Building Blocks for Two-Dimensional MetalâOrganic Frameworks

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Building Blocks for Two-Dimensional Metal-Organic Frameworks Confined at the Air-Water Interface: An Ab Initio Molecular Dynamics Study Ralph Koitz, Marcella Iannuzzi, and Juerg Hutter J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp510199k • Publication Date (Web): 04 Feb 2015 Downloaded from http://pubs.acs.org on February 9, 2015

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The Journal of Physical Chemistry

Building Blocks for Two-Dimensional Metal-Organic Frameworks Confined at the Air-Water Interface: An Ab Initio Molecular Dynamics Study Ralph Koitz, Marcella Iannuzzi,∗ and J¨urg Hutter Department of Chemistry, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland E-mail: [email protected],Phone:+41446354479

Abstract

Introduction

Two-dimensional molecular sheets are of prime interest in nanoscience and technology. A promising class of such materials is 2D metalorganic frameworks (MOFs), assembled by crosslinking precursors with metal ions. It was recently demonstrated that such MOFs can be synthesized from monomers confined at an air-water interface. In order to elucidate this process at the atomic scale, we study a large flat tris-terpyridine-derived molecule (TTPB) on a water surface using ab initio molecular dynamics. We investigate the properties of the molecule and examine its reaction with Zn ions from the liquid phase. The fluid substrate significantly stabilizes the adsorbate, while maintaining sufficient conformational flexibility to allow dynamic rearrangement and chemical reactions. The successful uptake and binding of ions is the first step towards linking TTPB molecules to dimers and large 2D MOFs.

Since the advent of graphene and related materials, two-dimensional sheet-like molecules have become the target of intense study. 1,2 Up to now, the large-scale fabrication of welldefined nanosheets has remained a challenge. A popular method to synthesize these materials is through the decomposition of precursor molecules on suitable surfaces. 3,4 However, some aspects of this approach limit its usefulness. Firstly, substrates are typically only reactive at the surface, leaving the bulk of the solid unutilized for the reaction. Secondly, the solid with its rigidly placed binding sites limits the conformational flexibility of adsorbed species and thus the range of accessible reaction products. 5,6 Thirdly, removing the final extended sheet from the support may be challenging. 7 Alternative approaches to preparing 2D sheets are thus highly desirable. A promising route towards preparing extended 2D coordination polymers or 2D metalorganic frameworks (MOFs) has recently been presented. 8,9 Suitable monomeric precursors can be confined on the surface of water, induced to form a dense layer by increasing lateral pressure, and linked with metal ions from the liquid phase to form large sheets. 8,9 This way the liquid-vapor interface of water is used as a “surface” on which the reaction is per-

Keywords Molecular Dynamics, Density Functional Theory, Water/Air Interface, Molecule Adsorption ∗

To whom correspondence should be addressed

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formed, while metal ions can be supplied from the bulk of the solution and used to assemble an extended sheet. The substrate can provide reactants from the bulk liquid by diffusion, lightly and dynamically stabilize adsorbates, and facilitate easy removal by drying. Therefore all three issues mentioned above are overcome. By systematically exploiting the advantages of liquids as a support for chemical reactions, it may be possible to prepare new materials or develop new routes towards relevant products such as tailored single-layer membranes, free-standing functional sheets and improved graphene analogs. To this end, we aim to gain insight into the relevant processes at the atomic scale using advanced computational methods. A number of recent simulation studies have explored the properties of water surfaces, 10–12 focusing particularly on hydrogen bonding at the interface, diffusivity, and ion solvation. Water slabs with adsorbed molecules are less widely studied, but some classical simulations of surfactants on water have been performed. 13 Self-assembly and reactions to produce large two-dimensional layers on solids constitute an important field in surface science have been exhaustively studied in recent years. 3,14–17 In contrast, the approach of using fluid surfaces to carry out reactions and produce extended sheets is currently largely unexamined, both experimentally and computationally. An exception to this has been the use of liquid interfaces for the formation of self-assembled monolayers, as well as the large field of surfactants and lipid bilayers. In these applications, however, no new bonds are formed and products are typically weakly bound aggregates. Previous investigations have focused either on photochemical processes involving molecules on water, 18,19 salts and ions at/near the surface, 20–22 and substance uptake across the gas/liquid interface. 23 Reactions such as the enzymatic cleavage of surfaceconfined esters have also been studied. 24 Large molecules “adsorbed” at the water surface and their reactions with solutes have not been examined thoroughly so far, particularly at the atomic scale. In this paper we study the atomic-scale de-

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tails of the first steps of the 2D MOF formation, aiming to rationalize the involved processes and guide future efforts. We use a large flat molecule, 1,3,5-tris(4’(2,2’:6’2”terpyridyl))benzene (TTPB), as a model monomer, and investigate its properties and reactivity on a water surface. We employ large-scale molecular dynamics (MD) simulations based on density functional theory (DFT). Our results provide insight into the unconventional approach of using liquid water as a surface and a solvent for reactions. In the first part of the article we focus on the dynamic structure of the molecule and subsequently its influence on the properties of the substrate. Then, we explore the reaction of surface-supported TTPB with Zn2+ ions from the liquid phase and characterize the free energy landscape of this transformation using the metadynamics (MTD) method. Finally, we take a closer look at the diffusion of Zn and its binding to the ligand.

Computational Details Fig. 1(a) and (b) show the molecular structure of the adsorbate molecule TTPB and a sketch of the simulation cell, respectively. We model the water surface as a periodic slab containing 482 water molecules in 30×30×16 ˚ A3 , and approximately 60 ˚ A of vertical vacuum space. The diameter of the TTPB molecule is approximately 19 ˚ A, which allows for a sufficient distance to decouple the periodic replicas of the molecule. We simulate three systems, free TTPB in the gas phase (TTPBg ), TTPB supported on water (TTPBw ), and supported TTPB with Zn2+ coordinated to the three terpyridines with dissolved Cl– counterions (TTPBw -Zn3 ). We perform ab initio molecular dynamics (AIMD) simulations based on DFT using the cp2k code. 25,26 The entire system is treated quantum-mechanically, representing valence and core electrons with Double-Zeta basis sets of the MOLOPT type 27 and Goedecker-TeterHutter pseudopotentials, 28 respectively. The D3 method by Grimme 29 is used to account for dispersion interactions in an approximate


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porting Information). In TTPBw -Zn3 , on the other hand, the coordination of the Zn ion by the 3 ligands leads to stable bonds that prevent large rotations. Therefore, all pyridyls remain in the cis conformation. Metal coordination can thus successfully and selectively restrain the torsional flexibility of the molecule. After the groups have adopted a stable conformation (cis or trans), they fluctuate around an average dihedral angle Φavg . In Fig. 2(b) and (c) we plot the distributions of the deviations ∆Φ = Φ − Φavg and ∆Θ = Θ − Θavg , aggregrated for all equivalent angles in the molecule, for TTPBg , TTPBw , and TTPBw -Zn3 . The distributions follow a clear trend in both instances, obvious from the shape of the curves, and well-represented also by the full width at half maximum (FWHM) of the peaks. The greatest fluctuation of angles occurs in the gas phase. The distribution significantly narrows once the molecule is brought into contact with the water surface. The peak width is reduced by almost 60% for ∆Φ. Coordination of Zn additionally increases the conformational rigidity of the molecule, keeping all fluctuations within 30◦ of the equilibrium angles and reducing the FWHM by 28%. The same trend holds for the dihedral angles Θ, where the distributions progressively narrow by 47 and 56% with adsorption and metal coordination. Overall, the ∆Θ distributions are narrower than those of ∆Φ, because the rotation of the whole terpyridine unit is sterically hindered compared to that of an invidiual pyridyl group.

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Figure 2: Inner (Θ) and outer (Φ) dihedral angles of TTPB (C-C-C-C and N-C-C-N, respectively. (a) Time-dependence of the six angles Φ in TTPBg ; (b) Distribution function of Φ angles’ deviation from their mean value, ∆Φ; (c) Distribution function of Θ angles’ deviation from their mean value, ∆Θ; Horizontal bars indicate estimates of the FWHM.

Another way to quantify the structural flexibility of TTPB is based on the deviation from the molecule’s equilibrium shape. For this purpose, we define the distortion ∆d as the mean distance over all atoms from a plane of best fit through the central benzene ring, ∆d =

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to the density data (from both sides) we obtain values for the bulk density ρ0 = (1.05 ± 0.01) g·cm-3 , the interface thickness δ = (1.46 ± 0.20) ˚ A and the location of the Gibbs’ dividing surface zG , at -16.34 ˚ A and -1.01 ˚ A (averages for TTPBw and TTPBw -Zn3 simulations). ρ0 and δ are nearly identical on both sides of the slab for TTPBw and TTPBw -Zn3 , and the averages agree well with previously reported data (δ = 1.1, 10 1.71 ˚ A, 11 ρ0 = 0.98, 10 1.01 11 ). The density profiles and fitted functions are presented in the Supporting Information. Based on the density profiles, we observe no significant effect of the adsorbate on the overall structure of the water slab.

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(a)

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We subdivide the water slab into 4 vertical regions with a thickness of 4 ˚ A (Fig. 4(a)), and investigate differences between the hydrogen bonding (H-bonding) network in the bulk and near the free and covered surfaces. An H-bond is defined by two H2 O molecules, for which dOO < 3.5 ˚ A and 6 OHO > 140◦ . 10 In the Hbond network, water molecules can be categorized based on the number of donor (D) and acceptor (A) H-bonds. Most are bound by 2–4 H-bonds: 1 donor/1 acceptor (DA), 2 donors/1 acceptor (DDA), 1 donor/2 acceptors (DAA), 2 donors/2 acceptors (DDAA). 10 Fig. 4(b) and (c) show the probabilities for the various donor/acceptor combinations as a function of depth for TTPBw and TTPBw Zn3 , respectively. In both simulations, the two middle regions contain predominantly DDAA species, as expected in bulk water. The probability of DDAA bonding in the middle regions ranges from 0.62 to 0.67, with slightly higher values found in the TTPBw simulation. This value is in line with previous reports. Using the BLYP functional, Baer et al. 10 determine a value around 0.5 using the same H-bond definition as this work, while other studies find 0.40 12 and 0.84, 11 using different definitions and methods. The free surface is characterized by an increased incidence of the other three species, reducing the number of DDAA by approximately one third (TTPBw ) to one quarter (TTPBw -

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Figure 4: Properties of the water slab. (a) Sketch of the vertical subdivision of the water slab into 4 regions I-IV. (b) Populations of water molecules in particular H-bond environments in the 4 regions, for TTPBw . (c) Populations of water molecules in particular Hbond environments in 4 regions, for TTPBw Zn3 . The error bars indicate the standard deviation of the H-bond population over 2 ps subblocks of the trajectories.


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Zn3 ). Qualitatively this agrees well with previous reports, 11 but it should be noted that we use a rather coarse vertical partitioning of the water slab, which may not fully capture the rapid change in the distribution of these species. For the case of TTPBw the surface covered by the adsorbate exhibits a distribution of H-bonds roughly half-way between those of the free surface and the bulk regions. Compared to the free surface, the fraction of DDAA species increases from 0.41 to 0.54, while the fraction of DA species decreases by approximately 50%. Compared to the bulk-like region, the fraction of DDAA species decreases from 0.65 and the occurrence of DA molecules roughly doubles. On the other hand, the DAA and DDA water molecules are approximately equally common on the free and covered surface sides, exhibiting increased probabilities compared to the bulk. In the TTPBw Zn3 simulations, the differences in DDAA frequencies between the covered and free surfaces are less pronounced.The fraction of DA species is about 40% lower on the TTPB-covered side and the occurrence of DAA species is somewhat higher. Furthermore, we note a small number of double-acceptor (AA) species on the TTPB-covered side, that are almost completely absent from all other vertical regions. Generally, the TTPBw and TTPBw -Zn3 systems exhibit quite similar depth-dependencies of their H-bond distributions. It should be noted that in the TTPBw -Zn3 simulation, a total of 6 Cl– ions are present in the bulk liquid, which may account for some of the differences in H-bonding patterns between the two simulations. Our results agree with previous conclusions about the distributions of H-bonds near the air-water interface. Furthermore, they suggest that the adsorbate has a subtle influence on the H-bonding pattern of near-surface water molecules, reordering them to affect the frequencies of different donor/acceptor combinations.

MOF is the coordination of metal ions, which serve as linkers between molecules. This process involves diffusion of the ions from the liquid phase to the surface and subsequent formation of 3 coordinative Nterpy –Zn bonds. Having established that Zn2+ binds to the terpy moieties of TTPB in a stable way (vide supra), we now examine the transition TTPB + Zn2+ (aq) ⇀ ↽ TTPB-Zn in detail, focusing on one isolated ion uptake rather than the saturation of all three ligand sites. We use MTD to accelerate the simulation of the process. As CVs, we choose the distance from Zn to the center of TTPB, dZn,TTPB and the Zn–N coordination number, nZn,N . Starting from a configuration with Zn2+ rather far away from the molecule, along the metadynamics trajectory dZn,TTPB slowly decreases and eventually nZn,N increases from 0 to 3, i. e. full coordination of Zn by terpyridine. As the simulation goes on, the bias potential drives Zn2+ out of the complex back into the solution. We can thus elucidate the molecular details of ion binding and describe the energy landscape of the process. Ion Insertion: Free Energy and Mechanism We plot the aggregated energy data as a function of the two CVs, as well as a minimumenergy path for the first Zn2+ uptake event in Fig. 5(a). The free energy profile along this path is shown in Fig. 5(b), with minima and barriers labelled for comparison. Starting from the bulk water (point 1) the ion has to escape a minimum of 430 kJ·mol-1 due to the solvation free energy, overcomes a barrier of about 165 kJ·mol-1 (point 2) and settles into a wider and deeper minimum somewhat closer to TTPB (-520 kJ·mol-1 , point 3). The ion then traverses a second barrier (point 4) until it is bound by the first pyridyl group (nZn,N =1). After a final, smaller barrier (127 kJ·mol-1 , point 6), TTPBw Zn3 is formed, characterized by a very narrow minimum with a depth of -388 kJ·mol-1 (point 7). The FES exihibits a number of minima. Most notable is a broad and deep “valley” with unco-

Ion Uptake from the Solution Once the TTPB monomer is confined at the water surface, the next step in the formation of a


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ordinated Zn (nZn,N ≈ 0) at the center of mass of TTPB up to dZn,TTPB ≈ 8 ˚ A with Zn solvated by H2 O in a bulk-like environment. There is also a broad minimum for nZn,N between 1 and 2 (point 5), where the ion is partially coordinated by TTPB and still has substantial freedom of movement. Among the configurations with coordinated Zn, full binding to the terpyridine unit has the narrowest and steepest minimum. We rationalize this situation with the competition between entropy and binding enthalpy. While configurations with a higher number of Zn–N bonds are energetically more stable, they also increasingly restrict the conformational freedom of TTPB and the movement of Zn, making these microstates entropically disfavored. Furthermore, the vastly larger number of configurations with Zn in solution increases the statistical probability of the unbound state, thus leading to deeper free energy minima for dissolved Zn. The unbound states are sampled more frequently by the MTD trajectory, producing the corresponding minima in the MTD representation of the free energy surface. The fact that the TTPBw -Zn3 complex is thermodynamically higher in energy than the unbound state does not preclude the formation of larger clusters and extended networks. In the single metal-linker complex Zn is coordinated by three pyridyl groups and the binding interaction is exceeded by the entropic contributions described above. When the Zn ion bridges two TTPB molecules, it is coordinated by six pyridyls, which should approximately double the binding enthalpy. Furthermore, when fully coordinated by pyridyls, Zn is shielded from water molecules, and less likely to re-enter the aqueous phase. Thus, the lower thermodynamic stability of TTPBw -Zn3 is due to the conditions imposed in the simulation, but should not adversely affect our conclusions on the mechanism of ion insertion as such. In this context, it would be instructive to study the formation of a TTPB-Zn-TTPB dimer as well as larger aggregates, which we plan to address in follow-up work. Nevertheless, the mechanism of ion insertion is a necessary first step towards the formation of 2D sheets.

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The 4 snapshots shown in in Fig. 5(c) illustrate the relevant steps of the binding process. Zn approaches the binding site (panel 1), and the first pyridyl group coordinates the ion (panel 2), effectively pulling it farther out from solution. Then the central ligand forms the second bond to Zn (panel 3). The final pyridine ring, taking advantage of the conformational flexibility allowed by the liquid substrate, rotates by 180◦ (between panels 2 and 4) and completes the 3-fold coordination. Solvation and Coordination of Zn2+ In order to bind to the molecule at the surface, Zn diffuses from the bulk towards the surface. We define the first solvation shell of Zn2+ as containing all water molecules whose oxygen atoms are within 2.2 ˚ A of the ion. Conceivably, the ion can either move independently of its solvation shell and transiently bind different H2 O molecules along the diffusion path, or move along with its first solvation shell, coordinated by the same H2 O over extended time and distance. The barriers for each of these mechanisms should determine the preferred one. As a first step towards answering this question, we plot the distribution of Zn–OH2 O (nZn,O ) versus the Zn–Nterpy coordination (Fig. 6(a)). As expected, there is an inverse relationship between the two quantities. In bulk-like situations with a 4- to 5-fold solvation of Zn, the ion is not coordinated by the Nterpy atoms. Conversely, the bound Zn retains on average two solvating water molecules that coordinate axially from below and at the fourth equatorial position on the open side of the terpyridine. Between the limits of full solvation and full complexation, the two coordination numbers are inversely correlated, with several spots of particularly high occurrence. The combinations of integer numbers 4/0, 3/0, 4/1, 3/1, 3/2, 2/2, 2/3 (nZn,O /nZn,N ) can be recognized in the plot. Thus, Zn maintains an environment of 4 to 5 ligands at all times. Particularly, while the ion is partially bound to 1 or 2 Nterpy , it remains coordinated by 3 water molecules. The structure of the bulk solvation shell of Zn2+ has not been unambiguously determined,


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Conclusions

in the ion’s solvation shell appears to be independent of the diffusional movement of Zn, occurring on longer timescales. Eventually, assisted by the intermittent binding to the ligand, the solvent shell would be fully exchanged for “other” H2 O molecules. (a)

Confining precursors at a liquid surface and subsequent crosslinking with metal ions from the solution has been shown to be a viable route to prepare tailored 2D networks. 8,9 In this article we examined a protoype monomer, TTPB, adsorbed on a water surface both with and without coordinated Zn ions. Compared to the gas phase, the molecule is significantly stabilized conformationally by adsorption on water. At the same time, the adsorbate has a noticeable influence on the liquid, causing H-bond distributions on the covered side of the water slab to resemble those in the bulk. The water subphase can act as a solvent for species that may react with the molecule on the surface. A dissolved Zn ion can overcome the free energy barrier keeping it in the liquid environment and react to form a stable coordination compound with the pyridyl ligands of TTPB. On a molecular scale, this happens in a step-wise process of successive binding to the three nitrogen atoms. Crucially, the terpyridine moiety needs to be mobile enough to allow the ligands to rotate into the all-cis conformation otherwise unfavorable. On the time scale of the simulation, the solvation shell of Zn remains stable even as the ion moves through the liquid. Our results provide insight into the first steps of 2D sheet formation on the liquid surface. As a next step, we plan to study more complex reactions in this context, particularly the formation of metal-bridged dimers and networks. These insights, combined with the presently available encouraging experimental results, may allow for further improvement of the available methods and lead to an increased number of tailored monolayer materials for applications in nanoscience.

Frequency 1e−3

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Figure 6: (a) Correlation of Zn solvation (Zn– OH2 O coordination) with Zn complexation by TTPB. Color bar shows frequency per x/y bin. (b) Time-resolved structure of the Zn solvation shell. Each horizontal bar represents one H2 O molecule in the vicinity of the metal. Colors indicate the O–Zn distance of that molecule over time. An estimate of the number of solvating molecules can be calculated by counting the number of black spots at a particular time.

Acknowledgement The authors gratefully acknowledge financial support from the Swiss National Science Foundation under grant no. 20021 140441 and generous computing resources from the Swiss National Supercomputing Center (project ID: S415). RK thanks the Japanese High-Performance Computing Initia-


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Supporting Information Available: Plots of dihedral angles in TTPBg , TTPBw and TTPBw -Zn3 as a function of time. Fitted density profiles of the water slabs. This material is available free of charge via the Internet at http://pubs.acs.org/.

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Building Blocks for Two-Dimensional MetalâOrganic Frameworks

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