S. Vasudevan

Guest molecules in Layered and Porous Solids

The insertion of guest molecules and polymers into inorganic host lattices, usually layered or porous solids, has been an attractive route for the synthesis of new materials as well as a source of new phenomena. Insertion could, in principle, lead to materials which combine the optical and electrical properties as well as chemical reactivity of the guest with the mechanical strength, thermal stability and electronic properties of the host and so possess properties which may not be achieved by either component separately. We have, in the last decade, been looking at guest molecules inserted by intercalation into the galleries of the layered metal chalcogenophosphates and by encapsulation in the voids of microporous zeolites. Confinement affects properties of both host and guest and our research efforts have been to understand the nature of these changes and how these reflect on host-guest interactions.

We have been investigating the intercalation chemistry of the layered metal chalcogenophosphates, MPX3 (M=Mn, Fe, Ni, Zn or Cd and X= S or Se). When M is a paramagnetic ion both crystallographic and magnetic lattices are two-dimensional and exhibit antiferromagnetic ordering at low temperatures. We have shown that this family of compounds are unique in that the spin-dimensionality can be controlled either by the choice of the transition metal or the chalcogen. The magnetic results have been rationalized as arising from a combination of spin-orbit coupling and distortion around the metal ion. Such a model is applicable since optical and vibrational spectroscopy have shown that the metal ligand bonding in these compounds are ionic.

The ionic nature of the metal ligand bond is reflected in the intercalation chemistry of the MPS3 compounds, which are quite different from other layered materials. They undergo a rather unusual ion-exchange intercalation reaction; solvated cationic guest species insert into the interlamellar space with an equivalent loss of divalent metal ions leaving random immobile vacancies in the layer. The guest cationic species and the neutral molecules solvating them may be independently exchanged, giving rise to a rich diversity of intercalation compounds. In addition when M is a paramagnetic ion intercalation leads to profound changes in magnetic properties.

We have looked at the organization and motion of guest species confined by intercalation within the galleries of insulating, diamagnetic CdPS3 using a variety of spectroscopic methods as well as dielectric and conductivity measurements. An attractive feature of this host that it is ‘inert’ and its role restricted to providing a charge compensated confining environment. The systems that we have looked at include intercalated hydrated alkali–metal cations and intercalated alkali-metal polymer-electrolytes formed by replacing the hydration shell of the intercalated alkali–metals by polyethylene-oxide or polypropylene oxide. We show that the properties of these guest species confined to the ‘flatland’ of the interlamellar region are quite different from those exhibited by these molecules in the bulk.

The internal surface of the galleries of the MPS3 componds can be ‘functionalized’ by intercalation of cationic surfactant species.This results in a significant change in their chemistry from one which is essentially hydrophillic to one that is ‘organophillic’. The organization of the hydrocarbon ‘tails’ of the surfactant depends on the grafting density and consequently different phases of the two-dimensionally confined hydrocarbon – monolayer, bilayer, interdigitated bilayer – can be realized. Small molecules, aliphatic alchols, phenols etc., can be solubilized in this two-dimensional surfactant giving rise to a unique (host-guest)-guest chemistry. We have also exploited the close similarity between the intercalated surfactant bilayer and biomembranes to show that bioactive molecules e.g. cholesterol can bind to the hydrocarbon ‘tail’ of the intercalated surfactant.

The properties of transition metal complexes and organometallics encapsulated within the channels and cages of zeolites have also been investigated. We have focussed on the ‘ship-in-a-bottle’ compounds which for steric reasons have to be assembled insitu by bringing metal and ligand within the zeolite void. In these componds the topology and dimensions of the void are likely to have a profound influence on the geometry that the encapsulated compound adopts, leading in turn to apprecaible changes inelectronic and magnetic properties. The Cobalt-trisbipyridyl complex ion, for example, when encapsulated within the super-cage of Zeolite-Y exhibits a thermally driven interconversion between a low-spin and high-spin state – a phenomena not observed for this ion either in the solid state or in solution. We show that this spin behaviour is intra-molecular in origin and arises because the Co-tris(bipyridyl) ion is forced to adopt a geometry different from that in the un-encapsulated state. Similarly square planar complexes -metal salens and phthalocyanines – exhibit a high-spin ground state on encapsulation since they are forced to adopt a non-planar geometry, necessary to accommodate them within the supercage of Zeolite-Y.

Representative Publications

1. P. A. Joy and S. Vasudevan, Magnetism in the layered transition metal thiophosphates, Phys. Rev. B46 (1992) 5425.
2. N. Chandrashekaran and S. Vasudevan, Magnetism, Exchange and Crystal Field parameters in the orbitally unquenched Ising antiferromagnet FePS3, Pramana (1994) 1.
3. P. Jeevanandan and S. Vasudevan, Magnetism in MnPSe3: a layered 3d5 antiferromagnet with unusually large X-Y anisotropy, J. Phys: Condensed Matter (1999) 3563.
4. P. A. Joy and S. Vasudevan, Optical Absorption Spectra of the layered transition metal thiophosphates, Phys. Rev. B46 (1992) 5134
5. P. A. Joy and S. Vasudevan, The intercalation reaction of pyridine with Manganese thiophospahte, MnPS3, J. Am. Chem. Soc. (1992) 7792.
6. P. A. Joy and S. Vasudevan, The intercalation of n-alkylamines in FePS3, Chem. Mater. (1993) 1182.
7. S. Wang, S. Vasudevan, E. P. Gianellis and D. B. Zax, Dynamics in a confined polymer electrolyte: a 7Li and 2H NMR study, J. Am. Chem. Soc. (1995) 7568.
8. S. K. Tiwary and S. Vasudevan, Regular versus alternating (FeS4)n chains: Magnetism in KFeS2 and CsFeS2, Phys. Rev. B56 (1997) 9563.
9. P. Jeevanandam and S. Vasudevan, Anamolous low frequency dispersion and dielectric relaxation in the layered intercalated compounds of Cd0.75PS3A0.5(H2O) (A=K,Cs), J. Chem. Phys. (1998) 1206.
10. P. Jeevanandam and S. Vasudevan, Conductivity and dielectric response in the ion-exchanged mono- and double-layer hydrates Cd0.75PS3Na0.5(H2O)y, J. Phys. Chem. (1998) 3082.
11. N. Arun, P. Jeevanandam, S. Vasudevan and K. V. Ramanathan, Motion of interlamellar hydrated Sodium ions in layered Cd0.75PS3Na0.5(H2O)y, J. Chem. Phys. (1999) 1231.
12. N. Arun, S. Vasudevan and K. V. Ramanathan, Orientation and motion of interlamellar water, J. Am. Chem. Soc. (2000) 6028.
13. N. Arun, S. Vasudevan and K. V.Ramanathan, Effect of the extent of hydration on motion of interlamellar water, J. Phys. Chem. (2000) 9091.
14. P. Jeevanandam and S. Vasudevan, Intercalation of alkali-metal polyethylene oxide polymer electrolytes in layered CdPS3, Chem. Mater. (1998) 1276.
15. P. Jeevanandam and S. Vasudevan, Conductivity of a confined polymer-electrolyte:Lithium poly-propylene oxide intercalated in layered CdPS3, J. Phys. Chem. (1998) 3082.
16. P. Jeevanandam and S. Vasudevan, Altenating current conductivity and electrical conductivity relaxation in an intercalated polymer electrolyte, J. Chem. Phys. (1998) 8102.
17. P. Jeevanandam and S. Vasudevan, Arrehenius and non-Arrhenius conductivities in intercalated polymer electrolytes, J. Chem. Phys. (1998) 8109
18. N. V. Venkataraman and S. Vasudevan, Conformation of an alkane chain in confined geometry:Cetyl Trimethyl Ammonium Ion intercalated in layered CdPS3, J. Phys. Chem. B104 (2000) 11179.
19. N. V. Venkataraman and S. Vasudevan, Conformation of methylene chains in an intercalated bilayer, J. Phys. Chem. B105 (2001) 1805.
20. N. V. Venkataraman and S. Vasudevan, Interdigitation of an intercalated surfactant bilayer, J. Phys. Chem. B105 (2001) 7639.
21. S. K. Tiwary and S. Vasudevan, Void geometry driven spin-crossover in Zeolite encapsulated Cobalt tris(bipyridyl) complex ion, Inorg. Chem. (1998) 5239.