PCM-14 is a dense coordination polymer formed from Ca(II) and an unusual organophosphonium ligand. The dehydrated framework contains 3-coordinate Ca(II) sites within catenated, chiral 3,3-connected nets. PCM-14 exhibits a stark CO2 sorption selectivity over H-2, N-2 and O-2. The maximum CO2 uptake was shown to be highly sensitive to the material pretreatment evacuation temperature.Welch Foundation F-1738Chemistr

Humphrey, Simon M.

Ibarra, Ilich A.

Lynch, Vincent M.

Tan, Kristina E.

Dalton Transactions

Ilich A. Ibarra

Kristina E. Tan

Vincent M. Lynch

Simon M. Humphrey

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CO2 adsorption properties of a Ca(ii)-based organophosphonium coordination material

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This article is published as part of the Dalton Transactions themed issue entitled:  Coordination chemistry in the solid state  Guest Editor Russell E. Morris  Published in Issue 14, Volume 41 of Dalton Transactions    Articles in this issue include:  Communications Highly oriented surface-growth and covalent dye labeling of mesoporous metal–organic frameworks  Florian M. Hinterholzinger, Stefan Wuttke, Pascal Roy, Thomas Preuße, Andreas Schaate, Peter Behrens, Adelheid Godt and Thomas Bein  Papers Supramolecular isomers of metal–organic frameworks: the role of a new mixed donor imidazolate-carboxylate tetradentate ligand  Victoria J. Richards, Stephen P. Argent, Adam Kewley, Alexander J. Blake, William Lewis and Neil R. Champness  Hydrogen adsorption in the metal–organic frameworks Fe2(dobdc) and Fe2(O2)(dobdc) Wendy L. Queen, Eric D. Bloch, Craig M. Brown, Matthew R. Hudson, Jarad A. Mason, Leslie J. Murray, Anibal Javier Ramirez-Cuesta, Vanessa K. Peterson and Jeffrey R. Long   Visit the Dalton Transactions website for the latest cutting inorganic chemistry www.rsc.org/publishing/journals/dt/ Published on 23 January 2012. Downloaded by University of Texas Libraries on 19/08/2016 19:18:09. View Article Online / Journal Homepage / Table of Contents for this issueDaltonTransactionsDynamic Article LinksCite this: Dalton Trans., 2012, 41, 3920www.rsc.org/dalton COMMUNICATIONCO2 adsorption properties of a Ca(II)-based organophosphoniumcoordination material†Ilich A. Ibarra, Kristina E. Tan, Vincent M. Lynch and Simon M. Humphrey*Received 23rd October 2011, Accepted 15th December 2011DOI: 10.1039/c2dt12011aPCM-14 is a dense coordination polymer formed from Ca(II)and an unusual organophosphonium ligand. The dehydratedframework contains 3-coordinate Ca(II) sites within cate-nated, chiral 3,3-connected nets. PCM-14 exhibits a starkCO2 sorption selectivity over H2, N2 and O2. The maximumCO2 uptake was shown to be highly sensitive to the materialpretreatment evacuation temperature.The development of materials that can efficiently and selectivelystore gases remains a major challenge for their future relevancein critical applications such as sequestration of CO2 and othergreenhouse gases.1 Porous coordination polymers are amongstthe most promising candidates in this regard, because their sorp-tion selectivity towards small molecule adsorbates is directlytunable as a function of the topology and chemical compositionof the micropores.2 Recent work in the field has shown that apropitious choice of inorganic and organic components cangreatly favour certain types of sorption selectivity. In particular,unsaturated metal sites3 and polar organic moieties4 have beenshown to favour selective physisorption of small molecules, bypromotion of stronger host–guest interactions. For this reason,our continued interest into organophosphine-based coordinationpolymers has recently focused on studies of the structure–func-tion relationship due to the presence of phosphonium (R3P+–R′)ligand-based polar species. It is possible to generate these groupspre-, or post-synthetically by direct reaction of ‘free’ phosphine-P of either the monomer, or a corresponding coordinationpolymer, with R′I species to yield the corresponding P(V) salt(Scheme 1). Here, the synthesis of a dense framework materialwith an unusual chiral structure is reported, using pre-syntheti-cally modified [R3P–Me]Cl in reaction with Ca(II). The metalnodes within this material can be rendered highly unsaturated byremoval of three OH2 ligands per metal. The resulting sorptionselectivity with different small molecule adsorbates is reported.Porous coordination polymers constructed from lighter alka-line-earth metals have been understudied in comparison to theplethora of examples of d-block-based materials, perhaps due totheir unpredictable coordination numbers and geometries andlimited reactivity.5 However, some recently reported Mg(II) fra-meworks have been shown to result in thermally stable, lighteropen-frameworks that are effective in a range of selective gasstorage applications.6 The Mg(II)-based phosphine coordinationmaterial, PCM-11, recently showed an unrivalled CO2 uptakecapacity at high pressure.7 Ca(II) centres in porous coordinationframeworks should be similarly attractive, and might also offerthe possibility to construct unusual framework types due to theexpanded coordination sphere. Ca(II) is also comparatively cheapand non-toxic. Relatively few examples of Ca(II) coordinationpolymers have been reported in the literature.5 Plater et al.reported the first coordination polymer based on Ca(II), [Ca(hbtc)·2H2O]∞, (hbtc2− = benzene-1,3,5-tricarboxylate);8 andsince then, the vast majority of the Ca(II)-based coordinationpolymers reported, have focused primarily on the description ofthe crystal structure.9 Only sparse efforts have been made to finddifferent applications for Ca(II)-based coordination polymers,such as catalysis,10 or water and organic solvent adsorption.11As shown previously, the trigonal pyramidal geometry of tris(p-carboxylated) triphenylphosphine results in the rational con-struction of coordination polymers with pseudo-cubic symmetrythat have P-sites at opposing corners of a cube, with inorganicnodes at alternating corners.12 In addition, due to the variableoxidation states and coordination numbers available to phos-phorus, alkylation of the free phosphine (R3P) with alkyl iodide(R′I) followed by HCl work-up affords the P(V) phosphoniumsalt ([R3PR′]Cl; Scheme 1) and concomitantly reduces theoverall charge on the fully deprotonated ligand from −3 to −2.Thus, fewer metal cation centres are needed to generate a neutralframework, without altering the geometry and primary functionScheme 1 Alkylation of the free phosphine (ptbc3− = tris(p-carboxy-late)triphenylphosphine) with methyl iodide affording the P(V) phos-phonium salt (mptbc2− = tris(p-carboxylate)triphenylmethylphosphonium) and the formation of PCM-14.†Electronic supplementary information (ESI) available: Additionalexperimental information, XRPD data corresponding to various pre-treat-ment conditions, TGA data, and X-ray data. CCDC reference numbers849683 for PCM-14 (180 K) and 849684 (375 K). For ESI and crystal-lographic data in CIF or other electronic format see DOI: 10.1039/c2dt12011aDepartment of Chemistry and Biochemistry, The University of Texas atAustin, 1 University Station A5300, Austin, TX 7812-0165, USA. E-mail:smh@cm.utexas.edu; Tel: +1-(512)-471-03123920 | Dalton Trans., 2012, 41, 3920–3923 This journal is © The Royal Society of Chemistry 2012Published on 23 January 2012. Downloaded by University of Texas Libraries on 19/08/2016 19:18:09. View Article Onlineof the ligand as a 3-connected node, since the ligand remains tet-rahedral at P.Low-temperature solvothermal reaction of Ca(OH)2 with theiodo salt of methylphosphonium trylbenzene-p-carboxylic acid(mptbcH3I; Scheme 1)12 in slightly basic DMF : EtOH : H2O(1 : 1 : 1) afforded large crystalline quantities of the complex[Ca(mptbc)(OH2)3·2H2O], hereafter referred to as PCM-14.‡Low temperature (−93 °C) X-ray structural determination of asingle crystal showed that PCM-14 crystallises into the rarelyobserved chiral cubic space group I213 (Z = 8; Fig. 1A, andassociated bond lengths and angles in Table 1).§ The polymer isbased on a simple 1 : 1 Ca2+ : mptc2− ratio and no halide ispresent. The P1–C1P phosphonium moiety and each Ca1 areresident on 3-fold sites such that only 1/3 of a complete ligandcomprises the asymmetric unit. Each phosphonium ligand iscoordinated to three symmetry-equivalent Ca1 sites via mono-dentate carboxylate interactions; it is noteworthy that the C7–O1–Ca1 angle is nearly linear at 173.7(3)°, which suggests ahighly ionic interaction cf. the expected 120° angle that wouldbe predicted from covalent bonding to an sp2-hybridized donorsite on carboxylate-O. Ca1 is roughly octahedral with a fac-arrangement of carboxylate contacts and three additional fac-OH2 ligands that are partially disordered over two sites in thesingle crystal at −93 °C (Fig. 1A; green bonds). With respect tothe overall resulting network type in PCM-14, the ligand pro-vides a 3-connected trigonal node at P1 that bridges to a moreuncommon 3-connected node at Ca1, with acute (92.6(1)°)angles. Two discrete chiral and catenated 3,3-c nets in PCM-14(Fig. 1B; pink/blue and green nets) result in a dense materialwith small pore voids that are occupied by two H2O solvent mol-ecules per repeat unit.The framework components themselves are not inherentlychiral: the origin of chirality in PCM-14 is a result of “handed-ness” of each individual 3,3-c network; the bulk of the crystalsare therefore expected to be a racemic mixture of enantiomor-phous pairs. However, inspection of the extended structure ofPCM-14 along (111) clearly shows small cavities flanked bythree Ca(II) atoms with a common ‘twist’ direction (Fig. 1C).To probe the extent to which PCM-14 could be desolvatedwith the retention of structure, a single crystal was first heated insitu to 102 °C by a stream of dry N2 and then allowed to equili-brate for 2 h, before a second X-ray structure was collected.§ Wewere surprised to find at this temperature that the crystallinitywas maintained, but only a proportion of the lattice H2O mol-ecules had been removed; the coordinated OH2 also remainedintact, but were no longer disordered over two sites. This indi-cated that both lattice and coordinated H2O in PCM-14 wasstrongly bound and would require removal under more forcingconditions.Thermogravimetric analysis (TGA) of PCM-14 under Hecarrier gas showed a continuous weight loss of approximately16.6 wt% over the temperature range 25–200 °C, which is inagreement with the complete loss of all lattice and coordinatedH2O (calc. 16.8 wt%). PCM-14 reaches a plateau above 200 °Cwith the onset of structure decomposition at 360 °C (see ESIFig. S1†). In situ variable-temperature powder X-ray diffraction(PXRD) measurements were used to further probe the dehy-dration behaviour of PCM-14, upon evacuation at 1 × 10−10 barat elevated temperatures (Fig. 2). This confirmed retention of theframework up to 200 °C, with excellent agreement between theexperimental PXRD data and the calculated pattern as simulatedfrom the single crystal data. At 250 °C under high vacuum, thematerial was decomposed.Fig. 1 (A) Expanded asymmetric unit of PCM-14 showing Ca(II)coordination to a single complete phosphonium ligand. Thermal ellip-soids are drawn at the 30% probability level and all hydrogen atomshave been omitted for clarity; (B) representation of the catenated 3,3-connected networks (Ca = blue; P = pink); (C) View of PCM-14 along(111) showing small channels flanked by Ca(II) sites; all water moleculesare omitted for clarity.Table 1 Selected bond distances (Å) and angles (°) for PCM-14Ca1–O1 2.263(3) Ca1–O1Ai 2.263(3)Ca1–O1Bii 2.263(3) Ca1–O3 2.47(1)Ca1–O3′ 2.271(7)O1–Ca1–O1Ai 92.65(12) O1–Ca1–O1Bii 92.65(12)O1–Ca1–O3 164.60(4) O1–Ca1–O3Ai 102.30(4)O1–Ca1–O3Bii 83.30(2) O1–Ca1–O3′ 173.40(6)O1–Ca1–O3′A 80.80(6) O1–Ca1–O3′ B 88.80(2)C1P–P1–C1 109.92(13) C1–P1–C1Aiii 109.02(13)Symmetry operations used to generate equivalent atoms: (i) 3/2 − z,1 − x, 1/2 + y; (ii) 1 − y, 1/2 −z, 3/2 − x; (iii) 1 − z, x − 1/2, 1/2 − y;(iv) 1/2 + y, 1/2 − z, 1 − x.Fig. 2 Variable temperature in situ PXRD patterns for PCM-14 and thecalculated pattern derived from the single-crystal structure data.This journal is © The Royal Society of Chemistry 2012 Dalton Trans., 2012, 41, 3920–3923 | 3921Published on 23 January 2012. Downloaded by University of Texas Libraries on 19/08/2016 19:18:09. View Article OnlineBased on the TGA and XRPD data, it seems reasonable toassume that treatment of PCM-14 under high vacuum at elevatedtemperature yields the fully desolvated and activated material(PCM-14a). This would invoke the generation of 3-coordinateCa(II) sites (Fig. 1C) that could potentially enhance gas uptake.Thus, a range of experiments were conducted to study the poros-ity in PCM-14a. Initial experiments using N2, H2 or O2 at 77 Kas the probe gas showed minimal uptake when PCM-14 was pre-activated at 180 °C and 1 × 10−10 bar (0.50 wt%, 0.76 moleculesper unit cell; 0.07 wt%, 1.48 molecules per unit cell; 0.39 wt%,0.52 molecules per unit cell, respectively). However, CO2 wasfound to be strongly adsorbed in PCM-14a at 196 K, suggestinga strong selectivity for CO2 (Fig. 3). CO2 has a similar kineticdiameter to the other probe analytes, which should equally beable to penetrate the pore openings. A significant desorption hys-teresis was also noted for CO2 (Fig. 3; open green diamonds).There are a number of possible explanations for the observedhysteresis that include the influence of polar P+-CH3 headgroupsin the pores. However, the most likely origin of CO2 trapping inthis instance is due to moderately strong physisorption betweenCO2 molecules and coordinatively-unsaturated Ca(II) sites.Based on the highly dense nature of PCM-14, the measured porevolume is correspondingly low (0.03 cm3 g−1). To further probethe CO2 capacity of PCM-14a, freshly-prepared samples werestudied as a function of outgasing temperature under vacuum.Samples activated between 25–150 °C and 1 × 10−10 showed nosignificant CO2 uptake (approx. 0.16 wt%; Fig. 4), whichsuggests the presence of coordinated water in the framework.However, upon increasing the temperature to 170 °C, theuptake rose sharply to 1.74 wt%. This must be due to the barrierfor complete dehydration having been reached, and suggestsmoderately strong Ca-OH2 coordination in the as-synthesizedmaterial. Further increasing the temperature to 180 °C and 1 ×10−10 bar resulted in optimal uptake of 2.92 wt% (correspondingto 2.82 molecules per unit cell). Higher activation temperaturesresulted in correspondingly lower CO2 uptake, most likely due tothe onset of structural collapse (Fig. 4). These results were alsoconfirmed by PXRD of the samples after each activation con-dition (see ESI Fig. S2†). Although PCM-14 is thermally stableup to 360 °C (under He at ambient pressure) and retains overallframework crystallinity up to 200 °C under similar conditions,the vacuum used to evacuate the samples during the activationprocess may accelerate the degradation of the material.13 It isevident from this study that routine exploration of pre-treatmentconditions is pivotal to maximization of gas uptake properties insuch coordination polymer materials, especially those containingsolvated metal sites or other polar framework residues.AcknowledgementsThe authors acknowledge The Welch Foundation for financialsupport (F-1738).Notes and references‡The ligand, [mptbcH3]Cl, [(H3C)P(C6H5-p-CO2H)3]Cl (20 mg,37 μmol)12 was dissolved in DMF : H2O : EtOH (1 : 1 : 1, 2.5 cm3), towhich was added a second solution of Ca(OH)2 (8 mg, 108 μmol) dis-solved in 2.5 cm3 of the same solvent. NaOH (0.10 cm3, 1.0 M) wasadded to the opaque slurry, which was sonicated (20 s). The mixture wasthen heated in a 20 cm3 sealed glass vial at 70 °C (graphite thermalbath) for 5 days. PCM-14 was formed cleanly and isolated by simpledecantation of the supernatant. The crystals were washed with DMF anddried in air prior to activation. (average yield 35 mg, 60% based on Ca).Found: C, 47.9; H, 4.25%; C22H25CaO11P requires: C, 49.2; H, 4.70%;νmax (KBr/cm−1): 1619 m, 1617 m, 1552 s, 1496 w, 1361 s, 1317 m,1106 m, 1016 m, 901 m, 840 m, 769 s, 734 s, 691 s. After heating asample of PCM-14 to 50 °C under N2 for 30 min, Found: C, 50.8; H,4.42%; C22H23CaO10P requires: C, 51.0; H, 4.47%.§Crystal data for PCM-14 (T = 180 K): C22H25CaO11P; MW = 532.44,space group I213, a = 17.1078(9), V = 5007.1(5), Z = 8, ρ = 1.413 gcm−3, R1 = 0.056 (0.091) (4088 independent reflections, I > 2σ(I)), wR2= 0.116 (0.135) (all data), GoF = 1.03. Crystal data for PCM-14 (T =375 K) C22H24CaO10.5P; MW = 527.46, space group I213, a = 17.124(2), V = 5020.9(10), Z = 8, ρ = 1.388 g cm−3, R1 = 0.059 (0.106) (1292independent reflections, I > 2σ(I)), wR2 = 0.143 (0.160) (all data), GOF= 1.04.1 J.-R. Li, Y. Ma, M. C. McCarthy, J. Sculley, J. Yu, H.-K. Jeong, P.B. Balbuena and H.-C. Zhou, Coord. Chem. Rev., 2011, 255, 1791;M. Hirscher, B. Panella and B. Schitz, Microporous Mesoporous Mater.,2010, 129, 335; S. Keskin, T. M. Van Heest and D. S. Sholl, Chem-SusChem, 2010, 3, 879; D. M. D’Alessandro, B. Smit and J. R. Long,Angew. Chem., Int. Ed., 2010, 49, 6058; G. Férey, C. Serre, T. Devic,G. Maurin, H. Jobic, P. L. Llewellyn, G.-D. Weireld, A. Vimont,M. Daturi and J.-S. Chang, Chem. Soc. Rev., 2011, 40, 550.2 S. Yang, G. S. B. Martin, J. J. Titman, A. J. Blake, D. R. Allan, N.R. Champness and M. Schröder, Inorg. Chem., 2011, 50, 9374; S. Yang,X. Lin, A. J. Blake, G. S. Walker, P. Hubberstey, N. R. Champness andM. Schröder, Nat. 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CO2 Adsorption Properties of a Ca(II)-Based Organophosphonium Coordination Material

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CO2 Adsorption Properties of a Ca(II)-Based Organophosphonium Coordination Material

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