Determination of Geometry Arrangement of Copper Ions in HKUST-1

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Determination of Geometry Arrangement of Copper Ions in HKUST-1 by XAFS During a Prolonged Exposure to Air Michela Todaro, Luisa Sciortino, Franco Mario Gelardi, and Gianpiero Buscarino J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07792 • Publication Date (Web): 13 Oct 2017 Downloaded from http://pubs.acs.org on October 16, 2017

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The Journal of Physical Chemistry

Determination of Geometry Arrangement of Copper Ions in HKUST-1 by XAFS During a Prolonged Exposure to Air

AUTHOR NAMES Michela Todaroa,b, Luisa Sciortinoa, Franco Mario Gelardia and Gianpiero Buscarinoa*

AUTHOR ADDRESS a

Dipartimento di Fisica e Chimica, Università di Palermo, 90123 Palermo, Italia;

b

Dipartimento di Fisica e Astronomia, Università di Catania, 95123 Catania, Italia.

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ABSTRACT We present an experimental investigation focused on the local structural changes taking place around Cu2+ ions in Metal-Organic Framework (MOF) HKUST-1 for different times of exposure to air by XAFS (X-rays Absorption Fine Structure). The analysis involves both XANES (X-ray Absorption Near Edge Structure) and EXAFS (Extended X-ray Absorption Fine Structure) regions around the Cu K-edge. Starting from the paddle-wheel structures proposed in literature, a more detailed description of the geometrical environment of Cu2+ ions is found. In particular, the paddlewheel structure of a fresh sample, that means a pristine HKUST-1 material with a single water molecule weakly adsorbed on each Cu2+ ion, has been disclosed for the first time. Furthermore, after 20 days of exposure to air, relevant structural changes with respect to the pristine sample have been evidenced. An activation process has demonstrated that these local changes are totally reversible, in agreement a recent model of the decomposition process of HKUST-1 proposed in literature.


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INTRODUCTION Metal-Organic Frameworks (MOFs) constitute a new class of high porous material, particularly interesting for adsorption of small molecules and gases.1,2 These compounds ensue from a connection of metal groups and organic linkers, giving as result a rigid material with remarkable surface area and large pore volume. 1,2 HKUST-1 is one of the most interesting MOF for its notable range of applications and for its fascinating physical-chemical properties. A dimer of copper ions in +2 oxidation state is coordinated by four carboxylate bridges, which are the terminal groups of BTC (= benzene 1,3,5-tricarboxylate) organic molecules.3 The result of the combination between organic molecule and metal ions is a cubic framework with pores of diameter of 9 Å and small pocket of diameter of 6 Å.3 The presence of cavities and pores makes HKUST-1 very efficient in adsorption of small molecules,4-9 gas separation,10-12 gas storage,13-15 catalysis,16 and many others17. Most of these chemical reactions occur near the Cu2(CO2)4 moiety, also known as paddle-wheel unit. The interaction with polar molecules as H2O and NH3 is favored especially near to the Cu2+ ions.4-5,18 The paddle-wheel structure of the as-synthetized material was already determined. It was found that each Cu2+ is in square-pyramidal geometry with four oxygen atoms of carboxylate bridges forming the planar base and a water molecule, weakly adsorbed on the fifth coordination site of Cu2+ in the axial direction, which represents the vertex of the pyramid.4-5,18-19 After the activation process, that is namely the removal of the apical water molecule, a change in the coordination geometry of Cu2+ was recognized, and consequently the metal ion results in a square-planar configuration.5,18 Recently, the effect of a prolonged interaction of HKUST-1 with air moisture was investigated. The outcomes revealed a decomposition process of the crystalline matrix which takes place in three different stages. During the first stage, of about 20 days, water molecules are accumulated in the cavities of the material producing a swelling of the crystalline matrix. For longer exposure times, the hydrolysis of Cu−O bonds in the paddle-wheels occurs, causing the break of the crystalline network.20 In spite of the first stage which is totally reversible, second and third stages of the 3 ACS Paragon Plus Environment


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decomposition process of HKUST-1 upon interaction with air moisture are irreversible. The possibility to recover the structure of HKUST-1 after 20 days of exposure to air can be a very relevant aspect to consider for the applicability of the material in industrial field. This result could be the basis for future works devoted to the identification of specific procedures able to stabilize the whole HKUST-1 volume, making it a suitable material for many important applications involving moisture and allowing easiest and cheapest ways to store this outstanding material in ordinary ambient condition. In our previous work, a different paddle-wheel structure was associated to each stage of this processes, using principally EPR (Electron Paramagnetic Resonance) spectroscopy results.20 In the present study, we aim to determine the local environment of copper ions in each stage of the decomposition process of HKUST-1 by exposure to air moisture by XAFS (X-rays Absorption Fine Structure) spectroscopy in both XANES (X-ray Absorption Near Edge Structure) and EXAFS (Extended X-ray Absorption Fine Structure) regions, and to refine the paddle-wheel structures.

EXPERIMENTAL SECTION HKUST-1 samples were prepared starting from commercial powder purchased by Sigma Aldrich. We considered three samples of HKUST-1, formerly exposed to air for different times in our laboratory at T=300 K and with 70% of relative humidity (RH). In particular, these samples were exposed to air for 0, 20, and 180 days respectively. The 0 days sample was exposed to air only for few minutes, so it represents a pristine material. In the following, such three samples are named using the number of days of exposure to air. Two clones of the first two samples were activated at T=400 K and P=5x10-2 mbar overnight in the Microtomo Furnace21 available at the beamline. For this reason, these two further samples are named 0 days_A and 20 days_A, respectively, to be distinguishable from the others not activated. Furthermore, we used Cu foil, Cu2O and CuO in powder form as reference compounds. 4 ACS Paragon Plus Environment

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All masses were weighed in such a way that the absolute absorption was about ~1.5 and the edge jump was about ~1. Since some recent studies on pressure effect on HKUST-1 have highlighted that the crystalline structure is easily damaged during the pellets preparation,22 we performed XAFS measurements on packed powders trapped into between two pieces of kapton tape, to be able to evaluate the only effect of air moisture on the crystalline matrix. XAFS measurements were performed on Cu K-edge (8979 eV) in transmission mode recording both XANES and EXAFS regions at BM08 beamline of ESRF facility (experiment number: MA2949). The white beam from the bending magnet source was monochromatized using a double crystal Si(311) monochromator with a photon flux up to 1010÷1011 ph/s, and an energy resolution ΔE/E≈10-5÷10-4 on a spot size about 1×1 mm2.23 In particular, a Pd mirror which cuts third and fifth harmonics was used to perform the experiment. Furthermore, the crystals of the monochromator were slightly detuned. The apparatus was constituted by two ionization chambers filled with a N2/Ar mixture at different composition for the incident I0 and transmitted I1 beam, respectively.24 All the measurements were performed at liquid-nitrogen temperature (T~80 K), using a Liquid-N2 cryostat to reduce effects of thermal disorder, and in high vacuum. For the energy/angle calibration, K-edge XANES spectra of pure Cu foil were collected simultaneously using a third ionization chamber. We acquired XAFS spectra in the range 8700-10213 eV, but with a different energy step depending on the energy region. In particular, XANES was acquired using an energy step of 0.2 eV while EXAFS with an energy step of 1, 2, 4 eV for the energy ranges delimited by the following values: 9010, 9080, 9379, 10213 eV. The integration time was fixed at 3 s for each energy range. Two scans for sample were acquired. The normalization of the absorption spectrum and extraction of the χ(k) was performed with Athena program.25 XANES region of pre-edges was analyzed by subtracting a straight line. The amplitude and phase of the scattering path were obtained by FEFF 8.126 using the single crystal data for the HKUST-1 structure3. For EXAFS fitting analysis we used Viper software.27 The fits 5 ACS Paragon Plus Environment


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were performed in module and imaginary component of the data in R-space. The best value for S02, was obtained fitting the spectra of Cu foil with six paths, fixing the coordination numbers to the ideal ones. The EXAFS spectra were weighted in k3∙χ(k) and Fourier transformed was in Δk=2.4-14 Å-1. Each spectrum was fitted with the first six paths.

RESULTS XANES SPECTRA In Figure 1, a comparison between the normalized absorption edges of all the samples of HKUST-1 exposed for different times to air moisture and the reference compounds is reported.

Figure 1. Normalized absorption μ(E) curve of three samples of HKUST-1 exposed to air for different times (0 days red, 20 days magenta, 180 days green), two activated samples (0 days_A black, 20 days_A blue), and the three reference compounds (Cu foil navy, Cu2O purple, and CuO orange).

The edge position of each sample reported in Table 1 is estimated by the evaluation of the maximum of the first derivative of the absorption spectra as usual.28 6 ACS Paragon Plus Environment

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Table 1: Edge Positions of three copper reference compounds and of the investigated HKUST-1 samples. Samples

E0 (eV)

Cu foil

8979.1

Cu2O

8980.4

CuO

8990.7

0 days_A

8990.3

0 days

8990.3

20 days_A

8990.4

20 days

8990.4

180 days

8990.3

As it emerges from the spectra reported in Figure 1, all the samples of HKUST-1 show nearly identical edge energies to that of the reference CuO, and hence the estimation of edge positions suggests the divalent oxidation state of copper ion in all the samples analyzed. The absorption edge region of the five samples is reported in Figure 2(a), in which three features are evident5,18,29: two pre-edges peaks (zoomed in the two insets), and the white line. For the pristine material (0 days sample), a typical pre-edge is observed at about 8977.7 eV and it is attributed to the 1s→3d quadrupole transition. 18,29Although the 1s→3d transition is forbidden by dipole selection rules, it is known in literature that the presence of this peak is due both to orbital 3d+4p mixing and to direct quadrupolar coupling.29-30 In this sense, its intensity gives clear information about the coordination number of the Cu2+ absorber atom. Especially the reduction of its intensity and a blue shift of its position are indicative of a reduction of coordination number of copper ion.5,18 7 ACS Paragon Plus Environment


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Moreover, the other pre-edge, often indicated as shoulder,18,29 is observed at about 8985.3 eV and it is related to the allowed 1s→4p dipolar shakedown transition.18,29 An increase of its intensity and a red-shift of its peak position are interpreted as an increase of the covalent character of the ligandcopper bond.18 While, the white line at 8998.9 eV is representative to the 1s→continuum transition. The uncertainty of peak position is the energy step (0.2 eV), while the uncertainty of peak intensity is a repeatability error ±5%. In Figure 2(b), the intensity of the peak of 1s→3d transition in function of its position is reported. It evidences a significant trend toward a red shift and an increasing of the intensity with the time of exposure to air moisture and, consequently, with increasing of the amount of water molecules in the matrix cavities. The peak position is 8978 eV for the activated samples (0 days_A and 20 days_A), whereas it shifts towards 8977.4 eV for the hydrated samples (20 days and 180 days), with a total red shift of 0.6 eV. This trend agrees very well with that recognized by Prestipino et al.18 during the progressive outgassing of hydrated HKUST-1 although our total shift does not match that of 0.2 eV founded by the authors. Analogously, we plot (Figure 2(c)) the intensity of the peak of 1s→4p transition as a function of its position. In this case, it is possible to observe a large reduction of the intensity and blue shift of the peak position with increasing of time of exposure to air. This transition shows a maximum intensity at 8985.3 eV for the activated (0 days_A and 20 days_A) and the pristine sample (0 days) and it shows a blue shift towards 8985.6 and 8985.5 eV for 20 days and 180 days samples, respectively. Once again we have found a good accordance with the trend reported in literature but with a different value of total shift. In fact, we observe an energy shift of 0.3 eV between the activated and the hydrated samples, with respect to a shift of 0.5 eV by Prestipino et al.18 By the inspection of the Figure 2(d) that reports the intensity of white line as a function of the energy position, two regions are clearly distinguishable: the region of the white line position of activated and pristine samples (around 8999 eV) and that of the samples exposed to air for 20 and


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180 days (around 8997.6 eV). Both the increase of the intensity of the white line and the red shift of the peak position, going from activated to hydrated samples, are in agreement with literature.5,18

Figure 2. Comparison between normalized absorption μ(E) curve of five samples of HKUST-1 exposed to air for different times (a) (the curves were vertically shifted for clarity); estimated peak intensity of 1s→3d transition, 1s→4p and white line transition in function of the energy position reported in (b), (c) and (d), respectively.

EXAFS ANALYSIS In order to obtain detailed information about the changes of local environment of Cu2+ ions in HKUST-1 samples at different times of exposure to moisture, we investigated the EXAFS signal of all the samples. 9 ACS Paragon Plus Environment


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The cluster used to fit data is only a portion of the matrix of HKUST-1, that within a distance of a few of Å from the absorber atom. Thus, we consider the paddle-wheel moiety with two axial water molecules, Cu2(CO2)4(H2O)2, as pictured in Figure 3. The reason is also related to the consciousness that the principal effect due to the interaction with water molecules is concentrated on paddle-wheel unit.20 Furthermore, this is well-rooted custom in literature.5,18

Figure 3. Cu2(CO2)4(H2O)2 cluster model used for EXAFS analysis. In this schematic representation, we have labeled the atoms involved in scattering path used in fitting procedure. Hydrogen atoms of water molecules are omitted for clarity. Copper ions are blue, while oxygen and carbon atoms are red and brown, respectively.

The scattering paths used in the EXAFS analysis are generated by FEFF 8.126 program and they include five single scattering (SS) and one multiple scattering (MS) path. In particular, we have considered: (i) the scattering path between Cuabs ion and the four basal oxygen atoms directly bonded to the Cuabs (Cuabs-O1); (ii) that one between Cuabs ions and the single oxygen atom pertaining the water molecule adsorbed on Cuabs (Cuabs-Ow); (iii) the scattering path between the 10 ACS Paragon Plus Environment

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two copper ions of the paddle-wheel (Cuabs-Cu); (iv) that one between Cuabs ions and the four carbon atoms of the carboxylate bridges (Cuabs-C); (v) the multiple scattering path which involves oxygen and carbon atoms (Cuabs-C-O1); (vi) the single scattering path between Cuabs ion and the four oxygen atoms directly bonded to the other copper ion of the same paddle-wheel (Cuabs-O2). These scattering paths are sufficient for simulating the Fourier transforms of EXAFS experimental data in 0.98÷3.09 Å range, in a good accordance with literature.5,18 The experimental Fourier Transform of EXAFS signals of the five investigated samples and the respective fits are presented in Figure 4 (a-e), in all modulus, real, and imaginary components. Furthermore, the optimized parameters obtained by fitting are summarized in Table 2. All the moduli reported in Figure 4 (a-e) are characterized by a main signal contribution at about 1.5 Å (the spectra are corrected in phase) and other contribution less intense at longer distance from the absorber Cuabs atom (see Figure S1 in supporting information). The spectra of the two activated samples (Figure 4(a-b)) look very similar to each other as well as to the pristine sample (0 days (Figure 4c)), on the other hand comparing to the most hydrated samples (20 days (Figure 4d) and 180 days (Figure 4e)) some evident differences appear. In particular, these changes characterize the signal above 2 Å, in the region pertaining to Cu-Cu interaction (see Figure S1 in supporting information).



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Figure 4. Fourier Transform of EXAFS signal of HKUST-1 samples at different times of exposure to air: 0 days_A (a), 20 days_A (b), 0 days (c), 20 days (d), 180 days (e). More precisely, we indicate with points the experimental data and with lines the best fit. The different colors correspond to modulus (black), real component (light blue) and imaginary component (red).


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Furthermore, in graphs of Figure 4 (a-e), it is possible to appreciate the high degree of compatibility between experimental and simulated EXAFS data in the fit range, not only with respect to the modulus but also with respect to real and imaginary components. The goodness of the fit results is also testified evaluating the R-factor values, reported in Table 2, which are lower than 8%. For each fit, we fixed the coordination number of scattering path of Cuabs-Cu to 1, according to the intact paddle-wheel structures proposed in previous studies.20,31

Table 2: EXAFS fitting results for the samples of HKUST-1 exposed to air for different times. Samples

Scattering path

Ncoor

R (Å)

σ2(Å2)

ΔE0 (eV)

R-factor

0 days_A

Cuabs-O1

3.6

1.94

0.004

2.1

5.24%

Cuabs-Ow

0.4

2.25

0.001

2.1

Cuabs-Cu

1.0

2.50

0.008

-6.0

Cuabs-C

3.6

2.85

0.007

2.1

Cuabs-C-O

7.2

2.98

0.001

2.1

Cuabs-O2

3.6

3.19

0.018

2.1

Cuabs-O1

3.8

1.95

0.005

3.1

Cuabs-Ow

0.8

2.25

0.001

3.1

Cuabs-Cu

1.0

2.54

0.006

2.3

Cuabs-C

3.8

2.80

0.009

3.1

Cuabs-C-O

7.6

3.02

0.001

3.1

Cuabs-O2

3.8

3.17

0.022

3.1

Cuabs-O1

3.7

1.94

0.004

2.6

Cuabs-Ow

0.5

2.23

0.001

2.6

Cuabs-Cu

1.0

2.46

0.008

-6.0

Cuabs-C

3.7

2.84

0.010

2.6

0 days

20 days_A


7.92%

5.63%


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20 days

180 days

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Cuabs-C-O

7.4

3.01

0.001

2.6

Cuabs-O2

3.7

3.17

0.027

2.6

Cuabs-O1

3.9

1.96

0.005

1.7

Cuabs-Ow

0.7

2.24

0.006

1.7

Cuabs-Cu

1.0

2.63

0.006

-5.0

Cuabs-C

3.9

2.90

0.006

1.7

Cuabs-C-O

7.7

3.03

0.001

1.7

Cuabs-O2

3.9

3.22

0.020

1.7

Cuabs-O1

3.7

1.97

0.005

2.5

Cuabs-Ow

0.7

2.26

0.003

2.5

Cuabs-Cu

1.0

2.60

0.010

-6.0

Cuabs-C

3.7

2.90

0.011

2.5

Cuabs-C-O

7.4

3.01

0.001

2.5

Cuabs-O2

3.7

3.27

0.016

2.5

4.50%

4.89%

For the six scattering paths used to simulate the experimental curve, the table reports the optimized parameters: (i) the coordination number (Ncoor); (ii) the absorber to scatterer distance (R); and (iii) the Debye−Waller factor (σ2); (iv) the energy shift (ΔE0); (v) the R-factor value, which is the minimized function of fitting parameters 27. Uncertainty is 10% for Ncoor, 0.02 Å for R, and on the last digit for σ2.

As further support to the analysis described above, the EXAFS fits were repeated using the same ΔE0 for all the paths. The outcomes (data not reported) are in agreement with that of Table 2, confirming their goodness.

DISCUSSION 14 ACS Paragon Plus Environment

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Both XANES and EXAFS analyzed data provide detailed local modification around the Cuabs atom during the entire period of exposure to air moisture, highlighting the main effects due to interaction with water molecules. It is useful to start our discussion from an activated sample: the 0 days_A sample. The analysis of positions of the two pre-edge peaks, 1s→3d transition at 8978 eV and 1s→4p transition at 8985.2 eV, and of the white line 8998.9 eV are compatible with those found by Prestipino et al.18 for the activated material at T=453 K. Furthermore, from EXAFS analysis the outgassing process occurs more efficient in our experiment than in those reported in literature. As a matter of fact, we obtain a smaller coordination numbers of the first two shells (Cuabs-O1=3.6 and Cuabs-Ow=0.4) that Prestipino at the same distances from the absorber atom that are reported in literature.18 Then, it is legitimate to consider that each Cu2+ ions in this sample is in square planar geometry. 0 days sample represents a pristine HKUST-1 material, exposed to air only for few minutes before XAFS measurements. It is a known fact that few minutes of exposure to ambient conditions are sufficient to reveal the presence of water molecules adsorbed on the open metal sites (OMS) of Cu2+ ions by NMR spectroscopy.32 Thus, it is reasonable to consider occupied by a water molecule the main part of the available OMS, but still empty the cavities of the matrix. To our knowledge, such a sample has never been characterized before by XAFS spectroscopy. According to our results, in XANES region the shape of white line and of the two pre-edges as well as their peak intensities and positions are indicative of small changes of coordination geometry around the Cuabs ion with respect to the 0 days_A sample. In particular, the increasing of intensity of the 1s→3d transition is indicative of a larger mixing with 4p orbitals, thus of a tendency toward a square-pyramidal geometry. This suggestion is supported by the resulting fitting of EXAFS signal. Indeed, the CuabsOw coordination number (0.8) and the Cuabs-Cu distance (2.54 Å) indicate the occurrence of an adsorption process of a single water molecule on each Cu2+ OMS. This could reasonably lead to think to each Cu2+ ion in square pyramidal geometry but not too far from the basal plane.



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In this geometry, the highest occupied energy level is 3dx2- y2,33 so the large overlap between this wave function with the 2p orbital of oxygen basal atoms of carboxylate bridges is strongly favored.34 After 20 days of exposure to air, it is possible to notice considerable spectroscopic changes in XANES and EXAFS regions with respect to those of 0 days sample. All the data indicate further significant modifies of the local geometry of the Cuabs ion occurred in this period. Indeed, it is well known that after 20 days of exposure to air moisture, all the cavities of crystalline matrix are full of water molecules. This was recognized by the observation of the swelling of matrix by XRD measurements and mostly by the reduction of the EPR triplet signal20 coming from the antiferromagnetic coupling of the two copper ions in each paddle-wheel.35 According to our previous hypothesis20, the large quantity of water molecules applies a pressure on the paddle-wheels and consequently a σ bond between the two copper ions is established making the paddle-wheel units diamagnetic. Therefore, it is easy to think to a compressed pyramidal geometry as the best paddle-wheel configuration to justify the spectroscopic differences with 0 days sample. The compressed pyramidal geometry involves a reduction of the distance of the axial ligand with respect to those of basal oxygens. Furthermore, an inversion of the orbital energy levels occurs, leading 3dz2 occupied by an unpaired electron spin above the 3dx2- y2.36 It would be a good explanation to the observed reduction of the EPR triplet signal, but it is not compatible with our EXAFS fitting results, because we have never found a shorter Cuabs-Ow distance than Cuabs-O1. On the contrary, all the changes observed in intensity and positions in the peaks of pre-edges and white line together with the significant raise of the Cuabs-Cu distance indicate a square-pyramidal geometry of the Cuabs after 20 days of exposure to air. The effect of the accumulation of water molecules inside the cavities of the matrix is to distance the two Cu2+ ions, making the environment of the Cuabs more pyramidal than in the pristine material. The obtained Cuabs-Cu distance of 2.63 Å agrees with that of 2.63 Å of the as-synthetized sample of Chui,3 2.64 Å of Prestipino,18 or 2.65 Å of Borfecchia5. 16 ACS Paragon Plus Environment

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From the point of view the energy level scheme, no differences are present with respect to the 0 days sample. This has led us to ask how to explain the disappearance of a large percentage of EPR triplet signal. Analyzing the system during 20 days in air, we suppose that the large amount of water molecules adsorbed inside the matrix cavities are bonded to other water molecules through H-bonds forming clusters, as already suggested by water adsorption experiment by other researchers.37 We propose that it is the presence of strong H-bonds between water molecules to have a crucial effect on the magnetic coupling between the copper ions in each paddle-wheel. This idea is driven by the fact that the water molecule donating the hydrogen atom has increased electron density in its lone pair region, which promotes hydrogen bond acceptance.38 In this way, a major contribution of electron density is localized around Cu2+. We conceive that this further negative charge may contribute to an enhancement of the antiferromagnetic coupling intra paddle-wheels. Moreover, the obtained increase of Cu-Cu distance for 0 days sample leads to a further increase of the absolute values of the coupling constant, as stated by some theoretical studies on the exchange coupling in carboxylate-bridged dinuclear copper(II) compounds.39 We hypothesize that the total effect is an increment of the antiferromagnetic coupling, with a consequent increase of the separation between ground and triplet state. As a result, the population of excited triplet state is lower than the pristine sample, and then the EPR signal is significantly reduced. To confirm our idea magnetic susceptibility measurements might be useful. By the experiment of X-rays absorption and as already proved by EPR spectroscopy, we can confirm that no hydrolysis process is involved during the first 20 days of exposure to air, then this first stage is totally reversible.20 The XAFS measurements on the 20 days_A sample show a total recovery of the parameters of the activated sample, indicating a square planar geometry and a shrinking of the main distances of the paddle-wheel, above all the Cuabs-Cu distance, in good accordance with the results reported in literature.5,18 This result finds location in the research field of the study of the stability of HKUST-1 in presence of water. Actually, a wide literature, of which the review of Walton and co-workers40 is a representative example, already exists. In particular, the 17 ACS Paragon Plus Environment


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reversibility of the process of water absorption is particularly relevant in the study of the stability of HKUST-1. For example, this aspect was already tested under particular temperature conditions or as a result of consecutive hydration/dehydration cycles.41 If the interaction with air moisture is prolonged until 180 days, both in XANES and EXAFS regions only small changes are evident with respect to the latter situation at 20 days. This outcome is not surprising because the XAFS spectrum contains the signal both from the pristine and the broken paddle-wheels, then it is representative of a mixed system. In fact, it is known that the last stage of the decomposition process, that is the stage involved the 180 sample, has effect just on the small percentage of the paddle-wheels of the matrix.

CONCLUSION We performed an experimental investigation by XAFS measurement at Cu K-edge in both XANES and EXAFS regions of the principal stages of the decomposition process of HKUST-1 upon exposure to air moisture. This study has enabled us to understand the changes of the local environment of the Cuabs ion during a prolonged interaction with air, together with the characterization of an activated sample of HKUST-1. As a result, two totally different paddle-wheel moieties have been distinguished, pertained to 0 days sample and 20 days sample. As a matter of fact, both paddle-wheel moieties consider a water molecule adsorbed on each Cu2+ ion, but the presence or not of further water molecules in the cavities and, in particular, the occurrence of Hbonds between these latter has a significant effect on the distance Cu-Cu and on the super-exchange antiferromagnetic coupling between them. In agreement with our previous work, we have proved the reversibility of the first stage of the decomposition process during the first 20 days, since the geometrical parameters of the activated samples are very closer each other.


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ASSOCIATED CONTENT Supporting Information Additional detail about the experimental EXAFS fitted curves are reported in Supporting Information.

AUTHOR INFORMATION Corresponding Author *Gianpiero Buscarino Dipartimento di Fisica e Chimica, Università di Palermo, Via Archirafi 36 - 90123 Palermo, Italia +3909123891725 [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT This work has been financially supported by European Synchrotron Radiation Facility (ESRF) (Experiment Number MA-2949). We acknowledge the ESRF for provision of synchrotron facilities and we thank Alessandro Puri, the local contact of the beamline BM08 of ESRF, for his assistance. We acknowledge the LAMP group (www.unipa.it/lamp) for stimulating discussion and useful advices.



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Figure 1. Normalized absorption µ(E) curve of three samples of HKUST-1 exposed to air for different times (0 days red, 20 days magenta, 180 days green), two activated samples (0 days_A black, 20 days_A blue), and the three reference compounds (Cu foil navy, Cu2O purple, and CuO orange). 239x176mm (300 x 300 DPI)

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Figure 2. Comparison between normalized absorption µ(E) curve of five samples of HKUST-1 exposed to air for different times (a) (the curves were vertically shifted for clarity); estimated peak intensity of 1s→3d transition, 1s→4p and white line transition in function of the energy position reported in (b), (c) and (d), respectively. 241x173mm (300 x 300 DPI)


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Figure 3. Cu2(CO2)4(H2O)2 cluster model used for EXAFS analysis. In this schematic representation, we have labeled the atoms involved in scattering path used in fitting procedure. Hydrogen atoms of water molecules are omitted for clarity. Copper ions are blue, while oxygen and carbon atoms are red and brown, respectively. 121x140mm (96 x 96 DPI)



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Figure 4. Fourier Transform of EXAFS signal of HKUST-1 samples at different times of exposure to air: 0 days_A (a), 20 days_A (b), 0 days (c), 20 days (d), 180 days (e). More precisely, we indicate with points the experimental data and with lines the best fit. The different colors correspond to modulus (black), real component (light blue) and imaginary component (red). 224x175mm (300 x 300 DPI)


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Determination of Geometry Arrangement of Copper Ions in HKUST-1

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