Characteristic Features of CO2 and CO Adsorptions to Paddle-Wheel

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Characteristic Features of CO and CO Adsorptions to Paddle-Wheel-Type Porous Coordination Polymer Jia-Jia Zheng, Shinpei Kusaka, Ryotaro Matsuda, Susumu Kitagawa, and Shigeyoshi Sakaki J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b02707 • Publication Date (Web): 08 Aug 2017 Downloaded from http://pubs.acs.org on August 9, 2017

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The Journal of Physical Chemistry

Characteristic Features of CO2 and CO Adsorptions to PaddleWheel-Type Porous Coordination Polymer Jia-Jia Zheng,†,‡ Shinpei Kusaka,† Ryotaro Matsuda,†,§ Susumu Kitagawa,*,†,∥ and Shigeyoshi Sakaki*,‡ †

Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan

‡

Fukui Institute for Fundamental Chemistry, Kyoto University, Nishi-hiraki cho, Takano, Sakyo-ku, Kyoto 606-8103, Japan

§

Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan ∥Department

of Synthetic Chemistry and Biological Chemistry, Graduate School of

Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan

1 ACS Paragon Plus Environment


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ABSTRACT: Adsorptions of CO, N2, NO, and CO2 in a paddle-wheel-type porous coordination polymer (PCP) [Cu(aip)]n (aip = 5-azidoisophthalate) were investigated with ONIOM[MP4(SDQ):ωB97XD] method using a model system consisting of two [Cu2(O2CC6H4-R)4] units (R = H and Me) and one [Cu2(O2CC6H4-R)4] unit; namely dimer and monomer models. The experimental CO adsorption position was reproduced well by the present calculation with the dimer model. For adsorptions of CO, N2, NO, and CO2 in the dimer model, the position of gas molecule deviates from the normal one that is found in the monomer model and becomes more distant from the surrounding phenyl group(s) of the neighbour [Cu(aip)] unit. For all these gas molecules, the calculated binding energy (BE) at the deviating adsorption position is larger than that at the normal one against our expectation that the normal position is the best for the gas adsorption. The deviation of gas adsorption position arises from the interaction between the organic linker (O2CC6H4-R moiety) and gas molecule. For all cases, the exchange repulsion with the organic linker decreases to a larger extent than the attractive electrostatic and dispersion interactions decrease, as going from the normal position to the deviating one. To enhance the binding energy of gas molecule, introduction of electron-donating substituent on phenyl moiety is computationally recommended for this PCP.


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INTRODUCTION Porous coordination polymers (PCPs) or metal organic frameworks (MOFs)1,2 have attracted recent attentions as potential materials for gas storage,3-6 gas separation,7-11 and catalysis.12-16 Because a variety of metal ions and organic linkers are available for synthesis, geometry and property of PCP can be well tuned for achieving the purpose.17 Actually, many PCPs with excellent adsorption ability for target gas molecule have been designed and synthesized by tuning geometries and properties of PCPs.7-11 To solve the CO2 problem, it is now important to adsorb CO2 molecule efficiently either from air or from large point sources such as factory and power plant.18 Thus, a lot of efforts have been made to synthesize PCPs excellent for CO2 adsorption.19-23 For those PCPs, the open metal site (OMS) has been often utilized for the interaction with CO2. Recently, one experimental work elucidated that CO2 molecule interacts with OMS through O atom in an η1-O coordination form and the M-O-C angle is 120º to 130º, depending on the M, unlike CO (the M-C-O angle is 180º), interestingly.24 CO2 adsorption to PCPs has been investigated by many computational studies such as atomistic and continuum modelling,25 classical molecular dynamics and Monte Carlo simulations,26-29 and the first principle and quantum mechanics calculations.30-33 However, the determining factor(s) for the CO2 adsorption and even the reason of the coordination structure of CO2 with OMS have been unclear. Another important purpose is to separate target gas molecule from mixture containing several kinds of gas molecules with similar physicochemical properties such as CO/N2 mixture. Flexible PCPs, which undergo structural transformation upon external stimuli such as adsorption/desorption of some particular gas molecule,34,35 have been recognized as a promising candidate for the separation of target gas molecule from mixture. The structural



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response of such flexible PCP to target gas molecule is an origin of the gate-opening gas adsorption which leads to significant increase in adsorption amount of target gas molecule. For instance, the gate-opening gas adsorption was recently applied to the separation of CO from N2 using a paddle-wheel-type PCP [Cu(aip)]n (aip = 5-azidoisophthalate, n = large number nearly infinite);36 this is named Cu(aip)-PCP hereafter. This PCP undergoes structural transformation upon CO adsorption/desorption, while the gate-opening does not occur in the N2 case. The difference in structural response between CO and N2 adsorptions was discussed in terms of the CO and N2 binding energies with the Cu-OMS, as follows: Though one Cu atom is coordinated with one oxygen atom of carboxylate of the neighbour paddle-wheel unit in addition to four oxygen atoms of carboxylates bridging two Cu atoms in the case of guest-free structure (Scheme 1a), CO molecule coordinates with the Cu-OMS to break this Cu-O bond (Scheme 1b) to induce significant change of structure. On the other hand, N2 does not strongly interact with the Cu-OMS to induce no structural transformation via the Cu-O bond breaking. One can expect that such gate-opening mechanism could work in CO2 separation from CO2/N2 mixture, if the CO2 adsorption with the Cu-OMS is stronger than the N2 adsorption and similar to the CO adsorption; it should be remembered that the binding energy is one factor for the gate-opening and other factors such as the morphology, surface termination, etc. are also important for the gate-opening. Therefore, the evaluation of binding energy of gas molecule with the Cu-OMS is important as one factor for understanding and predicting whether the gate-opening mechanism may work well in gas adsorption or not. One more interesting feature observed in the CO-adsorbed Cu(aip)-PCP (Scheme 1b) is that CO molecule interacting with the Cu-OMS is sandwiched by two phenyl moieties of a neighbour paddle-wheel unit and its adsorption position deviates from the normal


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adsorption one that is observed in the CO interaction with one paddle-wheel unit [Cu2(O2CC6H4-R)4], as shown in Schemes 1c and 1d; hereafter, the gas adsorption position in one [Cu2(O2CC6H4-R)4] unit is named “normal position”. It is likely that the deviating adsorption position in the Cu(aip)-PCP is induced by steric repulsion of CO with phenyl moieties of the neighbour paddle-wheel unit. If so, the binding energy of CO must become smaller in the Cu(aip)-PCP than that with one paddle-wheel unit. This suggests that the presence of organic linker is not favourable for CO adsorption. Thus, it is necessary to evaluate how much the binding energy decreases at such deviating adsorption position and to know what type of linker is favourable for gas adsorption. Computational approach at various levels of theory30-33,37-51 has been extensively carried out to investigate interactions between gas molecule and PCPs. The density functional theory (DFT) combined with dispersion-corrected functional52 has been often employed in those studies. However, the use of more accurate post-Hartree-Fock method such as MP2– MP4 and CCSD(T) is highly desirable, because the binding energy of gas molecule with PCP is determined by delicate balance among various interactions such as electrostatic (ES), charge transfer (CT), exchange repulsion (EXR), and dispersion (DIS) interactions. Because PCP is a very large system, one of the reasonable computational methods is the ONIOM procedure,53,54 in which whole system is calculated with such a low-cost method as DFT with dispersion-corrected functional and an important moiety is calculated with the post-Hartree-Fock method. We recently applied the ONIOM[MP2.5:M06-2X] method to the adsorptions of CO2 and CS2 into the Hofmann-type PCP {Fe(Pz)[Pt(CN)4]}n and successfully evaluated adsorption positions and binding energies of CO2 and CS2.47 Snurr and co-workers also employed the ONIOM procedure to evaluate CO2 binding energy with CPO-27.32 These successful examples suggest that the ONIOM method is useful in



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investigating the interaction of gas molecule with PCPs. In this work, we theoretically investigated interactions of such gas molecules as CO, N2, NO, and CO2 with a paddle-wheel type PCP [Cu(aip)]n using model systems consisting of one and two Cu paddle-wheel units [Cu2(O2CC6H4-R)4]2 (R = H and Me). Our purposes here are to investigate the adsorption structures of these gas molecules in the [Cu(aip)]n, to evaluate their binding energies, and to clarify the roles of Cu-OMS and organic linker in determining the adsorption structure and adsorption energy. It is also our intention to elucidate the reasons why the adsorption position in the [Cu(aip)]n deviates from the normal one but the binding energy is enhanced at such deviating adsorption position, which will be seen below.

COMPUTATIONAL DETAILS AND MODLES Though the real Cu(aip)-PCP is an infinite system (Schemes 1a and 1b; see Figure S1 in Supporting Information (SI) for the structure from another viewpoint), cluster model systems were employed here to apply post-Hartree-Fock method to the Cu(aip)-PCP. As shown in Scheme 1c, we employed a model system consisting of two Cu paddle-wheel units [Cu2(O2CC6H4-R)4]2 (R = H and Me), where the azido group in Cu(aip)-PCP was replaced by R for simplicity; this is named dimer model, Dm-R (R = H or Me), hereafter. The Cuα and Cuβ represent the outside Cu atom and the inside one of [Cu2(O2CC6H4-R)4]2, respectively; see Scheme 1c. Because the gas molecule at the β site is surrounded by two phenyl moieties in the Dm-R like that in the real Cu(aip)-PCP (Scheme 1c), the structural situation of the gas molecule in the Dm-R resembles that of gas molecule in the real Cu(aip)-PCP. Gas molecule at the α site is not surrounded at all by phenyl moiety, which is similar to that in a monomeric [Cu2(O2CC6H4-R)4] unit. For the comparison, we


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investigated the interaction of gas molecule with a monomeric [Cu2(O2CC6H4-R)4] model, which is named monomer model, Mm-R (Scheme 1d). These dimer and monomer models with gas molecule (L) are named Dm-R-L and Mm-R-L hereafter. In the geometry optimization of Dm-R and Dm-R-L, the position of Cuα was fixed to be the same as the experimental one but the other moiety was fully optimized. Because each Cu2+ (d9) in the Cu paddle-wheel unit has one unpaired electron, there are two possible spin states, open-shell singlet and triplet states in one Cu paddle-wheel unit. Previous theoretical studies show that the Cu paddle-wheel system has an open-shell singlet ground state, which is slightly more stable than triplet state.46,49 However, the binding energy of gas molecule with the Cu paddle-wheel unit is calculated to be almost the same between the triplet and open-shell singlet states.46,49 Thus, the calculation was carried out here at the high spin state for both monomer (S = 1) and dimer (S = 2) models. All geometry optimizations were performed by the density functional theory (DFT) with the ωB97XD functional.55 The 6-31G(d) basis sets were used for carboxylate group, where a set of diffuse functions was added to O atom. The 6-31G basis sets were used for phenyl and substituent groups (R= H and Me). For all gas molecules, the 6-31G(d) basis sets were used. For Cu, the LANL2DZ basis set was used to represent the valence electrons and the effective core potentials (ECPs) were used for replacing core electrons.56 Subsequent frequency calculations were carried out at the same level of theory.57 The ONIOM method53,54 was used to evaluate the binding energy of gas molecule with this PCP, because it is particularly reliable for incorporating dispersion interaction.58 A small model SM1 [Cu2(O2CH)4] was employed to evaluate the interaction of gas molecule with the Cu at the high quality level, as shown in Scheme 2. Another small model SM2 consisting of [Cu(O2C-Ph)2] (Scheme 2) was used to evaluate the interaction of gas



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molecule with two phenyl moieties at the high quality level, where one Cu(II) was involved to compensate negative charges of [(O2C-Ph)2]2-; see also Figure S2 in SI for SM1 and SM2. The binding energy (BEβ) of gas molecule adsorbed at the β site was calculated using Dm-R at a low level and SM1 and SM2 at a high level. The binding energy (BEα) of gas molecule at the α site was calculated using Dm-R at a low level and SM1 at a high-level; the superscripts “α” and “β” are used hereafter to represent the coordination sites; for instance, Lα means gas molecule L at the α site. The evaluation method of the binding energy is presented in SI; see page S3. In the ONIOM calculation, better basis sets were employed. The (311111/22111/411/11) basis set including two f polarization functions was employed for Cu, where its core electrons were replaced by Stuttgart-Dresden-Born (SDB) ECPs.59,60 The 6-311G(2d) basis sets were used for gas molecules. For other atoms, the 6-311G(d) basis sets were used, where a set of diffuse functions was added to O atoms of carboxylate group. The basis set superposition error (BSSE) was corrected by the counterpoise method.61 Natural bond orbital (NBO) analysis was made using the DFT method with the ωB97XD functional, where the better basis sets were employed. These calculations were performed with the Gaussian 09 program package.62 Localized molecular orbital (LMO) energy decomposition analysis63 was carried out with the GAMESS program,64 using SM1 and SM2 models.

RESULTS AND DISCUSSION Geometry and Binding Energy of CO2 with Mm-R Prior to the discussion of the interaction of gas molecule with Dm-R, we will briefly


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discuss first the interaction of CO2 molecule with Mm-R to understand the normal coordination structure of CO2 without interference by other moiety such as phenyl group in PCPs, because the CO2 interaction structure with OMS has not been discussed well compared to the CO and N2 coordinations.49 In Mm-H-CO and Mm-H-N2, CO and N2 take a position on the Cu-Cu axis with a linear η1-end-on coordination form, as shown in Figures 1a and 1b; see Table S1 in SI for details in optimized geometrical parameters. NO and CO2 also take an η1-end-on coordination form, but their coordination positions deviate from the Cu-Cu axis unlike CO and N2 (Figures 1c and 1d). Also, NO and CO2 approach one O atom of carboxylate (donated as OC), where the OC-Cu-N-O and OC-Cu-O-C dihedral angles are nearly 0°. These adsorption positions are named “normal position”, as defined above. Important geometrical parameters such as Cu-L distance and Cu-Cu-L angle differ little between R = H and Me, indicating that the R substituent influences little the adsorption structure of gas molecule. This is reasonable because the R substituent is distant from the adsorption site. Consistent with the small difference in geometry, the charge distribution in Mm-R-L is almost the same between R = H and Me; see Table S2 in SI. In the normal CO2 adsorption structure, the Cu-O-C angle is not linear but 117° (Figure 1d). This coordination structure agrees with the experimental structure of CO2-adsorbed CPO-27-M (M = Mg, Mn, Fe, Co, Ni, Cu, or Zn).24 It is likely that the CO2 adsorption position is determined by both of HOMO-LUMO interaction and charge distribution of CO2. As well known, the HOMO of CO2 is non-bonding π, which expands perpendicularly to the O-C-O axis unlike the HOMO (lone pair) of CO, as shown in Scheme 3. As a result, the O atom of CO2 wants to interact with the Cu atom and the Cu-O-C angle tends to take around 90° to provide a good orbital overlap between the HOMO of CO2 and the empty Cu 4pz orbital. Simultaneously, the positively charged C atom of CO2 is withdrawn towards the



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negatively charged OC atom of the carboxylate ligand (Scheme 3). As a result, the Cuβ-O-C angle becomes 117° and the Cuα-Cuβ-O angle is 167º in the normal position. Though NO coordination structure is different from the CO coordination one (Figure 1c), we omitted discussion because the NO interaction with transition metal complex has been discussed well.65,66 The ONIOM[MP4(SDQ):ωB97XD]-calculated binding energy (BE) of Mm-R-L decreases in the order CO > CO2 > NO > N2, as shown in Table 1; the R substituent influences little their binding energies. In comparison with previous experimental and computational studies on the HKUST-1,30,46,48,67 the binding energy is underestimated here but the decreasing trend of the binding energy (CO > CO2 > NO > N2) is the same as that of the previous works, suggesting that the method employed here describes well the relative binding energy of gas molecule with Cu-OMS. It is likely suggested that the gate-opening mechanism could be applied to CO2 adsorption because the CO2 binding energy is not different so much from the CO binding energy and much larger than that of N2.

Geometries and Binding Energies of CO, N2, NO, and CO2 Molecules with Dm-R Optimized geometries of Dm-R with such gas molecules as L = CO, N2, NO, and CO2, are shown in Figure 2; see Table S3 in SI for geometrical parameters. The CO molecule at the α site takes a linear η1-end-on coordination structure (Figure 2a), in which the carbon atom of COα takes a position on the Cuα-Cuβ axis. This is a typical adsorption structure observed in CO interaction with OMS of PCP.46,48,49 For the CO adsorption at the β site, on the other hand, the carbon atom takes a position deviating from the Cuα-Cuβ axis; the CuαCuβ-C and Cuβ-C-O angles are 169° and 170°, respectively, both of which differ from those of the normal adsorption structure. These angles agree with the experimental values in the


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CO-adsorbed Cu(aip)-PCP (168° and 175° for Cuα-Cuβ-C and Cuβ-C-O angles, respectively).36 Also, the Cuα-Cuβ, Cuβ-Cuβ, and the average Cu-O distances of Dm-H-CO agree with the experimental values, as shown in Table S3; see page S8 for the brief discussion of changes in the Cuα-Cuβ, Cuβ-Cuβ, and the average Cu-O distances by CO adsorption. For the N2 adsorption (Figure 2b), N2α takes a position on the Cuα-Cuβ axis with a linear η1-end-on coordination form but N2β takes a position deviating from the Cuα-Cuβ axis like the COβ adsorption, where the Cuα-Cuβ-N and Cuβ-N-N angles are 167° and 165°, respectively. NO adsorption position at the α site moderately deviates from the Cuα-Cuβ axis (the Cuβ-Cuα-Nα angle = 175°) like that in the Mm-H-NO (Figures 1c and 2c). At the β site, the NO adsorption position deviates from the Cuα-Cuβ axis (the Cuα-Cuβ-Nβ angle = 171°) more than does the NOα position (Figure 2d). Also, the dihedral angle OCβ-Cuβ-Nβ-Oβ changes to 42° from 0° in the normal one, where the OCβ represents the O atom of carboxylate which is close to the β site; see Figures 2a to 2d for OCβ. For the CO2 adsorption at the α site, the O atom takes a position deviating from the Cuα-Cuβ axis like that in Mm-H-CO2 (Figures 1d and 2d); however, the Cuβ-Cuα-Oα and Cuα-Oα-Cα angles of Dm-H-CO2 are moderately smaller than those of the Mm-H-CO2, suggesting that the CO2 position and orientation are flexible, as found in previous studies.48,68 At the β site, the CuαCuβ-Oβ and Cuβ-Oβ-Cβ angles (169° and 119°, respectively) of Dm-H-CO2 are slightly larger than those of the Mm-H-CO2 and the OCβ-Cuβ-Oβ-Cβ dihedral angle increases to 13° from 3°, indicating that CO2 position at the β site is distant from the phenyl moiety of the neighbour [Cu2(O2CC6H4-R)4] unit.69 The above results lead us to the reasonable explanation described below: If gas molecule at the β site existed on the Cuα-Cuβ axis like that in the normal position, the position of the gas molecule was close to the phenyl moiety of the neighbour



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[Cu2(O2CC6H4-R)4] unit to give rise to significant steric repulsion and destabilize the gas adsorption. To reduce such destabilization, the gas molecule has to take the deviating position from the Cuα-Cuβ axis that is distant from the phenyl moiety. The similar deviating position of gas adsorption is found in Dm-Me, too. Because the difference is not large and the same trend is observed between R = H and Me, we wish to skip the discussion of the difference between them. The ONIOM[MP4(SDQ):ωB97XD]-calculated binding energies (BEs) of these gas molecules with Dm-R are summarized in Table 1. The binding energy (BEα) at the α site is almost the same as that of L with Mm-R, as expected. However, the BEβ is unexpectedly larger than BEα for all these gas molecules. The larger BEβ value than BEα indicates that the gas adsorption is enhanced by the organic linker compared to that of one paddle-wheel unit. These results are surprising because the normal position is considered the best for the interaction of gas molecule with the Cu-OMS and the β-adsorption position deviates from the normal one probably due to the steric repulsion with the organic ligand. The BEβ decreases in the order CO2 ≈ CO > NO > N2 for Dm-H and CO > CO2 > NO > N2 for DmMe. Also, the BEβ value is larger for R = Me than for R = H, while the BEα is moderately smaller for R = Me than for R = H (Table 1). These results suggest that the Me substituent enhances the BEβ value by providing perturbation to the electronic structure of the phenyl moieties surrounding the gas molecule. Another interesting result is that the BEβ of CO2 is similar to that of CO, suggesting that the gate-opening mechanism could work in the CO2 adsorption to Cu(aip)-PCP.

Analysis of Binding Energies of Gas Molecules with Cu Center It is of considerable interest to elucidate what roles the Cu-OMS and the organic linker


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play in determining the gas adsorption position and the binding energy at the β site. As shown in Figure 3, the sum of MP2-calculated CO binding energies with SM1 and SM2 shows the energy minimum similar to that of the realistic model (Dm-H-CO); see Figures S3 and S4 in SI for ωB97XD-calculated potential energy surfaces and those of CO2 adsorption. These results indicate that these SM1 and SM2 models are useful for elucidating the reasons why the deviation of gas adsorption position occurs and why the BE becomes larger at the deviating position than at the normal one. The binding energies of gas molecules with SM1 were analysed at both the normal structure and the deviating one (LN and Lβ, respectively) in Table 2, using energy decomposition analysis with localized molecular orbital at the Hartree-Fock (HF) level.63 Geometrical parameters of the deviating structures were taken to be the same as those in the optimized structures of Dm-R-L. In this analysis, the binding energy BEHF at the HF level is decomposed into electrostatic interaction (ES), exchange repulsion (EXR), and the sum of charge transfer (CT), polarization (Pol), and other mixing terms (Mix); see Ref. 63 for the explanations of these terms. The dispersion term (DIS) is defined as the difference in binding energy between at the MP4(SDQ) and the HF levels. MP4(SDQ) For all gas molecules, the binding energy BE SM1 at the MP4(SDQ) level is

moderately smaller at the deviating position than at the normal one, as shown in Table 2; we define that the negative value represents the stabilization energy and vice versa in Tables 2 and 3. For the CO adsorption, the dispersion interaction differs little between the normal and deviating structures, indicating that the change in binding energy occurs at the Hartree-Fock level. The ES term becomes more negative (more attractive) but the EXR term becomes more positive (more repulsive) in the deviating structure than in the normal one, while the CT+POL+Mix and DIS terms become moderately more attractive.70 These



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results indicate that the CO binding energy is smaller at the deviating position than at the normal one because the EXR term becomes more repulsive than the other terms become more attractive. The EXR term arises from the overlap of occupied orbitals between gas molecule and Cu(aip)-PCP. As shown in Figure 4, the doubly occupied dπ and dπ* MOs of Cu(aip)-PCP expand on the xz and yz planes. The deviation of CO position from the normal one increases orbital overlap of the σ lone-pair orbital (HOMO) of CO with these dπ and dπ* MOs of Cu(aip)-PCP, and thereby, the EXR destabilization increases at the deviating structure. Because of such change in the EXR term, the interaction between the CO and the Cu site becomes weak at the deviating structure. For the N2 and NO adsorptions, on the other hand, the EXR becomes less repulsive at the deviating position than at the normal one, because the Cu-N distance is longer at the deviating position than at the normal one (Figures 2c and 2d).71 Because of the elongation of the Cu-N distance, the ES and CT+Pol+Mix terms become less attractive. For the NO case, the DIS also becomes less attractive. Because the sum of those attractive interactions MP4(SDQ) decreases more than the EXR decreases, the BE SM1 is smaller at the deviating position

than at the normal one. MP4(SDQ) For the CO2 interaction with SM1, the BE SM1 differs little between the normal

(CO2N) and deviating (CO2β) adsorption positions, suggesting that the CO2 adsorption structure is flexible, as mentioned above. The ES slightly decreases (less attractive), the EXR repulsion moderately decreases (less repulsive) and the CT+Pol+Mix slightly increases (more attractive), but the DIS changes little, as going from the CO2N to the CO2β, MP4(SDQ) leading to almost the same BE SM1 at both the normal and deviating positions. The

larger ES of CO2N arises from the electrostatic interaction between the positively charged C of CO2 and the negatively charge OC of the carboxylate shown in Scheme 3. For the CO2β,


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the Cuβ-O-C angle increases to 119° and the OCβ-Cuβ-O-C dihedral angle increases to 13°, which elongates the distance between the C of CO2 and the OC to decrease the ES stabilization. Because the Cuα-Cuβ-O angle increases at the deviating position, the O of CO2 approaches the Cuα-Cuβ axis. As a result, the orbital overlap of the non-bonding π (HOMO) of CO2 with the Cu 4pz orbital becomes larger and that with the 3dxz/3dyz orbitals become smaller at the deviating position, leading to the larger CT+Pol+MIX and smaller EXR terms.72 MP4(SDQ) In conclusion, the BE SM1 with the Cu center is smaller at the deviating position than MP4(SDQ) at the normal one except for CO2 in which the BE SM1 differs little between CO2N and

CO2β. This indicates that the larger BEβ value arises from some attractive interaction between gas molecule and organic linker.

Analysis of Binding Energies of Gas Molecules with Phenyl Moiety We made energy decomposition analysis of the interaction of gas molecule with SM2, MP4(SDQ) as shown in Table 3. The BE SM2 is positive at the normal position but decreases very

much to negative value at the deviating position for all the gas molecules examined here, indicating that the interaction with SM2 is the origin of the larger BE at the deviating MP4(SDQ) position than at the normal one. For instance, the BE SM2 of CO with SM2 is 5.76 kcal

mol-1 larger at the deviating position than at the normal one but that with SM1 is 0.35 kcal mol-1 smaller at the deviating position (Table 2). It is clearly concluded that the interaction with SM2 is responsible for the deviating adsorption position. For the CO adsorption, the EXR becomes much smaller (less repulsive) by 15.79 kcal mol-1 than the ES term decreases by 7.74 kcal mol-1, as going from CON to COβ. The CT+Pol+Mix term moderately changes. Based on these results, it appears that the small



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EXR term with the phenyl moieties is a main factor for the deviating adsorption position of CO. But, the change in the EXR is not enough to understand the reason why the CO binding energy is larger at the deviating position than at the normal one (Table 1); if only the EXR term between the CO and the phenyl moieties is the reason for taking the deviating position, the CO binding energy at the β site must be smaller than at the α site because CO molecule at the α site does not suffer at all from the EXR with the phenyl moieties. The other reason must exist for stabilizing the deviating position. It is noted that HF MP4(SDQ) the BE SM2 with SM2 is positive but the BE SM2 is negative at the deviating position,

indicating that the DIS term is important. Actually, the ES, EXR, and CT+Pol+Mix terms decrease by about 70% but the DIS decreases by about 30%, as going from CON to COβ (see values in parentheses in Table 3). In other words, the EXR with phenyl moieties is sensitive to the position change and decreases very much as going from the normal position to the deviating one, and the ES and CT+Pol+Mix decrease to a similar extent to the EXR but the DIS is not sensitive and decreases much less than the EXR. Thus, the CO binding energy is larger at the deviating position than at the normal one. The similar explanation can be presented for CO2, N2, and NO, in which the DIS term decreases much less than the ES, EXR, and CT+Pol+Mix terms decrease as going from the normal positon to the deviating one; see Table 3. These results suggest that the DIS term plays important role(s) in stabilizing gas adsorption in this PCP. Notably, the dispersion interaction plays crucial role(s) in many host-guest interactions of PCPs, as reported for H2 and benzene adsorptions in MOF-5.73,74 The above conclusion suggests that appropriate modification of phenyl moieties enhances gas adsorption to paddle-wheel PCPs. We investigated CO binding energy with two Cu paddle-wheel units [Cu2(O2CC6H4-R)4]2 employing several substituents R = CH3,


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CF3, OCH3, and tBu. The ONIOM[MP4(SDQ):ωB97XD]-calculated BE at the β site increases in the order CF3 (–7.19) < OCH3 (–7.26) ~ CH3 (–7.28) < tBu (–7.47), where in parentheses are BE (in kcal mol-1). This increasing order is almost parallel to the DIS term MP4(SDQ) with SM2 and the binding energy ( BE SM2 ) with SM2; see Table S6 for DIS and MP4(SDQ) terms. These results suggest that the use of electron-releasing substituent BE SM2

enhances the dispersion interaction and the CO binding energy.

CONCLUSIONS We theoretically investigated the interactions of CO, N2, NO, and CO2 with a porous coordination polymer Cu(aip)-PCP consisting of Cu paddle-wheel units, using ONIOM[MP4(SDQ):ωB97XD] method. The optimized adsorption structure of COβ in the realistic model Dm-R is in good agreement with the experimental structure of CO-adsorbed Cu(aip)-PCP, where the CO adsorption position deviates from the normal linear position. The similar deviation of adsorption position at the β site was found for N2, NO, and CO2 in Dm-R. For all these gas adsorptions, the gas molecule at the β site tends to move to the deviating position distant from the phenyl group of the neighbouring [Cu2(O2CC6H4-R)4] paddle-wheel unit. Interestingly, the ONIOM[MP4(SDQ):ωB97XD]-calculated binding energy (BEβ) of gas molecule at the β site is larger than those at the α site and the normal adsorption position; the BEβ decreases in the order CO2 ≈ CO > NO > N2 for R = H and CO > CO2 > NO > N2 for R = CH3. The similar BEβs of CO and CO2 suggest that the gate-opening adsorption of CO2 may occur in Cu(aip)-PCP, if the other factors such as morphology, crystal edge structure, etc. are favourable for the gate-opening. The energy decomposition analysis at the Hartree-Fock level revealed that the phenyl



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moiety of the organic linker plays an important role in deviating the adsorption position and enhancing the binding energy of gas molecule with this PCP. For all cases, the ES, EXR, and CT+Pol+Mix terms with the phenyl moiety become smaller at the deviating structure than at the normal one. However, the DIS term decreases much less than the ES, EXR, and CT+Pol+Mix terms. This means that the EXR is more sensitive to the position change of gas molecule than the DIS and decreases more than does the DIS as going from the normal position to the deviating one and thereby the binding energy becomes larger (more negative) at the deviating adsorption structure. This explanation suggests that the phenyl moiety enhances gas adsorption through the DIS term (i.e. dispersion interaction). Introduction of electron-donating substituents on the phenyl moiety is computationally predicted to enhance the binding energy, because such substituent induces large dispersion interaction with gas molecule. It is concluded that the presence of organic linkers surrounding an adsorbed gas molecule at the open-metal site leads to the deviation of adsorption position but increases the binding energy. These theoretical findings reveal the important roles of organic linkers on the adsorption of gas molecule into PCPs.

ASSOCIATED CONTENT Supporting Information Detailed equations in the ONIOM scheme, NBO charges of atoms and atomic groups in gas-adsorbed Dm-R and Mm-R, detailed structural parameters, additional presentations of real and computational models, results of LMO energy decomposition analysis for Lβ in Dm-Me, plots of ωB97XD-calculated binding energies of CO against deviation angles in SM1 and SM2 models, complete references of 54, 62, and 64, Cartesian coordinates and


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total energies of Mm-R and Dm-R with and without gas molecules. This material is free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors [email protected] [email protected]

Notes The authors declare no completing financial interest.

ACKNOWLEDGMENTS This work is supported by the PRESTO and ACCEL project of the Japan Science and Technology Agency (JST), and JSPS KAKENHI Grant-in-Aid for Young Scientists (B) (Grant No. 25870360), for Challenging Exploratory Research (Grant No. 25620187) and for Specially Promoted Research (Grant No. 25000007). iCeMS is supported by the World Premier International Research Initiative (WPI) of the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT). We are thankful for Institute for Molecular Science (Okazaki, Japan) to perform some of calculations with the computer systems.

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Table 1. Binding energy (BE, kcal mol-1) of gas molecule L (L = CO2, CO, N2, and NO) with Dm-R and Mm-R (R = H, Me) calculated by the ONIOM[MP4(SDQ):ωB97XD] method. a L CO N2 NO CO2 a

Mm-H BEMm –5.86 –3.13 –3.63 –4.99

Dm-H BEα BEβ –5.58 –6.11 –2.98 –3.31 –3.37 –4.55 –5.15 –6.16

Mm-Me BEMm –5.80 –3.11 –3.38 –4.82

Dm-Me BEα BEβ –5.38 –7.28 –2.84 –4.09 –3.10 –4.81 –4.38 –6.49

BEα and BEβ denote binding energies for Lα and Lβ with Dm-R. BEMm is BE of L with

Mm-R. Counterpoise correction was made.

Table 2. Various interaction terms (kcal mol-1) of gas molecule with the SM1.a L CON COβ N2N N2β NON NOβ CO2N CO2β a

HF BE SM1

–3.47 –2.91 –1.14 –0.75 0.03 –0.09 –3.94 –3.92

ES –14.14 –15.04 –6.23 –5.85 –5.50 –4.87 –10.96 –10.88

MP4(SDQ) EXR CT+Pol+Mix DIS BE SM1 16.97 –6.30 –3.42 –6.89 18.86 –6.74 –3.63 –6.54 8.21 –3.12 –2.57 –3.71 8.01 –2.91 –2.59 –3.34 9.30 –3.78 –3.86 –3.83 8.42 –3.64 –3.58 –3.67 11.19 –3.78 –1.42 –5.36 10.84 –3.87 –1.44 –5.36

The geometry was taken to be the same as that of R = H; see Table S3 in SI for R = Me.

The binding energy was analyzed at both the normal position (LN) and deviating one (Lβ).



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Table 3. Various interaction terms (kcal mol-1) of gas molecule with the SM2.a HF BE SM2 L CON 11.32

COβ

3.94

N2N

13.29

N2β

3.41

NON

9.05

NOβ

2.83

CO2N 9.55 CO2β a

2.46

ES –9.46 –2.72 (–71%)b –7.20 –1.94 (–73%) –5.22 –2.69 (–48%) –8.65 –3.42 (–60%)

EXR 23.02 7.23 (–69%) 22.26 5.72 (–74%) 17.03 6.51 (–62%) 20.64 6.78 (–67%)

CT+Pol+Mix –2.25 –0.74 (–67%) –1.78 –0.36 (–80%) –3.03 –0.99 (–67%) –2.44 –0.86 (–65%)

DIS –5.61 –3.99 (–29%) –7.09 –3.57 (–50%) –5.39 –3.61 (–33%) –5.15 –3.66 (–29%)

MP4(SDQ) BE SM2

5.71 –0.05 6.20 –0.16 3.66 –0.78 4.40 –1.20

The geometry was taken to be the same as that of R = H; see Table S3 in SI for R = Me.

The binding energy was analyzed at both the normal position (LN) and deviating one (Lβ). b

The extent of decrease in relevant interaction term from the normal position (LN) to

deviating one (Lβ) is presented in parentheses.


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Scheme 1. (a) Experimental structurea of activated Cu(aip)-PCP, (b) that of CO-adsorbed Cu(aip)-PCP, and (c) computational dimer model (Dm-R) of Cu(aip)-PCP with adsorbed gas molecule L, and (d) that of monomer model Mm-R.

a

These experimental structures

were re-drown from CIF files of experimental report (ref. 36).



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Scheme 2. Small models (SM1 and SM2) employed for high-level calculations in the ONIOM method.


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Scheme 3. Schematic representation of interaction between CO2 and Cu open-metal site.



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Figure 1. Optimized structures of Cu paddle-wheel unit with (a) CO, (b) N2, (c) NO, and (d) CO2 molecules. Distances are in angstroms. a R = H; see Table S1 for details and those for R = Me.


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Figure 2. Optimized structures of two Cu paddle-wheel units, Dm-H, with (a) CO, (b) N2, (c) NO, and (d) CO2 molecules. Distances are in angstroms. a R = H; see Table S3 for details and those for R = Me.



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Figure 3. MP2-calculated potential energy surfaces for CO interaction with SM1 and SM2.


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Figure 4. Overlap between the lone-pair orbital of CO and (a) dπ and (b) dπ* orbitals of Cu(aip)-PCP. Hydrogen atoms are omitted for clarity.



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TOC Graphic


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Characteristic Features of CO2 and CO Adsorptions to Paddle-Wheel

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