J. Phys. Chem. C 2010, 114, 21371–21377
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Hydrogen Physisorption in a Cu(II) Metallacycle Tanja Pietraβ,*,† Itza Cruz-Campa,‡ Justine Kombarakkaran,† Suman Sirimulla,§ Atta M. Arif,§ and Juan C. Noveron*,‡ Department of Chemistry, New Mexico Tech, Socorro, New Mexico 87801, United States, Department of Chemistry, UniVersity of Texas at El Paso, 500 West UniVersity AVenue, El Paso, Texas 79968-0513, United States, and Department of Chemistry, UniVersity of Utah, 315 South 1400 East Room 2020, Salt Lake City, Utah 84112-0850, United States ReceiVed: February 3, 2010; ReVised Manuscript ReceiVed: October 29, 2010
The interaction of molecular hydrogen with a novel microporous dinuclear Cu(II) complex, [bis-µ-di(4pyridyl)methanol-1,4,7-triazacyclononane copper(II)] triflate (1), and its derivatives formed from oxidation and solvent removal was studied with 2H NMR and density functional theory (DFT). The Cu-complex 1 was characterized with X-ray diffraction methods and consists of a dinuclear macrocycle that forms one-dimensional channels of 9.55 Å in diameter. 2H NMR studies of deuterium gas adsorption by 1 suggest that physisorption condensation of D2 occurs within two distinct microenvironments: in the interior and in-between the microtubular structures. The assignment of NMR resonances to specific adsorption sites is supported by spectral decomposition and analysis of the line widths and integrated signal intensities of the components. The dynamics of the system are probed by spin-lattice relaxation time measurements and spectral hole-burn experiments as a function of temperature and pressure. NMR and DFT calculations suggest that hydrogen uptake is mediated through interactions with the Cu(II) centers via dipole-ion interactions. Introduction The use of molecular hydrogen as the energy source for power driven technologies relies on the development of an effective hydrogen storage system that operates under low-pressure and high-capacity conditions.1 Recent reviews2-8 show that existing materials do not yet meet the targeted criteria for practical hydrogen storage in mobile applications, and that a need to increase our fundamental understanding of hydrogen physisorption in well-characterized molecular materials is essential. This will pave the way to finding structure-function relationships that inspire new material designs with hydrogen storage potential in the future. Microporous materials with metal-organic frameworks (MOFs) have been considered promising candidates for hydrogen storage.9-12 They comprise a large number of self-assembled, three-dimensional structures with open frameworks typically composed of coordination networks of transition metals and organic spacer linkers. Their porosity and chemical functionality can be adjusted by varying the metal type and the nature of the organic linkers, and they are suitable models to study physisorption of molecular hydrogen with nuclear magnetic resonance (NMR).10 In previous work, it has been suggested that the metal oxide clusters in MOFs are primarily responsible for hydrogen storage, while the organic linkers play only a secondary role related to porosity and surface area.13-15 On the other hand, in organometallic compounds with hydrogen binding capacity, H2 uptake is limited by the 18-electron rule.16 Hydrogen adsorption was suggested to be a cooperative process which is initiated by the * To whom correspondence should be addressed. (T.P.) Telephone: 575835-5586. Fax: 575-835-5364. E-mail:
[email protected]. (J.C.N.) Telephone: 915-747-7572. Fax: 915-747-5748. E-mail:
[email protected]. † New Mexico Tech. ‡ University of Texas at El Paso. § University of Utah.
metal site,17 and adsorption is strongest when hydrogen is oriented in the direction of coordinatively unsaturated sites on the metal center.18 In addition, there are reports that amine functionalities within cavities created by larger spacer linkers lead to larger binding energies and increased H2 uptake.19 To date, no material has been found that meets all requirements for an economically viable usage in terms of cost, weight, volume, efficiency, durability, refueling time, life-cycle, and codes and standards,3 at least not at ambient temperature and moderate pressures.18 Much of this work on hydrogen storage is based on calculations, and only a limited amount of experimental data is available. In this contribution, we use NMR spectroscopy to experimentally probe hydrogen storage sites and dynamics in a microporous crystalline material that exhibits coordinatively unsaturated Cu(II) sites. As shown in previous work,21 it is advantageous for NMR purposes to use deuterium gas 2H2 instead of 1H2 to probe microenvironments in porous materials, as diminished dipolar couplings in 2H NMR significantly enhance spectral resolution. In the present paper we report the hydrogen gas interactions with the microporous Cu(II) dinuclear metallacyclic compound [bis-µ-di(4-pyridyl)methanol-1,4,7-triazacyclononane copper(II)] 1 and its derivatives studied with NMR and DFT. Although, the copper and hydrogen interactions have been studied in the past,20,32 this paper describes the Cu(II)-H2 interactions in a well-characterized porous material as well as after air oxidation and solvent removal. Experimental Section All commercially available chemicals were purchased from Aldrich, Inc. and used as supplied. The compounds 1,4,7triazacyclononane22 and di(4-pyridyl)methanol23 were prepared according to previously reported methods. Solvents where distilled under anaerobic conditions. Infrared spectra were recorded on a Perkin-Elmer FT-IR spectrometer. Characteriza-
10.1021/jp104544r 2010 American Chemical Society Published on Web 11/19/2010
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tion of organic components was carried out with the NMR Varian XL-300 spectrometer. UV-vis spectra were recorded in the visible range (400-800 nm) on a Perkin-Elmer Lambda 35 spectrometer. UV WinLab v. 2.85.04 software was used to analyze the spectra. Synthesis of Cu Complex 1. In a 50 mL Schlenk flask, 1,4,7triazacyclononane (100 mg, 0.77 mmol) was dissolved in 3 mL of acetonitrile. This solution was slowly cannulated into a solution of copper(II) trifluoromethanesulfonate (280 mg, 0.77 mmol) in 3 mL of acetonitrile under an argon atmosphere. The combined mixture turned dark blue and was left to stir for 1 h. Afterward, di(4-pyridyl)methanol (18 mg, 0.77 mmol) dissolved in 3 mL of acetonitrile was slowly injected into the mixture, and the solution turned dark purple and was allowed to stir for 18 h. Addition of 7-10 mL of diethyl ether to the solution and storage for 24 h at -20 °C afforded microcrystals of 1 (271 mg, yield 43%). Single crystals were grown by diffusion of diethyl ether into a dilute acetonitrile solution under anaerobic and anhydrous conditions. UV-vis (acetonitrile) λmax ) 580 nm. Selected IR bands (KBr pellet, cm-1) 3433 (m, OH), 1616 (m, NdC), 1429 (m), 1250 (st), 1030 (m), 800 (s), 638 (m). Single-Crystal Structural Analysis. The X-ray crystallographic data of Cu complex 1 (a purple prism-shaped crystal 0.33 × 0.25 × 0.13 mm3 in size) were collected at 150(1) K on a Nonius Kappa CCD diffractometer equipped with Mo KR radiation (λ ) 0.71073 Å). The structure refinement was carried out by full-matrix least-squares on F2 and Fourier transform techniques, and location of hydrogen atoms and their isotropically refinement was done using SHELXTL-97 (Bruker-AXS, Inc. Madison, WI). The Cu complex 1 crystal was mounted on a glass fiber with traces of viscous oil and then transferred to the diffractometer. Ten frames of data were collected at 150(1) K with an oscillation range of 1°/frame and an exposure time of 20 s/frame. Indexing and unit cell refinement based on all observed reflection from those 10 frames indicated a triclinic P lattice. A total of 12 692 reflections (λmax ) 27.48°) were indexed, integrated, and corrected for Lorentz, polarization, and absorption effects using DENZO-SMN and SCALEPAC.24 Post refinement of the unit cell gave a ) 12.0834(5) Å, b ) 12.8184(5) Å, c ) 13.3568(5) Å, R ) 110.7808(19), β ) 103.2229(18), γ ) 101.3639(18), and V ) 1792.85(12) Å3. NMR Experiments. About 30 mg of sample was weighed into a 5 mm outer diameter Pyrex glass tube. The open end of the tube was connected via Swagelok fittings to a long piece of PFA tubing, which was attached to the vacuum rack. A tank of deuterium gas (Cambridge Isotope Laboratories, Andover, MA) was attached to this assembly via a three-way stopcock, so that the sample could be evacuated and exposed to deuterium gas while inside the NMR magnet. Prior to NMR experiments, the sample was evacuated overnight to a pressure of 10-1 kPa and then exposed to varying pressures of deuterium gas. NMR data were recorded on a Tecmag Apollo NMR spectrometer, operating at a Larmor frequency of 61.4 MHz for 2H and using a dual-resonance Bruker 5 mm high resolution, inverse probe. Temperature was controlled by allowing a stream of nitrogen gas, evaporated off a liquid nitrogen reservoir, to flow around the sample. The gas stream was heated to the desired temperature using a resistive heating coil controlled by an Omega Engineering (Stamford, CT) unit. Results and Discussion Synthesis and Self-Assembly. The dinuclear Cu(II) complex 1 was prepared when stoichiometric mixtures of 1,4,7-triazacyclononane, Cu(II) triflate, and di(4-pyridyl)methanol were
Pietraβ et al. TABLE 1: 2H NMR Line Widths after Spectral Decomposition of the Resonances of Deuterium Gas in Contact with 1ox at Different Temperatures and Pressures line width in Hz component
P [kPa]
295 K
273 K
253 K
233 K
1
136 446 653
129 42 32
94 38 27
91 39 24
75 40 25
2
136 446 653
30 37
62 45
46 52
23
136 446 653
85 72
37
3
reacted in acetonitrile under anaerobic and anhydrous conditions. This mixture underwent spontaneous self-assembly in solution to form the discrete metallacycles. UV-vis spectroscopy of 1 in acetonitrile gives rise to an absorbance λmax ) 580 nm. IR spectroscopy reveals two distinguishable bands at 3433 and 1616 cm-1 corresponding to the absorbance of the hydroxyl group (OH) and to the pyridyl (NdC) groups, respectively. Structures. Selected X-ray crystallographic data for the copper complex is compiled in Table 1. The structure of the dimer is shown in Figure 1a, the unit cell in Figure 1b, and a ball and stick view of the extended unit cell in Figure 1c. The structure reveals that the Cu(II) complex exhibits a rhomboidal shape with the Cu(II) ions acting as 90° linkers to the pyridyl rings of di(4-pyridyl)methanol (N1-Cu-N2′ angle is determined as 89.77°). The pyridyl rings are bent outward in order to compensate for the higher ring strain. The Cu-N distances are all 2.03 Å, except for one of the nitrogen atoms of the triazacyclononane (N3) which is separated from the copper center by 2.22 Å. The angle between C3-C6-C7 is 108.3°, indicating an sp3 hybridized carbon atom. Four triflate counterions are present per asymmetric unit of 1 dimer complex. These ionic species are located outside the parallelograms, filling up the void space between the complexes. For the cavity, a diagonal Cu-Cu distance of 9.55 Å and a diagonal C6-C6′ distance of 8.29 Å are observed. Dispersion forces and hydrogen bonds between Cu dimer complex 1 and its counterions (Figure 2) are responsible for arrangement of molecules in the crystal lattice. The structure reveals distorted solvent molecules located within the channels formed by the metallacycles. 2 H NMR Adsorption Studies of 2H2. Figures 3 and 4 show 2 H NMR spectra for 1. After exposure to air and prolonged exposure to vacuum, the spectral signature of 2H2 in contact with 1 changed, suggesting a structural modification of the parent compound (see Figure 4). Reasonable hypotheses for the observed changes include air oxidation of the most labile hydroxide functionality and solvent removal, respectively. In order to verify the oxidation hypothesis, we prepared the 1 crystals as described earlier, replacing the argon atmosphere with air. Indeed, the methanol functionality was oxidized to the corresponding ketone. However, ring closure did not occur when the crystals were formed from solution resulting in a nonporous structure, which suggests that the hydroxide functionality plays an important role in the self-assembly process. Once the ring structure is formed, it appears to be stable upon oxidation. Oxidation of the hydroxyl group in 1 was verified using IR spectroscopy. The IR spectra (see Figure 3) show the disappearance of the O-H band upon oxidation, accompanied by the emergence of a strong CdO band.
Hydrogen Physisorption in a Cu(II) Metallacycle
Figure 1. (a) Thermal ellipsoid plot of 1. H atoms and counterions are omitted for clarity. (b) Unit cell of 1. Triclinic space group P1, a ) 12.0834(5), b ) 12.8184(5), c ) 13.3568(5) Å, R ) 110.7808(19)°, β ) 103.2229(18)°, and γ ) 101.3639(18)°. (c) Solvent-free crystal packing of 1 (ball and stick view).
The retention of the porosity upon oxidation of the crystals, which depends on intact rings, was verified by NMR analysis (vide infra). Removal of solvent molecules led to structural collapse of the porous network as discussed in more detail below. Figure 4 shows 2H NMR spectra for the original form of 1 with the intact methanol unit exposed to deuterium gas, the oxidized ketone form 1ox, and the methanol form 1sf after prolonged exposure to vacuum at ambient temperature. Spectra shown in Figure 4 were recorded at ambient temperature and a pressure of 653 kPa, conditions which favor penetration of deuterium gas into the lattice (see Figure 5). Prolonged exposure to vacuum causes the shoulder, assigned to occluded gas, to diminish in size, suggesting structural collapse due to acetonitrile removal, which leads to less void space available for the deuterium gas molecules to occupy. Temperature- and pressure-dependent 2H NMR data of 1ox exposed to 2H2 are shown in Figure 5. At ambient temperature,
J. Phys. Chem. C, Vol. 114, No. 49, 2010 21373 the signal amplitude is weakest, and it increases both with decreasing temperature and increasing pressure. The line width, however, is greatest at ambient temperature. A more detailed analysis on line width and integrated signal intensity is provided below. At the highest temperature, the signal consists of a tall, narrow resonance that is accompanied by an upfield shoulder. This shoulder gradually disappears with decreasing temperature. Tentatively, we assign the tall, narrow resonance to deuterium gas in the interparticle and void gas space above the powder in the NMR sample tube. It should be noted that the volume occupied by the sample was smaller than that of the pick-up coil, so that also the overhead gas space contributes to the NMR signal. The shoulder is assigned to gas occluded in the void spaces of the crystal structure. Both the methanol and ketone forms give rise to two to three spectral components, depending on temperature and pressure (Figure 4). Spectral decomposition is somewhat arbitrary and was carried out with the minimum number of components necessary to obtain reasonably good fits while keeping the number of possible adsorption sites in the crystal structure in mind. Oxidation causes the shoulder components to appear on the upfield side of the gas peak, while the reverse is true for 1 (Figure 4). In addition, the chemical shift for the gas peak, component 1, is 6.8 ppm for the oxidized form, the same as that of free deuterium gas.22 For 1, however, this resonance is located at 7.9 ppm. In the oxidized form, components 2 and 3 are shifted upfield from the gas peak by 0.8 ppm and 1.4 ppm, respectively, while for 1 these shifts are 0.9 ppm and 2.5 ppm downfield. Note that peak positions are independent of temperature and pressure. When compared to free deuterium gas, the strong downfield shifts of all components in the methanol form suggest interaction of the gas with paramagnetic Cu2+. It is conceivable that in the ketone form a quinoid structure might exist, which would entail back-donation from the Cu center to the bridging moieties. If in fact this does occur, Cu2+ would become diamagnetic, explaining the absence of a paramagnetic shift of the deuterium resonances. This is also consistent with the greater line widths that are observed for 1 (see below). Sites associated with components 2 and 3 are most likely the interior of the microtubules and voids in between the macrocycles. Components 2 and 3 are observable at all temperatures and pressures for 1, and only at the higher temperatures and pressures for the oxidized form (Table 1), especially component 3. Component 2 is observed over a wider temperature and pressure range. Thus, sites corresponding to component 3 must either be harder to access, or possess lower adsorption energies. As is evident from Figure 4, in the methanol form, component 3 experiences the largest paramagnetic shift and must be in the greatest spatial proximity to the Cu centers. Most likely, this is the site within the metallacycles. Line widths of all three components generally are greater than those for the keto-form 1ox. In addition, and unlike in free deuterium gas, the line width of component 1 increases with decreasing temperature and pressure, which implies that hydrogen gas in the interparticle space must be in exchange with sites closer to the paramagnetic center. At lower temperatures and higher pressures, the reduced mobility of hydrogen molecules in the crystal voids extends the average interaction time of deuterium gas with the paramagnetic center, and the line width increases. In order to elucidate the origin of components 2 and 3, a more detailed analysis of line widths as function of temperature and pressure is required. Data for 1ox (Figure 5) are listed in
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Figure 2. Hydrogen bonds between Cu dimer complex 1 and its counterions responsible for arrangement of molecules in the crystal lattice.
Figure 3. IR spectra of 1 and 1ox (256 scans, 4 cm-1 resolution).
Figure 4. 2H NMR spectra (dashed lines) of 1, 1ox, and 1sf, exposed to an equilibrium pressure of 653 kPa deuterium gas at ambient temperature. Solid lines marked 1, 2, and 3 represent different contributions to the experimentally observed line shape after spectral decomposition and line fitting. The green solid line is the sum of the individual contributions.
Table 1. The line width of component 1, tentatively assigned to deuterium gas in the interparticle and overhead space, changes with temperature most notably at the lowest pressure of 136 kPa where it decreases from 129 to 75 Hz. Here, the line width decreases slightly as the temperature is dropped and decreases
strongly as pressure is increased. The line width of pure deuterium gas at 136 kPa decreases from 117 to 104 Hz between 295 and 233 K. Overall, the temperature dependence is slight, whereas increased pressure causes a pronounced drop in line width. To summarize, the temperature and pressure dependence of the line width of component 1 is very similar to that of free deuterium gas, confirming the tentative assignment to gas in the overhead or interparticle space. An analysis of the integrated signal intensity provides information about the extent of gas uptake. Without adsorption, an increase in pressure leads to a linear increase in signal intensity as the spin density in the coil region grows. In the presence of a sample, the overall signal intensity is smaller due to the dead volume taken up by the sample. In the absence of adsorption, the slope of the signal intensity as a function of pressure is identical to that of an empty cell. In the presence of adsorption, however, this slope will become steeper when the total spin density (of adsorbed and free gas) is greater than that of free gas alone. In this case, the total signal intensity may also exceed that of an empty cell, overcompensating for the reduction in available volume. The ratio of the slopes for the integrated signal intensity of adsorbed gas as a function of pressure (component 2 and 3) to that of free gas (component 1) is thus a measure for the ease of adsorption. If the ratio is greater than one, adsorption is occurring. For 1ox in the temperature range 295-253 K, this ratio is about 2.3 and is independent of temperature. At 233 K, however, the ratio drops to 1.3. The fraction occupying the channel sites, however, ranges from 0% at 233 K and 136 kPa to over 40% at 295 K and 653 kPa. From these data, we conclude that the process of adsorption is favored above 233 K, yet the absolute amount adsorbed is limited. Typically, adsorption is more favorable at lower temperatures. Simply put, this is due to competition between adsorption energy of the adsorbent and kinetic energy of the adsorbate. If the latter exceeds the former, adsorption is less likely to occur. Here, we observe the opposite trend. The occupation of sites corresponding to components 2 and 3 is favored if the temperature is greater or equal than 253 K. This implies that the deuterium molecules
Hydrogen Physisorption in a Cu(II) Metallacycle
Figure 5.
2
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H NMR spectra of 1ox exposed to different equilibrium pressures of deuterium gas at different temperatures.
must have sufficient kinetic energy to overcome a barrier to adsorption, most likely entry into the interior of the crystallites. Both increased kinetic energy and pressure facilitate this entry. For 1, the situation is different. Over the entire temperature range, the slopes are constant, within error, and the ratio of the slopes for the integrated signal intensity of adsorbed gas as a function of pressure (component 2 and 3) to that of free gas (component 1) is 0.3. The fraction of gas located in the interior of the crystal, however, is constant over the entire temperature and pressure range and is about 65%. These results indicate that while more deuterium is located in the void spaces of the crystal, the adsorption process is not affected by temperature and pressure. Moreover, the small ratio of 0.3 may indicate that some of the deuterium in the interior of the crystal may not be detectable by NMR, most likely because of rapid relaxation due to interaction with the paramagnetic Cu center. In conjunction with the fact that the line widths and shifts are greater in 1 than for 1ox, these results lead us to suggest that deuterium may be binding to the Cu center which only affords 5-fold coordination in the absence of deuterium. In addition, the adsorption energy must be greater than that for the oxidized form as the kinetic energy of the molecules does not affect the extent of deuterium uptake, which is also consistent with the picture of deuterium binding to Cu. As mentioned above, our data indicate the possibility of exchange between deuterium in the interparticle space and deuterium occluded in the crystal. Therefore, we investigated the dynamics of the system by measuring 2H NMR spin-lattice relaxation times, T1, as a function of temperature and pressure, shown in Figure 6 for 1ox. While at 295 K, T1 for D2 in 1ox is slightly shorter than that in the pure gas phase; at 253 and 233 K, the T1’s are close to equal. As shown above, at the lowest temperatures, deuterium penetration into the crystal is minor and the T1’s are expected to be close to those of the free gas, as observed. Above 253 K, 2 H NMR relaxation of deuterium is slowed when compared to the free gas due to the increased number of collisions, inhibiting the coupling of the total angular momentum of the molecule to that of the nuclear spin.26 2H nuclear spins in free deuterium gas relax due to the spin-rotation interaction, and T1 is linearly dependent on gas density and temperature.25 On the contrary, for 1, where deuterium is located in the interior of the crystal at all temperatures and pressures, T1 differs most strongly from that of the free gas at the lowest temperature and is shorter due to interaction with the paramagnetic Cu center.
Figure 6. 2H NMR spin-lattice relaxation times T1 of free deuterium gas (unfilled symbols, dashed lines) and of 1ox exposed to deuterium gas (filled symbols, full lines), at different temperatures and as a function of pressures. Error bars are smaller than the symbols. Straight lines represent linear fits.
As mentioned above, upon prolonged exposure to vacuum, the acetonitrile solvent molecules are apparently removed from the crystal, leading to structural collapse. If this is indeed the case, we expect that exchange between occluded deuterium gas and the interparticle space should be impeded. The exchange dynamics were probed using spectral hole burning28 where a narrow slice of a width of 100 Hz, centered on component 2, was excited with a soft, Gaussian-shaped 90° pulse, followed by a hard 90° pulse after a variable delay τ. For short values of τ, the magnetization of spins within the narrow excitation window of the Gaussian pulse is then oriented along the negative z-axis and is not detectable by NMR. The nuclear spin magnetization of deuterium molecules residing in areas where they did not experience the soft pulse is only excited by the hard pulse and gives rise to an NMR signal. A hole can only be burned if the broadening is inhomogeneous. The recovery of the spectral hole was shown to be a measure of rotational reorientation in glassy glycerol,28 and followed an exponential behavior. In our case, the filling of the spectral hole is a measure of physical diffusion29 that is intertwined with T1, if τ is sufficiently long for T1 relaxation to occur. The hole recovery time constant Tex at 295 K is plotted as a function of pressure in Figure 7. Tex was obtained by fitting the difference between the integrated signal intensity obtained after excitation with a hard pulse, and that after excitation of a soft 90° pulse followed by a hard 90° pulse, separated by a delay τ, as function of τ.
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As is evident from Figure 7, Tex is increasing with pressure, indicating a diffusion-limited process. The data can be fitted to a phenomenological equation of the form
T1 [ms] ) Tc [ms] × (1 - e-(P-Pon)/Pch)
(1)
where Pon is the pressure at the onset of the regime where exchange is diffusion limited and Pch is a characteristic pressure that describes how strongly T1 depends on pressure. Tc is a constant that describes the time at which the system has returned to its original state, be it through spin-lattice relaxation or completed exchange (whichever occurs first). The fit to data shown in Figure 7 yields Tc ) 39 ( 2 ms, Pon ) 348 ( 13 kPa, and Pch ) 61 ( 24 kPa. It should be noted that, at 295 K, Tc is much longer than T1. If solely relaxation dominated the filling of the spectral hole, Tc would be expected to be equal to about five times T1, where nuclear spin relaxation has reached close to 100%. Tc indeed is close to this time scale. However, if T1 solely dominated Tc, we would expect a linear relationship between Tex and pressure, which is not observed. DFT Calculations. An equilibrium geometry DFT calculation was carried out using Titan molecular modeling software.27 The lowest possible energy position of H2 gas near the Cu complex 1 was obtained. One dihydrogen molecule was placed at a distance of 1.4 Å from the Cu(II) center, and DFT calculations were obtained at the B3LYP/LACVP* level of theory. The LACVP* basis set consists of the Los Alamos effective core pseudopotential (ECP) polarization basis29 for Cu combined with the 6-31G* polarization basis set for H, C, N, and O. The global minimum position gave a final energy of -8.311 × 105 kcal/ mol. The calculation produced a geometry with nearest hydrogen (in H-H molecule) to the copper atom at a distance of 4.539 Å and a natural population analysis (NPA) charge of -0.027 and the farther hydrogen with a distance of 4.873 Å and a NPA charge of 0.027. This suggests that the H-H molecule forms an ion-dipole interaction with the Cu(II) center (Figure 8). Similar ion-dipole interactions between transition metals and small molecules have been proposed in the literature30,31 and have been observed in the adsorption of amides, alcohols, amines, and ethers on inorganic materials.32 Conclusions In conclusion, we have studied hydrogen interaction with copper(II) complex 1 and its derivatives in theory and experi-
Figure 8. Ion-dipole interactions between dihydrogen and Cu(II) center in 1 suggested by DFT calculations.
ment. The experimental studies show that there is significant deuterium adsorption on 1. This fact is supported by DFT calculations which reveal that there are ion-dipole interactions between the pentacoordinated Cu(II) centers and dihydrogen. The structure of 1 was characterized with X-ray crystallography, and it exhibits parallel microtubes of 9.55 Å in diameter. Deuterium adsorption studies with NMR spectroscopy indicate that physisorption occurs in the interior and in-between the microtubular structures. The extent of hydrogen uptake was strongly dependent on crystal morphology. Although the solvent molecules embedded in the space between the metallacycles can be removed upon prolonged exposure to vacuum at ambient temperature, our studies indicate a structural glass phase transition upon desolvation. The use of NMR in the study of gas adsorption allows one to gain insight into the adsorption process on a molecular level in order to complement macroscopic studies. Acknowledgment. This material is based upon work supported by the National Science Foundation under Grant No. EPS-0447691 and while serving at the National Science Foundation (T.P.). Financial support for J.C.N. and I.C.-C. from NSF 0649020 and 0748913, ACS-PRF 44703-GB3, and the UTEP-SEED grant is gratefully acknowledged. We also gratefully acknowledge a grant by Cambridge Isotope Laboratories, Inc. providing us with 2H2 used in this study, and we thank Dr. Kalugin for recording the IR spectra. References and Notes
Figure 7. 2H NMR integrated signal intensities for deuterium exposed to 1sf from a hole-burning experiment.
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