Synthesis, Crystal Structures, and Properties of Novel Lamellar

Dec 18, 2003 - ... The Royal Institution of Great Britain, 21 Albemarle Street, London, ...... balls (Ga), large white balls (P), small gray balls (O)...
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VOLUME 16, NUMBER 2

JANUARY 27, 2004

© Copyright 2004 by the American Chemical Society

Articles Synthesis, Crystal Structures, and Properties of Novel Lamellar Gallium Methylenediphosphonates Howard G. Harvey† and Martin P. Attfield*,‡ Davy-Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London, W1S 4BS, U.K., and UMIST Centre for Microporous Materials, Department of Chemistry, University of Manchester Institute of Science and Technology, P.O. Box 88, Manchester, M60 1QD, U.K. Received August 12, 2003. Revised Manuscript Received October 22, 2003

Three new gallium methylenediphosphonates: (C5H5NH)[Ga2(H2O)2(O3PCH2PO3)(O3PCH2PO3H)] (1) (monoclinic, P21/c, a ) 11.0848(2) Å, b ) 8.4441(2) Å, c ) 9.4419(1) Å, β ) 104.537(1)°, Z ) 2), [(CH3)4N][Ga2(H2O)2(O3PCH2PO3)(O3PCH2PO3H)] (2) (monoclinic, C2/c, a ) 22.1628(8) Å, b ) 8.5724(2) Å, c ) 9.4455(2) Å, β ) 95.486(3)°, Z ) 4), and (C5H5NH)[Ga(H2O)(O3PCH2PO3)] (3) (monoclinic, P21/a, a ) 9.5721(7) Å, b ) 8.6140(6) Å, c ) 12.894(1) Å, β ) 99.961(3)°, Z ) 4) have been synthesized by solvothermal methods in the presence of organocations, and their structures were determined using powder (1 and 2) and singlecrystal X-ray diffraction data (3). All three materials contain a novel corrugated [Ga(H2O)(O3PCH2PO3)]- layer motif composed of octahedral GaO5(OH2) units that are linked together by the diphosphonate groups. One of the oxygen atoms of each diphosphonate group protrudes into the interlamellar region where it hydrogen bonds to adjacent layers in 1 and 2 to form hydrogen-bonded framework structures, or to the pyridinium cations in the lamellar material 3. The stacking arrangement of the layers in 1 and 2 is shown to be dependent on the organocations present during the synthesis, and to form materials with one- or twotypes of hydrogen-bonded channels, respectively. Material 3 is an intermediate phase isolatable during formation of 1 and is convertible to the type of hydrogen-bonded framework of 1 by solvothermal, thermal, or solution ion-exchange methods.

Introduction Lamellar inorganic materials form an important class of materials and find increasing use as catalysts, * To whom correspondence should be addressed. Tel: 00-44-161200-4467. Fax: 00-44-161-200-4559. E-mail: [email protected]. † Davy-Faraday Research Laboratory. ‡ UMIST Centre for Microporous Materials.

absorbents, ion-exchangers, and hosts.1-8 One particular advantage of these systems over the chemically similar three-dimensional framework materials is that there is (1) Corma, A.; Fornes, V.; Guil, J. M.; Pergher, S.; Maesen, T. L. M.; Buglass, J. G. Microporous Mesoporous Mater. 2000, 38, 301. (2) Corma, A.; Fornes, V.; Pergher, S. B. Nature 1998, 396, 353. (3) van Olphen, H. An Introduction to Clay Colloid Chemistry; John Wiley and Sons: New York, 1963.

10.1021/cm034745q CCC: $27.50 © 2004 American Chemical Society Published on Web 12/18/2003

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less limitation on the size of the species that can interact with the intracrystalline region and active sites of the lamellar material. Lamellar materials can be delaminated or pillared to produce high-surface-area materials with the majority of their active sites or cations exposed at the crystal surface.9,10 Inclusion of an organic component in these materials has led to the formation of inorganic-organic hybrid lamellar materials whose potential benefits stem from the inclusion of both the inorganic and organic components into the product. In particular, the incorporation and modification of organic groups within the structures allows the possibility of rationally designing materials with specific chemical, hydrophobic/hydrophilic, and structural properties. Phosphonic acids [RPO(OH)2, where R is an organic group] and diphosphonic acids [(HO)2OPRPO(OH)2] are excellent precursors for the preparation of such hybrid lamellar materials, and such materials have shown great potential for application in areas such as sensing,11 ion sequestration,12 ion-exchange,13,14 and catalysis.15 Interest in lamellar materials has also focused recently on their role as direct precursors or intermediates in the formation of open-framework structures. Transformation of the layers may occur under solvothermal conditions, as exemplified by the conversion of aminecontaining zinc phosphate lamellar materials to openframework materials in which the final framework material may contain layers of the same structure16 or of a structure different from17 those found in the starting lamellar material. Transformation may also be induced by simple thermal treatment as used in the formation of the zeolite MCM-22 by calcination of a layered aluminosilicate precursor material.18 Calcination of such layers can also form different structural products depending on the nature of the layers. If the layers are flat then only one product can be formed. However, if the layers are corrugated, then structurally different products may result depending on the stacking of the layers before transformation. For this reason it is desirable to control the way in which the layers are stacked in order to form materials with different types of pore channels. Use of different organic amines as structure-directing agents can produce lamellar phases in which the corrugated layers are stacked in- and outof-phase relative to each other. Subsequent calcination of the materials will yield structurally different frame(4) McBride, M. B. Environmental Chemistry of Soils; Oxford University Press: New York, 1994. (5) Theng, B. K. G. The Chemistry of Clay Organic Reactions; John Wiley and Sons: New York, 1974. (6) Theng, B. K. G. Formation and Properties of Clay-Polymer Complexes; Elsevier: New York, 1979. (7) Dyer, A.; Chow, J. K. K.; Umar, I. M. J. Radioanal. Nucl. Chem. 1999, 242, 313. (8) Cool, P.; Clearfield, A.; Crooks, R. M.; Vansant, E. F. Adv. Environ. Res. 1999, 3, 151. (9) Corma, A.; Diaz, U.; Domine, M. E.; Fornes, V. Angew. Chem., Int. Ed. 2000, 39, 1499. (10) Corma, A.; Fornes, V.; Diaz, U. Chem. Commun. 2001, 2642. (11) Mallouk, T. E.; Gavin, J. A. Acc. Chem. Res. 1998, 31, 209. (12) Zhang, B.; Clearfield, A. J. Am. Chem. Soc. 1997, 119, 2751. (13) Kullberg; Clearfield, A. Solv. Extr. Ion Exch. 1989, 7, 527. (14) Kullberg; Clearfield, A. Solv. Extr. Ion Exch. 1990, 8, 187. (15) Segawa, K.; Ozawa, T. J. Mol. Catal., A 1999, 141, 249. (16) Natarajan, S. Chem. Commun. 2002, 780. (17) Choudhury, A.; Neeraj, S.; Natarajan, S.; Rao, C. N. R. J. Mater. Chem. 2002, 12, 1044. (18) Mochida, I.; Eguchi, S.; Hironaka, M.; Nagao, S. I.; Sakanishi, K.; Whitehurst, D. D. Zeolites 1997, 18, 142.

Harvey and Attfield

work materials. This is exemplified in the case of the corrugated ferrierite layers that are stacked 180° outof-phase relative to each other in the precursor lamellar material PREFER,19 formed in the presence of 4-amino2,2,6,6-tetramethylpiperidine, and the precursor lamellar material MCM-47,20 formed in the presence of tetramethylene bis(N-methylpyrrolidinium) cations, that contains ferrierite layers stacked in-phase. Upon calcination the former material produces the fully condensed zeolite ferrierite,19 whereas the latter forms a structurally different material whose structure has not been fully determined, but in which the siloxy/silanol groups are not fully condensed.21 During the course of our program to investigate new group IIIB metal diphosphonates22,23 we have discovered the first three gallium methylenediphosphonate materials: (C5H5NH)[Ga2(H2O)2(O3PCH2PO3)(O3PCH2PO3H)] 1, [(CH3)4N][Ga2(H2O)2(O3PCH2PO3)(O3PCH2PO3H)] 2, and (C5H5NH)[Ga(H2O)(O3PCH2PO3)] 3, that are structurally related phases formed from modifications to the synthetic conditions. The compounds all contain the novel corrugated [Ga(H2O)(O3PCH2PO3)]- layer motif. These layers are either bound by strong hydrogenbonding interactions to form a hydrogen-bonded framework material (1 and 2) or remain as individual layers in the truly lamellar material (3). The stacking arrangement of the layers is seen to be dependent on the structure-directing effect of the organocation in the synthesis allowing the rational design of framework materials with one- or two-types of hydrogen-bonded channels. We also show that material 3 behaves as an isolatable intermediate phase in the formation of 1 and is convertible to the type of hydrogen-bonded framework of 1 by solvothermal, thermal, and ion-exchange solution methods. Experimental Section Materials and Methods. The reagents used to synthesize all the materials were Ga2(SO4)3‚18H2O (Alfa Aesar), methylenediphosphonic acid (Lancaster), HF/ pyridine (70 wt %, Aldrich), and pyridine (Aldrich). All additional amines and alkali metal chlorides were obtained from Aldrich or Lancaster, and, like the aforementioned reagents, were used without further purification. Microprobe EDXA measurements were used to determine the Ga/P ratios in the as-synthesized samples and the alkali metal cation/Ga/P ratio of the reexchanged ion-exchanged sample of 3. Measurements were made on separate regions of the as-synthesized samples using an Oxford Instrument ISIS system (EDS) fitted with a JEOL 773 Super-probe operating with an accelerating voltage of 15 kV, a beam diameter of 2 µm, under a vacuum of 10-12 Torr. Measurements were made on separate regions of the ion-exchanged sample of 3 using a FEI Quanta 200 ESEM fitted with a large field detector with a cone attachment, operating with an accelerating voltage of 20 kV under a vacuum of 1 Torr. Magic-angle spinning solid-state nuclear magnetic resonance (MAS SSNMR) 31P spectra were recorded using a Bruker MSL 300 spectrometer. An 85% solution of H3PO4 was used (19) Schreyeck, L.; Caullet, P.; Mougenel, J. C.; Guth, J. L.; Marler, B. Microporous Mater. 1996, 6, 259. (20) Valyocsik, E. W. U.S. Patent 5068096, 1991. (21) Burton, A.; Accardi, R. J.; Lobo, R. F.; Falcioni, M.; Deem, M. W. Chem. Mater. 2000, 12, 2936. (22) Harvey, H. G.; Teat, S. J.; Attfield, M. P. J. Mater. Chem. 2000, 10, 2632. (23) Harvey, H. G.; Teat, S. J.; Tang, C. C.; Cranswick, L. M.; Attfield, M. P. Inorg. Chem. 2003, 42, 2428.

Synthesis of Lamellar Gallium Methylenediphosphonates as the reference with the spectrometer operating at a frequency of 121.50 MHz, a sample spinning speed of 20 kHz, and recycle delays of 30 s for phase pure samples of 1 and 3. Thermogravimetric analysis (TGA) data were collected using a Shimadzu TGA 50 thermogravimetric analyzer with the samples heated in open alumina crucibles under flowing nitrogen from 25 to 900 °C at a heating rate of 5 °C min-1. A Bruker D8 diffractometer with a 50M PSD, configured in capillary mode and equipped with an Huber HTC 9634 furnace, was used to collect the thermodiffraction data on 3. Data for each scan were collected from 5 to 40° 2θ with a step size of 0.0073° and a collection time of 4 s per step. The furnace temperature was manually adjusted such that measurements were taken at 30 °C intervals between 20 and 470 °C. Roomtemperature X-ray diffraction patterns of ion-exchanged samples of 3 were collected using a Siemens D500 diffractometer employing a Ge-monochromated Cu KR1 radiation or a Phillips X’Pert diffractometer employing Cu KR1+2 radiation and a RTMS X’Celerator detector. Inductively coupled plasma optical emission spectroscopy was used to determine the alkali metal cation/P ratio in the ion-exchanged samples of 3, except for the Cs-containing sample for which the Cs/P ratio was determined by atomic absorption spectroscopy. Sample Preparation. The sample of (C5H5NH)[Ga2(H2O)2(O3PCH2PO3)(O3PCH2PO3H)] 1 used for structure determination was synthesized by mixing together Ga2(SO4)3‚ 18H2O, methylenediphosphonic acid, HF, pyridine, and deionized water to form a reagent mixture of molar ratio 1:2.05: 6.16:38.8:102 which, after stirring, had an initial pH of 6.4. This reagent mixture was loaded in a 23-mL Teflon-lined steel autoclave and heated for 14 d at 160 °C. The product was washed with ethanol and acetone, separated by suction filtration, and had the form of a white polycrystalline powder. Microprobe analysis confirmed the presence of Ga and P in the ratio 1: 2. [(CH3)4N][Ga2(H2O)2(O3PCH2PO3)(O3PCH2PO3H)] 2 was synthesized from a synthesis gel similar to that used to synthesize 1 but which contained an additional organocationic species, tetramethylammonium hydroxide (25 wt % in H2O) added. Ga2(SO4)3‚18H2O, methylenediphosphonic acid, tetramethylammonium hydroxide, HF, pyridine, and deionized water were mixed together to form a reagent mixture of molar ratio 1:2.05: 3.97:6.14:38.7:102 which, after stirring, had an initial pH of 6.2. The reagent mixture was loaded in a 23-mL Teflon-lined steel autoclave and heated for 4 d at 150 °C. The product, which had the form of a crystalline white powder, was washed and separated by suction filtration. Microprobe analysis confirmed the presence of Ga and P in the ratio 1:2. Single crystals of (C5H5NH)[Ga(H2O)(O3PCH2PO3)] 3 for structure determination were synthesized using a synthesis gel composition similar to those above but with tripropylamine added. Ga2(SO4)3‚18H2O, methylenediphosphonic acid, tripropylamine, HF, pyridine, and deionized water were mixed together to form a reagent mixture of molar ratio 1:2.05:9.52: 8.88:39.0:102 which, after stirring, had an initial pH of 6.1. The reagent mixture was loaded in a 23-mL Teflon-lined steel autoclave and heated for 4 d at 150 °C. The product was formed as a bi-phasic mixture of needlelike crystals of 3 embedded within powdered 1. Microprobe analysis of the single crystals confirmed the presence of Ga and P in the ratio 1:2. Additional syntheses were performed to determine under what conditions compounds 1 and 3 could be produced as pure phases and what effect on the products formed arose from the addition of other organic amines to the initial synthesis gel. One series of samples was synthesized using a fixed gel composition but with each sample undergoing solvothermal treatment at a different temperature or duration, as summarized in Table 1. Another series of samples was synthesized in which different organic amines were added to the starting gel, as summarized in Table 2, and all the reaction mixtures were then treated under solvothermal conditions at 150 °C for 4 d. One of the reactions from Table 2 was then repeated and samples were withdrawn from the reaction vessel at different times to monitor the phases present as a function of time. The

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Figure 1. Final observed (crosses), calculated (lines), and difference profiles (lines) for (C5H5NH)[Ga2(H2O)2(O3PCH2PO3)(O3PCH2PO3H)] (1). Tick-marks show reflection positions. Table 1. Summary of the Conditions Imposed upon and Products Formed from the Reagent Mixture of Ga2(SO4)3‚18H2O, Methylenediphosphonic Acid, HF, Pyridine, and H2O with the Molar Ratio 1:2.05:6.16:38.8:102 temp (°C)

reaction duration (h)

product

160 150 150 150

336 96 72 18

pure 1 pure 1 3 & 1 (majority 3) majority of 3 & trace of 1

phases present in the extracted solid samples from this reaction are summarized in Table 3. Ion-exchanged and re-ion-exchanged materials derived from compound 3 were formed by refluxing 0.5 g of 3 with more than a 5 molar excess of the alkali metal halide in 20 cm3 of water at 80 °C with constant stirring. The solid was retrieved by suction filtration and washed thoroughly with copious amounts of deionized water. Ab initio Powder X-ray Structure Determination of [C5H5NH][Ga2(H2O)2(O3PCH2PO3)(O3PCH2PO3H)] 1 and [(CH3)4N][Ga2(H2O)2(O3PCH2PO3)(O3PCH2PO3H)] 2. The X-ray data used to determine the unit cell parameters of 1 were collected during a 12-h scan on a Siemens D500 X-ray diffractometer using Cu KR1 radiation (λ ) 1.54056 Å). The first 20 low-angle Bragg reflections were used to determine the monoclinic unit cell parameters using the auto-indexing program DICVOL24 contained within the CRYSFIRE suite of programs.25 Synchrotron X-ray data were collected on a sample contained in a 0.5-mm diameter Lindemann glass capillary tube mounted on the high-resolution X-ray powder diffractometer at station 2.3, Daresbury, SRS, UK. The incident X-ray wavelength was 1.25018 Å, selected using a Si(111) monochromator, and the capillary tube was spun during data collection to minimize preferred orientation and sampling effects. Data were collected in steps of 0.01° 2θ for periods of 2 s between 5.5 and 20° 2θ, 6 s between 20 and 50° 2θ, and 12 s between 50 and 80° 2θ. Inspection of the synchrotron X-ray diffraction pattern revealed the systematic absences were consistent with the space group P21/c. One region of the diffraction data was excluded between 10.21 and 10.59° 2θ to remove a minor spurious peak not attributable to that of the structure being determined. Structure factors were extracted from this diffraction data by the Le Bail method26 as imple(24) Louer, D. Acta Crystallogr. A. 1998, 54, 922. (25) Shirley, R. The CRYSFIRE System for Automatic Powder Indexing: User’s Manual; The Lattice Press: 41 Guildford Park Avenue, Guildford, Surrey GU2 5NL, England, 1999. (26) Le Bail, A.; Duroy, H.; Fourquet, J. Mater. Res. Bull. 1988, 23, 447.

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Table 2. Summary of the Products Formed from Synthesis Gels Containing Different Organic Amines Heated at 150 °C for 96 Hours

a

added amine

reactant ratioa

initial pH

product

none triethylamine tripropylamine tetramethylammonium hydroxide tetraethylammonium hydroxide tetrapropylammonium hydroxide

1:2.05:6.16:38.8:0:102 1:2.05:6.16:38.8:9.54:102 1:2.05:8.90:40.4:9.54:102 1:2.05:6.16:38.8:3.98:102 1:2.05:6.16:38.8:4.77:102 1:2.05:6.16:38.8:1.67:102

6.4 7.0 6.1 6.2 6.0 6.5

pure 1 pure 3 3 & 1 (majority 3) pure 2 3 & 1 (majority 3) pure 1

Reagent mixture of Ga2(SO4)3‚18H2O/Methylenediphosphonic acid/HF/pyridine/amine/H2O.

Table 3. Summary of the Products Formed at Different Times from Heating the Synthesis Gel of Ga2(SO4)3‚18H2O, Methylenediphosphonic Acid, HF, Pyridine, Tripropylamine, and H2O with the Molar Ratio 1:2.05:6.16:38.8:9.54:102 at 150 °C time (h)

products

15.5 45.6 140.1 210.6 305.1

pure 3 3 & small amount of 1 3 & 1 (majority 3) 3 & 1 (majority 1) pure 1

mented in the GSAS suite of programs.27 The background of the diffraction profile was fitted with fixed points in the profile. The peak profiles were described by a pseudo-Voigt function with additional terms used to account for anisotropic particle size and strain broadening effects. The extracted structure factors were used in the direct methods program SIRPOW97,28 via the EXPO interface,29 to provide the Ga, P, and some of the O atoms of the structure. This model was used as the starting model for the Rietveld refinement, again using the GSAS suite of programs.27 The remaining atoms were located from difference Fourier maps. Only three unique atoms were used to define the pyridine molecule, due to the symmetry relations within this space group, one atom (N/C(1)) of which was given an occupancy of half nitrogen and half carbon. Initially, soft restraints were applied to all the Ga-O, P-O/ C, and C-C/N distances within the structure with the soft restraint weighting factor fixed at a high value. As the refinement proceeded the soft restraint weighting factor was reduced to a final value of 3 for the latter cycles of the refinement. The final cycle of least squares refinement included the histogram scale factor, zero point error, lattice parameters, peak profile parameters, and positional and isotropic atomic displacement parameters for all atoms in the structure. The isotropic atomic displacement parameters of the Ga and P atoms and the O, N, and C atoms were constrained to have the same value during refinement. The final residuals and crystallographic data are given in Table 4, atomic coordinates and isotropic atomic displacement parameters are provided in the Supporting Information, and selected bond distances and angles are presented in Tables 5 and 6, respectively. The final observed, calculated and difference profiles are shown in Figure 1. The structure solution of 2 was performed in a manner analogous to that described for 1 except for following details. The unit cell parameters were determined using the autoindexing program TREOR-9030 within the CRYSFIRE indexing package.25 Synchrotron X-ray data were collected on a sample contained in a 0.5-mm diameter Lindemann glass capillary tube in steps of 0.01° 2θ with a time per step of 4 s between 5 and 25° 2θ, 7 s between 25 and 50° 2θ, and 14 s between 50 (27) Von Dreele, R. B.; Larson, A. C. GSAS, General Structure Analysis System; Regents of the University of California: LANSCE, Los Alamos National Laboratory, 1995. (28) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R.; Istituto di Ricerca per lo Sviluppo di Metodologie Cristallografiche (IRMEC): Bari, Italy, 1997. (29) Altomare, A.; Burla, M. C.; Camalli, M.; Carrozzini, B.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Rizzi, R. J. Appl. Crystallogr. 1999, 32, 339. (30) Werner, P. E. J. Appl. Crystallogr. 1985, B41, 418.

Table 4. Crystal Data and Structure Refinement Parameters for 1 and 2 formula formula weight temperature (K) wavelength (Å) crystal system space group a (Å) b (Å) c (Å) β (°) V (Å3) Z DC (gcm-3) number of reflections no. of fitted parameters Rp Rwp RF χ2

1

2

Ga2P4O14NC6H21 600.4 293 1.25018 monoclinic P21/c 11.0848(2) 8.4441(2) 9.4419(1) 104.537(1) 855.48(3) 2 2.273 963 56 0.0600 0.0751 0.1320 1.387

Ga2P4O14C7NH15 594.78 293 1.25078 monoclinic C2/c 22.1628(8) 8.5724(2) 9.4455(2) 95.486(3) 1786.3(1) 4 2.132 1012 51 0.0652 0.0877 0.1455 2.229

and 80° 2θ. Inspection of the synchrotron X-ray diffraction pattern revealed systematic absences consistent with the space group C2/c and the presence of a minor spurious peak, not attributable to that of the structure being determined, in the region 10.40 and 11.05° 2θ that was excluded from the analysis. Soft restraints were applied to all the Ga-O, P-O/ C, and C-C/N distances within the structure during structure refinement, and the soft restraint weighting factor was reduced to a final value of 5 in the latter cycles of the refinement. The final residuals and crystallographic data are given in Table 4, atomic coordinates and isotropic atomic displacement parameters are provided in the Supporting Information, and selected bond distances and angles are presented in Tables 5 and 6, respectively. The final observed, calculated, and difference profiles are shown in Figure 2. Single-Crystal Structure Determination of (C5H5NH)[Ga(H2O)(O3PCH2PO3)] 3. A suitable crystal of compound 3

Figure 2. Final observed (crosses), calculated (lines), and difference profiles (lines) for [(CH3)4N][Ga2(H2O)2(O3PCH2PO3)(O3PCH2PO3H)] (2). Tick-marks show reflection positions.

Synthesis of Lamellar Gallium Methylenediphosphonates

Chem. Mater., Vol. 16, No. 2, 2004 203

Table 5. Selected Bond Distances (Å) for Compounds 1, 2, and 3 1A Ga(1)-O(1) Ga(1)-O(2)a Ga(1)-O(3)a Ga(1)-O(4) Ga(1)-O(5)a Ga(1)-O(6)a P(1)-O(3) P(1)-O(6)a P(1)-O(7) P(1)-C(1)b P(2)-O(1)c P(2)-O(2) P(2)-O(4) P(2)-C(1)b C(2)-N(1) C(3)-N(1) C(2)-C(3)d

2B 1.969(6) 1.962(6) 1.966(6) 1.975(6) 2.113(6) 1.954(6) 1.558(6) 1.528(6) 1.533(6) 1.794(6) 1.528(6) 1.540(6) 1.550(6) 1.789(6) 1.365(6) 1.367(6) 1.393(6)

3C

Ga(1)-O(1)a Ga(1)-O(2)b Ga(1)-O(4)a Ga(1)-O(5) Ga(1)-O(6) Ga(1)-O(7)c P(1)-O(1) P(1)-O(2) P(1)-O(5) P(1)-C(1) P(2)-O(3) P(2)-O(4) P(2)-O(6) P(2)-C(1) N(1)-C(2) × 2 N(1)-C(3) × 2

1.963(6) 1.972(6) 1.937(6) 1.962(6) 1.951(6) 2.099(6) 1.532(6) 1.542(6) 1.550(6) 1.799(6) 1.547(6) 1.518(6) 1.526(6) 1.797(6) 1.485(5) 1.491(4)

Ga(1)-O(3) Ga(1)-O(6) Ga(1)-O(4) Ga(1)-O(2) Ga(1)-O(1) Ga(1)-O(5)a P(1)-O(3) P(1)-O(6)a P(1)-O(7) P(1)-C(1)b P(2)-O(1)b P(2)-O(2) P(2)-O(4)c P(2)-C(1) C(2)-N(1) C(2)-C(6) C(3)-N(1) C(3)-C(4) C(4)-C(5) C(5)-C(6)

1.929(6) 1.931(6) 1.960(7) 1.962(6) 1.976(6) 2.097(6) 1.531(6) 1.551(7) 1.509(7) 1.786(9) 1.515(6) 1.529(7) 1.546(6) 1.810(9) 1.33(1) 1.38(2) 1.35(1) 1.40(2) 1.38(1) 1.38(2)

A For 1: ax, -y + 1/ , z + 1/ ; bx, y - 1, z; c-x, -y, -z + 1; d-x + 1, -y + 1, -z + 1. B For 2: ax, -y + 1, z - 1/ ; b-x + 1/ , y - 1/ , -z 2 2 2 2 2 + 3/2; c-x - 1/2, -y + 1/2, -z + 1. C For 3: ax + 1/2, -y - 1/2, z; b-x + 1, -y, -z + 3; c-x + 1/2, y + 1/2, -z + 3.

Table 6. Selected Bond Angles (°) for Compounds 1, 2, and 3 1A O(2)a-Ga(1)-O(3)a O(2)a-Ga(1)-O(6)a O(3)a-Ga(1)-O(6)a O(2)a-Ga(1)-O(4) O(3)a-Ga(1)-O(4) O(6)a-Ga(1)-O(4) O(2)a-Ga(1)-O(1) O(3)a-Ga(1)-O(1) O(6)a-Ga(1)-O(1) O(4)-Ga(1)-O(1) O(2)a-Ga(1)-O(5)a O(3)a-Ga(1)-O(5)a O(6)a-Ga(1)-O(5)a O(4)-Ga(1)-O(5)a O(1)-Ga(1)-O(5)a O(6)a-P(1)-O(7) O(6)a-P(1)-O(3) O(7)-P(1)-O(3) O(6)a-P(1)-C(1)b O(7)-P(1)-C(1)b O(3)-P(1)-C(1)b O(1)c-P(2)-O(4) O(1)c-P(2)-O(2) O(4)-P(2)-O(2) O(1)c-P(2)-C(1)b O(4)-P(2)-C(1)b O(2)-P(2)-C(1)b C/ N(1)-C(2)-C(3) C(2)-C/ N(1)-C(3)d C(2)-C(3)-C/ N(1)d

2B 91.6(5) 173.6(7) 84.9(5) 85.3(5) 176.7(6) 94.7(7) 92.3(7) 87.7(6) 92.9(6) 91.2(6) 87.7(6) 92.6(6) 87.1(6) 88.4(6) 179.7(4) 104.9(7) 108.8(9) 112.5(9) 106(1) 111.0(8) 113.1(9) 106.8(9) 105.6(6) 108.8(9) 109.6(8) 113(1) 113(1) 118.0(7) 119.7(7) 117.7(7)

O(1)a-Ga(1)-O(2)b O(1)a-Ga(1)-O(4)a O(1)a-Ga(1)-O(5) O(1)a-Ga(1)-O(6) O(1)a-Ga(1)-O(7)c O(2)b-Ga(1)-O(4)a O(2)b-Ga(1)-O(5) O(2)b-Ga(1)-O(6) O(2)b-Ga(1)-O(7)c O(4)a-Ga(1)-O(5) O(4)a-Ga(1)-O(6) O(4)a-Ga(1)-O(7)c O(5)-Ga(1)-O(6) O(5)-Ga(1)-O(7)c O(6)-Ga(1)-O(7)c O(1)-P(1)-O(2) O(1)-P(1)-O(5) O(1)-P(1)-C(1) O(2)-P(1)-O(5) O(2)-P(1)-C(1) O(5)-P(1)-C(1) O(3)-P(2)-O(4) O(3)-P(2)-O(6) O(3)-P(2)-C(1) O(4)-P(2)-O(6) O(4)-P(2)-C(1) O(6)-P(2)-C(1) C(2)-N(1)-C(2)d C(2)-N(1)-C(3) × 2 C(2)-N(1)-C(3)d × 2 C(3)-N(1)-C(3)d

3C 91.7(5) 89.6(7) 87.1(5) 178.5(7) 88.4(7) 96.6(6) 96.0(7) 88.1(6) 175.3(7) 167.1(7) 91.9(5) 78.7(6) 91.5(6) 88.7(6) 92.0(5) 110(1) 109(1) 109(1) 108.0(6) 111.8(8) 109(1) 109.9(9) 117(1) 114.6(8) 95.9(8) 109(1) 109(1) 109.9(6) 109.7(3) 109.0(3) 109.5(6)

O(3)-Ga(1)-O(6) O(3)-Ga(1)-O(4) O(6)-Ga(1)-O(4) O(3)-Ga(1)-O(2) O(6)-Ga(1)-O(2) O(4)-Ga(1)-O(2) O(3)-Ga(1)-O(1) O(6)-Ga(1)-O(1) O(4)-Ga(1)-O(1) O(2)-Ga(1)-O(1) O(3)-Ga(1)-O(5)a O(6)-Ga(1)-O(5)a O(4)-Ga(1)-O(5)a O(2)-Ga(1)-O(5)a O(1)-Ga(1)-O(5)a O(7)-P(1)-O(3) O(7)-P(1)-O(6)a O(3)-P(1)-O(6)a O(7)-P(1)-C(1)b O(3)-P(1)-C(1)b O(6)a-P(1)-C(1)b O(1)b-P(2)-O(2) O(1)b-P(2)-O(4)c O(2)-P(2)-O(4)c O(1)b-P(2)-C(1) O(2)-P(2)-C(1) O(4)c-P(2)-C(1) N(1)-C(2)-C(6) N(1)-C(3)-C(4) C(5)-C(4)-C(3) C(4)-C(5)-C(6) C(2)-C(6)-C(5) C(2)-N(1)-C(3)

85.2(3) 169.4(3) 93.4(3) 94.2(3) 87.6(3) 96.3(3) 92.4(3) 177.6(3) 89.1(3) 92.6(3) 84.1(3) 91.9(3) 85.4(3) 178.3(3) 87.8(3) 109.4(4) 113.0(4) 110.5(4) 110.7(4) 105.4(4) 107.4(4) 112.3(4) 112.1(4) 107.4(4) 108.2(4) 110.8(4) 106.0(4) 120(1) 120(1) 118(1) 120(1) 120(1) 122.3(9)

A For 1: ax, -y + 1/ , z + 1/ ; bx, y - 1, z, c-x, -y, -z + 1; d-x + 1, -y + 1, -z + 1. B For 2: ax, -y + 1, z - 1/ ; b-x + 1/ , y - 1/ , -z 2 2 2 2 2 + 3/2; c-x - 1/2, -y + 1/2, -z + 1; d-x, y, -z + 1/2. C For 3: ax + 1/2, -y - 1/2, z; b-x + 1, -y, -z + 3; c-x + 1/2, y + 1/2, -z + 3.

was mounted on a Nonius Kappa CCD diffractometer with a Nonius FR591 rotating anode generator at the EPSRC national crystallography service, Southampton, U.K., and data were collected from it. The structure was solved by direct methods and refined by full-matrix least squares on F2 using the SHELXS-97 and SHELXL-97 programs, respectively.31,32 The atomic displacement parameters of all of the non-hydrogen

atoms were refined anisotropically. The hydrogen atoms of the water molecule were found from difference Fourier maps and all others were geometrically placed. The hydrogen atoms were refined in riding mode and their isotropic atomic displacement parameters were fixed at a value of 0.05 Å2. The crystallographic data and structure refinement parameters are summarized in footnote 33, atomic coordinates and equivalent

(31) Sheldrick, G. M. SHELXS-97, A Program for Crystal Structure Determination; University of Go¨ttingen: Germany, 1997. (32) Sheldrick, G. M. SHELXL-97, A Program for Crystal Structure Determination; University of Go¨ttingen: Germany, 1997.

(33) GaP2O7NC6H10 (3), MW ) 339.81, crystal dimensions 0.10 × 0.05 × 0.05 mm, monoclinic, space group P21/a, a ) 9.5721(7) Å, b ) 8.6140(6) Å, c ) 12.894(1) Å, β ) 99.961(3)°, V ) 1047.1(1) Å3, Z ) 4, Fcalc ) 2.155 gcm-3, µ ) 2.958 mm-1, 2θmax 54.88°, Mo KR radiation λ

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Figure 3. Ball-and-stick representation of the layer structure of compounds 1, 2, and 3 viewed perpendicular to the plane of the layer. Atom representation: large gray balls (Ga), large white balls (P), small gray balls (O), and small black balls (C). isotropic and anisotropic atomic displacement factors are provided in the Supporting Information, and selected bond distances and angles are presented in Tables 5 and 6.

Results Syntheses. The products formed from the various reactions are summarized in Tables 1, 2, and 3. The results presented in Table 1 show that phase 1 is the dominant phase formed at higher reaction temperatures and over longer reaction periods, with nearly pure phase 3 only occurring after a shorter reaction time at 150 °C. The products formed from the syntheses with various amines added, summarized in Table 2, produce either pure phase 1 or 3, or a mixture of these phases, except for the synthesis containing tetramethylammonium hydroxide from which pure phase 2 was formed. The phases resulting from synthesis containing tripropylamine stopped at various times during the reaction are given in Table 3 and show that phase 3 is the only phase formed after 15.5 h. As the reaction time increases, phase 1 begins to appear and the amount of phase 1 grows at the expense of phase 3 until pure phase 1 is formed after 305 h. Structure and Thermal Properties of (C5H5NH)[Ga2(H2O)2(O3PCH2PO3)(O3PCH2PO3H)] (1). The structure of (C5H5NH)[Ga2(H2O)2(O3PCH2PO3)(O3PCH2PO3H)] is shown in Figures 3 and 4. The main constituents of 1 are the corrugated [Ga(H2O)(O3PCH2PO3H0.5)]0.5- layers that lie in the (100) plane. The layer structure is shown in Figure 3 and is composed of octahedral Ga(1)O(1-6)6 units that are linked together within the layer by the diphosphonate groups. One of ) 0.71073 Å, ω scans, T ) 120 K, measured reflections 13977, independent reflections 2386, reflections included in refinement 2386, SORTAV absorption correction Tmax ) 0.99, Tmin ) 0.886, parameters refined ) 154, R1 [I > 4(I)] ) 0.070, R1 (all data) ) 0.1129, wR2 [I > 4(I)] ) 0.1762, wR2 (all data) ) 0.1897, ∆Fmax/min 1.297 and -1.043 eÅ-3.

the GaO6 apexes is occupied by a coordinated water molecule, O(5), and the remaining oxygen atoms originate from five different phosphonate (-PO3) groups of three separate methylenediphosphonate ligands. The three oxygen atoms associated with P(2), O(1, 2, and 4), form apexes of three separate GaO6 octahedra. The other phosphonate group, -P(1)O(3, 6, 7), of the same diphosphonate moiety is coordinated to two of the same three GaO6 octahedra with the remaining oxygen atom, O(7), protruding from the layer in the [100] direction. The 31P MAS SSNMR spectra of 1 (see Supporting Information) shows two peaks of roughly equal area, centered at 12.719 and 13.208 ppm, indicating the presence of two crystallographically independent phosphorus atoms in agreement with the crystallographic findings. Three-, four-, and eight-membered rings are formed within the layer structure, with the latter consisting of four GaO6 octahedra and four PO3C tetrahedra. Two Ga-O(5)H2 waters project into the pore space of these eight-membered rings. The diphosphonate units are aligned along the [100] axis and alternate in direction so that pairs of consecutive O(7) atoms in one layer protrude in the [100] and the [-100] directions. Half of the O(7) atoms must be protonated for charge neutrality of the overall structure so there is one proton associated with each pair of protruding O(7) atoms from adjacent layers. The interlayer O(7)‚‚‚‚O(7) distance is 2.29(2) Å, which is slightly shorter than typically reported34 and implies strong hydrogen bonding between the layers. The shortness of this hydrogen-bonding distance may also partly be an artifact of having to perform the Reitveld refinement with restraints applied to the bond lengths of the structural model.21 The corrugated layers are aligned in phase with each other, as seen in Figure 4, to form one type of hydrogen-bonded channel parallel (34) Steiner, T. Chem. Commun. 1999, 2299.

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Figure 4. Structure of 1 viewed in ball-and-stick mode along the z axis. Hydrogen-bonding close contact distances are represented as dotted lines. Atom representation: large gray balls (Ga), large white balls (P), small gray balls (O), and small black balls (C/N).

to the c axis. The pyridinium cations reside in the interlamellar region within the hydrogen-bonded framework and are disordered with the N atoms of the pyridinium cation being directed in one of two possible directions. This atom of the pyridinium ring was assigned as that containing the nitrogen atom as it is a distance of 2.84(2) Å from O(3) and 3.07(1) Å from O(6), and so is in the correct position to form hydrogen-bonding interactions with the framework oxygen atoms. Mass losses were observed in three main sections in the TGA of 1 (see Supporting Information). The first mass loss between 20 and 100 °C is attributed to the loss of surface-bound water species. The second mass loss of 7.05% between 100 and 190 °C is attributed to the loss of the coordinated water molecule [O(5)], calculated as 6.66%. The third mass loss is observed between 190 and 430 °C, with the maximum rate of loss at 345 °C. This mass loss is 10.45% and is assigned to the partial decomposition of the pyridinium cations, calculated to be 14.8%. A further mass loss of 2.32% is observed between 570 and 600 °C which is accounted for by the partial decomposition of the organic component of the methylenediphosphonate group and the remainder of the pyridinium species. Structure and Thermal Properties of [(CH3)4N][Ga2(H2O)2(O3PCH2PO3)(O3PCH2PO3H)] (2). The structure of [(CH3)4N][Ga2(H2O)2(O3PCH2PO3)(O3PCH2PO3H)] 2 is shown in Figure 5 and is similar to that of 1. As in 1, the main constituents of 2 are the corrugated [Ga(H2O)(O3PCH2PO3H0.5)]0.5- layers that lie in the (100) plane. These layers have the exact structural connectivity as those previously described for 1 and are connected together through hydrogen-bonds to form a hydrogen-bonded framework structure. However, the

corrugated layers in this material are aligned out of phase with each other, as seen in Figure 5, to form two types of hydrogen-bonded channels parallel to the c axis. Charge-balancing tetramethylammonium cations are found in the interlayer spaces of 2 occupying positions between the interlayer hydrogen bonds similar to those observed for the pyridinium cations in 1. The interlayer distance between the protruding oxygen atoms (O(3)‚‚ ‚‚O(3)) in 2 is 2.27(2) Å which is similar to the distance of 2.29(2) Å found in 1. Again the short hydrogenbonding distance implies that the interaction is relatively strong,34 but may also have a contribution from the refinement procedure of the structural model.21 The tetramethylammonium cations are arranged in two distinct orientations within 2 as seen in Figure 5. The TGA of 2 (see Supporting Information) consists of four major losses. The first, of 1.68% between 30 and 90 °C, is attributed to the mass loss of surface water. The second mass loss of 5.44% between 90 and 150 °C is assigned to the mass loss of the coordinated water H2O(7) calculated at 6.06%. The third mass loss of 11.54% between 270 and 370 °C is due to the mass loss of the tetramethylammonium cations, calculated at 12.47%. The fourth mass loss of 4.65% between 450 and 510 °C is attributed to the release of the organic component as the diphosphonate species decomposes; this is calculated at 4.72%. Structure and Thermal Properties of (C5H5NH)[Ga(H2O)(O3PCH2PO3)] (3). The structure of (C5H5NH)[Ga(H2O)(O3PCH2PO3)] 3 is shown in Figure 6 and consists of corrugated [Ga(H2O)(O3PCH2PO3)]- layers with a structural connectivity identical to those found in 1 and 2. The layers lie in the (001) plane and all the protruding diphosphonate oxygen atoms [O(7)] which terminate the layers are unprotonated, unlike those in

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Figure 5. Structure of 2 viewed in ball-and-stick mode along the z axis. Hydrogen-bonding close contact distances are represented as dotted lines. Atom representation: large gray balls (Ga), large white balls (P), small gray balls (O), and small black balls (C/N).

structures 1 and 2. This makes 3 a layered structure. Between the layers of 3 are the pyridinium cations that are hydrogen-bonded to the protruding diphosphonate oxygen atoms (O(7)) of the layers with a O(7)‚‚‚‚N(1) hydrogen-bonding contact distance of 2.541(8) Å. Each of these oxygen atoms is hydrogen-bonded to a pyridinium cation, as shown in Figure 6, which results in twice as many pyridinium cations occupying the interlayer region of 3 than 1. The larger number of pyridinium cations and the lack of hydrogen-bonding between the layers in 3, as compared to 1, results in the layer separation of 3 (12.894 Å) being greater than that found in 1 (11.085 Å). The bond distances and angles found in structure 3 are consistent with those found in

other gallophosphates and metal phosphonates.23,35,36 The 31P MAS SSNMR spectrum of 3 (see Supporting Information) contains two peaks of roughly equal area, centered at 15.096 and 9.934 ppm, suggesting the presence of two crystallographically independent phosphorus sites, which is consistent with the experimentally determined crystal structure. The TGA trace of 3 is shown in Figure 7 in which five distinct mass losses are revealed. The first, of 0.58%, between 25 and 130 °C is assumed to be surface adsorbed water. Three mass losses are then observed, merged together between 130 and 485 °C. The first of (35) Weigel, S. J.; Weston, S. C.; Cheetham, A. K.; Stucky, G. D. Chem. Mater. 1997, 9, 1293. (36) Loiseau, T.; Paulet, C.; Simon, N.; Munch, V.; Taulelle, F.; Ferey, G. Chem. Mater. 2000, 12, 1393.

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Figure 6. Structure of 3 viewed along the x axis with atomic displacement ellipsoids shown at 50% probability. Hydrogenbonding close contact distances are represented as dotted lines. Hydrogen atoms are omitted for clarity.

Figure 8. Thermodiffraction patterns for 3. Figure 7. TGA trace (black) and first derivative (gray) of the mass loss of 3.

these mass losses between 130 and 195 °C of 6.45% is attributed to the loss of the coordinated water, O(5), which is calculated to be 5.81%. The second and third mass losses merge together and are observed between 195 and 475 °C. These mass losses are attributed to the mass loss of the two pyridine molecules. The observed mass loss in this temperature region is 21.3%, which corresponds with the calculated value of 25.8%. The first derivative analysis of the TGA trace clearly indicates that the loss of the pyridine molecules appears in two stages. The final mass loss of 1.9% occurs in the

temperature range 475 to 690 °C and is assigned to the decomposition of the organic component of the methylenediphosphonate groups, calculated as 4.52%. The magnitude of this mass loss is less than that expected for the full decomposition of the methylenediphosphonate organic groups indicating that the organic components do not fully decompose or are not fully oxidized. X-ray Thermodiffraction Analysis of 3. The X-ray thermodiffraction patterns of 3 are shown in Figure 8 and show little change between 20 and 150 °C, except for a slight broadening of certain peaks at higher angles at the upper end of this temperature range. As the temperature increases from 170 to 290 °C a structural

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Figure 9. Normalized powder X-ray diffraction patterns of (from top to bottom) phase 3, phase 1, Cs-exchanged 3, Rbexchanged 3, K-exchanged 3, Na-exchanged 3, Li-exchanged 3, and 3 refluxed in pure H2O. Table 7. Resultant Elemental Ratios of Ion-Exchanged Samples of 3 elemental ratio Ga/P/Li Ga/P/Na Ga/P/K Ga/P/Rb Ga/P/Cs

observed ratio 1:2:0.196 1:2:0.400 1:2:0.568 1:2:0.427 1:2:0.446

percentage exchange (%)a 39.2 80.0 113.6b 85.4 89.3

a Exchange of pyridinium cations for alkali metal cations calculated assuming formula of compound 1. b Over-exchange of K+ cations results from exchange of some K+ cations for protons in the type of hydrogen-bonded structure of 1.

rearrangement is observed, and the peaks corresponding to phase 3 diminish in intensity while those of a new crystalline phase appear concurrently. This new phase is unknown and has a low-angle, high-intensity peak at 8.19° 2θ which is in a position similar to that of the (100) peak of phase 1. At 470 °C the new phase becomes X-ray amorphous. Ion-Exchange Properties of 3. The X-ray powder patterns of the samples of 3 treated under reflux in pure H2O and alkali metal chloride solutions are shown in Figure 9 and the compositions of the resultant materials are given in Table 7. The powder patterns of all the treated materials indicate that the materials are all still crystalline, although the crystallinity is significantly lower than that of the starting material 3. There is also a decrease in d-spacing for the first major peak, and hence interlayer separation, for all the treated materials compared with the starting material 3. The relative amount of alkali metal cations found in the resulting treated products increases from Li+ to K+ where a maximum amount of substitution is observed. The amount of exchange then decreases for rubidium and cesium, but it is noticeable that the larger cations are incorporated at a higher level than the smaller cations. The re-ion-exchange of the K-containing material for Rb+ cations was successful, yielding a material with a Ga/P/Rb/K ratio of 1:2:0.39:0.06, indicating that a high degree of Rb+ cation for K+ cation exchange occurred. Discussion The results of the different syntheses presented in Tables 1 and 3 indicate that phase 1 is the more thermodynamically stable phase compared to 3, as it is the only phase formed at higher temperatures and over

extended reaction times. Phase 3 appears to be an intermediate phase formed first in the preparation of 1 and is stable enough to be isolated. This is clearly indicated in the results presented in Table 3 in which phase 3 is initially present after 15.5 h as a pure phase, and subsequently phase 1 is seen to grow at the expense of phase 3 until after 305 h only phase 1 is present. Hence, the reaction appears to proceed by formation of phase 3 within the first 15.5 h which then converts to phase 1 over longer reaction periods. The conversion of phase 3 to 1 is easily envisaged when the similarities between the two structures are considered. Both consist of the same [Ga(H2O)(O3PCH2PO3)]- layer motif and interlamellar pyridinium cations. The corrugated [Ga(H2O)(O3PCH2PO3)]- layers of 3 contain unprotonated protruding diphosphonate oxygen atoms [O(7)] that are hydrogen-bonded to charge-compensating pyridinium cations, as seen in Figure 6. Conversion from phase 3 to phase 1 in structural terms simply requires the loss of half of the pyridinium cations in phase 3, as pyridine molecules, which allows the [Ga(H2O)(O3PCH2PO3)]layers to move closer together and become hydrogenbonded to each other. The protons in the resulting hydrogen bonds arise from the loss of the pyridine molecules from the structure. The formation of phase 1 is likely to be favored in enthalpic terms due to the formation of hydrogen-bonding interactions between the layers and in entropic terms as pyridine molecules are re-released into solution. The growth of phase 1 at the expense of phase 3 implies a direct solid-solid transformation may be occurring in this system. However, other mechanisms cannot be fully excluded, and in-situ diffraction and spectroscopic studies are required to fully distinguish between mechanistic possibilities for this transformation.37 These results do make it apparent that reaction duration can be used to control the product formed in this case between products 1 and 3. The results of the syntheses presented in Table 2 involving the addition of various organoamines to the synthesis gel result in phase 1 or 3 or a mixture of these two phases being formed, except for the reaction involving tetramethylammonium hydroxide. It is difficult to deduce any precise trend for the reactions that yield phase 1 or 3 but the presence of the organoamines, and maybe the slight change in pH they induce, does appear to increase the rate at which phase 3 is converted to phase 1 in the order triethylamine < tetraethylammonium hydroxide ∼ tripropylamine < no amine ∼ tetrapropylammmonium hydroxide. The formation of the different phase 2 in the presence of the smallest organoamine, tetramethylammonium hydroxide, is of note as this is the only organoamine of those tried that is able to incorporate itself into a crystalline structure at the expense of the more numerous pyridine solvent molecules. This is presumably due to all the other organoamines being too large to be incorporated into this type of hydrogen-bonded framework structure and not presenting alternative structures with lower free energies than those of phase 1 or 3. The effect the tetramethylammonium cations have on the arrangement of the stacking of the [Ga(H2O)(O3PCH2PO3)]- layers is also of interest. In compounds 1 and 3 the corrugated layers are stacked in phase (37) Francis, R. J.; O’Brien, S.; Fogg, A. M.; Halasyamani, P. S.; O’Hare, D.; Loiseau, T.; Ferey, G. J. Am. Chem. Soc. 1999, 121, 1002.

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relative to each other, as seen in Figures 4 and 6, which results in only one type of hydrogen-bonded channel being formed parallel to the z axis in 1. However, the inclusion of the tetramethylammonium cations in the structure of 2 results in the stacking of the corrugated layers out-of-phase with each other, as seen in Figure 5, and so two types of hydrogen-bonded channels are formed parallel to the z axis of 2. The structure of 2 with the associated stacking of the [Ga(H2O)(O3PCH2PO3)]- layers in an out-of-phase manner must be significantly more thermodynamically stable than phase 1 as it is formed readily as a pure phase even though formation of phase 1 should be competing under these reaction conditions. The tetramethylammonium cations are thus acting in a structure-directing role to form the structure of 2, with its associated out-of-phase layer arrangement, over the pyridinium-containing product 1 with its associated in-phase layer arrangement. Hence, inclusion of tetramethylammonium hydroxide to the synthesis exerts a structure-directing effect on the layer stacking in these materials and diverts the product formed from phase 1 and 3. The close relationship of the structure of the intermediate compound 3 to compound 1, and the theory that it converts during the reaction to phase 1, imply that it is likely to undergo conversion, other than under the solvothermal synthesis reaction conditions described, to form other hydrogen-bonded framework phases related to 1. This is clearly seen in both the thermodiffraction/ TGA and the ion-exchange results. The TGA trace, shown in Figure 7, shows a sharp mass loss between 130 and 195 °C, which is attributed to the loss of the coordinated O(5) water molecules. However, the thermodiffraction data shown in Figure 8 show little change in this temperature region, which indicates the gallium centers can adopt a 5-coordinate environment without a major structural rearrangement occurring. The TGA trace indicates a two-stage loss of the pyridine molecules from 3 between 195 and 475 °C with half of the pyridine species being removed from the structure at approximately 260 °C. The thermodiffraction patterns corroborate the TGA results in that the intensities corresponding to phase 3 begin to diminish at 170 °C and are gone by 290 °C. Over this temperature range the diffraction peaks of a new phase evolve that has a low-angle, high-intensity peak at 8.16° 2θ which is in a position identical to that of the (100) peak of phase 1. In fact, a small peak at 8.16° 2θ is observed prior to heating which is attributed to a trace impurity of 1 within the sample. Thus, it would seem that simple thermal treatment of 3 to 290 °C results in the loss of at least half the pyridine molecules and the structurally intact corrugated layers collapsing together to form the type of hydrogen-bonded framework found in phase 1. Poorer ordering of the remaining pyridinium cations and the presence of 5-coordinated gallium centers in the resulting phase mean that that this phase is not identical to as-synthesized phase 1. These results show that phase 3 is readily converted to a structure type similar to that of 1 by heating in air. The refluxing of phase 3 in water or alkali metal chloride solutions at 80 °C results in a structural transition similar to that induced by thermal treatment in air. The powder diffraction patterns of the refluxed samples all show the loss of the (001) peak at 7.0° 2θ and the formation of new phases with the first low-

angle, high-intensity peak in the d-spacing range 10.5 to 11.2 Å. These values compare closely to those observed for the compounds 1 and 2 of 10.71 and 10.99 Å, respectively. This again suggests that treatment of phase 3 in water or alkali metal chloride solutions at 80 °C results in loss of half of the interlayer pyridinium cations with the layers collapsing around the remaining pyridinium cations or metal cations, from the solution, to form the hydrogen-bonded framework type of structure found in phase 1. The incorporation of the metal cations from solution during the collapse of the structure indicates a high mobility of the interlayer pyridinium cations within phase 3. The ability of the alkali metal cations to incorporate themselves into the resultant material follows the typical trend of cationic mobility in aqueous solution.38 The exchange of the K+ cations for Rb+ cations in the K-exchanged material indicate that metal cations within the hydrogen-bonded type framework are also relatively mobile allowing a high degree of reexchange of the interlayer cations. This implies that phase 3 and its derivatives can be used as ion-exchange materials. Conclusions We have described the synthesis, structural characterization, and properties of three novel gallium methylenediphosphonate materials, (C5H5NH)[Ga2(H2O)2(O3PCH2PO3)(O3PCH2PO3H)] 1, [(CH3)4N][Ga2(H2O)2(O3PCH2PO3)(O3PCH2PO3H)] 2, and (C5H5NH)[Ga(H2O)(O3PCH2PO3)] 3, whose structures all contain corrugated [Ga(H2O)(O3PCH2PO3)]- layers. The use of organocations to structurally direct the stacking of these layers is shown to be possible for 1 and 2 to produce structures with differing numbers of hydrogen-bonded channels and is a methodology that should lead to the synthesis of many novel open-framework materials for a variety of systems, as has been shown for the aluminosilicate family of materials. Control of the reaction temperature and duration of reaction has allowed the intermediate phase 3 in the formation of 1 to be isolated, and through further treatment under different conditions has enabled the transformation of this material to 1 and structurally related materials. The isolation and transformation of material 3 provides further insight into the general understanding of the mode of formation and transformation of two- and three-dimensional extended structures. Acknowledgment. We thank Dr. A. Aliev (ULIRS), Dr. M. Odlyha (ULIRS), and Dr. A. Beard for collection of the SS MAS NMR, TGA, and microprobe data, respectively, and the EPSRC National Crystallography Service, University of Southampton, U.K., for collecting single crystal X-ray data for compound 3. M.P.A. thanks the Royal Society for provision of a University Research Fellowship, H.G.H. thanks EPSRC for provision of a quota award and for funding. Supporting Information Available: The 31P MAS SSNMR spectrum and TGA/first derivative trace of compound 1, the TGA/first derivative trace of compound 2, the 31P MAS SS NMR spectrum of compound 3 (PDF), and CIF files for compounds 1, 2, and 3. This material is available free of charge via the Internet at http://pubs.acs.org. CM034745Q (38) Atkins, P. W. Physical Chemistry; Oxford University Press: Oxford, 1978.