Coordination Compounds of Metal Ions in SolâGel Glasses

Chapter 36

Coordination Compounds of Metal Ions in Sol—Gel Glasses 1

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Renata Reisfeld and Christian K. Jørgensen 1

Department of Inorganic Chemistry, Hebrew University, 91904 Jerusalem, Israel Section of Chemistry, University of Geneva, 30 Quai Ansermet, CH 1211 Geneva 4, Switzerland

Downloaded by UNIV OF ARIZONA on April 5, 2017 | http://pubs.acs.org Publication Date: November 4, 1994 | doi: 10.1021/bk-1994-0565.ch036

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Sol-gel glasses (known since 1846) are a family of inorganic glasses or organic composites prepared at ambient temperature, usually from silicon alkoxides (sometimes mixed with other alkoxides). Hydrolysis and subsequent polycondensation of the precursor solution allows incorporation of specific cations or their pre-existing complexes. Detailed study of absorption and emission spectra, including lifetimes of their excited states, sometimes allows determination of the site symmetry and the coordination number, e.g., for cobalt (II). The ruthenium (II) tris(2,2'bipyridine) cation in glass shows a much higher yield offluorescencethan in solution, partly because of less triplet quenching. Lanthanides can show very high luminescence yields because no collisions occur between the species in an excited state and in its ground state. A few minerals (mainly related to volcanic activity) are glasses, such as the dark brown obsidian readily broken up for cutting tools. 3000 to 3500 years ago, Phoenicians prepared glasses by melting white sand (Si0 ) with calcium, sodium, or potassium carbonate (derived from calcite, Egyptian natron lakes, and wood ashes, respectively). Ancient sources suggest that making transparent quartz crystals was the goal, although the hottest fires could not go much above 1100°C (slightly above the melting points of the coinage metals). Later, lead oxide decreased the temperature needed for glass making, but nearly all multicomponent glasses made today are still prepared above 500°C. Another early motivation was to make brightly colored glass beads (/) imitating gemstones, e.g., found on Merovingian crowns 1500 years ago. For a long time such beads were objects of trade in all of Africa and the two Americas. As recently reviewed by Hench and West (2), an entirely different technique developed since 1846, in which silicon alkoxides such as Si(OCH ) and Si(OC2H ) [with the colloquial acronyms TMOS and TEOS] were hydrolyzed and subsequently underwent polycondensation under controlled conditions (losing alcohol vapor) to form quite clearly transparent, moderately viscous to almost vitreous materials. Such a glass has close relations to the limpid silica glass obtained today by melting quartz 2

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0097-6156/94/0565-0439$08.00/0 © 1994 American Chemical Society Kauffman; Coordination Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

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above 1700°C and, to a certain extent, to glassy stoichiometric N a P 0 , obtained by dehydration of N a H P 0 . A difference is that colored chromium (ΠΙ), cobalt (Π), nickel (II), or copper (II) must be introduced as finely divided phosphates or oxides during the fusion of monosodium phosphate, whereas sol-gel glasses prepared by hydrolysis and subsequent polycondensation can incorporate organic dyestufFs and luminescent compounds in tiny (20 to 100 nm) cavities or crevices and be spared from caramelization and pyrolysis by very carefully avoiding heating the sol-gel sample above, e.g., 120°C or 200°C. The metaphosphate glasses (3) have absorption spectra showing the 3d-group ions to be roughly octahedral, with the exception of the JahnTeller unstable 3d manganese (ΙΠ) and 3d copper (II). The behavior of octahedral 3d chromium (ΙΠ) has been reviewed at length (4, 5) and compared with 3d nickel (II) in fluoride glasses (6). 3

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By sufficient heating of a sol-gel glass, e.g., to 400 or 500°C, most absorption spectra of 3d- and 4f-group (lanthanide) ions become quite similar to conventional silicate glasses containing large ions Na(I), K(I), Ca(II), Ba(II), etc. as "network modifiers" (7, 7). At this point, the behavior of 3d cobalt (II) is atypical (8). Although pale raspberry-red (not fully dehydrated) samples have band positions closely similar to strawberry-red [ C o ( O H ) ] , the intensities are 2 to 5 times higher. Progressive loss of water and alcohol (ROH) during heating provides a sky-blue color but only with some 3 times higher molar extinction coefficients than the raspberry-red solvated form. This is a much smaller difference than the ratio (600/4.6 = 130) between the highest band of [C0CI4] " in 12M hydrochloric acid and of [ C o ( O H ) ] in water. It cannot be argued that exactly one definite blue cobalt (Π) species occurs in sol-gel glasses (9). However, the trend to three absorption bands close to 640, 590, and 525 nm (15600, 17000, and 19100 cm" ) is also known for several (10, 11) L C o X with two neutral ligands L (acetone, tryphenylphosphine, etc.) and two X = CI, Br, or NCS. Actually, such spectra are quite similar to Co(II) in cesium borate glass (7). The three rather broad bands in the visible region cannot be described as a weak deviation from the cubic point-group T . As early as 1958 unexpected deviations from predicted T^ energy levels were reported (12) for cobalt(u) syncrystallized in several spinel-type mixed oxides and for [CoCl ] " and [CoBr ] " in various solvents (IS, 14). In view of the Tanabe-Kamimura stability (5) of both the A ground state of regular tetrahedral Co(II)X and of the Pauli-related 3d Cr(III)X the deviations in the former case are unexpected (and have largely been neglected in the literature despite their frequent and conspicuous appearance). The "three-bands-in-the-red" syndrome cannot be more than half explained (75, 16) by nondiagonal elements of spin-orbit coupling between the highest of the two T (six Kramers doublets) and the almost (14) superposed E , T and also T (i.e., 2+3+3=8 Kramers doublets). In view of the identical symmetry type t in of all 3 orbitals of a given ρ shell and of the σ-antibonding orbitals [xy, xz, and yz] of the 3d shell, a weak amplitude of the inner shell 3p (known from photoelectron spectra (77, 18) to have ionization energies close to 68 to 72 eV) in the L C A O description of (t ) of Co(II)X might easily enhance spin-orbit coupling ζ is very close to 2 eV. The observed spreading out of the excited Kramers doublets in the red would be assisted by the squared amplitude of the Hartree-Fock ground configuration Is 2s 2p 3s 3p 3d today being recognized (19, 20) to be below 0.8 (if not below 0.7) in 7

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Kauffman; Coordination Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

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REISFELD & J0RGENSEN

the Schrôdinger solution. This effect would increase the importance of the interelectronic repulsion (20) operator mixing well-defined electron configurations to considerable amounts (in particular with Is 2s 2p 3s 3p 3d ). Parallel to the unavoidable configuration intermixing, the spin-allowed transitions of cobalt (II) in (fairly heated) sol-gel glasses suggest an energy dispersion of the three antibonding orbitals (xy), (xz), and (yz) that would be easy to accept in six-coordinate Mn(III) or copper(II) with a large intrinsic propensity to Jahn-Teller effect, or, for that matter, tetrahedral Cu(II). The observed 3-band spectra of Co(II) suggest a holohedrized symmetry (7, 8, 21) like D h but are also readily compatible with the point-groups C ; S (or even as low as C{) before holohedrization. This includes the maximum symmetry C of L C o X . A possible scenario is the sol-gel glass providing short Si-0 bonds, making the 3d shell of an adjacent cobalt(II) simultaneously σ- and π-antibonding, but also (probably nonlinear) Si-O-Si bonds not being linear ligators (Le., cylindrically symmetrical, at least to the extent that R S i O are) and hence the roughly coplanar Si OCo showing distinct energies of (xz), (yz), and (xy) orbitals. A plausible lower-symmetry scenario would be the intermediate between 4- and 5-coordinated C O ^ O N (in one extreme, all Ν distances are quite different). 2

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It is generally true for glasses at cryogenic temperatures that bands due to partly filled 3d or 4f shells do not narrow almost to lines, whereas, e.g., 3d chromium(III) on approximately octahedral sites in crystalline ruby A l _ C r 0 in emerald (beryl) B e A l _ C r S i 0 > and in microcrystalline glass ceramics (5) doped with Cr(III) often show very sharp absorption and fluorescence lines below -150°C (e.g., the lowest excited E level). This physical distinction can be easily explained by the glass formed from a liquid mixture (at an absolute temperature Τ so high that it is not fundamentally different from a molten crystal) becoming rapidly more viscous by cooling. A crystal growing at a definite point of fusion immediately shows a narrow (22) distribution of internuclear distances Cr-O, Al-O, Mg-O, etc. with amplitudes (roughly proportional with the square-root of T) like "thermal vibrations". But in glasses, ions of metallic elements M are almost immobile (as far as M - M distances are concerned) at a high Τ slightly below the apparent melting point of the glass. By further cooling, the absorption (and, if detectable, luminescence) spectra will give the impression of Τ being almost as high as when the internuclear distances "froze" in. The far larger vibrational (and Stokes) width suggests, in a sense, a high Τ not decreasing by cooling the sample. It is well known (23) that polytungstate [ W O ] " and molybdate [ M o O ] ' can wrap themselves around a central M such as Si(IV), P(V), or As(V) surrounded by a tetrahedral set of four oxygens at a short distance. Such heteropolytungstates (and -heteropolymolybdates) are also known, in which blue Co(II) can be oxidized to dark brown Co(III)0 and, with somewhat differing stoichiometry (24), octahedral purple Cr(III)0 and bluish green Co(III)0 . Trivalent lanthanides can be incorporated as in [ L W i O ] " , but also (25) Ce(IV), Pr(TV), Tb(IV), and Sf Am(IV) are known in the quite heavy anions [ M ( S i W 0 9 ) ] - . They may be fair models of polycondensed silicates. In the last few years sol-gel glasses have been applied in many novel inventions (2, 26) such as tunable solid-state lasers (containing photochemical resistant organic colorants with very high luminescence yields (27, 28)); nonlinear optical effects due to 3

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colorants or other pigments with specific physical characteristics (26, 29, 30) [also to be discussed in another volume of Structure and Bonding edited by the writers R.R. and C.K.J.]; and very small (31-34) translucid semiconductor (ZnS, CdS, C d S e ^ ^ , Agi, etc.), 1- to 10-nm size particles, colloquially named "quantum dots", for high resolution information storage (intended for various time-scales of duration). Organic colorants acting as antibases (Lewis acids) can be used as sensors (35) for analysis of minor constituents of air invading the sol-gel glass. In other instances (26) the behavior of strongly colored organic species is more comparable to large cationic complexes like the ruthenium(II)-2,2'-bipyridine species [Ru(bipy)3 ] showing much higher yields of luminescence (36) than in less viscous solvents allowing very frequent intermolecular collisions. For comparison with other ion-exchanging solids, the firm binding (26) of europium (ΙΠ) species with differing fluorescence lines (37) to heated sol-gel glasses is similar to absorption in Zv(HPO^)2 or on argillaceous montmorillonite [and conceivable examples of complexes of multidentate oxo ligands], whereas organic ion-exchange resins are analogous to the large Malachite Green (35, 38) or six-coordinate Ru(II) and Ir(ffl) cations in faintly heated sol-gel glasses (39). Many interesting areas of research remain open (5, 8, 40, 41, 42) for


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coordination chemists considering the immediately adjacent environment of the sol-gel glass as a multidentate ligand for colored and/or luminescent ions of transition elements and lanthanides. Acknowledgments We are grateful to the Swiss National Science Foundation for their grant (20.32127.91) making possible our collaboration and also that with the experimental staff in Jerusalem. Literature Cited 1. Weyl, W. A. Colored Glasses; Dawson's ofPallMall: London, 1959. 2. Hench, L. L.; West, J. K. Chem Rev. 1990, 90, 33. 3. Brawer, S. Α.; White, W. B. J. Chem. Phys. 1977, 67, 2043. 4. Reisfeld, R.; Kisilev, A. Chem. Phys. Lett. 1985, 115, 457. 5. Reisfeld, R.; Jørgensen, C. K. Struct. Bonding 1988, 69, 63. 6. Reisfeld, R.; Eyal, M.; Jørgensen, C. K.; Guenther, Α.; Bendow, B. Chimia 1986, 40, 403. 7. Reisfeld, R.; Jørgensen, C. K. Lasers and Excited States of Rare Earths; Springer: Berlin and New York, 1977. 8. Reisfeld, R.; Chernyak, V.; Eyal, M.; Jørgensen, C. K. In Proceedings by the Second International School on Excited States of Transition Elements; World Scientific: Singapore, 1992; pp 247-256. 9. Reisfeld, R.; Chernyak, V.; Eyal, M.; Jørgensen, C. K. Chem. Phys. Lett. 1989, 164, 307. 10. Fine, D. A. J. Am. Chem. Soc. 1962, 84, 1139. 11. Cotton, F. Α.; Faut, Ο. D.; Goodgame, D. M. L.; Holm, R. H. J. Am. Chem. Soc. 1961, 83, 1780.



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12. Schmitz-DuMont, O.; Brokopf, H.; Burkhardt, Κ. Z. Anorg. Allg. Chem. 1958, 295, 7. 13. Ballhausen, C. J.; Jørgensen, C. K. Acta Chem.Scand.1955, 9, 397 14. Cotton, F. Α.; Goodgame, D. M. L.; Goodgame, M. J. Am. Chem. Soc. 1961, 83, 4690. 15. Griffith, J. S. Theory of Transition-metalIons;Cambridge University Press: Cambridge, 1961. 16. Horrocks, W. De W.; Burlone, D. A. J. Am. Chem. Soc. 1976, 98, 6512. 17. Jørgensen, C. K.; Berthou, H. Mat. Fys. Medd. Dan. Vid Selskab (Copenhagen) 1972, 38, No.15. 18. Jørgensen, C. K. Struct. Bonding 1975, 24, 1. 19. Jørgensen, C. K. Chimia 1988, 42, 21. 20. Jørgensen, C. K. Comments Inorg. Chem. 1991, 12, 139. 21. Jørgensen, C. K. Modern Aspects of Ligand Field Theory; North Holland: Amsterdam, 1971. 22. Jørgensen, C. K. Top. Current Chem. 1989, 150, 1. 23. Pope, M. T. Heteropoly and Isopoly Oxometalates; Springer: Berlin and New York, 1983. 24. Schaeffer, C. E.; Jørgensen, C. K. J. Inorg.Nucl.Chem. 1958, 8, 143. 25. Jørgensen, C. K. In Handbook on the Physics and Chemistry of Rare Earths; Gschneidner, Κ. Α.; Eyring, L., Eds.; North-Holland: Amsterdam, 1988; Vol. 11; pp 197-292. 26. Reisfeld, R.; Jørgensen, C. K. Struct. Bonding 1992, 77, 207. 27. Reisfeld, R.; Brusilovsky, D.; Eyal, M.; Miron, E.; Burstein, Z.; Ivri, J. Chem. Phys. Lett. 1989, 160, 43. 28. Reisfeld, R.; Seybold, G. Chimia 1990, 44, 295. 29. Vogel, Ε. M.; Weber, M. J.; Krol, D. M. Phys. Chem. Glasses 1991, 32, 231. 30. Klein, L. C., Ed. Sol-gel Technology for Thinfilms,Fibers, Preforms, Electronics and Speciality Shapes. Noyes Publishers: Park Ridge NJ, 1988. 31. Rosetti, R.; Hull, R.; Gibson, J. M.; Brus, L. E. J. Chem. Phys. 1985, 82, 552. 32. Henglein, A. Chem. Rev. 1989, 89, 1861. 33. Steigerwald, M. L.; Brus, L. E. Acc. Chem. Res. 1990, 23, 183. 34. Minti, H.; Eyal, M.; Reisfeld, R.; Berkovic, G. Chem. Phys. Lett. 1991, 183, 277. 35. Chernyak, V.; Reisfeld, R. Sensors and Materials 1993, 4, 195. 36. Reisfeld, R.; Brusilovsky, D.; Eyal, M.; Jørgensen, C. K. Chimia 1989, 43, 385. 37. Levy, D.; Reisfeld, R.; Avnir, D. Chem. Phys. Lett. 1984, 109, 593. 38. Reisfeld, R.; Chernyak, V.; Jørgensen, C. K. Chimia 1992, 46, 148. 39. Slama-Schwok, Α.; Avnir, D.; Ottolenghi, M. J. Am. Chem. Soc. 1991, 113, 3984. 40. Avnir, D.; Braun, S.; Ottolenghi, M. In Supermolecular Architecture; Bein, T., Ed.; A.C.S. Symposium Series No. 499; Washington DC, 1992; pp 384-404. 41.Wu, S.; Ellerby, L.; Cohan, J. S.; Dunn, B.; El-Sayed, Μ. Α.; Valentine, J. S.; Zink, J. I. Chem. Mater. 1993, 5, 115. 42.Rottman,C.; Ottolenghi, M.; Zusman, R.; Lev, O.; Smith, M.; Gong, G.; Kagan, M. L.; Avnir, D. Material Letters 1992, 13, 293. RECEIVED December 27,

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Coordination Compounds of Metal Ions in SolâGel Glasses

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