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CRYSTAL GROWTH & DESIGN

Pyrazinetetracarboxylic Acid as an Assembler Ligand in Uranyl-Organic Frameworks

2008 VOL. 8, NO. 5 1689–1696

Bernardo Masci*,† and Pierre Thuéry*,‡ Dipartimento di Chimica, UniVersità “La Sapienza”, Box 34, Roma 62, Piazzale Aldo Moro 5, 00185 Roma, Italy, and CEA/Saclay, DSM/IRAMIS/SCM (CNRS URA 331), Bât. 125, 91191 Gif-sur-YVette, France ReceiVed December 18, 2007; ReVised Manuscript ReceiVed January 24, 2008

ABSTRACT: The reaction of uranyl nitrate with pyrazinetetracarboxylic acid (H4PZTC) has been investigated under different experimental conditions, and the crystal structures of the resulting complexes have been determined. In all cases, the uranyl ion is chelated in the ONO tridentate site, as in the complexes with pyridine-2,6-dicarboxylic acid, but much variety arises from the increased number of potential donor atoms. Hydrothermal synthesis in the presence of NEt4Br led to the complex [UO2(PZDC)(H2O)] (1), from in situ decarboxylation of H4PZTC into pyrazine-2,6-dicarboxylic acid (H2PZDC) upon prolonged heating. Complex 1crystallizes as ribbons held by bridging carboxylate groups and hydrogen bonds. In the presence of NaOH and under hydrothermal conditions, two species could be obtained: [(UO2)2(PZTC)(H2O)] · 2H2O (2) and [UO2Na2(PZTC)(H2O)4] (3). In these compounds, [UO2(PZTC)]n2n- linear chains with bis-chelating PZTC ligands are further assembled either into a twodimensional assemblage by other, carboxylate-bound bridging uranyl ions or into a three-dimensional framework by bridging [Na2(H2O)4]2+ dimers. In contrast, in [UO2K2(H2PZTC)2(H2O)2] · 2H2O (4) and the two isomorphous compounds [UO2K2(H2PZTC)2] · 4H2O (5) and [UO2Rb2(H2PZTC)2] · 4H2O (6), which were not obtained under hydrothermal conditions, the ligand retains two protons and is bound to only one uranyl ion. The [UO2(H2PZTC)2]2- units are then assembled into a three-dimensional framework by bridging, oxo-bonded potassium ions in 4 or in layers by the carboxylic/ate-bound alkalimetal ions in 5 and 6 (with formation of a three-dimensional framework in the case of Rb+ through weaker Rb-acid bonds). In the last case, narrow channels contain the hydrogen-bonded water molecules. These results evidence the remarkable assembling potential of the H4-xPZTCx- ligand, with all 10 donor atoms coordinated in complexes 2 and 3 (x ) 4) and bonding to as many as 7 metal atoms in 6 (x ) 2). Introduction The family of aromatic polycarboxylic acids is prominent in the search for assembler ligands suitable for designing metal-organic frameworks; one of its most remarkable members is pyrazinetetracarboxylic acid (denoted H4PZTC hereafter), which has been investigated with several first- and second-row transition-metal ions,1 whereas the related pyrazine-2,6-dicarboxylic acid (H2PZDC) is present in only a handful of crystal structures of alkaline-earth and first-row transition-metal ion complexes2 reported in the Cambridge Structural Database (CSD, version 5.28).3 However, in both cases, no complex with f element ions has been described, which is all the more surprising given that related ligands such as pyridine-2,6dicarboxylic or benzene-1,2,4,5-tetracarboxylic acids have been much investigated, in particular as complexants for the uranyl ion UO22+.4,5 H4PZTC can be viewed as an assemblage of two pyridine-2,6-dicarboxylic acid molecules defining two divergent, heterofunctional coordination sites2g and consequently as a potentially useful linker in the synthesis of uranium-organic frameworks (UOFs).6 It also presents some analogies with nonaromatic ONO donor ligands such as iminodiacetic acid, which are of importance in uranyl ion speciation studies.6q,7 With 10 donor atoms, H4PZTC presents one of the highest potential overall denticities among the ligands used in the design of metal-organic frameworks, which can be put into use in the synthesis of hybrid metal ion complexes. We report herein the synthesis and crystal structures of several uranyl complexes * To whom correspondence should be addressed. E-mail: bernardo.masci@ uniroma1.it (B.M.), [email protected] (P.T.). † Università “La Sapienza”. ‡ CEA/Saclay.

with H4-xPZTCx-, obtained under either hydrothermal or nonhydrothermal conditions, some of them including alkali-metal ions, and also the crystal structure of the uranyl complex with the ligand pyrazine-2,6-dicarboxylate, generated in situ by decarboxylation of the tetra acid under hydrothermal conditions, as previously reported.2g Experimental Section Synthesis. Caution! With uranium being a radioactiVe and chemically toxic element, uranium-containing samples must be handled with suitable care and protection. UO2(NO3)2 · 6H2O was purchased from Prolabo. Elemental analyses were performed by Analytische Laboratorien GmbH, Lindlar, Germany. Pyrazinetetracarboxylic Acid (H4PZTC). H4PZTC was synthesized according to a literature procedure.8 Its purity was checked through a 13 C NMR spectrum (50 MHz) in D2O and excess KOH, which only showed two peaks at δ 148.3 and 172.7. [UO2(PZDC)(H2O)] (1). H4PZTC (20 mg, 0.078 mmol), UO2(NO3)2 · 6H2O (40 mg, 0.080 mmol), NEt4Br (68 mg, 0.324 mmol), and demineralized water (2.2 mL) were placed in a 15 mL tightly closed glass vessel and heated at 180 °C under autogenous pressure (ca. 1.1 MPa). Lightyellow crystals of complex 1, resulting from in situ decarboxylation of PZTC into PZDC, were obtained within 2 weeks. The product was recovered after filtration and washed with water (15 mg, 41% yield). Anal. Calcd for C6H4N2O7U: C, 15.87; H, 0.89; N, 6.17. Found: C, 15.61; H, 0.95; N, 5.93. Complex 1 was also obtained in the same conditions, with NEt4Cl in place of NEt4Br as a potential templating agent. [(UO2)2(PZTC)(H2O)] · 2H2O (2). H4PZTC (27 mg, 0.105 mmol), UO2(NO3)2 · 6H2O (53 mg, 0.106 mmol), NaOH (14 mg, 0.35 mmol), and demineralized water (2.5 mL) were placed in a 15 mL tightly closed glass vessel and heated at 180 °C under autogenous pressure (ca. 1.1 MPa). Light-yellow crystals of complex 2 were obtained within 2 days, with a very low yield preventing elemental analyses from being performed. Repeated attempts at the synthesis of 2 never gave more than a few crystals.

10.1021/cg701246j CCC: $40.75  2008 American Chemical Society Published on Web 04/18/2008

1690 Crystal Growth & Design, Vol. 8, No. 5, 2008

Masci and Thuéry

Table 1. Crystal Data and Structure Refinement Details

chemical formula M (g mol-1) cryst syst space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Dcalcd (g cm-3) µ(Mo KR) (mm-1) F(000) reflns collcd indep reflns obsd reflns [I > 2σ(I)] Rint params refined R1 wR2 S ∆Fmin (e Å-3) ∆Fmax (e Å-3)

1

2

3

4

5

6

C6H4N2O7U 454.14 orthorhombic P212121 9.0184(4) 9.9048(6) 10.3237(7) 90 90 90 922.17(9) 4 3.271 17.627 808 23 403 1745 1723 0.031 147 0.025 0.065 1.032 -1.56 1.22

C8H6N2O15U2 846.21 monoclinic P21/m 5.8446(3) 19.1888(12) 6.8849(4) 90 102.164(4) 90 754.81(8) 2 3.723 21.521 744 23 098 1476 1394 0.023 130 0.026 0.069 1.087 -2.35 0.85

C8H8N2Na2O14U 640.17 triclinic P1j 5.5743(4) 6.6476(7) 10.3039(10) 76.091(5) 87.586(6) 80.911(6) 365.97(6) 1 2.905 11.237 296 15 695 1392 1387 0.047 124 0.023 0.057 1.070 -1.51 0.98

C16H12K2N4O22U 928.53 monoclinic P21/c 5.8717(3) 10.7693(3) 20.2364(9) 90 95.631(2) 90 1273.46(9) 2 2.422 6.814 884 35 837 2404 2161 0.027 205 0.021 0.054 1.049 -1.38 0.59

C16H12K2N4O22U 928.53 triclinic P1j 8.0517(3) 8.7548(4) 10.2542(4) 110.274(2) 107.093(3) 90.135(2) 643.71(5) 1 2.395 6.740 442 32 529 2448 2436 0.029 205 0.013 0.033 1.085 -0.94 0.62

C16H12N4O22Rb2U 1021.27 triclinic P1j 8.0501(5) 8.7608(6) 10.3486(5) 109.559(4) 107.331(4) 90.071(3) 652.35(7) 1 2.600 10.038 478 31 961 2477 2476 0.045 206 0.031 0.089 1.137 -1.90 0.84

[UO2Na2(PZTC)(H2O)4] (3). H4PZTC (20 mg, 0.078 mmol), UO2(NO3)2 · 6H2O (40 mg, 0.080 mmol), NaOH (4 mg, 0.1 mmol), NEt4Cl (68 mg, 0.324 mmol), and demineralized water (1.5 mL) were placed in a 15 mL tightly closed glass vessel and heated at 180 °C under autogenous pressure (ca. 1.1 MPa). After 4 days, the solution was allowed to slowly cool down, yielding light-yellow crystals of complex 3, which were recovered after filtration and washed with water (18 mg, 35% yield on the basis of U). Anal. Calcd for C8H8N2Na2O14U: C, 15.01; H, 1.26; N, 4.38. Found: C, 14.97; H, 1.26; N, 4.31. [UO2K2(H2PZTC)2(H2O)2] · 2H2O (4). A solution of UO2(NO3)2 · 6H2O (27 mg, 0.054 mmol) in MeOH (0.55 mL) was added dropwise to a stirred solution of H4PZTC (57 mg, 0.22 mmol), KOH (14 mg, 0.022 mmol), and NaOH (8.0 mg, 0.020 mmol) in H2O (8 mL). After heating at 60 °C for 10 min, the solution was allowed to stand and single crystals of 4 were collected after 12 days (4.0 mg, 8% yield on the basis of U). Anal. Calcd for C16H12K2N4O22U: C, 20.70; H, 1.30; N, 6.03. Found: C, 20.92; H, 1.42; N, 6.05. [UO2K2(H2PZTC)2] · 4H2O (5). A solution of UO2(NO3)2 · 6H2O (5.6 mg, 0.011 mmol) in MeOH (0.11 mL) was added dropwise to a stirred solution of H4PZTC (7.7 mg, 0.030 mmol) and KOH (5.3 mg, 0.082 mmol) in H2O (2 mL). After heating at 60 °C for 10 min, the solution was allowed to stand and single crystals of 5 were collected after 7 days (3.2 mg, 31% yield on the basis of U). Anal. Calcd for C16H12K2N4O22U: C, 20.70; H, 1.30; N, 6.03. Found: C, 22.06; H, 1.49; N, 6.20. [UO2Rb2(H2PZTC)2] · 4H2O (6). A solution of UO2(NO3)2 · 6H2O (9.0 mg, 0.018 mmol) in MeOH (0.18 mL) was added dropwise to a stirred solution of H4PZTC (12.3 mg, 0.048 mmol) and RbOH (0.26 g of 50% solution in H2O, 0.13 mmol) in H2O (2.5 mL). After heating at 60 °C for 10 min, the solution was allowed to stand and single crystals of 6 were collected after 10 days (2.9 mg, 16% yield on the basis of U). Anal. Calcd for C16H12N4O22Rb2U: C, 18.82; H, 1.18; N, 5.49. Found: C, 19.21; H, 1.25; N, 5.50. Crystallography. The data were collected at 100(2) K on a Nonius Kappa CCD area detector diffractometer9 using graphite-monochromated Mo KR radiation (λ ) 0.710 73 Å). The crystals were introduced in glass capillaries with a protecting “Paratone-N” oil (Hampton Research) coating. The unit cell parameters were determined from 10 frames and then refined on all data. The data (combinations of φ and ω scans giving complete data sets up to θ ) 25.7° and a minimum redundancy of 4 for 90% of the reflections) were processed with HKL2000.10 The structures were solved by direct methods (1, 3, and 4) or Patterson map interpretation (2, 5, and 6) with SHELXS-97, expanded by subsequent Fourier difference synthesis, and refined by full-matrix least squares on F2 with SHELXL-97.11 Absorption effects were corrected empirically with the program SCALEPACK.10 All nonhydrogen atoms were refined with anisotropic displacement parameters.

The hydrogen atoms bound to oxygen atoms were found on Fourier difference maps (except those of the water oxygen atom O9 in compound 2, which are probably disordered over three positions compatible with hydrogen bonding), and the carbon-bound hydrogen atoms in compound 1 were introduced at calculated positions; all were treated as riding atoms with a displacement parameter equal to 1.2 times that of the parent atom. The absolute configuration of complex 1 was determined from the value of the Flack parameter, -0.016(16).12 Crystal data and structure refinement parameters are given in Table 1 and selected bond lengths and angles in Table 2. The molecular plots were drawn with SHELXTL13 and Balls & Sticks.14

Results and Discussion The complex [UO2(PZDC)(H2O)] (1) was obtained accidentally, with the ligand PZDC2- resulting from the decarboxylation of the tetra acid derivative after prolonged heating at 180 °C. Such a reaction had previously been observed during the synthesis of copper(II) complexes in water at 120 °C.2g This ligand complexes the uranyl ion in a tridentate fashion, through the nitrogen atom located between the two acid groups and one oxygen atom from each of these groups (Figure 1), as observed with pyridine-2,6-dicarboxylate.4 The uranyl coordination sphere is completed by one water molecule and by a carboxylate oxygen atom from a neighboring unit, which gives the usual pentagonal-bipyramidal uranium environment. The U-N bond length, 2.521(6) Å, is in the lower part of the range spanned by its counterpart in the five-coordinate complexes with pyridine2,6-dicarboxylate reported in the CSD [2.513–2.609, average 2.54(2) Å], whereas the average U-O(carboxylate) bond length, 2.37(4) Å, matches the average value of 2.38(2) Å in these complexes; the U-O(water) bond length, 2.399(5) Å, is unexceptional [average value from the CSD 2.44(4) Å]. The five donor atoms define a mean plane with a root-mean-square (rms) deviation of 0.13 Å, with the uranium atom being displaced by 0.026(3) Å. The aromatic ring and the two carboxylate groups are nearly coplanar, with dihedral angles of 7.1(6) and 1.8(8)° and displacements from the aromatic plane lower than 0.16(1) Å for the oxygen atoms. Each ligand is thus bridging two metal atoms, in a µ2-1κ3O,O′,N:2κO′′ coordination mode. Such a ligating mode is also found in the helical polymer formed by uranyl with pyridine-2,6-dicarboxylate,15 whereas in

Pyrazinetetracarboxylic Acid as an Assembler Ligand

Crystal Growth & Design, Vol. 8, No. 5, 2008 1691

Table 2. Environment of the Metal Atoms in Compounds 1–6: Selected Bond Lengths (Å) and Angles (deg) 1

2

3

U-N1 U-O1 U-O3 U-O4′ U-O5 U-O6 U-O7 O1-U-N1 N1-U-O3 O3-U-O4′ O4′-U-O7 O7-U-O1 O5-U-O6 U1-N1 U1-O1 U1-O3 U1-O5

2.521(6) 2.354(5) 2.427(5) 2.319(5) 1.754(6) 1.749(5) 2.399(5) 63.3(2) 63.11(18) 74.71(18) 80.05(19) 79.68(18) 178.3(3) 2.683(5) 2.419(4) 2.411(4) 1.758(4)

O1-U1-N1 N1-U1-O3 O1-U1-O3′ O5-U1-O5′ U-N1 U-O1 U-O2 U-O4 O1-U-O1′ O2-U-N1 N1-U-O4 O2-U-O4′

59.26(15) 59.44(16) 63.84(15) 180 2.680(4) 1.773(4) 2.408(4) 2.417(3) 180 59.46(12) 59.85(13) 63.64(12)

U2-O2 U2-O4′′ U2-O6 U2-O7 U2-O8 O2-U2-O4′′ O4′′-U2-O4′′′′ O2-U2-O8 O6-U2-O7 Na-O3 Na-O5′′ Na-O5# Na-O6 Na-O6* Na-O7

2.390(4) 2.368(4) 1.773(7) 1.771(7) 2.391(6) 69.09(15) 78.3(2) 71.88(10) 180 2.312(5) 2.332(4) 2.589(5) 2.332(5) 2.406(4) 2.466(5)

4

U-N1 U-O1 U-O3 U-O9 O1-U-N1 N1-U-O3 O1-U-O3′ O9-U-O9′

2.694(2) 2.4780(19) 2.395(2) 1.7619(19) 58.08(6) 60.23(6) 62.98(6) 180

5

U-N1 U-O1 U-O3 U-O9 O1-U-N1 N1-U-O3 O1-U-O3′ O9-U-O9′

2.6696(17) 2.4507(17) 2.4626(18) 1.7649(16) 59.59(6) 59.84(6) 61.54(5) 180

6

U-N1 U-O1 U-O3 U-O9 O1-U-N1 N1-U-O3 O1-U-O3′ O9-U-O9′

2.670(5) 2.453(4) 2.466(4) 1.765(4) 59.65(14) 59.66(14) 61.86(14) 180

K-O1 K-O2 K-O3′ K-O4′ K-O5′′ K-O7′′′ K-O8# K-O9* K-O10 K-O1 K-O2 K-O3′ K-O4′ K-O2′′ K-O4# K-O5# K-O5′′′ K-O7′′ Rb-O1 Rb-O2 Rb-O3′ Rb-O4′ Rb-O2′′ Rb-O4# Rb-O5# Rb-O5′′′ Rb-O6* Rb-O7′′

2.841(2) 3.119(2) 2.727(2) 3.178(2) 2.751(2) 2.849(2) 2.882(2) 2.764(2) 2.864(2) 2.8124(19) 3.2998(17) 2.7734(18) 3.1367(16) 2.7542(19) 3.1300(17) 2.8935(17) 3.2328(16) 2.7549(18) 2.880(4) 3.342(4) 2.840(4) 3.170(4) 2.845(4) 3.191(5) 2.978(5) 3.192(5) 3.521(4) 2.826(4)

Symmetry codes: 1: ′ ) 1 - x, 1/2 + y, 3/2 - z. 2: ′ ) 1 - x, 1 - y, -z; ′′ ) 2 - x, 1 - y, 1 - z; ′′′ ) x, 3/2 - y, z; ′′′′ ) 2 - x, 1/2 + y, 1 - z. 3: ′ ) 2 - x, 1 - y, 1 - z; ′′ ) 1 - x, 2 - y, 1 - z; # ) x, 1 + y, -1 + z; * ) 2 - x, 3 - y, -z. 4: ′ ) -x, -y, -z; ′′ ) 1 + x, 1/2 - y, -1/2 + z; ′′′ ) -x, 1 - y, -z; # ) 1 - x, 1 - y, -z; * ) 1 - x, -y, -z. 5 and 6: ′ ) 1 - x, 1 - y, 1 - z; ′′ ) 1 - x, 2 - y, 2 - z; ′′′ ) 1 - x, 2 - y, 1 - z; # ) x, y, 1 + z; * ) 1 + x, y, 1 + z.

most cases of molecular complexes with this ligand, the uranyl ion is surrounded by two ligands with a six-coordinate environment or is comprised in a dimer doubly bridged by additional ligands;4b hydrothermal synthesis promotes the isolation of polymeric species with the ligand bridging three or four metal atoms.4d Complex 1 is different from the polymeric complexes of pyridine-2,6-dicarboxylic acid due to the presence of the second nitrogen atom in the ligand, which is uncoordinated but acts as an acceptor in a hydrogen bond involving the water ligand of a neighboring unit (Table 3). Each [UO2(PZDC)(H2O)] subunit is bound to two others by coordination bonds, as in the helical complex cited above. However, instead of being bound to one another so as to give a helical thread, these subunits are arranged in an approximately parallel fashion in 1, so as to form a zigzag chain or ribbon parallel to the b axis, which is also held by hydrogen bonds. This difference in arrangement between two closely related species with a similar overall formula exemplifies the extreme sensitivity of the architecture of UOFs to changes in the nature of the ligands when they bring about the possibility of different weak interactions happening. When viewed down the b axis, the ribbons in 1 are tilted with respect to one another and they are connected by hydrogen bonds between water and uncoordinated carboxylate oxygen atoms so as to form a three-dimensional framework held by weak interactions. Mixed copper(II)-lanthanide(III) complexes with PZDC have recently been reported, in which the ligand is bonded to the rare-earth ion in a tridentate fashion and to the copper atom by its second nitrogen atom,16 which constitutes a nice example of the use of heterofunctional ligands in the synthesis of heterometallic frameworks.4e In the present case, the preference of the uranyl ion for oxygen versus nitrogen donors makes the absence of coordination to the second nitrogen atom, which is not associated with proximate oxygen donors, quite unsurprising.

In complex 2, the asymmetric unit comprises two independent uranyl ions, which are in different environments. Atom U1 is located on an inversion center and is chelated in the ONO site of two tridentate PZTC4- ligands, as is frequently observed with pyridine-2,6-dicarboxylate,4 whereas atom U2, located on a symmetry plane, is chelated by two bidentate ligands, with the coordinated oxygen atoms pertaining to two carboxylate groups in adjacent positions on the ring, and is also bound to one water molecule (Figure 2). These uranyl ions are thus in six- and fivecoordinate equatorial environments, respectively, with hexagonal- and pentagonal-bipyramidal geometries for the uranium environments. The U1-N1 and average U1-O(carboxylate) bond lengths, 2.683(5) and 2.415(4) Å, are larger than those in 1 because of the increase in the coordination number, and they match the average values in the six-coordinate pyridine-2,6dicarboxylate complexes reported in the CSD, 2.65(4) and 2.44(4) Å, respectively. The average U2-O(carboxylate) and U2-O8 bond lengths, 2.379(11) and 2.391(6) Å, respectively, match their counterparts in complex 1. The equatorial donor atoms define mean planes with rms deviations of 0.23 and 0.07 Å for U1 and U2, respectively, with U2 displaced from the plane by 0.052(3) Å; these two mean planes make a dihedral angle of 53.56(13)°. The two carboxylate groups in the half-ligand make dihedral angles of 14.9(9) and 19.9(8)° with the mean aromatic plane, with the maximum displacement of oxygen atoms from this plane being 0.52(1) Å. To the best of our knowledge, this is the first instance of a PZTC complex in which all 10 donor atoms are coordinated; in the 3d transition-metal ion complexes reported in the CSD, two diametrically located carboxylic/ate groups are often uncoordinated and, when this is not the case, not all of the groups are bidentate and/or the nitrogen atoms are not both coordinated. The low water content of 2, as often results from the use of hydrothermal methods, certainly helps in this respect. The arrangement comprises chains

1692 Crystal Growth & Design, Vol. 8, No. 5, 2008

Masci and Thuéry Table 3. Hydrogen-Bonding Geometry in Compounds 1–6: Distances (Å) and Angles (deg)

1 2 3

4

5

6

O7 · · · O2′ O7 · · · N2′′ O8 · · · O9 O6 · · · O7′ O6 · · · O4′′ O7 · · · O2′′′ O7 · · · O5′′′′ O6 · · · O10′ O8 · · · O2 O10 · · · O11 O10 · · · O5′′ O11 · · · O4′′′ O6 · · · O10 O8 · · · O11 O10 · · · O2′ O10 · · · N2′′ O11 · · · O4′′′ O11 · · · O10′′′ O6 · · · O10 O8 · · · O11 O10 · · · O2′ O10 · · · N2′′ O11 · · · O4′′′ O11 · · · O10′′′

D· · ·A

D-H

H· · ·A

D-H · · · A

2.639(8) 2.840(9) 2.694(6) 2.827(6) 2.878(6) 2.890(5) 3.191(6) 2.504(3) 2.441(3) 2.715(3) 2.936(3) 2.808(3) 2.594(3) 2.521(2) 2.751(2) 2.961(2) 2.779(3) 2.794(2) 2.596(6) 2.536(6) 2.741(6) 2.965(7) 2.793(6) 2.795(6)

0.93 0.86 0.85 0.89 0.87 0.91 0.88 0.86 0.80 0.93 0.90 0.86 0.90 0.97 0.95 0.92 0.97 0.90 0.96 0.98 0.86 0.94 0.90 0.84

1.81 2.05 1.85 2.02 2.25 1.99 2.35 1.66 1.67 1.81 2.14 1.95 1.70 1.56 1.80 2.16 1.86 1.92 1.64 1.58 1.89 2.18 1.90 1.97

148 152 172 149 129 175 162 167 162 164 146 174 173 167 172 144 156 164 178 165 168 141 176 166

Symmetry codes: 1: ′ ) 1/2 - x, 2 - y, -1/2 + z; ′′ ) x, 1 + y, z; 3: ′ ) 1 + x, y, z; ′′ ) x, 1 + y, -1 + z; ′′′ ) 2 - x, 2 - y, -z; ′′′′ ) x, y, -1 + z. 4: ′ ) -1 + x, 1/2 - y, 1/2 + z; ′′ ) x, 1/2 - y, -1/2 + z; ′′′ ) -1 - x, -y, -z. 5 and 6: ′ ) x, y, -1 + z; ′′ ) -x, 2 - y, -z; ′′′ ) x, 1 + y, 1 + z.

Figure 1. Top: View of complex 1. Displacement ellipsoids are drawn at the 50% probability level. Symmetry codes: see Table 2. Middle: View of one ribbon. Bottom: View of the packing down the ribbon axis. The uranium coordination polyhedra are shown, and the other atoms are drawn as spheres of arbitrary radii. Carbon-bound hydrogen atoms are omitted. Hydrogen bonds are shown as dashed lines.

of [UO2(PZTC)]2- units that are connected to one another by the other oxygen-bound uranyl ions. This gives rise to the formation of sheets parallel to (1 0 1j), in which the U1 atoms are located in the mean plane, whereas the U2 atoms are located alternately above and below the plane along the b axis. These sheets are packed so as to form channels of approximate section 5.5 × 5 Å, directed along the a axis, in which the hydrogen-bonded water molecules are located. An analogous geometry, in which equimolar uranyl/ligand chains are connected by partially hydrated uranyl ions, is also found in [NEt4]2[(UO2)3(cit)2(H2O)2] · 2H2O (H4cit ) citric acid);17 this situation is also reminiscent of the structure of [(UO2)3(Hcit)2(H2O)3] · 2H2O, albeit the latter crystallizes as a three-dimensional framework.6l In all of these cases, the hydrated uranyl ions serve as spacers connecting more tightly packed moieties and channels are created between these spacers. Incorporation of alkali-metal ions in uranyl-containing systems is a powerful way for obtaining higher dimensionality systems, which is particularly due to the alkali-metal ion binding ability of the uranyl oxo groups. This has been extensively used in the case of uranyl calixarene complexes.18 More specifically, in the family of UOFs, mixed uranyl/alkali-metal complexes with pyridine-2,6-dicarboxylate have been described,4b and this

same ligand was used for the synthesis of mixed uranyl/ transition-metal ion complexes.4c–e The four complexes 3-6 contain both uranyl and alkali-metal ions; the first was obtained under hydrothermal conditions, and complex 4 was isolated while attempting to obtain uranyl complexes containing both sodium and potassium ions. The crystal structure of 3 is particularly interesting when compared to that of complex 2. The asymmetric unit in 3 contains one uranyl ion located on an inversion center, one sodium ion, one half-ligand, and two coordinated water molecules (Figure 3). The uranyl ion is complexed in the same way as atom U1 in compound 2, and the U-N and average U-O(carboxylate) bond lengths, 2.680(4) and 2.413(5) Å, respectively, are identical with those in 2. The equatorial environment of the uranium atom is a puckered plane, with a rms deviation of 0.25 Å close to that in 2. The sodium ion occupies the same coordination site as U2 in 2, being bound to two carboxylate oxygen atoms from one ligand. It is further bound to one carboxylate oxygen atom from another ligand and to three water molecules, which gives a distorted octahedral environment. One of the carboxylate oxygen atoms (O5) and two water molecules act as bridges between the sodium ions. The Na-O(carboxylate) bond lengths are in the range 2.312(5)–2.589(5) Å, with the larger one being associated with the asymmetrically bridging O5; these values are in agreement with the average Na-O(carboxylate) bond length from the CSD, 2.42(13) Å. The Na-O(water) bond lengths are curiously shorter for the bridging O6 than for the terminal O7 atom, but all are well within the range reported in the CSD [average values 2.39(9) and 2.42(8) Å for terminal and bridging water molecules, respectively]. As in complex 2, the PZTC4- ligand somewhat departs from planarity, with dihedral angles of 16.6(7) and 25.0(5)° between the carboxylic groups and the mean aromatic plane. It may be noticed that, once more, all 10 donor atoms of the ligand are coordinated, with each PZTC4- ligand being bound to six cations (two uranyl and four sodium ions). The arrangement thus consists of [UO2(PZTC)]n2n- chains running

Pyrazinetetracarboxylic Acid as an Assembler Ligand

Figure 2. Top: View of complex 2. The hydrogen bond is shown as a dashed line. Displacement ellipsoids are drawn at the 50% probability level. Symmetry codes: see Table 2. Bottom: View of one layer down the a axis, showing the uranium coordination polyhedra (green, sixcoordinate uranyl ions; yellow, five-coordinate uranyl ions); the other atoms are shown as spheres of arbitrary radii; the solvent molecules and hydrogen atoms are omitted for clarity.

along the [1 1j 0] direction, analogous to those in 2 but related to one another by translations instead of symmetry planes. These chains are connected to one another by [Na2(H2O)4]2+ centrosymmetric dimers, which replace the hydrated uranyl ions in 2, so as to form sheets parallel to (1 1 1). These sheets are further linked to one another by the water bridges, which gives a three-dimensional assemblage. The shortest separation between uranium atoms of adjacent chains in a sheet is slightly larger in 3 (10.84 Å) than in 2 (9.59 Å). In contrast with the previous case, no channels of significant size are formed (and the structure does not contain any solvent molecule). Although the structures of 2 and 3 present many similarities, particularly the presence of [UO2(PZTC)]n2n- chains, the replacement of the essentially planar uranyl linkers by octahedral and thus more threedimensional sodium ones results in the extension of a two- into a three-dimensional architecture. Compound 4 is represented in Figure 4; the two complexes 5 and 6 are isomorphous, and the rubidium species 6 is shown in Figures 5 and 6. In all cases, the asymmetric unit comprises one uranyl ion located on an inversion center, one alkali-metal ion, one H2PZTC2- ligand, and either one coordinated and one free water molecule (4) or two solvent–water molecules (5 and 6). The overall formula of complex 4 is thus identical with that of 5, but the difference in the numbers of coordinated and free

Crystal Growth & Design, Vol. 8, No. 5, 2008 1693

Figure 3. Top: View of complex 3. Hydrogen bonds are shown as dashed lines. Displacement ellipsoids are drawn at the 50% probability level. Symmetry codes: see Table 2. Bottom: View of the threedimensional assemblage down the chain axis; the sheets are viewed edge-on. The uranium coordination polyhedra are in yellow and the sodium atoms in blue; the other atoms are shown as spheres of arbitrary radii; the hydrogen atoms are omitted for clarity.

water molecules has important consequences on the structure. The uranyl ion is once more chelated by two tridentate ligands, with average U-N and U-O(carboxylate) bond lengths of 2.678(11) and 2.45(3) Å, respectively (three compounds included), showing a slight lengthening of the latter, by about 0.03 Å, with respect to its counterpart in 2 and 3, which is likely due to both carboxylate groups chelating the alkali-metal ion. The equatorial environment of the uranium atom is a less puckered plane than that in 2 and 3, with rms deviations of 0.17, 0.14, and 0.16 Å in 4-6, respectively. Each of the four carboxylate groups surrounding the uranyl ion chelates one of the two bis-chelated alkalimetal atoms located on each side of the complex. In compound 4, the potassium atom is bound to seven carboxylic/ate oxygen atoms (with only O1 and O3 as bridging atoms) pertaining to five different ligands and its coordination sphere is completed by one water molecule and by the oxo group of a uranyl ion. The length of the latter K-O(oxo) bond, 2.764(2) Å, is well within the range previously observed in other uranyl/potassium complexes, 2.658–3.23 [mean value 2.79(18)] Å. The alkali-metal ion in compounds 5 and 6 is only bound to carboxylic/ate oxygen atoms, some of them terminal (O7, the particular case of O6 will be discussed later), some bridging between two alkali-metal ions (O2, O4, and O5), and some bridging the uranyl and alkali-metal atoms

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Figure 4. Top: View of complex 4. Displacement ellipsoids are drawn at the 50% probability level. Symmetry codes: see Table 2. Bottom: View of the three-dimensional assemblage down the b axis. The uranium coordination polyhedra are in yellow and the potassium atoms in cyan; the other atoms are shown as spheres of arbitrary radii; the solvent molecules and hydrogen atoms are omitted for clarity.

Figure 5. Top: View of complex 6. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen bonds are shown as dashed lines. Symmetry codes: see Table 2. Bottom: View of one linear subunit showing the uranium coordination polyhedra in yellow and the rubidium atoms in green; the other atoms are shown as spheres of arbitrary radii; the solvent molecules and hydrogen atoms are omitted for clarity.

(O1 and O3), giving coordination numbers of 9 and 10 for K+ and Rb+, respectively. Considering the variety among the donor atoms in these three compounds, a large range of values for the M-O bond lengths is to be expected, that is, 2.73–3.30 [mean value 2.93(19)] Å for potassium (both compounds included) and 2.83–3.52 [mean value 3.1(2)] Å for rubidium [the average values from the CSD are 2.81(15) and 2.99(12) Å]. The two carboxylic groups that are not bound to uranyl are not deprotonated, and they are involved in intermolecular hydrogen bonds with the water molecules or, for atom O8 in 4, in an intramolecular hydrogen bond with the proximal carboxylate group. Not all of the donor atoms of the ligand are complexed, because N2 and one or two protonated oxygen atoms are uncoordinated; however, each H2PZTC2- ligand is bound to as many as six (4 and 5) or seven (6) cations due to the numerous bridging atoms. The PZTC molecule is further from planarity than that in the previous complexes, with one or two carboxylic groups being much more tilted than the others, but the carboxylate containing O3 and O4 is nearly coplanar with the ring. The four dihedral angles between the carboxylic/ate groups and the aromatic ring are 9.8(5), 4.1(3), 83.7(2), and 8.3(3)° in 4, 17.0(2), 3.3(3), 87.7(1), and 52.6(1)° in 5, and 17.7(5), 2.9(7), 85.6(2), and 53.4(4)° in 6. In compound 4, a threedimensional framework is formed that does not present any channel of significant size. Tightly packed sheets parallel to the ab plane can be seen, which are linked to one another by the bonding of potassium to the more tilted carboxylic group

and by hydrogen bonding. In 5 and 6, the [UO2(H2PZTC)2]2units are connected by dimers of O4-bridged alkali-metal atoms to form one-dimensional linear subunits directed along the c axis (Figure 5). Each alkali-metal ion dimer is bound to the adjacent ones along the b axis by four O2 and O5 bridges, which generates two-dimensional layers parallel to the bc plane. This is the highest dimensionality resulting from coordination bonds in the potassium complex. The protonated carboxylic atoms O6 and O8 point outside on both sides of the layers, which are separated from one another by about 8 Å. Their involvement in hydrogen bonds with the solvent–water molecules (O10 and O11), which are themselves hydrogen-bond donors toward nitrogen and carboxylate oxygen atoms, results in an extended hydrogen-bonding network connecting the layers to one another. In complex 6, the interlayer Rb-O6 contact, 3.521(4) Å, is slightly smaller than K · · · O6 in 5, 3.5680(17) Å, whereas the Rb+ ionic radius is about 0.08 Å larger than that of K+.19 It can thus be quite confidently considered as a coordinative bond, albeit weak (which is not surprising because O6 is protonated and also involved in hydrogen bonding and also because the strength of this bond is likely limited by the value of the interlayer distance). In addition to the weak forces present in 5, these bonds turn complex 6 into a three-dimensional framework (Figure 6), with however a much stronger cohesion in the two-dimensional subunits than between the layers. Narrow channels of approximate section 5 × 4.5 Å

Pyrazinetetracarboxylic Acid as an Assembler Ligand

Crystal Growth & Design, Vol. 8, No. 5, 2008 1695 Scheme 1. Coordination Mode of PZTC4- in Compounds 2 and 3 and H2PZTC2- in Compounds 4 and 6

Figure 6. Top: View of one layer in complex 6. Bottom: The threedimensional framework viewed down the c axis. The uranium coordination polyhedra are in yellow and the rubidium atoms in green; the other atoms are shown as spheres of arbitrary radii; the solvent molecules and hydrogen atoms are omitted for clarity.

running along the c axis and occupied by the water molecules are apparent in the structure of 6. Conclusions To the best of our knowledge, the results reported herein represent the first attempts to use the tetra acid H4PZTC (and also its derivative H2PZDC), of potential high overall denticity, in the synthesis of UOFs. Five complexes with PZTC4- or H2PZTC2- have been synthesized and structurally characterized; two of them, 2 and the mixed uranyl/sodium complex 3, were obtained under hydrothermal conditions, whereas the other three, 4 and the isomorphous complexes 5 and 6, were obtained under more ordinary conditions. In all cases, the overall denticity of the ligand is high indeed, with four (2) to seven (6) cations bound to each molecule and all 10 nitrogen and oxygen atoms coordinated in 2 and 3. This demonstrates the very high potential of this ligand as an assembler and its particularly good adaptation to uranyl coordination due to the presence of two planar ONO sites. The four different coordination modes of the ligand (if the slight difference between 5 and 6 is disregarded) are represented in Scheme 1. In all cases, the uranyl ion occupies the ONO site of the ligand and, when present, the alkali metals are bound to oxygen atoms only (uranyl ions being coordinated by the lateral O2 sites only when the structure is free from alkali-

metal ions), thus illustrating the interest of polyfunctional ligands for the synthesis of mixed metal ion complexes.4e This situation is similar to that observed in the mixed uranyl/rubidium or cesium complexes of pyridine-2,6-dicarboxylate.4b Linear [UO2(PZTC)]n2n- subunits are present in complexes 2 and 3, which are further linked to one another either by uranyl ions or by [Na2(H2O)4]2+ dimers to give two- (2) or three-dimensional (3) frameworks. Oxo bonding of the alkali-metal ion, which is a very frequent occurrence in other families of complexes,6l,m,18 is present only in the potassium complex 4, in which each H2PZTC2- ligand is bound to one uranyl ion and five potassium ions, giving a quite compact three-dimensional arrangement. The situation is different in complexes 5 and 6, in which the [UO2(H2PZTC)2]2- moieties are assembled into chains by bridging alkali-metal ions, with further bridging giving rise to two- (5) or three-dimensional (6) assemblages. In compounds 4–6, the departure of the ligand from planarity, with one or two carboxylic groups much more tilted than in compounds 2 and 3, clearly contributes to the formation of three-dimensional species, as it appears in Figures 4 and 6. The high denticity of the ligand generally results in very compact assemblages with formation at best of narrow channels in 2 and 6. Clearly, PZTC does not seem well suited for the synthesis of highly porous UOFs. The differences between 3 and the three compounds 4-6 may be due, in part, to the different sizes of the alkali-metal ions, resulting in very different coordination numbers (6 for Na+, 9 for K+, and 10 for Rb+), but it may also arise from the different conditions used during the synthesis. Unfortunately, we did not succeed in growing crystals of any of the mixed uranyl/alkali-metal ion complexes under both sets of experimental conditions (hydrothermal synthesis in the presence of KOH or RbOH gives amorphous precipitates). Nevertheless, it can be noticed that complexes 4-6 present some features that could well result from the use of nonhydrothermal methods, which are the presence of two remaining carboxylic groups and of uncoordinated nitrogen and oxygen atoms, and the higher content in solvent–water molecules (the latter being generally quite low in hydrothermally synthesized UOFs). Notwithstanding these variations in synthesis, the complexes reported here demonstrate the quite exceptional assembling properties of H4PZTC and its usefulness (as well as that of H2PZDC) in the design of UOFs and hybrid metal-organic frameworks. Supporting Information Available: Tables of crystal data, atomic positions and displacement parameters, anisotropic displacement parameters, and bond lengths and bond angles in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.

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