Regulation of NO Uptake in Flexible Ru Dimer Chain Compounds with

Nov 7, 2016 - On-demand design of porous frameworks for selective capture of specific gas molecules, including toxic gas molecules such as nitric oxid...
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Regulation of NO Uptake in Flexible Ru Dimer Chain Compounds with Highly Electron Donating Dopants Jun Zhang,†,‡ Wataru Kosaka,†,‡ Hiroki Fukunaga,†,‡ Susumu Kitagawa,§,∥,⊥ Masaki Takata,⊥,#,& and Hitoshi Miyasaka*,†,‡ †

Institute for Materials Research (IMR), Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan Department of Chemistry, Graduate School of Science, Tohoku University, 6-3 Aramaki-Aza-Aoba, Aoba-ku, Sendai 980-8578, Japan § Institute for Integrated Cell-Materials Science (iCeMS), Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan ∥ Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan ⊥ RIKEN SPring-8 Center, Sayo-gun, Hyogo 679-5148, Japan # Japan Synchrotron Radiation Research Institute/SPring-8, Sayo-gun, Hyogo 679-5198, Japan & Institute of Multidisciplinary Research for Advanced Materials (IMRAM), 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan ‡

S Supporting Information *

ABSTRACT: On-demand design of porous frameworks for selective capture of specific gas molecules, including toxic gas molecules such as nitric oxide (NO), is a very important theme in the research field of molecular porous materials. Herein, we report the achievement of highly selective NO adsorption through chemical doping in a framework (i.e., solid solution approach): the highly electron donating unit [Ru2(o-OMePhCO2)4] (o-OMePhCO2− = o-anisate) was transplanted into the structurally flexible chain framework [Ru2(4-Cl-2OMePhCO2)4(phz)] (0; 4-Cl-2-OMePhCO2− = 4-chloro-o-anisate and phz = phenazine) to obtain a series of doped compounds, [{Ru2(4-Cl-2-OMePhCO2)4}1−x{Ru2(o-OMePhCO2)4}x(phz)] (x = 0.34, 0.44, 0.52, 0.70, 0.81, 0.87), with [Ru2(o-OMePhCO2)4(phz)] (1) as x = 1. The original compound 1 was made purely from a “highly electron donating unit” but had no adsorption capability for gases because of its nonporosity. Meanwhile, the partial transplant of the electronically advantageous [Ru2(o-OMePhCO2)4] unit with x = 0.34−0.52 in 0 successfully enhanced the selective adsorption capability of NO in an identical structurally flexible framework; an uptake at 95 kPa that was 1.7−3 mol/[Ru2] unit higher than that of the original 0 compound was achieved (121 K). The solid solution approach is an efficient means of designing purposeful porous frameworks.

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INTRODUCTION Metal-organic frameworks (MOFs), or porous coordination polymers (PCPs), which consist of metal ions and organic linkers,1,2 have been considered as promising gas adsorbents that induce gas recognition because of their structural diversity, flexibility, and high potential for chemical activation.3 Indeed, some intriguing properties characteristic of MOFs/PCPs, such as gas storage,4−7 separation,8−13 catalysis,14,15 and others,16−18 have been discussed so far; however, rational control of physical and chemical activities of nanosized pores for the purpose of selective gas capture still remains a challenge. In many cases, the gas recognition capability has been strongly associated with functionalities of frameworks, which in general are incorporated in the process of framework design. For example, the introduction of open-metal sites that can possibly catch and activate specific molecules to react inside pores yields effective framework designs.19−24 Also, adopting electron-donating ability into frameworks could be useful, since this ability is © XXXX American Chemical Society

significant for the selective capture of molecules possessing electron-accepting characteristics (vice versa, adopting electronaccepting ability into frameworks could be useful for the selective capture of electron-donating molecules).25,26 A reversible structural transformation between nonporous and porous phases, namely, the use of structural flexibility involving gate-opening and -closing transformations, is available as another framework functionality. In addition to the intrinsic structural flexibility in each chosen framework, such a structural switch is strongly associated with characteristics of the gas used.27 However, it is still not easy to find appropriate materials that recognize gases effectively from a class of pure original MOFs/PCPs made following these framework designs. To address this issue, the solid solution approach has recently been focused on, enabling precise modulation of structural flexibility Received: September 30, 2016

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DOI: 10.1021/acs.inorgchem.6b02349 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry and other functionalities10,28,29 and maximizing the sorption properties in identical frameworks.30 This method is quite efficient for rationally designing on-demand functionality in MOFs/PCPs. Nevertheless, only a few reports on the rational utilization of a solid solution approach for selective gas adsorption properties have been presented.31,32 Here, we report the achievement of highly selective adsorption of nitric oxide (NO) by using a solid solution approach involving porous Ru dimer chain compounds, in which highly electron donating units were chemically doped into a structurally flexible framework (Figure 1). The original framework, consisting only of a highly electron donating unit,

had no adsorption capability for NO, which possesses an electron-accepting ability, because of its nonporosity; however, partial transplant of the electron-donating units into an identical structurally flexible framework successfully enhanced the selective adsorption capability for NO. The frameworks used in this work were chain-type Ru complexes composed of carboxylate-bridged paddlewheel-type diruthenium(II,II) complexes ([Ru2II,II(RCO2)4], where RCO2− stands for R-substituted carboxylate groups; abbreviated hereafter as [Ru2II,II]) and phenazine (phz) as a linker. These chain compounds with electron-donating [Ru2II,II] units (called D-MOFs) act as porous compounds,26,33−35 some of which selectively capture NO, as the electron-donating abilities of the [Ru2] units are tunable using the R groups of RCO2−.26,36−38 A chain compound with the formula [Ru2(4-Cl-2-OMePhCO2)4(phz)] (0; 4-Cl-2-OMePhCO2− = 4-chloro-2-anisate) is a NO adsorbent associated with a gate-opening structural transition from a nonporous form to a porous form.26 The [Ru2(4-Cl-2-OMePhCO2)4] unit (Figure 1a) has a relatively high electron-donating ability, because the addition of a methoxy group (MeO) in the ortho position of the benzoate ligand in the [Ru2] unit enhances its electron-donating nature, which is necessary for selective capture of NO; however, an electron-withdrawing Cl group is present in the para position.26 The analogous chain compound [Ru2(o-OMePhCO2)4(phz)] (1; o-OMePhCO2− = o-anisate) made from [Ru2(o-OMePhCO2)4] (Figure 1a) has nonporosity and rigidity; hence, it is unfortunately inactive for gas uptake, even for NO, although it should be much more electronically advantageous than 0 due to the lack of a Cl group on the RCO2− ligand.33 We can imagine that if the packing form of 1 were flexible, able to undergo a gate-opening transformation like that of 0, and consequently enabled to accommodate gases, 1 should exhibit NO uptake higher than that of 0. On the basis of this prediction, we employed a solid solution approach between these two compounds (Figure 1b): the [Ru2(o-OMePhCO2)4] unit with a higher electron-donating ability was site-doped into the structurally flexible 0, producing a solid solution series of [{Ru2(4-Cl-2-OMePhCO2)4}1−x{Ru2(o-OMePhCO2)4}x(phz)] (x = 0.34, 0.44, 0.52, 0.70, 0.81, 0.87, denoted hereafter as 0.34, 0.44, 0.52, 0.70, 0.81, and 0.87, respectively) (Figure 1b). Depending on the doping ratio x, the compounds were classified into three types of structural phases (Figure 1b): a flexible phase (i.e., 0-like structure for 0.34, 0.44, and 0.52), a rigid phase (i.e., 1-like structure for 0.87), and a mixed phase (for 0.70 and 0.81), where both phases coexist. Interestingly, the NO uptake was successfully increased in the flexible phase with an identical framework.

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RESULTS AND DISCUSSION Crystal Structures. The original 0 was obtained by drying its dichloromethane-solvated phase, [Ru2(4-Cl-2-OMePhCO2)4(phz)]·4CH2Cl2 (0′),26 and had a solvent-accessible volume of 460.3 Å3 corresponding to 32.3% of the total volume; 0′ experienced a prominent structural transformation to the totally nonporous form 0 associated with desolvation (Figure S1a in the Supporting Information).26 On the other hand, it is known that 1 is obtained without an inner crystallization solvent;33 however, by carefully treating crystals obtained in the same synthetic procedure, we found a new solvated phase, i.e. [Ru2(o-OMePhCO2)4(phz)]·2CH2Cl2 (1′), which easily loses its crystallization solvent (CH2Cl2) at room

Figure 1. Schematic representation of a porous material design based on a solid solution method of site-doping of highly electron-donating units. (a) Paddlewheel-type Ru dimer units, [Ru2II,II (4-Cl-2OMeCO2)4] and [Ru2II,II(o-OMeCO2)4], where the electron-donating abilities of the units are in the order [Ru2II,II(4-Cl-2-OMeCO2)4] < [Ru2II,II(o-OMeCO2)4]. (b) Schematic representation of NO gas adsorption processes described in this work. B

DOI: 10.1021/acs.inorgchem.6b02349 Inorg. Chem. XXXX, XXX, XXX−XXX

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bond distances and angles are given in Figure S5a and Table S2 in the Supporting Information. Note that the data quality for 0.70′ and 0.81′ is not very good, probably due to the mixing of isolated domains of 0′- and 1′-like phases even in a chain (vide infra). Similar to the case for 0′ and 1′, the oxidation state of the [Ru2] unit was assigned as [Ru2II,II] for all of the compounds, as confirmed by Ru−Oeq lengths (Table S2). The structural features of 1′ are basically the same as those of 0′,26 showing a chain structure with a [−{Ru2}−phz−] repeating unit (details of structures are summarized in Table S1 and Figure S4). The solvated compounds (0.34′, 0.44′, 0.52′, 0.70′, 0.81′, and 0.87′) underwent crystal to crystal phase transitions upon removal of the crystallization solvents (CH2Cl2) to produce their solvent-free compounds 0.34, 0.44, 0.52, 0.70, 0.81, and 0.87, respectively (Figure S6 in the Supporting Information). Most of the solvated 0′-like compounds turned into solventfree compounds similar to 0 (called the 0-like phase), corresponding to 0.34, 0.44, and 0.52, whereas only 0.87′, with a 1′-like structure, turned into a 1-like phase. Compounds 0.70 and 0.81, with borderline mixing ratios, revealed mixed phases containing 0- and 1-like structures, although their PXRD patterns indicate they are likely close to 0- and 1-like structures, respectively (Figure 2b). This fact indicates that [Ru2(4-Cl-2OMePhCO2)4]1−x and [Ru2(o-OMePhCO2)4]x are not uniformly distributed in 0.70′ and 0.81′ but likely form some domains composed of respective isolated species in a chain; hence, we did not deal with the gas sorption properties for 0.70 and 0.81. The structures of 0.34, 0.44, 0.52, and 0.87 were determined on the basis of synchrotron PXRD data obtained using the Rietveld refinement technique (Figure S7 in the Supporting Information), yielding 0-like (0.34, 0.44, and 0.52) and 1-like (0.87) structures that crystallized in the same triclinic P1̅ space group (Z = 1; Table S3 in the Supporting Information). To facilitate comparison of the cell constants between the solvated and desolvated compounds, the unit cell system can be transformed using cell vectors as a′ = a, b′ = −c, and c′ = b for 0.34, 0.44, and 0.52 and a′ = a, b′ = c, and c′ = b for 0.87 on the basis of the IUCR rule, where a, b, and c are the standard axis vectors for the original cell and a′, b′, and c′ are the transformed vectors, providing a new set of lattice constants (Figure S8 in the Supporting Information).39 After removal of crystallization solvents, the b and c unit cell axes for 0.34, 0.44, and 0.52 and the c axis for 0.87, which correspond to the chainstacking directions, were largely reduced, while the a axis cell

temperature after several minutes to form 1, with resolvation irreversibility (Figure S1b). The solvated compounds of the solid solution materials 0.34′, 0.44′, 0.52′, 0.70′, 0.81′, and 0.87′ were prepared by mixing a benzene solution (top layer) of phenazine and a dichloromethane solution (bottom layer) containing [Ru2(4Cl-2-OMePhCO2)4(THF)2] and [Ru2(o-OMePhCO2)4(THF)2] in mixing ratios of 3:1, 1:1, 9:11, 3:7, 1:4, and 3:17, respectively. The final formula of [{Ru2(4-Cl-2OMePhCO2)4}1−x{Ru2(o-OMePhCO2)4}x(phz)] (x = 0.34, 0.44, 0.52, 0.70, 0.81, 0.87) was confirmed by elemental analysis after desolvation (see the Experimental Section), which was not contradictory with the results of site-occupancy optimization for the para position of benzoate ligands in single-crystal X-ray diffraction analyses. All of the solvated phases of the solid solution compounds were thus structurally characterized (Figure S2−S4 in the Supporting Information). Their calculated powder X-ray diffraction (PXRD) patterns (Figure 2a) clearly demonstrate

Figure 2. XRPD patterns of compounds: (a) XRPD simulated patterns of the solvated phases, which were estimated from single-crystal X-ray crystallography data; (b) XRPD patterns of the dried phases prepared by drying the corresponding solvated phases at 373 K under vacuum for 12 h. The data were obtained using synchrotron radiation at the BL44B2 beamline in Spring-8 (λ = 0.80 Å).

that the structures of the compounds can be classified into two types: a 0′-like phase (0 ≤ x ≤ 0.70; 0.34′, 0.44′, 0.52′, and 0.70′) (Figure 2a and Figure S3) and a 1′-like phase (0.81 ≤ x ≤ 1; 0.81′ and 0.87′) (Figure 2a and Figure S4). The selected

Figure 3. Adsorption (closed) and desorption (open) isotherms for (a) CO2 at 195 K and (b) O2 at 90 K in 0, 0.34, 0.44, 0.52, 0.87, and 1. C

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Figure 4. Adsorption properties for NO gas at 121 K in 0, 0.34, 0.44, 0.52, 0.87, and 1: (a) adsorption (closed symbols) and desorption (open symbols) isotherms; (b) comparison of gas uptake in the solid solution series with the original compounds 0 and 1. In the flexible series (i.e., the 0like series), the NO uptake is increased and the O2 uptake is decreased, although the CO2 uptake is constant, in comparison to those in 0, respectively; therefore, the gas selectivity for NO is more pronounced in the solid solution series of the 0-like series (0.34, 0.44, and 0.52).

treatment affected a bulk phenomenon dependent on the doping ratio x. Sorption Properties of NO. Figure 4a shows the gas adsorption isotherms for NO at 121 K, exhibiting a two-step gate-opening pressure as observed in the gas adsorption isotherms of 0.26 The gate-opening pressure trend was found to be the same as those for CO2 and O2 (Table S4 in the Supporting Information), reflecting the fact that the structural natures of the solid solution compounds varied depending on the doping ratio x. A remarkable characteristic of the solid solution compounds 0.34, 0.44, and 0.52 is observable in the final uptake of NO, which was found to be 11.4, 10.7, and 10.1 mol/[Ru2] unit at 121 K for 0.34, 0.44, and 0.52, representing a 1.7−3.0 mol/ [Ru2] unit increase relative to the final uptake for 0 (Figure 4b). As discussed previously,26 NO molecules can interact with phz moieties, and the NO adsorption capability of 0 was observed to be superior to that of an isostructural [Rh2] analogue.33 Specifically, this result indicates that charge transfer (CT) from [Ru2] to phz is a key for trapping NO molecules inside the pores of these types of compounds (indeed, the CT band was observed at 1.54 eV for 0, a much lower energy than the 1.99 eV energy of the CT band observed for the [Rh2] analogue).33 The solid solution compounds 0.34, 0.44, and 0.52 have electron-donating abilities higher than that of 0 in an identical “flexible framework”, due to the partial transplant of the [Ru2(oOMePhCO2)4] unit. This electronic advantage of the framework could enable NO accommodation in 0.34, 0.44, and 0.52 higher than that in 0 (Figure 4b). Indeed, the CT bands of the solid solution compounds shift slightly to lower energies with increasing doping ratio x (Figure S9 in the Supporting Information). Thus, the solid solution technique, with introduction of higher electron-donating units in a flexible framework, successfully enhanced NO capture. In Situ IR Spectra Measurement under NO. To investigate the chemical situation of the NO molecules inside the pores, in situ infrared (IR) spectroscopy under an NO atmosphere at 121 K was conducted for 0.44. Figure 5a,b gives in situ IR spectra for the adsorption and desorption processes, respectively, where the pressure points (i)−(vii) at which the IR spectra were obtained are indicated in the NO sorption isotherm of Figure 5c. Similarly to the adsorption spectrum of 0,26 the vibrational mode of the NO molecules was observed in three regions: band A (1820−1920 cm−1), band B (1660−1760

constant corresponding to the chain-running direction remained almost unchanged (Figure S8 and Table S3), resulting in unit cell shrinkage to 71.7%, 76.5%, 75.6%, and 81.7% of the volumes of the solvated lattices of 0.34, 0.44, 0.52, and 0.87, respectively. The structural change is mainly due to the shrinkage of the solvent-accessible volume, which remains a void with volumes of 25.4, 37.8, 46.1, and 22.4 Å3 for 0.34, 0.44, 0.52, and 0.87, corresponding to 2.4%, 3.5%, 4.3%, and 2.2% of the cell volumes, respectively. Since 0 has no void space,26 the existence of void spaces with volumes in the order 0.34 < 0.44 < 0.52, although they are small, demonstrates the effect of chemical doping. In addition, 0.34, 0.44, and 0.52 could be regarded as on-target compounds, since they successfully combine the characteristics of the flexible structure found in the 0-like structures with the higher electron-donating ability resulting from the doped [Ru2(o-OMePhCO2)4] unit originally used for 1. Sorption Properties of CO2 and O2. Gas adsorption measurements were conducted for 0.34, 0.44, 0.52, and 0.87. Figure 3 shows the gas adsorption isotherms for CO2 at 195 K and O2 at 90 K, where the curves for 0 and 1 from refs 26 and 33, respectively, are also depicted: 0 shows one-step gateopening adsorption for O2 and CO2,26 whereas 1 does not adsorb any gases.33 Similar to the case for 0, compounds 0.34, 0.44, and 0.52 show one-step gate-opening adsorption behaviors with gate-opening pressures of 1.7, 7.3, and 15.3 kPa for CO2 and of 0.2, 5.4, and 15.5 kPa for O2, respectively, yielding the trend 0.34 < 0.44 < 0.52 (Figure 3). This trend could be explained by the doping of [Ru2(o-OMePhCO2)4], which somewhat enhanced the rigidity of framework packing; thus, the partial doping treatment efficiently affected a bulk phenomenon. The amounts of CO2 absorbed at 95 kPa reached 2.6, 2.6, and 2.5 mol/[Ru2] unit at 195 K for 0.34, 0.44, and 0.52, respectively (Figures 3a and 4b and Table S4 in the Supporting Information); these amounts are almost the same because the physical characteristics of the gate-opened micropores in each compound could be basically identical at 195 K. In contrast, the amounts of O2 absorbed at 95 kPa were 3.5, 2.5, and 0.5 mol/[Ru2] unit at 90 K for 0.34, 0.44, and 0.52, respectively (Figures 3b and 4b and Table S4), since the rigidity of framework packing from the [Ru2(o-OMePhCO2)4] dopants is more characteristic in 0.44 and 0.52 at such low temperatures; these data thus indicate that the partial doping D

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mode of gaseous NO (i.e., bulk NO), band B is associated with the absorbed NO molecules at the first gate opening, and band C is associated with NO molecules trapped in both the first and second gate openings. The shift to lower frequencies from band A to bands B and C indicates that NO molecules interact with frameworks inside pores somewhat involving charge transfer. In other words, the framework’s electron-donating ability plays an important role in the sorption process for NO. Despite being advantageous electronically, 0.87 adsorbs a small amount of CO2 (16 mL (STP) g−1, corresponding to 0.7 mol/[Ru2] unit), no O2, and no NO gas (Figures 3 and 4), as also observed for 1.33 The lack of gas adsorption is due to the lack of a flexible framework required for gate opening in the compound. Thus, two types of materials, 0- and 1-like compounds, demonstrate the importance of both high electron-donation ability and structural flexibility in achieving selective NO trapping in these gate-open-type MOF/PCP materials.

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CONCLUSIONS Although a framework with a highly electron donating ability was prepared, it would be ineffective as a porous material if it had no porosity; however, if this building unit were transplanted into a porous material, the transplanted solid solution material could have both porosity and a highly electron donating ability. This concept was demonstrated in this study for use in NO capture in a series of solid solution materials with gate-open-type Ru dimer chain compounds. The original compound 1 has a high potential for capturing NO molecules because of its preferable electron-donating characteristics but unfortunately has no porosity. Therefore, the electronically active component ([Ru2(o-OMePhCO2)4]) of 1 was transplanted into the structurally flexible material [Ru2(4Cl-2-OMePhCO2)4(phz)] (0). A series of solid solution compounds, [{Ru2(4-Cl-2-OMePhCO2)4}1−x{Ru2(o-OMePhCO2)4}x(phz)] (x = 0.34, 0.44, 0.52, 0.70, 0.81, 0.87) were thus synthesized. The crystal structures were dependent on the unit mixing ratio x: for 0 ≤ x ≤ 0.52, the structure (i.e., 0-type structure) was similar to that of 0 with the flexibility of the framework exhibiting gate-opening/-closing behavior, whereas for 0.87 ≤ x ≤ 1, the structure (i.e., 1-like structure) was similar to that of 1 with the rigidity of framework packing and no porosity. The 0-like series showed gate-opening adsorption for CO2, O2, and NO. The total uptake of CO2 was almost unchanged from the performance of 0, whereas the uptake of O2 was dependent on x: the flexibility of framework packing was dominant. Finally, the NO uptake was enhanced in the doped 0-like compounds relative to 0; their superior electrondonating abilities characteristically appeared. Consequently, the NO gas recognition capability vs CO2 and O2 was much more emphasized in the solid solution series (Figure 4b). The present study revealed that the solid solution approach for our D-MOF compounds enables control of the structural flexibility and electronic environment inside pores. This type of approach provides a promising methodology for the design of functional MOFs/PCPs for use in gas-separation applications.32 We believe that the method described herein will help to solve complex problems in nanospace science.

Figure 5. In situ IR spectra in the frequency range of 1400−2000 cm−1 for 0.44 measured under a NO gas atmosphere: (a) IR spectra in the absorption process where the red, green, cyan, and blue spectra were measured at points (i)−(iv) in (c), respectively; (b) IR spectra during the desorption process, where the blue, cyan, green, and red spectra were measured at points (iv)−(vii) in (c), respectively; (c) sorption isotherms for NO in 0.44 at 121 K measured using this in situ technique.

cm−1), and band C (1550−1630 cm−1). During the adsorption process (Figure 5a), the NO pressure increases monotonically with increasing intensity of band A; the intensity of band B increases steeply at low pressures