Understanding an Order–Disorder Phase Transition in Ionothermally

ARTICLE pubs.acs.org/crystal

Understanding an Order
Disorder Phase Transition in Ionothermally Synthesized Gallium Phosphates Jacob H. Olshansky, Samuel M. Blau, Matthias Zeller,† Joshua Schrier, and Alexander J. Norquist* Department of Chemistry, Haverford College, Haverford Pennsylvania 19041, United States † Department of Chemistry, Youngstown State University, Youngstown Ohio 44555, United States

bS Supporting Information ABSTRACT: Two new organically templated gallium phosphates were synthesized under ionothermal conditions. Single crystals were grown from a mixture of Ga(NO3)3 3 H2O, H3PO4, and either 1-methylpiperazine or 2-methylpiperazine, which were allowed to react in 1-ethyl-3-methyl-imidazolium bromide or 1-butyl-3-methyl-imidazolium bromide at 150 °C for 4 days. The use of 1-methylpiperazine and 2-methylpiperazine resulted in compounds that both contain [Ga(HPO4)2/2(PO4)2/2]n2n
chains and extensive hydrogen-bonding networks. Solvent effects associated with ionothermal versus hydrothermal conditions in this system result in marked differences in structure and stoichiometry. [1-MethylpiperazineH2][Ga(HPO4)(PO4)] exhibits a phase transition from an ordered monoclinic structure at 100 K to a disordered, averaged structure at 298 K, as determined using both single crystal X-ray diffraction and density-functional calculations of the total energy.

’ INTRODUCTION Although the synthesis of templated metal oxides1
5 has been dominated by the use of hydrothermal or solvothermal techniques for the past 20 years, recent advances in ionothermal syntheses6,7 have resulted in the formation of a host of new materials. The use of low melting ionic liquids can be beneficial in several ways, including essentially eliminating the autogenous pressures generated in hydrothermal reactions, providing a means to tailor solvent polarity and ionic conductivity, and increasing solvent preorganization while retaining good solvating properties. In addition, chiral induction in microporous materials can be achieved through the use of chiral ionic liquids.8
10 Notable systems in which ionic liquids have been employed include aluminophosphates,11
17 zinc phosphates,18,19 gallium phosphates,11,20 borates,21,22 oxyfluorides,23 and metal oxalates.24
26 A host of organically templated gallium phosphates and fluorophosphates have been reported in the past 15 years.5,27
34 This mature system contains an exceptionally diverse range of inorganic architectures and has yielded valuable information about formation mechanisms in such compounds.5,35,36 This report contains an analysis of two main reaction parameters in the synthesis of templated gallium phosphates. First, solvent effects associated with ionothermal versus hydrothermal conditions are probed by substituting ionic liquids for solvent water in a reaction gel we recently reported.34 Second, the structure directing role of two related amines is investigated through analysis of the resulting reaction products. The ionothermal synthesis, structure, and characterization of two new templated gallium phosphates is reported here. r 2011 American Chemical Society

[C5H14N2][Ga(HPO4)(PO4)] (1) contains 2-methylpiperazinium dications, whereas [C5H14N2][Ga(HPO4)(PO4)] (2) contains 1-methylpiperazinium dications. Single crystal X-ray diffraction data were collected on both compounds at 100 and 298 K, owing to the presence of a phase transition in compound 2 from a monoclinic ordered structure at 100 K to a disordered orthorhombic structure at 298 K. The absence of such a phase transition in compound 1 is discussed in the context of hydrogenbonding interactions.

’ EXPERIMENTAL SECTION Ga(NO3)3 3 H2O (99.9%), H3PO4 (85%), 2-methylpiperazine (95%, 2-mpip), 1-methylpiperazine (99%, 1-mpip), and HFaq (48%) were purchased from Aldrich, and 1-ethyl-3-methyl-imidazolium bromide (97%, EMIm Br) and 1-butyl-3-methyl-imidazolium bromide (97%, BMIm Br) were purchased from Fluka. All reagents were used as received. Synthesis. All reactions were conducted in 23 mL poly(fluoroethylene-propylene) lined pressure vessels. Reactions were heated to 150 °C over 30 min and allowed to soak for 4 days. The reaction mixtures were then cooled to room temperature at a rate of 6 °C h
1 to promote the growth of large single crystals. Autoclaves were opened in air, and products were recovered through filtration. Reaction yields ranged between 55
60%, based upon Ga. Recrystallized EMIm Br and BMIm Br was removed by washing with cold water. Received: March 16, 2011 Revised: May 2, 2011 Published: May 04, 2011 3065

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Table 1. Crystallographic Data for [2-mpipH2][Ga(HPO4)(PO4)] (1a and 1b) and [1-mpipH2][Ga(HPO4)(PO4)] (2a and 2b)

a

[2-mpipH2][Ga(HPO4)

[2-mpipH2][Ga(HPO4)

[1-mpipH2][Ga(HPO4)

[1-mpipH2][Ga(HPO4)

(PO4)] (1a)

(PO4)] (1b)

(PO4)] (2a)

(PO4)] (2b)

formula

C5H15GaN2O8P2

C5H15GaN2O8P2

C5H15GaN2O8P2

C5H15GaN2O8P2

fw

362.85

362.85

362.85

362.85

space group

P-1 (No. 2)

P-1 (No. 2)

Pnnm (No. 58)

P21/n (No. 14)

a, Å

8.477(5)

8.451(2)

15.1432(17)

8.899(4)

b, Å

8.795(5)

8.777(2)

8.9454(10)

15.087(7)

c, Å

8.929(5)

8.928(2)

8.9730(10)

8.923(4)

R, deg β, deg

92.248(5) 95.825(5)

92.195(3) 95.709(3)

90 90

90 90.114(7)

γ, deg

98.076(5)

98.509(3)

90

90

V, Å3

654.7(6)

650.7(3)

1215.5(2)

1198.0(10)

Z

2

2

4

4

Fcalc, g cm
3

1.838

1.852

1.983

2.012

λ, Å

0.71073

0.71073

0.71073

0.71073

T, K

298(2)

100(2)

298(2)

100(2)

μ, mm
1 R1a

2.393 0.0252

2.393 0.0262

2.562 0.0474

2.600 0.0526

wR2b

0.0666

0.0722

0.0933

0.1176

R1 = Σ||Fo|
Fc||/Σ|Fo|. b wR2 = [Σw(Fo2
Fc2)2/[Σw(Fo2)2]1/2.

[2-mpipH2][Ga(HPO4)(PO4)] (1). Synthesized through the reaction of 0.0793 g (2.90  10
4 mol) of Ga(NO3)3 3 H2O, 0.2646 g (2.65  10
3 mol) of 2-mpip, and 0.3018 g (3.08  10
3 mol) of H3PO4 in 1.9993 g (9.13  10
3 mol) of 1-butyl-3-methylimidazolium bromide. Colorless blocks. Elemental microanalysis for 1 obsd (calc): C 16.24(16.54); H 4.28(4.14); N 7.54(7.72). IR data: N
H 1464, 1479, and 1628 cm
1, C
H 3014 cm
1, P
O 1052, 1120 cm
1. [1-mpipH2][Ga(HPO4)(PO4)] (2). Synthesized through the reaction of 0.0749 g (2.74  10
4 mol) of Ga(NO3)3 3 H2O, 0.2618 g (2.2  10
3 mol) of 1-mpip, and 0.3072 g (3.14  10
3 mol) of H3PO4 in 2.0033 g (9.15  10
3 mol) of 1-butyl-3-methylimidazolium bromide. Colorless blocks. Elemental microanalysis for 2 obsd (calc): C 16.34(16.54); H 4.14(4.14); N 7.47(7.72). IR data: N
H 1454, 1470, and 1617 cm
1, C
H 2994 cm
1, P
O 1039, 1112 cm
1. Single Crystal X-ray Diffraction. Data were collected using a Bruker AXS Smart Apex CCD diffractometer with Mo KR radiation (λ = 0.71073 Å). Single crystals were mounted on a Mitegen micromesh mount using a trace of mineral oil. Data were collected at both 100(2) and 298(2) K for each compound. Frames were collected, indexed, and processed, and the files were scaled and corrected for absorption using APEX2,37 Cell Now,38 and Twinabs.39 The heavy atom positions were determined using SIR92.40 All other non-hydrogen sites were located from Fourier difference maps. All non-hydrogen sites were refined using anisotropic thermal parameters using full matrix least-squares procedures on Fo2 with I > 3σ(I). Hydrogen atoms were placed in geometrically idealized positions. All calculations were performed using Crystals.41 Relevant crystallographic data are listed in Table 1. Powder X-ray Diffraction. Powder diffraction patterns were recorded on a GBC-Difftech MMA powder diffractometer. Samples were mounted on aluminum plates. Calculated powder patterns were generated from single crystal data using ATOMS v. 6.0.42 Infrared Spectroscopy. Infrared measurements were obtained using a Perkin-Elmer FT-IR Spectrum 1000 spectrophotometer. Samples were diluted with spectroscopic grade KBr and pressed into pellets. Scans were run over the range of 400
4000 cm
1. Thermogravimetric Analysis. Thermogravimetric analyses (TGA) were conducted using a Q500 thermogravimetric analyzer from TA Instruments. Samples were contained within a platinum crucible and

Table 2. Relative Total Energies of 2a and 2b relative total energies (eV) compound

V100 (1198 Å3)

V298 (1215.5 Å3)

2a 2b

9.753 0

9.297
1.996

heated in nitrogen at 10 °C min
1 to 950 °C. TGA traces are available in the Supporting Information. Total Energy Calculation. Solid-state electronic structure calculations were performed using ABINIT 6.0.3,43 using the Perdew
Burke
Ernzerhof generalized gradient approximation (PBE-GGA) exchange-correlation functional, norm-conserving Trollier
Martins pseudopotentials, and a planewave basis set with energy cutoff of 35 hartree and utilizing the experimental crystal structures. The Brillouin zone was sampled by an 8  8  8 Monkhorst
Pack grid. Relative total energies are reported in Table 2.

’ RESULTS [2-mpipH2][Ga(HPO4)(PO4)] (1) and [1-mpipH2][Ga(HPO4)(PO4)] (2) were synthesized using BMIm Br or EMIm Br as solvent, and both in the presence and absence of HFaq. No differences in the composition, crystal quality, or yield were observed between ionic liquids or with the addition of HFaq. Under no reaction conditions explored was fluoride incorporated into the products. Single crystal X-ray diffraction data were collected on compounds 1 and 2 at both 100 and 298 K. The high (1a) and low (1b) temperature structures of compound 1 are isostructural, with the only difference observed being a ∼1% reduction in unit cell parameters upon cooling. In contrast, the high temperature (2a) structure of compound 2 crystallizes in the orthorhombic space group Pnnm (No. 58), whereas the low temperature (2b) structure crystallizes in the monoclinic space group P21/n (No. 14). These two distinct structures were observed in the same single 3066

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Figure 1. [Ga(HPO4)2/2(PO4)2/2]n2n
chains. Purple and green polyhedra represent [PO4] and [GaO4], respectively. Red and gray spheres represent oxygen and hydrogen, respectively.

crystal on the same mounting. The following relationships between the orthorhombic and monoclinic structures are observed; ao = bm, bo = am, co = cm, where the subscripts o and m refer to orthorhombic and monoclinic, respectively. Crystallographic data are provided in Table 1. [2-mpipH2][Ga(HPO4)(PO4)] (1) and [1-mpipH2][Ga(HPO4)(PO4)] (2) both contain tetrahedrally coordinated Ga3þ centers and phosphate tetrahedra. The Ga
O bonds range between 1.817(6) and 1.858(7) Å, and the P
Obridging bonds range between 1.538(5) and 1.5650(13) Å. The P
O(H) bonds are longer (1.5342(17) to 1.549(5) Å) than the P
Oterminal bonds (1.485(5) to 1.521(5) Å). Bond valence sums44,45 were calculated for structures 1a, 1b, and 2b. No bond valence sums were calculated for 2a because the observed disorder obscures true bond lengths. The values on the Ga3þ and P5þ centers range between 3.05 and 3.11, and 4.74 and 4.95, respectively, in good agreement with the assigned oxidation states. Full tables of bond valence sums are available in the Supporting Information. The [GaO4], [HPO4]2
, and [PO4]3
tetrahedra link to create [Ga(HPO4)2/2(PO4)2/2]n2n
chains. See Figure 1. The [HPO4]2
and [PO4]3
tetrahedra bridge between neighboring [GaO4] centers. This chain type has been observed in several compounds containing a range of metal centers,4,11,46
56 most notably as a secondary building unit in the formation of various aluminum phosphate structures.57 In addition, Morris et al. have recently reported the synthesis of this chain connectivity using ionothermal methods.11 The [Ga(HPO4)2/2(PO4)2/2]n2n
chains lie parallel to one another in both compounds 1 and 2. Three-dimensional packing figures of compound 1 and both the orthorhombic (2a) and monoclinic (2b) structures of compound 2 are shown in Figure 2. A single packing figure is provided for compound 1 because the high (1a) and low (1b) temperature structures are isostructural. The organic amine cations reside between [Ga(HPO4)2/2(PO4)2/2]n2n
chains in each compound. The [2-mpipH2]2þ cations in compound 1 are disordered over a central inversion center, resulting in 50% occupancy of the methyl group. Two distinct hydrogen-bonding networks are observed in compounds 1 and 2. (P)O
H 3 3 3 O(P) interactions are observed between neighboring [Ga(HPO4)2/2(PO4)2/2]n2n
chains. The two distinct hydrogens that participate in these interactions in compound 1, H1 and H2, both reside on inversion centers at (0, 0, 0) and (12,12,12), with Wyckoff symbols of 1a and 1h, respectively. This constrains the hydrogen atoms to be equidistant between the two oxides (two O5 or two O7 sites), as shown in Figure 2. The use of two half occupied hydrogen atoms, placed on either side of the inversion centers, was found to result in a less suitable refinement. The analogous hydrogen atoms in compound 2 are each localized on a specific oxygen site and reside on a mirror plane (4g) and general position (4e), respectively,

Figure 2. Three-dimensional packing in (a) 1, (b) 2a, and (c) 2b. Hydrogen atoms on the organic amines have been removed for clarity. Purple and green polyhedra represent [PO4] and [GaO4], respectively. Red, white, blue, and gray spheres represent oxygen, carbon, nitrogen, and hydrogen atoms, respectively.

for 2a and 2b. The second hydrogen-bonding network in each compound involves both the protonated organic amines and [Ga(HPO4)2/2(PO4)2/2]n2n
chain oxides. Oxide ligands that exhibit higher nucleophilicities, as determined using bond valence sums, have a greater propensity to accept hydrogen bonds from either the organic amines or [HPO4]2
tetrahedra.58
62 Figures of the interchain [Ga(HPO4)2/2(PO4)2/2]n2n
hydrogen-bonding structures are available in the Supporting Information. Thermogravimetric analyses of compounds 1 and 2 revealed amine decompositions between 100 and 560 °C and between 175 and 500 °C, respectively. These observed mass losses agree with calculated values 27.7% obsd (27.9% calc) for 1 and 26.4% obsd (27.8% calc) for 2.

’ DISCUSSION [2-mpipH2][Ga(HPO4)(PO4)] (1) and [1-mpipH2][Ga(HPO4)(PO4)] (2) were synthesized from reactions gels with compositions of 1 Ga3þ:10 amine:10 H3PO4:x HFaq (x = 0 to 4) in either BMIm Br or EMIm Br. No dependencies on ionic liquid structure or fluoride concentration are observed. However, analogous hydrothermal reactions resulted in marked differences in product structure,34 with [2-mpipH2]2[Ga3F(PO4)4] 3 nH2O containing isotypic [Ga3F(PO4)4]n4n
layers in which a central F
is shared between three neighboring gallium centers. In addition, each [PO4] tetrahedron has one hanging P
O bond 3067

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Crystal Growth & Design and bridges three adjacent gallium sites. Morris et al.12
15 have studied the effects of water and fluoride addition on ionothermal syntheses of a range of aluminophosphates and recently proposed that increased mineralizer (H2O or HF) concentrations favor the formation of structures with fewer hanging P
O bonds. The differences in structure between [2-mpipH2][Ga(HPO4)(PO4)] (1) and [2-mpipH2]2[Ga3F(PO4)4] 3 nH2O, which stem from a transition between ionothermal and hydrothermal conditions, reflect the trends observed by Morris et al.12
15 and Tian et al.,63 with a decrease from two hanging P
O bonds per [PO4] to just one after the addition of water. It should be noted that the small amount of water present in Ga(NO3)3 3 H2O likely results in some autogenous pressure64 and precludes the use of open reaction vessels. Strong similarities exist between the structures of compounds 1 and 2. First, the presence of isotypic [Ga(HPO4)2/2(PO4)2/2]n2n
chains is likely the result of charge density matching between the gallium phosphate building units and the two closely related amines with similar pKas and charge densities.5,35 Second, the use of nearly identical reaction gel compositions resulted in identical product stoichiometries. Correlations between reaction gel compositions and product formulas are well-known in related systems.62,65
70 Third, analogous three-dimensional hydrogenbonding networks are created through N
H 3 3 3 O and O
H 3 3 3 O interactions. Despite these similarities, marked differences between compounds 1 and 2 are observed. The position of the methyl group on the [1-mpipH2]2þ and [2-mpipH2]2þ cations in compounds 1 and 2 is the source of variations in the three-dimensional packing, hydrogen bonding, and the absence or presence of phase transitions in compounds 1 and 2, respectively. Crystallographic disorder of the [R-2mpipH2]2þ and [S-2-mpipH2]2þ cations in compound 1 results in the imposition of inversion symmetry within these cations. The inclusion of both [R-2-mpipH2]2þ and [S-2-mpipH2]2þ cations on a single cation site is reasonable because a racemic source of the enantiomers was used and the position of the methyl group does not affect hydrogen bonding between the cations and [Ga(HPO4)2/2(PO4)2/2]n2n
chains. Finally, the orientations of the [2-mpipH2]2þ and [1-mpipH2]2þ cations in compound 1 and 2 differ. In both compounds, the orientations of the methyl groups are perpendicular to the direction of [Ga(HPO4)2/2(PO4)2/2]n2n
chain propogation; see Figure 2. This results in a twisting of the [2-mpipH2]2þ cations, with respect to the [Ga(HPO4)2/2(PO4)2/2]n2n
chains in 2, that is not observed in 1. The effects of this twisting are responsible for the presence of a phase transition in 2 and the absence of one in 1. To fully understand the phase transition in compound 2, an analysis of the differences between 2a and 2b is first required. The [Ga(HPO4)2/2(PO4)2/2]n2n
chains in 2a are significantly higher in symmetry than those in 2b. See Figure 3. The Ga3þ and P5þ centers in 2a lie on 2-fold axes and mirror planes, respectively, whereas the same cations all lie on general positions in 2b. The [PO4] tetrahedra in 2b are significantly rotated about the P2
O6 bond axis with respect to the same polyhedra in 2a. In addition, the [HPO4] tetrahedra are twisted in 2b. The twisting of these tetrahedra results in marked distortions of the Ga3þ sites, in an up
down
up motif. Also, inspection of the [Ga(HPO4)2/2 (PO4)2/2]n2n
chains in 1, 2a, and 2b along the direction of propagation reveals interesting differences. See Figure 4. O
Ga
Ga
O torsion angles of exactly 0° are observed in 2a, where non-zero torsion angles are found in both 1 and 2b.

ARTICLE

Figure 3. Ball-and-stick representations of the [Ga(HPO4)2/2(PO4)2/2]n2n
chains in (a) 2a and (b) 2b. The horizontal lines lie at (x, y, 0) in (a) 2a and at (x, 1/2, z) in (b) 2b, and the vertical black lines lie at (x, y, 0) and (x, y, 1/2) in both (a) 2a and (b) 2b. Black arrows in (b) indicate the direction of atomic distortions in 2b.

It initially appears that compound 2 exists in both high and low symmetry phases. However, a better description is that the room temperature structure (2a) is massively disordered, and the model represented in Figure 2b and Figure 3a is an average of the 2b structure and its mirror image in the ab plane. This assignment is based upon an analysis of the thermal ellipsoids in 2a, and torsion angles and total energy calculations of both 2a and 2b. The Ga3þ thermal ellipsoids in 2a are clearly elongated along the direction of the distortions discussed above; see Figure 5. Rotation of the [PO4] tetrahedra about the P2
O4 bond results in oxygen (O5 and O6  2) ellipsoids that are elongated about the rotation while the O4 ellipsoids remain far more spherical. The twisting of the [HPO4] tetrahedra is manifested in the O1 and O3 ellipsoids. Superimposing 2b structure and its mirror results in the thermal ellipsoid elongations present in 2a. As noted above, the O
Ga
Ga
O torsion angles in the 2a [Ga(HPO4)2/2(PO4)2/2]n2n
chains are exactly 0°. Of 19 reported compounds that contain chains with identical connectivities, 2a is the only structure in which these O
M
M
O torsion angles are exactly 0°,4,11,46
57 suggesting that the observed structure is an average. Total energy calculations were performed on 2a and 2b. Values of total energies relative to 2b, using the experimental unit cell volumes at 100 and 298 K, are provided in Table 2. Two outcomes are observed. First, the total energy of 2a is so much higher than 2b at both 100 and 298 K that its Boltzmann probability is essentially zero. This supports the hypothesis that 3068

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Figure 4. Ball-and-stick representations of the [Ga(HPO4)2/2(PO4)2/2]n2n
chains in (a) 1, (b) 2a, and (c) 2b, viewed along the direction of chain propagation. O
Ga
Ga
O torsion angles are shown (deg). Purple, green, red, and gray spheres represent phosphorus, gallium, oxygen, and hydrogen, respectively.

Figure 5. Thermal ellipsoid plots (50% probability) of the [Ga(HPO4)2/2(PO4)2/2]n2n
chains in 2a. Black arrows indicate the directions of ellipsoid elongation.

the crystallographic model in 2a does not represent the microscopic structure and is instead an average of lower energy geometries. Second, thermal expansion/contraction of the unit cells does not qualitatively change this large energy difference. This rules out the possibility that structure 2a is stabilized by the different unit cell dimensions and instead supports the hypothesis that it is a thermally weighted average structure. The increases in total energy of both 2a and 2b between V100 and V298 result from the well-known underestimation of unit cell parameters by GGA.71 From this evidence, we conclude that only local order is present at 298 K with domain sizes that are too small to be observed using single crystal X-ray diffraction. As a result, an average structure (2a) is observed. As the temperature decreases, interactions between domains become more important and their sizes increase. At 100 K, these domains are now sufficiently large that no additional symmetry is imposed on the model and an ordered structure (2b) is observed. However, the structure of 2b exists as a pseudomerohedral twin, with a twinning ratio close to 1:1. If one could cool the crystal to very low temperatures, one might expect a single (or very few) domains to encompass the entire crystal, eliminating the twinning. As such, the structures found at 100 and 298 K represent two points in an order-to-disorder continuum. As stated above, one single crystal on the same mounting was used in the X-ray diffraction experiments from which structures 2a and 2b were derived. This implies that 2b represents one of two opposite orientations that interconvert at finite temperature

Figure 6. Ball-and-stick representation of the three-dimensional packing of 2b. Hydrogen-bonding interactions are shown as dashed lines. Selected C 3 3 3 O distances are shown (Å). Black arrows indicate the direction of phosphate rotation.

and that this interconversion is necessary for both 2a and 2b to be observed in the same single crystal. Inspection of the [1-mpipH2]2þ cation environments in 2b allows for the identification of the interactions that are most important during an interconversion. The [1-mpipH2]2þ cations lie roughly perpendicular to the direction of [Ga(HPO4)2/2(PO4)2/2]n2n
chain propagation in 2b, as shown in Figure 6. These cations interact with the gallium phosphate chains through a series of hydrogen bonds. Although stronger N
H 3 3 3 O hydrogen bonds are formed between the cations and anionic chains, they are oriented approximately perpendicular to the directions of chain propagation and are unlikely to strongly influence the orientations of [PO4] and [HPO4] tetrahedra. Instead, the C
H 3 3 3 O hydrogen bonds play a more pivotal role in the observed phase transition because such interactions are formed with O2 and O5. It is these oxide sites that most strongly influence the [PO4] and [HPO4] 3069

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bond valence sums for 1a, 1b, and 2b. An X-ray crystallographic information file (CIF) is available for [2-mpipH2][Ga(HPO4)(PO4)] (1a and 1b) and [1-mpipH2][Ga(HPO4)(PO4)] (2a and 2b). This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel: (610) 896 2949. Fax: (610) 896 4963. E-mail: [email protected].

Figure 7. Ball-and-stick representation of the three-dimensional packing of 1. Hydrogen-bonding interactions are shown as dashed lines.

orientations. Despite being both longer and weaker than (N/O/F)
H 3 3 3 O hydrogen bonds, the importance and ubiquity of C
H 3 3 3 O interactions in supramolecular chemistry are wellknown.72
75 The [P(2)O4] tetrahedra in 2b are rotated clockwise or counterclockwise, resulting in the unequal C
H 3 3 3 O hydrogen bond distances shown in Figure 6. Conversion to the opposite orientation, as dictated by the twin law observed in 2b, requires only subtle twisting of the [PO4], [HPO4] and [1-mpipH2]2þ components that interact through C
H 3 3 3 O hydrogen bonds. The twin law in 2b is a 180° rotation about the a axis. The absence of a similar phase transition in compound 1 can also be attributed to hydrogen bonding. The [2-mpipH2]2þ cations reside between [Ga(HPO4)2/2(PO4)2/2]n2n
chains, just as in compound 2. However, the position of the methyl group forces a twist in the [2-mpipH2]2þ cations. These cations cannot lie perpendicular to the gallium phosphate chains, as observed in 2a and 2b, because no room exists for the methyl groups. Instead, the [2-mpipH2]2þ cations twist, as shown in Figure 7. In contrast to 2a and 2b, much stronger N
H 3 3 3 O hydrogen bonds are formed between the [2-mpipH2]2þ cations and O7, fixing the [P(2)O4] orientation. Reversal of the rotation in this tetrahedron would require substantial structural rearrangement, which precludes the presence of the type of disorder found in 2a.

’ CONCLUSION C
H 3 3 3 O hydrogen bonding is responsible for the existence of an order-to-disorder phase transition in the organically templated gallium phosphate [1-mpipH2][Ga(HPO4)(PO4)] (2). The rotations of [HPO4] and [PO4] tetrahedra are largely governed by these weak interactions and are responsible for the formation of a low symmetry, monoclinic structure at 100 K and a disordered, averaged structure at 298 K. In addition, the use of ionothermal conditions results in a greater number of hanging P
O bonds versus analogous reactions conducted under hydrothermal conditions. ’ ASSOCIATED CONTENT

bS

Supporting Information. Figures of interchain hydrogen bonding, thermogravimetric traces for all structures, and tables of

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