Soft X-ray photoreduction has been observed in K3[Fe(CN)6], [Co(acac)3] and [Cp2Fe][BF4]

Collison, David

Garner, Catherine David

McGrath, Catherine M.

Mosselmans, J. Fred Willem

Roper, Mark D.

Seddon, Jon M. W

Sinn, Ekkehard

Young, Nigel A.

English

Missouri University of Science and Technology (Missouri S&T): Scholars' Mine

Missouri University of Science and Technology Scholars' Mine Chemistry Faculty Research & Creative Works Chemistry 01 Jan 1999 Soft X-ray Photochemistry at the L₂,₃-edges in K₃[Fe(CN)₆], [Co(acac)₃] and [Cp₂Fe][BF₄] David Collison Catherine David Garner Catherine M. McGrath J. Fred Willem Mosselmans et. al. For a complete list of authors, see https://scholarsmine.mst.edu/chem_facwork/1695 Follow this and additional works at: https://scholarsmine.mst.edu/chem_facwork  Part of the Chemistry Commons Recommended Citation D. Collison et al., "Soft X-ray Photochemistry at the L₂,₃-edges in K₃[Fe(CN)₆], [Co(acac)₃] and [Cp₂Fe][BF₄]," Journal of Synchrotron Radiation, International Union of Crystallography (IUCr), Jan 1999. The definitive version is available at https://doi.org/10.1107/S0909049598014964 This Article - Journal is brought to you for free and open access by Scholars' Mine. It has been accepted for inclusion in Chemistry Faculty Research & Creative Works by an authorized administrator of Scholars' Mine. This work is protected by U. S. Copyright Law. Unauthorized use including reproduction for redistribution requires the permission of the copyright holder. For more information, please contact scholarsmine@mst.edu. Conference Proceedings 585 J. Synchrotron Rad. (1999). 6, 585-587 Soft X-ray photochemistry  at the ~,3-edges in K3[Fe(CN)s], [Co(acac)3] and [Cp2Fe][BF4]t David Collison, c C. David Garner, c Catherine M. McGrath," J. Frederick W. Mosselmans, b Mark D. Roper, b Jon. M. W. Seddon," Ekk. Sinn" and Nigel A. Young" Department of Chemistry, The University of Hull, Hull. HU6 7RX, UK, bCCLRC Daresbury Laboratory, Warrington, Cheshire. WA4 4AD, UK, CDepartment of Chemistry, The University of Manchester, Oxford Road, Manchester. M13 9PL, UK. Email:n. a. young @ chem. hull. ac. uk Soft X-ray photoreduction has been observed in K3[Fe(CN)6], [Co(acac)3] and [Cp2Fe][BF4]. of recording in-situ calibration spectra simultaneously (Collison et al. (1997)). The Fe L-edge data were calibrated against he L3- edge features of K4[Fe(CN)6] at 710.35 and 712.05 eV (Peng et al. (1995)) and the Co L-edge spectra were calibrated using values of the L3-edge peaks in K3[Co(CN)6] at 782.7 and 785.5 eV. The sampling depth is related to the electron escape depth and is of the order of 30 - 100 A. All of the compounds were obtained from commercial sources (Aldrich, BDH, M&B) and used as supplied. 3. Results and Discussion Fig.l shows the Fe L-edge spectra observed for K3[Fe(CN)6] at room temperature. Fig. 1 (a) shows the initial spectrum, which is in good agreement with that previously published, (Cartier dit Moulin et al., (1992)) with the sharp feature at 707.1 eV being assigned to a transition from the core 2p orbitals into the hole in the t2s orbital in d 5 low spin Fe(III). The spectra immediately above it (Fig. l(b)) are successive spectra recorded for 23 min each at the same sample position. It is immediately obvious that significant changes are occurring, with the loss of intensity of the sharp feature at 707.1 eV, the gradual shift to lower energy of the Keywords: L-edge; photoreduction; soft X-ray; K,[Fe(CN),]; [Co(acac),]; [Cp2Fe][BF,]. 1, Introduction 3d L-edge X-ray absorption spectroscopy is potentially a very powerful tool for the investigation of electronic structure in first row transition metal compounds. The well resolved spectra are very sensitive to oxidation state, spin state, ligand f ield and local environment, because there is a strong interaction between the 2p core-hole and the valence 3d orbitals. However, as part of a detailed investigation of the applicability of 3d L-edge XAFS to the study of 3d transition metal complexes we have recently observed spectral changes occurring whilst recording L2.3-edge X-ray absorption spectra. In particular, we have observed soft X- ray induced photoreduction in some vanadium complexes (Collison et al. (1998a)), photoreduction a d photoisomerisation in nickel dithiocarbamate nd xanthate complexes (Collison et al. (1998b)), and soft X-ray induced spin transitions at low temperatures (< 50 K), and photochemistry at all temperatures (50 - 300 K) in [Fe(phen)2(NCX)2] complexes (phen = 1,10- phenanthroline, and X = S, Se) (Collison et al. (1997)). In this paper we report further examples of soft X-ray induced photochemistry in a number of iron and cobalt compounds. L .  °m 2. Experimental The 3d L-edge data were collected using TEY (both channeltron and drain current) at station 5U.1 on the undulator beamline at the Daresbury SRS, and the details of the experimental facility have been described in detail previously, including the capability ' Soft X-ray photochemistry at the L2.3 edges of 3d transition metal complexes, Part 3. For Part 2 see Collison et al., 1998b. I I I 710 720 730 Photon Energy (eV) Figure 1 Fe L-edge spectra of K3[Fe(CN)6] (a) initial spectrum, (b) successive 23 min spectra, (c) final spectrum, (d) Fe L-edge spectrum of K4[Fe(CN)6]. (~) 1999 International Union of Crystallography Printed in Great Britain - all rights reserved Journal of Synchrotron Radiation ISSN 0909-0495 (~) 1999 586 Conference Proceedings other L 3 and L 2 features, and a reduction in the overall intensity of both the L 3 and I-,2 peaks. The final spectrum is shown in Fig. l(c), and there is a very considerable difference between this and Fig. l(a). These changes and the similarity between the final spectrum (Fig. l(c)) and that of K4[Fe(CN)6] (Fig. l(d)) are indicative of a reduction in oxidation state as the 2p binding energy is reduced with lower oxidation states, and the intensity of the transitions is directly related to the number of electron holes in the 3d manifold (Wang et al. (1998)). Significantly, there were no observable changes during multiple scans at the Fe L-edge for K.~[Fe(CN)6]. Whilst the final spectra of the photoreduction of K~[Fe(CN)6] and K4[Fe(CN)6] are not identical, the similarities are strong enough to confirm that in this case when K3[Fe(CN)6] is exposed to soft X-rays it is photoreduced to a species that appears to be very similar to [Fe(CN6)] 4. In order to understand more fully the causes of soft X-ray photochemistry we carded out a series of further, detailed experiments on K3[Fe(CN)6]. These have shown that if the sample is exposed to visible light (400 - 700 nm, 200 W Hg-Xe lamp) there is no appreciable change in the Fe L-edge spectra. However, the rate of photoreduction isdependent on the photon flux and the photon energy of the soft X-rays as well as the sample temperature. There appears to be a reasonably linear relationship between photon flux and rate of °m = t . .  ° , - i  .= photoreduction with 2 scans at ca.  230 mA beam current displaying the same extent of change as 3 scans with a ca. 175 mA beam current. If instead of scanning through the spectral energy range, the photon energy is tuned to a particular value and the sample left for a length of time equivalent to one scan, then it is possible to assess how much of a resonance ffect there is in the photoreduction. When this was carried out using a photon energy of 697.5 eV, less photoreduction was observed than when 707.0 or 710.9 eV photon energies were used. The rate of change observed for the latter two photon energies was comparable to that observed after scanning through the whole spectrum. Therefore, the photoreduction is greater if one of the L-edge absorption bands is illuminated. To check the temperature dependence a fresh sample was cooled to ca.  50 K, and the observed rate of photoreduction was much reduced, with 3 scans at 50 K showing about half the change observed after 2 scans at 298 K. Whilst we have previously observed both photon flux and photon energy dependence in the detailed study of [Niw(S2CNEt2)3][BF4], temperature dependence was not observed (Collison et al. (1998(b))). Whilst the Fe L-edge spectrum of K3[Fe(CN)6] has been reported previously, (Cartier dit Moulin et  al. (1992)) there was no report of any photochemistry. The observation of soft X-ray induced photoreduction i  K3[Fe(CN)6] is no surprise, as there have been several reports of detailed studies into the kinetics of photoreduction during XPS experiments, (Oku (1993), Oku (1994)). Prolonged exposure to soft X-rays during sequential scanning of K3[Co(CN)6] (Fig. 3(c)) did not result in any spectral changes. However, we have observed significant changes occurring in the spectra of [Co(acac)3] on prolonged exposure to soft X-rays. Fig. 2(a) shows the initial spectrum recorded for [Co(acac)3], with the successive spectra shown in Fig. 2(b) and the final spectrum in Fig. 2(c). As in the case of K3[Fe(CN)6] there are very significant changes occurring when [Co(acac)3] is left exposed to soft X- rays. The changes observed are consistent with a reduction from a Co(III) centre to a Co(II) centre, and this is confirmed by the striking comparison of the final spectrum (Fig. 2(c)) with that of [Co(acac)2] shown in Fig. 2(d), which itself showed no changes on subsequent exposure to soft X-rays. When the experiments were carded out at ca.  50 K, there was still appreciable photoreduction, of the order of half of that observed at room temperature. Therefore, whilst reducing the temperature atwhich the experiment is carried out, may reduce the likelihood of photochemistry, this is not necessarily always going to be the case. As for Ka[Fe(CN)6], photoreduction of [Co(acac)3] is not unexpected as the photoreduction of [Co(acac)3] in organic solvents by near-ultraviolet irradiation to give [Co(acac)2] has been previously reported (Filipescu and Way (1969)). I I I 780 790 800 Photon Energy (eV) Figure 2 Co L-edge spectra of [Co(acac)3] (a) initial spectrum, (b) successive 20 min spectra, (c) final spectrum, (d) Co L-edge spectrum of [Co(acach]. To assess the relationship between the ease of photoreduction a d known oxidising properties, the metal L-edge spectra of [Cp2Fe][BF4] and [Cp2Co][PF6] were recorded (Fig. 3). The initial (lower) spectrum of [Cp2Fe][BF4] has a low energy feature similar to that in the spectrum of K3[Fe(CN)6], consistent with an at82e2g 3 electronic configuration. (Deeming (1982)) On further exposure to soft X-rays the spectra change, with the most marked differences being the change in relative intensities of the low energy feature and the main L3 peak, the change in branching ratio and the shift of ca.  0.2 eV of the main L 3 peak to lower energy. Whilst it would have been attractive to measure the spectra of [Cp2Fe] this is too volatile for use in the UHV chamber. Therefore, the spectrum of [Cp2Co][PF~] (Fig. 3(b)) Conference Proceedings 587 was collected as this is expected to have a similar L-edge spectrum as both will be low spin d 6 complexes, and this was confirmed by comparison with the ISEELS spectrum of [Cp2Fe] (Hitchcock et al. (1990)). Whilst it is not completely clear whether the photoproduct of [Cp2Fe][BF4] leads to a formal Fe(ll) product, there is evidence that [Cp2Fe][BF4] is photochemically altered on exposure to soft X-rays. In contrast, we could detect no difference in the successive spectra of [Cp2Co][PF6], and this correlates well with the known oxidising and reducing powers of [Cp2Fe][BF4] and [Cp2Co][PF6], respectively. 4. Conclusions It is clear from this work that soft X-ray photochemistry is probably more prevalent and pernicious than previously recognised. Therefore, we recommend that steps such as multiple scans at the same sample position using a simultaneous in-situ calibration device be taken either to confirm or to rule out its presence in spectra. One possible explanation for the lack of previously reported photochemistry in 3d L-edge experiments is that the experiments described herein were carded out on an I I I 780 790 800 i ( (c) I I I 710 720 730 Photon  Energy  (eV)  F igure  3 (a) Successive Fe L-edge spectra of [Cp2Fe][BF4], (b) Co L-edge spectrum of [Cp2Co][PF6] and (c) Co L-edge spectrum of K3[Co(CN)6]. undulator beamline with focusing optics, which will deliver a higher photon flux to the sample compared to a bending magnet beamline. Therefore, there might be severe implications for the use of 3 rd generation synchrotron sources for carrying out 3d L- edge experiments, and in particular those that use micro-focusing techniques. We gratefully acknowledge: the EPSRC/SERC for an Advanced Fellowship (NAY), equipment and SRS beamtime (GR/J 34200), personnel (CMMcG, GR/K 64662) and a post-graduate studentship (JMWS); The Royal Society for a University Research Fellowship (DC); The University of Hull and The University of Manchester for equipment; and the Director of the Synchrotron Radiation Division at Daresbury Laboratory for access to synchrotron and computing facilities. References Cartier dit Moulin, C, RUdolf, P., Flank, A. M. and Chen, C. T. (1992) J.Phys. Chem., 96, 6196-8. Collison, D., Garner, C. D., McGrath, C. M., Mosselmans, J. F. W., Roper, M. D., Seddon, J. M. W., Sinn, E. and Young, N. A. (1997), J.Chem. Soc., Dalton Trans, 4371-4376. Coilison, D., Garner, C. D., Grigg, J., McGrath, C. M., Mosselmans, J. F. W., Pidcock, E., Roper, M. D., Seddon, J. M. W., Sinn, E., Tasker, P. A., Thornton, G., Walsh, J. F. and Young, N. A. (1998a), J. Chem. Soc., Dalton Trans, 2199-2204. Collison, D., Garner, C. D., McGrath, C. M., Mosselmans, J. F. W.. Pidcock, E., Roper, M. D., Searle, B. G., Seddon, J. M. W., Sinn, E. and Young, N. A. (1998b), J. Chem. Soc., Dalton Trans, in press. Deeming, A. J. (1982) Chapter 31.3 in Comprehensive Organometallic Chemistry, Eds Wilkinson, G., Stone, F. G. A. and Abel, E. W., Pergamon. Filipescu, N. and Way, H. (1969) Inorg. Chem., 8, 1863-1867. Hitchcock, A. P., Wen, A. T. and Ruhl, E. (1990), Chem. Phys., 147, 51- 63. Oku, M. (1993) J. Chem. Soc. Farad. Trans., 89, 743-748. Oku, M. (1994) J. Electron Spect. Relat. Phenom., 67, 401-407. Peng, G, van Elp, J. Jang, H., Que Jr., L., Armstrong, W. H. and Cramer, S. P. (1995)J. Am. Chem. Soc., 117, 2515-2519. Wang, H., Ge, P. Riordan, C. G., Brooker, S., Woomer, C. G., Collins, T., Melendres, C. A., Graudejus, O, Bartlett, N. and Cramer, S. P. (1998) J. Phys. Chem. B., 102, 8343-8346. (Received 10 August 1998; accepted 12 November 1998) 

Soft X-ray Photochemistry at the L₂,₃-edges in K₃[Fe(CN)₆], [Co(acac)₃] and [Cp₂Fe][BF₄]

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Soft X-ray Photochemistry at the L₂,₃-edges in K₃[Fe(CN)₆], [Co(acac)₃] and [Cp₂Fe][BF₄]

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