Porous and low-dimensional molybdates

Current Opinion in Solid State and Materials Science 4 (1999) 133–139

Porous and low-dimensional molybdates Yan Xu* School of Science, Nanyang Technological University, Singapore 259756, Singapore

Abstract Major advancement in the development of porous and low-dimensional molybdates lies on the realisation of directed self-assembly of molecular clusters through tethering units. Combining the hydrothermal technique with structure-directing templates functioning as charge-compensating ions, ligands, covalent network constituents and cationic scaffolding provides a promising strategy leading to this goal. Research effort is continuously driven by the curiosity of the range of structural versatility of composite molybdates evolving from the introduction of templating agents and heteroatoms.  1999 Elsevier Science Ltd. All rights reserved.

1. Introduction The purpose of this review is to highlight the outstanding achievements in the development of porous (threedimensional) and low-dimensional (one- and two-dimensional) non-molecular composite molybdates in the past 1–2 years. Discussion is restricted to crystalline molybdates where molybdenum exists in either 61 or reduced oxidation states. The review is organised by identifying the nature and roles assumed by structure-directing templates during the assembly of non-molecular solids. Combining the hydrothermal techniques with structuredirecting templates provides an essential element for the realisation of solid architecture design, and ultimately for tuning of the catalytic, electronic, magnetic and optical properties of these materials. Achievements in this light are reviewed by commenting on relevant publications (Table 1). Development of the porous and low-dimensional molybdates by using other methods which show outstanding structural features and interesting properties is also reviewed. A brief account is dedicated to an extended area of research, e.g. synthesis of mesoscopic oxomolybdate and non-oxide molybdenum molecular materials, in an anticipation of the construction of non-molecular materials in which the coexistence of classical and quantum effects can be observed.

2. Assembly of molybdates: impact of amines Considerable attention has focused on molybdenum *Corresponding author. E-mail address: [email protected] (Y. Xu)

trioxide-based solid materials due to their rich chemical reactivity and structural complexity. While the design of solid materials remains elusive, the introduction of organic amines provides a possibility for structural modification with retained structural and chemical properties of synthetic precursors. The roles assumed by amine templates are, broadly speaking, twofold: (i) as charge-compensating ions and (ii) as ligands coordinated to the covalent backbones of molybdates. An improved understanding of these templating roles leads to a better-directed assembly process. What follows is a discussion of recent discoveries emphasising the rationale´ of synthesis and the structural features of new materials. Exploiting the roles of aromatic amines and searching for non-conventional templates characterises the main streams of recent research. (4,49-H 2 bpy)[Mo 7 O 22 ]?H 2 O 1 is hydrothermally synthesised using 4,49-bipyridine (4,49bpy) under acidic conditions to preclude the coordination of pyridyl nitrogen atoms to oxide skeleton, resulting in its incorporation as bipyridinium ions [*1]. The structure of 1 22 consists of ladder-shaped Mo 7 O 22 layers of hMoO 6 j 21 octahedra, interlamellar H 2 O molecules and 4,49-H 2 bpy 22 cations oriented nearly perpendicular to the Mo 7 O 22 layers to maximise the electrostatic interactions between counterions. Driven by the curiosity of the structural roles that might be assumed by aminoacids, (NH 4 ) 2 [Mo 4 O 13 ] 2 is isolated from acidic hydrothermal reaction containing 4-aminobenzoic acid [*2]. The structure of 2 is composed of the 22 Mo 4 O 13 layers of hMoO 6 j octahedra and hMoO 5 j square pyramids, and interlamellar NH 41 cations. 4-aminobenzoic acid is not included in 2 in part due to its incompatible charge density with molybdenum polyoxoanions. Instead, NH 1 4 is incorporated presumably through its charge-com-

1359-0286 / 99 / $ – see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S1359-0286( 99 )00017-0

Y. Xu / Current Opinion in Solid State and Materials Science 4 (1999) 133 – 139

134 Table 1 A list of selected solids Solid

Formula

Dimension

Method of preparation

Ref

1 2 3 4 5 6 7 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

(4,49-H 2 bpy)[Mo 7 O 22 ]?H 2 O (NH 4 ) 2 [Mo 4 O 13 ] (CN 3 H 6 )(NH 4 )[Mo 3 O 10 ] (2,29-bpy)[MoO 3 ] (2,29-bpy)[Mo 2 O 6 ] (2,29-bpy) 2 [Mo 3 O 9 ] (4,49-Hbpa) 2 [Mo 4 O 13 ] (enH) 6 [Co 3 Mo 4 P4 O 28 ] Tl 2 (MoO 3 ) 3 PO 3 CH 3 M 2 Th 4 (MoO 4 ) 9 Cs 5 Mo 8 AsO 30 H 2 ?2H 2 O Cs 7 Mo 8 AsO 30 Cs 2 Mo 2 As 2 O 12 ?H 2 O Mo 3 As 2 O 15 H 2 Na 0.5 K 6.5 [Mo 8 V6 O 38 (VO 4 )]?12.5H 2 O A 2 Mo 3 TeO 12 [hFeCl(2,29-bpy)jMoO 4 ] [hFe(2,29-bpy)j 2 Mo 3 O 12 ]?0.25H 2 O [hFe(2,29-bpy)j 2 Mo 4 O 15 ] [hNi(2,29-bpy) 2 jMo 4 O 13 ] [hCu(2,29-bpy)jMo 2 O 7 ] [hCo(2,29-bpy)jMo 3 O 10 ] [hCu(en) 2 j 2 Mo 8 O 26 ] [hCu 3 (4,7-phen) 3 j 2 Mo 14 O 45 ] [hCu(dpe)jMoO 4 ] (enH 2 ) 2 [hCu(en)(OH 2 )jMo 5 P2 O 23 ]?4H 2 O (enH 2 )[hCu(en) 2 jMo 5 P2 O 22 (OH 2 )]?2H 2 O [hCu(en)(enH)j 2 Mo 5 P2 O 23 ]?3H 2 O [hCu(4,49-bpy)j 4 Mo 8 O 26 ] [hNi(4,49-bpy) 2 (H 2 O) 2 j 2 Mo 8 O 26 ] [hCu(4,49-bpy)j 4 Mo 15 O 47 ]?8H 2 O [hCu 2 (triazolate) 2 (H 2 O) 2 jMo 4 O 13 ] (H 3 O) 12 h[MoO 2.5 (H 2 O)][Mo 36 O 108 (NO) 4 (H 2 O) 16 ] [MoO 2.5 (H 2 O)]j?44H 2 O Rb 6 [(Mo 9 V3 O 6 )(PO 4 ) 10 (H 2 PO 4 ) 3 (OH) 9 ]?8.5H 2 O WMoP2 O 11 LiW1.32 Mo 0.68 P2 O 11 Rb 2 WMo 2 P3 O 17 Ag 0.7 Mo 3 PO 11 CsMo 8 P4 O 33 AMoPO 5 Cl TeMo 5 O 16 NdMo 6 O 12

2-D 2-D 1-D 1-D 1-D 1-D 1-D 3-D 2-D 3-D 1-D 2-D 3-D 3-D 1-D 2-D 1-D 1-D 1-D 1-D 1-D 2-D 2-D 2-D 3-D 1-D 1-D 2-D 0-D 3-D 2-D 1-D 1-D

hydrothermal hydrothermal hydrothermal hydrothermal hydrothermal hydrothermal hydrothermal hydrothermal hydrothermal ceramic hydrothermal ceramic ceramic solution solution hydrothermal hydrothermal hydrothermal hydrothermal hydrothermal hydrothermal hydrothermal hydrothermal hydrothermal hydrothermal hydrothermal hydrothermal hydrothermal hydrothermal hydrothermal hydrothermal hydrothermal solution

[*1] [*2] [*2] [*4] [*4] [*4] [*5] [**6] [8] [9] [*10] [*10] [*11] [12] [*13] [14] [*15] [*15] [*15] [*16] [*16] [*16] [*17] [*18] [**19] [*20] [*20] [*20] [*21] [*21] [*21] [*22] [*27]

3-D 3-D 3-D 3-D 3-D 3-D 2-D 3-D 3-D

hydrothermal ceramic ceramic ceramic ceramic ceramic ceramic ceramic ceramic

[**28] [29] [30] [31] [32] [33] [34] [35] [36]

35 36 37 38 39 40 41 42 43

pensating role. Interlamellar separation of 2 is closely related to the size of NH 1 4 ions. The charge density of guanidinium ion (CN 3 H 61 ) is intermediate to those of Na 1 , NH 41 , enH 21 and hexH 21 2 2 ions which are known to function as templating agents in one-dimensional solids of Na(NH 4 )[Mo 3 O 10 ], (NH 4 ) 2 [Mo 3 O 10 ], (enH 2 )[Mo 3 O 10 ] [3] and (hexH 2 ) [Mo 4 O 13 ], respectively. Therefore, it can be expected that low-dimensional guanidinium-analogues may exist. This expectation is realised by the isolation of onedimensional (CN 3 H 6 )(NH 4 )[Mo 3 O 10 ] 3 [*2]. It consists 22 of the Mo 3 O 10 chains of hMoO 6 j octahedra, interchain 1 NH 4 and CN 3 H 61 cations oriented perpendicular to the 22 Mo 3 O 10 chains giving rise to large interchain separation.

Extensive H-bond networks are found between counterions in the structures of 1–3. The structural role of amines as ligands forming an integral part of molybdate skeletons is made possible in weak acidic to neutral reaction medium. The successful crystallisation of (2,29-bpy) m [MoO 3 ] n family 4–6 [*4], (4,49-Hbpa) 2 [Mo 4 O 13 ] 7 [*5] and (4,49-bpy) 0.5 [MoO 3 ] 8 demonstrates this point. Solids 4–6 have one-dimensional structures assembled using 2,29-bipyridine (2,29-bpy). The chain structure of (2,29-bpy)[MoO 3 ] 4 is built up of hMoO 4 N 2 j octahedra and the chains of (2,29-bpy)[Mo 2 O 6 ] 5 and (2,29-bpy) 2 [Mo 3 O 9 ] 6 are composed of hMoO 4 N 2 j octahedra and hMoO 4 j tetrahedra in 1:1 and 2:1 ratios, respectively. 2,29-bpy molecules are incorporated as neu-

Y. Xu / Current Opinion in Solid State and Materials Science 4 (1999) 133 – 139

Fig. 1. A space-filling view of (enH) 6 [Co 3 Mo 4 P4 O 28 ] 9 showing the unusual seven-ring pores.

tral chelating groups to Mo centers forming hMoO 4 N 2 j octahedra. The crystallisation of 4–6 cannot be accomplished in the absence of Fe 21 and Mn 21 . The number of N donors of amine templates and their orientations subtly impact upon solid structures as demonstrated by the hydrothermal crystallisation of 7 and 8, using a bent-rod 4,49-bipyridylamine (4,49-bpa) and a linear-rod 4,49-bipyridine (4,49-bpy), respectively. The one-dimensional structure of 7 consists of (4,49Hbpa) 2 [Mo 4 O 13 ] chains of hMoO 5 Nj octahedra and hMoO 5 j square pyramids. The porous framework structure of (4,49-bpy) 0.5 [MoO 3 ] 8 is constructed from layers of hMoO 5 Nj octahedra and hMoO 5 j square pyramids linked by 4,49-bpy. It may also be described in terms of alternating MoO 3 layers and aromatic organic bilayers. The use of amine ligands serves to link up molybdate components to form extended organomolybdate solids; on the other hand, they impose steric constraints that block further condensation of organomolybdates and cause poor crystal packing. A strategy used to facilitate the condensation of molecular precursors is to increase negative charges by replacing Mo 61 with low oxidation-state ions. This is normally accompanied by concurrent iteration of amine templates together with the use of F 2 ions as demonstrated by the synthesis of porous (enH) 6 [Co 3 Mo 4 P4 O 28 ] 9 using en [**6]. The framework structure of 9 is constructed from CoMo 2 P units of hMoO 6 j and hCoO 6 j octahedra and hPO 4 j tetrahedra resulting in interlocking cavities and sevenmember ring intersecting channels filled with enH 1 ions (Fig. 1). The successful isolation of 9 may be attributed to the compatible charge density of ethylenediammonium ions with molybdenocobalt phosphate coupled with the appropriate use of F 2 ions. Attempts to use other amine templates, so far, prove to be unfruitful.

3. Assembly of molybdates: impact of metal cations In the effort to search for templates possessing specific charge density, the rational use of metal cations provides a promising alternative as demonstrated by the discovery of

135

zeotyped chiral cobalt phosphates, MCoPO 4 [7]. This remarkable success is largely owing to the recognition of charge density differences between cobalt phosphates and porous AlPOs, and in consequence, the design of porous MCoPO 4 using similar amines to AlPOs is difficult. Extrapolating this idea to the heteromolybdate systems results in the assembly of porous and low-dimensional metal cation heteromolybdates. Tl 2 (MoO 3 ) 3 P 51 O 3 CH 3 10 is a two-dimensional solid hydrothermally mediated using Tl 1 [8]. It consists of layers of hMoO 6 j octahedra and peripheral hPO 3 CH 3 j tetrahedra, and interlamella Tl 1 cations. Porous 1 1 M 2 Th 41 4 (MoO 4 ) 9 (M5Cu , Li ) 11 [9], prepared using the classical ceramic methods of high temperature calcination, is constructed from hMoO 4 j tetrahedra and hThO 9 j polyhedra. Cages where exchangeable Cu 1 and Li 1 ions reside are generated from polyhedral connectivity. Cs 5 Mo 8 As 51 O 30 ?H 2 2H 2 O 12, Cs 7 Mo 8 As 51 O 30 13 and Cs 2 Mo 2 As 51 2 O 12 ?H 2 O 14 are low-dimensional and porous molybdenum arsenates [*10,*11]. The chain structure of 12 is constructed from unprecedent [AsMo 8 O 30 H 2 ] 52 clusters bridged through Cs 1 cations. The dehydrated form of 12 is a two-dimensional solid, 13, in which layer structures consist of [AsMo 8 O 30 ] 72 clusters and bridging Cs 1 cations. Solid 14, obtained by prolonged heating of Cs 2 Mo 2 As 2 O 13 H 2 ?H 2 O and Cs 4 Mo 6 As 2 O 27 H 4 ?2.5H 2 O cluster compounds [*11], has a porous structure built up from hMoO 6 j octahedra and hAsO 4 j tetrahedra. Cavities in 14 are interconnected to form open channels that accommodate Cs 1 cations and H 2 O molecules. In contrast, Mo 3 As 251 O 15 H 2 15 [12], prepared using a solution method, is a three-dimensional solid composed of hMoO 6 j and hMoO 5 OHj octahedra and hAsO 4 j tetrahedra, but without accessible internal volume. The absence of pores may be a consequence of framework neutrality. The synthesis of one-dimensional Na 0.5 K 6.5 [Mo 8 V 641 O 38 (V 51 O 4 )]?12.5H 2 O 16 provides a novel example of which materials design is effected through the controlled linkage of preformed heteropolyoxomolybdenum clusters [*13]. The chain structure of 16 is constructed from trans-vanadium capped a-Keggin type hMo 8 V6 O 38 (VO 4 )j units. Exploitation of the structural versatility of heteromolybdates in the presence of metal cation templates leads to the successful syntheses of twodimensional molybdenum tellurites A 2 Mo 3 TeO 12 (A5 1 NH 1 4 ,Cs ) 17 [14]. Summing up, the appropriate use of metal cations induces the crystallisation of porous and low-dimensional heteromolybdate solids of which internal volume is critically controlled by the size of metal cations.

4. Assembly of molybdates: impact of metal–amine complexes Transition metal–amine complexes may function as structure-directing templates by providing building units.

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This suggests an innovative approach to the assembly of molybdate clusters through the linkage of bulky metal– amine complex groups. The essential element in the realisation of this approach includes the use of the hydrothermal technique as demonstrated by the isolation of solids 18–33. Observations to date suggest that structural versatility may evolve from the introduction of various types of transition metal–amine complex bridging units. Incorporating transition metals, Fe 21 , Ni 21 , Cu 21 and Co 21 , into the hydrothermal mixtures containing 2,29-bpy affords six low-dimensional solids [hFeCl(2,29bpy)jMoO 4 ] 18, [hFe(2,29-bpy)j 2 Mo 3 O 12 ]?0.25H 2 O 19, [hFe(2,29-bpy)j 2 Mo 4 O 15 ] 20, [hNi(2,29-bpy) 2 jMo 4 O 13 ] 21, [hCu(2,29-bpy)jMo 2 O 7 ] 22 and [hCo(2,29-bpy)jMo 3 O 10 ] 23 [*15,*16]. Solids 18–20 are one-dimensional in which chains are constructed from hMoO 4 j tetrahedra and hFeXN 2 O 3 j octahedra (X5Cl for 18 and O for 19 and 20). The packing of organoironmolybdate chains gives rise to similar cylindrical units of which each solid has distinct interchain separations controlled by the position and orientation of 2,29-bpy molecules. Solids 21 and 22 are one-dimensional where chains are constructed from b[Mo 8 O 26 ] 42 clusters and hNiN 4 O 2 j octahedra in 21, and hMoO 6 j octahedra, hMoO 5 j square pyramids and peripheral hCuN 2 O 4 j octahedra in 22. Solid 23 consists of twodimensional layers of hMoO 4 j tetrahedra, hMoO 5 j square pyramids, and hMoO 6 j and hCoN 2 O 4 j octahedra. Exploring the range of structural types evolving from the use of different amines including en, 4,7-phenanthroline (4,7-phen) and 1,2-trans-(4-pyridyl)ethene (dpe) results in the isolation of [hCu(en) 2 j 2 Mo 8 O 26 ] 24 [*17], [hCu 3 (4,7-phen) 3 j 2 Mo 14 O 45 ] 25 [*18] and [hCu(dpe)jMoO 4 ] 26 [**19]. Solids 24 and 25 have twodimensional structures where layers consist of g[Mo 8 O 26 ] 42 clusters and hCuN 4 O 2 j 21 octahedra in 24, and 42 the chains of b-[Mo 8 O 26 ] clusters and hCu 3 (4,731 phen) 3 j groups, and interlocking [Mo 6 O 19 ] 22 clusters in 25. Solid 26 has a porous framework structure constructed from hCu(dpe)j 21 chains and hMoO 4 j tetrahedra (Fig. 2). Practicing the same approach in the en-Cu 21 molybdenophosphate system gives rise to three low-dimensional solids (enH 2 ) 2 [hCu(en)(OH 2 )jMo 5 P2 O 23 ]?4H 2 O 27, (enH 2 )[hCu(en) 2 jMo 5 P2 O 22 (OH 2 )]?2H 2 O 28 and [hCu(en)(enH)j 2 Mo 5 P2 O 23 ]?3H 2 O 29 [*20]. The structures of 27–29 are assembled from [Mo 5 P2 O 23 ] 62 clusters and hCu(en)O x j 21 polyhedra where connectivity between the two building units controls the dimensionality of solid architectures. Solids 27 and 28 are one-dimensional where the spiral-shaped chains of 27 are composed of [Mo 5 P2 O 23 ] 62 clusters and hCuN 2 O 2 (H 2 O)j square pyramids and the zig-zag chains of 28 of [Mo 5 P2 O 22 (OH 2 )] 42 clusters and hCuN 4 O 2 j octahedra. The structure of 29 consists of porous layers of [Mo 5 P2 O 23 ] 62 clusters and hCuN 3 O 2 j square pyramids, and pillaring H 2 O molecules. Rapid advancement in crystal engineering of metal-

Fig. 2. A ball-and-stick representation of the three-dimensional framework of [hCu(dpe)jMoO 4 ] 26 constructed from hCu(dpe)j 21 chains and hMoO 4 j tetrahedra.

dipodal organonitrogen ligand complexes inspires the identification of a new structural role of transition metal– amine complexes as cationic scaffolding. Typical molybdates demonstrating this role are [hCu(4,49-bpy)j 4 Mo 8 O 26 ] 30, [hNi(4,49-bpy) 2 (H 2 O) 2 j 2 Mo 8 O 26 ] 31, [hCu(4,49bpy)j 4 Mo 15 O 47 ]?8H 2 O 32 and [hCu 2 (triazolate) 2 (H 2 O) 2 jMo 4 O 13 ] 33 (triazolate51,2,4-triazole) [*21,*22]. Solids 30 and 31 consist of the linear chains of hCu(4,49-bpy)j 1 and the branched chains of hNi(4,49-bpy) 2 (H 2 O) 2 j 21 respectively with entrenched [Mo 8 O 26 ] 42 clusters. In 31, e-[Mo 8 O 26 ] 42 clusters which are coordinated to cationic scaffoldings are distorted due to steric constraints. Solid 32 contains alternating layers of hCu(4,49-pby)j 1 and [Mo 15 O 47 ] 42 . The structure of 33 is characterised by the three-dimensional cationic framework of hCu 2 (triazolate) 2 (H 2 O) 2 j 21 with entrenched one-dimensional [Mo 4 O 13 ] 22 chains. The initial success in building molybdenum polyanions into porous and low-dimensional cationic scaffolding indicates a fertile area of study with development of novel composite molybdates through the structure-directing effect of metal–amine complexes. Further advancement in this light will be facilitated by the concurrent growth of polyoxometalate chemistry.

5. Assembly of reduced molybdates Reduced polyoxomolybdate molecular materials are a focus of contemporary research due to their potential

Y. Xu / Current Opinion in Solid State and Materials Science 4 (1999) 133 – 139

electrical, optical and magnetic properties [23,24]. Recent success in the isolation of molybdate clusters containing 154 and 176 [25,26] molybdenum atoms represents a remarkable move towards nanomaterials synthesis. How can we make use of the vast amount of quasi-preorganised building units to design porous and low-dimensional nonmolecular composite molybdates with specific properties? This is a question of immense general interest that calls for immediate attention. The synthesis of one-dimensional (H 3 O) 12 h[MoO 2.5 (H 2 O)] [Mo 36 O 108 (NO) 4 (H 2 O) 16 ][MoO 2.5 (H 2 O)]j?44H 2 O 34, using an aqueous solution method, provides a promising example towards this goal [*27]. The polymeric chains of 34 are constructed from linking [Mo 36 O 108 (NO) 4 (H 2 O) 16 ] 122 clusters through [(H 2 O)O 2 MoOMoO 2 (H 2 O)] polyhedra. A closely related example is the porous Rb 6 [(Mo 9 V3 O 6 )(PO 4 ) 10 (H 2 PO 4 ) 3 (OH) 9 ]?8.5H 2 O 35, hydrothermally assembled using Rb 1 cations [**28]. The framework structure of 35 contains hexagonal 12-ring channels built up from tetranuclear units of Mo 51 and Mo 31 / V 31 octahedra linked through hPO 4 j tetrahedra (Fig. 3). The association of reduced Mo and W appears to favour the construction of porous tungstenomolybdenum phosphates as demonstrated by the crystallisation of WMoP2 O 11 36 [29], LiW1.32 Mo 0.68 P2 O 11 37 [30] and Rb 2 WMo 2 P3 O 17 38 [31] using the ceramic methods. Polyhedral connectivity between the anionic chains of [MPO 8 ] in 36, [M 2 P2 O 15 ] in 37 and enantiomorphic [M 3 P3 O 20 ] in 38 generates channels that accommodate counterions (M5Mo,W). Solids 39–43 are porous and low-dimensional examples of reduced molybdates. The framework structure of Ag 0.7 Mo 3 PO 11 39 consists of ReO 3 -type slabs linked by hPO 4 j units forming hexagonal channels occupied by Ag 1 cations [32]. The porous structure of CsMo 8 P4 O 33 40 is built up from hMo 4 O 20 j units, hMo 2 O 7 j ditetrahedra,

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hMoO 6 j octahedra and hPO 4 j tetrahedra where cages are 1 occupied by Cs cations [33]. AMoPO 5 Cl 41 is a twodimensional solid where layers are constructed from hMoO 5 Clj octahedra and hPO 4 j tetrahedra [34]. TeMo 5 O 16 42, prepared using the ceramic methods, consists of a three-dimensional framework of hMoO 6 j octahedra with hexagonal channels occupied by Te 41 ions [35]. NdMo 6 O 12 43 has a porous structure constructed from alternating hMo 3 j chains and hMo 6 O 12 j double-rutile chains [36]. It exhibits unidirectional semiconductivity derived from well-defined cation-defect sequence of 31 31 Nd –Nd along one-dimensional channels.

6. Non-oxide molybdenum solids Extrapolating the strategy from porous and low-dimensional molybdates synthesis to non-oxide molybdenum materials synthesis is an obvious extension, however, it is not clear that it could be successful. Nevertheless, combining oxide with non-oxide molybdenum chemistry provides a good starting point. The cluster compound Mo 12 S 12 O 30 H 24 44 consists of hMo 12 j rings based on 21 [Mo 2 S 2 O 2 ] building units [37]. It is characterised by the cyclic arrangement of hMo 12 j clusters and a reversible self-condensation reaction, suggesting the feasibility of cluster-reorganisation for non-molecular thiomolybdates synthesis. [(n-Bu) 4 N] 4 [Mo 4 Cu 10 S 16 O 3 ]?H 2 O 45, [(nBu) 4 N] 4 [Mo 4 Cu 10 S 18 O]?H 2 O 46 and hMAg 3 S 3 [S 2 P(OCH 2 CH 3 ) 2 ]j(S)(Ph 3 P) 3 (M5W,Mo) 47 are cluster compounds with potential bioactivity and optical properties [38,39]. Recent synthesis of Ni 2 Mo 3 N 48 marks the initial success towards understanding the unusual structures and properties of ternary and higher molybdenum nitride materials [40]. Overall, the discovery of non-oxide molybdenum molecular clusters provides useful building blocks for the design of non-molecular porous and low-dimensional solids through the unit-reconstruction principle.

7. Conclusions

Fig. 3. A ball-and-stick representation of the hexagonal 12-ring channels of Rb 6 [(Mo 9 V3 O 6 )(PO 4 ) 10 (H 2 PO 4 ) 3 (OH) 9 ]?8.5H 2 O 35 viewed along the c-axis.

Contemporary research has focused on structural modification and design of porous and low-dimensional molybdates through the power of self-assembly. While a variety of methods are available for preparing these materials, combining the hydrothermal technique with structure-directing templates proves to be a fundamental strategy leading to desired solid architectures. The broad goal of these studies is to create accessible internal volume that allows easy passage of guest molecules and ultimately generate bifunctional composite molybdates. This goal is partially accomplished with the aid of organic and organic–inorganic bridging units in conjunction with the introduction of structure-directing templates. While it

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Y. Xu / Current Opinion in Solid State and Materials Science 4 (1999) 133 – 139

remains difficult to navigate through the large parameter space defining designed synthesis, the structure-directing role of templates through their charge-compensating effects is manifest. A more complete understanding of this role will have to await the discovery of additional examples. It can be anticipated that future developments are likely to stress on the following aspects: (i) Directed synthesis of molybdates using tailor-made templates with specific steric, optical and chemical properties. (ii) Directed assembly of micro- and nano-molybdate clusters with welldefined properties for non-molecular materials synthesis. (iii) Crystal engineering of non-oxide molybdenum solids. (iv) Exploitation of the application potentials of molybdates as to provide feedback for better directed materials engineering. Finally, a curious, yet underdeveloped area that needs immediate attention concerns the structural role of large organic templates for constructing solid architectures of non-molecular molybdates, especially those of isopolyoxomolybdate composite materials.

8. Notation Molybdates Porous and (hetero)polyoxomolybdates

low-dimensional

iso-

Acknowledgements I am grateful to Dr. TJ White for useful discussion, Professors NK Goh and PY Lee for their encouragement, and JJ Lu and BB Yan for their help in preparation of the reference list, and the Nanyang Technological University, Singapore, for financial support (RP 23 / 96XY).

References Papers of particular interest, published within the annual period of review, have been highlighted as: * of special interest; ** of outstanding interest [*1] Zapf PJ, Haushalter RC, Zubieta J. Crystal engineering of inorganaic / organic composite solids: the structure-directing role of aromatic ammonium cations in the synthesis of the ‘step’layered molybdenum oxide phase [4,49-H 2 bpy][Mo 7 O 22 ]H 2 O. Chem Commun 1997;321–2, A directed synthesis of a novel two-dimensional network through the charge-compensating effect of aromatic ammonium cation rendered under acidic reaction conditions. [*2] Lu JJ, Xu Y. Investigation into the hydrothermalassembly of molybdenum polyoxoanion solids in the presence of 4-aminobenzoic acid, guanidinium carbonate and aminoacetic acid. Chem Mater 1998;10:4141–7, Use of non-conventional templates to direct the hydrothermal assembly of two low-dimensional networks.

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