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Optimised hydrothermal synthesis of multi-dimensional hybrid coordination polymers containing flexible organic ligands
Progress in Solid State Chemistry 33 (2005) 113e125 www.elsevier.com/locate/pssc
Optimised hydrothermal synthesis of multi-dimensional hybrid coordination polymers containing flexible organic ligands Filipe A. Almeida Paz a,*, Jo~ao Rocha a, Jacek Klinowski b, Tito Trindade a, Fa-Nian Shi a, Luı´s Mafra a b
a Department of Chemistry, University of Aveiro, CICECO, 3810-193 Aveiro, Portugal Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK
Abstract In this paper, we summarise our recent research interest in the hydrothermal synthesis and structural characterisation of multi-dimensional coordination polymers. The use of N-(phosphonomethyl)iminodiacetic acid (also referred to as H4pmida) in the literature as a versatile chelating organic ligand is briefly reviewed. This molecule plays an important role in the formation of centrosymmetric dimeric [V2O2(pmida)2]4 anionic units, which were first used by us as building blocks to construct novel coordination polymers. Starting with [V2O2(pmida)2]4 in solution, we have isolated [M2V2O2(pmida)2 (H2O)10] species (where M2þ ¼ Mn2þ, Co2þ or Cd2þ) via the hydrothermal synthetic approach, which were then employed for the construction of [CdVO(pmida)(4,4#-bpy)(H2O)2]$(4,4#-bpy)0.5$(H2O), [CoVO(pmida)(4,4#-bpy)(H2O)2]$(4,4#-bpy)0.5, [Co(H2O)6][CoV2O2(pmida)2(pyr)(H2O)2]$2(H2O) and [Cd2V2O2(pmida)2(pyr)2(H2O)4]$4(H2O) by the inclusion of bridging organic ligands in the reactive mixtures, such as pyrazine (pyr) and 4,4#-bipyridine (4,4#-bpy). These materials can contain channel systems, and exhibit magnetic behaviour, not only due to the V4þ centres but also to the transition metal centres which establish the links between neighbouring dimeric [V2O2(pmida)2]4 anionic units. A closely related anionic moiety, [Ge2(pmida)2(OH)2]2, was engineered to allow the study of such crystalline hybrid materials using one- and two-dimensional high-resolution solid-state NMR. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Hydrothermal synthesis; Metal-organic frameworks; Coordination polymers; Crystal engineering; Transition metal centres; PMIDA
* Corresponding author. E-mail address: [email protected] (F.A.A. Paz). 0079-6786/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.progsolidstchem.2005.11.033
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1. Introduction Chemistry is used by scientists to modify Nature in accordance with their wishes. With the control of the atom arrangement in individual molecules being now a reality in modern science, and with the most recent technological advances in methods for structural characterisation, researchers have turned their attention to the large supramolecular frameworks which, by their physical and chemical nature, constitute a wonderful intellectual and experimental challenge. In any supramolecular network, the topology and properties are simultaneously controlled by the chemical nature of the individual building blocks and by their relative orientation in the solid state. Research in this broad field of science, usually referred to as crystal engineering, started with the work of Desiraju [1] and Etter [2] on organic crystals assembled via (usually strong) hydrogen bonds. However, these interactions are only in the range of tens of kJ mol1, thus rendering the isolated materials susceptible to thermal decomposition at relatively low temperatures, or even to framework demolition by the action of an adequate solvent. At the beginning of the 1990s, a qualitative jump was made when Hoskins and Robson [3] started to mimic Nature from the most fundamental topological point of view. They used the much stronger and highly directional coordinative interactions (several hundreds of kJ mol1) to engineer the first multi-dimensional diamondoid-type metal-organic frameworks (known as MOF, coordination polymers or, more recently, as coordination frameworks). The volume of research in the field of coordination polymers is directly reflected in the exponential growth of the number of publications in international journals, which is at the moment well over two thousand (Scheme 1) [4]. This worldwide effort is motivated by the large variety of architectures, their interesting properties (high porosity, hosteguest exchange, gas storage, photoluminescence, catalysis, non-linear optical properties, clathration, photochromism, chirality and magnetic properties) and potential applications as functional materials. Several excellent reviews on the various topics related to coordination polymers are available [5e13].
Scheme 1. Number of scientific publications featuring ‘‘metal-organic framework’’, ‘‘coordination polymer’’ and/or ‘‘coordination framework’’ in their title, abstract or keywords. Source: ISI Web of Science [29].
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Following our interest in the design and synthesis of these novel coordination polymers [14e26], we wish to summarise our most recent efforts towards the rational temperaturecontrolled hydrothermal synthesis of modular materials with interesting architectures, obtained in high yields and as phase-pure samples directly from the autoclave. We focus our attention on the centrosymmetric dimeric [V2O2(pmida)2]4 anionic unit, originally reported by Crans and collaborators [27], in the formation of several multi-dimensional frameworks [21,23e26]. We also emphasise our research on a recently engineered anionic unit, [Ge2(pmida)2(OH)2]2, closely related to [V2O2(pmida)2]4, which has the great advantage of not containing paramagnetic V4þ centres [28]. 2. Optimisation of the hydrothermal synthesis ‘‘Hydrothermal’’ synthetic approaches to the isolation of novel materials tacitly refer to a reaction carried out in aqueous media at a temperature higher than 100 C and a pressure above 1 atm. In essence, this is inspired by Nature itself, as many minerals are formed under such conditions. Hydrothermal methods offer a number of advantages not attainable by other conventional synthetic methods. This is true, not only for the particular example of coordination polymers, but also for any other type of material. For example, such experimental conditions can, for many systems, overcome the high activation energies necessary to start nucleation of thermodynamically metastable phases. In the particular case of the synthesis of hybrid materials, the physical and chemical properties of water in the temperature and pressure ranges usually employed permit an increase of the solubility of heavily organic molecules. However, these methods are usually driven by fast kinetics of nucleation and crystal growth, leading in the vast majority of cases to poorly crystalline and insoluble powders. Exact knowledge of the structure of hybrid materials can only, so far, be fully achieved by X-ray crystallography, which requires good-quality single-crystals with manageable dimensions. This is the main reason why, until very recently, synthetic approaches such as (1) slow diffusion of the reactants into a polymeric matrix, (2) diffusion from the gas phase, (3) evaporation of the solvent at ambient or reduced temperatures, (4) precipitation or recrystallisation from a mixture of solvents, and (5) temperature controlled cooling, have been preferred over the hydrothermal synthetic approach. To overcome the limitations concerning the isolation of good-quality single-crystals of hybrid materials, special hydrothermal experimental conditions for each reactive system must be applied, which implies optimisation of the process involving variables such as the composition of the reactive mixture (including the starting chemicals and their relative mole ratios) and the temperature profile used for the synthesis [20]. The latter is rather complex, as it includes several other factors such as the duration of the synthesis, the temperature employed and the cooling process of the reaction vessels [20]. We first reported such optimisation for the three-fold interpenetrated, 3D and modular [Cd(BPhDC)(BPE)]$(H2O) compound (where H2BPhDC is biphenyl-4,4#-dicarboxylic acid, and BPE is 1,2-bis-(4-pyridyl)ethane) [14]. Later, a much more detailed study of the hydrothermal conditions needed to isolate the 3D [Cd(NDC)(H2O)] diamondoid-type material was reported (where H2NDC is 2,6-naphtalenedicarboxylic acid) [20]. It was shown that: (1) ‘‘mild’’ hydrothermal conditions (i.e. temperatures ca. 150 C) and slow cooling of the autoclaves usually lead to highly crystalline hybrid materials; (2) the usual quenching of the reaction vessel usually leads to microcrystalline materials; (3) a systematic change in the composition of the reactive mixture is of crucial importance for isolating an optimal chemical range which leads to a phase-pure [Cd(NDC)(H2O)] material. More recently, and using the same organic ligand
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with Yb3þ cations to produce a 3D pseudo-five-connected network, [Yb2(NDC)3(H2O)]$(H2O)2 (4455 topology), we have further demonstrated that a fine-tuning of the optimisation process can significantly reduce the synthesis time for large quantities of a phase-pure material to just about 4 h [18]. 3. Multi-dimensional coordination polymers 3.1. Organic building blocks: flexibility versus rigidity Studies undertaken to optimise the hydrothermal synthetic approach were, in the vast majority of the cases, based on the use of highly rigid ditopic organic ligands such as H2BPhDC and/ or H2NDC (see above) [14,18,20]. Indeed, the reduction of conformational flexibility of the organic ligands tends to lead preferentially to the formation of a particular network for a given chemical composition. When organic ligands start to exhibit a number of degrees of flexibility, supramolecular isomerism can occur [7], which in most cases directs interesting novel topologies. We have shown that the system formed by 1,3,5-benzenetricarboxylic acid (H3BTC), Cd2þ cations with BPE or 4,4#-trimethylenedipyridine (TMD) can lead to 1 or 3 frameworks, respectively, when the flexibility of the bipyridine derivative is increased from two to three methylene groups between the 4-pyridyl aromatic rings [30,31]. Multidentate chelating organic ligands are much less commonly used in the assembly of multi-dimensional coordination polymers. Indeed, chelating and highly flexible ‘‘arms’’ lead preferentially to the formation of discrete aggregates due to the elimination of all coordination sites of metal centres, as revealed by a search in the Cambridge Structural Database [32,33]. However, by controlling the degree of chelation it is theoretically possible to exert some control over the dimensionality of the isolated frameworks. We have used diethylenetriaminepentacetic (H5DTPA) and nitrilotriacetic acids (H3NTA) as precursors for large and highly flexible multidentate organic ligands which incorporate several carboxylic acid groups and central N-donor atoms. When used in conjunction with Cd2þ cations and 4,4#-bipyridine (4,4#-bpy) in the presence of TEA, three novel compounds were produced [15,19]. We concluded that the multidentate nature of these two flexible organic ligands seems to prevent the formation of coordination polymers with high dimensionality (i.e. 2D and 3D) [15,19]. Indeed, only with H3NTA a metastable 2D porous structure was successfully isolated, [Cd4(4,4#-bpy)5(NTA)2 (H2O)4]$(NO3)2$(H2O)x, which proved to be highly unstable and converted into a 1D chain, [Cd4(4,4#-bpy)4(NTA)2(H2O)10]$(4,4#-bpy)$(NO3)2$(H2O)8 [19], structurally very similar to the compound with H5DTPA residues, [Cd4(4,4#-bpy)3(HDTPA)2(H2O)4]$14(H2O) [15]. Moreover, the protonation level of H5DTPA seems to depend on the amount of TEA added to the reactive mixture. 3.2. N-(Phosphonomethyl)iminodiacetic acid: a versatile chelating organic ligand N-(Phosphonomethyl)iminodiacetic acid (H4pmida) is an intermediate in the important process of glyphosate production, a herbicide in worldwide use [34]. This organic molecule is a derivate of nitrilotriacetic acid (H3NTA), with two carboxylic acid groups and a phosphonate group at the end of the third chelating arm (Scheme 2). The presence of the phosphonate functional group can confer, on the one hand, additional interesting flexible coordination properties (mainly due to the presence of three tetrahedrally distributed oxygen atoms) and, on the other, can help to stabilise the isolated structures due to extra coordinative bonds with neighbouring
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O
HO
N
P OH
117
CO2H
CO2H
Scheme 2. N-(Phosphonomethyl)iminodiacetic acid (H4pmida).
metal centres. Searches in the literature and in the Cambridge Structural Database [32,33] produced just a handful of structures which incorporate this organic molecule: Na4[V2O2(pmida)2]$10H2O and Na8[V2O2(pmida)2]2$16H2O, both reported by Crans and collaborators [27]; [Co2(pmida)(H2O)5]$H2O, [Zn2(pmida)(CH3CO2H)]2$H2O and several zirconium derivatives by Clearfield and co-workers [35,36]; {K2[CoO3(pmida)]}6$xH2O [37]; [Pb2(pmida)]$1.5H2O [38]; [Co2(pmida)(H2O)5(4,4#-bpy)]$14H2O [39]; [Cu(H2pmida)(phen)]$3H2O (where phen ¼ 1,10-phenanthroline) [40]; [Mn(H2pmida)(H2O)] [41]. Partially deprotonated residues of N-(phosphonomethyl)iminodiacetic acid, Hpmida3, with the hydrogen atom located in the phosphonate group, were used to isolate discrete binuclear anionic centrosymmetric [M2(Hpmida)2(pyr)(H2O)2]2 moieties (Fig. 1; where M2þ ¼ Co2þ and Ni2þ; pyr stands for pyrazine), which close pack with well-known 1D cationic [M(pyr)(H2O)4]2nþ coordination polymers [26]. Magnetic measurements on these materials n show that the decrease of cT with decreasing temperature deviates from simple paramagnetic Curie-like behaviour, with this being attributed to crystal field effects due to the influence of the different ligands in the various M2þ coordination environments [26]. More recently, we have described the first 1D neutral coordination polymer containing Fe2þ metal centres and bridging Hpmida3 organic ligands (Fig. 2), [Fe(Hpmida)(H2O)2], in which the repetition of
Fig. 1. Schematic representation of the anionic [M2(Hpmida)2(pyr)(H2O)2]2 moieties present in the crystal structures of [M(pyr)(H2O)4][M2(Hpmida)2(pyr)(H2O)2]$2(H2O) (where M2þ ¼ Co2þ and Ni2þ; pyr stands for pyrazine). For further details see reference [26].
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Fig. 2. Schematic representation of the 1D coordination polymer [Fe(Hpmida)(H2O)2] running along the [0 0 1] direction of the unit cell. For further details see reference [25].
the {Fe(Hpmida)(H2O)2} hybrid monomer along the [0 0 1] direction is assured by the proton˚ [25]. ated phosphonate groups which impose a physical Fe/Fe separation of ca. 6.32 A 3.3. [V2O2(pmida)2]4: a robust building block The structures reported by Crans and co-workers [27] contain a centrosymmetric dimeric [V2O2(pmida)2]4 anionic unit in which pmida4 traps vanadium(IV) centres inside three five-membered rings formed by the two carboxylate groups and the phosphonate group, thus ˚ (Fig. 3). In that report, imposing a spatial separation between the metal centres of ca. 5.22 A [V2O2(pmida)2]4 was isolated in the solid state while crystallising with Naþ cations and several water molecules of crystallisation, which led us to believe that if these counterions could be replaced at the synthesis stage by transition metal centres (such as Mn2þ, Co2þ, Cd2þ), the anionic unit could then be used as a rigid building block to construct multi-dimensional
Fig. 3. Mixed polyhedral and ball-and-stick representations of the dimeric anionic centrosymmetric [V2O2(pmida)2]4 moieties as reported by Crans and co-workers [27].
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coordination polymers. Indeed, such a strategy would, on the one hand, reduce the probability of occurrence of supramolecular isomerism [7] and, on the other, the frameworks would have a typical modular nature, known to facilitate the 3D self-assembly and crystallisation of multidimensional coordination polymers [13]. Neutral centrosymmetric [M2V2O2(pmida)2(H2O)10] species were isolated (where M2þ ¼ Mn2þ, Co2þ or Cd2þ) [21], revealing that just as in Na4[V2O2(pmida)2]$10H2O and Na8[V2O2(pmida)2]2$16H2O [27], the transition metal centres also preferentially coordinate to the exo-PeO bonds of the phosphonate groups (Fig. 4). The presence of five water molecules coordinated to the transition metal cations and axially positioned relative to the core of the [M2V2O2(pmida)2(H2O)10] species, led us to believe that they could be labile if bridging organic ligands were included in the reactive mixtures. The inclusion of 4,4#-bpy ditopic bridging organic ligands directed the formation of the first 3D modular frameworks containing [V2O2(pmida)2]4 moieties [21], [CdVO(pmida)(4,4#bpy)(H2O)2]$(4,4#-bpy)0.5$(H2O) and [CoVO(pmida)(4,4#-bpy)(H2O)2]$(4,4#-bpy)0.5 (Fig. 5), with the extra water molecule of crystallisation present in the former structure being the only difference between the two materials. To achieve high dimensionality, the carboxylate groups of the [V2O2(pmida)2]4 moieties need to be involved in coordinative bonds with the transition metal centres of neighbouring [M2V2O2(pmida)2(H2O)4] species, leading to undulated [MVO(pmida)(H2O)2] plane nets placed in the bc plane (Fig. 6a; M2þ ¼ Cd2þ and Co2þ). Ditopic 4,4#-bpy molecules are axially coordinated to the transition metal centres (Fig. 6b), establishing the links between adjacent [MVO(pmida)(H2O)2] layers (Fig. 5b) and ultimately leading to the 3D porous structures previously mentioned (Fig. 5a). However, the gap between layers imposed by the 4,4#-bpy bridge is such that the 1D channel system running along the [1 0 0] direction is fully blocked by the presence of uncoordinated 4,4#-bpy molecules (Fig. 5a) [21]. The use of smaller bridging organic molecules would, in principle, lead to very similar frameworks but with narrower channels. By replacing the 4,4#-bipyridine organic molecules
Fig. 4. Mixed polyhedral and ball-and-stick representations of the neutral centrosymmetric [M2V2O2(pmida)2(H2O)10] moieties (where M2þ ¼ Mn2þ, Co2þ or Cd2þ). For further details see reference [21].
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Fig. 5. Mixed polyhedral and ball-and-stick representations of the crystal packing of [CdVO(pmida)(4,4#bpy)(H2O)2]$(4,4#-bpy)0.5$(H2O) and [CoVO(pmida)(4,4#-bpy)(H2O)2]$(4,4#-bpy)0.5, viewed along the (a) [1 0 0] and (b) [0 0 1] directions of the unit cell. In (b) the bridging 4,4#-bpy organic molecules are represented as blue pillars between [MVO(pmida)(H2O)2] layers (where M2þ ¼ Cd2þ and Co2þ). For more details see reference [21].
for pyrazine (pyr), a novel material was synthesised and formulated as [Co(H2O)6][CoV2O2 (pmida)2(pyr)(H2O)2]$2(H2O) (Fig. 7) [24]. As in the previous frameworks, the anionic binuclear [V2O2(pmida)2]4 building unit appears intact in the crystal structure and directly connected to the Co2þ metal centres via the exo-PeO bonds, establishing direct links between adjacent [Co(pyr)(H2O)2]2nþ cationic polymers which run along the [1 0 0] direction n (Fig. 7a). Such an arrangement results in the formation of a 2D anionic [CoV2O2(pmida)2(pyr) (H2O)2]2n layer placed in the ab plane which alternates along the [0 0 1] direction with hexan aquocobalt(II) complexes and water molecules of crystallisation (Fig. 7b) [24]. The structural features observed in [Co(H2O)6][CoV2O2(pmida)2(pyr)(H2O)2]$2(H2O) appear to originate in a deficiency of pyrazine ligand, thus resulting in excess of free [Co(H2O)6]2þ cations within the reactive gel. By increasing the mole ratio of pyrazine, a novel
Fig. 6. Schematic representation of the (a) undulated [MVO(pmida)(H2O)2] plane net placed in the bc plane (where M2þ ¼ Cd2þ and Co2þ), and (b) their connection with adjacent layers via the ditopic bridging organic 4,4#-bpy molecules (represented and blue rods). For more details see reference [21].
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Fig. 7. Mixed polyhedral and ball-and-stick representations of the (a) 2D anionic [CoV2O2(pmida)2(pyr)(H2O)2]2n layer n and (b) crystal packing viewed along the [1 0 0] direction of [Co(H2O)6][CoV2O2(pmida)2(pyr)(H2O)2]$2(H2O). For more details see reference [24].
3D framework-type material was isolated in large quantities directly from the autoclave contents and formulated as [Cd2V2O2(pmida)2(pyr)2(H2O)4]$4(H2O) by single-crystal X-ray diffraction (Fig. 8) [23], with a topology identical to that of the dense silicate moganite (by taking the Cd2þ cations and the centre of mass of [V2O2(pmida)2]4 as the nodes of the framework). As expected, the smaller distance between N-donor atoms from pyrazine favoured the formation of a channel system with a smaller cross-section (elongated six-membered
Fig. 8. Mixed polyhedral and ball-and-stick representation of the crystal structure of [Cd2V2O2(pmida)2(pyr)2 (H2O)4]$4(H2O) viewed in perspective along the [1 0 0] direction of the unit cell [23]. The highly disordered water molecules present within the elongated 6-membered rings have been omitted for clarity.
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apertures), but also running along the [1 0 0] direction of the unit cell. These channels contain water molecules moderately hydrogen-bonded to the framework (not shown in Fig. 8), which were found to be highly disordered. Magnetic measurements revealed that the susceptibility of the compound could be reasonably well fitted throughout the whole temperature range assuming an antiferromagnetic interaction within the [V2O2(pmida)2]4 moieties, with no inter-dimer interaction (Fig. 9). The intra-dimer magnetic exchange parameter J/kB was fitted to 17 K, and the magnetic moment of vanadium was calculated as 1.71 mB, which is in good agreement with the expected value (1.73 mB). 3.4. [Ge2(pmida)2(OH)2]2: a NMR-friendly potential building block Even though MOFs are isolated as solids, to date chemists have completely relied on X-ray diffraction methods (particularly single crystal techniques) to investigate the detailed structural aspects of such materials. This situation contrasts with the research done on both organic and inorganic materials, for which a substantial amount of important chemical information has been gathered by using advanced high-resolution NMR. However, in order to start applying such techniques, the building blocks used for the construction of coordination frameworks need to be amenable to NMR studies, i.e. must not contain paramagnetic centres (such as V4þ) and have available and useful nuclei such as 1H, 13C and 31P. Therefore, starting with the above mentioned [V2O2(pmida)2]4 building block as a model, we have recently engineered a closely related anionic moiety, [Ge2(pmida)2(OH)2]2 (Fig. 10), which was also isolated using hydrothermal approaches, (C4H12N2)[Ge2(pmida)2(OH)2]$4H2O (where C4H12N2þ 2 ¼ piperazinium cations) [28]. This potential building block was studied using several one- and two-dimensional high-resolution solid-state NMR techniques, some specifically designed to improve resolution of the 1H resonances [28]. As demonstrated in Fig. 11, the 13C resonances attributed to [Ge2(pmida)2(OH)2]2 are well resolved, thus producing what can be seen as a ‘‘fingerprint’’ of the moiety. If a new multi-dimensional coordination polymer is to be isolated from this
Fig. 9. Magnetic susceptibility of powdered [Cd2V2O2(pmida)2(pyr)2(H2O)4]$4(H2O) collected at 100 Oe and fitted with a model assuming dimmer magnetic interactions between V4þ centres.
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Fig. 10. Mixed polyhedral and ball-and-stick representation of the binuclear anionic [Ge2(pmida)2(OH)2]2 complex. For further details see reference [28].
building block and no single crystals are available, the detailed solid-state NMR information contained in reference [28] will be important to probe structural aspects of the isolated material. 4. Conclusions The various synthetic procedures, which are readily available and lead to coordination polymers, offer nowadays a great degree of freedom to the materials researcher. Indeed, the versatility in the self-assembly of the primary building blocks (i.e., organic ligands and metal centres) is very high, with this being obviously reflected in the vast heterogeneity of types of materials reported to date. This should provide a valuable hint for how the field should evolve
Fig. 11. 13C{1H} RAMP-CP spectra of [Ge2(pmida)2(OH)2]2 with a contact time (CT) of (top) 2000 ms and (bottom) 20 ms, recorded at 100.62 MHz. Asterisks depict the spinning sidebands of the C]O groups. The polyhedral representation of the binuclear anionic [Ge2(pmida)2(OH)2]2 complex is provided in the top-right corner. For more details see reference [28].
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in the next few years: the use of simple and highly robust building blocks, which can give some degree of control over the three-dimensional self-assembly of a given coordination polymer, should be preferred over the sporadic ‘‘shake-and-bake’’. Functionalisation of the materials will then be easier and more controllable as, at first glance, it is only necessary to manipulate the ‘‘brick’’ of the framework. We are now systematically using the anionic [V2O2(pmida)2]4 building block to construct novel frameworks which simultaneously exhibit some degree of porosity and magnetic properties, either arising from the V4þ centres or from other transition metal cations structurally belonging to the frameworks. The use of other techniques to probe structural aspects of coordination polymers, such as solid-state NMR, is also being pursued.
Acknowledgements We are grateful to FEDER, POCTI (Portugal) and to the Portuguese Foundation for Science and Technology (FCT) for financial support and the postdoctoral and Ph.D. research grants Nos. SFRH/BPD/9309/2002 (to F.-N.S.) and SFRH/BD/13858/2003 (to L.M.).
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Optimised hydrothermal synthesis of multi-dimensional hybrid coordination polymers containing flexible organic ligands Filipe A. Almeida Paz a,*, Jo~ao Rocha a, Jacek Klinowski b, Tito Trindade a, Fa-Nian Shi a, Luı´s Mafra a b
a Department of Chemistry, University of Aveiro, CICECO, 3810-193 Aveiro, Portugal Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK
Abstract In this paper, we summarise our recent research interest in the hydrothermal synthesis and structural characterisation of multi-dimensional coordination polymers. The use of N-(phosphonomethyl)iminodiacetic acid (also referred to as H4pmida) in the literature as a versatile chelating organic ligand is briefly reviewed. This molecule plays an important role in the formation of centrosymmetric dimeric [V2O2(pmida)2]4 anionic units, which were first used by us as building blocks to construct novel coordination polymers. Starting with [V2O2(pmida)2]4 in solution, we have isolated [M2V2O2(pmida)2 (H2O)10] species (where M2þ ¼ Mn2þ, Co2þ or Cd2þ) via the hydrothermal synthetic approach, which were then employed for the construction of [CdVO(pmida)(4,4#-bpy)(H2O)2]$(4,4#-bpy)0.5$(H2O), [CoVO(pmida)(4,4#-bpy)(H2O)2]$(4,4#-bpy)0.5, [Co(H2O)6][CoV2O2(pmida)2(pyr)(H2O)2]$2(H2O) and [Cd2V2O2(pmida)2(pyr)2(H2O)4]$4(H2O) by the inclusion of bridging organic ligands in the reactive mixtures, such as pyrazine (pyr) and 4,4#-bipyridine (4,4#-bpy). These materials can contain channel systems, and exhibit magnetic behaviour, not only due to the V4þ centres but also to the transition metal centres which establish the links between neighbouring dimeric [V2O2(pmida)2]4 anionic units. A closely related anionic moiety, [Ge2(pmida)2(OH)2]2, was engineered to allow the study of such crystalline hybrid materials using one- and two-dimensional high-resolution solid-state NMR. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Hydrothermal synthesis; Metal-organic frameworks; Coordination polymers; Crystal engineering; Transition metal centres; PMIDA
* Corresponding author. E-mail address: [email protected] (F.A.A. Paz). 0079-6786/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.progsolidstchem.2005.11.033
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1. Introduction Chemistry is used by scientists to modify Nature in accordance with their wishes. With the control of the atom arrangement in individual molecules being now a reality in modern science, and with the most recent technological advances in methods for structural characterisation, researchers have turned their attention to the large supramolecular frameworks which, by their physical and chemical nature, constitute a wonderful intellectual and experimental challenge. In any supramolecular network, the topology and properties are simultaneously controlled by the chemical nature of the individual building blocks and by their relative orientation in the solid state. Research in this broad field of science, usually referred to as crystal engineering, started with the work of Desiraju [1] and Etter [2] on organic crystals assembled via (usually strong) hydrogen bonds. However, these interactions are only in the range of tens of kJ mol1, thus rendering the isolated materials susceptible to thermal decomposition at relatively low temperatures, or even to framework demolition by the action of an adequate solvent. At the beginning of the 1990s, a qualitative jump was made when Hoskins and Robson [3] started to mimic Nature from the most fundamental topological point of view. They used the much stronger and highly directional coordinative interactions (several hundreds of kJ mol1) to engineer the first multi-dimensional diamondoid-type metal-organic frameworks (known as MOF, coordination polymers or, more recently, as coordination frameworks). The volume of research in the field of coordination polymers is directly reflected in the exponential growth of the number of publications in international journals, which is at the moment well over two thousand (Scheme 1) [4]. This worldwide effort is motivated by the large variety of architectures, their interesting properties (high porosity, hosteguest exchange, gas storage, photoluminescence, catalysis, non-linear optical properties, clathration, photochromism, chirality and magnetic properties) and potential applications as functional materials. Several excellent reviews on the various topics related to coordination polymers are available [5e13].
Scheme 1. Number of scientific publications featuring ‘‘metal-organic framework’’, ‘‘coordination polymer’’ and/or ‘‘coordination framework’’ in their title, abstract or keywords. Source: ISI Web of Science [29].
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Following our interest in the design and synthesis of these novel coordination polymers [14e26], we wish to summarise our most recent efforts towards the rational temperaturecontrolled hydrothermal synthesis of modular materials with interesting architectures, obtained in high yields and as phase-pure samples directly from the autoclave. We focus our attention on the centrosymmetric dimeric [V2O2(pmida)2]4 anionic unit, originally reported by Crans and collaborators [27], in the formation of several multi-dimensional frameworks [21,23e26]. We also emphasise our research on a recently engineered anionic unit, [Ge2(pmida)2(OH)2]2, closely related to [V2O2(pmida)2]4, which has the great advantage of not containing paramagnetic V4þ centres [28]. 2. Optimisation of the hydrothermal synthesis ‘‘Hydrothermal’’ synthetic approaches to the isolation of novel materials tacitly refer to a reaction carried out in aqueous media at a temperature higher than 100 C and a pressure above 1 atm. In essence, this is inspired by Nature itself, as many minerals are formed under such conditions. Hydrothermal methods offer a number of advantages not attainable by other conventional synthetic methods. This is true, not only for the particular example of coordination polymers, but also for any other type of material. For example, such experimental conditions can, for many systems, overcome the high activation energies necessary to start nucleation of thermodynamically metastable phases. In the particular case of the synthesis of hybrid materials, the physical and chemical properties of water in the temperature and pressure ranges usually employed permit an increase of the solubility of heavily organic molecules. However, these methods are usually driven by fast kinetics of nucleation and crystal growth, leading in the vast majority of cases to poorly crystalline and insoluble powders. Exact knowledge of the structure of hybrid materials can only, so far, be fully achieved by X-ray crystallography, which requires good-quality single-crystals with manageable dimensions. This is the main reason why, until very recently, synthetic approaches such as (1) slow diffusion of the reactants into a polymeric matrix, (2) diffusion from the gas phase, (3) evaporation of the solvent at ambient or reduced temperatures, (4) precipitation or recrystallisation from a mixture of solvents, and (5) temperature controlled cooling, have been preferred over the hydrothermal synthetic approach. To overcome the limitations concerning the isolation of good-quality single-crystals of hybrid materials, special hydrothermal experimental conditions for each reactive system must be applied, which implies optimisation of the process involving variables such as the composition of the reactive mixture (including the starting chemicals and their relative mole ratios) and the temperature profile used for the synthesis [20]. The latter is rather complex, as it includes several other factors such as the duration of the synthesis, the temperature employed and the cooling process of the reaction vessels [20]. We first reported such optimisation for the three-fold interpenetrated, 3D and modular [Cd(BPhDC)(BPE)]$(H2O) compound (where H2BPhDC is biphenyl-4,4#-dicarboxylic acid, and BPE is 1,2-bis-(4-pyridyl)ethane) [14]. Later, a much more detailed study of the hydrothermal conditions needed to isolate the 3D [Cd(NDC)(H2O)] diamondoid-type material was reported (where H2NDC is 2,6-naphtalenedicarboxylic acid) [20]. It was shown that: (1) ‘‘mild’’ hydrothermal conditions (i.e. temperatures ca. 150 C) and slow cooling of the autoclaves usually lead to highly crystalline hybrid materials; (2) the usual quenching of the reaction vessel usually leads to microcrystalline materials; (3) a systematic change in the composition of the reactive mixture is of crucial importance for isolating an optimal chemical range which leads to a phase-pure [Cd(NDC)(H2O)] material. More recently, and using the same organic ligand
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with Yb3þ cations to produce a 3D pseudo-five-connected network, [Yb2(NDC)3(H2O)]$(H2O)2 (4455 topology), we have further demonstrated that a fine-tuning of the optimisation process can significantly reduce the synthesis time for large quantities of a phase-pure material to just about 4 h [18]. 3. Multi-dimensional coordination polymers 3.1. Organic building blocks: flexibility versus rigidity Studies undertaken to optimise the hydrothermal synthetic approach were, in the vast majority of the cases, based on the use of highly rigid ditopic organic ligands such as H2BPhDC and/ or H2NDC (see above) [14,18,20]. Indeed, the reduction of conformational flexibility of the organic ligands tends to lead preferentially to the formation of a particular network for a given chemical composition. When organic ligands start to exhibit a number of degrees of flexibility, supramolecular isomerism can occur [7], which in most cases directs interesting novel topologies. We have shown that the system formed by 1,3,5-benzenetricarboxylic acid (H3BTC), Cd2þ cations with BPE or 4,4#-trimethylenedipyridine (TMD) can lead to 1 or 3 frameworks, respectively, when the flexibility of the bipyridine derivative is increased from two to three methylene groups between the 4-pyridyl aromatic rings [30,31]. Multidentate chelating organic ligands are much less commonly used in the assembly of multi-dimensional coordination polymers. Indeed, chelating and highly flexible ‘‘arms’’ lead preferentially to the formation of discrete aggregates due to the elimination of all coordination sites of metal centres, as revealed by a search in the Cambridge Structural Database [32,33]. However, by controlling the degree of chelation it is theoretically possible to exert some control over the dimensionality of the isolated frameworks. We have used diethylenetriaminepentacetic (H5DTPA) and nitrilotriacetic acids (H3NTA) as precursors for large and highly flexible multidentate organic ligands which incorporate several carboxylic acid groups and central N-donor atoms. When used in conjunction with Cd2þ cations and 4,4#-bipyridine (4,4#-bpy) in the presence of TEA, three novel compounds were produced [15,19]. We concluded that the multidentate nature of these two flexible organic ligands seems to prevent the formation of coordination polymers with high dimensionality (i.e. 2D and 3D) [15,19]. Indeed, only with H3NTA a metastable 2D porous structure was successfully isolated, [Cd4(4,4#-bpy)5(NTA)2 (H2O)4]$(NO3)2$(H2O)x, which proved to be highly unstable and converted into a 1D chain, [Cd4(4,4#-bpy)4(NTA)2(H2O)10]$(4,4#-bpy)$(NO3)2$(H2O)8 [19], structurally very similar to the compound with H5DTPA residues, [Cd4(4,4#-bpy)3(HDTPA)2(H2O)4]$14(H2O) [15]. Moreover, the protonation level of H5DTPA seems to depend on the amount of TEA added to the reactive mixture. 3.2. N-(Phosphonomethyl)iminodiacetic acid: a versatile chelating organic ligand N-(Phosphonomethyl)iminodiacetic acid (H4pmida) is an intermediate in the important process of glyphosate production, a herbicide in worldwide use [34]. This organic molecule is a derivate of nitrilotriacetic acid (H3NTA), with two carboxylic acid groups and a phosphonate group at the end of the third chelating arm (Scheme 2). The presence of the phosphonate functional group can confer, on the one hand, additional interesting flexible coordination properties (mainly due to the presence of three tetrahedrally distributed oxygen atoms) and, on the other, can help to stabilise the isolated structures due to extra coordinative bonds with neighbouring
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O
HO
N
P OH
117
CO2H
CO2H
Scheme 2. N-(Phosphonomethyl)iminodiacetic acid (H4pmida).
metal centres. Searches in the literature and in the Cambridge Structural Database [32,33] produced just a handful of structures which incorporate this organic molecule: Na4[V2O2(pmida)2]$10H2O and Na8[V2O2(pmida)2]2$16H2O, both reported by Crans and collaborators [27]; [Co2(pmida)(H2O)5]$H2O, [Zn2(pmida)(CH3CO2H)]2$H2O and several zirconium derivatives by Clearfield and co-workers [35,36]; {K2[CoO3(pmida)]}6$xH2O [37]; [Pb2(pmida)]$1.5H2O [38]; [Co2(pmida)(H2O)5(4,4#-bpy)]$14H2O [39]; [Cu(H2pmida)(phen)]$3H2O (where phen ¼ 1,10-phenanthroline) [40]; [Mn(H2pmida)(H2O)] [41]. Partially deprotonated residues of N-(phosphonomethyl)iminodiacetic acid, Hpmida3, with the hydrogen atom located in the phosphonate group, were used to isolate discrete binuclear anionic centrosymmetric [M2(Hpmida)2(pyr)(H2O)2]2 moieties (Fig. 1; where M2þ ¼ Co2þ and Ni2þ; pyr stands for pyrazine), which close pack with well-known 1D cationic [M(pyr)(H2O)4]2nþ coordination polymers [26]. Magnetic measurements on these materials n show that the decrease of cT with decreasing temperature deviates from simple paramagnetic Curie-like behaviour, with this being attributed to crystal field effects due to the influence of the different ligands in the various M2þ coordination environments [26]. More recently, we have described the first 1D neutral coordination polymer containing Fe2þ metal centres and bridging Hpmida3 organic ligands (Fig. 2), [Fe(Hpmida)(H2O)2], in which the repetition of
Fig. 1. Schematic representation of the anionic [M2(Hpmida)2(pyr)(H2O)2]2 moieties present in the crystal structures of [M(pyr)(H2O)4][M2(Hpmida)2(pyr)(H2O)2]$2(H2O) (where M2þ ¼ Co2þ and Ni2þ; pyr stands for pyrazine). For further details see reference [26].
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Fig. 2. Schematic representation of the 1D coordination polymer [Fe(Hpmida)(H2O)2] running along the [0 0 1] direction of the unit cell. For further details see reference [25].
the {Fe(Hpmida)(H2O)2} hybrid monomer along the [0 0 1] direction is assured by the proton˚ [25]. ated phosphonate groups which impose a physical Fe/Fe separation of ca. 6.32 A 3.3. [V2O2(pmida)2]4: a robust building block The structures reported by Crans and co-workers [27] contain a centrosymmetric dimeric [V2O2(pmida)2]4 anionic unit in which pmida4 traps vanadium(IV) centres inside three five-membered rings formed by the two carboxylate groups and the phosphonate group, thus ˚ (Fig. 3). In that report, imposing a spatial separation between the metal centres of ca. 5.22 A [V2O2(pmida)2]4 was isolated in the solid state while crystallising with Naþ cations and several water molecules of crystallisation, which led us to believe that if these counterions could be replaced at the synthesis stage by transition metal centres (such as Mn2þ, Co2þ, Cd2þ), the anionic unit could then be used as a rigid building block to construct multi-dimensional
Fig. 3. Mixed polyhedral and ball-and-stick representations of the dimeric anionic centrosymmetric [V2O2(pmida)2]4 moieties as reported by Crans and co-workers [27].
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coordination polymers. Indeed, such a strategy would, on the one hand, reduce the probability of occurrence of supramolecular isomerism [7] and, on the other, the frameworks would have a typical modular nature, known to facilitate the 3D self-assembly and crystallisation of multidimensional coordination polymers [13]. Neutral centrosymmetric [M2V2O2(pmida)2(H2O)10] species were isolated (where M2þ ¼ Mn2þ, Co2þ or Cd2þ) [21], revealing that just as in Na4[V2O2(pmida)2]$10H2O and Na8[V2O2(pmida)2]2$16H2O [27], the transition metal centres also preferentially coordinate to the exo-PeO bonds of the phosphonate groups (Fig. 4). The presence of five water molecules coordinated to the transition metal cations and axially positioned relative to the core of the [M2V2O2(pmida)2(H2O)10] species, led us to believe that they could be labile if bridging organic ligands were included in the reactive mixtures. The inclusion of 4,4#-bpy ditopic bridging organic ligands directed the formation of the first 3D modular frameworks containing [V2O2(pmida)2]4 moieties [21], [CdVO(pmida)(4,4#bpy)(H2O)2]$(4,4#-bpy)0.5$(H2O) and [CoVO(pmida)(4,4#-bpy)(H2O)2]$(4,4#-bpy)0.5 (Fig. 5), with the extra water molecule of crystallisation present in the former structure being the only difference between the two materials. To achieve high dimensionality, the carboxylate groups of the [V2O2(pmida)2]4 moieties need to be involved in coordinative bonds with the transition metal centres of neighbouring [M2V2O2(pmida)2(H2O)4] species, leading to undulated [MVO(pmida)(H2O)2] plane nets placed in the bc plane (Fig. 6a; M2þ ¼ Cd2þ and Co2þ). Ditopic 4,4#-bpy molecules are axially coordinated to the transition metal centres (Fig. 6b), establishing the links between adjacent [MVO(pmida)(H2O)2] layers (Fig. 5b) and ultimately leading to the 3D porous structures previously mentioned (Fig. 5a). However, the gap between layers imposed by the 4,4#-bpy bridge is such that the 1D channel system running along the [1 0 0] direction is fully blocked by the presence of uncoordinated 4,4#-bpy molecules (Fig. 5a) [21]. The use of smaller bridging organic molecules would, in principle, lead to very similar frameworks but with narrower channels. By replacing the 4,4#-bipyridine organic molecules
Fig. 4. Mixed polyhedral and ball-and-stick representations of the neutral centrosymmetric [M2V2O2(pmida)2(H2O)10] moieties (where M2þ ¼ Mn2þ, Co2þ or Cd2þ). For further details see reference [21].
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Fig. 5. Mixed polyhedral and ball-and-stick representations of the crystal packing of [CdVO(pmida)(4,4#bpy)(H2O)2]$(4,4#-bpy)0.5$(H2O) and [CoVO(pmida)(4,4#-bpy)(H2O)2]$(4,4#-bpy)0.5, viewed along the (a) [1 0 0] and (b) [0 0 1] directions of the unit cell. In (b) the bridging 4,4#-bpy organic molecules are represented as blue pillars between [MVO(pmida)(H2O)2] layers (where M2þ ¼ Cd2þ and Co2þ). For more details see reference [21].
for pyrazine (pyr), a novel material was synthesised and formulated as [Co(H2O)6][CoV2O2 (pmida)2(pyr)(H2O)2]$2(H2O) (Fig. 7) [24]. As in the previous frameworks, the anionic binuclear [V2O2(pmida)2]4 building unit appears intact in the crystal structure and directly connected to the Co2þ metal centres via the exo-PeO bonds, establishing direct links between adjacent [Co(pyr)(H2O)2]2nþ cationic polymers which run along the [1 0 0] direction n (Fig. 7a). Such an arrangement results in the formation of a 2D anionic [CoV2O2(pmida)2(pyr) (H2O)2]2n layer placed in the ab plane which alternates along the [0 0 1] direction with hexan aquocobalt(II) complexes and water molecules of crystallisation (Fig. 7b) [24]. The structural features observed in [Co(H2O)6][CoV2O2(pmida)2(pyr)(H2O)2]$2(H2O) appear to originate in a deficiency of pyrazine ligand, thus resulting in excess of free [Co(H2O)6]2þ cations within the reactive gel. By increasing the mole ratio of pyrazine, a novel
Fig. 6. Schematic representation of the (a) undulated [MVO(pmida)(H2O)2] plane net placed in the bc plane (where M2þ ¼ Cd2þ and Co2þ), and (b) their connection with adjacent layers via the ditopic bridging organic 4,4#-bpy molecules (represented and blue rods). For more details see reference [21].
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Fig. 7. Mixed polyhedral and ball-and-stick representations of the (a) 2D anionic [CoV2O2(pmida)2(pyr)(H2O)2]2n layer n and (b) crystal packing viewed along the [1 0 0] direction of [Co(H2O)6][CoV2O2(pmida)2(pyr)(H2O)2]$2(H2O). For more details see reference [24].
3D framework-type material was isolated in large quantities directly from the autoclave contents and formulated as [Cd2V2O2(pmida)2(pyr)2(H2O)4]$4(H2O) by single-crystal X-ray diffraction (Fig. 8) [23], with a topology identical to that of the dense silicate moganite (by taking the Cd2þ cations and the centre of mass of [V2O2(pmida)2]4 as the nodes of the framework). As expected, the smaller distance between N-donor atoms from pyrazine favoured the formation of a channel system with a smaller cross-section (elongated six-membered
Fig. 8. Mixed polyhedral and ball-and-stick representation of the crystal structure of [Cd2V2O2(pmida)2(pyr)2 (H2O)4]$4(H2O) viewed in perspective along the [1 0 0] direction of the unit cell [23]. The highly disordered water molecules present within the elongated 6-membered rings have been omitted for clarity.
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apertures), but also running along the [1 0 0] direction of the unit cell. These channels contain water molecules moderately hydrogen-bonded to the framework (not shown in Fig. 8), which were found to be highly disordered. Magnetic measurements revealed that the susceptibility of the compound could be reasonably well fitted throughout the whole temperature range assuming an antiferromagnetic interaction within the [V2O2(pmida)2]4 moieties, with no inter-dimer interaction (Fig. 9). The intra-dimer magnetic exchange parameter J/kB was fitted to 17 K, and the magnetic moment of vanadium was calculated as 1.71 mB, which is in good agreement with the expected value (1.73 mB). 3.4. [Ge2(pmida)2(OH)2]2: a NMR-friendly potential building block Even though MOFs are isolated as solids, to date chemists have completely relied on X-ray diffraction methods (particularly single crystal techniques) to investigate the detailed structural aspects of such materials. This situation contrasts with the research done on both organic and inorganic materials, for which a substantial amount of important chemical information has been gathered by using advanced high-resolution NMR. However, in order to start applying such techniques, the building blocks used for the construction of coordination frameworks need to be amenable to NMR studies, i.e. must not contain paramagnetic centres (such as V4þ) and have available and useful nuclei such as 1H, 13C and 31P. Therefore, starting with the above mentioned [V2O2(pmida)2]4 building block as a model, we have recently engineered a closely related anionic moiety, [Ge2(pmida)2(OH)2]2 (Fig. 10), which was also isolated using hydrothermal approaches, (C4H12N2)[Ge2(pmida)2(OH)2]$4H2O (where C4H12N2þ 2 ¼ piperazinium cations) [28]. This potential building block was studied using several one- and two-dimensional high-resolution solid-state NMR techniques, some specifically designed to improve resolution of the 1H resonances [28]. As demonstrated in Fig. 11, the 13C resonances attributed to [Ge2(pmida)2(OH)2]2 are well resolved, thus producing what can be seen as a ‘‘fingerprint’’ of the moiety. If a new multi-dimensional coordination polymer is to be isolated from this
Fig. 9. Magnetic susceptibility of powdered [Cd2V2O2(pmida)2(pyr)2(H2O)4]$4(H2O) collected at 100 Oe and fitted with a model assuming dimmer magnetic interactions between V4þ centres.
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Fig. 10. Mixed polyhedral and ball-and-stick representation of the binuclear anionic [Ge2(pmida)2(OH)2]2 complex. For further details see reference [28].
building block and no single crystals are available, the detailed solid-state NMR information contained in reference [28] will be important to probe structural aspects of the isolated material. 4. Conclusions The various synthetic procedures, which are readily available and lead to coordination polymers, offer nowadays a great degree of freedom to the materials researcher. Indeed, the versatility in the self-assembly of the primary building blocks (i.e., organic ligands and metal centres) is very high, with this being obviously reflected in the vast heterogeneity of types of materials reported to date. This should provide a valuable hint for how the field should evolve
Fig. 11. 13C{1H} RAMP-CP spectra of [Ge2(pmida)2(OH)2]2 with a contact time (CT) of (top) 2000 ms and (bottom) 20 ms, recorded at 100.62 MHz. Asterisks depict the spinning sidebands of the C]O groups. The polyhedral representation of the binuclear anionic [Ge2(pmida)2(OH)2]2 complex is provided in the top-right corner. For more details see reference [28].
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in the next few years: the use of simple and highly robust building blocks, which can give some degree of control over the three-dimensional self-assembly of a given coordination polymer, should be preferred over the sporadic ‘‘shake-and-bake’’. Functionalisation of the materials will then be easier and more controllable as, at first glance, it is only necessary to manipulate the ‘‘brick’’ of the framework. We are now systematically using the anionic [V2O2(pmida)2]4 building block to construct novel frameworks which simultaneously exhibit some degree of porosity and magnetic properties, either arising from the V4þ centres or from other transition metal cations structurally belonging to the frameworks. The use of other techniques to probe structural aspects of coordination polymers, such as solid-state NMR, is also being pursued.
Acknowledgements We are grateful to FEDER, POCTI (Portugal) and to the Portuguese Foundation for Science and Technology (FCT) for financial support and the postdoctoral and Ph.D. research grants Nos. SFRH/BPD/9309/2002 (to F.-N.S.) and SFRH/BD/13858/2003 (to L.M.).
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