Discrepant gas adsorption in isostructural heterometallic coordination polymers: strong dependence of metal identity

Abstract

Two isostructural porous 3d–4d materials [M(cyclam)Pd(2,4-pydc)₂·4H₂O]_n (2,4-H₂pydc = pyridine-2,4-dicarboxylate acid) (1, M = Ni and 2, M = Zn) and five other isostructural complexes {[M′(cyclam)]₂[M(2,5-pydc)₃]·xH₂O}_n (2,5-H₂pydc = pyridine-2,5-dicarboxylate acid) (3, M = Co, M′ = Ni; 4, M = M′ = Ni; 5, M = Zn, M′ = Ni; 6, M = Ni, M′ = Zn and 7, M = M′ = Zn) were synthesized. Both complexes 1′ and 2′ show high selective CO₂ capture over CH₄ and N₂. The comparisons of different gas sorption properties between 1′ and 2′ strongly suggest that metal identity has a great influence on the polarity of frameworks, but just the polar molecules respond to the polarity changes of different frameworks. The comparisons of CO₂ sorption properties between five other isostructural complexes 3′–7′ also strongly suggest that metal identity has great influence on polarity of frameworks and further the CO₂ uptake.

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Introduction

In order to reduce air pollution and therefore improve human health, our society pays increasing attention to the capture and separation of greenhouse gas – CO₂.¹ Compared with traditional porous materials such as carbon nanostructures, zeolites and mesoporous silica, porous coordination polymers (PCPs) represent a new class of crystallized porous materials, and they have exhibited great potential for gas storage and gas separation.² Because CO₂ capture and separation are mainly based on adsorbate–surface interactions between CO₂ and frameworks, great efforts thus have been made to synthesize new PCPs with enhanced CO₂ affinity.³ One of the ongoing endeavors to effectively study the mechanism for gas capture and adsorptive gas separation is to systematically compare isostructural PCPs with various building units (organic linkers or metal ions).^4,5 Isostructural coordination polymers with the same metals but different organic linkers are easily obtained, and gas sorption or separation property mutation induced by the different organic linkers have been studied thoroughly.⁴ Nevertheless, metal ions, as the cores of coordination complexes, have a significant effect on their physical and chemical properties (for example, photoluminescence,⁶ catalysis⁷ and gas adsorption⁵). Moreover, due to the uncontrollable coordination chemistry of metal ions in the synthetic reaction, isostructural coordination polymers with different metals are rare. Due to their different quadrupole moment and polarizability of gases, selective capture of one specific gas will mainly effected by the interaction between the gas and frameworks.⁸ To our knowledge, there has been no previous report that unambiguously studies the gas adsorption properties of various polar frameworks via metal substitution.

The chemical attributes of organic ligands, such as coordination ability, geometry and relative orientation of the donor groups, play a very important role in constructing polymer framework topology. Perhaps the most ubiquitous functional groups used for the synthesis of coordination polymers are the pyridyl group⁹ and carboxylic acid.¹⁰ It is worth stressing that the coexistence of pyridyl and carboxylate groups (dual functionality) imparts these ligands with special networking abilities.¹¹ Especially, the presence of the pyridyl donor and the carboxylate group in the adjacent position (i.e. pyridine-2,n-dicarboxylate and pyrazine-2-carboxylate) furnishes an additional functionality, the chelating effect, which enables these ligands to be used for constructing stable metallotectons that can further coordinate to other metal centers.¹² These metallotectons have two kinds of Lewis-base coordination groups: 2-carboxylate (group B) and n-carboxylate (n = 3, 4 or 5; group A) moieties (Scheme 1). Group A has a stronger bridging capability than group B because of its strong electron-donating and electrostatic power. Using these metallotectons has resulted in the formation of a very large number of heterometallic coordination polymers with intriguing structures and functional properties.¹³ In the assembly of coordination polymer networks, utilization of a macrocyclic complex [for example, M(cyclam)(ClO₄)₂] as a metal building block will simplify the coordination chemistry of metal ions and make extending directions of the network easily controllable. Especially, following change of metal ions embedded in the macrocyclic ligand, the isostructural complexes with different metals will be easily obtained. Furthermore, macrocyclic complexes are able to prevent the interpenetration of networks due to their bulkiness.¹⁴ Thus, we mainly focused on using macrocyclic complexes as metal source for the construction of TMCPs.

Scheme 1 Observed coordination modes of (2,n-pydc)–metal complexes and polymers.

Experimental

Materials and physical measurements

All reagents and solvents employed were commercially available and used as supplied without further purification. Powder XRD patterns were recorded with Cu Kα radiation by using a PANalytical X'Pert PRO diffractometer. TGA analyses of the complexes were performed on a NETZSCH STA 409 PC Simultaneous Thermal Analyzer under an N₂ atmosphere at a scan rate of 10 °C min⁻¹. Elemental analysis was performed on an Elementar vario EL III analyzer. IR (KBr) spectra were recorded on the Nicolet FT-IR spectrophotometer. All the gas sorption experiments were measured with an IGA-002 gas adsorption instrument.

Synthesis

[Zn(cyclam)Pd(2,4-pydc)₂·4H₂O]_n2. Pd(2,4-Hpydc)₂ (1.00 mmol, 0.44 g) was added to an aqueous solution of NaOH (2.00 mmol, 0.08 g). After stirring for half an hour, an aqueous solution of [Zn(cyclam)](ClO₄)₂ (1.00 mmol, 0.46 g) was slowly added. A colourless precipitate was formed immediately. The mixture was stirred for a further 2 h to ensure completion of the reaction. The solid was collected by filtration, washed with H₂O and THF, and dried in air to afford a pale yellow powder. A solution of [Zn(cyclam)](ClO₄)₂ (0.1 mmol) in H₂O (2.5 mL) was carefully layered on the top of a solution of [Pd(2,4-Napydca)₂)] (0.1 mmol) in H₂O–ethylene glycol (4 : 1, 2.5 mL). Colourless needle-shaped crystals suitable for X-ray crystallographic analysis were obtained after few days. Yield: 93%. Anal. Calcd for C₂₄H₃₈N₆O₁₂ZnPd: C, 37.22; H, 4.95; O, 24.79. Found: C, 37.15; H, 5.03; O, 24.86. IR (KBr, cm⁻¹): 3451 (m), 3215 (w), 1664 (s), 1624 (w), 1382 (s), 1316 (s), 1100 (s), 930 (m), 871 (w), 778 (w), 746 (w), 688 (w).

{[Ni(cyclam)]₂[Co(2,5-pydc)₃]·4H₂O}_n3. Co[Co(2,5-Hpydc)₃]₂·6H₂O (0.50 mmol, 0.64 g) was added to an aqueous solution of NaOH (3.00 mmol, 0.12 g). After stirring for half an hour, filtering and an aqueous solution of [Ni(cyclam)](ClO₄)₂ (2.00 mmol, 0.92 g) was added and stirred for an hour. The resulting solution was evaporated, and block-shaped crystals suitable for X-ray crystallographic analysis were obtained after few days. The solid was collected by filtration, washed with H₂O and dried in air to afford a pink powder. Yield: 83%. Anal. Calcd for C₄₁H₆₅N₁₁O₁₆CoNi₂: C, 43.03; H, 5.73; O, 22.37. Found: C, 42.96; H, 5.83; O, 22.44.

{[Ni(cyclam)]₂[Ni(2,5-pydc)₃]·4H₂O}_n4. The synthesis of 4 was carried out similarly to that of 3 except for using Ni[Ni(2,5-Hpydc)₃]₂·6H₂O (0.50 mmol, 0.64 g) instead of Co[Co(2,5-Hpydc)₃]₂·6H₂O. Yield: 91%. Anal. Calcd for C₄₁H₆₅N₁₁O₁₆Ni₃: C, 43.04; H, 5.73; O, 22.37. Found: C, 42.95; H, 5.82; O, 22.46.

{[Ni(cyclam)]₂[Zn(2,5-pydc)₃]·4H₂O}_n5. The synthesis of 5 was carried out similarly to that of 3 except for using Zn[Zn(2,5-Hpydc)₃]₂·6H₂O (0.50 mmol, 0.65 g) instead of Co[Co(2,5-Hpydc)₃]₂·6H₂O. Yield: 91%. Anal. Calcd for C₄₁H₆₅N₁₁O₁₆ZnNi₂: C, 42.79; H, 5.69; O, 22.24. Found: C, 42.76; H, 5.60; O, 22.18.

{[Zn(cyclam)]₂[Ni(2,5-pydc)₃]·7H₂O}_n6. The synthesis of 6 was carried out similarly to that of 3 except for using Ni[Ni(2,5-Hpydc)₃]₂·6H₂O (0.50 mmol, 0.64 g) instead of Co[Co(2,5-Hpydc)₃]₂·6H₂O and [Zn(cyclam)](ClO₄)₂ (2.00 mmol, 0.92 g) instead of [Ni(cyclam)](ClO₄)₂. Yield: 91%. Anal. Calcd for C₄₁H₇₁N₁₁O₁₉NiZn₂: C, 40.65; H, 5.91; O, 25.09. Found: C, 40.60; H, 6.01; O, 25.01.

{[Zn(cyclam)]₂[Zn(2,5-pydc)₃]·5H₂O}_n7. The synthesis of 7 was carried out similarly to that of 3 except for using Zn[Zn(2,5-Hpydc)₃]₂·6H₂O (0.50 mmol, 0.65 g) instead of Co[Co(2,5-Hpydc)₃]₂·6H₂O and [Zn(cyclam)](ClO₄)₂ (2.00 mmol, 0.92 g) instead of [Ni(cyclam)](ClO₄)₂. Yield: 91%. Anal. Calcd for C₄₁H₆₇N₁₁O₁₇Zn₃: C, 41.65; H, 5.71; O, 23.01. Found: C, 41.58; H, 5.79; O, 23.10.

Crystal structure analysis

Diffraction data of 2 were collected on a Bruker APEX DUO diffractometer equipped with triumph-monochromated Cu Kα radiation (λ = 1.54178 Å). Diffraction data of 3–7 and A1–A3 were collected on a Bruker Smart APEX CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). These data were collected at 173 K (A3, 293 K) temperature and the structures were solved by direct methods and subsequently refined on F² by using full-matrix least-squares techniques (SHELXL).¹⁵ SADABS¹⁶ absorption corrections were applied to the data. There are disordered water molecules in the asymmetric unit of 2, which cannot be refined properly, so the SQUEEZE algorithm was run to remove them before the structures were refined to convergence. According to elemental analysis (EA), IR spectra and TGA analyses, the solvent molecules were confirmed to be H₂O. All the non-hydrogen atoms were refined anisotropically, and hydrogen atoms were located at calculated positions. However, hydrogen atoms of water molecules cannot be found. In the asymmetric unit of A2, a water molecule coordinated to Cu1 was disordered and it was divided into two parts (81 : 19). A summary of the crystallographic data are given in Table 1.

Table 1 Crystallographic data and structure refinement results for compounds 2–7, A1–A3

Compounds	2	3	4	5	6	7	A1	A2	A3
a R 1 = Σ||Fo| − |Fc||/Σ|Fo|, wR2 = {Σ[w(Fo^2 − Fc^2)^2]/Σ[w(Fo^2)^2]}^1/2.
Formula	C24H30N6O8PdZn	C41H65N11O16CoNi2	C41H65N11 O16Ni3	C41H65N11O16ZnNi2	C41H71N11O19NiZn2	C41H67N11O17Zn3	C24H36N6O11CuNi	C24H34N6O10Cu2	C24H50N6O18CuNi
Formula weight	702.31	1144.39	1144.17	1150.83	1211.54	1182.17	706.84	693.65	832.95
Crystal system	Monoclinic	Triclinic	Triclinic	Triclinic	Triclinic	Triclinic	Monoclinic	Triclinic	Triclinic
Space group	C2/c	P1̄	P1̄	P1̄	P1̄	P1̄	P21/c	P1̄	P1̄
a, Å	23.8588(12)	11.4952(9)	11.4505(8)	11.4882(8)	11.5109(14)	11.5056(8)	13.8978(8)	8.2345(9)	9.706(4)
b, Å	18.6112(9)	11.5504(8)	11.5123(8)	11.5521(8)	11.5916(14)	11.6391(8)	15.0595(9)	13.4315(15)	10.184(4)
c, Å	8.4514(5)	21.6426(16)	21.6191(14)	21.6308(16)	21.717(3)	21.8286(15)	15.5605(9)	13.4515(15)	10.607(4)
α, °	90	94.3510(10)	94.2070(10)	94.3610(10)	91.170(2)	91.6950(10)	90	65.4050(10)	62.712(4)
β, °	100.598(3)	91.2180(10)	91.5310(10)	91.3170(10)	94.449(2)	94.3000(10)	115.0970(10)	81.539(2)	85.470(5)
γ, °	90	113.6270(10)	113.6120(10)	113.5840(10)	113.834(2)	113.4320(10)	90	85.676(2)	69.691(4)
V, Å^3	3688.8(3)	2620.9(3)	2599.3(3)	2618.8(3)	2638.3(6)	2668.9(3)	2949.3(3)	1337.9(3)	869.8(6)
Z	4	2	2	2	2	2	4	2	1
D c, g cm^−3	1.265	1.450	1.462	1.459	1.525	1.471	1.592	1.722	1.590
μ, mm^−1	5.111	1.099	1.151	1.241	1.337	1.413	1.427	1.659	1.237
θ range/°	3.03–62.49	0.95–27.54	0.95–28.25	0.95–27.01	0.94–28.28	0.94–27.51	1.62–27.73	1.67–28.26	2.17–25.03
GOF on F^2	1.088	0.995	1.081	1.065	1.066	1.081	0.942	1.146	1.023
R 1 ^a [I > 2σ(I)]	0.0424	0.0519	0.0569	0.0492	0.0497	0.0528	0.0362	0.0390	0.0648
wR2^a (all data)	0.1312	0.1947	0.1977	0.1527	0.1611	0.1935	0.0867	0.1589	0.1773
Δρmax, Δρmin, e Å^−3	0.822, −1.310	0.774, −0.734	1.609, −1.025	0.621, −0.522	1.138, −0.973	1.209, −0.948	0.407, −0.334	0.916, −0.765	1.012, −1.468

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Results and discussion

Synthesis and design

Recently, the porous TMCPs complexes [Ni(cyclam)Pd(2,4-pydc)₂·4H₂O]_n have been reported by us.¹⁷ Importantly, the varied metal ions have been confirmed to have a great influence on the synthesis of these complexes. Our further efforts to extend this series to other first-row transition metals {Ni(II) ions of [Ni(cyclam)Pd(2,4-pydc)₂·4H₂O]_n may be substituted by other cations} using a similar synthesis protocol, resulted in either noncrystalline substances (Mn²⁺, Co²⁺) or nonporous precipitates with different crystal structures (Cu²⁺, Fig. S1, ESI†), with Zn²⁺ being the only exception. This is not surprising due to the different coordination chemistry of various metal cations. It is well known that Cu(II) favors a square planar arrangement similar to Pd(II). If the Pd(II) ions of Pd(2,4-pydc)₂ were replaced by Cu(II), whether the complexes with the structures similar to [M(cyclam)Pd(2,4-pydc)₂]_n (M = Ni, Zn) will be obtained. In view of this, the metallotectons Cu(2,4-Hpydc)₂, Cu(2,5-Hpydc)₂ and their further constructing complexes [Ni(cyclam)Cu(2,5-pydc)₂·3H₂O] (A1), [Cu(cyclam)Cu(2,5-pydc)₂·2H₂O] (A2) and [Ni(cyclam)Cu(2,4-pydc)₂·10H₂O]_n (A3) were synthesized, but the extending structures were largely different from [M(cyclam)Pd(2,4-pydc)₂]_n. In complexes A1 and A2, both group A and group B moieties of [Cu(2,5-pydc)₂]²⁻ did not connect with [M(cyclam)]²⁺, which resulted in the formation of their discrete structures (Fig. S2 and S3, ESI†). Nevertheless, in complex A3, group B connected with [Ni(cyclam)]²⁺ and group A remained idle. Compared with complexes 1 and 2, complex A3 also holding the 1D linear chain structure, but axial positions of Cu(II) were occupied by H₂O molecules and no M⋯M interaction existed between the two chains, which resulted in its nonporous structure. Based on the preferred octahedral coordination environment of Co(II), Ni(II) and Zn(II), we also developed a series metallotectons M[M(2,5-Hpydc)₃]₂·6H₂O [M = Co(II), Ni(II), Zn(II)], in which the M²⁺ is octahedrally coordinated with three N and three O atoms from three 2,5-Hpydc⁻ moieties.

Crystal structures

Crystal structures of 1–2

In the current work, we mainly focus on these two isostructural structure types: type I and type II. Complexes [M(cyclam)Pd(2,4-pydc)₂·4H₂O]_n (1–2) have the type I structure, and complexes {[M′(cyclam)]₂[M(2,5-pydc)₃]·xH₂O}_n (3–7) have the type II structure. Both complexes 1–2 were constructed from the metallotectons Pd(2,4-Hpydc)₂ and M(cyclam)(ClO₄)₂. The powder X-ray diffraction patterns (PXRD) (Fig. 1c) for complexes 1 and 2 accord well with each other, confirming that the two 3d–4d coordination polymers are indeed isostructural and they may be an ideal system to study the impact of metal cation exchange on polarity changes of frameworks and their gas adsorption properties. In such structures, all crystallize in the monoclinic space group C2/c with the Pd(II) and M(II) (M = Ni or Zn) atoms lying on independent inversion centres, and with each asymmetric unit containing one complete 2,4-pydc²⁻ ligand and half a cyclam ligand (crystallographic data are summarized in Table 1). Each Pd(II) cation lies in a square planar environment with the two oxygen atoms and the two nitrogen atoms of 2,4-pydc²⁻ ligands in the equatorial plane, leaving two vacant axial positions. Nevertheless, M(II) takes the classical octahedral coordination environment [MN₄O₂] formed by four nitrogen atoms from cyclam and two carboxylate oxygen atoms of 2,4-pydc²⁻, respectively. The carboxylate group at the 2-position together with the nitrogen atom chelate to the Pd(II) cation and the 4-position carboxylate group is coordinated to M(II), which results in the formation of linear chains which come from two different directions to cross each other through Pd⋯Pd interaction and form rhombus grid with dihedral angle 74.430° for 1 and 76.871° for 2 (Fig. 1a). The packing of these linear chains generates a 1D channel with petal-like cylindrical pores (Fig. 1b).

Fig. 1 (a) View showing the stacking of linear chains. (b) View of 1D channels present in the structure [M(cyclam)Pd(2,4-pydc)₂]_n (H atoms were omitted for clarity, C, gray; O, red; N, blue; Pd, orange; M, green.) (c) PXRD for 1 and 2 depicting the isostructural relationship.

Crystal structures of 3–7

The one-pot reaction of [M(2,5-pydc)₃]⁴⁻ [M = Co(II), Ni(II), Zn(II)] and [M′(cyclam)](ClO₄)₂ [M′ = Ni(II), Zn(II)] in solvent H₂O at room temperature yielded type II complexes. The block-shaped crystals suitable for X-ray crystallographic analysis were obtained in high yield after few days. All the structures of 3, 4, 5, 6 and 7 crystallize in the triclinic space group P1̄ (crystallographic data are summarized in Table 1), and their asymmetric units contain one [M(2,5-pydc)₃]⁴⁻ and two [M′(cyclam)]²⁺. The coordination modes of [M(2,5-pydc)₃]⁴⁻ in all the complexes are given in Fig. S5, ESI.† Each [M(2,5-pydc)₃]⁴⁻ acts as a μ₃ bridge to link three [M′(cyclam)]²⁺ ions in which one 2,5-pydc²⁻ moiety remains idle, the 5-carboxylate groups of one 2,5-pydc²⁻ and the 2-, 5-carboxylate groups from the other 2,5-pydc²⁻ moieties are coordinated with [M′(cyclam)]²⁺ with monodentate η¹:μ₂ bridging modes. [Corresponding bond lengths (Å) of complexes 3–7 are shown in Table S1, ESI†]. Although [M(2,5-pydc)₃]⁴⁻ acts as a μ₃ building block, the directions of coordinated 2-carboxylate and 5-carboxylate groups from the same 2,5-pydc²⁻ moieties are in the inverse positions. So it finally resulted in the formation of 1D ladder-shaped coordination polymers (Fig. 2). The 1D ladder frameworks extended to 2D frameworks through H bonds between the 1D ladder frameworks and [M′(cyclam)]²⁺ (Fig. 3).

Fig. 2 (a and b) View showing the 1D ladder shapes (the H atoms were omitted for clarity, C, gray; O, red; N, blue; M, yellow; M′, green.)

Fig. 3 H bonds between the 1D ladder frameworks and [M′(cyclam)]²⁺.

Adsorption properties

Gas sorption of 1 and 2

The guest molecules can be removed by heating 1 and 2 under vacuum at 100 °C for 6 h, and the dehydrated complexes 1′ [Ni(cyclam)Pd(2,4-pydc)₂]_n and 2′ [Zn(cyclam)Pd(2,4-pydc)₂]_n will be obtained, respectively. Upon removal of guest molecules, guest-free complexes 1′ and 2′ remained highly crystalline and isostructural, as confirmed by PXRD analysis (Fig. S6, ESI†). Surface area analysis by N₂ sorption was performed on evacuated 1′ and 2′, which indicated their permanent microporosity (Fig. S10, ESI†). Gas (CO₂, CH₄, and N₂) sorption isotherms for complexes 1′ and 2′ were measured up to 1 atm at 273 and 298 K (Fig. 4). Fitting the Langmuir equations to the CO₂ adsorption isotherm (at 273 K, P/P₀ = 0.05–0.25) gave estimated surface areas of 276 m² g⁻¹ for 1′ and 308 m² g⁻¹ for 2′. Because surface area is reported as an area per given mass, this value for a complex with a lighter metal should be much higher in a series of isostructural materials. Nevertheless, herein, the varied surface areas may be resulted by their different pore sizes induced by Zn²⁺ with larger ionic radius than Ni²⁺ [As seen from the longer distance of adjacent Zn⋯Zn in 2 (18.369 Å) than Ni⋯Ni in 1 (18.157 Å)]. At room temperature, these two complexes showed similar CO₂ sorption capacity (34.1 cm³ g⁻¹ for 1′ and 34.6 cm³ g⁻¹ for 2′, 1 atm). Following the decreasing temperature, CO₂ adsorption capacity for these complexes increased greatly (54.4 cm³ g⁻¹ for 1′ and 57.1 cm³ g⁻¹ for 2′ at 273 K, 1 atm). In particular, the enthalpy of CO₂ adsorption for 1′ and 2′ were estimated from the sorption isotherms at 273 and 298 K using the virial equation to understand the strong affinity of two complexes toward CO₂. At zero coverage, the enthalpy of CO₂ adsorption is 26.2 kJ mol⁻¹ for 1′ and 25.9 kJ mol⁻¹ for 2′ (Fig. 5a), which indicates the existence of sufficient interactions between the quadrupolar CO₂ molecules and frameworks. The increase in the CO₂ adsorption capacity with increasing pressure and decreasing temperature is indicative of physisorption in a microporous solid, which indicates this material may be applied in pressure or temperature swing adsorption processes. The CH₄ sorption capacity of these two complexes were smaller than CO₂, with the amounts of CH₄ uptake reaching 5.8 cm³ g⁻¹ for 1′ and 5.3 cm³ g⁻¹ for 2′ per gram at 1.0 atm, 298 K (10.3 cm³ g⁻¹ for 1′ and 10.1 cm³ g⁻¹ for 2′ at 273 K, 1 atm). The isosteric heat of CH₄ adsorption for 1′ and 2′ also were calculated from the Clausius–Clapeyron equation (Fig. 5b). Unlike CO₂ and CH₄, N₂ gas was minimally absorbed in 1′ and 2′ up to 1 atm (1.97 cm³ g⁻¹ for 1′ and 1.92 cm³ g⁻¹ for 2′ at 298 K, 1 atm; 3.00 cm³ g⁻¹ for 1′ and 3.78 cm³ g⁻¹ for 2′ at 273 K, 1 atm). In the industrial application process of PCPs, besides the adsorption capacity, the selectivity is a principal property relevant to adsorptive gas separation. Fig. 4 also shows that complexes 1′ and 2′ can preferentially adsorb CO₂ over CH₄ and N₂ and provide separation capability of CO₂/N₂ and CO₂/CH₄. The single-component CO₂/N₂ separation ratios of 1′ at 0.1 and 1 atm (298 K) are 25.7 : 1 (v : v), 17.3 : 1 (v : v), respectively, and for CO₂/CH₄ they are 10.6 : 1 (v : v), 5.9 : 1 (v : v) at 0.1 and 1 atm (298 K), respectively. Complex 2′ has a similar selectivity with CO₂/N₂ 24.1 : 1 (v : v), 18.0 : 1 (v : v) at 0.1 and 1 atm (298 K) and CO₂/CH₄ 9.8 : 1 (v : v), 6.5 : 1 (v : v) at 0.1 and 1 atm (298 K), respectively. Nevertheless, at 273 K, the CO₂/N₂ and CO₂/CH₄ selectivity for 1′ and 2′ are larger than the separation ratios of corresponding gas pairs at 0.1 atm, but lower than that at 1 atm (Table S3, ESI†). The CH₄/N₂ selectivity for 1′ and 2′ also were shown in Table S3, ESI.† The high selective capture of CO₂ than CH₄ and N₂ may be ascribed to CO₂ having a larger quadrupole moment (13.4 × 10⁻⁴⁰ cm²vs. 4.7 × 10⁻⁴⁰ cm² for N₂, CH₄ is non-polar) and CH₄ adsorbs preferentially over N₂ due to its higher polarizability (26.3 × 10⁻²⁵ cm³ for CO₂vs. 17.6 × 10⁻²⁵ cm³ for N₂ and 26.0 × 10⁻²⁵ cm³ for CH₄). The influencing factors on adsorptive gas separation include the molecular sieving effect, the thermodynamic equilibrium effect, and the kinetic effect.^1a The highest Q_st (CO₂) value for 1′ (26.2 kJ mol⁻¹) and 2′ (25.9 kJ mol⁻¹) show that here the preferential adsorbate–surface interaction based thermodynamic equilibrium effect is operative.

Fig. 4 (a) Gas sorption curves [273 K (red) and 298 K (purple)] of 1′. (b) Gas sorption curves [273 K (blue) and 298 K (green)] of 2′. Square, CO₂; star, CH₄, round, N₂.

Fig. 5 Isosteric heats of gas adsorption (Q_st) values for 1′ (blue) and 2′ (red): (a) CO₂ and (b) CH₄.

All CO₂ isotherms are of type I according to the IUPAC classification.¹⁸ The CO₂ sorption isotherms for complexes 1′ and 2′ measured at 298 K show that CO₂ uptake of 2′ is always slightly less than 1′, which may be ascribed to the lighter atomic weight of Ni than Zn (Fig. 6a). Nevertheless, at 273 K, CO₂ uptake of 2′ is also lower than 1′ below 0.7 atm, but with higher uptake above 0.7 atm. N₂ sorption capacity of 1′ and 2′ are almost the same at 298 K. Furthermore, at 273 K, the N₂ sorption capacity of 2′ is obviously superior to 1′ (Fig. 6c). The varied CO₂ and N₂ uptake of 1′ and 2′ is interpreted as specific interactions between the quadrupolar CO₂, N₂ molecules and the polar groups in the pore channels of different frameworks [the N–H distance of Zn-cyclam (0.907 Å) in 2 is much longer than that of Ni-cyclam (0.880 Å)] in 1]. The CH₄ isotherms show that 1′ shows higher CH₄ uptake than 2′ under all conditions (Fig. 6b), which may be ascribed to the fact that CH₄ is non-polar (the non-polar molecules don't respond to the polarity changes of different frameworks) and here just the varied atom weight played a key role.

Fig. 6 The comparisons of different gas sorption properties between 1′ and 2′: (a) CO₂, (b) CH₄ and (c) N₂.

Gas sorption of 3–7

Using similar methods, we successfully synthesized a series complexes {[M′(cyclam)]₂[M(2,5-pydc)₃]·xH₂O}_n. The powder X-ray diffraction patterns (PXRD) and IR spectra (Fig. 7) for complexes 3–7 accord well with each other, confirming that these five complexes are indeed isostructural and they may be an ideal system to study the impact of metal identity on polarity changes of frameworks and their gas adsorption properties. Heating 3–7 under vacuum at 150 °C for 12 h, the dehydrated complexes {[M′(cyclam)]₂[M(2,5-pydc)₃]}_n3′–7′ will be obtained, respectively (Fig. S8 and S9, ESI†). CO₂ sorption isotherms for complexes 3′–7′ were measured up to 1 atm at 273 and 298 K (Fig. 8). At room temperature, these series of complexes showed largely varied CO₂ sorption capacity (10.8 cm³ g⁻¹ for 3′, 15.2 cm³ g⁻¹ for 4′, 7.4 cm³ g⁻¹ for 5′, 1.8 cm³ g⁻¹ for 6′, 3.3 cm³ g⁻¹ for 7′, 1 atm) and all these CO₂ isotherms at 298 K are of type I according to the IUPAC classification. Here, the metal identity has great influence on polarity of frameworks (Table S1, ESI†) and further the CO₂ adsorption. As we know, the CO₂ sorption capacity of coordination polymers is greatly influenced by the coordinatively unsaturated metal centers (UMCs) and polarity of their frameworks. In these complexes, one [M′(cyclam)]²⁺ remains uncoordinated and interacting with the frameworks through H bonds. Complexes 3′, 4′ and 5′ contain the same [Ni(cyclam)]²⁺ but with varied [M(2,5-pydc)₃]⁴⁻ building blocks and the varied M ions have a great influence on their CO₂ sorption capacity (4′ having the largest amount, 3′ medium and 5′ small). Complexes 4′ and 6′ holding the same [Ni(2,5-pydc)₃]⁴⁻ but with varied [M′(cyclam)]²⁺ building blocks and the varied M′ ions also have great influence on their CO₂ sorption capacity (4′ holding larger CO₂ sorption capacity than 6′). This phenomenon also was found in the CO₂ sorption of complexes 5′ and 7′. These complexes constructed by [Ni(cyclam)]²⁺ holding larger CO₂ sorption capacity than with [Zn(cyclam)]²⁺ building blocks, which may be ascribed to the larger interaction between CO₂ molecules and coordinatively unsaturated Ni centers than Zn metals. Following the decreasing temperature, CO₂ adsorption capacity for these complexes increased greatly (14.0 cm³ g⁻¹ for 3′, 24.8 cm³ g⁻¹ for 4′, 14.2 cm³ g⁻¹ for 5′, 3.9 cm³ g⁻¹ for 6′, 14.8 cm³ g⁻¹ for 7′, at 273 K, 1 atm), which indicates that these materials can also be applied in pressure or temperature swing adsorption processes.

Fig. 7 (a) PXRD patterns for complexes 3–7; (b) IR spectrum of complexes 3–7.

Fig. 8 The comparisons of CO₂ sorption properties between 3′–7′: (a) 298 K, 1 atm; (b) 273 K, 1 atm.

Unlike CO₂, CH₄ and N₂ gas were minimally absorbed in 4′ up to 1 atm (2.62 cm³ g⁻¹ for CH₄ and 0.69 cm³ g⁻¹ for N₂ at 298 K, 1 atm; 3.37 cm³ g⁻¹ for CH₄ and 1.20 cm³ g⁻¹ for N₂ at 273 K, 1 atm). Besides the adsorption capacity, the selectivity is a principal property relevant to adsorptive gas separation. Fig. 9 shows that complex 4′ can preferentially adsorb CO₂ over CH₄ and N₂ and providing separation capability of CO₂/N₂ and CO₂/CH₄. The single-component CO₂/CH₄ separation ratios of 4′ at 273 K and 298 K (1 atm) are 7.3 : 1 (v : v), 5.8 : 1 (v : v), respectively, and for CO₂/N₂ they are 20.7 : 1 (v : v), 22.0 : 1 (v : v) at 273 K and 298 K (1 atm), respectively.

Fig. 9 Gas sorption curves [298 K (red) and 273 K (blue)] of 4′. Square, CO₂; star, CH₄, round, N₂.

Conclusions

In summary, two type I complexes [M(cyclam)Pd(2,4-pydc)₂·4H₂O]_n (1–2) and five type II complexes {[M′(cyclam)]₂[M(2,5-pydc)₃]·xH₂O}_n (3–7) were synthesized and characterized. The high selective capture of CO₂ over CH₄ and N₂ indicates that both complexes 1′ and 2′ are promising materials for the separation and purification of CO₂ from CO₂/CH₄ or CO₂/N₂ mixtures. In addition, the comparisons of different gas sorption properties between 1′ and 2′ strongly suggest that metal identity has great influence on polarity of frameworks and further the adsorption of polar gas molecules, but the non-polar molecules don't respond to the polarity changes of different frameworks. The non-polar molecules uptake of isostructural materials with different metals will just be influenced by the varied atom weight. The comparisons of CO₂ sorption properties between five type II complexes also strongly suggest that metal identity has a great influence on the polarity of frameworks and further on CO₂ uptake. In addition, the high selective capture of CO₂ over CH₄ and N₂ indicates that complex 4′ is also promising material for the separation and purification of CO₂ from CO₂/CH₄ or CO₂/N₂ mixtures.

Acknowledgements

This work was supported by Shanghai Science and Technology Committee (08DZ2270500 and 08DJ1400103), Shanghai Leading Academic Discipline Project (B108) and the National Basic Research Program of China (2009CB825300 and 2010DFA41160).

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Footnote

† Electronic supplementary information (ESI) available: Crystal structures of complexes A1, A2 and A3; powder XRD patterns, TGA curves and gas sorption curves. CCDC 851502 (2), 889537 (3), 889538 (4), 889539 (5), 889540 (6), 889541 (7), 889542 (A1), 889543 (A2), –889544 (A3). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c2ce26072g

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