Electron Spin Resonance Study of Nitroxide Radical Adsorption at

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Electron Spin Resonance Study of Nitroxide Radical Adsorption at Cupric Ions in the Metal-Organic Framework Compound Cu3(btc)2 Bettina Jee,† Kathrin Koch,† Lutz Moschkowitz,† Dieter Himsl,‡ Martin Hartman,‡ and Andreas P€oppl*,† † ‡

Institut f€ur Experimentelle Physik II, Universit€at Leipzig, Linnestrasse 5, D-04103 Leipzig, Germany Erlangen Catalysis Resource Center (ECRC), Friedrich-Alexander-Universit€at Erlangen-N€urnberg, Egerlandstrasse 3, D-91058 Erlangen, Germany

bS Supporting Information ABSTRACT: The Cu(II) pairs in the paddle-wheel building blocks of the metal-organic framework compound Cu3(btc)2 give rise to an antiferromagnetic spin state with an electron spin resonance (ESR)-silent S = 0 ground state. However, the adsorption of di-tert-butyl nitroxide (DTBN) radicals leads to the formation of an unusual nitroxide ESR spectrum and later, upon thermal treatment of the samples, to distinct paramagnetic Cu(II) centers, whose ESR signals can be observed at temperatures below 70 K. Various scenarios for the suppression of the antiferromagnetic coupling of the Cu(II) ions in the paddle-wheel units by interaction with the nitroxide and the subsequent formation of these S = 1/2 copper centers are discussed. SECTION: Surfaces, Interfaces, Catalysis

u3(btc)2, also known as HKUST-1,1,2 is one of the most investigated porous metal-organic framework (MOF) materials and among the first commercially available representatives of this new class of organic-inorganic hybrid materials. Although discovered 10 years ago, HKUST-1 is still a material of high interest due to its outstanding properties in gas storage,3 delivery,4 and separation5 processes, its magnetism,2 as well as its catalytic activity in cyanosilylation reactions.6 The network of Cu3(btc)2 (Figure 1a) belongs to the cubic space group Fm3m.1 Antiferromagnetically coupled Cu(II)2 clusters are coordinated by carboxylate groups to form so-called paddle-wheel units (Figure 1b), which are connected by the benzene 1,3,5-tricarboxylate (btc) linker molecules to form a three-dimensional porous network with two different interconnected pores. These pores with approximate diameters of 0.9 and 0.7 nm as measured between the oxygen atoms of the carboxylate groups6 are large enough to accommodate also bulky guest molecules. In the dehydrated Cu3(btc)2 material, the cupric ions in the paddlewheel units possess coordinatively unsaturated axial binding sites that may serve as adsorption sites for guest molecules with some electron-pair-donating capacity. Concerning its magnetic properties, Cu3(btc)2 exhibits pronounced antiferromagnetism due to the Cu(II) ion pair building blocks, which leads to a drastic increase of the magnetic susceptibility for temperatures above 100 K.2 The antiferromagnetically coupled Cu(II) ion pairs in the paddle-wheel units have an excited S = 1 electron spin state and an electrons spin resonance (ESR)-silent S = 0 ground state.7 In principle, the excited state is sufficiently populated at temperatures above 100 K to allow for ESR detection. Because of the wide variety of possible applications, in particular, in gas storage, delivery, separation, and sensing for

C

r 2011 American Chemical Society

Cu3(btc)2 and for MOFs in general, it is of fundamental interest to explore the formation of adsorption complexes on a microscopic scale. For Cu3(btc)2, the modification of its magnetic properties by adsorption of suitable guest molecules is of particular interest. Here, we report the adsorption of paramagnetic ditert-butyl nitroxide (DTBN-(C4H9)2NO) radicals8 (Figure 1c) at the antiferromagnetically coupled Cu(II) ion pairs in the paddle-wheel units of dehydrated Cu3(btc)2. DTBN was selected for these adsorption studies because its size allows the radical to enter the pores of the Cu3(btc)2 framework and its NO group acts as an electron pair donor and therefore may be expected to interact with the cupric ions. All adsorption experiments have been done on dehydrated MOF materials. The formed paramagnetic DTBN adsorption complexes and the resulting changes of the local magnetic properties of the MOF materials were then characterized by continuous-wave (cw) ESR and hyperfine sublevel correlation spectroscopy (HYSCORE). Details about the sample preparation and spectroscopic studies are summarized in the Supporting Information. Figure 2 illustrates the changes in the low-temperature cw ESR spectra of Cu3(btc)2 upon adsorption of DTBN. In these experiments, a low loading of 1 DTBN molecule per 45 paddle-wheel units was used to avoid broadening of the ESR spectra due to dipole-dipole interactions between the radicals. Prior to DTBN adsorption, Cu3(btc)2 displays only a faint signal of minor residual extraframework monomeric Cu(II) species (Figure 2a) which were not transformed into Cu(II)2 clusters Received: December 9, 2010 Accepted: January 24, 2011 Published: February 02, 2011 357

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Figure 1. Schematic representation of the structures of Cu3(btc)2 and the di-tert-butyl nitroxide (DTBN) molecule showing (a) the Cu3(btc)2, (b) the Cu(II) paddle-wheel building block, and (c) the DTBN radical.

Figure 2. The cw ESR spectra of DTBN adsorbed over Cu3(btc)2 at 6 K for (a) dehydrated Cu3(btc)2, (b) immediately after adsorption of DTBN, (c) after heating the DTBN-loaded sample at 363 K for 16 h, and (d) after subsequent evacuation at 393 K for 16 h. The spectra were recorded with different receiver gains, as indicated in the figure.

Figure 3. The cw ESR spectra of Cu3(btc)2 upon adsorption of DTBN and heating at 363 K for 16 h, showing the presence of three Cu(II) species A, B, and C; (a) simulated and (b) experimental spectra at 6 K. The asterisk indicates the magnetic field position where the HYSCORE spectra have been recorded.

during the synthesis process and reside in the pores of the Cu3(btc)2 network.7 Immediately after DTBN adsorption, an intense and almost isotropic ESR signal R is observed at T = 6 K with an electronic Zeeman splitting parameter of g = 2.016 and a peak-to-peak ESR line width of ΔBpp = 5.2 mT (Figure 2b). The shape and g value of signal R differ significantly from the typical ESR signal of the DTBN radical, which exhibits a three-line 14N hyperfine (hf) pattern and an axially symmetric g tensor with principal values g^ = 2.0058 and g|| = 2.0023.9 Moreover, in contrast to the radical ESR spectrum, signal R of DTBN adsorbed on Cu3(btc)2 cannot be detected at room temperature, where only the broad signal of the excited S = 1 state of the antiferromagnetically coupled Cu(II) pairs in the paddle-wheel units is observed (see Supporting Information, Figure S3). Signal R becomes observable only for temperatures below 110 K. This rather unusual behavior for an ESR signal of a radical species is further illustrated by the temperature dependence of its intensity, which significantly deviates from the Curie law valid for noninteracting paramagnetic species, the increase of the line width at T > 60 K (see Supporting Information, Figure S4 and S5), and the upward shift of the g value in comparison with those reported for DTBN.9 We assume that signal R is due to DTBN radicals

that are present in high local concentrations in areas near the surface of the MOF particles, where they are weakly adsorbed at open axial binding sites of Cu(II) ions from the paddle-wheel units. At elevated temperatures, the adsorbed radicals interact more and more with the S = 1 state of the Cu(II) pairs. As a result, the radical spectrum broadens and cannot be detected anymore for T > 110 K. The low-temperature ESR spectrum of the DTBN-loaded sample changes drastically after heating the sample to 363 K for 16 h (Figure 2c). The single-line signal R observed immediately after adsorption is completely lost, and the typical well-resolved ESR powder pattern of monomeric cupric ion species emerges and is observable for temperatures below 70 K. Spectral simulations reveal that the spectrum consists of a superposition of the spectra of a major Cu(II) species A and two minor species B and C with a concentration ratio of 1:0.5:0.15 (Figure 3). The Cu(II) spin Hamiltonian parameters as estimated from spectral simulations using the EasySpin ESR simulation package10 are summarized in Table 1. The Cu(II) parameters of species A and C suggest a square-planar coordination geometry of the cupric ions, whereas those of species B are more indicative of a squarepyramidal coordination symmetry.11,12 We have to note that 358

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Table 1. Spin Hamiltonian Parameters at 6 K of Cu(II) Ions Cu3(btc)2 after DTBN Adsorption and after Heating the Sample at 363 K for 16 ha

a

species

gxx,yy

gzz

-1 ACu xx,yy [cm ]

-1 ACu zz [cm ]

A

2.052

2.224

0.0025

0.0201

B

2.054

2.280

0.0021

0.0166

C

2.052

2.274

0.0024

0.0188

-1 Errors: Δgii = (0.002; ΔACu xx,yy = (0.0004 cm .

comparable Cu(II) spectra with slightly different concentration ratios between the three species are also observed without further thermal treatment after storing the DTBN-adsorbed samples at room temperature for a sufficiently long time of about 5 months (see Supporting Information, Figure S6). The same species are likewise observed at even lower DTBN loadings (1 DTBN molecule per 180 paddle-wheel units) but with comparable concentration ratios here (see Supporting Information, Figure S7). Subsequent evacuation of the samples at 393 K for 16 h results in an almost complete disappearance of the ESR signals of the three Cu(II) species (Figure 2d). In addition, the formation of further ESR signals which might be attributed to the nitroxide or another Cu(II) species have not been observed. Therefore, we conclude that evacuation at elevated temperatures leads to the removal of the DTBN radical without formation of a substantial amount of additional extraframework Cu(II) ions and without a collapse of the Cu3(btc)2 framework as verified by X-ray diffraction (XRD) powder patterns recorded before DTBN adsorption and after thermal removal of the nitroxide (see Supporting Information, Figure S8). The described adsorption and desorption steps are reproducible and have been observed for several cycles. In order to probe the local environment of the Cu(II) species observed for the DTBN-loaded samples after heating to 363 K, HYSCORE spectra were recorded at 6 K. A typical spectrum is presented in Figure 4 and shows a proton cross peak ridge at (14.8,14.8) MHz whose extension along the frequency offdiagonal provides a maximum 1H hf coupling of approximately 3 MHz. However, the framework protons of the btc linker have a maximum hf coupling of less than 2.4 MHz.13 Therefore, we assign the ridge to the methyl protons of the adsorbed DTBN radical. The presence of DTBN in the vicinity of the Cu(II) ion centers is likewise indicated by a number of cross peaks at lower frequencies from two 14N (IN = 1) nuclei N1 and N2 indicated by the red and blue areas in Figure 4. The cross-peak pattern of N1 with three intense diagonal peaks at ν-,N1 = 0.98 MHz, ν0,N1 = 2.94 MHz, and νþ,N1 = 3.92 MHz in the (þ,þ) quadrant of the 2D spectrum is characteristic of 14N near the cancellation regime, frequently observed for remote nitrogen atoms at the X band.14 We determine a nuclear quadrupole (nq) coupling constant of KN1 = 1.2 MHz with an asymmetry parameter of ηN1 = 0.43 from these nq frequencies caused by the nuclear spin transitions within one electron spin manifold according to14 K¼

ν- þ ν þ 6

η¼

ν0 2K

Figure 4. HYSCORE spectrum at 6 K of Cu3(btc)2 upon adsorption of DTBN and heating at 363 K for 16 h. Two spectra with pulse delays of τ = 104 and 136 ns were recorded at 342.8 mT, and the sum of the two spectra is displayed. Red and blue areas indicate cross peaks of nitrogen nuclei N1 and N2, respectively, and green areas mark peaks of unknown origin. (An enlarged plot of the 14N region of the HYSCORE spectrum is presented in the Supporting Information, Figure S9.)

because the 14N hf coupling appears to be too small for such a binding mode. Using14 2 2 2 1=2 νR;β dq ¼ 2½ðνN ( Aiso =2Þ þ K ð3 þ η Þ

ð2Þ

we derive with νRdq = νþ,N1 from the frequency νβdq = 6.2 MHz of the so-called double quantum (dq) transition in the other electron spin manifold an effective hf coupling parameter12 of only AN1 = 2.6 MHz for N1. In addition, the HYSCORE spectrum displays further cross peaks that do not belong to the red-marked correlation features of N1. The most striking features are two pronounced cross peaks at (4.7,-7.7) and (7.7,-4.7) MHz (marked blue in Figure 4) in the (þ,-) quadrant that we assign to the dq frequencies νRdq and νβdq of a second 14N nucleus N2. Here, eq 2 provides AN2 = 4.4 MHz and K2N2(3 þ η2N2) = 4.2 MHz2. We assign these couplings to a nitrogen of a further DTBN molecule, presumably adsorbed in the proximity of one of the two other minor Cu(II) species B and/or C. One more set of cross peaks at (4.4,11.8) and (11.8,4.4) MHz together with a diagonal peak at 4.4 MHz indicated by green color in Figure 4 can be assigned to neither a third nitrogen nuclei nor to protons. Here, we may only speculate about their origin. They might be caused by the hf interaction of the Cu(II) centers with a neighboring 63Cu(ICu=3/2) nucleus as their spacing of about 7.4 MHz is close to twice the 63Cu Larmor frequency 2νCu = 7.7 MHz. This assumption would imply a copper hf coupling of approximately 15 MHz. However, further studies are required to support the interpretation proposed here, and a more detailed analysis of the very complex HYSCORE spectra including orientation-selective HYSCORE and pulsed electron nuclear double resonance experiments will be given elsewhere. In principle, three interaction schemes between the DTBN nitroxide molecules and the MOF framework can be envisaged in

ð1Þ

They are comparable to the nq parameters recently published for other nitroxides.15 Therefore, we may assign N1 to the nitrogen atom of a DTBN molecule located in the vicinity of presumably the major Cu(II) species A. However, a direct coordination of the NO group of the nitroxide to the cupric ion seems to be unlikely 359

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The Journal of Physical Chemistry Letters order to explain the formation of paramagnetic Cu(II) species in Cu3(btc)2 with its antiferromagnetically coupled Cu(II) ion pairs in the paddle-wheel units upon adsorption of the radicals. (I) There could be an electron transfer of the single electron in the antibonding Π*y molecular orbital (MO) at the NO group of the DTBN molecule adsorbed at the open axial binding site of one of the two cupric ions in the paddle-wheel unit to the nextneighboring Cu(II) ion, leading to a diamagnetic 3d10 closedshell electronic configuration and consequently leaving the electron spin of the second Cu(II) ion in the pair with its 3d9 configuration uncompensated. Such an electron transfer would imply the existence of a potential barrier that could be overcome by thermal excitation of the electron in the Πy* MO of the DTBN. However, the detection of paramagnetic Cu(II) species not only after heating the sample to 363 K but also after equilibrating the sample at room temperature for a prolonged time does not seem to support an activated electron-transfer process across a potential barrier. (II) Alternatively, an antiferromagnetic coupling between the electron spin of the cupric ion and that of the axially binding nitroxide could develop, leaving, likewise, the electron spin at the second Cu(II) uncompensated even without any electron transfer. Such complexes between magnetic metal ions and free nitroxide radical ligands have been frequently discussed, and their magnetic properties were analyzed in detail.16-19 However, an axial binding mode of the nitroxide at cupric ions is reported to result in a ferromagnetic coupling between the unpaired electron spins at the radical and at the ion,19 which makes the formation of S = 1/2 Cu(II) centers with the unpaired electron spin almost entirely localized at one metal ion very unlikely. Otherwise, an equatorial coordination of the nitroxide radical to the Cu(II) ion gives rise to an antiferromagnetic coupling between the spins,19 which will lead to a more likely scenario in the present case. In this scenario (III), the DTBN nitroxide radicals distribute more uniformly over the entire MOF sample with proceeding time, a diffusion process which can be stimulated by thermal treatment. The initially axially coordinating radicals switch to an equatorial binding mode with the Cu(II) ions, where the oxygen of the NO group of the DTBN replaces an equatorially binding carboxylate oxygen of the paddle-wheel unit. We like to emphasize that the paddle-wheel units in Cu3(btc)2 are known to be chemically fragile, and the carboxylate oxygen atoms can be easily substituted by oxygen20 or nitrogen21 donor atoms of polar adsorbates. Then, for such a reorganized Cu(II) pair-DTBN adsorption complex, the antiferromagnetic coupling between the electron spin at the equatorially binding radical and that of the cupric ion of the paddle-wheel where the radical is adsorbed will result in a diamagnetic S = 0 spin state, leaving the spin of the second Cu(II) in the pair uncompensated. Consequently, the resulting overall DTBN-Cu(II) paddle-wheel adsorption complex will be paramagnetic, with an S = 1/2 ground state where the unpaired electron is mainly localized at the second Cu(II) ion. This ion may retain its coordination to four carboxylate oxygen atoms in a square-planar coordination geometry, as indicated by the Cu(II) spin Hamiltonian parameters of the major species A. The nitrogen of the DTBN radical equatorially coordinating to the other Cu(II) ion of the ion pair would have the typical characteristic of a remote nitrogen with small isotropic 14N hf coupling, as observed by HYSCORE spectroscopy for the two nitrogen nuclei N1 and N2. The minor Cu(II) species B and C resemble then more distorted Cu(II) paddle-wheel units whose existence does not seem unlikely for such defect sites in the MOF

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framework. Later evacuation of the sample at elevated temperatures may lead to the removal of the nitroxide and the restoration of the undistorted Cu(II) paddle-wheel units. The presented experimental results support the later scenario III with an equatorial binding of the radicals to Cu(II) ion pairs. Further support for a formation of such a kind of nitroxideCu(II) pair complex is expected from future quantum chemical computations on DTBN-Cu(II) paddle-wheel model clusters.

’ ASSOCIATED CONTENT

bS

Supporting Information. Experimental details of sample preparation, characterization, and spectroscopic measurements. Nitrogen adsorption isotherm and IR spectra of Cu3(btc)2, ESR spectra of DTBN freshly adsorbed over Cu3(btc)2 at 300 and 6 K as well as of DTBN in frozen toluene, temperature dependences of the intensities and line widths of the ESR spectra of DTBN freshly adsorbed over Cu3(btc)2, ESR spectra of DTBN adsorbed on Cu3(btc)2 after storing the samples at room temperature for 5 months, ESR spectra of DTBN adsorbed on Cu3(btc)2 for different loadings, XRD powder pattern of dehydrated Cu3(btc)2 before DTBN adsorption and after thermal removal of the nitroxide radicals in vacuum, and the enlarged part of the 14 N cross-peak region of the HYSCORE spectrum taken after adsorption of DTBN and heating the sample at 363 K. This material is available free of charge via the Internet at http://pubs. acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel. þ49 341 9732608. Fax: þ49 341 97 32649.

’ ACKNOWLEDGMENT The authors thank the DFG for financial support in the frame of the Priority Program 1362. ’ REFERENCES (1) Chui, S. S.-Y.; Lo, S. M.-F.; Charmant, J. P. H.; Orpen, A. G.; Williams, I. D. A Chemically Funtionalizable Nanoporous Material [Cu3(TMA)2(H2O)3]n. Science 1999, 283, 1148–1150. (2) Zhang, X. X.; Chui, S.S.-Y.; Williams, I. D. Cooperative Magnetic Behavior in the Coordination Polymers [Cu3(TMA)2L3] (L=H2O, Pyridine). J. Appl. Phys. 2000, 87, 6007–6009. (3) Prestipino, C.; Regli, L.; Vitillo, J. G.; Bonino, F.; Damin, A.; Lamberti, C.; Zecchina, A.; Solari, P. L.; Kongshaug, K. O.; Bordiga, S. Local Structure of Framework Cu(II) in HKUST-1 Metallorganic Framework: Spectroscopic Characterization upon Activation and Interaction with Adsorbates. Chem. Mater. 2006, 18, 1337–1346. (4) Xiao, B.; Wheatley, P. S.; Zhao, X.; Fletcher, A. J.; Fox, S.; Rossi, A.G .; Megson, I. L.; Bordiga, S.; Regli, L.; Thomas, K. M.; et al. HighCapacity Hydrogen and Nitric Oxide Adsorption and Storage in a Metal-Organic Framework. J. Am. Chem. Soc. 2007, 129, 1203–1209. (5) Hartmann, M.; Kunz, S.; Himsl, D.; Tangermann, O.; Ernst, S.; Wagener, A. Adsorptive Separation of Isobutene and Isobutane on Cu3BTC2. Langmuir 2008, 24, 8634–8642. (6) Schlichte, K.; Kratzke, T.; Kaskel, S. Improved Synthesis, Thermal Stability and Catalytic Properties of the Metal-Organic Framework Compound Cu3(BTC)3. Microporous Mesoporous Mater. 2004, 73, 81–88. (7) P€ oppl, A.; Kunz, S.; Himsl, D.; Hartmann, M. CW and Pulsed ESR Spectroscopy of Cupric IOns in the Metal-Organic Framework Compound Cu3(BTC)2. M. J. Phys. Chem. C 2008, 112, 2678–2684. 360

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