A two-dimensional flexible porous coordination polymer based on Co(II) and terpyridyl phosphine oxide

Abstract

Two porous coordination polymers, Co-Py2PO and Co-Py2PO′, have been obtained via reactions of in situ oxidized terpyridyl phosphines and cobalt(II) nitrate. Single-crystal structure analysis reveals that Co-Py2PO′ has a three-dimensional (3D) structure with (8·10²)(8³) net topology, while the structural analysis of Co-Py2PO reveals a 2D polycatenated framework consisting of two sets of parallel 2D networks of (6³) net topology. For Co-Py2PO′, guest CHCl₃ molecules are locked in the channels along the b axis by P(O)Ph₂ phenyl rings and coordinated nitrate ions, and cannot be released out of the host framework. In contrast, the as-synthesized 2D framework Co-Py2PO readily loses its solvated guest molecules to give an activated amorphous material for sorption. Activated Co-Py2PO has a flexible structure and shows dynamic adsorption behaviours towards various guests (N₂ and benzene). The flexibility may arise from the combination of layer–layer motion and the local flexibility of the P–C bond of –P(O)Ph₂ groups.

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Introduction

Porous coordination polymers (PCPs) or metal–organic frameworks (MOFs) composed of metal ions and organic ligands have been developed as an important class of porous materials. Among them, dynamic/flexible MOFs and/or soft porous materials have received much attention due to their flexible and responsive properties.¹ Based on the materials’ ‘softness’ and stimuli responsiveness, unique functional properties have been obtained for this family of materials. Possible applications of flexible metal–organic frameworks include separation, catalysis, sensing, and biomedicine.¹ Coordination polymers made up of two-dimensional (2D) structures are a group of interesting flexible porous materials.² The subnets of interdigitated and stacked 2D frameworks are connected to each other only by weak forces, and thus can drift, relocate, or shift. Therefore 2D frameworks have a more marked flexibility than 3D structures.³ Because the subnetwork movements can be easily induced by guest inclusion, 2D frameworks show the breathing phenomenon and responsiveness to guest adsorption.⁴ They can respond to external stimuli by changing their porosity and show flexibility-associated properties such as gas adsorption,⁵ stepwise gas adsorption,⁶ selective adsorption/separation,⁷ and ion exchange.⁸ These dynamic responses are closely related to the structural changes, including sliding of 2D grids,⁹ structural expansion/shrinkage along the stacking direction,¹⁰ and bond formation/cleavage.¹¹

In the design and synthesis of flexible 2D frameworks, N-donor or O-donor-based ligands have been used widely. Although phosphines and phosphine oxides are important ligands in catalysis and coordination chemistry and have recently been used to construct porous coordination polymers,^12,13 phosphine-based ligands have rarely been exploited in flexible 2D frameworks.¹⁴ Phosphine-based 2D frameworks may be particularly interesting because the rotation of –PPh₂ phenyl rings is an important, specific source of flexibility. Herein we report the syntheses and structures of one 2D and one 3D porous coordination polymers based on Co(II) and terpyridyl phosphine oxides. Their sorption properties were investigated and interestingly the 2D framework shows remarkable flexibility behaviour.

Results and discussion

Synthesis and crystal structures

A reaction of Py2P with Co(NO₃)₂·6H₂O in MeCN–CHCl₃ at 80 °C for two days led to the formation of (Py2PO)₄Co₄(NO₃)₈·xMeCN·xCHCl₃·xH₂O (denoted Co-Py2PO). When Py2P′ was used as the starting ligand, (Py2PO′)Co(NO₃)₂·CHCl₃ (denoted Co-Py2PO′) was obtained. During the reactions, Py2P and Py2P′ were oxidized in situ to form Py2PO and Py2PO′, respectively. Py2PO and Py2PO′ are considered as 3-connecting rigid terpyridyl phosphine oxide ligands for Co(II) (and other harder 3d ions) with two outer pyridyl N donors and one P=O O donor, leaving the N donor of the central pyridine ring uncoordinated¹⁵ (Scheme 1). In comparison to this, the spacer between terpyridyl and –P(O)Ph₂ groups is one phenyl ring longer for Py2P′ than for Py2P. The oxidation of Py2P and Py2P′ was evidenced using FT-IR and mass spectra. FT-IR spectra showed characteristic bands of –P=O at 1167 and 1162 cm⁻¹ for Co-Py2PO and Co-Py2PO′, respectively (Fig. S1 and S2†). Electrospray ionization time-of-flight (ESI TOF) mass spectroscopy showed only the signals of oxidized ligands (no signals of the ligands) after digesting the Co(II) complexes with HCl, suggesting complete oxidation. In addition, both the FT-IR spectra showed the strong characteristic band of nitrate anions at around 1384 cm⁻¹. Interestingly, Co-Py2PO′ could be prepared directly using the oxidized ligand Py2PO′, while Co-Py2PO could not be obtained using Py2PO under similar conditions (see ESI and Fig. S3–S6 for characterization data of Py2PO and Py2PO′†). A similar phenomenon was observed in another phosphine oxide framework.¹⁶

Scheme 1 Oxidation of Py2P and Py2PO to form Py2P′ and Py2PO′, respectively.

The structure of Co-Py2PO was unambiguously identified by single-crystal X-ray analysis. Co-Py2PO crystallizes in the triclinic crystal system within space group P1̄. The asymmetric unit contains two oxidized phosphine ligands Py2PO, two Co²⁺ and four NO₃⁻ coordinated anions. Two independent Co²⁺ ions in the asymmetric unit have similar coordination environments, adopting a CoO₄N₂ distorted octahedral geometry. Each Co²⁺ ion is six-coordinated by one P=O oxygen atom and two pyridyl nitrogen atoms from three different Py2PO ligands and three oxygen atoms from two nitrate ions (Fig. 1a). For the nitrate ions, one nitrate ion connects with Co²⁺ through a monodentate O atom in a η¹-mode and the other nitrate ion binds through two oxygen atoms in a η²-mode. Each Py2PO ligand binds to three Co²⁺ ions with one P=O O and two outer pyridyl N donors, leaving the central pyridyl N donor uncoordinated. Overall, the Co²⁺ ions as three-connected nodes are bridged by 3-connected Py2PO ligands to form a 2D honeycomb (hcb) structure of (6³) net topology (Fig. 1b). The Co⋯Co distances separated by Py2PO are different depending on different donors, and are 14.07 Å (separated by P=O O and pyridyl N donors) and 12.93 Å (separated by two pyridyl N donors). Every two undulating 2D layers are interlaced in a parallel fashion by stacking with offset of their average planes to result in 2D→2D 2-fold parallel polycatenation (Fig. 1c–e). The resulting flat catenated 2D layers show a thickness of ca. 15.04 Å. The 2D catenated layers stack in an ABCABC packing mode. Solvated molecules (MeCN, CHCl₃ and H₂O) are located between adjacent 2D catenated layers. Upon removal of the solvated molecules, there is 25.7% void space to the total crystal volume (total potential solvent area volume 967.9 ų per unit cell volume 3765.9 ų) calculated using PLATON.¹⁷ Thus Co-Py2PO represents a 2D framework with interlayer cavity rather than intralayer cavity.¹⁸

Fig. 1 X-ray structure of Co-Py2PO showing (a) the distorted octahedral coordination geometry around Co²⁺ ions highlighting two independent Co²⁺ ions in the asymmetric unit, (b) part of the 2D layer viewed along the ac plane of the 2D honeycomb (6³) network with the solvated molecules omitted for clarity, (c) part of the 2D catenated layers viewed along the ac plane with nitrate ions and solvated molecules omitted for clarity, (d) topological diagram of 2D catenated layers, (e) part of the 2D catenated layers viewed along the ac plane, highlighting the –P(O)Ph₂ phenyl groups on the surface.

Single crystal X-ray diffraction analysis shows that Co-Py2PO′ crystallizes in the orthorhombic group Pna2₁. The asymmetric unit contains one oxidized phosphine ligand Py2PO′, one Co²⁺ and two NO₃⁻ coordinated anions. Each Py2PO′ ligand binds to three Co²⁺ ions with one P=O O and two outer pyridyl N donors like Py2PO, while each Co²⁺ is coordinated with three ligands to form a 3D network. The Co²⁺ ion is six-coordinated with a distorted octahedral CoO₄N₂ coordination geometry (Fig. 2a) like that in Co-Py2PO. Every Co²⁺ ion coordinates with one P=O O and two pyridyl N donors from three different ligands and three O donors from a chelating nitrate and a monodentate nitrate ion. Net topological analysis reveals a 3D 2-nodal 3-connected net with a 1 : 1 stoichiometry if both Co²⁺ and Py2PO′ are simplified as 3-connecting nodes. The point (Schläfli) symbol is (8·10²)(8³) calculated using OLEX¹⁹ (Fig. 2c). Its vertex symbol is (8₂·10₆·10₂)(8·8·8). 1D channels are formed along the b axis (Fig. 2b). –P(O)Ph₂ phenyl rings and coordinated nitrate ions are pointed to the channel center. CHCl₃ guest molecules are accommodated inside the channels (Fig. 2d), which accounts for 16.2% solvent-accessible voids (total potential solvent area volume is 631.3 ų per unit cell volume 3907.9 ų) calculated using PLATON.¹⁷

Fig. 2 X-ray structure of Co-Py2PO′ showing (a) the distorted octahedral coordination geometry around Co²⁺ ions, (b) tubular channels along the b axis, (c) (8·10²)(8³) topological diagram, and (d) one tubular channel along the b axis to host CHCl₃ guest molecules.

Framework stability

Thermogravimetric analysis (TGA) showed that Co-Py2PO released the guest solvated molecules from room temperature to ca. 200 °C (Fig. 3). The weight loss of 19.4% corresponds to the loss of four CHCl₃, two MeCN and ten H₂O solvated molecules per Co₄(Py2PO)₄ unit (calcd: 21.0%). The framework of Co-Py2PO was stable up to 280 °C. In contrast, unexpectedly no loss of solvated CHCl₃ molecules was observed for Co-Py2PO′ up to ca. 250 °C and then the framework of Co-Py2PO′ began to collapse upon further heating (Fig. S7†). Different thermostability of Co-Py2PO and Co-Py2PO′ may be explained by their structures. For Co-Py2PO the solvated molecules are located between 2D catenated layers and easily escape from the cavity. However, for Co-Py2PO′ the solvated CHCl₃ molecules are firmly locked in the channels along the b axis by P(O)Ph₂ phenyl rings and coordinated nitrate ions.

Fig. 3 TG curves of Co-Py2PO and Co-Py2PO′.

X-ray power diffraction (XRPD) was performed to study the framework stability of Co-Py2PO (Fig. 4) and Co-Py2PO′ (Fig. 5). For Co-Py2PO′, the pattern for the as-synthesized bulk material closely matches the simulated one from the single-crystal analysis, indicating stable framework and good phase purity at room temperature. In comparison, the pattern of Co-Py2PO at room temperature was found to deviate from the simulated to some degree, probably due to its 2D nature and easy loss of solvent molecules. In addition, XRPD for the activated Co-Py2PO (activated at 50 °C under vacuum for 16 h) reveals a broad and featureless pattern. This suggests that the long-range order in the as-synthesized material was lost and a crystalline-to-amorphous phase transformation occurred. The amorphous material could not be reversed to crystalline even after the activated Co-Py2PO was immersed in the mother liquor where Co-Py2PO crystallized for one week. Considering the structural characteristics of the present framework, we speculate that the activated material may undergo an interlayer shift/distortion of the 2D catenated layers after the guest molecules are removed.

Fig. 4 XRPD patterns of Co-Py2PO, (a) simulated from CIF, (b) as-synthesized, (c) activated at 50 °C under vacuum for 16 h, and (d) immersing the activated sample in mother liquor for one week.

Fig. 5 XRPD patterns of Co-Py2PO′, (a) simulated from CIF and (b) as-synthesized.

Sorption study

The above results reveal that Co-Py2PO readily loses guest molecules between its 2D networked layers to get activated Co-Py2PO, but Co-Py2PO′ firmly holds guest molecules in its pore channels. To study the porosity of Co-Py2PO, N₂ sorption analysis at 77 K was performed for activated Co-Py2PO (activated at 50 °C under vacuum for 16 h) (Fig. 6). Interestingly, the isotherm shows a gradual increase in gas uptake and exhibits a large desorption hysteresis loop. The uptake reaches 114 cm³ g⁻¹ without saturation at 1 bar. Moreover, the desorption does not trace the adsorption one. It loses 55 cm³ N₂ gradually up to P/P₀ = 0.016 and rapidly loses 59 cm³ N₂ below P/P₀ = 0.016. Hysteresis in the sorption profiles suggests the presence of a high diffusion barrier for N₂ in Co-Py2PO. Such a remarkable breathing hysteric behaviour is rarely observed for non-polar N₂.²⁰ In comparison, CO₂ adsorption of Co-Py2PO at 195 K revealed a type I profile indicative of its microporous nature. The adsorption capacity at 1 bar is 123 cm³ g⁻¹. The Langmuir and Brunauer–Emmett–Teller surface areas calculated from the CO₂ profile are 689 and 479 m² g⁻¹, respectively. The pore volume was calculated to be 0.157 cm³ g⁻¹ and the pore width is centred around 7.8 Å according to the Horvath–Kawazoe method. Selective sorption of CO₂ over N₂ indicates that the interlayer micropore surface is overall polar due to coordinated nitrate ions.

Fig. 6 N₂ and CO₂ adsorption (solid symbols) and desorption (open symbols) isotherms of Co-Py2PO.

Co-Py2PO was further subjected to vapour adsorption (Fig. 7). The sorption isotherms of methanol and acetonitrile vapours measured at 298 K exhibit type I profiles with hysteresis, showing that polar molecules may readily diffuse into the interlayer cavity. The MeCN isotherm displays a more marked hysteresis than the MeOH one. The overall uptake capacities are 17.3 wt% and 18.3 wt% for MeOH and MeCN, respectively.

Fig. 7 Methanol, acetonitrile, benzene and cyclohexane adsorption (solid symbols) and desorption (open symbols) isotherms of Co-Py2PO at 298 K.

The sorption studies of benzene and cyclohexane were also carried out. At the beginning range of the sorption isotherm, both the amounts of benzene and cyclohexane adsorbed are negligible. This suggests that it is difficult for both the molecules to diffuse into the interlayer cavity. For benzene, a sudden increase in uptake appears when the relative pressure P/P₀ is 0.15. The amount adsorbed is 24.5 wt% when P/P₀ reaches 1, which is equivalent to the adsorption of 9.0 benzene molecules per Co₄(Py2PO)₄ unit. In contrast to the stepwise benzene adsorption, the amount adsorbed for cyclohexane increased gradually in the tested pressure range and a final uptake is 12.8 wt% (4.1 molecules per Co₄(Py2PO)₄ unit) at P/P₀ = 1. The different sorption behaviours for benzene and cyclohexane are probably due to different interactions between the adsorbent and the adsorbates. For Co-Py2PO, π–π interactions between guest benzene molecules and the –P(O)Ph₂ phenyl rings located on the surface of 2D catenated layers provide the favourable driving force for the adsorption of guest benzene molecules. As P/P₀ increases and approaches the gate point (P/P₀ = 0.15), the π–π interactions trigger the dynamic movement of 2D catenated layers to lead to a sudden increase in benzene uptake. In addition, the dynamic motion is probably related to the rotation of –P(O)Ph₂ phenyl rings via the P–C bond. In comparison, it is difficult to form such interactions between cyclohexane and the 2D catenated layers, so a different isotherm with a gradually increasing amount was obtained. Thus the flexibility herein arises from the combination of layer–layer motion and local flexibility of P–C bonds of –P(O)Ph₂ groups. Additionally, the large hysteresis loop for both benzene and cyclohexane indicates that the 2D catenated layers restrict the release of guest molecules at high pressure. Such a 2D structure may be developed for the selective sorption of benzene over cyclohexane, which is important in industrial separation.²¹

Conclusions

In summary, two porous coordination polymers, Co-Py2PO′ and Co-Py2PO, have been synthesized based on Co(II) and terpyridyl phosphine oxides. The terpyridyl phosphine oxides were generated by in situ oxidation of terpyridyl phosphines, Py2P and Py2P′. The terpyridyl phosphine oxides coordinated with Co(II) to yield the porous materials. Both the porous coordination polymers are based on 3-connected Co(II) ions and 3-connected terpyridyl phosphines. Co-Py2PO′ has a 3D structure with (8·10²)(8³) net topology, while Co-Py2PO has a 2D polycatenated framework consisting of two sets of parallel 2D networks of (6³) net topology. Co-Py2PO′ is not able to release its guest CHCl₃ molecules out of the host framework because they are locked in the channels along the b axis by –P(O)Ph₂ phenyl rings and coordinated nitrate ions. In contrast, as-synthesized crystalline Co-Py2PO readily loses its guest solvated molecules to obtain an amorphous activated material for sorption. Activated Co-Py2PO has a flexible structure and shows gated adsorption behaviours towards various guests (N₂ and benzene). The unprecedented sorption behaviours are related to layer–layer motion and the rotational motion of –P(O)Ph₂ phenyl groups with respect to the P–C bond.

Experimental

Chemicals and solvents were obtained from commercial sources and used as received unless otherwise stated. Py2P²² and Py2P′²³ were synthesized according to the reported methods. NMR spectra were obtained using a Bruker Avance III spectrometer. Mass spectra were recorded using a Bruker maXis 4G ESI-Q-TOF mass spectrometer. X-ray powder diffraction data were collected using a Bruker D8 ADVANCE diffractometer at 40 kV and 40 mA with a Cu-target tube and a graphite monochromator (λ = 1.5418 Å). Thermo analyses were performed under an N₂ atmosphere at a heating rate of 10 °C min⁻¹ with a NETZSCH TG 209F3 system. Adsorption measurements were performed using a Quantachrome Autosorb-iQ₂ or Autosorb-iQ analyzer. Before sorption measurements, the as-synthesized sample of Co-Py2PO was degassed at 50 °C for 16 h under high vacuum.

Synthesis of Co-Py2PO

A solution of Co(NO₃)₂·6H₂O (2.9 mg, 0.01 mmol) in MeCN (2 mL) was added to a solution of Py2P (5.0 mg, 0.01 mmol) in CHCl₃ (2 mL) while stirring. The reaction mixture was stirred for 20 min at room temperature to yield a pink suspension. Then the suspension was heated in a closed vial at 80 °C. Red block crystals were obtained after five days (3.5 mg, 40%). Microanal. found (calcd) for C₁₄₀H₁₂₆N₂₂P₄Co₄O₃₈Cl₁₂ ((Py2PO)₄Co₄(NO₃)₈·4CHCl₃·2MeCN·10H₂O): C, 47.70 (47.97); H, 3.79 (3.63); N, 8.32 (8.80)%. IR (KBr, cm⁻¹): 3402br, 1603m, 1534w, 1436w, 1384s, 1166.6w, 1120w, 1066w, 1022w, 880w, 827m, 725w, 696w, 655s, 547m.

Synthesis of Co-Py2PO′

Method 1: A solution of Co(NO₃)₂·6H₂O (2.9 mg, 0.01 mmol) in MeCN (1 mL) was added to a solution of Py2P′ (4.3 mg, 0.075 mmol) in CHCl₃ (1 mL) while stirring. The reaction mixture was stirred for 20 min at room temperature to yield a pink suspension. Then the suspension was heated in a closed vial at 80 °C. Red block crystals were obtained after five days (3.5 mg, 53% based on ligand). Method 2: A solution of Co(NO₃)₂·6H₂O (2.9 mg, 0.01 mmol) in MeCN (1 mL) was added to a solution of Py2PO′ (4.3 mg, 0.075 mmol) in CHCl₃ (1 mL) while stirring. The reaction mixture was stirred for 20 min at room temperature to yield a pink suspension. Then the suspension was heated in a closed vial at 80 °C. Red block crystals were obtained after two days (2.5 mg, 38% based on ligand). Microanal. found (calcd) for C₄₀H₂₉N₅PCoO₇Cl₃ ((Py2PO′)Co(NO₃)₂·CHCl₃): C 53.76 (54.17); H, 3.47 (3.30); N, 7.97 (7.90)%. IR (KBr pellets, cm⁻¹): 3389br, 2916br, 1797w, 1603s, 1545w, 1459s, 1384s, 1307m, 1162m, 1120m, 1067w, 1023w, 874w, 822m, 750w, 555m, 529w.

X-ray structure determination

X-ray crystallographic data of Co-Py2PO and Co-Py2PO′ were collected at 150(2) K using an Oxford Gemini S Ultra diffractometer equipped with a graphite monochromated Enhance Cu X-ray source (λ = 1.54178 Å). The structure was solved by direct methods following difference Fourier syntheses, and refined by the full-matrix least-squares method against F₀² using SHELXTL software. All non-hydrogen atoms were refined with anisotropic thermal parameters while hydrogen atoms were introduced into the final refinement model in calculated positions with isotropic thermal parameters. For Co-Py2PO, one CHCl₃ and one H₂O solvated molecules display disorder, which are refined with fractional occupancy and modelled, and the hydrogen atoms of solvated water molecules are not added. Selected bond lengths and bond angles are given in Tables S1 and S2†.

Crystallographic data for Co-Py2PO: C₁₄₃H₁₁₁N₂₄P₄Co₄O₃₁Cl₉M_w = 3340.21, triclinic, P1̄, a = 14.1110(5), b = 15.3337(5), c = 18.7087(6) Å, α = 78.679(3), β = 84.134(2), γ = 71.745(3)°, V = 3765.9(2) ų, Z = 1, D = 1.548 g cm⁻³, μ = 5.942 mm⁻¹, λ = 1.54178 Å, T = 150(2) K, F(000) = 1706, 24 009 reflections were collected (12 690 were unique) for 2.41 < θ < 66.03, R(int) = 0.0644, R = 0.0974, wR₂ = 0.2077 [I > 2σ(I)], R = 0.0700, wR₂ = 0.1851 (all data) for 993 parameters with 3 restraints, GOF = 1.025. CCDC 1023746.

Crystallographic data for Co-Py2PO′: C₄₀H₂₉N₅PCoO₇Cl₃M_w = 887.93, orthorhombic, Pna2₁, a = 23.8620(6), b = 8.5930(1), c = 19.0584(5) Å, α = β = γ = 90°, V = 3907.85 (15) ų, Z = 4, D = 1.509 g cm⁻³, μ = 6.204 mm⁻¹, λ = 1.54178 Å, T = 150(2) K, F(000) = 1812, 6609 reflections were collected (4484 were unique) for 3.70 < θ < 62.36, R(int) = 0.0513, R = 0.0611, wR₂ = 0.1472 [I > 2σ(I)], R = 0.0812, wR₂ = 0.1588 (all data) for 514 parameters with 1 restraint, GOF = 1.027. CCDC 1023747.

Acknowledgements

This work was funded by the 973 Program (2012CB821701), the NSFC (21273007, 21350110212 and 91222201), the Program for New Century Excellent Talents in University (NCET-13-0615) and the Fundamental Research Funds for the Central Universities (14lgpy05).

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Footnote

† Electronic supplementary information (ESI) available: FT-IR spectra, characterization data of Py2PO and Py2PO′, selected bond lengths and bond angles. CCDC 1023746 and 1023747. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4qi00138a

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