CRYSTAL GROWTH & DESIGN
Hydrogen-Bonded Structures Formed from the Reaction of 1,3,5-Benzene-triphosphonic Acid and Adamantane Deyuan Kong,† Jiyong Yao,† Abraham Clearfield,*,† and Jerzy Zon‡ Department of Chemistry, Texas A&M UniVersity, College Station, Texas 77842-3255, and Faculty of Chemistry, Department of Organic Chemistry, Wroclaw UniVersity of Technology, 50-370 Wroclaw, Poland
2008 VOL. 8, NO. 8 2892–2898
ReceiVed December 20, 2007; ReVised Manuscript ReceiVed February 6, 2008
ABSTRACT: The triphosphonic acid 1,3,5-benzene-triphosphonic acid (BTP; 1,3,5-[(HO)2PO]3C6H3) was allowed to react with different ratios of adamantane. Three compounds with amine to BTP ratios of 2:1, 4:1, and 6:1 were isolated, and their crystal structures were determined by single crystal X-ray diffraction. In the 2:1 compound the BTP molecules transferred two protons to the amines to form a complex of composition [C10H15NH3]22+[C6H3(PO3H)2PO3H2]2- · 2H2O. This compound formed zig-zagged chains of BTP molecules hydrogen bonded to each other with the amine cations and water molecules interspersed between the chains. The 4:1 complex also formed chains through hydrogen bonding between the individual BTP molecules, but the chains are separated from each other by a double layer of protonated amines. In the 6:1 compound, all the protons are transferred to the adamantane molecules so that there is no hydrogen bonding between BTP molecules. Rather, a very complex hydrogen bond system between amine-BTP, amine-water and water-BTP results. In all three complexes, all the protonated amino groups hydrogen bond by utilizing all three of their protons as donors. The pKa values of the BTP protons were determined by analysis of the titration curve. Introduction 1,3,5-Benzenetricarboxylic acid (BTC), also termed trimesic acid, is a multidentate ligand having a planar structure.1 Its behavior in the presence of amines wherein one, two, or three protons are transferred has been summarized as part of a review article by Moulton and Zaworotko.2 A variety of layered structures with pendant amines and interesting packing arrangements are described. Because of its trigonal symmetry and complementary in-plane directional forces, trimesic acid tends to form multidimensional metal-organic frameworks (MOFs) containing a large variety of metal centers.3,4 More recently, Ferey et al.5 have synthesized a number of chromium BTC complexes. The culmination of this research is the synthesis of MIL ∼ 101 a complex of oxochromium clusters and 1,4benzenedicarboxylic acid (terephthalic acid).6 Our own interests have centered around the use of phosphonic acids.7 These ligands are more complex than carboxylic acids, possessing three bonding oxygens and two replaceable protons. In the case of diphosphonic acids we have been able to prepare porous pillared three-dimensional materials with surface areas of 300–430 m2/g and pores in the range of 10–20 Å diameter.8 The pillars are either 1,4-biphenylenebis(phosphonic acid), the analogous triphenyldiphosphonic acid, and an ether type, H2O3PC6H4-O-C6H4PO3H2. The metals involved are Zr(IV), Sn(IV), and Al(III). The pore sizes of the tin derivatives prepared from ether phosphonic acid could be regulated by utilizing alcohols of increasing chain length as solvents in the solvothermal reactions. In retrospect we have felt that the phosphonic acid analogue of trimesic acid, 1,3,5-benzene triphosphonic acid (BTP), would be an interesting ligand to examine. The only triphosphonic acid we have used previously is nitrilotris(methylphosphonic acid) (NTP) with nonplanar molecular building blocks to construct three-dimensional hydrogen-bonding systems.9 Studies with this * To whom correspondence should be addressed. † Texas A&M University. ‡ Wroclaw University of Technology.
ligand clearly demonstrated that the products of reactions with amines strongly depended upon the number of protons transferred to the amines. The monodeprotonation of NTP led to the formation of three-dimensional hexagonal networks. Double deprotonation of NTP resulted in the formation of onedimensional chain structures with exceptionally strong hydrogen bonds, O · · · O 2.452(6), 2,559(7), 2.531(6) Å. In this paper we explore the reaction BTP with a primary amine, adamantane, in which the NH2 group is attached to a bulky carbon network. Mehring10 prepared crystals of BTP in a methanol–water solvent in the presence of 4-(dimethylamino)pyridine at reflux temperature to obtain [1-{(HO)2PO}-3,5-{(HO)PO2}2C6H3]2-[4(MeN)C5H4NH+]2 and [1,3{(HO)2PO}2-5{(HO)PO2}C6H3]-[4(Me2N)C5H4NH]+. Subsequently, the structure of the BTP propyl ester-CHCl3 adduct was crystallized and its structure was determined.11 Work from our laboratory described the structures BTP formed on reaction with 2,2′-bipyridine and 4,4′bipyridine.12 Two copper derivatives were also described structurally.13 Experimental Section All reagents used in this study were obtained from Aldrich (98–99% stated purity) and utilized without further purification. The synthesis of BTP was provided in detail previously.11 Briefly, 1,3,5-benzene tribromide was treated with triethyl phosphite in 1,3-diisopropylbenzene at 180 °C using NiBr2 as a catalyst. The ethyl ester was then converted to BTP by refluxing in strong HCl for 14 h. Synthesis of Adamantane - 1,3,5-Benzene Phosphonic Acid Derivatives. The general procedure for the synthesis was to dissolve 50 mg (0.157 mmol) of BTP in 2 mL of H2O. A fixed ratio of amine, starting with 1:1 and ending at 8:1, was dissolved in 5 mL of ethanol, added to the BTP solution and stirred overnight. The remaining residue was filtered off, and the filtrate was allowed to evaporate slowly at room temperature. Colorless or light yellow crystals were deposited. X-ray Crystallography. Data collections (2.53° < θ < 24.27) were performed at 110 K on a Bruker Smart CCD-1000 diffractometer with Mo KR (λ ) 0.71073 Å) radiation using a stream of N2 gas as coolant. Data reduction and refinement were performed with the SAINT program and multiscan corrections were applied.14 Crystal structures were solved by direct methods and refined with full matrix least-squares (SHELXTL-
10.1021/cg701253u CCC: $40.75 2008 American Chemical Society Published on Web 07/10/2008
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Table 1. Crystal Data for Adamantane-BTP Complexes ratio Ada:BTP empirical formula formula weight temperature (K) wavelength (Å) crystal system space group unit cell dimensions
volume (Å3) Z density (calculated) (Mg/m3) absorption coefficient (mm-1) F(000) crystal size Theta range for data collection (°) reflections collected independent reflections completeness to theta ) 24.71° absorption correction data/restraints/parameters goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) largest diff peak and hole
2:1 C26N2H47011P31 1313.13 120(2) 0.71073 orthorhombic P2(1)2(1)2(1) a ) 10.974(3) Å b ) 13.124(4) Å c ) 21.098(6) Å R ) 90° β ) 90° γ ) 90° 3038.4(14) 4 1.435 0.257 1400 0.20 × 0.04 × 0.02 mm3 1.83 to 24.71 15242 5153 [R(int) ) 0.0350] 99.9% none 5153/0/409 1.067
4:1 C49H101N4016P3 1075.43 110(2) 0.71073 triclinic P1j a ) 11.862(5) Å b ) 15.663(7) Å c ) 16.830(8) Å R ) 62.422(7)° β ) 80.885(8)° γ ) 75.348(7)° 2679(2) 2 1.358 0.181 1192 0.60 × 0.09 × 0.02 mm3 2.29 to 24.71 25082 9086 [R(int) ) 0.0310] 99.6% none 9086/0/1051 1.037
6:1 C68H136N6O25P3 1530.74 110(2) 0.71073 triclinic P1j a ) 11.002(8) Å b ) 19.264(13) Å c ) 20.600(14) Å R ) 72.623(12)° β ) 84.532(13)° γ ) 76.059(12)° 4043(5) 2 1.257 0.150 1662
R1 ) 0.0323, wR2 ) 0.0802 R1 ) 0.0345, wR2 ) 0.0817 0.421 and -0.210 e Å-3
R1 ) 0.0470, wR2 ) 0.1084 R1 ) 0.0595, wR2 ) 0.1143 0.628 and -0.559 e Å-3
R1 ) 0.0900, wR2 ) 0.2198 R1 ) 0.1136, wR2 ) 0.2450 1.539 and -0.573 e Å-3
97) with atomic coordinates and anisotropic thermal parameters for all non-hydrogen atoms.15 Hydrogen atoms for O and N were located in difference Fourier maps and refined isotropically with the O-H and N-H distances set to provide the best fit to the X-ray data. Hydrogens in the water molecules were set geometrically. Crystallographic data are presented in Table 1. Potentiometric Titration of BTP. A Corning digital pH meter, fitted with Fisher full-range blue-glass and a Fisher Calomel reference electrode, was used for poteniometric titrations. A Metrohm 10 mL capacity piston buret was used for precise delivery of the standard KOH. The solution to be titrated was contained in a 75 mL jacketed glass cell thermostatted at 25 ( 0.05 °C by a circulating constant temperature water bath. The pH calibrations were performed with standardized KOH in aqueous solutions to measure hydrogen ion concentrations directly (pH ) -log[H+]). The ionic strength of the medium was adjusted to 0.100 M by the addition of KCl solution. Titrations were conducted in the manner described by Martell and Motekaitis.6 Cell solutions were purged with a purified argon stream to create an inert atmosphere. Standard base was added in increments to the BTP solution from the Metrohm buret. The concentration of the BTP solution was 1 × 10-3 M. The pKw for the aqueous system defined as log ([H+][OH-]) at the ionic strength employed was found to be 13.78.16 Protonation
1.91 to 24.71 37833 13725 [R(int) ) 0.0509] 99.6% none 13725/0/1469 1.071
constants from the direct titration were calculated from the potentiometric data with the program BEST developed by the Martell group.16 Species distribution diagrams were computed from the measured equilibrium constants with SPE and plotted with SPEPLOT.16
Results The titration curve for BTP is shown in Figure 1 and its analysis is in Figure 1 (right panel). Of the six protons in BTP, three have high pK values of 8.56, 7.43, and 6.71 for the first three protons, respectively. The removal of these three protons results in a negatively charged anion H3L3- that is the dominant species in the pH range 3–6. The pK values then drop to 2.61, and 1.44 for protons 4 and 5 at higher pH. H2L4-, H2L5-, and L-6 are the major species above pH 6. However, the fully deprotonated ligand first appears at pH 7 and increases rapidly to pH 10 and above. X-ray Structures: Compound 1, [C10H15NH3]2+[C6H3(PO3H)2PO3H2]2- · 2H2O. The numbering scheme and thermal ellipsoids of the atoms comprising the unique portion of the
Figure 1. Potentiometric titration curve of 1,3,5-benzenetris(phosphonic acid) (left) and mathematical analysis of the titration curve (right) showing the species distribution as a function of pH.
2894 Crystal Growth & Design, Vol. 8, No. 8, 2008
Figure 2. A view of the BTP molecules in compound 1 that are close to zero in the c-axis direction and straddle the ab plane. Another such grouping exists at c ∼ ½ (see Figure 3). Color scheme: phosphonate tetrahedra, yellow; N, blue; O, red; C, grey; H, white.
unit cell are given in Figure 2. In the formation of compound 1, two protons were transferred to the amine groups from each BTP unit. This transfer manifested itself in the length of the P-O bonds. Those oxygens which remained bonded to hydrogen were significantly longer than those without protons attached. For example P(1)-O(3), 1.491(2) Å, P(1)-O(4), 1.508(2) Å, P(1)-O(2), 1.579(2) Å. Similarly in P(2), O(5) retained the proton, 1.575(2) Å and O(1) and O(6) were shorter. In the P(3) group both O(8) and O(9) are bonded to protons with essentially identical bond distances of 1.540(2) and 1.541(2) Å, respectively, while the remaining P(3)-O(7) bond distance is 1.504(2) Å. Figure 2 is a view down the long c-axis, showing the disposition of the BTP groups in the ab plane. The phenyl rings lie almost in the ab plane. The three BTP molecules at the bottom half of the unit cell are related to each other by the 21 axis parallel to the a-axis at ¼b. These phosphonate groups are hydrogen bonded to each other as indicated by the dashed lines between them (Table 2). This grouping of three BTP is the self-assembled repeat pattern along the a-axis. There is a break in the motif along the b-axis direction by insertion of water molecules and protonated amino groups. O(8) and O(9) in phosphonate group P(3) hydrogen bond, respectively, to O(4) in P2 and O(1) in P(1) as acceptors. These bonds are seen as broken lines in Figure 2 most prominently from a yellow tetrahedron whose center is at ∼½a and 0.17b. The oxygen on the right at the base of the triangle is O(9) hydrogen bonding to O(1) in P(1) and O(8) to the left hydrogen bonding to O(4) in P(2). In turn O(5)-H · · · O(7) and O(2)-H · · · O(7) eminate from P(2) and P(1), respectively, to O(7) in our designated P(3) tetrahedron. The same hydrogen bonds are evident in Figure 3 in either of the upper or lower left-hand corners. O(1)W is hydrogen bonded to O(6) in P(2) and O(1) in P(1) in opposite directions. O(1)W is in turn hydrogen bonded as acceptor to O(2)W and N(2). This grouping of three BTP groups at the c-axis near zero in Figure 3 is rotated 180° about a 21 axis parallel to the b-axis at
Kong et al.
¼c and translated ½b. This places a new grouping of three BTP at ½b, distant from the first grouping and centered about ½c (Figure 3). The disposition of the phosphonate groups is as expected from Figure 2. The two located at the ends of the a-axis have their base pointed outward and their apice pointed in the positive c-axis direction. The BTP group near the center of the a-axis exhibits the opposite disposition. There are eight adamantane molecules in the unit cell, four of the N(1) variety and four N(2). They are all protonated and hydrogen bond as donors to two phosphonate oxygens and one water molecule (Figures 3 and 4). N(1) hydrogen bonds to O(3) and O(5) and to O(3)W. N(2) hydrogen bonds to O(1) and O(3) and O(1)W. The disposition of the nitrogens in Figure 3 are two N(1) in the upper half at about ½a, one pointing to the phosphonate groups above and one below. In the lower half the two N(1) adamantane molecules are half in and half out of the unit cell and similarly disposed up and down. The reverse is true for the N(2) adamantane molecules being near the edges in the upper and near ½a in the lower half. Figure 4 shows the disposition of the nitrogens along the b-axis and their proximity to water molecules as well as the phosphonate oxygens. This view clearly reveals that the chains of BTP groups as pictured in Figure 1 are separated from each other by water molecules in a complex hydrogen-bond arrangement. Compound 2, [C10H15NH3]44+[C6H5(PO3H)2PO3]4- · 5H2O · C2H5OH · CH3OH1. In compound 2, four protons were transferred to the adamantane groups from a single BTP group. In this synthesis a mixture of ethanol, methanol, and water was used as solvent. As a result close to one molecule each of these two alcohols was incorporated into the unit cell. The presence of so much solvent resulted in a certain amount of disorder. In the refinement two positions were required for O(4)W and O(5)W, and a pancake shape thermal ellipsoid was refined for the methanol molecule. Figure 5 is a view of the structure projected down the a-axis onto the bc plane showing the arrangement of the BTP molecules grouped about c ) ½. The phenyl rings are lying almost parallel or slightly tilted to the ab plane. The BTP groups form chains running along the b-axis direction in which the phenyl rings are almost equally divided by the a-axis (Figure 6). The chains are tied together directly by hydrogen bonds, P(3)-O(8)-H · · · O(7)P(3)′, 2.595(3) Å and P(2)-O(5)-H · · · O(3)P(1), 2.484(2) Å in which the donor and acceptor atoms are in adjacent BTP molecules. These are the two shortest hydrogen bonds listed in Table 3. The adamantane molecules are aligned in a bilayer in which all the -NH3+ groups point in the direction of the BTP chains with a van der Waals gap between the adamantane molecules. A set of four amino groups occupy the lower half of the unit cell along the c-axis (Figure 6) with approximate b-axis parameters of N2 and N3 at ½b, N4, 0.07b, and N1 at 0.85b. An equivalent set of four adamantane molecules reside in the upper half of the unit cell related to those in the lower half by a center of symmetry. This distribution along the b-axis is clearly seen in Figure 5. The BTP groups form chains running in the b-axis direction tied together by the POH · · · O hydrogen bonds listed above. However, Figure 6 shows a gap between the chains in the a-axis direction. The water molecules are mainly spread through this gap along the b-axis at near ½a. Only O3W is outside this area with a-axis parameters of approximately 0.16 and 0.84 as seen in Figure 6. Two of the amino nitrogens N(1) and N(3) have a-axis parameters near ½a with N(4) and N(2) at about ¼ and ¾a. N(3) hydrogen bonds to O(1) and O(2) in the P(1) tetrahedron, but in adjacent BTP molecules across the gap. N(3) also hydrogen bonds to O(1)W, which in turn
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Table 2. Hydrogen Bonds (Å) and Angles (°) for BTP-Adamantane Compound with Amine:BTP 2:1a D-H · · · A
d(D-H)
d(H · · · A)
d(D · · · A)
