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Robust building blocks for inorganic crystal engineering
Inorganica Chimica Acta 359 (2006) 1255–1262 www.elsevier.com/locate/ica
Robust building blocks for inorganic crystal engineering Christer B. Aakero¨y
a,*
, John Desper a, Brock Levin a, Jesu´s Valde´s-Martı´nez
b
a
b
Department of Chemistry, Kansas State University, Manhattan, KS 66506, USA Instituto de Quı´mica, Universidad Nacional Auto´noma de Me´xico, Circuito Exterior, Ciudad Universitaria, Coyoaca´n 04510, D.F., Mexico Received 11 August 2005; received in revised form 29 September 2005; accepted 29 September 2005 Available online 10 November 2005
Abstract We have prepared two ligands, 4- and 5-carboxylic acid pyrimidine, and synthesized and crystallographically characterized seven coordination complexes thereof. The need for potentially structurally disruptive counterions is eliminated by deprotonation of the carboxylic acid moieties; the carboxylates present in each structure act as counterions and balance the charge on the divalent metal ions leading to charge-neutral complex ions. The four new M(II)-complexes with 4-carboxylic acid pyrimidine (M = Ni, Cu, Zn, and Co) are isostructural as are the three M(II) complexes with 3-carboxylic acid pyrimidine (M = Ni, Cu, and Zn), indicating robust and reliable coordination modes for both ligands. Ó 2005 Elsevier B.V. All rights reserved. Keywords: X-ray crystal structures; Supramolecular chemistry; Hydrogen bonding; Pyrimidine carboxylate; Coordination complexes
1. Introduction Crystal engineering (supramolecular chemistry geared towards the solid state) has experienced intense activities in recent years [1], but it is still necessary to continue to explore correlations between intermolecular interactions and structural consequences in simple systems to better understand fundamental recognition and assembly processes [2]. In order to improve protocols for reliable intermolecular synthesis, it is particularly useful to have access to a wide variety of suitable bridging ligands or supramolecular connectors that allow for the directed assembly of individual building blocks into infinite architectures with desired topologies [3,4]. To this end, we report on the synthesis of two ligands, 5-carboxylic acid pyrimidine [5] (1), 4-carboxylic acid pyrimidine [6] (2) (Scheme 1), and the crystal structures of seven coordination complexes thereof. These multi-functional ligands are attractive for a variety of reasons. They are versatile enough to be used to form both coordination polymers [7], when coordination to *
Corresponding author. Tel.: +1 785 532 6665; fax: +1 785 532 6666. E-mail addresses: [email protected] (C.B. Aakero¨y), jvaldes@ servidor.unam.mx (J. Valde´s-Martı´nez). 0020-1693/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2005.09.051
more than one metal ion takes place, and coordination compounds linked together with hydrogen bonds [8] either carboxylic acid dimers or carboxylic acid pyrimidine O–H N hydrogen bonds. With these types of architectures, it is possible to use the intrinsic geometry of the metal ion, and the directionality imparted by the hydrogen bonds to neighboring ligands to generate infinite 1-D, 2-D, or 3-D networks [9]. By changing the substitution position of the carboxylic acid, it is also possible to control coordination modes. Ligand–metal interactions may take place via one or both of the heterocyclic nitrogen atoms and/or via the carboxylate. The change in substitution may also lead to differences in thermal stability and solubility that may be taken advantage of in well-planned structures. The resulting networks may be free of counterions if the ligand is deprotonated and a 2+ metal ion is used. This could lead to coordination polymers similar to those formed by picolinic acid [10], nicotinic acid [11] and isonicotinic acid [12]. However, if the carboxylic acids remain protonated, there is the potential to form coordination compounds linked together by hydrogen bonds. It is somewhat difficult to predict, a priori, what the exact hydrogen-bonded networks may look like. There is only
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C.B. Aakero¨y et al. / Inorganica Chimica Acta 359 (2006) 1255–1262
a
COOH N
COOH
b
N N
N
Scheme 1. (a) 4-Carboxylic acid pyrimidine (2) and (b) 5-carboxylic acid pyrimidine (1).
one hydrogen bond donor (carboxylic acid), but three hydrogen bond acceptors (two nitrogen atoms, carbonyl) and some, or all, of these acceptors may be involved in coordination to the metal. Also, coordination to a metal ion via one of the nitrogen atoms may deactivate the pyrimidine ring, rendering the other nitrogen atom effectively inert (in a supramolecular sense). Interplay between coordination site and number, ring electronics and hydrogen bond donating ability leads to a plethora of different possible structures. In order to obtain information about the preferred coordination modes of these ligands, we report on the crystal structures of seven coordination compounds. 2. Experimental Starting materials were purchased from Aldrich and used without further purification. Melting points were determined on a Fisher–Johns melting point apparatus and are uncorrected.
solid dissolved in 50 ml water. A second filtration was done to remove any remaining SeO2. Concentration of the aqueous solution gave 4-carboxylic acid pyrimidine. M.p. 226– 230 °C (7.9 g, 80%). 1H NMR (DMSO): 9.379 singlet (1H), 9.072 doublet (1H), 8.014 doublet (1H). IR 1700 cm 1 (C@O), 1393 cm 1, 1297 cm 1. 2.3. Synthesis of trans-nickel(II) tetraaqua-bis(pyrimidine5-carboxylate) (1a) 5-Carboxylic acid pyrimidine (35 mg, 0.28 mmol) was dissolved in ethanol and added to an aqueous solution of the NiCl2 Æ 6H2O (67 mg, 0.28 mmol). The solvent was allowed to evaporate slowly until green prisms precipitated (Fig. 1(a)). M.p. >300 °C. 2.4. Synthesis of trans-copper(II) tetraaqua-bis(pyrimidine5-carboxylate) (1b) To an ethanolic solution of 5-carboxylic acid pyrimidine (35 mg, 0.28 mmol) was added CuSO4 Æ 5H2O (71 mg, 0.28 mmol) in water. Slow evaporation of the solvent gave blue-green prisms of 1b (Fig. 1(b)). M.p. >300 °C. 2.5. Synthesis of trans-zinc(II) tetraaqua-bis(pyrimidine-5carboxylate) (1c) A solution of 5-carboxylic acid pyrimidine (35 mg, 0.28 mmol) in ethanol was allowed to react with ZnBr2
2.1. Synthesis of 5-carboxylic acid pyrimidine (1) A solution of 5-bromopyrimidine (2.1 g, 13 mmol) in dry THF (120 ml) was cooled to 110 °C using ethanol and liquid nitrogen. Pre-cooled butyllithium (12.5 ml, 1.6 M in hexanes, 20 mmol) was added dropwise to the solution and the resulting yellow solution was stirred 20 min at 110 °C. This solution was then poured into an ether solution containing dry ice. The mixture was allowed to warm to room temperature and hydrolyzed with water. The ether phase was extracted six times with water and then with 1 N sodium hydroxide. The combined aqueous layers were acidified to pH 2–3 with 6 N hydrochloric acid. Upon acidification, an off-white solid precipitated from the solution. The solid was collected via suction filtration and washed with water to afford 5-carboxylic acid pyrimidine. M.p. 267–270 °C. (1.13 g, 68%). 1H NMR (DMSO): 9.397 doublet (1H), 9.220 doublet (2H), IR: 1700 cm 1 (C@O), 1310 cm 1, 1593 cm 1. 2.2. Synthesis of 4-carboxylic acid pyrimidine (2) To a solution of 4-methylpyrimidine (7.5 g, 80 mmol) in pyridine (85 ml) was added SeO2 (13.4 g, 120 mmol). The solution was then stirred at 50–60 °C for 2 h and then at 80–85 °C for 3.5 h. The heating was stopped and the reaction allowed to reach ambient temperature overnight. The solution was filtered, then concentrated, and the resulting
Fig. 1. Thermal ellipsoid of 1a–d (50% probability level).
C.B. Aakero¨y et al. / Inorganica Chimica Acta 359 (2006) 1255–1262
(63 mg, 0.28 mmol) in water. As the solvent evaporated, colorless prisms of 1c precipitated (Fig. 1(c)). M.p. >300 °C. 2.6. Synthesis of trans-cobalt(II) tetraaqua-bis(pyrimidine5-carboxylate) (1d) An aqueous solution of CoCl2 Æ 6H2O (67 mg, 0.28 mmol) was mixed with a solution of 5-carboxylic acid pyrimidine (35 mg, 0.28 mmol) in ethanol. The resulting solution was allowed to evaporate until amber prisms precipitated (Fig. 1(d)). M.p. >300 °C. 2.7. Synthesis of trans-nickel(II) diaqua-bis(pyrimidine-4carboxylate)hydrate (2a) A mixture of 4-carboxylic acid pyrimidine (35 mg, 0.28 mmol) in ethanol and an aqueous solution of the NiCl2 Æ 6H2O (67 mg, 0.28 mmol) was allowed to evaporate. Green prisms of 2a suitable for single-crystal X-ray diffraction precipitated (Fig. 2(a)). M.p. >300 °C. 2.8. Synthesis of trans-copper(II) diaqua-bis(pyrimidine-4carboxylate)hydrate (2b) To an aqueous solution of CuSO4 Æ 5H2O (71 mg, 0.28 mmol) was added 4-carboxylic acid pyrimidine (35 mg, 0.28 mmol) in ethanol. Slow evaporation of the solvent afforded cyan prisms of 2b (Fig. 2(b)). M.p. >300 °C.
Fig. 2. Thermal ellipsoid plot of 2a–c (50% probability level).
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2.9. Synthesis of trans-zinc(II) diaqua-bis(pyrimidine-4carboxylate)hydrate (2c) A solution of 4-carboxylic acid pyrimidine (35 mg, 0.28 mmol) in ethanol and an aqueous solution of ZnBr2 (63 mg, 0.28 mmol) were mixed and the resulting solution allowed to evaporate. As the solvent was reduced, colorless prisms of 2c precipitated (Fig. 2(c)). M.p. >300 °C. 2.10. X-ray crystallography X-ray data were collected on a Bruker SMART 1000 four-circle CCD diffractometer using a fine-focus molybdenum Ka tube. Data were collected using SMART [13]. Initial cell constants were found by small widely separated ‘‘matrix’’ runs. Preliminary Laue´ symmetry was determined from axial images. Generally, an entire hemisphere of reciprocal space was collected regardless of Laue´ symmetry. Scan speed and scan width were chosen based on scattering power and peak rocking curves. Unless otherwise noted, 0.3° scans were used. Unit cell constants and orientation matrix were improved by least-squares refinement of reflections thresholded from the entire dataset. Integration was performed with SAINT [14], using this improved unit cell as a starting point. Precise unit cell constants were calculated in SAINT from the final merged dataset. Lorenz and polarization corrections were applied, and data were corrected for absorption using the multi-scan technique of SADABS. Laue´ symmetry, space group, and unit cell contents were found with XPREP. Data were reduced with SHELXTL [15]. The structures were solved in all cases by direct methods without incident. All C–H hydrogen atoms were assigned to idealized positions and were allowed to ride. ˚ . Hydrogen atom 1a: Data were truncated to 0.77 A coordinates for the two water molecules were allowed to refine. 1b: Hydrogen atom coordinates for the two water molecules were allowed to refine. Data were corrected for absorption. 1c: Hydrogen atom coordinates for the two water molecules were allowed to refine. Data were corrected for absorption. 1d: Hydrogen atom coordinates for the two water molecules were allowed to refine. Data were corrected for absorption. 2a: Hydrogen atom coordinates for the two water molecules were allowed to refine. Data were corrected for absorption. 2b: 0.2° wide scans were used. Hydrogen atom coordinates for the water molecule were allowed to refine. Data were corrected for absorption. 2c: Hydrogen atom coordinates for the two water molecules were allowed to refine. Data were corrected for absorption. Crystallographic data are given in Table 11, a summary of unit-cell dimensions in Tables 2 and 4, hydrogen-bond 1 CCDC 285333–285339 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif.
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Table 1 Crystallographic data for 1a–d and 2a–c 1a
1b
1c
1d
2a
2b
2c
Systematic name
tetraaquonickel(II) bis(pyrimidine5-carboxylate) (C5H3N2O2)2, Ni,(H2O)4 C10H14N4NiO8 376.96 green prism >300 monoclinic P2(1)/c, 2 6.5252(9) 12.2691(16) 8.5446(11) 90 97.137(2) 90 678.77(16) 1.844 203(2) 0.71073 1.483 0.025/0.012 2.92 28.25
tetraaquocopper(II) bis(pyrimidine5-carboxylate) (C5H3N2O2)2, Cu, (H2O)4 C10H14CuN4O8 381.79 green prism >300 monoclinic P2(1)/c, 2 6.2953(17) 12.384(3) 8.960(3) 90 100.809(5) 90 686.1(3) 1.848 203(2) 0.71073 1.645 0.035/0.019 2.84 28.16
tetraaquozinc(II) bis(pyrimidine5-carboxylate) (C5H3N2O2)2, Zn, (H2O)4 C10H14N4O8Zn 383.62 colorless prism >300 monoclinic P2(1)/c, 2 6.5417(12) 12.458(2) 8.5530(15) 90 97.711(4) 90 690.7(2) 1.845 203(2) 0.71073 1.831 0.062/0.024 2.91 28.55
tetraaquocobalt(II) bis(pyrimidine5-carboxylate) (C5H3N2O2)2, Co, (H2O)4 C10H14CoN4O8 377.18 amber prism >300 monoclinic P2(1)/c, 2 6.5374(10) 12.4346(19) 8.5370(13) 90 97.831(3) 90 687.50(18) 1.822 203(2) 0.71073 1.301 0.035/0.023 2.91 28.72
tetraaquonickel(II) bis(pyrimidine4-carboxylate) (C5H3N2O2)2, Ni,(H2O)4 C10H14N4NiO8 376.96 green prism >300 triclinic P 1, 1 5.6001(11) 7.1115(14) 9.4415(19) 81.162(3) 88.118(3) 70.054(3) 349.18(12) 1.793 203(2) 0.71073 1.441 0.027/0.010 2.18 28.21
diaquocopper(II) bis(pyrimidine4-carboxylate) (C5H3N2O2)2, Cu, (H2O)2 C10H10CuN4O6 345.76 aqua plate >300 triclinic P 1, 1 6.3199(7) 6.9486(8) 8.5504(10) 98.158(6) 104.302(7) 114.405(9) 318.42(6) 1.803 203(2) 0.71073 1.751 0.047/0.025 2.56 28.16
tetraaquozinc(II) bis(pyrimidine4-carboxylate) (C5H3N2O2)2, Zn, (H2O)4 C10H14N4O8Zn 383.62 colorless prism >300 triclinic P 1, 1 5.5916(12) 7.1526(14) 9.526(2) 81.242(4) 87.924(4) 70.466(4) 354.83(13) 1.795 203(2) 0.71073 1.782 0.0385/0.0117 2.16 28.28
4653 1596 1410 >2r(I) 0.0266 0.0802
4554 1558 1476 >2r(I) 0.0242 0.0694
4828 1610 1510 >2r(I) 0.0225 0.0639
4723 1610 1431 >2r(I) 0.0266 0.0734
2375 1537 1506 >2r(I) 0.023 0.0636
2390 1431 1417 >2r(I) 0.0371 0.097
2463 1578 1528 >2r(I) 0.0289 0.0793
Formula moiety Empirical formula Molecular weight Color, habit Melting point (°) Crystal system Space group, Z ˚) a (A ˚) b (A ˚) c (A a (°) b (°) c (°) ˚ 3) Volume (A Density (g/cm3) Temperature (K) X-ray wavelength l (mm 1) Rmerg before/after Hmin (°) Hmax (°) Reflections Collected Independent Observed Threshold expression R1(observed) wR2(all)
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Code
C.B. Aakero¨y et al. / Inorganica Chimica Acta 359 (2006) 1255–1262 Table 2 Unit cell parameters for 1a–d a
b
6.532(2) 12.305(4) 6.2953(17) 12.384(3) 6.5417(12) 12.458(2) 6.5374(10) 12.4346(19) ˚ Distances in A and angles in °. 1a 1b 1c 1d
c
b
Volume
8.562(3) 8.960(3) 8.5530(15) 8.5370(13)
97.060(7) 100.809(5) 97.711(4) 97.831(3)
683.0(4) 686.1(3) 690.7(2) 687.50(18)
geometries are provided in Table 3. Thermal ellipsoids and labeling schemes are shown in Figs. 1(a)–(d) and 2(a)–(c). 3. Results Compounds 1a–d are all isostructural. In each case, the metal-ion is in a six-coordinate octahedral geometry. There are two anionic pyrimidine-based ligands per metal, both
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coordinated in the axial position with four water molecules in the plane. The H2O M bond lengths are, in the case of ˚. the Ni, Zn, and Co complexes, very similar, 2.03–2.07 A However, the copper(II) complex, 1b, displays significant Jahn–Teller distortion with Cu O distances of 1.97 and ˚ , respectively. The pyrimidine ligand is coordinated 2.29 A to one metal center via a single M–N bond, whereas the second nitrogen atom does not participate in coordination to a metal ion. It does, however, accept a hydrogen bond from one of the coordinated water molecules (Fig. 3). The M N bond distances in this series of complexes 1a– d, do not display any unexpected distances and fall in the ˚ . In each case, the carboxylic acid moiety range 2.04–2.19 A is deprotonated, which means that the ligand also serves as a counterion and the overall complex ion is neutral. The four coordinated water molecules are engaged in an extensive network of hydrogen bonds.
Table 3 ˚ and ° Hydrogen bond parameters for 1a–d and 2a–c in A D–H A
d(H A)
d(D A)
\(DHA)
Symmetry transformations used to generate equivalent atoms:
1a
O(1)–H(1A) N(3)#2 O(1)–H(1B) O(4)#3 O(2)–H(2A) O(3)#4 O(2)–H(2B) O(4)#5
2.10(3) 1.87(2) 1.99(3) 1.91(3)
2.852(2) 2.692(2) 2.656(2) 2.743(2)
161(2) 174(3) 177(3) 169(2)
#2: x + 1, y + 1/2, z + 1/2 #3: x 1, y 1/2, z + 1/2 #4: x + 2, y, z #5: x, y 1/2, z + 1/2
1b
O(1)–H(1A) N(3)#2 O(1)–H(1B) O(4)#3 O(2)–H(2A) O(3)#4 O(2)–H(2B) O(4)#5
2.08 1.96 1.82 1.9
2.8839(18) 2.7332(16) 2.6226(17) 2.7048(16)
164.4 154.1 163.8 162.9
#2: x + 1, y + 1/2, z + 3/2 #3: x 1, y + 1/2, z + 1/2 #4: x + 2, y + 1, z + 1 #5: x, y + 1/2, z + 1/2
1c
O(1)–H(1A) N(3)#2 O(1)–H(1B) O(4)#3 O(2)–H(2A) O(3)#4 O(2)–H(2B) O(4)#5
2.05 1.9 1.85 1.93
2.8471(16) 2.6715(15) 2.6498(16) 2.7280(15)
161.4 154.2 162.3 159.8
#2: x + 1, y + 1/2, z + 3/2 #3: x 1, y + 1/2, z + 1/2 #4: x + 2, y + 1, z + 1 #5: x, y + 1/2, z + 1/2
1d
O(1)–H(1B) O(4)#2 O(1)–H(1A) N(3)#3 O(2)–H(2B) O(4)#4 O(2)–H(2A) O(3)#5
1.9 2.07 1.94 1.84
2.6724(16) 2.8363(18) 2.7280(17) 2.6428(18)
154.2 154.2 159.1 162.2
#2: x 1, y + 1/2, z + 1/2 #3: x + 1, y + 1/2, z + 3/2 #4: x, y + 1/2, z + 1/2 #5: x + 2, y + 1, z + 1
2a
O(1S)–H(1A) O(2S) O(1S)–H(1B) N(1)#2 O(2S)–H(2A) O(2)#3 O(2S)–H(2B) O(2)#4
1.96(2) 2.02(2) 2.10(2) 1.97(2)
2.7345(17) 2.8223(16) 2.8448(17) 2.8098(16)
171(2) 171.6(19) 171(2) 178(2)
#2: x + 1, y 1, z #3: x + 1, y + 1, #4: x 1, y, z
O(3)–H(3A) N(1)#2 O(3)–H(3B) O(4) O(4)–H(4A) O(2)#3 O(4)–H(4B) O(2)#4
2.11(3) 2.10(2) 2.16(3) 1.98(3)
2.8885(17) 2.8026(19) 2.8788(17) 2.8355(18)
172(2) 170(2) 166(3) 174(2)
O(1S)–H(1A) O(2S) O(1S)–H(1B) N(1)#2 O(2S)–H(2A) O(2)#3 O(2S)–H(2B) O(2)#4
2.00(3) 2.00(3) 2.04(4) 2.08(3)
2.746(2) 2.831(2) 2.825(2) 2.850(2)
172(3) 169(3) 177(3) 169(3)
2b
2c
#2: x + 1, y
z
1, z
#3: x, y, z #4: x 1, y, z
1
#2: x + 1, y 1, z #3: x 1, y, z #4: x + 1, y + 1,
z
Table 4 Unit cell data for 2a–c a 5.6001(11) 5.6470(12) 5.5916(12) ˚ and angles in °. Distances in A 2a 2b 2c
b
c
a
b
c
Volume
7.1115(14) 6.9889(15) 7.1526(14)
9.4415(19) 9.622(2) 9.526(2)
81.162(3) 78.914(4) 81.242(4)
88.118(3) 86.973(4) 87.924(4)
70.054(3) 71.370(3) 70.466(4)
349.18(12) 353.11(13) 354.83(13)
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Fig. 3. Extended hydrogen-bonded network of complex ions in 1a (1b–d all display the same intermolecular structures).
Fig. 4. Arrays of complex ions in 2c (2a and b display the same intermolecular structures) interlinked by hydrogen-bonds to non-coordinated water molecules.
Compounds 2a–c are all isostructural. However, the coordination chemistry in this series of complexes is very different compared to that found in 1a–d due to the positioning of the carboxylic acid moiety with respect to the nearest nitrogen atom in the ring. The combination of a carboxylate group with N(3) creates a chelating ligand that subsequently results in di-aqua-bis-chelated metal complex (Fig. 4). The bond distances between the metal ions in 2a–c and the chelating ligand are unremarkable. The two coordinated water molecules occupy the axial positions in all three complexes. Again, H2O M bond lengths are very ˚, similar for the Ni and Zn complexes, 2.04 and 2.07 A respectively, whereas the Cu complex is heavily distorted ˚. with a H2O M bond distance of 2.44 A 4. Discussion The ligand 5-carboxylic acid pyrimidine coordinates to the metal ion via the nitrogen atom every time, and each complex is overall neutral with a consistent formula [ML2(H2O)4]. The predictability in coordination mode of
1 is a very encouraging discovery as it may provide opportunities for building extended inorganic–organic networks in a predictable manner. There has only been one exception to the coordination mode of 1 reported herein – the carboxylate moiety is then acting as a bridge between two metal ions in a dinuclear Co-complex [16]. 4-Carboxylic acid pyrimidine is deprotonated and acts as an effective chelating ligand. Due to the 1 charge, there is no need for a counterion with these divalent metal ions. The remaining nitrogen atom on the ligand does not participate in any coordination chemistry, but it does act as a hydrogen-bond acceptor for a non-coordinated water molecule. The two coordinated water molecules give rise to a complicated hydrogen-bonded network that also includes the non-coordinated oxygen atom of the carboxylate moiety. The behavior of this ligand is similar to that of the pyrazine carboxylates (Scheme 2) in which there is only one example (out of a total of nine crystal structures) where both nitrogen atoms are simultaneously involved in ligand–metal coordination [17]. This study provides an opportunity to explore the effect substitution has on the coordination mode of the ligand. Comparing ligands 1 and 2 to related pyridine-based carboxylates shows a variety of coordination possibilities (Graph 1) [17]. Nicotinate and isonicotinate have the ability to coordinate via only the nitrogen atom, or to two metal ions, one via the nitrogen atom and the other via an oxygen atom from the carboxylate moiety [18]. When using the oxophilic Ni ion, coordination can take place only via an oxygen atom, with the nitrogen atom remaining uncoordinated. Picolinate no doubt owes some of its popularity as a ligand to its predictability. It coordinates to a metal ion via the N and O chelate every time. It was expected that the 5-carboxylic acid pyrimidine would coordinate in a manner similar to nicotinic acid and isonicotinic acid, while 4-carboxylic acid pyrimidine would coordinate like picolinic acid, forming the chelate through both the heterocyclic nitrogen atom in the 3 position of the ring and one of the oxygen atoms of the carboxylate/carboxylic acid (Scheme 3). In fact, 2 does behave exactly as expected, forming the chelate with the metal ions every time. However, 1 shows a surprising amount of predictability, coordinating in a mono-hapto fashion via a pyrimidine nitrogen atom every time. Given that isonicotinate and nicotinate coordinate through the carboxylate frequently, it is somewhat unexpected that 1 shows no coordination via its carboxylate
N O
N O
Scheme 2. Pyrazine carboxylic acid with a chelating nitrogen atom– carboxylate binding site and an additional heterocyclic nitrogen atom binding site.
C.B. Aakero¨y et al. / Inorganica Chimica Acta 359 (2006) 1255–1262
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Graph 1. Summary of coordination site for pyridine and pyrimidine carboxylates with respect to Co, Ni, Cu, and Zn ions.
O
O
M
O
O
M
O
O
a
N
N
N
M
acting with M(II) ions, and as their coordination modes robust and reliable, we are currently pursuing the supramolecular chemistry of these ligands by replacing coordinated water molecules with linear connectors such that the assembly of complex ions can be controlled.
M
Acknowledgment O b
O
N M
O
c
N
O
N
We are grateful for generous financial support from the National Science Foundation (CHE-0078996).
M
References M O
O
O
O
N
N
N
N
M O
O
N
N
d
M
M
Scheme 3. Some possible coordination modes for (a) nicotinate/isonicotinate, (b) picolinate, (c) 4-carboxylate pyrimidine and (d) 5-carboxylate pyrimidine.
moiety. It should be noted that our sample size is very small (only three structures), so different coordination modes may well arise under different conditions or with different metal ions. As a comparison, when pyrimidine-based ligands (excluding fused-ring systems and chelating ligands) are used in coordination chemistry simultaneous coordination to both nitrogen atoms takes place less than 30% of the time. It is therefore not that surprising that 1 and 2 coordinate via only one nitrogen atom in these structures. Since ligands 1 and 2 do not require the presence of potentially structurally disruptive counterions when inter-
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Robust building blocks for inorganic crystal engineering Christer B. Aakero¨y
a,*
, John Desper a, Brock Levin a, Jesu´s Valde´s-Martı´nez
b
a
b
Department of Chemistry, Kansas State University, Manhattan, KS 66506, USA Instituto de Quı´mica, Universidad Nacional Auto´noma de Me´xico, Circuito Exterior, Ciudad Universitaria, Coyoaca´n 04510, D.F., Mexico Received 11 August 2005; received in revised form 29 September 2005; accepted 29 September 2005 Available online 10 November 2005
Abstract We have prepared two ligands, 4- and 5-carboxylic acid pyrimidine, and synthesized and crystallographically characterized seven coordination complexes thereof. The need for potentially structurally disruptive counterions is eliminated by deprotonation of the carboxylic acid moieties; the carboxylates present in each structure act as counterions and balance the charge on the divalent metal ions leading to charge-neutral complex ions. The four new M(II)-complexes with 4-carboxylic acid pyrimidine (M = Ni, Cu, Zn, and Co) are isostructural as are the three M(II) complexes with 3-carboxylic acid pyrimidine (M = Ni, Cu, and Zn), indicating robust and reliable coordination modes for both ligands. Ó 2005 Elsevier B.V. All rights reserved. Keywords: X-ray crystal structures; Supramolecular chemistry; Hydrogen bonding; Pyrimidine carboxylate; Coordination complexes
1. Introduction Crystal engineering (supramolecular chemistry geared towards the solid state) has experienced intense activities in recent years [1], but it is still necessary to continue to explore correlations between intermolecular interactions and structural consequences in simple systems to better understand fundamental recognition and assembly processes [2]. In order to improve protocols for reliable intermolecular synthesis, it is particularly useful to have access to a wide variety of suitable bridging ligands or supramolecular connectors that allow for the directed assembly of individual building blocks into infinite architectures with desired topologies [3,4]. To this end, we report on the synthesis of two ligands, 5-carboxylic acid pyrimidine [5] (1), 4-carboxylic acid pyrimidine [6] (2) (Scheme 1), and the crystal structures of seven coordination complexes thereof. These multi-functional ligands are attractive for a variety of reasons. They are versatile enough to be used to form both coordination polymers [7], when coordination to *
Corresponding author. Tel.: +1 785 532 6665; fax: +1 785 532 6666. E-mail addresses: [email protected] (C.B. Aakero¨y), jvaldes@ servidor.unam.mx (J. Valde´s-Martı´nez). 0020-1693/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2005.09.051
more than one metal ion takes place, and coordination compounds linked together with hydrogen bonds [8] either carboxylic acid dimers or carboxylic acid pyrimidine O–H N hydrogen bonds. With these types of architectures, it is possible to use the intrinsic geometry of the metal ion, and the directionality imparted by the hydrogen bonds to neighboring ligands to generate infinite 1-D, 2-D, or 3-D networks [9]. By changing the substitution position of the carboxylic acid, it is also possible to control coordination modes. Ligand–metal interactions may take place via one or both of the heterocyclic nitrogen atoms and/or via the carboxylate. The change in substitution may also lead to differences in thermal stability and solubility that may be taken advantage of in well-planned structures. The resulting networks may be free of counterions if the ligand is deprotonated and a 2+ metal ion is used. This could lead to coordination polymers similar to those formed by picolinic acid [10], nicotinic acid [11] and isonicotinic acid [12]. However, if the carboxylic acids remain protonated, there is the potential to form coordination compounds linked together by hydrogen bonds. It is somewhat difficult to predict, a priori, what the exact hydrogen-bonded networks may look like. There is only
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a
COOH N
COOH
b
N N
N
Scheme 1. (a) 4-Carboxylic acid pyrimidine (2) and (b) 5-carboxylic acid pyrimidine (1).
one hydrogen bond donor (carboxylic acid), but three hydrogen bond acceptors (two nitrogen atoms, carbonyl) and some, or all, of these acceptors may be involved in coordination to the metal. Also, coordination to a metal ion via one of the nitrogen atoms may deactivate the pyrimidine ring, rendering the other nitrogen atom effectively inert (in a supramolecular sense). Interplay between coordination site and number, ring electronics and hydrogen bond donating ability leads to a plethora of different possible structures. In order to obtain information about the preferred coordination modes of these ligands, we report on the crystal structures of seven coordination compounds. 2. Experimental Starting materials were purchased from Aldrich and used without further purification. Melting points were determined on a Fisher–Johns melting point apparatus and are uncorrected.
solid dissolved in 50 ml water. A second filtration was done to remove any remaining SeO2. Concentration of the aqueous solution gave 4-carboxylic acid pyrimidine. M.p. 226– 230 °C (7.9 g, 80%). 1H NMR (DMSO): 9.379 singlet (1H), 9.072 doublet (1H), 8.014 doublet (1H). IR 1700 cm 1 (C@O), 1393 cm 1, 1297 cm 1. 2.3. Synthesis of trans-nickel(II) tetraaqua-bis(pyrimidine5-carboxylate) (1a) 5-Carboxylic acid pyrimidine (35 mg, 0.28 mmol) was dissolved in ethanol and added to an aqueous solution of the NiCl2 Æ 6H2O (67 mg, 0.28 mmol). The solvent was allowed to evaporate slowly until green prisms precipitated (Fig. 1(a)). M.p. >300 °C. 2.4. Synthesis of trans-copper(II) tetraaqua-bis(pyrimidine5-carboxylate) (1b) To an ethanolic solution of 5-carboxylic acid pyrimidine (35 mg, 0.28 mmol) was added CuSO4 Æ 5H2O (71 mg, 0.28 mmol) in water. Slow evaporation of the solvent gave blue-green prisms of 1b (Fig. 1(b)). M.p. >300 °C. 2.5. Synthesis of trans-zinc(II) tetraaqua-bis(pyrimidine-5carboxylate) (1c) A solution of 5-carboxylic acid pyrimidine (35 mg, 0.28 mmol) in ethanol was allowed to react with ZnBr2
2.1. Synthesis of 5-carboxylic acid pyrimidine (1) A solution of 5-bromopyrimidine (2.1 g, 13 mmol) in dry THF (120 ml) was cooled to 110 °C using ethanol and liquid nitrogen. Pre-cooled butyllithium (12.5 ml, 1.6 M in hexanes, 20 mmol) was added dropwise to the solution and the resulting yellow solution was stirred 20 min at 110 °C. This solution was then poured into an ether solution containing dry ice. The mixture was allowed to warm to room temperature and hydrolyzed with water. The ether phase was extracted six times with water and then with 1 N sodium hydroxide. The combined aqueous layers were acidified to pH 2–3 with 6 N hydrochloric acid. Upon acidification, an off-white solid precipitated from the solution. The solid was collected via suction filtration and washed with water to afford 5-carboxylic acid pyrimidine. M.p. 267–270 °C. (1.13 g, 68%). 1H NMR (DMSO): 9.397 doublet (1H), 9.220 doublet (2H), IR: 1700 cm 1 (C@O), 1310 cm 1, 1593 cm 1. 2.2. Synthesis of 4-carboxylic acid pyrimidine (2) To a solution of 4-methylpyrimidine (7.5 g, 80 mmol) in pyridine (85 ml) was added SeO2 (13.4 g, 120 mmol). The solution was then stirred at 50–60 °C for 2 h and then at 80–85 °C for 3.5 h. The heating was stopped and the reaction allowed to reach ambient temperature overnight. The solution was filtered, then concentrated, and the resulting
Fig. 1. Thermal ellipsoid of 1a–d (50% probability level).
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(63 mg, 0.28 mmol) in water. As the solvent evaporated, colorless prisms of 1c precipitated (Fig. 1(c)). M.p. >300 °C. 2.6. Synthesis of trans-cobalt(II) tetraaqua-bis(pyrimidine5-carboxylate) (1d) An aqueous solution of CoCl2 Æ 6H2O (67 mg, 0.28 mmol) was mixed with a solution of 5-carboxylic acid pyrimidine (35 mg, 0.28 mmol) in ethanol. The resulting solution was allowed to evaporate until amber prisms precipitated (Fig. 1(d)). M.p. >300 °C. 2.7. Synthesis of trans-nickel(II) diaqua-bis(pyrimidine-4carboxylate)hydrate (2a) A mixture of 4-carboxylic acid pyrimidine (35 mg, 0.28 mmol) in ethanol and an aqueous solution of the NiCl2 Æ 6H2O (67 mg, 0.28 mmol) was allowed to evaporate. Green prisms of 2a suitable for single-crystal X-ray diffraction precipitated (Fig. 2(a)). M.p. >300 °C. 2.8. Synthesis of trans-copper(II) diaqua-bis(pyrimidine-4carboxylate)hydrate (2b) To an aqueous solution of CuSO4 Æ 5H2O (71 mg, 0.28 mmol) was added 4-carboxylic acid pyrimidine (35 mg, 0.28 mmol) in ethanol. Slow evaporation of the solvent afforded cyan prisms of 2b (Fig. 2(b)). M.p. >300 °C.
Fig. 2. Thermal ellipsoid plot of 2a–c (50% probability level).
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2.9. Synthesis of trans-zinc(II) diaqua-bis(pyrimidine-4carboxylate)hydrate (2c) A solution of 4-carboxylic acid pyrimidine (35 mg, 0.28 mmol) in ethanol and an aqueous solution of ZnBr2 (63 mg, 0.28 mmol) were mixed and the resulting solution allowed to evaporate. As the solvent was reduced, colorless prisms of 2c precipitated (Fig. 2(c)). M.p. >300 °C. 2.10. X-ray crystallography X-ray data were collected on a Bruker SMART 1000 four-circle CCD diffractometer using a fine-focus molybdenum Ka tube. Data were collected using SMART [13]. Initial cell constants were found by small widely separated ‘‘matrix’’ runs. Preliminary Laue´ symmetry was determined from axial images. Generally, an entire hemisphere of reciprocal space was collected regardless of Laue´ symmetry. Scan speed and scan width were chosen based on scattering power and peak rocking curves. Unless otherwise noted, 0.3° scans were used. Unit cell constants and orientation matrix were improved by least-squares refinement of reflections thresholded from the entire dataset. Integration was performed with SAINT [14], using this improved unit cell as a starting point. Precise unit cell constants were calculated in SAINT from the final merged dataset. Lorenz and polarization corrections were applied, and data were corrected for absorption using the multi-scan technique of SADABS. Laue´ symmetry, space group, and unit cell contents were found with XPREP. Data were reduced with SHELXTL [15]. The structures were solved in all cases by direct methods without incident. All C–H hydrogen atoms were assigned to idealized positions and were allowed to ride. ˚ . Hydrogen atom 1a: Data were truncated to 0.77 A coordinates for the two water molecules were allowed to refine. 1b: Hydrogen atom coordinates for the two water molecules were allowed to refine. Data were corrected for absorption. 1c: Hydrogen atom coordinates for the two water molecules were allowed to refine. Data were corrected for absorption. 1d: Hydrogen atom coordinates for the two water molecules were allowed to refine. Data were corrected for absorption. 2a: Hydrogen atom coordinates for the two water molecules were allowed to refine. Data were corrected for absorption. 2b: 0.2° wide scans were used. Hydrogen atom coordinates for the water molecule were allowed to refine. Data were corrected for absorption. 2c: Hydrogen atom coordinates for the two water molecules were allowed to refine. Data were corrected for absorption. Crystallographic data are given in Table 11, a summary of unit-cell dimensions in Tables 2 and 4, hydrogen-bond 1 CCDC 285333–285339 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif.
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Table 1 Crystallographic data for 1a–d and 2a–c 1a
1b
1c
1d
2a
2b
2c
Systematic name
tetraaquonickel(II) bis(pyrimidine5-carboxylate) (C5H3N2O2)2, Ni,(H2O)4 C10H14N4NiO8 376.96 green prism >300 monoclinic P2(1)/c, 2 6.5252(9) 12.2691(16) 8.5446(11) 90 97.137(2) 90 678.77(16) 1.844 203(2) 0.71073 1.483 0.025/0.012 2.92 28.25
tetraaquocopper(II) bis(pyrimidine5-carboxylate) (C5H3N2O2)2, Cu, (H2O)4 C10H14CuN4O8 381.79 green prism >300 monoclinic P2(1)/c, 2 6.2953(17) 12.384(3) 8.960(3) 90 100.809(5) 90 686.1(3) 1.848 203(2) 0.71073 1.645 0.035/0.019 2.84 28.16
tetraaquozinc(II) bis(pyrimidine5-carboxylate) (C5H3N2O2)2, Zn, (H2O)4 C10H14N4O8Zn 383.62 colorless prism >300 monoclinic P2(1)/c, 2 6.5417(12) 12.458(2) 8.5530(15) 90 97.711(4) 90 690.7(2) 1.845 203(2) 0.71073 1.831 0.062/0.024 2.91 28.55
tetraaquocobalt(II) bis(pyrimidine5-carboxylate) (C5H3N2O2)2, Co, (H2O)4 C10H14CoN4O8 377.18 amber prism >300 monoclinic P2(1)/c, 2 6.5374(10) 12.4346(19) 8.5370(13) 90 97.831(3) 90 687.50(18) 1.822 203(2) 0.71073 1.301 0.035/0.023 2.91 28.72
tetraaquonickel(II) bis(pyrimidine4-carboxylate) (C5H3N2O2)2, Ni,(H2O)4 C10H14N4NiO8 376.96 green prism >300 triclinic P 1, 1 5.6001(11) 7.1115(14) 9.4415(19) 81.162(3) 88.118(3) 70.054(3) 349.18(12) 1.793 203(2) 0.71073 1.441 0.027/0.010 2.18 28.21
diaquocopper(II) bis(pyrimidine4-carboxylate) (C5H3N2O2)2, Cu, (H2O)2 C10H10CuN4O6 345.76 aqua plate >300 triclinic P 1, 1 6.3199(7) 6.9486(8) 8.5504(10) 98.158(6) 104.302(7) 114.405(9) 318.42(6) 1.803 203(2) 0.71073 1.751 0.047/0.025 2.56 28.16
tetraaquozinc(II) bis(pyrimidine4-carboxylate) (C5H3N2O2)2, Zn, (H2O)4 C10H14N4O8Zn 383.62 colorless prism >300 triclinic P 1, 1 5.5916(12) 7.1526(14) 9.526(2) 81.242(4) 87.924(4) 70.466(4) 354.83(13) 1.795 203(2) 0.71073 1.782 0.0385/0.0117 2.16 28.28
4653 1596 1410 >2r(I) 0.0266 0.0802
4554 1558 1476 >2r(I) 0.0242 0.0694
4828 1610 1510 >2r(I) 0.0225 0.0639
4723 1610 1431 >2r(I) 0.0266 0.0734
2375 1537 1506 >2r(I) 0.023 0.0636
2390 1431 1417 >2r(I) 0.0371 0.097
2463 1578 1528 >2r(I) 0.0289 0.0793
Formula moiety Empirical formula Molecular weight Color, habit Melting point (°) Crystal system Space group, Z ˚) a (A ˚) b (A ˚) c (A a (°) b (°) c (°) ˚ 3) Volume (A Density (g/cm3) Temperature (K) X-ray wavelength l (mm 1) Rmerg before/after Hmin (°) Hmax (°) Reflections Collected Independent Observed Threshold expression R1(observed) wR2(all)
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Code
C.B. Aakero¨y et al. / Inorganica Chimica Acta 359 (2006) 1255–1262 Table 2 Unit cell parameters for 1a–d a
b
6.532(2) 12.305(4) 6.2953(17) 12.384(3) 6.5417(12) 12.458(2) 6.5374(10) 12.4346(19) ˚ Distances in A and angles in °. 1a 1b 1c 1d
c
b
Volume
8.562(3) 8.960(3) 8.5530(15) 8.5370(13)
97.060(7) 100.809(5) 97.711(4) 97.831(3)
683.0(4) 686.1(3) 690.7(2) 687.50(18)
geometries are provided in Table 3. Thermal ellipsoids and labeling schemes are shown in Figs. 1(a)–(d) and 2(a)–(c). 3. Results Compounds 1a–d are all isostructural. In each case, the metal-ion is in a six-coordinate octahedral geometry. There are two anionic pyrimidine-based ligands per metal, both
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coordinated in the axial position with four water molecules in the plane. The H2O M bond lengths are, in the case of ˚. the Ni, Zn, and Co complexes, very similar, 2.03–2.07 A However, the copper(II) complex, 1b, displays significant Jahn–Teller distortion with Cu O distances of 1.97 and ˚ , respectively. The pyrimidine ligand is coordinated 2.29 A to one metal center via a single M–N bond, whereas the second nitrogen atom does not participate in coordination to a metal ion. It does, however, accept a hydrogen bond from one of the coordinated water molecules (Fig. 3). The M N bond distances in this series of complexes 1a– d, do not display any unexpected distances and fall in the ˚ . In each case, the carboxylic acid moiety range 2.04–2.19 A is deprotonated, which means that the ligand also serves as a counterion and the overall complex ion is neutral. The four coordinated water molecules are engaged in an extensive network of hydrogen bonds.
Table 3 ˚ and ° Hydrogen bond parameters for 1a–d and 2a–c in A D–H A
d(H A)
d(D A)
\(DHA)
Symmetry transformations used to generate equivalent atoms:
1a
O(1)–H(1A) N(3)#2 O(1)–H(1B) O(4)#3 O(2)–H(2A) O(3)#4 O(2)–H(2B) O(4)#5
2.10(3) 1.87(2) 1.99(3) 1.91(3)
2.852(2) 2.692(2) 2.656(2) 2.743(2)
161(2) 174(3) 177(3) 169(2)
#2: x + 1, y + 1/2, z + 1/2 #3: x 1, y 1/2, z + 1/2 #4: x + 2, y, z #5: x, y 1/2, z + 1/2
1b
O(1)–H(1A) N(3)#2 O(1)–H(1B) O(4)#3 O(2)–H(2A) O(3)#4 O(2)–H(2B) O(4)#5
2.08 1.96 1.82 1.9
2.8839(18) 2.7332(16) 2.6226(17) 2.7048(16)
164.4 154.1 163.8 162.9
#2: x + 1, y + 1/2, z + 3/2 #3: x 1, y + 1/2, z + 1/2 #4: x + 2, y + 1, z + 1 #5: x, y + 1/2, z + 1/2
1c
O(1)–H(1A) N(3)#2 O(1)–H(1B) O(4)#3 O(2)–H(2A) O(3)#4 O(2)–H(2B) O(4)#5
2.05 1.9 1.85 1.93
2.8471(16) 2.6715(15) 2.6498(16) 2.7280(15)
161.4 154.2 162.3 159.8
#2: x + 1, y + 1/2, z + 3/2 #3: x 1, y + 1/2, z + 1/2 #4: x + 2, y + 1, z + 1 #5: x, y + 1/2, z + 1/2
1d
O(1)–H(1B) O(4)#2 O(1)–H(1A) N(3)#3 O(2)–H(2B) O(4)#4 O(2)–H(2A) O(3)#5
1.9 2.07 1.94 1.84
2.6724(16) 2.8363(18) 2.7280(17) 2.6428(18)
154.2 154.2 159.1 162.2
#2: x 1, y + 1/2, z + 1/2 #3: x + 1, y + 1/2, z + 3/2 #4: x, y + 1/2, z + 1/2 #5: x + 2, y + 1, z + 1
2a
O(1S)–H(1A) O(2S) O(1S)–H(1B) N(1)#2 O(2S)–H(2A) O(2)#3 O(2S)–H(2B) O(2)#4
1.96(2) 2.02(2) 2.10(2) 1.97(2)
2.7345(17) 2.8223(16) 2.8448(17) 2.8098(16)
171(2) 171.6(19) 171(2) 178(2)
#2: x + 1, y 1, z #3: x + 1, y + 1, #4: x 1, y, z
O(3)–H(3A) N(1)#2 O(3)–H(3B) O(4) O(4)–H(4A) O(2)#3 O(4)–H(4B) O(2)#4
2.11(3) 2.10(2) 2.16(3) 1.98(3)
2.8885(17) 2.8026(19) 2.8788(17) 2.8355(18)
172(2) 170(2) 166(3) 174(2)
O(1S)–H(1A) O(2S) O(1S)–H(1B) N(1)#2 O(2S)–H(2A) O(2)#3 O(2S)–H(2B) O(2)#4
2.00(3) 2.00(3) 2.04(4) 2.08(3)
2.746(2) 2.831(2) 2.825(2) 2.850(2)
172(3) 169(3) 177(3) 169(3)
2b
2c
#2: x + 1, y
z
1, z
#3: x, y, z #4: x 1, y, z
1
#2: x + 1, y 1, z #3: x 1, y, z #4: x + 1, y + 1,
z
Table 4 Unit cell data for 2a–c a 5.6001(11) 5.6470(12) 5.5916(12) ˚ and angles in °. Distances in A 2a 2b 2c
b
c
a
b
c
Volume
7.1115(14) 6.9889(15) 7.1526(14)
9.4415(19) 9.622(2) 9.526(2)
81.162(3) 78.914(4) 81.242(4)
88.118(3) 86.973(4) 87.924(4)
70.054(3) 71.370(3) 70.466(4)
349.18(12) 353.11(13) 354.83(13)
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Fig. 3. Extended hydrogen-bonded network of complex ions in 1a (1b–d all display the same intermolecular structures).
Fig. 4. Arrays of complex ions in 2c (2a and b display the same intermolecular structures) interlinked by hydrogen-bonds to non-coordinated water molecules.
Compounds 2a–c are all isostructural. However, the coordination chemistry in this series of complexes is very different compared to that found in 1a–d due to the positioning of the carboxylic acid moiety with respect to the nearest nitrogen atom in the ring. The combination of a carboxylate group with N(3) creates a chelating ligand that subsequently results in di-aqua-bis-chelated metal complex (Fig. 4). The bond distances between the metal ions in 2a–c and the chelating ligand are unremarkable. The two coordinated water molecules occupy the axial positions in all three complexes. Again, H2O M bond lengths are very ˚, similar for the Ni and Zn complexes, 2.04 and 2.07 A respectively, whereas the Cu complex is heavily distorted ˚. with a H2O M bond distance of 2.44 A 4. Discussion The ligand 5-carboxylic acid pyrimidine coordinates to the metal ion via the nitrogen atom every time, and each complex is overall neutral with a consistent formula [ML2(H2O)4]. The predictability in coordination mode of
1 is a very encouraging discovery as it may provide opportunities for building extended inorganic–organic networks in a predictable manner. There has only been one exception to the coordination mode of 1 reported herein – the carboxylate moiety is then acting as a bridge between two metal ions in a dinuclear Co-complex [16]. 4-Carboxylic acid pyrimidine is deprotonated and acts as an effective chelating ligand. Due to the 1 charge, there is no need for a counterion with these divalent metal ions. The remaining nitrogen atom on the ligand does not participate in any coordination chemistry, but it does act as a hydrogen-bond acceptor for a non-coordinated water molecule. The two coordinated water molecules give rise to a complicated hydrogen-bonded network that also includes the non-coordinated oxygen atom of the carboxylate moiety. The behavior of this ligand is similar to that of the pyrazine carboxylates (Scheme 2) in which there is only one example (out of a total of nine crystal structures) where both nitrogen atoms are simultaneously involved in ligand–metal coordination [17]. This study provides an opportunity to explore the effect substitution has on the coordination mode of the ligand. Comparing ligands 1 and 2 to related pyridine-based carboxylates shows a variety of coordination possibilities (Graph 1) [17]. Nicotinate and isonicotinate have the ability to coordinate via only the nitrogen atom, or to two metal ions, one via the nitrogen atom and the other via an oxygen atom from the carboxylate moiety [18]. When using the oxophilic Ni ion, coordination can take place only via an oxygen atom, with the nitrogen atom remaining uncoordinated. Picolinate no doubt owes some of its popularity as a ligand to its predictability. It coordinates to a metal ion via the N and O chelate every time. It was expected that the 5-carboxylic acid pyrimidine would coordinate in a manner similar to nicotinic acid and isonicotinic acid, while 4-carboxylic acid pyrimidine would coordinate like picolinic acid, forming the chelate through both the heterocyclic nitrogen atom in the 3 position of the ring and one of the oxygen atoms of the carboxylate/carboxylic acid (Scheme 3). In fact, 2 does behave exactly as expected, forming the chelate with the metal ions every time. However, 1 shows a surprising amount of predictability, coordinating in a mono-hapto fashion via a pyrimidine nitrogen atom every time. Given that isonicotinate and nicotinate coordinate through the carboxylate frequently, it is somewhat unexpected that 1 shows no coordination via its carboxylate
N O
N O
Scheme 2. Pyrazine carboxylic acid with a chelating nitrogen atom– carboxylate binding site and an additional heterocyclic nitrogen atom binding site.
C.B. Aakero¨y et al. / Inorganica Chimica Acta 359 (2006) 1255–1262
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Graph 1. Summary of coordination site for pyridine and pyrimidine carboxylates with respect to Co, Ni, Cu, and Zn ions.
O
O
M
O
O
M
O
O
a
N
N
N
M
acting with M(II) ions, and as their coordination modes robust and reliable, we are currently pursuing the supramolecular chemistry of these ligands by replacing coordinated water molecules with linear connectors such that the assembly of complex ions can be controlled.
M
Acknowledgment O b
O
N M
O
c
N
O
N
We are grateful for generous financial support from the National Science Foundation (CHE-0078996).
M
References M O
O
O
O
N
N
N
N
M O
O
N
N
d
M
M
Scheme 3. Some possible coordination modes for (a) nicotinate/isonicotinate, (b) picolinate, (c) 4-carboxylate pyrimidine and (d) 5-carboxylate pyrimidine.
moiety. It should be noted that our sample size is very small (only three structures), so different coordination modes may well arise under different conditions or with different metal ions. As a comparison, when pyrimidine-based ligands (excluding fused-ring systems and chelating ligands) are used in coordination chemistry simultaneous coordination to both nitrogen atoms takes place less than 30% of the time. It is therefore not that surprising that 1 and 2 coordinate via only one nitrogen atom in these structures. Since ligands 1 and 2 do not require the presence of potentially structurally disruptive counterions when inter-
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