Supramolecular Architecture - American Chemical Society

Chapter 8

Design and Chemical Reactivity of Low-Dimensional Solids Some Soft Chemistry Routes to New Solids

Downloaded by UNIV OF ARIZONA on August 6, 2012 | http://pubs.acs.org Publication Date: July 14, 1992 | doi: 10.1021/bk-1992-0499.ch008

Jean Rouxel Institute des Matériaux de Nantes, Centre National de la Recherche Scientifique UMR No. 110, Université de Nantes, 2 rue de la Houssinière, 44072 Nantes Cedex, France Low dimensional solids can be regarded as built up from a stacking of slabs or from a juxtaposition of fibers. These are infinite two dimensional or one-dimensional giant molecules weakly bonded to form a three dimensional architecture. General considerations about the stability of such arrangements will be followed by a critical discussion of various approaches introduced in the form of particular examples. Low dimensional solids have a fascinating chemical reactivity. It is largely a low-temperature "soft -chemistry"mostly based on redox processes or acido-basic reactions, that are reversible. It is therefore a powerful tool for the preparation of new solids that can be low dimensional compounds but also large open frameworks with empty cages or tunnels. This contribution will be primarily devoted to these subjects. To a large extent low-dimensional solids represent by themselves supermolecular architectures in which slabs or fibers, that can be regarded as giant two- or one-dimensional molecules, are weakly bound together to build a three dimensional arrangement. These solids show very specific physical properties (1-4) (charge density waves (CDW) instabilities, low-dimensional magnetism...). They also have a fascinating chemical reactivity (5) largely associated with the so-called "chimie douce", i.e, soft chemistry done close to room temperature that has three main components (i) redox intercalation/deintercalation processes, (ii) acido-basic reactions followed by structural recondensations, (iii) grafting reactions in which the van der Waals gaps between slabs or fibers are considered as many internal surfaces of these solids. The reversibility attached to some of these processes makes them useful for the design of new low0097-6156/92/0499-0088$07.50/0 © 1992 American Chemical Society

In Supramolecular Architecture; Bein, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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Design and Chemical Reactivity of Low-Dimensional Solids

dimensional solids. A l l are at the origin of new synthesis routes in solid state chemistry.

I - General outlines for the crystal chemistry of low-dimensional solids. The so-called van der Waals gap which separates slabs or fibers is bound on each side by atomic layers of the same nature, usually anionic layers (like in the transition metal dichalcogenides M X where the slabs can be regarded as [XMX] sandwiches). This has two major consequences : (i) the surrounding of the layers or rows of metal atoms by die non-metal element implies a chemical formula M X ^ with a rather high non-metal content, the value of η increasing when going from two-dimensional (2D) to one dimensional (ID) arrangements. (ii) there is a repulsion between these similar anionic layers on each side of the van der Waals gap. This partially fixes the slab to slab separation and consequendy the low-dimensional character of the structure. The more ionic the materials, the stronger the tendency to exhibit lowdimensionality through anion- anion repulsion with separation of the sheets or fibers, but also the less stable the structure. The limit of the low-dimensionality character is fixed by the intrinsic instability that it generates. Therein lies the difficulty of synthesis of these phases, which can rely only on qualitative approaches. To counter the slab to slab repulsion, the bonding through the interslab or the interfiber spacings must be stronger than die simple van der Waals interaction. It has been recently shown that an interaction between the 3p orbitals of sulfur atoms of one slab and the empty states provided by the metal atoms in the neighbouring slab, is to some extend responsible for the structural cohesion of layered chalcogenides and for the existence of polytypes (6). In the case of more ionic oxides, the repulsion would be stronger and very destabilizing. It is observed that the M 0 oxides have the rutile or fluorite structures and not the layered structures of the parent dichalcogenides. Low-dimensional oxide structures have to be stabilized by separating the slabs or fibers by counter-ions like in layered N a ^ o C ^ phases (7) or pseudo one-dimensional K O 3 0 M O O 3 blue bronzes of molybdenum (8). Stability is also achieved by reducing the formal charge of the anions through a protonation of oxide ions (to give layered hydroxides like brucite Mg(OH) ), or with transition metals in a high oxidation state ( M 0 O 3 , V 0 ) . In the latter case there is a strong polarization of the electrons of the anions towards the interior of the slabs. Finally, layered silicates represent some compromise among all of the above situations. Extra cations are often located between slabs that can be partially protonated. In addition, one remarks that the tetrahedra are always at die external part of a composite slab, the heart of the slab being made from the octahedra. The tetrahedra, with the shortest anion-


2

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SUPRAMOLECULAR ARCHITECTURE

90

cation distances, represent another approach to achieve a strong polarization of the electronic density and a stabilization of the structure. Such a situation is obtained in the recently prepared layered phosphatoantimonic acids and salts with slabs built up from internal [Sb0 ] octahedra and external [P0 ] tetrahedra (9-10). With the exception of these particular situations which allow the stabilization of a low-dimensional structure in the case of oxides, a convenient way to get layered or one dimensional arrangements is to decrease the ionicity of the bonds. A low-dimensional arrangement may then become stable (sulfides as compared to oxides). However, at the same time the ionicity of the bonds is decreased, there will be a loss in the low-dimensionality character of the structure as mentioned above. A layered framework is more easily stabilized in the case of selenides than that of sulfides but on the other hand the low-dimensional character of the properties is less evident. The stabilization of low-dimensional arrangements when going down a column in the periodic table may also reach a limit with the heaviest elements (tellurium for example), for which the loss of directionality of the bonds may lead to three dimensional metals. Between the extreme cases of ionic oxides and quasi-metallic, but mostly 3D, tellurides, sulfides and selenides represent the most favorable domain for the design of low-dimensional solids. Similar conclusions can be drawn for neighbouring columns. However structures involving sulfides and selenides may result in additional problems that arise from a redox competition, now possible, between d cationic levels and sp anionic ones. This can be easily understood when looking at the classical band scheme for transition metal chalcogenides (figure la,b,c). Between a valence band which is essentially sp anionic in character and antibonding levels mostly issued from the corresponding cationic levels, the d orbitals of the metal, split by the crystal field, play an essential role in the physical properties. But they also govern the stability of the layered arrangements and even their stoichiometrics. With sulfur, a maximum cationic oxidation state of four is observed in the case of IVB elements leading to a d ° configuration and no electronic conductivity. Z r S with a C d l structural type and an octahedral coordination of the metal is a semi-conductor. It shows a broad and empty t band. A d configuration in the case of its neighbour Nb, lowers the symmetry which becomes trigonal prismatic with a half fdled d 2 (a^) band, and a metallic behaviour. Immediately after with a d configuration and the same symmetry, M o S and W S are diamagnetic semiconductors. Going further to the right, one would expect to come back to an octahedral symmetry for a layered M n S for example (no stabilization through a distortion for a d configuration). But that phase cannot be obtained. Indeed, when going to the right of the periodic table the d levels progressively decrease in energy and may enter the sp valence band. If such a situation occurs for an empty d level


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Figure 1 - General band scheme models for Transition metal chalcogenides (a). Particular cases of Z r S (b), N b S and M o S (c), d-cationic levels and sp anionic band at the end of a period (d). 2

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2

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92

it will be filled at the expense of the valence band, at the top of which holes will appear (11) (figure Id). Chemically speaking this means that the cation is reduced and that the anion is oxidized via the formation of anionic pairs. Following that scheme, one goes from layered structures, such as T i S with T i and 2S ~ to pyrites and marcasites with F e and (S ) ~ for example. Selenium being less electronegative than sulfur, the top of the sp valence band is situated at a higher energy, and such a transition is observed earlier in selenides as compared to sulfides. 4 +

2

2 +

2

2

2


II - Dimensionality and compositional variations Niobium triselenide was obtained during a careful study of the Nb-Se system on the selenium rich side (12). The increased Se/Nb ratio resulted in a chain like arrangement of [NbSe ] trigonal prisms that are stacked on top of each other instead of being arranged in layers like in N b S e (figure 2). The chains are irregular due to the presence of a (Se )"" pair. However this structure is very complicated due to complex couplings between chains which modify the simplest d cationic - sp anionic redox competition expected to give Nb Se"(Se )~". One is left with three chains (d Se-Se values are 2.37, 2.48 and 2.91 Â , Fig. 2c). The chalcogen pair behaves as an electron reservoir. According to the population of its antibonding level (which governs its length), it takes or gives less or more electrons to the adjacent metallic chain. The two chains with the shortest intra-chain Se-Se distances are conducting and show charge density waves respectively below 59 and 140 K . The third chain is an insulating one. As compared to NbSe , the increase of the Se/Nb ratio, in NbSe , is accommodated by the structure through a diminution of its dimensionality. This a consequence of the possible condensation modes of polyhedra in order to form slabs or chains with an increasing number of unshared corners. Let us consider now the M P S 3 phases. They are layered materials with slabs built up from sharing edges of octahedra, like in T i S , except that the octahedra occupancy is 2/3 by M cations and 1/3 by (P-P) pairs. Once again a study of the chalcogen rich side of M-P-S systems leads to series of new low-dimensional compounds, most of them presenting one-dimensional arrangements. Extended studies have been done in the case of the P-V-S and P-Nb-S systems. Although presenting either a dimensionality or a chemical composition different from each other, the compounds derive from the same basic building unit which is a tetracapped biprism [ M S ] (figure 3a). A l l these biprisms are bonded together to form infinite puckered [ M S ] chains (figure 3b). Then the bonding between these chains takes place through [PS ] tetrahedra and the way such bondings are made determines the dimensionality of the phases which is for example : (i) I D in P V S (13) when the M S lines are linked two by two (figure 3c) 6

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8. ROUXEL

Ο Se (y = 1/4) •

Se ( y = 3/4)

ο Nb (y= 1/4) • Nb (y = 3/4)

Figure 2 - Two-dimensional NbSe (a) and one dimensional NbSe3 (along the chains (b) and perdicular to them (c)) 2


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Figure 3 - M S bicapped biprism (a), formation of (M Sg) chains from M S biprisms (b). Association of these chains by [P S ] and [P S ] groups in I D P V S (c), 2D P N b S (d) and 2D P N b S (e). The little rectangles represent a cross section of the [ M S ] chains. 2

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95 Design and Chemical Reactivity of Low-Dimensional Solids

(ii) 2D in P N b S Q (14) when two P S groups unite the chains on both their sides (figure *e) (iii) 2D in Ρ Ν ο 8 (15) when longer [PS -S-PS ] connecting groups link the chains above and sideways to each other (figure 3d). These are supermolecular architectures based on different connections of similar chains. Many other structures based on 3D arranged bicapped biprisms have been found in these series. However the most fascinating compound is T a P S (16). It is built up from T a S capped biprisms, as above, but these are connected by P S tetrahedra which associate an upper corner of one biprism to a lower corner of the adjacent biprism (figure 4). This leads to a helix which has four biprisms per repeat distance. Such a biprism helix can be either right handed or left handed. It contains an other helix which is a sulfur helix, a S chain, also right handed. In the unit cell there are two big intertwined right handed helices and two small left handed ones which give much smaller tunnels that are empty. The space group is P4 2 2. But there is another form with space group P 4 2 2 which is left handed for the big helices and right handed for the small ones. P N b S g , T a P S , and V P S are other members of the series with related open structure arrangements (17). In situ polymerizations are certainly possible in a similar way as in some zeolites. 2

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I l l - Tuning with the ionicity of bonds. NbSea is not a true one dimensional conductor. It remains metallic below the C.D.W. transitions which means that the Fermi surface is not completely destroyed (inperfect nesting condition). Due to the importance of this compound (first observation of the depinning of a C.D.W.) it was of great interest to increase the low-dimensional character. A s seen above this is basically a problem of increasing the ionicity of the chemical bonds in the corresponding framework. One has to change the niobium for the more electropositive tantalum, and the selenium for the more electronegative sulfur. T a S , which was already known with an orthorhombic unit cell (18), demonstrates effectively to the increased I D character in three ways : (i) a transition at 210 Κ which goes as far as a metal to semi-conductor transition (19). The superstructure is commensurate with the initial lattice. (ii) important pretransitional effects manifested by diffuse lines which condense at the transition into spots between the main Bragg rows of spots. These pretransitional effects imply that the C.D.W. exists dynamically above the transition but without phase coherence from one chain to another. (iii) the existence of a polytype. Like in the case of most lowdimensional compounds polytypism is expected in trichalcogenides. It expresses the fact that slabs or fibers weakly bound to each other can 3


In Supramolecular Architecture; Bein, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992. 2

1 2

4

Figure 4(a) - M S biprisms are connected by P S tetrahedra (right) to form intertwinned helix (left), each of them containing an other helix of sulfur (centre).


% η

Ο ε π ci

ci

σ\

97 Design and Chemical Reactivity of Low-Dimensional Solids


8. ROUXEL


98


behave as indépendant units in a true low-dimensional structure. Polytypism usually associates the same chains or slighdy different ones in new arrangements. No polytype was observed in the case of NbSe . However trials were successful for T a S . This is once more in agreement with the increased low-dimensional character of T a S as compared to NbSe . In addition to the orthorhombic form, a monoclinic T a S was found (20). The two structures are built from groups of four T a S chains (21) as shown on figure 5 (chains with 2.835, 2.068 and 2.105 À S-S distances are present). The b monocline axis is equal to the c orthorhombic axis. This corresponds to the height of one [TaS ] prism. The monoclinic form shows also a metal to semi-conductor transition at 240 Κ (22), but the superstructure is now incommensurate with the initial sublattice. A still more critical interplay with the ionicity of bonds concerns the tetrachalcogenides. The mineral patronite V S presents a I D organization with S rectangles built up from true (S ) "" pairs, rotating around a - V - V - chain and giving a rectangular antiprismatic coordination to the metal atoms. Vanadium is in the 4 oxidation state but there is a d ^ d pairing which leads to a diamagnetic semi-conducting situation. Long and short V - V distances alternate along the chain with a doubling in that direction of the lattice period which can be taken as a classical illustration of a Peierls distortion. If tantalum and niobium homologues of V S could be prepared, an increased ID character is to be expected due to the more electropositive character of the metal. In addition more expanded d orbitals could favor an electronic derealization along the chains. Direct combinations of the elements in evacuated silica tubes at various temperatures between 400 and 600°C have been unsuccessful. Even with the help of pressure it has not been possible, up to now, to prepare N b S and T a S . The impossibility to realize such a synthesis is certainly due to the increased ionicity of the bonds which destabilizes the one-dimensional structural arrangement. We are beyond the stability limit. The solution is either to decrease the tonicities by changing sulfur by selenium or tellurium, or to separate the chains by counter ions. Working with selenium is not sufficient. One has to go to tellurium to stabilize a binary tetrachalcogenide of niobium and tantalum (these tetratellurides have a complicated incommensurate structure (23-24). In the case of selenium a counter ion is needed to stabilize the structural arrangement. This is the situation of the tetrachalcogenoiodides of niobium and tantalum (25). There is a complete series of (MSe ) I phases with M = Nb, T a and η = 1/3, 1/2, 10/3... In the common structural type [MSe ] chains are separated by columns of I" anions, both running parallel to die c axis of unit cells which are generally tetragonal (figure 6). Large distances (-6.70 Â ) separate the M X chains. The various compounds differ not only by the amount of iodine but also by the sequence of long and short metal-metal distances along the chains, and by the way S e rectangles 3

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In Supramolecular Architecture; Bein, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992. 3

3

Figure 5 - The two T a S polytypes are built up from similar groups of T a S chains (sections perpendicular to the chains direction). The solid lines figure the different associations of similar T a S chains in the two structures (they do not represent metal-metal bonds). 3

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orthorhombic

TaS



100


cote ζ Nb φ = I Se

8 2 5

4 2

58 75 92 ® 37 63 L© 13 87 Ο 0 © 1 8 84 # 5 0 ® 3 3 67

Figure 6 - Niobium (and tantalum) halogeno-tetrachalcogenides show rectangular antiprismatic M X chains separated by Γ columns. The arrows indicate the relative orientations of two successive X planes above, and below, a metal atom. 4

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8. ROUXEL


rotate around the chains to form the rectangular antiprismatic coordination of the metal. Every two adjacent units [Se ] ~ have a dihedral angle of 45° measured between the arrows that characterize their orientation (minimization of the repulsion between their π and π * orbitals along with obtaining the best set of symmetry adapted orbitals to interact with the metal orbitals). But the relative displacement of such successive rectangles can refer to a right - or left - handed rotation. Thus (NbSe ) I is a (1 2 3 4 3 2)_ tetrachalcogenide (rotation in the same way between rectangles 1, 2, 3,4, then inverse rotation back to 3 and 2 before coming back to 1 and starting a new repeat unit) (TaSe ) I and (NbSe ) I are (12 3 4 ) and (1 2 3 4 5 6 5 4 3 2) chalcogenides, respectively. In the M-Se combination all the metal orbitals are involved, except d 2 which remains isolated and builds a d 2 band along the chain. This band governs the I D electronic properties of the system (26). Assuming that each iodine takes one electron, the average number of d electrons per metal is (n-l)/n with a corresponding filling of the d 2 band of f = (n-l)/2n, possibly leading to a distortion increasing the repeat distance by a factor 1/f. A s η increases the ά^ι band of (MXj.) I phases becomes nearly half-filled. Indeed ( M X ) I phases are C D W materials. Among them (TaSe )2l and ( N b S e ) I have shown interesting features, particularly a memory effect in the C D W depinning experiments in the latter case (27). Vanadium tetrasulfide constitutes finally an example of the n - > ° o limit. It has consequently the 2c insulating Peierls distortion. 4

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IV - Counter ions and counter chains. The tetraselenoiodide phases just described above represent a successful attempt to stabilize chains by counter ions. That structure also recalls the organisation of Krogman's salts in coordination chemistry, or organic conductors like tetraselenotetracene 1^ phases (28). Most of the time the counter ion is a cation. From that point of view the so-called blue bronzes of molybdenum, known for a long time represent a symmetrical situation with respect to the ( M X ) I derivatives, the counter ion being a cation instead of Γ species. With a general formulation A Q Q M O 0 (A = K , Rb, Ή ) they show complex chains built from groups of [ M o 0 ] distorted octahedra (8,29). It is worth noticing that most of these structure are stabilized by big counter ions which have the strongest favorable influence on the Madelung energies. Most of the time die corresponding arrangements cannot be obtained for small counter ions (CI" in the case of tetraselenides, L i in the blue bronzes). Instead of being stabilized by counter ions chain-like arrangements could probably become stable when different chains requiring opposite conditions for a stabilization are associated in the same structure. For example [NbSe ] chains are stabilized by negative Γ columns in 4

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(NbSe ) I phases. But, in A M o * S e compounds (30, 31), M o S e chains described below are stabilized by positive A cations (A = alkalimetal or thallium). Attempts have been made to associate in the same structure chains having opposite stability requirements. They have been essentially unsuccessful. However, even when "clever" approaches have not yet given the expected result, the concept is correct as proved by the recent characterization of a crystal with a mixed chain structure in a batch corresponding to preparations made in the Nb-Se-Br system. The formulation is Nb3Se )Br (32) and the structure is built from an alternation of [NbSe ] chains centered along a two-fold axis and [NbSe Br] chains running a 2γ axis parallel to the former one (figure 7). Se-Br exchanges on certain sites explain the rather complicated formulation. 4

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V - Redox soft-chemistry preparations. Cationic intercalation chemistry is based on coupled ion-electron transfer reactions. +

x

χ ( A + e") + Host -> A Host " These reactions represent an important part of the chemical reactivity of low-dimensional solids. They are reversible. In that case A ions are extracted and the host is reoxydized. A scale of reagents able to intercalate, or deintercalate, a given alkalimetal (lithium) in or from various host structures has been proposed (33). Interesting is the fact that the deintercalation reaction can be the base of a real concept to define a new and very important synthetic route in solid state chemistry. The idea is to consider a ternary compound A M Y (usually Υ = Ο, S, Se) that can prepared by classical solid state chemistry as the intercalation of A ions in a hypothetical M Y host structure. Removing A ions by electrochemistry or by using chemical reagents such as iodine in acetonitrile solutions in the case of lithium, yields M y Y which can be a new compound or a new structural form of an already known phase. For example one can get V S (34) from L i V S or a new form of T i S (named cubic T i S ) from the spinel C u T i S (35, 36). A critical discussion of this approach is possible under the light of the general considerations of section I. V S and T i S are favorable cases from the point of view of both the ionicity of bonds and the d-sp redox competition. But let us consider oxides. N a T i 0 presents the a N a F e 0 structure, like N a T i S . It can be regarded as an occupancy of octahedral sites by N a ions between pseudo [ T i 0 ] slabs similar to those of T i S . Removing A ions is possible. A t the beginning the process is reversible. It suggests that one could end up with a layered T i 0 similar to T i S . But this is not the case. After a critical extent in deintercalation titanium moves irreversibly from the [ T i 0 ] "slabs" to the van der Waals gap x

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8. R O U X E L

— Z= i


C axis

•

Se

Ο Br Θ Nb

Z=o

[NbSe ]

[NbSe BrJ

4

|1 '7 -*? ··«•··

0+)

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Supramolecular Architecture - American Chemical Society

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