Insights into the Molecular Assembly of Zeolitic Imidazolate

Article pubs.acs.org/cm

Insights into the Molecular Assembly of Zeolitic Imidazolate Frameworks by ESI-MS Ivy H. Lim, Wolfgang Schrader, and Ferdi Schüth* Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr, North Rhine-Westphalia, Germany S Supporting Information *

ABSTRACT: ZIFs are an interesting class of MOFs with zeolite-like topologies, owing to their structural similarities. Rational design and synthesis of ZIFs are highly challenging due to limited understanding of their formation process from solution, as previous studies were largely on crystal growth and properties. In this contribution, we describe a systematic approach to synthesize well-defined ZIF-8 crystals at different rates of nucleation by adjusting zinc to 2-methylimidazole ratios. For the first time, we report discrete chemical species detected in ZIF-8 synthesis and trace their transition with time using ESI-MS. Cravillon et al. have previously described three essential steps in ZIF-8 nucleation: (i) complex formation, (ii) complex deprotonation, and (iii) ligand exchange and oligomerization. In our work, we were able to identify species undergoing the various steps and correlate their evolution to the different nucleation rates. Applying ESI-MS in both positive and negative modes, we identified two species, “[Zn4(C4N2H5)4(C4N2H6)2(NO3)3]+” and “[Zn4(C4N2H5)5(C4N2H6)5(NO3)4(CH3OH)]−”, with changes in intensities corresponding to onset of nucleation processes. These species could potentially form the four-membered ring which is a basic structural unit in the sodalite framework. The usefulness of ESI-MS in studying ZIF formation is further demonstrated by the successful application of this technique to probe ZIF-67 syntheses.

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INTRODUCTION Metal−organic frameworks (MOFs), often also called porous coordination polymers, are an exciting class of highly porous materials with record surface areas and porosities.1−5 Within this material class, zeolitic imidazolate frameworks (ZIF)6−8 are a prominent family. ZIFs are built up from Me(Imi) 4 tetrahedra, formed by the coordination between Me2+ cations (for instance Zn2+ or Co2+) and imidazolate (Imi) anions where the imidazolate units form the bridges between the metal centers. The angle between adjacent tetrahedra (Me-Imi-Me) in ZIFs is similar to the Si−O−Si bond angle (145°) found in many zeolites. It thus comes as no surprise that a substantial number of ZIF materials, which were discovered via conventional6 and high-throughput methodologies,9 have zeolite-like topologies. In contrast to many MOFs, ZIFs possess excellent thermal and chemical stabilities.6 ZIF-8 (zinc 2-methylimidazolate or Zn(Imi)2), in particular, was described to have even higher hydrothermal stability than ordered MCM and SBA type mesoporous silica materials.6,10 Thus, ZIF-8 has attracted substantial interest for applications in areas such as gas storage or separation and catalysis.11 The design and synthesis of ZIFs are mainly based on the coordination chemistry of the metal centers and the ligands, with somewhat greater flexibility and controllability than © 2015 American Chemical Society

conventional zeolites, due to the possibility of specific ligand design. Nevertheless, the discovery of new ZIF materials is still to a large extent limited to trial and error, and the exact control of crystal structures and morphologies still remains challenging. It is therefore highly important to understand the formation process of such materials from solution in order to provide a rational basis for preparing well-defined ZIFs. The formation of ZIF-8 has been explored using a variety of different in situ techniques, such as X-ray diffraction (XRD),12 static light scattering,13 small-angle and wide-angle X-ray scattering (SAXS/WAXS),14 and ex-situ techniques, such as scanning electron microscopy (SEM)12,15 and transmission electron microscopy (TEM)16 with emphasis mainly placed on evaluating particle formation, sizes, structures, and morphologies. At the molecular level, in situ atomic force microscopy (AFM) was used to study the growth on ZIF-8 crystal surfaces. From the analysis of substep heights, the heterogeneous assembly process was found to involve simple monomeric species.17 Received: February 16, 2015 Revised: March 31, 2015 Published: April 15, 2015 3088

DOI: 10.1021/acs.chemmater.5b00614 Chem. Mater. 2015, 27, 3088−3095

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Chemistry of Materials

= 1:4), due to the relatively diluted system and the fact that methanol is a good solvent for ESI-MS, both facilitating the MS-analysis. Dynamic light scattering (DLS) analysis (see Figure 6) confirmed the rapid formation of monodispersed particles upon mixing the reagents which gradually increase in size, in line with reported data using in situ SAXS and WAXS techniques.14 TEM images of isolated ZIF-8 materials show uniform particles with average sizes of approximately 30−40 nm, again in line with values reported by Cravillon et al.14 Mass spectrometric analysis of the solutions proceeded without prior dilution to avoid possible interference with the dynamics of speciation, i.e. neat samples from the synthesis mixtures were introduced directly into the mass spectrometer. A representative spectrum, displaying the full range of positively charged molecular species detected immediately after mixing, is shown in Figure 1. The chemical compositions of these species were determined by comparing the experimental isotopic distributions with theoretical spectra (see inset in Figure 1).

To the best of our knowledge, so far there are no reports providing direct information on chemical composition of solution species during the formation of ZIFs and the correlation of these species to nucleation and growth of the crystals. However, insight into the speciation of solution species could yield valuable information which may provide a firmer basis for rational design and synthesis of ZIF materials. Electrospray ionization mass spectrometry (ESI-MS) is an interesting characterization method highly useful for gaining chemical information on speciation in solution. It has been applied to study MOF synthesis of metal carboxylates in a reaction mixture containing Mg(NO3)2·6H2O and (+)-camphoric acid (H2cam) where a key structural species with the composition [Mg2(Hcam)3]+ was detected.18 Cryospray mass spectrometry is a gentle ionization technique and has also been applied to probe prenucleating MOF synthesis mixtures of cis,trans-1,3,5-triaminocyclohexane with copper(II) salts.19 Key species that correlate to some of the structural units identified with extensive X-ray crystallographic studies were successfully characterized. These initial reports were largely focused on using MS as a complementary technique to confirm identified structural species. They have highlighted the potential of this method to investigate the mechanism of MOF formation. We have previously developed a systematic approach to monitor solid state formation of zeolitic materials using ESIMS.20 In this contribution, we present the first application of this technique to understand the molecular assembly of ZIF-8 under well-defined synthetic conditions.

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EXPERIMENTAL SECTION

Preparation of ZIF-8 Crystals with Different Sizes. ZIF-8 were prepared by dissolving 734.4 mg of Zn(NO3)2·6H2O (99%, SigmaAldrich) and corresponding amounts of 2-methylimidazole (HImi, 99%, Aldrich) in 50 mL of methanol (MeOH, ≥99.9%, Sigma-Aldrich) each. The latter solution was then poured into the former under vigorous magnetic stirring to form solutions with Zn:HImi ratios of 2:1, 1:1, 3:4, 1:2, 1:4, and 1:8. The clear solutions were allowed to stand undisturbed for 24 h. ZIF-8 crystals were recovered by centrifugation followed by washing with MeOH and recentrifugation. The purification step was repeated three times, and the material was dried overnight at 50 °C. Preparation of ZIF-67. ZIF-67 was prepared using Co(NO3)2· 6H2O (≥98%,, Sigma-Aldrich), in a manner similar to ZIF-8, with Co:HImi of 2:1, 1:1, and 1:4. Analysis of ZIF-8 and ZIF-67 Synthetic Solutions. All ESI-MS and ESI-MS/MS experiments were performed on a Waters Q-Tof Ultima hybrid mass spectrometer combining a quadrupole and a timeof-flight analyzer. The instrument is equipped with a Z-spray source alignment operating with a capillary voltage of 3.5 kV, desolvation temperature of 150 °C, and a sample flow rate of 5 μL·min−1. The evolution of particle sizes in solution was determined by DLS using a Malvern Zetasizer Nano-ZS instrument using laser radiation of wavelength 633 nm and power of 4 mW. The scattered light was measured at a backscattering angle of 173°. Characterization Methods. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) micrographs of the purified solids were recorded using Hitachi HF2000 and Hitachi S5500 instruments, respectively. Powder X-ray diffraction (PXRD) measurements were performed on a STOE Stadi P transmission X-ray diffractometer using CuKα radiation.

Figure 1. Positive ion spectrum obtained with full range of species (Zn:HImi = 1:4, magnified 20 times from m/z 700−2000 for optimum display of speciation). The insert depicts confirmation of chemical composition for m/z 644 [Zn2(C4N2H6)4(NO3)3]+ detected by ESIMS (red: theoretical spectrum, black: experimental spectrum).

Ligand coordination to a Zn ion in solution can occur via lone pairs of pyridinic and deprotonated pyrrolic nitrogen atoms on Imi, the single oxygen atom of H2O or MeOH, and the three oxygen atoms in NO3−. Higher oligomers containing more than one Zn ion can therefore be achieved by bridging through bidentate imidazolate ions or via two oxygen atoms on a single NO3− ion. In the case of ZIF-8 synthesis, it is expected that all Zn ions are bridged by imidazolate linkers. The signal at a mass to charge ratio (m/z) 644 can be due to either the gas phase protonation of neutral species containing (2Zn2+ + C4N2H5− + 3C4N2H6 + 3NO3−) or to a species containing (2Zn2+ + 4C4N2H6 + 3NO3−) both with adjacent Zn ions linked by NO3−. The formation of the former species would involve a gas phase protonation of uncoordinated pyridinic nitrogen which is commonly observed in ESI-MS analyses.21 While coordination of a Zn ion with deprotonated pyrrolic nitrogen is relatively easy, it is unlikely to have the pyridinic nitrogen on the same imidazolate remaining uncoordinated. It would be more probable for coordination

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RESULTS AND DISCUSSION Analysis of Species during ZIF-8 Formation. The rapid room temperature synthesis of ZIF-8 in methanol, developed by Cravillon et al.,14 was adopted as a starting point (Zn:HImi 3089


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From the analysis of this system it is clear that various oligomeric species are present in solutions during ZIF-8 synthesis. However, under these initial conditions used, particles are formed rapidly upon mixing the reagents, in agreement with literature data where it is reported that nucleation and growth processes under these conditions proceed within 1 min.14 In order to monitor the development of species it is necessary to study systems involving a longer prenucleation phase. ZIF-8 Synthesis at Lower Crystallization Rate. Highly controlled crystallization of ZIF-8 is possible if the reactivity of the species is modulated with auxiliary monodentate ligands.13 However, these systems use solvents, such as dimethylformamide, and a second ligand which could greatly complicate ESIMS analysis. Size-controlled syntheses were reported for aqueous solutions by increasing the HImi content, resulting in smaller crystals.22−25 Most of these syntheses, though, employ a large excess of HImi or require additional bases (to avoid formation of other crystalline phases), resulting in rapid crystallization processes which makes them unsuitable for monitoring the development of solution species. There is a general trend that decreasing HImi content gives rise to slower nucleation kinetics, so that this appeared as a possibility for ESI-MS analysis during crystal formation. However, an in-depth study involving systematic syntheses of well-defined and monodispersed ZIF-8 at lower HImi content without using additional ligands has not been carried out so far. Thus, in order to identify a system undergoing relatively slow homogeneous nucleation and to simplify speciation in the methanolic solution, we systematically studied a range of Zn to HImi ratios. All samples isolated from the batches with different Zn/HImi ratios possess the well-defined sodalite (sod) topology, as confirmed by powder X-ray diffraction (PXRD) analysis (see Figure 4).

of imidazole to precede imidazolate formation by deprotonation. However, from tandem mass spectrometric (MS/MS) analysis of this parent ion (see Figure 2), one cannot effectively

Figure 2. MS/MS spectrum of parent ion with m/z 644 and molecular formula [Zn2(C4N2H6)4(NO3)3] in ZIF-8 synthesis. Δm/z of 82 and 190 denote a loss of C4N2H6 and Zn(NO3)2 fragments, respectively.

differentiate the two species. In order to shed more light on the mode of coordination and the origin of the charges, a 1:1 mixture of Zn(NO3)2 and 1-methylimidazole in methanol was prepared and directly analyzed with ESI-MS (see Figure 3).

Figure 3. Positive ion spectrum obtained with 1:1 mixture of Zn:1methylimidazole (magnified 5 times from m/z range 300−1000 for optimum display of speciation).

The monodentate 1-methylimidazole cannot act as bridging ligand, and upon coordination with Zn ion, it does not possess any available lone pair on the nitrogen atoms for gas phase protonation. This should suppress species with bridging imidazolate, and the net positive charge can only be contributed by the Zn ion. In the ESI-MS of such mixtures, only species containing up to two Zn ions were detected, albeit also these at low intensity. Since the monodentate 1-methylimidazole ligand does not contribute to the linkage of two Zn ions nor possesses nitrogen atoms which can be protonated, the four positive charges in this case are only compensated by three nitrate ions, so one positive charge is left and no gas-phase ionization would be required, and the species containing two Zn can be linked by an auxiliary NO3− ligand. The ease of monodentate NO3− coordination to Zn2+ ion accounts for the presence of solely single charge species observed throughout the analyses.

Figure 4. PXRD comparison of ZIF-8 materials prepared with different metal to ligand ratios (top to bottom: 1:8, 1:4, 1:2, 3:4, 1:1, and 2:1).

Increasing the concentration of HImi resulted in broadening of the reflections, indicating a decrease in crystal size, which is supported by TEM analysis (see Figure 5). This suggests that a lower rate of nucleation can be achieved at lower HImi content. At all ratios, monodispersed crystals were obtained, denoting the absence of secondary nucleation. From visual inspection, precipitation was evident within 5 min of mixing the reagents at Zn:HImi ratios of 3:4 and higher 3090


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At Zn:HImi of 2:1, first particles were only detected after about 27 min, and a slower rate of growth was observed as compared to the ratio of 1:1. However, the first particle detected is of a much bigger relative size. Particle formation at earlier synthesis times cannot be completely ruled out, since few smaller particles might not be detected due to very low concentration of nuclei formed during the initial phase. Nevertheless, from the DLS data it is evident that the rate of nucleation is in the order 1:4 > 1:1 > 2:1 which is supported by the final crystal sizes obtained (see Figure 7). The slower rate of

Figure 5. TEM images of ZIF-8 prepared with different metal to ligand ratios (left to right, top to bottom: 2:1, 1:1, 3:4, 1:2, 1:4, and 1:8).

content of HImi. At the ratio of 1:1 and 2:1, the onset of precipitation was delayed to about 10 and 50 min, respectively. Visual inspection only gives a first indication of the rate of crystal formation. In order to follow the process in more detail, dynamic light scattering (DLS) was performed on ZIF-8 synthesis batches with Zn:HImi of 2:1, 1:1, and 1:4 to detect the appearance of first particles and the change of their sizes with time. At Zn:HImi of 1:4, particles were detected already in the first measurement at 5 min, with excellent count rates per second (Figure 6), and they increase gradually in size. This indicates

Figure 7. SEM images of ZIF-8 crystals prepared with different Zn:HImi ratios (top to bottom: 2:1, 1:1, and 1:4).

nucleation at 2:1 results in fewer nuclei, which can correspondingly grow to larger, well-defined rhombic-dodecahedral ZIF-8 crystals. The very narrow particle size distribution of the crystals suggests the occurrence of a nucleation event followed by growth of the particles. If nucleation would extend over a longer time period, one would expect a less sharp particle size distribution, since crystals nucleated later would have less time for growth and thus would only grow to smaller size. This, together with the longer induction period at low HImi, shown from the DLS data, suggests that ESI-MS could be used for such systems to monitor species evolution leading to nucleation, as a clear reaction solution precedes first particle formation. ESI-MS as a Tool To Probe Solution Dynamics of ZIF-8 Synthesis Mixtures at Different Zn:HImi Ratios. Before the development of species during the crystallization for the slower systems at low HImi concentration is described, first the speciation in reaction mixtures with different Zn:HImi ratios shall be discussed. Figure 8 shows the relative intensities of different species containing only one Zn2+ ion 5 min after mixing. Increasing concentration of HImi corresponded directly to an overall increase in relative intensity and a higher extent of coordination of HImi to Zn2+, as would be expected for higher HImi concentration. With increasing HImi concentration, the fraction of Zn1 species without HImi or Imi ([Zn(NO3) + H2O)]+, denoted by

Figure 6. Graph of relative size against synthesis time at Zn:HImi ratios 1:4 (blue), 1:1 (red), and 2:1 (green) detected by DLS.

the presence of a high density of such particles. Nucleation and growth thus occurs at an early stage of the synthesis, in agreement with published data.14 Analysis for ratios of 1:1 and 2:1 required longer acquisition time to achieve reliable data, suggesting a lower density of particles formed. At 1:1, particles were detected already in the first measurement after 5 min. They increased substantially in size during the next 20 min, followed by a more gradual increase thereafter. 3091


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equilibrium is attained rapidly; after a reaction time of 5 min at which the first spectrum was recorded, almost no change in species distribution is observed. Corresponding to the high HImi concentration, species with up to four HImi ligands coordinated to the Zn center, “[Zn(HImi)4(NO3)]+” − albeit at low concentration −, are observed. At a Zn:HImi ratio of 1:1, complex formation is also rapid, the highest overall intensity of Zn1 species is observed already after 5 min. However, then the intensity of Zn1 species falls, which could be due to either deprotonation setting in (leading to uncharged species) and/or condensation reactions occurring, which would consume Zn1 species in favor of higher oligomers. As expected, at this ratio with a lower HImi concentration than in the 1:4 case, species are mainly made up of one or two ligands coordinated to Zn with a less significant amount involving three or four ligands. Further lowering the content of HImi results in a delayed increase in intensity of the primary complex, and slower intensity reduction, with the main species consisting of one or two ligands. In the case of Zn:HImi of 1:1 and 2:1, changes in intensities of uncoordinated Zn species (see Supporting Information Figure S1) are relatively insignificant, ruling out the possibility that the intensity decrease over time is due to the dissociation reaction (step (i) in Scheme 1). More insight into the formation of higher oligomers is obtained from the analysis of the intensity of deprotonated species with time (Figure 10). Such species are rapidly formed and consumed at Zn:HImi of 1:4 and 1:1. This suggests that dimerization and further reactions proceed relatively quickly, due to the high concentration of species containing the imidazolate which is able to bridge two Zn2+ ions. As observed before (Figure 9), deprotonated species form at a slower rate at Zn:HImi of 2:1, which in turn would also delay the oligomerization process. A direct observation of the oligomerization reaction is possible, if species containing more than one Zn2+ ion are analyzed. For the rapidly evolving system at Zn:HImi of 1:4, dimers are observed at the highest concentration already after 5 min (Figure 11), and their concentration diminishes with time, as nucleation and growth proceeds in the reaction mixture. They probably undergo further oligomerization to form nuclei. Alternatively, as crystals form − and thus effective concentrations of solution species decrease − they could also dissociate to form monomeric units for crystal growth. However, this appears to be less probable, since the concentration of monomeric species does not change much (Figures 9 and 10). The longer induction time observed at ratios of 1:1 and 2:1 is evident also in the development of dimeric species, as their intensities initially increase with time, indicating the initial first bridging of Zn2+ ions by an imidazolate linker, followed by more prominent depletion, caused by further oligomerization. Possible species involved in building up the framework of ZIF-8 should involve Zn centers interconnected by 2methylimidazolate ligands. These species were identified, and their intensities were plotted against reaction time for the ratio of 2:1 which required the longest induction time (see Figure 12). Molecular species containing up to four Zn units ([Zn4(C4N2H5)4(C4N2H6)2(NO3)3]+ denoted by ″[Zn4(μImi)4(HImi)2(NO3)3]+″ or ″[Zn4(μ-Imi)3(Imi)(HImi)2(NO3)3]+″ were detected, and their intensities decrease with time after the maximum at 20 min. The changes in intensity are likely due to the nucleation process, as the formation of particles occurs within a similar time span. The

Figure 8. Relative intensities of Zn1 species at various Zn:HImi ratios detected by ESI-MS during the first 5 min of reaction. Zn, HImi, Imi, and nitrate ions are denoted by “Zn”, “HImi”, “Imi”, and “NO3″, respectively.

″[Zn(NO 3 )(H2 O)″) is reduced, indicating a preferred coordination of HImi species. Moreover, in all cases, ZnHImi2NO3 (m/z 290, [Zn(C4N2H6)2(NO3)]+) was detected with high intensity. Since this species corresponds stoichiometrically to the final ZIF-8 material Zn(MeIm)2 in terms of metal and organic ligand content, it is not surprising that this is a major species in all the synthesis mixtures; one may consider it as the most basic unit for ZIF-8 formation. We then proceeded to analyze the temporal evolution of species at low HImi concentration where the reaction is sufficiently slow. Cravillon et al.15 have discussed the effects of modulating ligands on the nucleation process. For the reactions in solution they considered the deprotonation equilibrium in addition to the coordination equilibria, which combined to give three essential steps: (i) ligand coordination to form the basic complex, (ii) deprotonation of complexes, and (iii) exchange of ligands, resulting in dimers and larger oligomers (Scheme 1). Scheme 1. Basic Reactions Involved in ZIF-8 Formationa

a

HImi, Imi, and L denote HImi, Imi, and auxiliary ligands such as NO3‑, H2O, and MeOH. Adapted from ref 13.

These basic reactions are highly relevant to understanding the dynamics at different ratios of Zn to HImi, as excess HImi influences the coordination equilibria. Different solution pH at different HImi concentration in turn affects deprotonation and oligomerization process. ESI-MS should in principle allow the analysis of the development of relevant species with time. In Figure 9, the intensities of different species containing one Zn2+ ion at different times for the three Zn:HImi ratios of 1:4, 1:1, and 2:1 are plotted. At an excess of HImi (1:4), nucleation and growth is very rapid, as discussed above, and the coordination 3092


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Figure 9. Plot of intensity at various reaction times of species formed by coordination of HImi to one Zn at Zn:HImi of 1:4, 1:1, and 2:1 (left to right).

Figure 10. Plot of intensity at various reaction times of Zn1 species containing Imi ligand responsible for forming dimers at Zn:HImi of 1:4, 1:1, and 2.1 (left to right).

Figure 11. Plot of intensity at various reaction times of dimeric species formed by bridging of Imi ligand between two Zn at Zn:HImi of 1:4, 1:1, and 2.1 (left to right).

chemical composition of ([Zn4(C4N2H5)4(C4N2H6)2(NO3)3]+, which has its highest concentration at 5 min and then continuously decreases in intensity, is particularly interesting, as it can potentially form the four-membered ring which is a basic structural unit in the sodalite framework. In ZIF-8 synthesis, zinc can be coordinated tetrahedrally by up to four imidazolate ligands, which would lead to an overall negative charge. Such negatively charged species cannot be detected in the positive ESI-MS mode which has been discussed so far but in the ESI-MS negative mode. This was indeed observed, as shown in Figure 13c. Species with up to five Zn units were detected at increasing coordination of imidazolate/deprotonation of imidazole with time. For Zn:HImi of 2:1 the overall intensity of such oligomers increases to a maximum, followed by depletion after 70 min, supporting

the trends observed in positive mode ESI-MS suggesting a slower nucleation process than at high HImi concentration. In comparison, molecular species with up to seven Zn units were detected at Zn:HImi of 1:1 (see Figure 13b), which also deplete with time, but at higher rate. Since species with six or seven Zn units are not detected at Zn:HImi = 2:1, they may not be critical for the nucleation process, and their disappearance could just be due to the overall reduced effective concentrations in solution as crystal growth proceeds. At the ratio of 1:4 (see Figure 13a), oligomers containing up to seven Zn units were also observed. Due to the fast reactions at this ratio, no pronounced changes are observed, and the persistence of higher oligomers is probably related to the high concentration of bridging species which would favor oligomerization. It is noteworthy that species resulting from octahedral coordination 3093


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topology. Reported room temperature syntheses of ZIF-67 involve the use of additional bases24,26 or a large excess of ligands (cobalt to 2-methylimidazole, Co:HImi of 1:58).27 These indicate that the protonation/deprotonation equilibrium might be a key factor in ZIF-67 nucleation and growth. However, due to the success in the analysis of ZIF-8 formation, we carried out the synthesis of ZIF-67 in a similar manner as that of ZIF-8, i.e. at Co:HImi of 1:4, 1:1, and 1:2. Unlike ZIF-8 synthesis, though, ZIF-67 was only obtainable at the ratio of 1:4. The overall ESI-MS spectra after 20 min are shown in Figure 14. A transition of species is observed with increasing content

Figure 12. Plot of intensity at various reaction times of oligomers formed by linkage of Zn2+-ions by 2-methyl imidazolate ligands, detected by positive mode ESI-MS at Zn:HImi of 2:1.

of zinc were also detected in the negative ESI-MS mode. This is due to the presence of increasing content of free NO3− in solution as complex formation and oligomerization proceeds. Since these auxiliary ligands are not effective linkers, they can eventually be removed from the oligomer during the deprotonation process. At all investigated ratios, [Zn4(C4N2H5)5(C4N2H6)5(NO3)4 + CH3OH]− (denoted by ″[Zn4(μ-Imi)3(Imi)2(HImi)5(NO3)4(CH3OH)]−″) was detected which eventually depletes in all solutions at around the time at which particles form. This species consists of additional imidazole ligands on top of those required for bridging existing Zn units, allowing possibilities of ligand exchange and formation of larger oligomers. While this is certainly no proof, the fact is suggestive of a key role that such a species or related structures may have in the formation of ZIF8. ESI-MS allows detailed insight in the nature of the species in solution during crystal formation of metal organic framework materials, and this insight may help in better understanding the principles governing the formation of certain structures. In order to extend the method to other materials, we have also explored the formation of ZIF-67. This material is a cobalt based analogue of ZIF-8, and it also possesses the sodalite

Figure 14. Comparison of negative mode ESI-MS spectra taken during ZIF-67 synthesis after 20 min carried out at Co:HImi ratios of 1:4, 1:1, and 2:1 (from top to bottom), highlighting differences in speciation.

of HImi (from 2:1 to 1:4). At low content of HImi (2:1), the spectrum is dominated by species consisting of cobalt and nitrate ions, indicating the low degree of coordination with

Figure 13. Plot of intensity at various reaction times of possible oligomers formed by linkage of Imi ligand detected by negative mode ESI-MS at Zn:HImi of (a) 1:4, (b) 1:1, and (c) 2:1. Assignments of peaks were based on linear chain species (enlarged version see Figure S11). 3094


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HImi ligands. With increased content of HImi at 1:1, coordination sets in more readily. However, deprotonation is still not evident, and the prominent species contain Co units linked via nitrate ions. At sufficiently high content of HImi and basicity (1:4), deprotonation and condensation processes lead to cobalt imidazolate formation, giving rise to oligomers bridged by imidazolate units, which are essential for the formation of ZIF frameworks. This system certainly requires more intensive investigation for detailed insight in the formation process. The initial results, however, already suggest that also the ZIF-67 would be amenable to ESI-MS studies.

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CONCLUSION ESI-MS analysis during the formation of ZIF-8 has allowed detailed insight in the nature of the species present in solution during nucleation and crystallization. For reactions at low HImi concentration, which lead to relatively low crystallization rates and large, well developed ZIF-8 crystals, the temporal evolution of solution species could be well analyzed. The schematic processes suggested by Cravillon et al.14 to occur during ZIF-8 formation (Scheme 1) could be monitored directly, and several of the species postulated were observed in situ by ESI-MS during ZIF-8 formation. The temporal evolution of some species, such as the Zn4(C4N2H5)5(C4N2H6)5(NO3)4, is suggestive of an important role of this or related species in the nucleation process. The current data do not yet allow a firm conclusion in this respect, and more work is required for a detailed understanding of the elementary processes involved in solid state formation. Here, the combination of ESI-MS and ESI-MS/MS with additional techniques to corroborate and complement the results could be a powerful combination for the observation of such processes. Nevertheless, the establishment of the nature of the solution species during crystal formation is certainly a good starting point for further experimental and theoretical work.

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ASSOCIATED CONTENT

S Supporting Information *

Plot of uncoordinated Zn1 species with time, experimental, and theoretical spectra of “[Zn4(C4N2H5)4(C4N2H6)2(NO3)3]+” and “[Zn4(C4N2H5)5(C4N2H6)5(NO3)4(CH3OH)]−”, TEM, XRD of ZIF-67, and enlarged images of Figure 8−13 with extended Y-axis. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank Bernd Spliethoff and Hans-Josef Bongard from the electron microscopy department for their kind help with the TEM and SEM analyses.

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REFERENCES

(1) Stock, N.; Biswas, S. Chem. Rev. 2012, 112, 933. (2) Meek, S. T.; Greathouse, J. A.; Allendorf, M. D. Adv. Mater. 2011, 23, 249. (3) Furukawa, H.; Cordova, K. E.; O’Keeffe, M.; Yaghi, O. M. Science 2013, 341, 974. (4) Ferey, G. Chem. Soc. Rev. 2008, 37, 191. 3095


Insights into the Molecular Assembly of Zeolitic Imidazolate

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