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Avoiding cross-linking in iron-polyphosphazene metallo-polymers
Inorganic Chemistry Communications 51 (2015) 1–3
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Avoiding cross-linking in iron-polyphosphazene metallo-polymers Ross J. Davidson a, Eric W. Ainscough a,⁎, Andrew M. Brodie a,⁎, Mark R. Waterland a, Harry R. Allcock b, Mark D. Hindenlang b, Guy N. L. Jameson c a b c
Chemistry — Institute of Fundamental Sciences, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealand Department of Chemistry, The Pennsylvania State University, University Park, PA 16802, USA MacDiarmid Institute for Advanced Materials and Nanotechnology, Department of Chemistry, University of Otago,PO Box 56, Dunedin 9054, New Zealand
a r t i c l e
i n f o
Article history: Received 8 September 2014 Received in revised form 13 October 2014 Accepted 16 October 2014 Available online 18 October 2014 Keywords: Metallo-polyphosphazene Iron thiocyanate complex M ssbauer spectroscopy X-ray crystallography
a b s t r a c t Two new polyphosphazene ligands containing 1,10-phenanthrolin-2-olate (L1) and 2,2 -bipyridine-6-olate moieties (L2) with 5,5 -di-tert-butylbiphenyl-2,2 -bis(olate) co-substituents were synthesised and then reacted with Fe(Pyridine)4(NCS)2. Variable temperature Mössbauer and electronic absorbance spectroscopies were used to establish the physical behaviour of the new iron-polyphosphazenes. By attaching two bidentate ligands to a geminal phosphorus atom a pseudo tetradentate ligand can be formed that prevents cross-linking when iron is coordinated to the polyphosphazene. © 2014 Elsevier B.V. All rights reserved.
Spin crossover (SCO) materials have long been suggested to have a wide range of applications such as qubits for quantum computers or for mass data storage [1,2]. This is due to the materials' ability to change spin-states (and therefore magnetic behaviour) with external stimuli, e.g. heat pressure, and light [2]. Despite this potential they suffer from one key difficulty, these materials are often crystalline, making deposition difficult and expensive. To date, the only reported SCO-grafted polymers consisted of tridentate ligands attached to the polymer backbone. Upon coordination to either iron(II) or iron(III), the polymers cross-link, forming insoluble solids [3–5]. As a means of avoiding crosslinking, a tetradentate ligand or higher could be employed; however, these are more difficult to synthesise and attach to a polymer. Phosphazenes provide an elegant solution since by attaching two bidentate ligands to a phosphazene in a geminal arrangement, it is possible to build a tetradentate ligand. In this report, we have extended our work with 1,10-phenanthroline (phen) substituted phosphazenes, N3P3(bph)2(OPhen)2 [6], by synthesising the 2,2 -bipyridine (bpy) analogue and their respective polymers, followed by coordination of iron(II)-dithiocyanate. As a prelude to polymer synthesis, N3P3(2,2 -biphenolate)2(2,2 bipyridine-6-olate)2 [N3P3(bph)2(Obpy)2] (Scheme 1) was synthesised (details are given in S1) to establish the reactivity of 6-(pyridine-2-yl) pyridine-2(1H)-one (HObpy) [7] towards phosphazenes. Its crystal structure (Fig. S2.1) shows both Obpy groups attached in a geminal ⁎ Corresponding authors. E-mail addresses: [email protected] (E.W. Ainscough), [email protected] (A.M. Brodie).
http://dx.doi.org/10.1016/j.inoche.2014.10.011 1387-7003/© 2014 Elsevier B.V. All rights reserved.
fashion, indicating an interaction between the adjacent pyridine nitrogen and the geminal phosphorus atom, similar to that of pyridoxysubstituted phosphazenes [8,9] (see S2 for selected bond lengths and angles). The two phosphazene polymers with pendant 1,10-phenanthrolin2-olate (L1) or 2,2 -bipyridine-6-olate (L2) moieties were synthesised by first substituting 80% of the phosphorus atoms with 5,5 -tert-butyl2,2 -biphenol (tBubph) [10], followed by reacting the sodium salt of either HObpy or HOphen (Scheme 2; Supplementary S1 gives experimental details). The elemental analysis of both L1 and L2 suggests that the polymers contain residual NaCl, which is a common feature for post-polymerisation synthesis of polyphosphazenes [5,11]. Attempts were made to synthesise the analogous polymers to L1 and 2 L using 2,2′-biphenol (bph) in place of tBubph due to the increased steric hindrance of the tBu groups. Although the substitution of bph proceeded rapidly, once the polymer was reacted with either HObpy or HOphen it rapidly decayed into what are assumed to be smaller phosphazene cyclomers [9]. The difference in polymer stabilities may be due to: i) the tBu groups of tBubph produce enough steric hindrance to prevent interactions that promote decay reactions; and ii) bph polymers tend to show block-like substitution patterns [12]. The random distribution of the tBubph is shown in broad signals observed by 31P NMR. Once completely substituted, only a single broad signal is observed, as opposed to the typically sharp signals observed with bph-substituted polymers (see Supplementary S3) [12]. The six-coordinated(4N and 2 NCS) iron-containing polyphosphazenes, L1–Fe and L2–Fe, were synthesised by reacting Fe(Pyridine)4(NCS)2 with the appropriate polymers This approach had the benefit of avoiding anion
2
R.J. Davidson et al. / Inorganic Chemistry Communications 51 (2015) 1–3
Scheme 1. The substituted tricyclophosphazene, N3P3(bph)2(Obpy)2.
metathesis on the polymer and restricted the iron to a coordination sphere with no more than two phen or bpy ligands. This is supported by the colour of the solution and solubility of the complexes. Unlike the metallo-polymers previously reported by us [5,13] both L1–Fe and L2–Fe, could be dissolved in THF and chloroform and dried repeatedly, suggesting little or no cross-linking. Both iron-polyphosphazenes showed similar 57Fe Mössbauer spectra at 291 K consisting of two overlapping quadrupole doublets in a ratio of approximately 1:4 with parameters corresponding to HS FeII and FeIII (see Fig. 1). As the temperatures are decreased to 4.6 K, the quadrupole doublets show an increase in isomer shift of ca. 0.15 mm s−1 caused by the second order Doppler shift (parameters are given in Supplementary S4). Apart from the isomer shift, the quadrupole doublet corresponding to FeII remains the same. The HS FeIII, however, partly relaxes into a broad magnetic component with a wide hyperfine field distribution. The hyperfine field distributions for the two polymers are given in Supplementary S4. The presence of the iron(III) species is likely due to the use of Fe(Py)4 (NCS)2 [14] which is unstable and in the presence of the impurities
Fig. 1. The Mössbauer spectra of L1–Fe (1) and L2–Fe (2), measured at 291 and 4.6 K. The spectra can be deconvoluted into two species, HS FeII (red line) and HS FeIII (blue line). At low temperatures the HS FeIII species partly relaxes into a broad magnetic component (blue dashed line).
contained in the polymer and would readily convert to the oxidised species [15]. This could be avoided by reacting FeCl2·4H2O with the free ligand followed by anion metatheses [15,16]. However, we have previously described attempts at anion exchange with non-coordinated anions resulting in incomplete reactions [5] which will be exacerbated
Scheme 2. Synthetic scheme for the synthesis of the polyphosphazene ligands: i) tBubph and NaH; ii) NaH and HOphen (L1); iii) NaH and HObpy (L2).
R.J. Davidson et al. / Inorganic Chemistry Communications 51 (2015) 1–3
when coordinated anions are involved. The electronic absorbance of the iron-polyphosphazenes clearly point to the presence of HS Fe(II) in chloroform solution, viz. an absorption band near 1100 nm (see Supplementary, Fig. S5.1) typically associated with a d–d spin forbidden transition [17–19]. The spectra show no change between −25 °C and 50 °C. Resonance Raman data were not obtainable due to the complexes' fluorescence. In this study the reaction of two polymeric ligands containing Obpy and Ophen with iron demonstrates an effective strategy for preventing cross-linking upon metal coordination and the important role that co-substituents play in the stability of the polymer. Acknowledgements We thank the Massey University Research Fund for financial support and a PhD Scholarship (to R.J.D.). We also thank Associate Professor S.G. Telfer and the MacDiarmid Institute for Advanced Materials and Nanotechnology for the use of the Spider Diffractometer, Professor K.C. Gordon and Dr R. Horvath (The University of Otago) for Resonance Raman data and G. Weal (The University of Otago) for help in collecting Mössbauer data. Appendix A. Supplementary material CCDC 1017933 contains the supplementary crystallographic data for N3P3(bph)2(Obpy)2. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_ request/cif or from the CCDC, 12 Union Road, Cambridge, CB21EZ, UK; E-mail: [email protected]. Supplementary data S1-S5 associated with this article can be found in the online version at http://dx.doi. org/10.1016/j.inoche.2014.10.011.
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References [1] O. Kahn, C.J. Martinez, Science 279 (1998) 44–48. [2] P. Gutlich, H.A. Goodwin, Spin Crossover in Transition Metal Compounds I, 2004, pp. 1–47. [3] B. Djukic, P.A. Dube, F. Razavi, T. Seda, H.A. Jenkins, J.F. Britten, M.T. Lemaire, Inorg. Chem. 48 (2009) 699–707. [4] B. Djukic, M.T. Lemaire, Inorg. Chem. 48 (2009) 10489–10491. [5] R.J. Davidson, E.W. Ainscough, A.M. Brodie, G.B. Jameson, M.R. Waterland, H.R. Allcock, M.D. Hindenlang, B. Moubaraki, K.S. Murray, K.C. Gordon, R. Horvath, G.N.L. Jameson, Inorg. Chem. 51 (2012) 8307–8316. [6] E.W. Ainscough, A.M. Brodie, G.B. Jameson, C.A. Otter, Polyhedron 26 (2007) 460–471. [7] D.B. Moran, G.O. Morton, J.D. Albright, J. Heterocycl. Chem. 23 (1986) 1071–1077. [8] E.W. Ainscough, A.M. Brodie, A. Derwahl, S. Kirk, C.A. Otter, Inorg. Chem. 46 (2007) 9841–9852. [9] S. Kirk, Cyclo- and Polyphosphazenes Containing 2-Oxypyridine Moieties Coordinated to Selected Transition Metals, Massey University, Palmerston North, New Zealand, 2008. [10] T. Yamato, K. Hasegawa, Y. Saruwatari, L.K. Doamekpor, Chem. Ber. Recl. 126 (1993) 1435–1439. [11] R.J. Davidson, E.W. Ainscough, A.M. Brodie, G.B. Jameson, M.R. Waterland, H.R. Allcock, M.D. Hindenlang, Polyhedron 85 (2015) 429–436. [12] H.R. Allcock, Chemistry and Applications of Polyphosphazenes, John Wiley & Sons Inc., Hoboken, 2003. [13] R.J. Davidson, E.W. Ainscough, A.M. Brodie, M.R. Waterland, H.R. Allcock, M.D. Hindenlang, B. Moubaraki, K.S. Murray, K.C. Gordon, R. Horvath, G.N.L. Jameson, Inorg. Chem. Commun. 37 (2013) 158–161. [14] C.D. Burbridge, D.M. Goodgame, Inorg. Chim. Acta 4 (1970) 232–234. [15] Y. Suffren, F.-G. Rollet, O. Levasseur-Grenon, C. Reber, Polyhedron 52 (2013) 1081–1089. [16] A.D. Naik, B. Tinant, K. Muffler, J.A. Wolny, V. Schunemann, Y. Garcia, J. Solid State Chem. 182 (2009) 1365–1376. [17] M. Konno, M. Mikamikido, Bull. Chem. Soc. Jpn. 64 (1991) 339–345. [18] R. Driver, W.R. Walker, Aust. J. Chem. 20 (1967) 1375–1383. [19] C.M. Harris, H. Wong, E. Sinn, H.R.H. Patil, S. Kokot, Aust. J. Chem. 25 (1972) 1631–1643.
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Avoiding cross-linking in iron-polyphosphazene metallo-polymers Ross J. Davidson a, Eric W. Ainscough a,⁎, Andrew M. Brodie a,⁎, Mark R. Waterland a, Harry R. Allcock b, Mark D. Hindenlang b, Guy N. L. Jameson c a b c
Chemistry — Institute of Fundamental Sciences, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealand Department of Chemistry, The Pennsylvania State University, University Park, PA 16802, USA MacDiarmid Institute for Advanced Materials and Nanotechnology, Department of Chemistry, University of Otago,PO Box 56, Dunedin 9054, New Zealand
a r t i c l e
i n f o
Article history: Received 8 September 2014 Received in revised form 13 October 2014 Accepted 16 October 2014 Available online 18 October 2014 Keywords: Metallo-polyphosphazene Iron thiocyanate complex M ssbauer spectroscopy X-ray crystallography
a b s t r a c t Two new polyphosphazene ligands containing 1,10-phenanthrolin-2-olate (L1) and 2,2 -bipyridine-6-olate moieties (L2) with 5,5 -di-tert-butylbiphenyl-2,2 -bis(olate) co-substituents were synthesised and then reacted with Fe(Pyridine)4(NCS)2. Variable temperature Mössbauer and electronic absorbance spectroscopies were used to establish the physical behaviour of the new iron-polyphosphazenes. By attaching two bidentate ligands to a geminal phosphorus atom a pseudo tetradentate ligand can be formed that prevents cross-linking when iron is coordinated to the polyphosphazene. © 2014 Elsevier B.V. All rights reserved.
Spin crossover (SCO) materials have long been suggested to have a wide range of applications such as qubits for quantum computers or for mass data storage [1,2]. This is due to the materials' ability to change spin-states (and therefore magnetic behaviour) with external stimuli, e.g. heat pressure, and light [2]. Despite this potential they suffer from one key difficulty, these materials are often crystalline, making deposition difficult and expensive. To date, the only reported SCO-grafted polymers consisted of tridentate ligands attached to the polymer backbone. Upon coordination to either iron(II) or iron(III), the polymers cross-link, forming insoluble solids [3–5]. As a means of avoiding crosslinking, a tetradentate ligand or higher could be employed; however, these are more difficult to synthesise and attach to a polymer. Phosphazenes provide an elegant solution since by attaching two bidentate ligands to a phosphazene in a geminal arrangement, it is possible to build a tetradentate ligand. In this report, we have extended our work with 1,10-phenanthroline (phen) substituted phosphazenes, N3P3(bph)2(OPhen)2 [6], by synthesising the 2,2 -bipyridine (bpy) analogue and their respective polymers, followed by coordination of iron(II)-dithiocyanate. As a prelude to polymer synthesis, N3P3(2,2 -biphenolate)2(2,2 bipyridine-6-olate)2 [N3P3(bph)2(Obpy)2] (Scheme 1) was synthesised (details are given in S1) to establish the reactivity of 6-(pyridine-2-yl) pyridine-2(1H)-one (HObpy) [7] towards phosphazenes. Its crystal structure (Fig. S2.1) shows both Obpy groups attached in a geminal ⁎ Corresponding authors. E-mail addresses: [email protected] (E.W. Ainscough), [email protected] (A.M. Brodie).
http://dx.doi.org/10.1016/j.inoche.2014.10.011 1387-7003/© 2014 Elsevier B.V. All rights reserved.
fashion, indicating an interaction between the adjacent pyridine nitrogen and the geminal phosphorus atom, similar to that of pyridoxysubstituted phosphazenes [8,9] (see S2 for selected bond lengths and angles). The two phosphazene polymers with pendant 1,10-phenanthrolin2-olate (L1) or 2,2 -bipyridine-6-olate (L2) moieties were synthesised by first substituting 80% of the phosphorus atoms with 5,5 -tert-butyl2,2 -biphenol (tBubph) [10], followed by reacting the sodium salt of either HObpy or HOphen (Scheme 2; Supplementary S1 gives experimental details). The elemental analysis of both L1 and L2 suggests that the polymers contain residual NaCl, which is a common feature for post-polymerisation synthesis of polyphosphazenes [5,11]. Attempts were made to synthesise the analogous polymers to L1 and 2 L using 2,2′-biphenol (bph) in place of tBubph due to the increased steric hindrance of the tBu groups. Although the substitution of bph proceeded rapidly, once the polymer was reacted with either HObpy or HOphen it rapidly decayed into what are assumed to be smaller phosphazene cyclomers [9]. The difference in polymer stabilities may be due to: i) the tBu groups of tBubph produce enough steric hindrance to prevent interactions that promote decay reactions; and ii) bph polymers tend to show block-like substitution patterns [12]. The random distribution of the tBubph is shown in broad signals observed by 31P NMR. Once completely substituted, only a single broad signal is observed, as opposed to the typically sharp signals observed with bph-substituted polymers (see Supplementary S3) [12]. The six-coordinated(4N and 2 NCS) iron-containing polyphosphazenes, L1–Fe and L2–Fe, were synthesised by reacting Fe(Pyridine)4(NCS)2 with the appropriate polymers This approach had the benefit of avoiding anion
2
R.J. Davidson et al. / Inorganic Chemistry Communications 51 (2015) 1–3
Scheme 1. The substituted tricyclophosphazene, N3P3(bph)2(Obpy)2.
metathesis on the polymer and restricted the iron to a coordination sphere with no more than two phen or bpy ligands. This is supported by the colour of the solution and solubility of the complexes. Unlike the metallo-polymers previously reported by us [5,13] both L1–Fe and L2–Fe, could be dissolved in THF and chloroform and dried repeatedly, suggesting little or no cross-linking. Both iron-polyphosphazenes showed similar 57Fe Mössbauer spectra at 291 K consisting of two overlapping quadrupole doublets in a ratio of approximately 1:4 with parameters corresponding to HS FeII and FeIII (see Fig. 1). As the temperatures are decreased to 4.6 K, the quadrupole doublets show an increase in isomer shift of ca. 0.15 mm s−1 caused by the second order Doppler shift (parameters are given in Supplementary S4). Apart from the isomer shift, the quadrupole doublet corresponding to FeII remains the same. The HS FeIII, however, partly relaxes into a broad magnetic component with a wide hyperfine field distribution. The hyperfine field distributions for the two polymers are given in Supplementary S4. The presence of the iron(III) species is likely due to the use of Fe(Py)4 (NCS)2 [14] which is unstable and in the presence of the impurities
Fig. 1. The Mössbauer spectra of L1–Fe (1) and L2–Fe (2), measured at 291 and 4.6 K. The spectra can be deconvoluted into two species, HS FeII (red line) and HS FeIII (blue line). At low temperatures the HS FeIII species partly relaxes into a broad magnetic component (blue dashed line).
contained in the polymer and would readily convert to the oxidised species [15]. This could be avoided by reacting FeCl2·4H2O with the free ligand followed by anion metatheses [15,16]. However, we have previously described attempts at anion exchange with non-coordinated anions resulting in incomplete reactions [5] which will be exacerbated
Scheme 2. Synthetic scheme for the synthesis of the polyphosphazene ligands: i) tBubph and NaH; ii) NaH and HOphen (L1); iii) NaH and HObpy (L2).
R.J. Davidson et al. / Inorganic Chemistry Communications 51 (2015) 1–3
when coordinated anions are involved. The electronic absorbance of the iron-polyphosphazenes clearly point to the presence of HS Fe(II) in chloroform solution, viz. an absorption band near 1100 nm (see Supplementary, Fig. S5.1) typically associated with a d–d spin forbidden transition [17–19]. The spectra show no change between −25 °C and 50 °C. Resonance Raman data were not obtainable due to the complexes' fluorescence. In this study the reaction of two polymeric ligands containing Obpy and Ophen with iron demonstrates an effective strategy for preventing cross-linking upon metal coordination and the important role that co-substituents play in the stability of the polymer. Acknowledgements We thank the Massey University Research Fund for financial support and a PhD Scholarship (to R.J.D.). We also thank Associate Professor S.G. Telfer and the MacDiarmid Institute for Advanced Materials and Nanotechnology for the use of the Spider Diffractometer, Professor K.C. Gordon and Dr R. Horvath (The University of Otago) for Resonance Raman data and G. Weal (The University of Otago) for help in collecting Mössbauer data. Appendix A. Supplementary material CCDC 1017933 contains the supplementary crystallographic data for N3P3(bph)2(Obpy)2. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_ request/cif or from the CCDC, 12 Union Road, Cambridge, CB21EZ, UK; E-mail: [email protected]. Supplementary data S1-S5 associated with this article can be found in the online version at http://dx.doi. org/10.1016/j.inoche.2014.10.011.
3
References [1] O. Kahn, C.J. Martinez, Science 279 (1998) 44–48. [2] P. Gutlich, H.A. Goodwin, Spin Crossover in Transition Metal Compounds I, 2004, pp. 1–47. [3] B. Djukic, P.A. Dube, F. Razavi, T. Seda, H.A. Jenkins, J.F. Britten, M.T. Lemaire, Inorg. Chem. 48 (2009) 699–707. [4] B. Djukic, M.T. Lemaire, Inorg. Chem. 48 (2009) 10489–10491. [5] R.J. Davidson, E.W. Ainscough, A.M. Brodie, G.B. Jameson, M.R. Waterland, H.R. Allcock, M.D. Hindenlang, B. Moubaraki, K.S. Murray, K.C. Gordon, R. Horvath, G.N.L. Jameson, Inorg. Chem. 51 (2012) 8307–8316. [6] E.W. Ainscough, A.M. Brodie, G.B. Jameson, C.A. Otter, Polyhedron 26 (2007) 460–471. [7] D.B. Moran, G.O. Morton, J.D. Albright, J. Heterocycl. Chem. 23 (1986) 1071–1077. [8] E.W. Ainscough, A.M. Brodie, A. Derwahl, S. Kirk, C.A. Otter, Inorg. Chem. 46 (2007) 9841–9852. [9] S. Kirk, Cyclo- and Polyphosphazenes Containing 2-Oxypyridine Moieties Coordinated to Selected Transition Metals, Massey University, Palmerston North, New Zealand, 2008. [10] T. Yamato, K. Hasegawa, Y. Saruwatari, L.K. Doamekpor, Chem. Ber. Recl. 126 (1993) 1435–1439. [11] R.J. Davidson, E.W. Ainscough, A.M. Brodie, G.B. Jameson, M.R. Waterland, H.R. Allcock, M.D. Hindenlang, Polyhedron 85 (2015) 429–436. [12] H.R. Allcock, Chemistry and Applications of Polyphosphazenes, John Wiley & Sons Inc., Hoboken, 2003. [13] R.J. Davidson, E.W. Ainscough, A.M. Brodie, M.R. Waterland, H.R. Allcock, M.D. Hindenlang, B. Moubaraki, K.S. Murray, K.C. Gordon, R. Horvath, G.N.L. Jameson, Inorg. Chem. Commun. 37 (2013) 158–161. [14] C.D. Burbridge, D.M. Goodgame, Inorg. Chim. Acta 4 (1970) 232–234. [15] Y. Suffren, F.-G. Rollet, O. Levasseur-Grenon, C. Reber, Polyhedron 52 (2013) 1081–1089. [16] A.D. Naik, B. Tinant, K. Muffler, J.A. Wolny, V. Schunemann, Y. Garcia, J. Solid State Chem. 182 (2009) 1365–1376. [17] M. Konno, M. Mikamikido, Bull. Chem. Soc. Jpn. 64 (1991) 339–345. [18] R. Driver, W.R. Walker, Aust. J. Chem. 20 (1967) 1375–1383. [19] C.M. Harris, H. Wong, E. Sinn, H.R.H. Patil, S. Kokot, Aust. J. Chem. 25 (1972) 1631–1643.