated assuming that Cp2TiCl2 is the only compound which is formed during the photolysis. Another photoproduct is characterized by an absorption which extends from the visible to the UV and increases in intensity towards shorter wavelength. This apparent absorption is obviously caused by colloidal particles which scatter the light. At higher concentrations of Cp2TiS5 (~ 10~ 2 M ) the colloid in the photolyzed solution becomes visible in a Tyndall effect. The colloid which can be removed by centrifuging the photolyzed solution consists o f elemental sulfur. It is identified by its absorption spectrum at Amax = 278 and 266 nm in cyclohexane [11]. The photolysis is not influenced by the presence o f oxygen. Irradiation o f argon-saturated solutions leads to the same results. The progress o f the photolysis is monitored by measuring the decrease o f absorbance at A = 492 nm. The efficiency o f the photoreaction depends on the wavelength o f irradiation. Quantum yields are 4> = 0.02 at A,n- = 366 nm and 0.005 at Ajn- = 436 nm.

    4. Discussion

    The electronic structures and spectra of various titanocene derivates with the general formula Cp2TiX 2 have been studied in detail. All low-energy transitions o f these pseudo-tetrahedral complexes

    are o f the LM CT type and involve the promotion o f an electron from the Cp- or X - ligands to the d° metal center [5, 7, 9, 10]. I f X “ is rather reducing (e.g. X = Br, alkyl, aryl) the longest-wavelength absorptions are assigned to X “ —> TiIV LM CT transitions. Cp2TiS5 belongs to the Cs symmetry class and all MOs are o f the a' and a" type [12]. Since all transitions between these orbitals are allowed [13] the absorptions o f Cp2TiSs are rather intense (Fig. 1). By comparison with the spectra o f other titanocene derivates [5, 7, 9, 10] and in agreement with previous calculations [ 1 2 ] the longest-wavelength band o f Cp2TiS5 at Amax = 492 nm is assigned to the lowest-energy S52- —> TiIV LM CT transition. The bands at shorter wavelengths are also o f the S52- —> TiIV LM CT type, but Cp- —► TiIV LM CT absorptions may occur in the same energy region.

    The photolysis o f Cp2TiSs in CH2CI2 leads to the formation o f Cp2TiCL and elemental sulfur. This observation can be rationalized on the basis o f a reactive Ss2~ —> TiIV LM CT state. LM CT excitation is followed by a two-electron reductive elimination at TiIV with concomitant extrusion o f S5 which is finally converted to Sg as the stable form o f elemental sulfur. This reductive elimination can proceed as a concerted process or in two consecutive one- electron transfer steps. In the absence o f a suitable scavenger a recombination o f the starting complex occurs in analogy to its photochemical formation [8 ]. However, chlorinated alkanes are well known to function as efficient oxidants in photoredox reactions o f transition metal complexes [5, 14], This reaction competes successfully with the recombination. The reduction o f R-Cl leads to the release o f chloride which is available for coordination at the metal atom while the radical R undergoes secondary reactions. The overall photolysis o f Cp2TiSs can then be rationalized by the following equation:

    Cp2TiIVS5 + 2 CH2C12Cp2TiIVCl2 + S5 + 2 CH2C1

    In this context it is quite interesting that the photoreduction o f some other titanocene derivates in chlorocarbons is also associated with the reoxidation by the solvent [6,9,10]. In the case o f Cp2TiIV[^-NCRun(CN )5]26- M M CT excitation leads to a one-electron reduction o f T iIV to yield Cp2Tim[/i-NCRun(C N )5]3- which is scavenged by CHCI3 and finally converted to Cp2TiIVCl[^- NCRun(CN )5]3- [6 ], The photolysis o f Cp2TiMe2

  • 226 H. Kunkely - A . Vogler • Photoreactivity o f Titanocene Pentasulfide

    produces several products including Cp2 T i(M e)C l and Cp2 T iC l2 [9]. It seems that these photoreactions involve competing one- and two-electron reductions o f T iIV with subsequent reoxidation by CHCI3 .

    The quantum yield o f the photolysis o f Cp2 TiS 5 is dependent on the wavelength o f irradiation. A lthough an obvious explanation is not available presently it is feasible that higher-energy S52~ —>

    T iIV LM C T excited states are more reactive. In the case o f M S4n~ with M = V v , M oVI, W VI and ReVH an analogous behavior has been observed [3].

    Acknowledgment

    Support o f this research by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie is gratefully acknowledged.

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    [2] A. G. Wedd, in A. Müller, B. Krebs (eds): Sulfur, Studies in Inorganic Chemistry 5, p. 181, Elsevier, Amsterdam (1984).

    [3] A. Vogler, H. Kunkely, Inorg. Chem. 27,504 (1988).[4] A. Müller, E. Diemann, Adv. Inorg. Chem. 31, 89

    (1987).[5] G. L. Geoffroy, M. S. Wrighton, Organometallic

    Photochemistry, Academic Press, New York (1979).[6 ] H. Kunkely, A. Vogler, Inorg. Chim. Acta 254, 195

    (1997) and references cited therein.[7] M. R. M. Bruce, A. Kenter, T. R. Tyler, J. Am. Chem.

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    [11] J. B. Meyer, T. V. Oommen, D. Jensen, J. Phys. Chem. 75, 912(1971).

    [12] J. L. Petersen, D. L. Lichtenberger, R. Fenske, L. F. Dahl, J. Am. Chem. Soc. 97, 6433 (1975).

    [13] C. Herzberg, Molecular Spectra and Molecular Structure, Van Nostrand Reinhold Company, New York (1966).

    [14] A. Vogler, H. Kunkely, in K. Kalyanasundaram and M. Grätzel (eds): Photosensitization and Photocatalysis Using Inorganic and Organometallic Compounds, Kluwer, Dordrecht, Netherlands (1993).