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Photochemical oxidative addition of hydrazinium cations to tetracyanoplatinate(II) induced by OSCT excitation
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Inorganic Chemistry Communications 11 (2008) 413–414 www.elsevier.com/locate/inoche
Photochemical oxidative addition of hydrazinium cations to tetracyanoplatinate(II) induced by OSCT excitation Horst Kunkely, Arnd Vogler * Institut fu¨r Anorganische Chemie, Universita¨t Regensburg, D-93040 Regensburg, Germany Received 21 November 2007; accepted 29 December 2007 Available online 8 January 2008
Abstract The hydrazinium cations N2H62+/N2H5+ form ion pairs with [Pt(CN)4]2. Outer-sphere charge transfer excitation (PtII ? N2 H2þ 6 / 2 IV 2þ þ N2 Hþ ) induces an oxidative addition: N H /N H PtðCNÞ ? [Pt (CN) (NH ) ] and [Pt(CN) (NH )(NH )], respectively. 2 2 4 3 2 4 3 2 5 6 5 4 Ó 2008 Elsevier B.V. All rights reserved. Keywords: Charge transfer; Photochemistry; Platinum; Hydrazinium; Oxidative addition
2þ Hydrazine and its protonated cations N2 Hþ 5 and N2 H6 are well known as reducing agents. However, they are also oxidants [1]. The combination of reducing and oxidizing properties are reflected by the facile disproportionation: 3N2H4 ? 4NH3 + N2. The acceptor properties of N2H4 are enhanced by protonation since the positive charge stabilizes the LUMO which is r-antibonding with regard to the N–N bond of hydrazine. Accordingly, N2 Hþ 5 and N2 H2þ in combination with a reducing anion are expected 6 to form redox-active ion pairs. Such ion pairs are characterized by outer-sphere charge transfer (OSCT) absorptions [2–5]. We explored this possibility and selected the 2 ion pair N2 H2þ for the present study. Since 6 [Pt(CN)4] 2 [Pt(CN)4] is a 2e reductant and is well known to undergo oxidative additions, OSCT excitations may then lead to a photolysis according to the equation 2 II N2 H2þ ! ½ðNH3 Þ2 PtIV ðCNÞ4 6 ½Pt ðCNÞ4
ð1Þ
This photoreaction would be rather interesting in its own right. Moreover, oxidative additions of hydrazine or its protonated forms seem to be generally unknown, also as thermal processes. In addition, interconversions between hydrazine and ammonia play an important role in nitrogen fixation [6,7]. In the context of the present study it should *
Corresponding author. Tel.: +49 941 943 4716. E-mail address: [email protected] (A. Vogler).
1387-7003/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2007.12.037
be noted that photochemical reductive eliminations of [PtIV(CN)4X2]2 with X = halide and azide initiated by LMCT excitation are well known to yield [PtII(CN)4]2 [8,9]. Upon mixing of aqueous solutions of hydrazinium sulphate and Ba[Pt(CN)4] 4H2O (e.g. from Strem or Aldrich) a stoichiometric precipitation of BaSO4 takes place ðN2 H6 ÞSO4 þ Ba½PtðCN4 Þ ! BaSO4 # þðN2 H6 Þ½PtðCNÞ4
ð2Þ
The remaining solution is slightly acidic owing to the parþ tial protolysis of N2 H2þ 6 to N2 H5 . Accordingly, the ion pairs 2 2þ þ N2 H6 [Pt(CN)4] and N2 H5 [Pt(CN)4]2 should be present. These ion pairs can also be generated by simply mixing (N2H6)SO4 and K2[Pt(CN)4]. The anion [Pt(CN)4]2 is photochemically and thermally very stable in solution [8,9] and displays a characteristic absorption spectrum with kmax = 278 (e = 1600 M1 cm1), 254 (12 300), 239 (sh, 1800) and 215 (22 500) nm. Addition of diluted acids does not change this spectrum nor does it affect the thermal or photochemical stability of [Pt(CN)4]2. Hydrazine is essentially transparent above 200 nm [10]. Protonation is expected to shift the VUV absorptions of N2H4 to even shorter wavelength (see below). Aqueous solutions of K2SO4 do also not absorb light above 200 nm. Upon addition of (N2H6)SO4 to aqueous solutions of K2[Pt(CN)4] a small but distinct increase of the absorption occurs. A new band appears at kmax = 285 nm which is resolved in the difference spectrum.
414
H. Kunkely, A. Vogler / Inorganic Chemistry Communications 11 (2008) 413–414
Aqueous solutions of (N2H6)SO4 (owing to the absence of any absorption above 200 nm) and aqueous solutions of [Pt(CN)4]2 are completely light stable. Accordingly, upon irradiation with white light (kirr > 200 nm) the separate solutions do not display any spectral changes in the UV or visible spectral region. In contrast, the mixed solutions are light sensitive. The photolysis is associated with spectral changes (Fig. 1) which essentially indicate the disappearance of [Pt(CN)4]2. However, at 278 nm an inflection shows up which becomes more prominent upon addition of NaOH to the photolyzed solution. Unfortunately, the clear isosbestic points (Fig. 1) disappear when the irradiation is continued indicating secondary photolysis. We suggest that the primary photolysis proceeds according to the equations N2 H2þ 6 ½PtðCNÞ4
2
2 N2 Hþ 5 ½PtðCNÞ4
! ½ðNH3 Þ2 PtðCNÞ4 ! ½ðNH3 ÞðNH2 ÞPtðCNÞ4
ð3Þ
ð4Þ
The spectrum of [PtIV(NH3)2(CN)4] is quite featureless and consists only of an increasing absorption from approximately 300 nm towards shorter wavelength [11]. However, [(NH3)2Pt(CN)4] is characterized by acidic ammonia ligands which undergo deprotonation to amide as a ligand. This deprotonation is favoured in alkaline solution. The amide complex [(NH3)(NH2)Pt(CN)4] shows an absorption at kmax = 277 nm which disappears again upon acidification [11]. The lowest-energy transitions of hydrazine are of the nr* and rr* type involving promotions from the lone pairs at the nitrogen atoms and N–N r-bonding pair, respectively, to the N–N r* orbital [10]. The longest-wavelength absorption appear at kmax = 194 nm. These high energies are partially caused by the large structural reorganization which is associated with the extension of the N–N bond owing to the population of the N–N r* orbital. Upon protonation of hydrazine the lone pairs are blocked and the nr* transitions are not any more present. However, the positive charge of N2 H2þ 6 stabilizes also other MOs including the LUMO (N–N r*). It follows that N2 H2þ 6 becomes a stronger CT acceptor (or reductant [1]) in comparison to
Fig. 1. Spectral changes during the photolysis of 9.76 105 M (N2H6)Pt(CN)4 in water at room temperature after 0 min (a), 60 and 120 min (b) irradiation times with white light (Osram HBO 200 W/2 lamp), 1 cm cell.
N2H4. Accordingly, it is not unexpected that the ion pairs 2 of N2 H2þ and N2 Hþ display OSCT 6 5 with [Pt(CN)4] bands. The optical OSCT transition at 285 nm is then associated with a one-electron transfer from [Pt(CN)4]2 to þ N2 H2þ 6 /N2 H5 . The N–N bond will be stretched but not yet broken. Platinum(II) (d8) is oxidized to Pt(III) (d7) which may increase its coordination number to 5 or 6 [9]. Back electron transfer as well as thermal transfer of a second electron can now take place. Finally, the (pseudo-) octahedral d6 complex [PtIV(CN)4(NH3)2] is formed as product of the photochemical oxidative addition. Generally, oxidative additions of suitable oxidants X–X such as chlorine and bromine to square-planar d8 complexes lead to the addition of the reduced species as new ligands X in the octahedral d6 complexes. However, it is also conceivable that coordinating solvents will finally replace these new ligands. It follows that the ions pairs 2 2 N2 H2þ and N2 Hþ may undergo a 6 [Pt(CN)4] 5 [Pt(CN)4] redox reaction yielding not only [Pt(CN)4(NH3)2] but also [Pt(CN)4(NH3)(H2O)] or [Pt(CN)4(H2O)2] and ammonia. Indeed, we can presently not exclude the formation of [Pt(CN)4(NH3)(H2O)] because its spectral properties would be very similar to those of [Pt(CN)4(NH3)2]. Owing to the presence of acidic ammonia ligands both complexes would exhibit the 277 nm band upon addition of hydroxide [11]. This band is attributed to a amide-to-PtIV LMCT transition. An unambiguous discrimination between [Pt(CN)4(NH3)2] and [Pt(CN)4(NH3)(H2O)] as photoproducts is also hampered by the secondary photolysis which may just lead to the aquation of the diamine complex yielding [Pt(CN)4(NH3)(H2O)] and/or [Pt(CN)4(H2O)]. However, the significance of the present work should not suffer from this restriction. þ In conclusion, the hydrazinium ions N2 H2þ 6 /N2 H5 form 2 ion pairs with [Pt(CN)4] which are characterized by an þ PtII ? N2 H2þ 6 /N2 H5 OSCT transition. OSCT excitation initiates an oxidative addition of the hydrazinium ions to [Pt(CN)4]2 generating [Pt(CN)4(NH3)2] or [Pt(CN)4(NH3)(H2O)] as a subsequent aquation product. References [1] N.N. Greenwood, A. Earnshaw, Chemistry of Elements, Pergamon Press, Oxford, 1984. [2] A. Vogler, H. Kunkely, Coord. Chem. Rev. 229 (2002) 147. [3] A. Vogler, H. Kunkely, Top. Curr. Chem. 158 (1990) 1. [4] R. Billing, D. Rehorek, H. Hennig, Top. Curr. Chem. 158 (1990) 151. [5] V. Balzani, N. Sabbatini, F. Scandola, Chem. Rev. 86 (1986) 319. [6] R.R. Eady, Chem. Rev. 96 (1996) 3031. [7] D. Sellmann, A. Hille, A. Roesler, F.W. Heinemann, M. Moll, G. Brehm, S. Schneider, M. Reiher, B.A. Hess, W. Bauer, Chemistry 10 (2004) 819. [8] A. Vogler, A. Kern, J. Hu¨ttermann, Angew. Chem. 90 (1978) 554. [9] A. Vogler, A. Kern, B. Fußeder, J. Hu¨ttermann, Z. Naturforsch. 33 b (1978) 1352. [10] A. Hopkirk, J.A. Salthouse, R.W.P. White, J.C. Whitehead, F. Winterbottom, Chem. Phys. Lett. 188 (1992) 399. [11] I.I. Chernyaev, A.V. Babkov, N.N. Zheligovskaya, Russ. J. Inorg. Chem. 8 (1963) 1279.
Inorganic Chemistry Communications 11 (2008) 413–414 www.elsevier.com/locate/inoche
Photochemical oxidative addition of hydrazinium cations to tetracyanoplatinate(II) induced by OSCT excitation Horst Kunkely, Arnd Vogler * Institut fu¨r Anorganische Chemie, Universita¨t Regensburg, D-93040 Regensburg, Germany Received 21 November 2007; accepted 29 December 2007 Available online 8 January 2008
Abstract The hydrazinium cations N2H62+/N2H5+ form ion pairs with [Pt(CN)4]2. Outer-sphere charge transfer excitation (PtII ? N2 H2þ 6 / 2 IV 2þ þ N2 Hþ ) induces an oxidative addition: N H /N H PtðCNÞ ? [Pt (CN) (NH ) ] and [Pt(CN) (NH )(NH )], respectively. 2 2 4 3 2 4 3 2 5 6 5 4 Ó 2008 Elsevier B.V. All rights reserved. Keywords: Charge transfer; Photochemistry; Platinum; Hydrazinium; Oxidative addition
2þ Hydrazine and its protonated cations N2 Hþ 5 and N2 H6 are well known as reducing agents. However, they are also oxidants [1]. The combination of reducing and oxidizing properties are reflected by the facile disproportionation: 3N2H4 ? 4NH3 + N2. The acceptor properties of N2H4 are enhanced by protonation since the positive charge stabilizes the LUMO which is r-antibonding with regard to the N–N bond of hydrazine. Accordingly, N2 Hþ 5 and N2 H2þ in combination with a reducing anion are expected 6 to form redox-active ion pairs. Such ion pairs are characterized by outer-sphere charge transfer (OSCT) absorptions [2–5]. We explored this possibility and selected the 2 ion pair N2 H2þ for the present study. Since 6 [Pt(CN)4] 2 [Pt(CN)4] is a 2e reductant and is well known to undergo oxidative additions, OSCT excitations may then lead to a photolysis according to the equation 2 II N2 H2þ ! ½ðNH3 Þ2 PtIV ðCNÞ4 6 ½Pt ðCNÞ4
ð1Þ
This photoreaction would be rather interesting in its own right. Moreover, oxidative additions of hydrazine or its protonated forms seem to be generally unknown, also as thermal processes. In addition, interconversions between hydrazine and ammonia play an important role in nitrogen fixation [6,7]. In the context of the present study it should *
Corresponding author. Tel.: +49 941 943 4716. E-mail address: [email protected] (A. Vogler).
1387-7003/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2007.12.037
be noted that photochemical reductive eliminations of [PtIV(CN)4X2]2 with X = halide and azide initiated by LMCT excitation are well known to yield [PtII(CN)4]2 [8,9]. Upon mixing of aqueous solutions of hydrazinium sulphate and Ba[Pt(CN)4] 4H2O (e.g. from Strem or Aldrich) a stoichiometric precipitation of BaSO4 takes place ðN2 H6 ÞSO4 þ Ba½PtðCN4 Þ ! BaSO4 # þðN2 H6 Þ½PtðCNÞ4
ð2Þ
The remaining solution is slightly acidic owing to the parþ tial protolysis of N2 H2þ 6 to N2 H5 . Accordingly, the ion pairs 2 2þ þ N2 H6 [Pt(CN)4] and N2 H5 [Pt(CN)4]2 should be present. These ion pairs can also be generated by simply mixing (N2H6)SO4 and K2[Pt(CN)4]. The anion [Pt(CN)4]2 is photochemically and thermally very stable in solution [8,9] and displays a characteristic absorption spectrum with kmax = 278 (e = 1600 M1 cm1), 254 (12 300), 239 (sh, 1800) and 215 (22 500) nm. Addition of diluted acids does not change this spectrum nor does it affect the thermal or photochemical stability of [Pt(CN)4]2. Hydrazine is essentially transparent above 200 nm [10]. Protonation is expected to shift the VUV absorptions of N2H4 to even shorter wavelength (see below). Aqueous solutions of K2SO4 do also not absorb light above 200 nm. Upon addition of (N2H6)SO4 to aqueous solutions of K2[Pt(CN)4] a small but distinct increase of the absorption occurs. A new band appears at kmax = 285 nm which is resolved in the difference spectrum.
414
H. Kunkely, A. Vogler / Inorganic Chemistry Communications 11 (2008) 413–414
Aqueous solutions of (N2H6)SO4 (owing to the absence of any absorption above 200 nm) and aqueous solutions of [Pt(CN)4]2 are completely light stable. Accordingly, upon irradiation with white light (kirr > 200 nm) the separate solutions do not display any spectral changes in the UV or visible spectral region. In contrast, the mixed solutions are light sensitive. The photolysis is associated with spectral changes (Fig. 1) which essentially indicate the disappearance of [Pt(CN)4]2. However, at 278 nm an inflection shows up which becomes more prominent upon addition of NaOH to the photolyzed solution. Unfortunately, the clear isosbestic points (Fig. 1) disappear when the irradiation is continued indicating secondary photolysis. We suggest that the primary photolysis proceeds according to the equations N2 H2þ 6 ½PtðCNÞ4
2
2 N2 Hþ 5 ½PtðCNÞ4
! ½ðNH3 Þ2 PtðCNÞ4 ! ½ðNH3 ÞðNH2 ÞPtðCNÞ4
ð3Þ
ð4Þ
The spectrum of [PtIV(NH3)2(CN)4] is quite featureless and consists only of an increasing absorption from approximately 300 nm towards shorter wavelength [11]. However, [(NH3)2Pt(CN)4] is characterized by acidic ammonia ligands which undergo deprotonation to amide as a ligand. This deprotonation is favoured in alkaline solution. The amide complex [(NH3)(NH2)Pt(CN)4] shows an absorption at kmax = 277 nm which disappears again upon acidification [11]. The lowest-energy transitions of hydrazine are of the nr* and rr* type involving promotions from the lone pairs at the nitrogen atoms and N–N r-bonding pair, respectively, to the N–N r* orbital [10]. The longest-wavelength absorption appear at kmax = 194 nm. These high energies are partially caused by the large structural reorganization which is associated with the extension of the N–N bond owing to the population of the N–N r* orbital. Upon protonation of hydrazine the lone pairs are blocked and the nr* transitions are not any more present. However, the positive charge of N2 H2þ 6 stabilizes also other MOs including the LUMO (N–N r*). It follows that N2 H2þ 6 becomes a stronger CT acceptor (or reductant [1]) in comparison to
Fig. 1. Spectral changes during the photolysis of 9.76 105 M (N2H6)Pt(CN)4 in water at room temperature after 0 min (a), 60 and 120 min (b) irradiation times with white light (Osram HBO 200 W/2 lamp), 1 cm cell.
N2H4. Accordingly, it is not unexpected that the ion pairs 2 of N2 H2þ and N2 Hþ display OSCT 6 5 with [Pt(CN)4] bands. The optical OSCT transition at 285 nm is then associated with a one-electron transfer from [Pt(CN)4]2 to þ N2 H2þ 6 /N2 H5 . The N–N bond will be stretched but not yet broken. Platinum(II) (d8) is oxidized to Pt(III) (d7) which may increase its coordination number to 5 or 6 [9]. Back electron transfer as well as thermal transfer of a second electron can now take place. Finally, the (pseudo-) octahedral d6 complex [PtIV(CN)4(NH3)2] is formed as product of the photochemical oxidative addition. Generally, oxidative additions of suitable oxidants X–X such as chlorine and bromine to square-planar d8 complexes lead to the addition of the reduced species as new ligands X in the octahedral d6 complexes. However, it is also conceivable that coordinating solvents will finally replace these new ligands. It follows that the ions pairs 2 2 N2 H2þ and N2 Hþ may undergo a 6 [Pt(CN)4] 5 [Pt(CN)4] redox reaction yielding not only [Pt(CN)4(NH3)2] but also [Pt(CN)4(NH3)(H2O)] or [Pt(CN)4(H2O)2] and ammonia. Indeed, we can presently not exclude the formation of [Pt(CN)4(NH3)(H2O)] because its spectral properties would be very similar to those of [Pt(CN)4(NH3)2]. Owing to the presence of acidic ammonia ligands both complexes would exhibit the 277 nm band upon addition of hydroxide [11]. This band is attributed to a amide-to-PtIV LMCT transition. An unambiguous discrimination between [Pt(CN)4(NH3)2] and [Pt(CN)4(NH3)(H2O)] as photoproducts is also hampered by the secondary photolysis which may just lead to the aquation of the diamine complex yielding [Pt(CN)4(NH3)(H2O)] and/or [Pt(CN)4(H2O)]. However, the significance of the present work should not suffer from this restriction. þ In conclusion, the hydrazinium ions N2 H2þ 6 /N2 H5 form 2 ion pairs with [Pt(CN)4] which are characterized by an þ PtII ? N2 H2þ 6 /N2 H5 OSCT transition. OSCT excitation initiates an oxidative addition of the hydrazinium ions to [Pt(CN)4]2 generating [Pt(CN)4(NH3)2] or [Pt(CN)4(NH3)(H2O)] as a subsequent aquation product. References [1] N.N. Greenwood, A. Earnshaw, Chemistry of Elements, Pergamon Press, Oxford, 1984. [2] A. Vogler, H. Kunkely, Coord. Chem. Rev. 229 (2002) 147. [3] A. Vogler, H. Kunkely, Top. Curr. Chem. 158 (1990) 1. [4] R. Billing, D. Rehorek, H. Hennig, Top. Curr. Chem. 158 (1990) 151. [5] V. Balzani, N. Sabbatini, F. Scandola, Chem. Rev. 86 (1986) 319. [6] R.R. Eady, Chem. Rev. 96 (1996) 3031. [7] D. Sellmann, A. Hille, A. Roesler, F.W. Heinemann, M. Moll, G. Brehm, S. Schneider, M. Reiher, B.A. Hess, W. Bauer, Chemistry 10 (2004) 819. [8] A. Vogler, A. Kern, J. Hu¨ttermann, Angew. Chem. 90 (1978) 554. [9] A. Vogler, A. Kern, B. Fußeder, J. Hu¨ttermann, Z. Naturforsch. 33 b (1978) 1352. [10] A. Hopkirk, J.A. Salthouse, R.W.P. White, J.C. Whitehead, F. Winterbottom, Chem. Phys. Lett. 188 (1992) 399. [11] I.I. Chernyaev, A.V. Babkov, N.N. Zheligovskaya, Russ. J. Inorg. Chem. 8 (1963) 1279.