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Studies on the photoredox behavior of copper(II) acetato complexes with tripodal 4N ligands in methanol solution
Inorganic Chemistry Communications 39 (2014) 31–33
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Studies on the photoredox behavior of copper(II) acetato complexes with tripodal 4N ligands in methanol solution Christa Hirtenlehner a, Elisa Tordin a,b, Uwe Monkowius a, Manuela List c, Günther Knör a,1 a b c
Institut für Anorganische Chemie, Johannes Kepler Universität Linz (JKU), 4040 Linz, Austria Institut für Physikalische Chemie, Johannes Kepler Universität Linz (JKU), 4040 Linz, Austria Institut für Chemische Technologie Organischer Stoffe, Johannes Kepler Universität Linz (JKU), 4040 Linz, Austria
a r t i c l e
i n f o
Article history: Received 27 September 2013 Accepted 25 October 2013 Available online 5 November 2013 Keywords: Copper Tripodal ligands Carboxylates Photolysis Nanoparticles
a b s t r a c t The reactivity of copper(II) acetate complexes carrying the fully methylated tris(2-aminoethyl)amine ligand Me6tren under UV- and visible-light irradiation was studied in methanol solution. In the presence of water, the formation of colloidal copper nanoparticles occurred, indicating an irreversible decomposition of the copper(II) precursor under UV-light exposure. In dry methanol, a photoredox reaction involving the acetate ligand was observed, which leads to intermediate copper(I) complex formation. The potential of these processes for photocatalytic applications is briefly discussed and evaluated. © 2013 Elsevier B.V. All rights reserved.
The reactivity of copper(II) acetate complexes carrying the fully methylated tris(2-aminoethyl)amine ligand Me6tren under UV- and visible-light irradiation was studied in methanol solution. In the presence of water, the formation of colloidal copper nanoparticles occurred, indicating an irreversible decomposition of the copper(II) precursor under UV-light exposure. In dry methanol, a photoredox reaction involving the acetate ligand was observed, which leads to intermediate copper(I) complex formation. The potential of these processes for photocatalytic applications is briefly discussed and evaluated. The redox chemistry of copper complexes has received increasing attention in the past years. Recently, photochemical and catalytic reactions of copper complexes have for example been studied for cycloadditions [1], C–H activation processes [2], and bio-inspired dioxygen activation [3]. In this context, copper as an earth-abundant and environmentally benign first row transition metal could be a highly desirable component for building up novel photoassisted reaction cycles for solar fuel production and alkane activation [4–6]. We therefore started to test out the stability and photoredox behavior of various copper(II) complexes under continuous wave irradiation, with the goal in mind to photogenerate the corresponding copper(I) species in solution, and to combine this process with a subsequent biomimetic oxygen activation step. Here we wish to report our results on the photoreactivity of the distorted trigonal bipyramidally coordinated complex cation [Cu(Me6tren)(CH3COO)]+ carrying the tripodal ligand Me6tren = tris [2-(dimethylamino)ethyl]amine (Scheme 1). [2]
1
E-mail address: [email protected] (G. Knör). Fax: +43 732 2468 9681.
1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.inoche.2013.10.044
The blue-green complex cation [Cu(Me6tren)(CH3COO)]+ was studied in argon saturated methanol solution. In the visible and NIR spectral region, the compound displays a broad absorption band with a maximum at 859 nm and a weak shoulder at around 700 nm (Fig. 1), which is typical for the metal centered (MC) dd-transitions of trigonalbipyramidal copper(II) complexes [7,8]. It can therefore be assumed that the [Cu(Me6tren)(CH3COO)]+ cation retains the solid-state coordination geometry determined by X-ray crystallography (Appendix A) also at room temperature in solution. In the UV-spectral region, where typically the charge transfer spectra of d9complexes carrying carboxylato ligands are situated [8], the compound displays an intense absorption band with a maximum at 292 nm, which is tentatively assigned to the ligand-to-metal charge transfer (LMCT) transition involving predominately the acetate copper(II) moiety. For the UV-photolysis experiments, a high-pressure mercury short arc lamp with a suitable colored glass band-pass filter was used in order to avoid a direct irradiation of free acetate ions in solution, which are expected to absorb below 260 nm [9]. In the presence of water (Fig. 1), the photolysis leads to a gradual bleaching of the [Cu(Me6tren)(CH3COO)]+ absorption bands, and two new peaks appear at around 400 and 580 nm. While the band at 400 nm results from unidentified degradation products, the occurrence of a brownish-red light-scattering dispersion and the spectral pattern emerging in the visible region with a surface plasmon absorption maximum at 580 nm are characteristic for the formation of colloidal copper nanoparticles [10,11]. These data indicate that a copper(I) species may be formed upon UV-light exposure, which undergoes an irreversible disproportionation in the presence of water. Irradiation at longer
32
C. Hirtenlehner et al. / Inorganic Chemistry Communications 39 (2014) 31–33
Scheme 1. a) Structural formula of the tripodal 4N ligand Me6tren. b) Molecular structure of the [Cu(Me6tren)(CH3COO)]+ cation showing the trigonal bipyramidal coordination of the copper(II) center.
wavelengths led to similar results, however with much longer reaction times. At irradiation wavelengths above 436 nm permanent spectral changes such as those shown in Fig. 1 could not be detected in our experiments. When the photolysis of [Cu(Me6tren)(CH3COO)]+ was carried out in dry methanol as a solvent, the formation of metallic copper did not occur anymore even under prolonged irradiation times of more than one day (Fig. 2). After several hours of irradiation, a partial bleaching of the dd-transitions in the visible spectral region was observed, which is consistent with a copper(II) to copper(I) photoreduction process leaving the Cu(Me6tren)-moiety of the complex intact. Obviously, the tripodal amine ligand is able to stabilize the colorless cuprous form of the starting compound and to prevent the complex from disproportionation under these irradiation conditions. It is important to note that in a recently published exhaustive study including also the copper(II) acetate complex of a similar tripodal ligand no such formation of a copper(I) species could be achieved upon 365 nm irradiation, while sensitization with an attached benzophenone chromophore was very successful [1]. In our case, we therefore assume that direct excitation in the charge transfer region of the [Cu(Me6tren)(CH3COO)]+ cation situated below 365 nm is responsible for the spectral changes shown in Fig. 2.
Fig. 1. Electronic absorption spectra of [Cu(Me6tren)(CH3COO)]+ in methanol in the presence of water before (black line) and after (red line) exposure to UV-light (HBO 100 W, Schott UG-11 filter, 1-cm cell, c = 4.9 × 10−4 M). Inset: colloidal copper nanoparticles formed after photolysis.
To learn more about the (photo)redox reactivity of the copper(II) compound, we carried out additional CV-, EPR- and ESI mass spectrometry experiments with dark and irradiated samples in different solvents (supporting information). The cyclovoltammogram of the cation measured in acetonitrile solution (Fig. 3) displays a quasireversible process at +255 mV vs. NHE, which can be ascribed to the cupric to cuprous redox couple. While we could not obtain clear evidence for the copper(I) photogeneration from EPR-spectra due to the remaining signal of unreacted copper(II) species under steady state irradiation, a comparison of the electrospray mass spectra of dark and irradiated samples was more illustrative (Appendix A). The positive ESI-spectrum of the starting compound shows the M+ signal of the [Cu(Me6tren)(CH3COO)]+ cation at m/z = 352.27 (100%, calc. for C14H33N4O2Cu m/z = 352.19). In UV-irradiated samples, a series of further signals resulting from photoproducts appeared, indicating that a reaction involving the acetate ligand took place. Among the most prominent new features are signals at m/z = 324.20 (M − CO)+, corresponding to a replacement of acetate against a methoxy group, but also a significant peak at m/z = 293.27 (40%) characteristic for the formation of the cuprous Cu(Me6tren)+ cation, and a (M + O)+ signal at m/z = 368.20 suggesting a reaction of the copper(I) complex with traces of O2. No formaldehyde formation was detected with a colorimetric test, which indicates that the methanol solvent is not acting as a donor for the copper(II) reduction. A similar UVphotoreactivity upon charge transfer excitation has been reported before
Fig. 2. Spectral changes of a solution of [Cu(Me6tren)(CH3COO)]+ in absolute methanol under inert conditions at 298 K (HBO 100 W, Schott UG-11 filter, 1-cm cell, c = 2.1 mM).
C. Hirtenlehner et al. / Inorganic Chemistry Communications 39 (2014) 31–33
33
Acknowledgement The Austrian Science Fund (ERA-Chemistry project I316 “Selective Photocatalytic Hydroxylation of Inert Hydrocarbons”) is gratefully acknowledged for the financial support. We thank Marek Havlicek for recording EPR spectra. Appendix A. Supplementary data
Fig. 3. Cyclic voltammogram of 6.6 mM Cu(Me6tren)(CH3COO)]+ in MeCN at room temperature with 0.1 M Bu4NPF6 supporting electrolyte and a Pt-disk working electrode recorded at a scan rate of 50 mV s−1 (potential vs. Fc/Fc+).
for other copper(II) carboxylato systems irradiated in alcohol solution [12,13]. Taking together our present results and the data available in the literature, we therefore suggest that direct LMCT-excitation of the [Cu(Me6tren)(CH3COO)]+ cation leads to an intermediate copper(I) formation and oxidative decomposition of the acetate ligand. Because of the rather low quantum yield estimated (φ b 10−4), however, no further attempts were made to elucidate a detailed mechanism. In summary, we found some evidence that in contrast to a recent study [1] it is possible to photoreduce copper(II) acetate complexes with tris(2-aminoethyl)amine ligands upon UV-light irradiation. Such a reactivity may be blocked, however, if the tripodal 4N ligand contains chromophores such as aromatic groups absorbing strongly in the LMCTspectral region situated around 290 nm. Due to the short threshold wavelength, moderate efficiency and irreversible formation of metallic copper in the presence of water, the potential for long-term photocatalytic applications of unmodified copper acetate derivatives carrying tripodal 4N ligands does not seem very promising, unless an efficient sensitization of the charge transfer state manifold of such complexes is achieved [4], or a different kind of photoreactivity is introduced as described elsewhere [1].
Crystallographic data: The complex crystallizes as the dichloromethane solvate. Formula C15H3Cl3CuN4O2, fw 473.36, orthorhombic, P212121, a 12.838(1), b 13.174(1), c 13.684(2) Å, V 2314.2(4) Å [3], Z 4, T 230 K, d(calc) 1.359 g cm− 3, R1 (all data) 0.026, wR2 (all data) 0.066. The full crystallographic information in CIF format has been deposited with the Cambridge Crystallographic Data Centre (CCDC 962758). Selected bond lengths [Å] and angles [°]: Cu1–O1 1.936(3), Cu1–N1 2.020(3), Cu1–N2 2.163(3), Cu1–N3 2.156(3), Cu1–N4 2.194(3), O1– Cu1–N1 171.1(2), N2–Cu–N3 119.6(1), N2–Cu1–N4 121.2(1), N3–Cu1– N4 116.7(1), N1–Cu1–N3 85.3(1). Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.inoche.2013.xx.xxx. References [1] L. Harmand, S. Cadet, B. Kauffmann, L. Scarpantonio, P. Batat, G. Jonusauskas, N.D. McClenaghan, D. Lastécouères, J.-M. Vincent, Angew. Chem. 124 (2012) 7249–7253. [2] E. Tordin, M. List, U. Monkowius, S. Schindler, G. Knör, Inorg. Chim. Acta 402 (2013) 90–96. [3] T. Hoppe, S. Schaub, J. Becker, C. Würtele, S. Schindler, Angew. Chem. 125 (2013) 904–907. [4] G. Knör, U. Monkowius, Adv. Inorg. Chem. 63 (2011) 235–289. [5] T. Kern, M. Zabel, U. Monkowius, G. Knör, Inorg. Chim. Acta 374 (2011) 632–636. [6] T. Kern, U. Monkowius, M. Zabel, G. Knör, Eur. J. Inorg. Chem. (2010) 4148–4156. [7] E.V. Rybak-Akimova, A.Y. Nazarenko, L. Chen, P.W. Krieger, A.M. Herrera, V.V. Tarasov, P.D. Robinson, Inorg. Chim. Acta 324 (2001) 1–15. [8] A.B.P. Lever, Inorganic Electronic Spectroscopy, 2nd ed. Elsevier, Amsterdam, 1984. 356. [9] G. Ferraudi, Inorg. Chem. 17 (1978) 2506–2508. [10] J. Hambrock, R. Becker, A. Birkner, J. Weiß, R.A. Fischer, Chem. Commun. (2002) 68–69. [11] P. Pootawang, N. Saito, S.Y. Lee, Nanotechnology 24 (2013) 055604. [12] O. Horváth, K.L. Stevenson, Charge Transfer Photochemistry of Coordination Compounds, VCH, Weinheim, 1993. 53. [13] E.M. Glebov, V.F. Plyusnin, V.P. Grivin, S.A. Krupoder, T.I. Liskovskaya, V.S. Danilovich, J. Photochem. Photobiol. A Chem. 133 (2000) 177–183.
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Studies on the photoredox behavior of copper(II) acetato complexes with tripodal 4N ligands in methanol solution Christa Hirtenlehner a, Elisa Tordin a,b, Uwe Monkowius a, Manuela List c, Günther Knör a,1 a b c
Institut für Anorganische Chemie, Johannes Kepler Universität Linz (JKU), 4040 Linz, Austria Institut für Physikalische Chemie, Johannes Kepler Universität Linz (JKU), 4040 Linz, Austria Institut für Chemische Technologie Organischer Stoffe, Johannes Kepler Universität Linz (JKU), 4040 Linz, Austria
a r t i c l e
i n f o
Article history: Received 27 September 2013 Accepted 25 October 2013 Available online 5 November 2013 Keywords: Copper Tripodal ligands Carboxylates Photolysis Nanoparticles
a b s t r a c t The reactivity of copper(II) acetate complexes carrying the fully methylated tris(2-aminoethyl)amine ligand Me6tren under UV- and visible-light irradiation was studied in methanol solution. In the presence of water, the formation of colloidal copper nanoparticles occurred, indicating an irreversible decomposition of the copper(II) precursor under UV-light exposure. In dry methanol, a photoredox reaction involving the acetate ligand was observed, which leads to intermediate copper(I) complex formation. The potential of these processes for photocatalytic applications is briefly discussed and evaluated. © 2013 Elsevier B.V. All rights reserved.
The reactivity of copper(II) acetate complexes carrying the fully methylated tris(2-aminoethyl)amine ligand Me6tren under UV- and visible-light irradiation was studied in methanol solution. In the presence of water, the formation of colloidal copper nanoparticles occurred, indicating an irreversible decomposition of the copper(II) precursor under UV-light exposure. In dry methanol, a photoredox reaction involving the acetate ligand was observed, which leads to intermediate copper(I) complex formation. The potential of these processes for photocatalytic applications is briefly discussed and evaluated. The redox chemistry of copper complexes has received increasing attention in the past years. Recently, photochemical and catalytic reactions of copper complexes have for example been studied for cycloadditions [1], C–H activation processes [2], and bio-inspired dioxygen activation [3]. In this context, copper as an earth-abundant and environmentally benign first row transition metal could be a highly desirable component for building up novel photoassisted reaction cycles for solar fuel production and alkane activation [4–6]. We therefore started to test out the stability and photoredox behavior of various copper(II) complexes under continuous wave irradiation, with the goal in mind to photogenerate the corresponding copper(I) species in solution, and to combine this process with a subsequent biomimetic oxygen activation step. Here we wish to report our results on the photoreactivity of the distorted trigonal bipyramidally coordinated complex cation [Cu(Me6tren)(CH3COO)]+ carrying the tripodal ligand Me6tren = tris [2-(dimethylamino)ethyl]amine (Scheme 1). [2]
1
E-mail address: [email protected] (G. Knör). Fax: +43 732 2468 9681.
1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.inoche.2013.10.044
The blue-green complex cation [Cu(Me6tren)(CH3COO)]+ was studied in argon saturated methanol solution. In the visible and NIR spectral region, the compound displays a broad absorption band with a maximum at 859 nm and a weak shoulder at around 700 nm (Fig. 1), which is typical for the metal centered (MC) dd-transitions of trigonalbipyramidal copper(II) complexes [7,8]. It can therefore be assumed that the [Cu(Me6tren)(CH3COO)]+ cation retains the solid-state coordination geometry determined by X-ray crystallography (Appendix A) also at room temperature in solution. In the UV-spectral region, where typically the charge transfer spectra of d9complexes carrying carboxylato ligands are situated [8], the compound displays an intense absorption band with a maximum at 292 nm, which is tentatively assigned to the ligand-to-metal charge transfer (LMCT) transition involving predominately the acetate copper(II) moiety. For the UV-photolysis experiments, a high-pressure mercury short arc lamp with a suitable colored glass band-pass filter was used in order to avoid a direct irradiation of free acetate ions in solution, which are expected to absorb below 260 nm [9]. In the presence of water (Fig. 1), the photolysis leads to a gradual bleaching of the [Cu(Me6tren)(CH3COO)]+ absorption bands, and two new peaks appear at around 400 and 580 nm. While the band at 400 nm results from unidentified degradation products, the occurrence of a brownish-red light-scattering dispersion and the spectral pattern emerging in the visible region with a surface plasmon absorption maximum at 580 nm are characteristic for the formation of colloidal copper nanoparticles [10,11]. These data indicate that a copper(I) species may be formed upon UV-light exposure, which undergoes an irreversible disproportionation in the presence of water. Irradiation at longer
32
C. Hirtenlehner et al. / Inorganic Chemistry Communications 39 (2014) 31–33
Scheme 1. a) Structural formula of the tripodal 4N ligand Me6tren. b) Molecular structure of the [Cu(Me6tren)(CH3COO)]+ cation showing the trigonal bipyramidal coordination of the copper(II) center.
wavelengths led to similar results, however with much longer reaction times. At irradiation wavelengths above 436 nm permanent spectral changes such as those shown in Fig. 1 could not be detected in our experiments. When the photolysis of [Cu(Me6tren)(CH3COO)]+ was carried out in dry methanol as a solvent, the formation of metallic copper did not occur anymore even under prolonged irradiation times of more than one day (Fig. 2). After several hours of irradiation, a partial bleaching of the dd-transitions in the visible spectral region was observed, which is consistent with a copper(II) to copper(I) photoreduction process leaving the Cu(Me6tren)-moiety of the complex intact. Obviously, the tripodal amine ligand is able to stabilize the colorless cuprous form of the starting compound and to prevent the complex from disproportionation under these irradiation conditions. It is important to note that in a recently published exhaustive study including also the copper(II) acetate complex of a similar tripodal ligand no such formation of a copper(I) species could be achieved upon 365 nm irradiation, while sensitization with an attached benzophenone chromophore was very successful [1]. In our case, we therefore assume that direct excitation in the charge transfer region of the [Cu(Me6tren)(CH3COO)]+ cation situated below 365 nm is responsible for the spectral changes shown in Fig. 2.
Fig. 1. Electronic absorption spectra of [Cu(Me6tren)(CH3COO)]+ in methanol in the presence of water before (black line) and after (red line) exposure to UV-light (HBO 100 W, Schott UG-11 filter, 1-cm cell, c = 4.9 × 10−4 M). Inset: colloidal copper nanoparticles formed after photolysis.
To learn more about the (photo)redox reactivity of the copper(II) compound, we carried out additional CV-, EPR- and ESI mass spectrometry experiments with dark and irradiated samples in different solvents (supporting information). The cyclovoltammogram of the cation measured in acetonitrile solution (Fig. 3) displays a quasireversible process at +255 mV vs. NHE, which can be ascribed to the cupric to cuprous redox couple. While we could not obtain clear evidence for the copper(I) photogeneration from EPR-spectra due to the remaining signal of unreacted copper(II) species under steady state irradiation, a comparison of the electrospray mass spectra of dark and irradiated samples was more illustrative (Appendix A). The positive ESI-spectrum of the starting compound shows the M+ signal of the [Cu(Me6tren)(CH3COO)]+ cation at m/z = 352.27 (100%, calc. for C14H33N4O2Cu m/z = 352.19). In UV-irradiated samples, a series of further signals resulting from photoproducts appeared, indicating that a reaction involving the acetate ligand took place. Among the most prominent new features are signals at m/z = 324.20 (M − CO)+, corresponding to a replacement of acetate against a methoxy group, but also a significant peak at m/z = 293.27 (40%) characteristic for the formation of the cuprous Cu(Me6tren)+ cation, and a (M + O)+ signal at m/z = 368.20 suggesting a reaction of the copper(I) complex with traces of O2. No formaldehyde formation was detected with a colorimetric test, which indicates that the methanol solvent is not acting as a donor for the copper(II) reduction. A similar UVphotoreactivity upon charge transfer excitation has been reported before
Fig. 2. Spectral changes of a solution of [Cu(Me6tren)(CH3COO)]+ in absolute methanol under inert conditions at 298 K (HBO 100 W, Schott UG-11 filter, 1-cm cell, c = 2.1 mM).
C. Hirtenlehner et al. / Inorganic Chemistry Communications 39 (2014) 31–33
33
Acknowledgement The Austrian Science Fund (ERA-Chemistry project I316 “Selective Photocatalytic Hydroxylation of Inert Hydrocarbons”) is gratefully acknowledged for the financial support. We thank Marek Havlicek for recording EPR spectra. Appendix A. Supplementary data
Fig. 3. Cyclic voltammogram of 6.6 mM Cu(Me6tren)(CH3COO)]+ in MeCN at room temperature with 0.1 M Bu4NPF6 supporting electrolyte and a Pt-disk working electrode recorded at a scan rate of 50 mV s−1 (potential vs. Fc/Fc+).
for other copper(II) carboxylato systems irradiated in alcohol solution [12,13]. Taking together our present results and the data available in the literature, we therefore suggest that direct LMCT-excitation of the [Cu(Me6tren)(CH3COO)]+ cation leads to an intermediate copper(I) formation and oxidative decomposition of the acetate ligand. Because of the rather low quantum yield estimated (φ b 10−4), however, no further attempts were made to elucidate a detailed mechanism. In summary, we found some evidence that in contrast to a recent study [1] it is possible to photoreduce copper(II) acetate complexes with tris(2-aminoethyl)amine ligands upon UV-light irradiation. Such a reactivity may be blocked, however, if the tripodal 4N ligand contains chromophores such as aromatic groups absorbing strongly in the LMCTspectral region situated around 290 nm. Due to the short threshold wavelength, moderate efficiency and irreversible formation of metallic copper in the presence of water, the potential for long-term photocatalytic applications of unmodified copper acetate derivatives carrying tripodal 4N ligands does not seem very promising, unless an efficient sensitization of the charge transfer state manifold of such complexes is achieved [4], or a different kind of photoreactivity is introduced as described elsewhere [1].
Crystallographic data: The complex crystallizes as the dichloromethane solvate. Formula C15H3Cl3CuN4O2, fw 473.36, orthorhombic, P212121, a 12.838(1), b 13.174(1), c 13.684(2) Å, V 2314.2(4) Å [3], Z 4, T 230 K, d(calc) 1.359 g cm− 3, R1 (all data) 0.026, wR2 (all data) 0.066. The full crystallographic information in CIF format has been deposited with the Cambridge Crystallographic Data Centre (CCDC 962758). Selected bond lengths [Å] and angles [°]: Cu1–O1 1.936(3), Cu1–N1 2.020(3), Cu1–N2 2.163(3), Cu1–N3 2.156(3), Cu1–N4 2.194(3), O1– Cu1–N1 171.1(2), N2–Cu–N3 119.6(1), N2–Cu1–N4 121.2(1), N3–Cu1– N4 116.7(1), N1–Cu1–N3 85.3(1). Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.inoche.2013.xx.xxx. References [1] L. Harmand, S. Cadet, B. Kauffmann, L. Scarpantonio, P. Batat, G. Jonusauskas, N.D. McClenaghan, D. Lastécouères, J.-M. Vincent, Angew. Chem. 124 (2012) 7249–7253. [2] E. Tordin, M. List, U. Monkowius, S. Schindler, G. Knör, Inorg. Chim. Acta 402 (2013) 90–96. [3] T. Hoppe, S. Schaub, J. Becker, C. Würtele, S. Schindler, Angew. Chem. 125 (2013) 904–907. [4] G. Knör, U. Monkowius, Adv. Inorg. Chem. 63 (2011) 235–289. [5] T. Kern, M. Zabel, U. Monkowius, G. Knör, Inorg. Chim. Acta 374 (2011) 632–636. [6] T. Kern, U. Monkowius, M. Zabel, G. Knör, Eur. J. Inorg. Chem. (2010) 4148–4156. [7] E.V. Rybak-Akimova, A.Y. Nazarenko, L. Chen, P.W. Krieger, A.M. Herrera, V.V. Tarasov, P.D. Robinson, Inorg. Chim. Acta 324 (2001) 1–15. [8] A.B.P. Lever, Inorganic Electronic Spectroscopy, 2nd ed. Elsevier, Amsterdam, 1984. 356. [9] G. Ferraudi, Inorg. Chem. 17 (1978) 2506–2508. [10] J. Hambrock, R. Becker, A. Birkner, J. Weiß, R.A. Fischer, Chem. Commun. (2002) 68–69. [11] P. Pootawang, N. Saito, S.Y. Lee, Nanotechnology 24 (2013) 055604. [12] O. Horváth, K.L. Stevenson, Charge Transfer Photochemistry of Coordination Compounds, VCH, Weinheim, 1993. 53. [13] E.M. Glebov, V.F. Plyusnin, V.P. Grivin, S.A. Krupoder, T.I. Liskovskaya, V.S. Danilovich, J. Photochem. Photobiol. A Chem. 133 (2000) 177–183.