Novel pyrazolate-based copper(II) [2×2] grid complexes: Synthesis, structure and properties

Inorganica Chimica Acta 392 (2012) 322–330

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Novel pyrazolate-based copper(II) [22] grid complexes: Synthesis, structure and properties Sergey O. Malinkin a,⇑, Yurii S. Moroz a,e, Larysa V. Penkova a, Matti Haukka b, Agnieszka Szebesczyk c, Elzbieta Gumienna-Kontecka c, Vadim A. Pavlenko a, Ebbe Nordlander d, Franc Meyer f, Igor O. Fritsky a a

Department of Chemistry, Kiev National Taras Shevchenko University, Volodymyrska Street 64, 01601 Kiev, Ukraine Department of Chemistry, University of Eastern Finland, P.O. Box 111, FI-80101 Joensuu, Finland Faculty of Chemistry, University of Wroclaw, F. Joliot-Curie Street 14, 50-383 Wroclaw, Poland d Inorganic Chemistry Research Group, Chemical Physics, Center for Chemistry and Chemical Engineering, Lund University, P.O. Box 124, SE-22100 Lund, Sweden e Department of Chemistry, Syracuse University 2-044 Center for Science & Technology Syracuse, 13244 NY, USA f Institut für Anorganische Chemie Georg-August-Universität Göttingen Tammannstrasse 4, 37077 Göttingen, Germany b c

a r t i c l e

i n f o

Article history: Received 28 December 2011 Received in revised form 5 March 2012 Accepted 8 March 2012 Available online 24 March 2012 Keywords: Pyrazole ligands Copper(II) Grid complexes Magnetic properties

a b s t r a c t 5-Acetyl-substituted pyrazole-3-carboxylic acid (H2L) forms [22] grid-like tetranuclear Cu(II) complexes with four five-coordinated copper(II) ions bridged by pyrazolate groups. Despite a significant dissociation of [Cu4L4(H2O)4]4H2O (1) in aqueous solution1, it was possible to substitute the coordinated water molecules by pyridine ligands or azide anions. The resulting tetranuclear complexes [Cu4L4Py4]2H2O (2) and Na4[Cu4L4(N3)4]7MeOH (3) were isolated and studied by X-ray diffraction analysis. In 2 and 3 the azide anions or pyridine molecules complete the distorted square-pyramidal coordination of each copper(II) center. Magnetic susceptibilities of the obtained compounds have been measured by SQUID techniques. Simulation of the data using a Heisenberg spin Hamiltonian approach showed that the bridges between the metals mediate weak intramolecular antiferromagnetic coupling (J in the range 13.3 to 17.1 cm1) and lead to a singlet ground state in all cases. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Grid-type coordination compounds, which consist of a regular array of metal ions sandwiched between two or more perpendicular sets of parallel ligand molecules, have attracted great interest in recent years [1–6]. Numerous [nn] and [nm] grids (where n and m = 2–5) containing d- and p-metal ions have been reported to date, and some of them were shown to have interesting optical and magnetic properties including spin crossover [2,7–11]. One of the requirements for the development of novel materials for high-density information storage is switchability on the molecular level. Magnetic grids have the number of electrons on the molecule that can be controlled electrostatically with a gate voltage, which opens up the possibility of single-molecule spin switching [12]. This class of complexes provide a matrix-like array of addressable sites whose size is even smaller than quantum dots [14]. Such systems attract particular interest as potential ‘‘quantum cellular automata’’ (QCA) with four dots located at the vertices of a square [2c]. In many cases it is difficult to control the size and geometry of large molecular metal clusters, but it has been demonstrated that predetermined metal-organic grid structures can be produced by ⇑ Corresponding author. E-mail address: [email protected] (S.O. Malinkin). 0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ica.2012.03.021

self-assembly processes using suitable ligand scaffolds [7,13]. The power of self-assembly to create ordered nanostructures of intriguing technological potential has been underlined through the investigation of metalloorganic grid complexes [14]. Despite the excellent bridging properties of the pyrazole heterocycle, only a few examples of pyrazolate complexes with a grid-like structure have been reported. Those complexes were usually based on symmetrical pyrazole ligands bearing the same substituents at the 3,5-positions of the heterocycle, except for the first synthesized pyrazolate molecular grid [3]. Compartmental pyrazole ligands with different donor substituents in the 3- and 5-positions are still surprisingly rare, although such ligands can be successfully used for obtaining oligonuclear heterometallic species [15]. In the present work, we report the synthesis and investigation of [22] molecular grids based on the synthetically easily accessible 5-acetyl-substituted pyrazole-3-carboxylic acid (H2L) [16]. Our investigation focuses on two main aspects: the role of the nuclearity and structural features of the obtained complexes on their magnetic and spectroscopic properties. 2. Experimental All chemicals were purchased from commercial sources and used as received. The ligand 5-acetyl-4-methyl-pyrazole-3-carboxylic acid (H2L) was synthesized according to the published

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procedure [16] and fully characterized by a variety of spectroscopic methods.

Table 1 Crystal data and structure refinement parameters for complexes 22H2O and 37MeOH.

2.1. Preparations 2.1.1. Synthesis of [Cu4(L)4(H2O)4](H2O)4 (1) The complex (1) was earlier obtained in our group [1]. The method of preparation was slightly changed and improved to avoid a large amount of impurities in the target product. To a suspension of the ligand H2L (0.10 g, 0.6 mmol) in water, (10 ml) 1.2 ml of aqueous NaOH (1 M) was added. The resulting mixture was added to a solution of Cu(ClO4)26H2O (0.22 g, 0.6 mmol) in H2O (10 ml). The reaction mixture was heated for 30 min at 80 °C with constant stirring. The resulting deep blue solution was left for crystallization at ambient temperature. Square block dark blue crystals suitable for X-ray diffraction were isolated after standing for several days (yield 0.14 g, 74%). ESI-MS+, m/z (%): 458.8 (38), {[Cu2L2]+H+]}+; 480.8 (100), {[Cu2L2] + Na+]}+; 940.6 (29), {[Cu4L4] + Na+}+. IR (KBr, cm1) 3402 w (H–O), 1636 s (macetyl C–O), 1607 sh (mas C–O), 1303 (ms C–O), 1233 m, 1079 m, 968 m, 845 m. UV–Vis (H2O): d–d 657 nm, p–p 260 nm. UV–Vis (DR): 700 nm. Elemental Anal. Calc for C28H40Cu4N8O20: C, 31.64; H, 3.79; N, 10.54. Found: C, 31.22; H, 3.47; N, 10.34%. 2.1.2. Synthesis of [Cu4(L)4Py4]2H2O (2) The ligand H2L (0.10 g, 0.6 mmol) was added to a solution of CuCl22H2O (0.102 g, 0.6 mmol) in a H2O:CH3OH mixture [80:20 v/v] (15 ml). The reaction mixture was heated for 10 min with constant stirring at 80 °C until complete dissolution of the ligand occurred. To the resulting deep blue solution aqueous pyridine (3 ml, 1 M) was added and the reaction mixture left at room temperature. Square block blue crystals suitable for X-ray diffraction were isolated after standing for few days (yield 0.15 g, 75 %). IR (KBr, cm1) 3400 w (H-O), 1640 s (macetyl C–O), 1607 sh (mas C– O), 1301 (ms C–O), 1224 m, 1073 m, 963 m, 841 m, 768 and 702 m (d C–H, pyridin). UV–Vis (DR): 710 nm. Elemental Anal. Calc. for C48H48Cu4N12O14: C, 45.35; H, 3.81; N, 13.22. Found: C, 45.70; H, 3.42; N, 13.43%. 2.1.3. Synthesis of Na4[Cu4(L)4(N3)4]7MeOH (3) To a solution of 1 (0.10 g, 0.078 mmol) in methanol (10 ml) was added a solution of NaN3 (0.025 g, 0.391 mmol) in H2O (1 ml). The reaction mixture was heated for 10 min with constant stirring at 50 °C. The crystals of 3 were formed upon slow diffusion diethyl ester into the resulting green solution for 24 h (yield 0.060 g, 65 %). IR (KBr, cm1) 3440 (H–O), 2075 (N–N), 1632 (macetyl C–O), 1609 (mas C–O), 1305 (ms C–O), 1231, 1077, 965. UV–Vis (H2O): d–d 658 nm, p–p 380, 270 nm. UV–Vis (DR): 660 nm. Elemental Anal. Calc. for C28H24Cu4N20Na4O12: C, 28.53; H, 2.05; N, 23.77. Found: C, 28.21; H, 2.50; N, 23.43%. 2.2. Physical measurements Mass spectra were recorded with a Bruker APEX IV spectrometer (HRMS, ESI). IR spectra from KBr pellets were recorded on a Perkin Elmer FT-IR Spectrum BX II spectrometer. Elemental analyses were performed by the analytical laboratory of the Faculty of Chemistry of Wroclaw University using an Elementar vario EL III instrument. UV–Vis spectra were recorded on a Cary 50 spectrophotometer. Variable-temperature magnetic susceptibility data (2–300 K) were acquired on a powdered sample using a Quantum Design MPMS-5S SQUID magnetometer. Corrections for the diamagnetism of the ligand were applied using Pascal’s constants. The powdered samples were contained in a gel bucket and fixed in a nonmagnetic sample holder. X-Band EPR spectra were

Formula MW (g mol1) Space group T (K) a (Å) b (Å) c (Å) a (°) b (°) c/(o) V (Å3) Z q(calc) (g cm3) l (mm1) Data/restraints/param. Goodness-of-fit R1 [I > (r(I)] wR2 (all data)

22H2O

37MeOH

C48H52Cu4N12O16 1307.22 P2/n 100(2) 19.3907(9) 13.5643(4) 21.5049(9) 90.00 114.120(5) 90.00 5162.4(4) 4 1.682 1.709 14 922/0/729 0.894 0.0591 0.0827

C35H50Cu4N20Na4O20 1408.61 C2/c 100(2) 22.063(8) 11.815(4) 22.336(8) 90 111.27(4) 90 5426(4) 4 1.724 1.669 7909/27/401 0.976 0.0381 0.1027

recorded on a Bruker ESP 300E spectrometer equipped with a Bruker NMR gaussmeter ER 035 M and a Hewlett Packard microwave frequency counter HP 5350B. 2.3. X-ray structure determination and refinement The crystallographic measurements were performed on a Nonius KappaCCD automated four-circle diffractometers with graphite-monochromated Mo Ka radiation. Low-temperature data were collected using an Oxford Cryosystems cooler. Data collection, cell refinement, data reduction and analysis were carried out using the Nonius KappaCCD software and Denzo/Scalepack [17]. All structures were solved by direct methods using SHELXS-97 and refined by a full-matrix least-squares technique based on F2 using SHELXL-97 [18] with anisotropic thermal parameters for all non-H atoms. The hydrogen atoms that were attached to the carbon atoms were placed in calculated positions and were refined by using the riding model, and the hydrogen atoms of the solvent molecules were located from the difference maps. Relevant crystal data are given in Table 1. 3. Results and discussion 3.1. Synthesis and structural characterization of complexes The synthetic interconversions discussed in this work are summarized in Scheme 1. The ligand H2L shows a sharp IR absorption at around 1700 cm1 assigned to the m(C@O) acetyl and carboxyl stretches which serves as a convenient spectroscopic marker and results in a characteristic bathochromic shift upon complex formation. Reaction of Cu(II) salts with H2L in water solutions gave [Cu4L4 (H2O)4]4H2O (1) complex that could be easily isolated with good yield in crystalline state (see Section 2). It was found that in water/methanol solution with pH above 7, complex 1 partially dissociates into the binuclear species (Supplementary materials). Despite the dissociation, the remaining tetranuclear species form in subsequent reactions with four equivalents of pyridine or NaN3 new grid-type coordination compounds 2 and 3, respectively (Scheme 1). Moreover, according to the data of EPR and UV–Vis spectrophotometric titrations, the formation of [Cu2L2(OH)2]2 (10 ) could occur at high pH (Supplementary materials). Since complex 10 was not isolated as an individual compound, this reaction of substitution was only postulated.

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O HO

O

N NH

H 2L CuX2 H2O 1:1 X= ClO4-, Ac-

2[Cu2L2(H2O)2]

[Cu4(L)4(H2O)4]·4H2O (1) 4py

4N3-

[Cu4(L)4(py)4]

Na4[Cu4(L)4(N3)4]

(2)

(3)

Scheme 1. Synthetic interconversions discussed in this work.

Characteristic shifts in the IR spectra confirm that the azide and carboxylate groups are coordinated to the metal ions in a monodentate fashion [19–20]. The molecular structures of [Cu4L4Py4]2H2O (2) and Na4[Cu4L4(N3)4]7MeOH (3), the single crystals of which were obtained from methanol–water solutions, were elucidated by X-ray crystallography. The molecular structures of 1, 2 and 3 are similar to the reported grid-like compounds: [Cu4(LC)4(Im)4]4Dma9H2O (dma = dimethyl-amine) based on pyrazole-3,5-dicarboxylic acid [4] and [{CuLB(H2O)}4]12H2O based on methoxy-carbonyl-pyrazole carboxylic acid [3]. The coordinated pyrazolate ligands have normal C–C and C–N distances [21–22], while C–O bond lengths (1.231–1.245 and 1.276–1.292 Å) of the deprotonated carboxylate groups are in accordance with monodentate coordination modes [23]. Selected bonds and angles for 2 and 3 are summarized in Table 2. The crystals of complexes 1 and 2 contain tetranuclear neutral complex molecules with S4 and C2 symmetries for [Cu4L4(H2O)4] and [Cu4L4(Py)4], respectively, and lattice water molecules. In the crystal packing of 2, two structurally slightly different grid-like cores are found. The difference is notable because of the subtle variations in intermetallic separations and chelate angles (around 0.151 Å and 2–3°, respectively). Also the O3–Cu1–O4, O1–Cu2– O6 angles are larger than the corresponding angles in the second isomer by 9.20(6)° and 6.97(6)°. The structures of the tetranuclear [22] grid-type complexes are composed of four copper(II) ions, four ligands [L]2 and four metal-bound solvent molecules with the following CuCu0 separations: Cu(1)Cu(10 ) is 4.0600(4) Å for 1, Cu(1)Cu(2) and Cu(3)Cu(4) are 4.2416(4) and 4.2332(3) Å for 2. The distances between the bridged ions in 2 are longer than those observed in [{CuLB(H2O)}4]12H2O (4.098-4.115 Å) [3], but are similar to the intermetallic separations in [Cu4(LC)4(Im)4]4Dma9H2O (4.195– 4.229 Å) [4]. The diagonal metal-metal separations in 1 have a value of 5.0814(5) Å and are thus significantly longer than those in 2 (3.7469(5)–3.8980(5) Å) and in all reported related structures. This feature reflects the variety of twisting of the boat-like tetranuclear core. All copper(II) ions are five-coordinated in distorted squarepyramidal fashion (s = 0.15 for 1 and s = 0.18 for 2) [24], which is typical for grid-like Cu(II) complexes (Fig. 1) [1–6]. Each of the

2 Cu(1)–Cu(2) Cu(1)–Cu(1)#1 Cu(2)–Cu(2)#1 N(1)–Cu(1) N(2)–Cu(2) N(3)–Cu(1) N(4)–Cu(2) N(5)–Cu(4) N(6)–Cu(3) N(7)–Cu(4) N(8)–Cu(3) N(9)–Cu(1) N(10)–Cu(4) O(4)–Cu(1)–N(3) N(3)–Cu(1)–N(1) O(4)–Cu(1)–N(9) N(1)–Cu(1)–N(9) O(4)–Cu(1)–O(3) N(3)–Cu(1)–O(3) N(1)–Cu(1)–O(3) N(9)–Cu(1)–O(3) O(1)–Cu(2)–N(2) N(4)–Cu(2)–N(2) O(1)–Cu(2)–N(11) N(4)–Cu(2)–N(11) O(1)–Cu(2)–O(6) N(4)–Cu(2)–O(6) N(2)–Cu(2)–O(6) N(11)–Cu(2)–O(6)

4.2416(4) 3.8980(5) 3.8404(5) 1.9968(16) 1.9996(16) 1.9906(16) 1.9780(16) 1.9846(16) 1.9881(16) 1.9942(16) 1.9844(16) 2.0174(17) 2.0149(17) 81.25(6) 95.27(6) 88.16(6) 96.56(7) 97.19(6) 101.46(6) 75.07(6) 89.11(6) 82.18(6) 94.66(6) 89.35(7) 95.30(7) 96.21(6) 75.54(6) 98.87(6) 92.72(6)

Cu(3)–Cu(4) Cu(3)–Cu(3)#2 Cu(4)–Cu(4)#2 N(11)–Cu(2) N(12)–Cu(3) O(1)–Cu(2) O(3)–Cu(1) O(4)–Cu(1) O(6)–Cu(2) O(7)–Cu(3) O(9)–Cu(4) O(10)–Cu(4) O(12)–Cu(3) O(7)–Cu(3)–N(6) N(8)–Cu(3)–N(6) O(7)–Cu(3)–N(12) N(8)–Cu(3)–N(12) O(7)–Cu(3)–O(12) N(8)–Cu(3)–O(12) N(6)–Cu(3)–O(12) N(12)–Cu(3)–O(12) O(10)–Cu(4)–N(7) N(5)–Cu(4)–N(7) O(10)–Cu(4)–N(10) N(5)–Cu(4)–N(10) O(10)–Cu(4)–O(9) N(5)–Cu(4)–O(9) N(7)–Cu(4)–O(9) N(10)–Cu(4)–O(9)

4.2332(3) 3.7469(5) 3.7557(5) 2.0305(17) 2.0040(17) 1.9401(14) 2.3642(15) 1.9599(14) 2.3425(14) 1.9545(14) 2.3279(14) 1.9545(14) 2.3451(15) 81.76(6) 96.65(7) 89.10(6) 96.21(7) 87.99(6) 76.25(6) 100.58(6) 93.82(6) 81.79(6) 95.85(6) 90.18(6) 96.14(7) 89.24(6) 76.10(6) 100.51(6) 95.81(6)

3 Cu(1)–Cu(2)#2 Cu(2)–Cu(2)#1 Cu(1)–N(3) Cu(1)–O(1) Cu(1)–N(5) Cu(1)–N(1) Cu(1)–O(5) N(3)–Cu(1)–O(1) N(3)–Cu(1)–N(5) O(1)–Cu(1)–N(5) N(3)–Cu(1)–N(1) O(1)–Cu(1)–N(1) N(5)–Cu(1)–N(1) N(3)–Cu(1)–O(5) O(1)–Cu(1)–O(5) N(5)–Cu(1)–O(5) N(1)–Cu(1)–O(5)

4.2383(13) 3.7917(14) 1.968(2) 1.9791(17) 1.981(2) 1.989(2) 2.352(2) 175.48(8) 97.01(9) 85.89(8) 96.11(8) 81.60(8) 164.06(9) 76.40(7) 100.06(7) 92.60(9) 99.26(8)

Cu(1)–Cu(1)#1

3.736(2)

Cu(2)–N(8) Cu(2)–N(2)#1 Cu(2)–O(3) Cu(2)–N(4)#2 Cu(2)–O(6)#1 N(3)–Cu(1)–O(1) N(8)–Cu(2)–N(2)#1 N(8)–Cu(2)–O(3) N(2)#1–Cu(2)–O(3) N(2)#1–Cu(2)–N(4)#2 O(3)–Cu(2)–N(4)#2 N(8)–Cu(2)–O(6)#1 N(2)#1–Cu(2)–O(6)#1 O(3)–Cu(2)–O(6)#1 N(4)#2–Cu(2)–O(6)#1

1.969(2) 1.972(2) 1.9839(18) 1.995(2) 2.394(2) 175.48(8) 99.45(9) 84.72(8) 173.15(8) 95.01(8) 81.56(8) 90.34(9) 75.43(7) 99.23(7) 99.72(8)

Symmetry transformations used to generate equivalent atoms: for 2, #1 x + 1/2, y, z + 1/2; #2 x + 3/2, y, z + 1/2; for 3, #1 x + 1, y, z + 1/2; #2 x + 1, y + 1, z + 1/2.

doubly deprotonated ligands provides two donor atoms to two metal ions including either the pyrazolate nitrogen and the carboxylate oxygen [d(Cu–Neq) = 1.9495(16)–1.9906(16) Å and d(Cu– Oeq) = 1.9519(14) –1.9599(14) Å], or the pyrazolate nitrogen and the carbonyl oxygen atom [d(Cu–Neq) = 1.9682(16) –1.9996(16) Å and d(Cu–Oax) = 2.3279(14) – 2.3938(15) Å]. The latter long Cu–O interactions reflect the typical Jahn-Teller elongation [2b]. The additional equatorial fifth site is usually occupied by the solvent molecule with distances of 1.9676(15) and 2.0040(17) – 2.0305(17) Å for water and pyridine, respectively. Each copper(II) atom forms two five-membered chelate rings, both adopting a planar conformation, with two ligands that include the carboxylic oxygen of one ligand and the carbonyl oxygen from another. The mean planes of the chelate rings intersect at the nearly right angle 85.3–89.5°. As a result of the grid structure, 12-membered-4-metal rings that contain the pyrazolate bridging N–N atoms and Cu(II)ions are present in the complexes. The crystal packings in 1 and 2 are almost identical: the complex molecules are connected to each other via intermolecular

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325

Fig. 1. View of the molecular structures of 1 (a) and 2 (b). All hydrogen atoms have been omitted for clarity.

Table 3 Hydrogen bonds for 2 and 3 (Å and °). D–H. . .A

d(D–H)

d(H. . .A)

d(D. . .A)

<(DHA)

2 O(1W)–H(1W). . .O(4W) O(2W)–H(2W). . .O(5)#1 O(4W)–H(4W). . .O(8) O(1W)–H(1V). . .O(10)#1 O(2W)–H(2V). . .O(2) O(4W)–H(4V). . .O(1W)#2 O(3W)–H(3W). . .O(2) O(3W)–H(3V). . .O(5)#3

0.97 0.97 0.97 0.97 0.97 0.97 0.97 0.97

1.87 1.98 1.81 2.71 2.31 1.87 1.91 2.00

2.823(3) 2.809(2) 2.767(2) 3.386(2) 2.942(2) 2.820(3) 2.874(2) 2.949(2)

168.3 142.5 167.3 127.4 122.3 165.6 174.4 167.2

3 O(7)–H(8). . .O(8) O(8)–H(7). . .N(7)#3

0.86 0.94

2.05 1.94

2.910(4) 2.854(4)

171.9 163.6

Symmetry transformations used to generate equivalent atoms: for 2, #1 x, y  1, z; #2 x + 3/2, y, z + 1/2; #3 x + 1/2, y  1, z + 1/2; for 3, #3 x + 1/2, y  1/2, z + 1/2.

hydrogen bonds that involve the coordinated and solvated water molecules and the non-coordinating carboxylate-O atoms (Table 3). Thus, the units are stacked along the crystallographic x and y (for 1), and y axes (for 2), forming column-like structures. Also, in 2 the columns are associated via a p-stacking intermolecular

interaction between the two pyridine molecules with a distance of 3.284 Å between the centroid of one pyridine ring (C39C40C41C42C43N12) and the mean plane of another (C29C30C31C32C33N9) (Fig. 2). X-ray crystal structure analysis shows that compound 3 consists of discrete tetranuclear [Cu4L4(N3)4]4 complex anions, four sodium cations as well as seven lattice methanol molecules (Fig. 3). The tetranuclear anion is based on a slightly distorted Cu4 tetrahedron that is very similar to complexes 1 and 2. In 3, Cu1 is located on the apical position of the mean pyramid, and the other three Cu ions lie in the basal plane. The separations between Cu(II) ions are 3.736(2)–3.7917(14) Å for diagonal and 4.2383(13) Å for adjacent atoms. All copper ions are five-coordinated with N3O2 donor sets. Each metal ion has a distorted square-pyramidal geometry (s = 0.23) with the basal plane being defined by two nitrogen atoms from the pyrazolate ring, one oxygen atom from the carboxylate and one azide nitrogen atom. The coordination sphere of Cu(II) is completed by one oxygen atom from the acetyl oxygen of the neighboring L2 ligand. The four L2 ligands bind to the metal ions in a head-to-tail arrangement [2b] to give a distorted [22] grid-type geometry; four azide ligands are further attached to Cu(II) ions to form the cluster with C2 symmetry in the same manner as it was shown for 2 (vide

Fig. 2. Fragments of the crystal packing in 2: view along crystallographic y axis (a) and hydrogen bonds and p-interaction between two pyridine molecules (b). O atoms of solvate water are drawn as spheres of arbitrary radii. Hydrogen bonds are indicated by dashed lines. (H atoms have been omitted for clarity.)

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via hydrogen bonding interaction paralel to xy crystallographic planes. The hydrogen bond parameters for 3 are displayed in Table 2. The structures of 1, 2, 3 could be implied as the four copper(II) atoms are connected into an unusual Cu4N4O4 (1) and Cu4N8 (2 and 3) U-like core [4] without endogenous atoms through the eight pyrazole nitrogen atoms. Side-views of the central cores of all three complexes, showing only the copper ions with their surrounding N,O-donor atoms, are presented in Fig. 5. These representations reflect the roughly tetrahedral arrangement of the metal ions in the complexes. A more pronounced twist in 2 and 3 is apparent, which is similar to the situation in a reported [22] grid complex with square-pyramidal coordination environment of each copper(II) ion with {N5} arrangement [2b]. 3.2. Spectral properties and solution study

Fig. 3. View of the molecular structure of 3. All hydrogen atoms have been omitted for clarity.

supra) and some reported complexes [2b,3] and in contrast to 1, [Cu4(HLA)4]8H2O (S4) [2a] and [Cu4(LC)4(Im)4]4Dma9H2O (S1) [4]. It is noteworthy that pairs of sodium cations connect adjacent grid-like motifs involving the coordinated methanol molecules along the crystallographic y axis and form a column-like architecture (Fig. 4). In addition, the columns are stabilized by hydrogen bonding with the coordinated methanol oxygen atoms as donors and the terminal azide nitrogen atoms as acceptors. Thus, the one-dimensional chains are inter-connected into 2D square sheets

IR spectra were recorded for solid samples of 1, 2 and 3 in the range 4000–400 cm1 and do not have many significant differences. They have strong and broad bands in the 1640–1632, 1609–1607 and 1305–1301 cm1 regions, which are assigned respectively to the vibrations of the coordinated acetyl group and asymmetric and symmetric vibrations of the monodentately bound carboxyl group [20] of the ligand. The bands with high intensity at 2075 cm1 for 3 can be assigned to the m(N–N) stretching vibration that is in accordance with the presence of monodentately coordinated azide-anions in the complex. Complexes 1 and 3 are soluble in water, methanol and mixtures of these solvents, but 2 is insoluble in almost all common solvents; thus the solution study for 2 could not be performed. The ESI mass spectrum of 1 (Fig. S3a) showed that two different types of species, with the composition [M2L2] and [M4L4], are present in solution. Moreover, a reaction mixture containing the ligand and Cu(ClO4)2H2O in 1:1 stoichiometric ratio resulted in the same ESI MS spectrum. The formation of the dinuclear species is expected

Fig. 4. Fragment of the crystal packing. All hydrogen atoms have been omitted for clarity.

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327

Fig. 5. Side-view projections of 1 (a), 2 (b), and 3 (c), depicting the respective spatial arrangements of the tetranuclear metal cores of each complex.

due to the realization of one of the most favorable binding modes for pyrazole ligands with monodentate substituents in 3,5-positions of the heterocycle, namely bis(pyrazolato)bridged dinuclear cores. Such coordination compounds have been already reported [25–28]. As complex 2 has poor solubility, the ESI-MS spectrum of the reaction mixture between 1 and pyridine in equimolar ratio was recorded in methanol (1103 mM). There are two new peaks with m/z = 538.0 and m/z = 997.9, which have been assigned to the monocharged dinuclear {[Cu2L2(py)]+H+}+ and the tetranuclear {[Cu4L4(py)]+H+}+ cationic species, respectively (Fig. S3b). The intensities of the peaks are in agreement with the calculated isotopic patterns. This confirms the formation of the pyridine containing complexes, which can be precursors to 2. For complexes 3 and 10 , the signals corresponding to the tetranuclear or binuclear species have not been observed in ESI-MS spectra of methanolic solutions, presumably, because of the dissociation of complexes upon a significant dilution or the decomposition of the species upon ionization, caused by their high negative charge (4- or 2-). However, evidence for the high stability of 3 in water/methanol solution comes from EPR spectroscopy (vide infra). The UV–Vis spectrum of an aqueous solution of 3 shows a wide band with kmax = 658 nm, which is typical for d–d transitions of copper(II) ions with distorted square-pyramidal {N3O2}-donor chromophore [29]. The spectrum of the powdered sample (kmax = 660 nm) confirms the identity of the metal coordination environment in the solid state and in solution. But for 1 the observed d–d band in aqueous solution is shifted to 657 nm in comparison to the band observed in the spectrum of the powdered sample (700 nm), which probably is a result of the complex dissociation into bimetallic species, as found by the ESI MS study (Scheme 2). This is also in accordance with the EPR data that show the presence of species with a strong antiferromagnetic interaction between the metal centers, which is typical for the doubly pyrazolate bridged copper(II) ions [30]. The X-band EPR spectra of solid 1–3 have been recorded at 120 K and show strong broad signals around 3200 G (g  2.1). In case of 3 an additional very weak ‘‘half-field’’ signal at 1600 G appears that is typical for systems with magnetically coupled copper(II) ions (Fig. 6a) [2]. The EPR spectrum of the diluted frozen solution of 3 at 120 K (Fig. 6a) is similar to that observed in the solid state, confirming the high stability of the tetrametallic grid-type core. The EPR spectrum of solid 1 is different to that recorded for its frozen solution, but a weak signal centered at 3250 G indicates the presence of tetranuclear species (Fig. 6b), which probably is a consequence of an equilibrium between [M2L2] and [M4L4]. pH titrations in aqueous solution were performed in order to check the

Scheme 2. Proposed coordination mode of bimetallic species.

Fig. 6. X-Band EPR spectra of 3 (a) and 1 (b): dashed line—solid at 120 K, solid line— frozen solution of complexes in water/glycol (50/50, v/v) (0.003 mol L1, 120 K).

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Fig. 7. Plots of M (open light blue circles) and MT (open red circles) versus temperature for 1 (a), 2 (b) and 3 (c) at 5000 G. The solid lines represent the calculated curve fits. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

formation of polynuclear species in the Cu(II)/H2L system; they were monitored by EPR and UV–Vis spectroscopy (Supplementary materials). It was found that complex 1 was present in solution in neutral medium, and transformed into hydroxo-derivatives at pH >7. It should be noted that the formed dinuclear species has low stability in comparison to related complexes with the 2:2-type coordination mode [15,25,27,31–32], probably because of the presence of a weak donor (the acetyl group) that decreases the ability of the ligand to form the doubly bridged pyrazolate complexes and increases the chance of the formation of self-assembled compounds with a grid-like structure. Thus, an equilibrium between di- and tetranuclear species arises in water/methanol solutions. It was shown above that these species could not be distinguished by UV–Vis spectroscopy due to similar coordination environments (N2O2) of the copper(II) ions, while EPR spectroscopy helps only to observe the appearance of 1 in small amounts, as the signals of the pyrazolate-bridged complexes are very broad and overlapped. Despite the significant dissociation, 1 and 2 were successfully isolated because of the lower solubility of the tetranuclear complexes in comparison to the dinuclear species. In complex 3 the coordinated azide anions might stabilize the tetranuclear core in solution.

metal ions (the position of the maxima in the vM curves are 35 K for 1, 30 K for 2 and 40 K for 3). Experimental data were simulated by using a fitting procedure to the appropriate Heisenberg–Diracvan-Vleck (HDvV) spin Hamiltonian for isotropic exchange coupling and Zeeman splitting (Eq. (1)), assuming equal J values for all four metal-metal bridges [33,34].

^ ¼ 2J R^Sj  ^Si þ g l R^Si H 1 B

ð1Þ

A Curie–Weiss-behaved paramagnetic impurity q that presumably causes the increase of vM at very low temperatures and temperature-independent paramagnetism (TIP) with value of 2.4  104 cm3 mol1 were included according to the equation v = (1  q)v + qvmono + TIP(fixed). The solid lines in Fig. 7 represents the best fits with the following parameters: g = 2.13 ± 0.01, J = 14.6 ± 0.1 cm1 and q = 1.2% (R = 7.2  105) for 1, g = 2.14 ± 0.01, J = 13.3 ± 0.1 cm1 and q = 5.7% (R = 4.5  104) for 2, g = 2.11 (fixed), J = 17.1 ± 0.1 cm1and

3.3. Magnetic properties Magnetic susceptibilities for 1, 2 and 3 were measured in the temperature range of 1.8–300 K at two different magnetic fields (2000 and 5000 G). No significant field dependence was observed for each complex. Fig. 7 shows the magnetic susceptibility v and the product vT versus temperature plots. The observed vMT and leff values at 295 K are 1.62 cm3 K mol1 (3.64 lB) for 1, 1.58 cm3 K mol1 (3.56 lB) for 2 and 1.31 cm3 K mol1 (3.20 lB) for 3, which closely match the value expected for four uncoupled Cu(II) ions (3.69 lB for g = 2.13). In all cases vMT tends toward zero at low temperature and the vM curve passes through a maximum, which is clear evidence of antiferromagnetic coupling between the

Scheme 3. Graphical representation of dihedral angels used for magneto-structural correlation.

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S.O. Malinkin et al. / Inorganica Chimica Acta 392 (2012) 322–330 Table 4 Selected structural and magnetic data for tetranuclear pyrazolate-bridged copper(II) grid complexes. Complexa

Distance Cu–Cu (Å)b

Angle (Cu–N–N) (°)b

(dCu-Base) (°)b

(dACu-NN0 ) (°)b

(dBCu-NN0 ) (°)b

J (cm1)

Ref.

[Cu4(HLA)4]8H2O [{CuLB(H2O)}4]12H2O [Cu4(LC)4(Im)4]4Dma9H2O 1 2 3

4.08 4.11 4.19 4.06 4.23 4.21

133.96 137.82 135.4 133.87 136.26 135.59

89.55 87.66 88.89 88.20 89.21 88.37

89.88 88.63 82.85 89.49 84.74 84.29

1.63 3.77 12.0 10.90 15.66 10.68

8.2 12.3 13.49 14.6 13.3 17.1

[2a] [3] [4] this work

a H3LA = N,N0 -bis(2-pyridylmethyl)pyrazole-3,5-dicarboxamide; H2LB = 5-methoxy-carbonylpyrazole-3-ccarboxylic acid; H3LC = 3,5-pyrazoledicarboxylic acid, Im = imidazole, Dma = dimethyl-amine. b Average values.

q = 7.2% (R = 3.2  104) for 3; where R = R[(vMT)obs  (vMT)calc]2/ R[(vMT)obs]2. The g values for 1 and 2 are in reasonable agreement with the EPR data discussed above. The moderate antiferromagnetic coupling observed for all complexes can be a result of a large r in-plane overlap between the coplanar dx2 y2 magnetic orbitals and the bridging units. It is known that the magnitude and the nature of the interaction in Cu(l-pz)Cu complexes depends on several structural parameters such as the Cu–N–N angle (at angle value of 130.5 the most effective interaction is expected) and Cu  Cu0 separation [3,5], the dihedral angles (dACu–NN0 and dBCu–NN)0 (a deviation from perpendicular and coplanar orientations of the pyrazolate plane with respect to the basal planes of adjacent metal ions as shown in Scheme 3) [2], and the bending angles dpz–bend [35]. Those parameters essentially reflect the asymmetry or the distortion of the Cu(l-pyrazolato)Cu central core, which effects the overlap between the magnetic orbitals of metal ions and the orbitals of the ligand. In the present work from analysis of structural and magnetic data two basic factors that determine the value of magnetic coupling in tetranuclear pyrazolate systems have been found (Table 4). According to X-ray data of 1 – 3 the Cu–N–N angles and CuCu0 separation are similar in all cases (133.96– 137.82° and 4.06–4.23 Å), and hence the complexes do not show major differences in |J| values. Also values of the dihedral angles between the basal planes of the copper(II) centers are around 87.66–89.55° and do not reveal a distinct correlation with the strength of antiferromagnetic coupling. It appears that deviations of the dihedral angle dACu-NN0 and dBCu-NN0 from 90° or 0° generally lead to stronger antiferromagnetic coupling (see Table 3), though magnetic and structural features of 3 do not follow this trend. On the other hand, the correlation between the ligand field strength of substituents or additional ligands and J values of the corresponding complexes can be observed. Thus, complexes 1–3 can be arranged into the following order with respect to the value of |J|: 3 > 1 > 2, which corresponds to the decrease in the ligand field strength of the additional ligands in the following raw: N3 < H2O < py. Also values of antiferromagnetic coupling for previously reported complexes confirm this observation. However, the reduction of |J| is conditioned by the nature of substituents in 3- and 5-positions of pyrazolate heterocycle. In conclusion it should be noted that a min value of the coupling constant is observed for complexes containing the strongest field ligands and, correspondingly, a max value of the coupling constant should be anticipated for Cu(II) species formed by the low field ligands. In general, the copper complexes with bridged imidazole [36], 1,2,4-triazole or 1,2,4-triazolato [37] ligands are characterized by the antiferromagnetic interaction in all cases. Thus, the obtained J values are unexceptionally negative for triazolate complexes. By comparison to these five-membered ring systems, it is apparent that pyrazolate complexes have the capacity to propagate the antiferromagnetic exchange more efficiently than 1,2,4-triazole or 1,2,4-triazolate [37]. This can be attributed to the presence of the

third electronegative nitrogen atom in the triazole ring, which renders the occurrence of polarizing spins within the ring and thus limits the magnetic exchange [38]. This trend is observed for the binuclear species, results in a magnetic interaction parameter |J| of 36 – 118 cm1 [39], which is significantly lower than those exhibited by the related pyrazolate-bridged compounds (181– 214 cm1) [40]. It is to be expected that a strong antiferromagnetic coupling would occur in these cases as the magnetic d(x2  y2) orbitals are coplanar and overlap with the r-orbitals of the coplanar bridging pyrazolate or triazolate fragments. This is a common type of complexes and it is well understood how antiferromagnetism arises in such cases [41]. However, the antiferromagnetic interactions between copper(II) ions in the grid complexes are comparable for both heterocycles [37,5]. CNDO/2 calculations on a model of monopyrazolatebridged dicopper complexes have been performed [3], for the purpose of explaining less strength of antiferromagnetic coupling in these compounds. Thus, it has been shown that the exchange interactions in a binuclear bridged pyrazolate complex have been further decreased due to the partial coplanarity between the bridging pyrazole plane and the basal planes of the copper atoms where the bridging pyrazole plane has been parallel to one neighboring copper basal plane but perpendicular to another. In contrast, the range of |J| is 38–117 cm1 for the imidazolate bridged complexes is more narrow in comparison with the pyrazolate compounds [36], and the antiferromagnetic exchange parameter in the grid complexes is larger than those observed for the pyrazolate and triazolate grids. This could be conditioned by the location of donor atoms in a heterocycle ensuring the more efficient aniferromagnetic interaction. The mechanism of the superexchange coupling which is realized across the nitrogen lone-pairs, has been discussed by Kolks et al. [42]. In conclusion, a simple magnetostructural correlation could not be performed in tetranuclear bridged Cu(II) complexes. The values of J parameter have not revealed a strict dependence on the angles between the CuL4 coordination and heterocycles planes or the coordination angles and distances between metal ions, thus disfavoring the p-exchange path and indicating the r-superexchange pathways cannot be the only mechanism for exchange coupling in these compounds. Also, the influence of additional ligands on the strength of antiferromagnetic coupling has been demonstrated only for pyrazolate moieties, it is not observed for imidazole or triazole complexes.

4. Conclusion A series of new Cu(II) complexes with 3-acetyl-4-methyl-1Hpyrazole-5 carboxylic acids have been synthesized and characterized by different physical methods. It has been found that the isolated compounds have [22] grid architectures according to a

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single crystal X-ray diffraction study. The obtained complexes are the products of the substitution of the coordinated water molecules in 1 by pyridine, hydroxyl and azide anions. The weak donor strength of the acetyl group and more stable 2:2 coordination mode are basic factors that contributed the reluctance to produce the [22] species. Complex 1 dissociates into the dinuclear species in solution, on the other hand, the presence of the azide anions stabilizes the tetranuclear core which is confirmed by the detection of the tetranuclear species during the solution study. The present structural studies show that some parameters of grid geometry, in particular the distortion of the M4 unit, are remarkably sensitive to the variations in the substituents in the base of the square-pyramidal environment of Cu(II) ions. These changes result in differences in efficiency of magnetic exchange of the obtained compounds (the tetranuclear Cu(II) complexes have CuCu0 separations of about 4.1 Å and are antiferromagnetically coupled). It has been found that the strength of ligand field defined by the substituents in pyrazolate heterocycle and additional ligands are the most crucial factors in distinguishing the value of the coupling constant in the tetranuclear core. For the first time, the [22] grid copper(II) complex has been used as a precursor for the synthesis of substituted derivatives with grid-like structures. Moreover, compound 3 contains monodentate azide groups that are well-known linkers used in synthesis of oligonuclear complexes [43]. Thus, the structural peculiarities and stability of obtained compounds permit further investigations dealing with the use of the complexes as building blocks for preparation of new large clusters or polymeric materials and heterometallic species. Acknowledgements The financial support from the State Fund for Fundamental Researches of Ukraine (Grant No. F40.3/041), the Polish Ministry of Science and Higher Education (1012/S/WCH/11/2), the Swedish Institute (Visby Program) and the DFG (SFB 602, project A16) is gratefully acknowledged. Appendix A. Supplementary material CCDC 851846 (for 2) and 851847 (for 3) contain 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. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ica.2012.03.021. References [1] S. Malinkin, I.A. Golenya, V.A. Pavlenko, M. Haukka, T.S. Iskenderov, Acta Crystallogr. E. 67 (2011) m1260. [2] (a) J. Klingele, A.I. Prikhod’ko, G. Leibeling, S. Demeshko, S. Dechert, F. Meyer, J. Chem. Soc., Dalton Trans. 20 (2007) 2003; (b) J.I. van der Vlugt, S. Demeshko, S. Dechert, F. Meyer, Inorg. Chem. 47 (2008) 1576; (c) B. Schneider, S. Demeshko, S. Dechert, F. Meyer, Angew. Chem. 122 (2010) 9461 (Angew. Chem. Int. Ed. 49 (2010) 9274). [3] H. Zhang, D. Fu, F. Ji, G. Wang, K. Yu, T. Yao, J. Chem. Soc., Dalton Trans. 19 (1996) 3799. [4] X. Feng, L.-Y. Wang, J.-S. Zhao, B. Liu, J.-G. Wang, X.-G. Shi, Inorg. Chim. Acta 362 (2009) 5127. [5] K.L.V. Mann, E. Psillakis, J.C. Jeffery, L.H. Rees, N.M. Harden, J.A. McCleverty, M.D. Ward, D. Gatteschi, F. Totti, F.E. Mabbs, E.J.L. McInnes, P.C. Riedi, G.M. Smith, J. Chem. Soc., Dalton Trans. 3 (1999) 339.

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