CW and Pulsed ESR Spectroscopy of Cupric Ions in the Metal

The metal−organic framework (MOF) compound Cu3(BTC)2(H2O)3·xH2O (BTC = benzene 1,3,5-tricarboxylate) was prepared by solvothermal synthesis under ...
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J. Phys. Chem. C 2008, 112, 2678-2684

CW and Pulsed ESR Spectroscopy of Cupric Ions in the Metal-Organic Framework Compound Cu3(BTC)2 Andreas Po1 ppl,*,† Sebastian Kunz,‡ Dieter Himsl,‡ and Martin Hartmann‡ UniVersita¨t Leipzig, Fakulta¨t fu¨r Physik und Geowissenschaften, Linne´ strasse 5, D-04103 Leipzig, Germany, UniVersita¨t Augsburg, AdVanced Materials Science, Institut fu¨r Physik, UniVersita¨tsstrasse 1, D-86159 Augsburg, Germany ReceiVed: October 15, 2007; In Final Form: NoVember 23, 2007

The metal-organic framework (MOF) compound Cu3(BTC)2(H2O)3‚xH2O (BTC ) benzene 1,3,5-tricarboxylate) was prepared by solvothermal synthesis under ambient pressure and structurally characterized by powder X-ray diffraction and nitrogen adsorption at 77 K. X- and Q-band CW electron spin resonance and hyperfine sublevel correlation spectroscopies were used to explore the coordination state and location of the Cu(II) ions in the porous coordination polymer. Cupric ions were found to be present in two different chemical environments: (a) Cu(II)2 clusters in the paddle-wheel building blocks of the Cu3(BTC)2 network, giving rise to an antiferromagnetically coupled spin state in accordance with previous susceptibility measurements (J. Appl. Phys. 2000, 87, 6007). However, the cross-linking of the paddle wheels by the BTC linker leads to an additional spin exchange among dimers as evidenced by the characteristics of the S ) 1 ESR signal of their excited spin state. (b) In addition, paramagnetic monomer Cu(II) species are accommodated in the pore system of Cu3(BTC)2. They coordinate to adsorbed water molecules and form [Cu(H2O)6]2+ complexes, which are inhomogeneously distributed over the Cu3(BTC)2 pore system.

1. Introduction Since the work by Yaghi et al.,1,2 Fe´rey et al.,3 and others, metal-organic framework (MOFs) compounds have attracted considerable attraction in the past decade. In general, these coordination polymers are composed of two basic structural building units, transition metal ion complexes or metal ion clusters and connecting organic linker molecules such as di- or tricarboxylic acids.4 Characteristic features of MOF compounds are a crystalline three-dimensional open framework with large pore diameters and high specific micropore volumes, high metal ion content, and high structural diversity controlled by the use of a variety of different organic linkers and metal ion clusters.4 These structural properties make MOFs attractive materials for potential applications in gas storage, separation by selective adsorption, and catalysis.4-10 Here, they compete with conventional inorganic zeolite or aluminophosphate based molecular sieves. Moreover, the use of open-shell paramagnetic transition metal ions in the MOF synthesis allows the development of novel porous materials with additional magnetic properties11,12 featuring antiferromagnetism,13 ferromagnetism,14,15 and ferrimagnetism,16 as well as guest-controlled magnetic ordering17,18 and spin crossover effects.19 Such low-density magnetic materials are in particular interesting for future application for magnetic sorting, as magnetic sensors, and multifunctional materials.12 Among the several hundred different MOF structures known today, Cu3(BTC)2(H2O)3‚xH2O (BTC ) benzene 1,3,5-tricarboxylate) is one of the first coordination polymers that has been intensively studied because of its specific magnetic,20 catalytic,21 and adsorption22 properties. The network of Cu3(BTC)2 belongs to the cubic space group Fm3m.23 Antiferromagnetically coupled * Corresponding author. E-mail: [email protected]. † Universita ¨ t Leipzig, Fakulta¨t fu¨r Physik und Geowissenschaften. ‡ Universita ¨ t Augsburg, Advanced Materials Science, Institut fu¨r Physik.

Cu(II)2 clusters20 are coordinated by carboxylate groups to form a so-called paddle-wheel unit. In this paddle-wheel unit, the copper dimers are arranged by four carboxylate groups to a square, whereas water molecules weakly bind to the residual axial binding site of the Cu(II) ions (Figure 1a). The Cu(II) dimers are connected by the BTC linker molecules to form a three-dimensional porous network (Figure 1b) with interconnected cages having an approximate diameter of 1.17 nm as measured between the oxygen atoms of the carboxylate groups.21 The axial water molecules can be easily removed from the Cu(II) dimers by a moderate heat treatment in vacuum to form structurally well-defined accessible Lewis acid copper sites for catalytic applications.21 Concerning its magnetic properties, Cu3(BTC)2 exhibits pronounced antiferromagnetism due to the dimeric Cu(II) building blocks, which leads to a drastic increase of the magnetic susceptibility for temperatures above 100 K.20 For lower temperatures, the susceptibility rises again below 70 K, which has been interpreted in terms of a weak ferromagnetic coupling between the dimers.20 Preliminary ESR experiments indicated the existence of residual extraframework paramagnetic monomeric Cu(II) species,24 which may significantly influence in particular the lowtemperature magnetic properties of Cu3(BTC)2 as well as its selectivity as a catalyst. Hence, detailed knowledge about the state of the cupric ions is of utmost importance for better understanding of the reported material properties of this coordination polymer. In the present paper, the Cu(II) ions in Cu3(BTC)2 are characterized by X- and Q-band continuous wave (CW) electron spin resonance (ESR) and pulsed ESR spectroscopy. Here, temperature-dependent CW ESR experiments allow us to study directly the antiferromagnetically coupled cupric ions in the dimeric building units of the Cu3(BTC)2 framework. On the other hand, field sweep detected electron spin echo (FS ESE) and hyperfine sublevel correlation25 (HYSCORE) spec-

10.1021/jp7100094 CCC: $40.75 © 2008 American Chemical Society Published on Web 01/29/2008

Cupric Ions in Metal-Organic Framework Cu3(BTC)2

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Figure 1. Schematic representation of the structure of Cu3(BTC)2(H2O)3 showing (a) the Cu(II) dimer paddle-wheel building block and (b) the Cu3(BTC)2(H2O)3 network. Only the oxygen atoms of the water molecules binding axially to the Cu(II) ions are shown in (a).

troscopy will be employed together with CW ESR in order to determine the structure of the monomeric extraframework Cu(II) complexes in this material. 2. Experimental Section Sample Preparation. The synthesis of Cu3(BTC)2 was carried out under ambient pressure using a method developed in our laboratory: A suspension of 0.84 g of trimesic acid and 1.75 g of Cu(NO3)2‚3H2O in 50 mL EtOH was refluxed under rigorous stirring. The synthesis mixture was kept under these conditions for 48 h and was subsequently cooled down to room temperature. The obtained blue powder was recovered by filtration and washed once with water and a second time with EtOH. Thereafter, the sample was dried under vacuum at 100 °C for 12 h and then kept under dry nitrogen until further use. Structural Characterization. The sample was characterized by powder X-ray diffraction using a Siemens D5000 diffractometer operated at 40 kV and 30 mA using monochromatic Cu KR radiation (λCu KR ) 0.15405 nm). Scattering experiments were conducted at 2θ values from 4° to 50° with step sizes of 0.1°. Nitrogen adsorption isotherms were recorded at 77 K on a Micromeritics ASAP 2010 sorption analyzer. Prior to the measurements, the samples were outgassed for 12 h at 100 °C under vacuum. The chemical analysis has been performed with a Vario EL 3 elemental analyzer and a Vista-MPX CCD simultaneous ICP-OES instrument, both from Varian. Spectroscopic Measurements. Q-band CW ESR experiments have been performed on a Bruker EMX 10-40 spectrometer. All X-band CW ESR, FS-ESE, and HYSCORE experiments have been recorded on a Bruker ESP 380 spectrometer. For the temperature-dependent CW ESR intensity measurements, an X-band rectangular dual mode cavity was employed for comparison of the signal intensity with an ultramarine standard sample. Spectral simulations of the CW ESR powder patterns were done using the EasySpin ESR simulation package.26 Twopulse FS ESE and HYSCORE spectra were recorded at 6 K using nonselective pulses of tπ/2 ) 16 and tπ ) 32 ns. To avoid suppression effects in the proton HYSCORE experiments, the spectra were recorded with two different pulse delays τ ) 104 and 136 ns and added. A 170 × 170 data matrix was sampled, and two-dimensional (2D) Fourier transformed (FT) magnitude spectra are displayed. The simulations of the HYSCORE spectra were calculated in the time domain by exact diagonalization of

Figure 2. X-band ESR spectra of Cu3(BTC)2(H2O)3 at 6 K: (a) CW ESR spectrum, (b) FS ESE spectrum, (c) first derivative of the FS ESE spectrum, and (d) simulated spectrum.

the spin Hamiltonian. For further information concerning the simulation procedure, we refer to an earlier paper.27 3. Results Sample Characterization. The powder X-ray diffraction pattern (cf. Supporting Information) is in nice agreement with the powder patterns published in the literature so far confirming the successful synthesis of Cu3(BTC)2. Nitrogen adsorption data (cf. Supporting Information) reveal a specific pore volume of 0.62 cm3/g and a specific surface area of 1624 m2/g, which are indicative of a high-quality Cu2(BTC)3 material. The chemical analysis of the synthesized sample (Cu ) 31.56 wt %, C ) 36.90 wt %, H ) 0.95 wt %) is in good agreement with the theoretical chemical composition (Cu ) 31.51 wt %, C ) 35.73 wt %, H ) 0.99 wt %). Monomeric Cu(II) species. Low-temperature ESR experiments were performed at X-band frequencies in order to explore the potential presence of paramagnetic copper ions in Cu3(BTC)2. The measured CW ESR and FS ESE spectra are illustrated in Figure 2. In addition, the first derivative of the FS ESE spectrum is presented for comparison with the CW ESR experiment. The spectra show the typical anisotropic ESR powder patterns of Cu(II) ions having an electron spin S ) 1/2

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Po¨ppl et al.

and interacting with the ICu ) 3/2 nuclear spin of the copper nuclei (species A). However, distinct differences between CW and ESE detection are observed with respect to the line width of the cupric ion spectra. The FS ESE spectrum (Figure 2b,c) is nicely resolved, whereas the CW spectrum in Figure 2a seems to be considerably broadened and the Cu hyperfine coupling (hfc) is barely resolved. This indicates that the broadening of the CW ESR spectrum is not caused by some inhomogenous line broadening effects but rather by relaxation processes, presumably due to high local Cu(II) concentrations. Because of its better resolution, the first derivative FS ESE spectrum was used to estimate the Cu(II) spin Hamiltonian parameters of species A by comparison with spectral simulations on the basis of an ESR standard spin Hamiltonian

H ˆ ) βeBgSˆ + Sˆ ACuIˆCu

(1)

The two terms with the tensor g and the hfc tensor ACu describe the electron Zeeman and Cu hf interaction. All other symbols have their usual meaning. The analysis yielded slightly orthorhombic tensors g and ACu with principal values gxx ) Cu -1 2.059, gyy ) 2.063, gzz ) 2.377, and ACu xx ) 0.0010 cm , Ayy ) 0.0014 cm-1, and AzzCu ) 0.0148 cm-1 for the Cu(II) species A. We have to note that the signal observed in the FS ESE spectrum corresponds to only a fraction of all monomeric Cu(II) centers in the sample. However, comparable values for the parameters gzz and ACu zz can be determined from the gzz spectral region of the less resolved CW ESR spectrum in Figure 2a. Therefore it seems justified to assume that the Cu(II) spin Hamiltonian parameters as determined from the better resolved FS ESE spectrum are representative for all monomeric cupric ion species. HYSCORE spectroscopy was employed to probe the coordination environment of the Cu(II) ion species A in Cu3(BTC)2(H2O)3‚xH2O. The experimental HYSCORE spectrum (Figure 3a) displays, except an intense cross-peak ridge at the proton Larmor frequency νH ) 14.3 MHz with a width of about 3 MHz, pronounced broad proton cross-peak ridges at (12.3, 17.3) and (17.3, 12.3) MHz, which are indicative for substantial anisotropic hf interactions between the Cu(II) ions and 1H nuclei from coordinating ligand molecules. The shape of these ridges implies that the relation AHiso < TH⊥ holds between the isotropic (AHiso) and anisotropic (TH⊥ ) proton hfc parameters.28 From the maximum shift of the cross-peak ridges ∆νSmax ) 0.67 MHz from the ν1 ) -ν2 frequency axis toward higher frequencies in the 2D spectrum, we determine TH⊥ ) 5.0 MHz according to the relation28

TH⊥

(

)

S 2 8∆νmaxνH ) 3 x2

1/2

(2)

The inner and outer end positions of the proton ridges provide estimates AHxx,yy ) -3.4 MHz and AHzz ) 11.6 MHz for the principal values of the hfc tensor AH, which translate into AHiso ) 1.6 MHz using TH⊥ ) 5 MHz and the relations AHiso ) AHxx,yy + TH⊥ or AHiso ) AHzz - 2TH⊥ . The values AHiso and TH⊥ are typical for water molecules coordinating equatorially to the Cu(II) ion.28 Simulations of the HSYCORE spectrum of these strongly coupled protons with the deduced parameter set (Figure 3b) give a reasonable accordance with the shape and position of the pronounced cross-peak ridges observed in the experimental spectrum and support the analysis of the 1H hfc data. Dimeric Cu(II) Species. The Cu(II) ions of the copper dimer in the paddle-wheel building blocks of the Cu3(BTC)2 frame-

Figure 3. 1H HYSCORE spectra of the monomeric Cu(II) ion species A in Cu3(BTC)2(H2O)3 at 6 K: (a) experimental and (b) simulated spectrum. The experimental spectrum has been recorded at the gxx,yy spectral position at 335.9 mT of the Cu(II) ESR powder pattern. In the H simulation of the HYSCORE spectrum, proton hfc parameters Axx,yy ) H -3.4 MHz and Azz ) 11.6 MHz were used and the angle between the z axes of the tensors g and AH was set to 90°. The inner and outer end positions of the proton ridges are indicated by their corresponding H principal values Axx,yy and AHzz of the proton hfc tensor in the experimental spectrum.

work are known to couple antiferromagntically, resulting in an S ) 0 singlet ground and an S ) 1 excited triple state. Therefore, they are expected to be ESR silent at lower temperatures. The typical singlet-triplet splitting of Cu(II) dimers is in the range J ) -150 to -200 cm-1. Therefore, their ESR active excited S ) 1 state will be populated at temperatures above approximately 70 K only. As the reported29 fine structure parameters of their S ) 1 state are comparable to the microwave frequency at X-band (9.5 GHz), we have studied their ESR response at Q-band frequencies (34 GHz). Figure 4 illustrates a series of representative temperature-dependent Q-band ESR

Cupric Ions in Metal-Organic Framework Cu3(BTC)2

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Figure 5. Q-band ESR spectra of Cu3(BTC)2(H2O)3 at 100 K showing the superposition of the spectra from the species A, B, and C: (a) experimental spectrum and (b) simulated spectrum of species B (parameters are given in text).

Figure 4. Temperature-dependent Q-band ESR spectra of Cu3(BTC)2(H2O)3.

spectra of Cu3(BTC)2(H2O)3. At temperatures below 90 K, only the broadened powder pattern of the paramagnetic Cu(II) monomer complexes (species A) with gxx ) 2.059, gyy ) 2.063, and gzz ) 2.377 is detected besides a narrow free radical signal at g ) 2.003. The Cu hfc in the gzz spectral region of the Cu(II) ESR powder spectrum is no longer resolved at Q-band frequencies, presumably due to g strain effects. The spectra become more complex for elevated temperatures between 90 and 180 K, where a superposition of the gxx,yy component of the cupric ion monomer spectrum with a multicomponent signal (species B) and a broad isotropic resonance (species C) is observed. For temperatures above 160 K, the spectra display only the isotropic signal of C, which has a Lorentzian line shape, with a g value g ) 2.164 and a peak-to-peak line width ∆Bpp ) 69 mT at room temperature. The superposition of the spectra of the three species at intermediate temperatures is particularly obvious in the spectrum recorded at 100 K (Figure 5a). The spectrum of B is indicative for a powder pattern of an S ) 1 spin system described by the spin Hamiltonian

H ˆ ) βeBgSˆ + Sˆ DSˆ

(3)

with the traceless zero field splitting tensor D. We have to note that no hf splitting could be resolved for species B. Therefore, a hfc term was omitted in eq 3. An estimate of the spin Hamiltonian parameters of species B by spectral simulations (Figure 5b) provided gxx,yy ) 2.060, gzz ) 2.369, D ) 0.320 cm-1, and E < 0.004 cm-1. Here the zero field splitting parameters D and E relate as usual to the principal values of

the tensor D according to D ) 3/2 Dzz and E ) 1/2 (Dxx - Dyy). The typical edge singularities of the ESR powder pattern of a S ) 1 spin system30 are illustrated by the indexed labels B in Figure 5, where the indices x, y, z indicate the principal values of the tensor D, and the numbers 1, 2 are referring to the two allowed electron spin transitions ∆MS ) (1. The label B|∆MS)2| defines the position of the ∆MS ) (2 half field transition of the S ) 1 spin system. Finally, we have explored the temperature dependence of the signal of species C to obtain more information about its chemical nature. Whereas its g value is temperature independent, the line width in the Q-band spectra decreases slightly from ∆Bpp ) 79 mT to ∆Bpp ) 69 mT with rising temperature in the interval 180 K e T e 298 K. The relative intensity of signal C was investigated by double integration of the corresponding ESR spectra at X-band frequency using a dual mode cavity. This allowed us to compare the obtained intensities with a standard sample in order to eliminate any changes in the Q factor of the cavity. The normalized intensities increase with rising temperatures between 170 and 290 K and seem to reach a maximum at about 320 K, as shown in Figure 6. We have to note that a reliable intensity measurement was not feasible below 180 K because of the superposition of the ESR signal of species C with those of A and B. 4. Discussion ESR and HYSCORE spectroscopies are powerful tools for the exploration of the structure and localization of paramagnetic transition metal ion complexes in porous materials.31 The Cu(II) spin Hamiltonian parameters usually provide some rough guide to the overall coordination geometry of the metal ion based on extensive studies of known cupric ion compounds.32 In -1 for tetrahedral general, ACu zz increases from ≈0.007 cm symmetry, through elongated distorted octahedral and squarepyramidal symmetry, to ≈0.017 cm-1 for square-planar symmetry, whereas gzz decreases from 2.516 to 2.245 for this sequence of coordination symmetries.32 But caution is needed in deriving the coordination geometry only from the Cu(II) spin Hamiltonian parameters, as both ACu zz and gzz depend likewise on the type of coordinating ligand. However, together with the hfc of coordinating ligand nuclei determined by for instance

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Figure 6. Normalized ESR signal intensity IESR of species C measured at X-band frequencies. The solid line corresponds to a best fit of the experimental data using eq 4.

HYSCORE spectroscopy and a comparison with data from known Cu(II) complexes in porous materials, the ESR parameters provide detailed information about the structure and location of the Cu(II) species in the Cu3(BTC)2(H2O)3 coordination polymer. On the basis of the determined spin Hamiltonian parameters, we deduce a coordination of the paramagnetic Cu(II) ion of species A to six oxygen ligand atoms in an elongated distorted octahedral symmetry.32 The parameters are typical for [Cu(H2O)6]2+ complexes found in various porous materials31,33 and indicate the formation of a hexaaqua copper(II) complex. The proton hfc data determined by HYSCORE spectroscopy support this assignment. Both parameters AHiso ) 1.6 MHz and TH⊥ ) 5.0 MHz are in good agreement with previously reported proton hf interactions28 of the equatorially water molecules of [Cu(H2O)6]2+. The TH⊥ value translates into a reasonable copper proton distance of 0.25 nm for such equatorial water molecules. The protons from the axially coordinating water molecules with Cu-H distances of approximately 0.33 nm provide dipolar hfc parameters of 2.2 MHz and will contribute to the intense proton cross-peak ridge with a width of about 3 MHz at the 1H Larmor frequency in the HYSCORE spectrum in Figure 3a. However, more distant protons from noncoordinating water molecules and the BTC linkers will likewise contribute with their small hfc to this cross-peak. It seems natural to assume that the [Cu(H2O)6]2+ complexes are formed from residual extraframework monomeric Cu(II) ions, which have not been transformed into Cu(II)2 clusters during the synthesis process and will subsequently coordinate to free water molecules adsorbed within the Cu3(BTC)2 network. Taking into account the approximate size of the hexaaqua copper(II) complex (0.6 nm × 0.7 nm) and comparing it with the diameter of the Cu3(BTC)2 cages (1.17 nm), it seems worthwhile to note that the pores of this coordination polymer can easily accommodate such a complex. As comparable Cu(II) spin Hamiltonian parameters could be derived from the FS ESE and the CW ESR spectrum, we have to conclude that the same species is observed in the two experiments. Then dipole-dipole interactions among [Cu(H2O)6]2+ complexes present in high local concentrations and presumably formed in neighbored cages may account for the substantial line broadening of the Cu(II) CW ESR spectrum. Consequently, the paramagnetic complexes and their parent Cu(II) ions must be nonuniformly distributed within the Cu3(BTC)2(H2O)3 network. Obviously, the response in the FS ESE spectrum must

Po¨ppl et al. be caused by sufficiently magnetically diluted [Cu(H2O)6]2+ species located in areas with low local Cu(II) monomer concentrations. The ESR signals of species B and C could be only observed for temperatures T g 90 K. Therefore, we tentatively assign both to antiferromagnetically coupled metal ion clusters. Indeed the spectrum of species B could be convincingly described by an S ) 1 spin system whose evaluated spin Hamiltonian parameters gxx,yy ) 2.060, gzz ) 2.369, and D ) 0.320 cm-1 compare well with earlier reported parameters on various Cu(II) dimers.29 Hence it seems justified to assign species B to the triplet state of the antiferromagnetically coupled Cu(II)2 clusters in the paddle-wheel units of Cu3(BTC)2. However, the disappearance of its signal at T g 180 K is unusual for an antiferromagnetic spin state and can only be understood in context with the third observed species C. The g value of 2.164 of the isotropic signal of C falls in the range of the mean g values of the [Cu(H2O)6]2+ complex (species A, gav ) 2.166) and of the Cu(II) dimers (species B, gav ) 2.163) in the paddle-wheels of the Cu3(BTC)2 network. In principle, at higher temperatures, both species might cause spectrum C. So an isotropic spectrum with gav ) 2.166 could be expected for fast reorientation of the paramagnetic [Cu(H2O)6]2+ complexes due to a tumbling motion at elevated temperatures in accordance with the observed decrease in the line width of spectrum C with rising temperatures. However, the observed increase of the signal intensity with increasing temperature (Figure 6) is in contradiction with an interpretation of C in terms of a paramagnetic species. The experimental result rather indicates likewise an antiferromagnetic spin state for species C. An analysis of the temperature dependence of the normalized ESR signal intensity IESR of species C taken as a measure for the concentration of the suspected antiferromagnetic spin state using the Bleaney-Bowers equation for exchange-coupled dimers34

IESR ∝

1 kBT(3 + exp(-2J/kBT))

(4)

provides a reasonable fit to the experimental data with an exchange coupling constant J ) -185 cm-1. The value of J is in accordance with exchange coupling constants of other antiferromagnetically coupled Cu(II) dimers.29 The evaluated exchange coupling constant together with the agreement between the g value of C and the mean g value of species B allows us to assign signal C to the exited S ) 1 state of Cu(II)2 clusters of the paddle-wheel units. We have to note that J is here somewhat overestimated by about 30 cm-1 in comparison with susceptibility measurements on Cu3(BTC)2 by Zhang et al.20 This discrepancy between ESR and susceptibility data was already pointed out by Bleaney29 although it is not fully understood yet. The occurrence of just an isotropic signal for the exited state of the Cu(II) dimers with Lorentzian line shape at elevated temperatures instead of the anisotropic spectrum of an S ) 1 spin systems indicates that an additional spin exchange is taking place between the interconnected Cu(II) dimers in the paddle-wheel units across the BTC linker molecules, most likely promoted by their π-bonding system. Likewise, the observed decrease of the line width with increasing temperature can be explained by the onset of such an additional exchange processes and has been already reported for other exchanged narrowed spin systems.30,35 Taking into account just the width of the resolved anisotropic S ) 1 spectrum of the Cu(II)2 clusters (species B) and neglecting dipole-dipole interactions between the Cu(II) dimers, we roughly estimate that an exchange

Cupric Ions in Metal-Organic Framework Cu3(BTC)2 coupling constant in the order of about 0.5 cm-1 will be sufficient to lead to such an exchanged narrowed line. The disappearance of the anisotropic S ) 1 spectrum of the of the Cu(II)2 clusters (species B) for T g 180 K can be also explained within this model. Whereas the Cu3(BTC)2 network behaves like a magnetically diluted system at lower temperatures (90 K e T < 180 K), the population of the excited S ) 1 states of the Cu(II) dimers increases with rising temperature and consequently the average distance between the dimers being in the excited triplet state decreases too. In turn, this will lead to a more and more efficient exchange process between the dimers in the Cu3(BTC)2 network and in that way to an enhanced exchange narrowing resulting in an increase of the intensity of the isotropic signal on the expense of the anisotropic spectrum. We have to note that the zero field splitting parameter D ) 0.320 cm-1 of the Cu(II) dimer of the paddle-wheel unit in Cu3(BTC)2 as estimated from the anisotropic S ) 1 spectrum at T < 180 K is slightly smaller than that of the Cu(II)2 clusters (D ) 0.347 cm-1) in a copper(II)-terephthalate compound36 constituted by the same paddle-wheel units. The value of D depends among other parameters on the copper-copper distance in the paddle-wheel unit as well as on the total spin density at the two Cu(II) ions. If we assume that the details of the paddlewheel structure are the same in both compounds, then the spin density at the Cu(II) ions shall be lower in the Cu3(BTC)2 network. This implies a larger delocalization of the spin density into the aromatic linker molecules for Cu3(BTC)2, which might promote an efficient spin exchange among the Cu(II) dimers at higher temperatures where their triplet state is highly populated. Finally, we may derive a rough estimate for the concentration ratio between the copper dimers and monomers. The intensity ratio between the [Cu(H2O)6]2+ spectrum A at 6 K and the Cu(II) dimer spectrum C at 320 K as obtained by double integration of the corresponding CW ESR spectra is about 1.4. Taking into account the 1/T temperature dependence of the paramagnetic susceptibility for the monomer, we estimate that no more than 3% of the overall observed copper ions are present as monomers in the studied sample. Preliminary magnetization measurements37 gave a comparable monomer concentration. 5. Conclusions Cupric ions are found to be present in the coordination polymer Cu3(BTC)2(H2O)3‚xH2O in two different states, as antiferromagnetically coupled Cu(II)2 clusters in the paddlewheel units of the MOF network as well as extraframework paramagnetic Cu(II) monomer species located in its pores. In comparison to compounds with isolated antiferromagnetically coupled Cu(II) dimers, the three-dimensional cross linking of the paddle-wheel units in Cu3(BTC)2 opens an additional spin exchange transfer path between Cu(II)2 clusters across the BTC linker molecules, resulting in an exchanged narrowed ESR signal of the dimers at room temperature. The presence of such exchange paths across the organic linkers in MOFs is crucial for their potential exploitation with respect to the design of novel porous magnetic materials. In the studied MOF Cu3(BTC)2(H2O)3‚xH2O, the extraframework Cu(II) ions coordinate to the water adsorbed in the pore structure and form [Cu(H2O)6]2+ complexes. The Cu(II) monomer species are inhomogeneously distributed over the pore system and seem to be a remnant of the synthesis process. Therefore, special care must be taken in the analysis of any catalytic and magnetic susceptibility data as these paramagnetic open shell ions may significantly influence the catalytic activity of the Cu3(BTC)2 as well as its macroscopic measurable magnetic properties, in particular, at low temperatures.

J. Phys. Chem. C, Vol. 112, No. 7, 2008 2683 Finally we conclude that the combination of CW and pulsed ESR spectroscopy is a very powerful tool to characterize the state of the transition metal ions not only in conventional diluted paramagnetic systems. As shown here, the CW EPR experiment reveals detailed information on a molecular level about strongly magnetically coupled ions such as the Cu(II) dimers in the framework of the studied coordination polymer Cu3(BTC)2. Alternatively, pulsed EPR studies of such materials may offer the unique potential to determine precisely the chemical nature and location of minor paramagnetic species, which are not involved in the magnetic coupling phenomena in the first place but may also significantly influence the properties of MOF materials. Acknowledgment. We are grateful to R. Kirmse and J. Griebel, Universita¨t Leipzig, for preliminary results on the copper(II)-terephtalate compound, and we thank J. Haase, R. Bo¨ttcher, Universita¨t Leipzig, and A. Kassiba, Universite´ du Maine, for helpful discussions. Supporting Information Available: Powder X-ray diffraction pattern and nitrogen adsorption isotherm of the Cu3(BTC)2 sample. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Yaghi, O. M.; Davids, C. E.; Li, G. M.; Li, H. L. J. Am. Chem. Soc. 1991, 119, 2861. (2) Li, H. L.; Eddaoudi, M.; O’Keeffe, M.; Yaghi, O. M. Nature 1999, 402, 276. (3) Fe´rey, G.; Mellot-Draznieks, C.; Serre, C.; Millange, F.; Dutour, J.; Surble´, S.; Margiolaki, I. Science 2005, 309, 2040. (4) Kitagawi, S.; Kitaura, R.; Nore, S. Angew. Chem., Int. Ed. 2004, 43, 2334. (5) Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O’Keeffe, M.; Yaghi, O. M. Science 2002, 295, 469. (6) Chac, H. K.; Siberio-Perez, D. Y.; Kim, J.; Go, Y. B.; Eddaoudi, M.; Matzger, A. J.; O’Keeffe, M.; Yaghi, O. M. Nature 2004, 427, 523. (7) Mu¨ller, U.; Schubert, M.; Teich, F.; Pu¨tter, H.; Schierle-Arndt, K.; Pastre´ J. Mater. Chem. 2006, 16, 626 (8) Prestipino, C.; Regli, L.; Vitillo, G.; Bonino, F.; Damin, A.; Lamberti, C.; Zecchina, A.; Solari, P. L.; Kongshang, K. O.; Bordiga, S. Chem. Mater. 2006, 18, 1337. (9) Chen, B.; Ockwig, N. W.; Millward, A. R.; Contreras, D. S.; Yaghi, O. M. Angew. Chem. 2005, 117, 4823. (10) Millward, A. R.; Yaghi, O. M. J. Am. Chem. Soc. 2005, 127, 17998. (11) Rosseinsky, M. J. Microporous Mesoporous Mater. 2004, 73, 15. (12) Maspoch, D.; Ruiz-Molin, D.; Veciana J. J. Mater. Chem. 2004, 14, 2713. (13) Poulsen, R. D.; Bentien, A.; Chevalier, M.; Iversen, B. B. J. Am. Chem. Soc. 2005, 127, 9156. (14) Riou-Cavellec, M.; Albinet, C.; Livage, C.; Guillou, N.; Nogue`s, M.; Grene`che, J. M.; Fe´rey, G. Solid State Scie. 2002, 4, 267. (15) Guillou, N.; Livage, C.; Drillon, M.; Fe´rey, G. Angew. Chem., Int. Ed. 2003, 42, 5314. (16) Li, J.-T.; Tao, J.; Huang, R.-B.; Zheng, L.-S.; Yuen, T.; Lin, C. L.; Varughese, P.; Li, J. Inorg. Chem. 2005, 44, 4448. (17) Barthelet, K.; Marrot, J.; Riou, D.; Fe´rey, G. Angew. Chem., Int. Ed. 2002, 41, 281. (18) Maspoch, D.; Vidal-Gancedo, J.; Ruiz-Molina, D.; Rovina, C.; Veciana, J. J. Phys. Chem. Solids 2004, 65, 819. (19) Halder, J. G.; Kepert, C. J.; Moubaraki, B.; Murray, K. S.; Cashion, J. D. Science 2002, 298, 1762. (20) Zhang, X. X.; Chui, S. S.-Y.; Williams, I. D. J. Appl. Phys. 2000, 87, 6007. (21) Schlichte, K.; Kratzke, T.; Kaskel, S. Microporous Mesoporous Mater. 2004, 73, 81. (22) Wang, Q. M.; Shen, D. M.; Bu¨low, M.; Lau, M. L.; Deng, S. R.; Fitch, F. R.; Lemcoff, N. O.; Semanscin, J. Microporous Mesoporous Mater. 2002, 55, 217. (23) Zhang, X. X.; Lo, S. M.-F.; Charmant, J. P. H.; Orpen, A. G.; Williams, I. D. Science 1999, 238, 1148. (24) Bo¨hlmann, W.; Po¨ppl, A.; Sabo, M.; Kaskel, S. J. Phys. Chem. B 2006, 110, 20177.

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