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Binuclear manganese(III) complexes of an unsymmetric pyrazolate-based compartmental ligand with hard donor set
Inorganica Chimica Acta 363 (2010) 3036–3040
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Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Binuclear manganese(III) complexes of an unsymmetric pyrazolate-based compartmental ligand with hard donor set Larysa Penkova a, Serhiy Demeshko b, Vadim A. Pavlenko a, Sebastian Dechert b, Franc Meyer b,*, Igor O. Fritsky a,** a b
Department of Chemistry, Kiev National Taras Shevchenko University, 64 Vladimirska Str., 01601 Kiev, Ukraine Institut für Anorganische Chemie, Georg-August-Universität Göttingen, Tammannstr. 4, 37077 Göttingen, Germany
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Article history: Received 17 February 2010 Received in revised form 16 April 2010 Accepted 24 April 2010 Available online 5 May 2010 Dedicated to Prof. Animesh Chakravorty on the occasion of his 75th Birthday Keywords: Manganese(III) Binuclear complexes Antiferromagnetic interaction Crystal structure Pyrazolate ligands
a b s t r a c t The synthesis, crystal structure and magnetic properties of manganese(III) binuclear complexes [MnIII2(L–3H)2(CH3OH)4]2CH3OH (1) and [MnIII2(L–3H)2(Py)4]2Py (2) (L = 3-[(1E)-N-hydroxyethanimidoyl]-4-methyl-1H-pyrazole-5-carboxylic acid) are reported. The ligand contains two distinct donor compartments formed by the pyrazolate-N and the oxime or the carboxylic groups. The complexes were characterized by X-ray single crystal diffraction, revealing that both 1 and 2 consist of dinuclear units in which the two metal ions are linked by double pyrazolate bridges with a planar {Mn2N4} core. Cryomagnetic measurements show antiferromagnetic interaction with g = 1.99, J = 3.6 cm1, H = 2.02 K for 1 and g = 2.00, J = 3.7 cm1, H = 1.43 K for 2. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Polynuclear manganese complexes are attracting significant interest for several reasons. Firstly, they play key roles in metallobiosites such as the oxygen evolving complex (OEC) of photosystem II or the active center of manganese catalase (Mn-CAT) [1–3]. Secondly, polynuclear manganese(III) complexes are very prominent in the field of molecular magnetism, due to the large single-ion anisotropy that is usually associated with this Jahn–Teller d4 metal ion [4]. Unravelling the magnetic coupling between manganese(III) ions that are linked by commonly used bridging units is therefore of general importance. It is well established that pyrazoles, 1,2,4-triazoles, pyridazines and phthalazines are able to span two metal ions through the diazine or diazole (@N–N@) groups, respectively [5]. Such binuclear metal arrangements are particularly interesting because these bridging moieties provide a metal–metal separation of 3.5–4.5 Å, which is reminiscent of the situation in bimetallic biosites. Pyrazole derived binucleating ligands that possess two chelating arms attached to the 3- and 5-positions of the heterocyclic ring allow to further tune metal metal separations and other characteristics * Corresponding author. Fax: +49 551 393063. ** Corresponding author. Tel./fax: +380 44 2393393. E-mail addresses: [email protected] (F. Meyer), [email protected] (I.O. Fritsky). 0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2010.04.038
of the bimetallic core [6–8]. However, only very few manganese(III) complexes of this ligand class have yet been reported [9,10]. Furthermore, relatively few unsymmetric pyrazole-centered ligands with different chelate arms in the 3- and 5-positions have been investigated so far [11]. We here introduce a new pyrazole-derived ligand L (Chart 1) that features two bidentate coordination compartments with relatively hard donor set, namely with a carboxylate and an oximic chelate arm, which are well suited for manganese(III). Crystal structures and some physico-chemical properties of two bis(pyrazolato)-bridged dimanganese(III) complexes are reported. 2. Experimental Solvents were purified by established procedures. All other chemicals were purchased from commercial sources and were used as received. Microanalyses were performed by the Analytical Laboratory at the Institute of Inorganic Chemistry at GeorgAugust-Universität Göttingen. IR spectra (as KBr pellets) were recorded with a Digilab Excalibur, and mass spectra were measured with a Finnigan MAT 8200 (ESI-MS). NMR spectra were recorded with a Bruker Avance 500, and chemical shifts were calibrated to the residual proton and carbon signal of the solvent (DMSO-d6: dH = 2.49, dC = 39.7 ppm). Magnetic data were measured with a Quantum-Design MPMS-5S SQUID magnetometer at 0.2 and 0.5 T
L. Penkova et al. / Inorganica Chimica Acta 363 (2010) 3036–3040
O HO
N
OH
NH
N
L Chart 1.
in the range from 2 to 295 K. The powdered samples were contained in a gel bucket and fixed in a nonmagnetic sample holder. X-Band EPR spectra were 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. Caution! Although no problems were encountered in this work, transition metal perchlorate complexes are potentially explosive and should be handled with proper precautions. Synthesis of 3-[(1E)-N-hydroxyethanimidoyl]-4-methyl-1H-pyrazole-5-carboxylic acid (L): 5-Acetyl-4-methyl-1H-pyrazole-3-carboxylic acid [12] (3 g, 0.018 mol), NH2OHHCl (1.86 g, 0.027 mol), and CH3COONa (2.80 g, 0.027 mol) were dissolved in water (50 ml). The mixture was stirred for 2 h, and the pH was adjusted to 4 by slow addition of aqueous HCl (1:1). A white precipitate formed, which was left to stand for 10–12 h, separated by filtration and dried under vacuum. Yield: 2.2 g (67%); white solid, m.p. 220 °C. IR (KBr, cm1) 1690 (s), 1640 (m), 1590 (s), 1400 (m), 1370 (m), 1300 (m), 1260 (m), 1240 (s), 1200 (m), 1130 (w), 1070 (w), 1005 (m), 960 (w), 925 (w), 790 (m), 725 (w), 650 (w). 1 H NMR (500 MHz, DMSO-d6): d = 2.13 (s, 3H, CH3), 2.34 (s, 3H, CH3), 11.08 (br, 1H, NOH). 13C NMR (500 MHz, DMSO-d6): d = 29.9 (CH3); 30.6 (CH3); 121.1; 125.6; 129.2 (3CPz). MS (EI, 70 eV): m/z (%) = 182 (75) [MH]+. Anal. Calc. for C7H9N3O3: C, 45.90; H, 4.95; N, 22.94. Found: C, 45.23; H, 5.36; N, 22.44%. [MnIII2(L–3H)2(CH3OH)4]2CH3OH (1) and [MnIII2(L–3H)2(Py)4] 2Py (2): To a solution of L (46 mg, 0.25 mmol) in methanol (20 ml) was added Mn(CH3COO)32H2O (67 mg, 0.25 mmol). The mixture was kept in a sonifier at room temperature for 5 min.
Table 1 Crystal data and structure refinement for 1 and 2.
Empirical formula Formula weight (g/ mol) Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dcalc (g cm3) l (mm1) F(0 0 0) T (K) h range (°) Reflections collected Reflections unique Goodness-of-fit (GoF) R1/wR2* [I > 2r(I)] R1/wR2* (all data)
*
[MnIII2(L– 3H)2(CH3OH)4]2CH3OH (1)
[MnIII2(L– 3H)2(Py)4]2Py (2)
C20H36N6O12Mn2 662.43
C44H42N12O6Mn2 944.78
triclinic P1 8.0217(7) 9.1712(8) 10.6930(9) 86.768(7) 73.846(7) 66.642(6) 692.39(10) 1 1.589 0.982 344 133(2) 1.99–27.32 13 544
triclinic P1 10.0581(10) 10.9889(11) 11.0912(11) 91.285(8) 113.080(8) 106.438(8) 1069.24(18) 1 1.467 0.655 488 133(2) 1.95–24.65 15 713
3123 1.058
3584 1.041
0.0294/0.0752 0.0434/0.1114 0.0376/0.0776 0.0603/0.1181 n h i h io1=2 R1 ¼ RðF o F c Þ=RF o ; wR2 ¼ R wðF 2o F 2c Þ2 =R wðF 2o Þ2 .
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The resulting green solution was filtered and salted-out with ether (in case of 1) or with pyridine (in case of 2) to gradually yield green crystals of 1 (77 mg, 93%) or 2 (113 mg, 96%), respectively. 1: IR (KBr, cm1) 3460 (br, s), mC@N 1650 (vs), mC@O 1600 (m), 1524 (m), 1431 (m), 1381 (m), 1321 (m), 1278 (vs), 1231 (s), 1138 (w), mN–O 1092 (s), 1055 (s), 964 (m), 855 (m), 801 (m), 732 (m), 651 (m), 542 (m), 462 (m), 430 (w). ESI–MS (DMF) m/z (%) = 639 (100) [Mn2(L–3H)2(CH3OH)3DMF–1H]. Anal. Calc. for C20H36N6O12Mn2: C, 36.26; H, 5.48; N, 12.69. Found: C, 35.79; H, 4.63; N, 13.18%. 2: IR (KBr, cm1) 3450 (br, s), 1881 (w), mC@N 1653 (vs), mC@O 1597 (m), 1518 (w), 1441 (s), 1403 (m), 1376 (w), 1314 (m), 1262 (vs), 1219 (s), 1152 (w), mN–O 1088 (m), 1036 (m), 1007 (m), 941 (w), 841 (s), 804 (w), 752 (m), 725 (m), 702 (s), 640 (m), 527 (w), 458 (m), 424 (m). Anal. Calc. for C44H42N12O6Mn2: C, 55.94; H, 4.48; N, 17.79. Found: C, 55.81; H, 4.36; N, 17.63%. Crystal structure determination: Details of the data collection and processing, structure analysis and refinement are summarized in Table 1. Diffraction data were collected on a Xcalibur-3 diffractometer (x and u scans) equipped with graphite-monochromated Mo Ka radiation (k = 0.71073 Å). The data were corrected for Lorentz-polarization effects and for the effects of absorption (analytical method using a multifaceted crystal model). The structures were solved by direct methods and refined by full-matrix leastsquares methods on F2 using the SHELX 97 set of programs [13]. Assignment of each of the metal atoms is based on refinement and on comparison of the bond lengths with the literature values.
3. Results and discussion 3.1. Synthesis and structural characterization of complexes The new ligand L is readily obtained from 5-acetyl-4-methyl1H-pyrazole-3-carboxylic acid and hydroxylamine, and manganese(III) complexes are best obtained from the reaction of the ligand with Mn(CH3COO)32H2O in a sonifier. Green crystals of 1 and 2 were grown by slow diffusion of diethyl ether (1) or pyridine vapour (2) into methanol solutions of the crude complexes, and the crystal structures were determined by X-ray crystallography (Fig. 1, Table 2). The crystals of the complexes consist of the centrosymmetric neutral molecules [Mn2(L–3H)2(CH3OH)4] (1) or [Mn2(L–3H)2(Py)4] (2) and two solvate methanol or pyridine molecules for 1 or 2, respectively. The complex ions are composed of two manganese(III) ions (the distance Mn(1) Mn(10 ) (x + 1,y + 1,z + 1) is 3.9544(6) Å and 3.9932(9) Å for 1 and 2, respectively), two ligands in their triply deprotonated form ([L– 3H]3) and four metal-bound solvent molecules. All manganese(III) ions are found six-coordinate with the typical Jahn–Teller situation of four relatively short and two significantly longer bonds. Each of the triply deprotonated ligands [L– 3H]3provides both manganese center with two donors, namely the pyrazolate nitrogen and the carboxylate oxygen donors [d(Mn–N) = 2.020(1)–2.022(2) Å and d(Mn–O) = 1.973(1)– 1.942(2) Å] or the pyrazolate nitrogen and the oxime oxygen donors [d(Mn–N) = 1.978(1)–1.972(2) Å and d(Mn–O) = 1.833(1)– 1.852(2) Å], which occupy the basal plane of the axially elongated octahedron. Apical sites are occupied by solvent molecules (viz. methanol [2.216(1)–2.235(1) Å] or pyridine [2.340(2)–2.367(3) Å] for 1 and 2, respectively). The manganese atom forms a five-membered chelate ring [Mn(1)–O(20 /40 )–C(70 )–C(30 )–N(20 )] with the [L– 3H]3 scaffold when the coordination involves the oxygen atom of the carboxylate group, but a six-membered chelate ring [Mn(1)– N(1)–C(2)–C(5)–N(3)–O(1)] when the oxygen atom of the oxime group is the coordinating atom. As a result of the double pyrazolate bridging a six-membered central bimetallic ring [Mn(1)–N(1)–
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Fig. 1. View of the molecular structures of 1 and 2. All protons have been omitted for clarity.
Table 2 Selected interatomic distances (Å) and angles (°) for 1 and 2. 1 Mn(1)–O(1) Mn(1)–O(40 ) Mn(1)–O(6) Mn(1)–O(7) Mn(1)–N(1) Mn(1)–N(20 ) Mn(1) Mn(10 ) N(1)–N(2) N(3)–O(1) O(1)–Mn(1)–O(40 ) O(1)–Mn(1)–O(6) O(1)–Mn(1)–O(7) O(1)–Mn(1)–N(1) O(1)–Mn(1)–N(20 ) O(40 )–Mn(1)–O(7) O(40 )–Mn(1)–O(6) O(40 )–Mn(1)–N(1) O(40 )–Mn(1)–N(20 ) O(7)–Mn(1)–O(6) N(1)–Mn(1)–O(6) N(1)–Mn(1)–O(7) N(1)–Mn(1)–N(20 ) N(20 )–Mn(1)–O(6) N(20 )–Mn(1)–O(7)
2 1.833(1) 1.973(1) 2.235(1) 2.216(1) 1.978(1) 2.020(1) 3.9544(6) 1.338(2) 1.355(2) 92.45(6) 89.18(6) 92.25(6) 89.11(6) 172.87(6) 90.55(5) 89.71(5) 178.43(6) 80.42(5) 178.53(5) 90.16(6) 89.54(5) 98.02(6) 90.56(6) 88.06(6)
Mn(1)–O(1) Mn(1)–O(20 ) Mn(1)–N(1) Mn(1)–N(20 ) Mn(1)–N(4) Mn(1)–N(5) Mn(1) Mn(10 ) N(1)–N(2) N(3)–O(1) O(1)–Mn(1)–O(20 ) O(1)–Mn(1)–N(1) O(1)–Mn(1)–N(20 ) O(1)–Mn(1)–N(4) O(1)–Mn(1)–N(5) O(20 )–Mn(1)–N(1) O(20 )–Mn(1)–N(20 ) O(20 )–Mn(1)–N(4) O(20 )–Mn(1)–N(5) N(1)–Mn(1)–N(20 ) N(1)–Mn(1)–N(4) N(1)–Mn(1)–N(5) N(20 )–Mn(1)–N(4) N(20 )–Mn(1)–N(5) N(4)–Mn(1)–N(5)
1.852(2) 1.942(2) 1.972(2) 2.022(2) 2.340(2) 2.367(3) 3.9932(9) 1.340(3) 1.391(4) 92.63(9) 89.12(9) 174.23(9) 89.77(9) 88.31(9) 178.16(1) 81.60(9) 89.99(9) 89.68(9) 96.64(9) 90.58(9) 89.82(9) 90.48(9) 91.39(9) 178.03(9)
Symmetry transformations used to generate equivalent atoms: 0 x + 1, y + 1, z + 1.
N(2)–Mn(10 )–N(10 )–N(20 )] is present in the complexes. The mean planes of the {MnN2O2} and {Mn2N4} motifs are rigorously coplanar, and the manganese(III) atoms lie in the mean-square planes of the complexes by symmetry. Interestingly, in both complexes 1 and 2 the ligands strands are oriented antiparallel with respect to each other, i.e., each Mn ion is coordinated by one carboxylate and one oxime oxygen atom. Formation of only this isomer that has an inversion center is evidently dictated by thermodynamic reasons. In the case of the unobserved C2v isomer with parallel ligand strands, the two metal ions would have different coordination environments, with one of them sur-
rounded by three six-membered chelate rings. Such situation causes significant conformational tension, and consequently the C2v isomer is disfavored both in solution and in the solid state. Xray check of the unit cell parameters of randomly chosen single crystals from the synthetic crops of both 1 and 2 confirmed the presence of only one crystal phase corresponding to the Ci isomer. While the planar central [Mn2(L–3H)2] cores in 1 and 2 are basically identical, crystal packing is significantly different and is obviously determined by the solvent molecules that are bound in axial positions. In 1 individual complex molecules are associated via intermolecular hydrogen bonds that involve the OH of the coordinated methanol molecules and the non-coordinating carboxylateO and oxime-N atoms (Fig. 2, Table 3). In contrast, the neutral complex molecules of 2 are united into chains along the crystallographic b -axis by means of offset p-stacking interactions between the pyridine rings (interplanar spacing 3.396 Å) (Fig. 3) [14]. Diffuse reflectance spectra of both complexes show an intense charge transfer at 280 nm. The broad bands observed at ca. 415 nm and 600 nm can be assigned to 5B1g ? 5Eg, 5B1g ? 5B2g or 5 B1g ? 5A1g ligand field transitions, respectively, in accordance with a distorted octahedral arrangement around the manganese(III) ions [15]. Solid-state magnetic susceptibilities of powdered samples of the complexes were measured in the temperature range 2–295 K. The temperature dependence of vMT is depicted in Fig. 4. The room temperature values for vMT are 6.42 and 5.98 cm3 K mol1 for 1 and 2, respectively, which is close to the expected value for two uncoupled high-spin Mn(III) (S = 2) ions (Calc. vMT = 6.00 cm3 K mol1). vMT decreases upon lowering the temperature, very gradually in the high-temperature region but more abruptly at low temperature, down to 0.48 and 0.69 cm3 K mol1 at 2 K for 1 and 2, respectively. Such behavior suggests the presence of antiferromagnetic coupling between the manganese(III) ions. Experimental data for complexes 1 and 2 were modelled by using a fitting procedure to the appropriate Heisenberg–Dirac– van-Vleck (HDvV) spin Hamiltonian for isotropic exchange coupling and Zeeman splitting, equation (1) [16]
L. Penkova et al. / Inorganica Chimica Acta 363 (2010) 3036–3040
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Fig. 2. Fragment of the crystal structure of 1.
Table 3 Hydrogen bond distances (Å) and angles (°) for 1. D–H A 00
O(6)–H(6) N(3 ) O(7)–H(7) O(8) O(8)–H(8) O(5000 )
d(D–H)
d(H A)
d(D A)
\(DHA)
0.81(3) 0.71(3) 0.75(3)
1.91(3) 1.95(3) 2.01(3)
2.717(2) 2.647(2) 2.756(2)
174(3) 166(3) 173(3)
Symmetry transformations used to generate equivalent atoms: 00 x + 1, y, z + 1; x + 1, y 1, z.
000
^ ¼ 2J^S1 ^S2 þ g l ð^S1 þ ^S2 ÞB H B
ð1Þ
Temperature-independent paramagnetism (TIP) and a Curie-behaved paramagnetic impurity (PI) were included according to vcalc = (1 PI)v + PIvmono + TIP [17]. For both complexes slightly better agreement with experimental data was obtained when intermolecular interactions were considered in a mean field approach by using a Weiss temperature H [16,18]. Best simulation parameters are g = 1.99, J = 3.6 cm1, H = 2.02 K for 1 and g = 2.00, J = 3.7 cm1, H = 1.43 K for 2 [19]. The Weiss temperature H relates to intermolecular interactions zJ0 of 0.7 cm1 for 1 and 0.5 cm1 for 2, where J0 is the interaction parameter between two nearest neighbour magnetic centers and z is the number of nearest neighbours [20]. Given the very similar, planar {Mn2N4} cores in 1 and 2, it is not surprising that intramolecular antiferro-
magnetic coupling in both complexes is basically identical. On the other hand, mutual orientation of the {Mn2N4} cores in the crystals is very different and is obviously dictated by the intermolecular Hbonding contacts (in 1) or p-stacking (in 2), which leads to fundamentally different intermolecular magnetic interactions, either antiferromagnetic (in 1) or ferromagnetic (in 2). These two systems thus provide valuable benchmark data for magnetic coupling in planar bis(pyrazolato)-bridged dimanganese(III) cores and at the same time demonstrate the significant effect of weak intermolecular contacts on the bulk magnetic properties. It should be noted that magnetic coupling in a bis(pyrazolato)-bridged dimanganese(II) complex is much weaker (J = 0.40 cm1) [9]. 4. Conclusions In summary, two new pyrazolato-bridged dinuclear manganese(III) complexes of an unsymmetrical ligand have been prepared. The ligand contains two distinct compartments that are formed by the pyrazolate-N and the oxime or the carboxylic groups. The complexes were characterized by X-ray single crystal diffraction, revealing both 1 and 2 consist of dinuclear units in which the two metal ions are doubly linked by pyrazolate bridges with a planar {Mn2N4} core. The magnetic behavior of these manganese(III) complexes is characterized by intradinuclear superexchange interactions that give rise to moderate antiferromagnetic
Fig. 3. Fragment of the crystal structure of 2.
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L. Penkova et al. / Inorganica Chimica Acta 363 (2010) 3036–3040
Fig. 4. Plots of vMT vs. temperature for 1 (left) and 2 (right) at 0.2 T; the solid lines represent the calculated curve fits.
coupling between the dimanganese(III) units, whereas distinct crystal packing is reflected by different intermolecular interactions transmitted by weak H-bonding or p-stacking contacts.
[8]
Supplementary material CCDC 766011 and 766012 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.
[9]
Acknowledgements
[10]
Financial support by the Deutsche Forschungsgemeinschaft (SFB 602, project A16) is gratefully acknowledged. We also thank the State Fund for Fundamental Research of Ukraine for financial support (Grant No. F28/241-2009).
[11]
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Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Binuclear manganese(III) complexes of an unsymmetric pyrazolate-based compartmental ligand with hard donor set Larysa Penkova a, Serhiy Demeshko b, Vadim A. Pavlenko a, Sebastian Dechert b, Franc Meyer b,*, Igor O. Fritsky a,** a b
Department of Chemistry, Kiev National Taras Shevchenko University, 64 Vladimirska Str., 01601 Kiev, Ukraine Institut für Anorganische Chemie, Georg-August-Universität Göttingen, Tammannstr. 4, 37077 Göttingen, Germany
a r t i c l e
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Article history: Received 17 February 2010 Received in revised form 16 April 2010 Accepted 24 April 2010 Available online 5 May 2010 Dedicated to Prof. Animesh Chakravorty on the occasion of his 75th Birthday Keywords: Manganese(III) Binuclear complexes Antiferromagnetic interaction Crystal structure Pyrazolate ligands
a b s t r a c t The synthesis, crystal structure and magnetic properties of manganese(III) binuclear complexes [MnIII2(L–3H)2(CH3OH)4]2CH3OH (1) and [MnIII2(L–3H)2(Py)4]2Py (2) (L = 3-[(1E)-N-hydroxyethanimidoyl]-4-methyl-1H-pyrazole-5-carboxylic acid) are reported. The ligand contains two distinct donor compartments formed by the pyrazolate-N and the oxime or the carboxylic groups. The complexes were characterized by X-ray single crystal diffraction, revealing that both 1 and 2 consist of dinuclear units in which the two metal ions are linked by double pyrazolate bridges with a planar {Mn2N4} core. Cryomagnetic measurements show antiferromagnetic interaction with g = 1.99, J = 3.6 cm1, H = 2.02 K for 1 and g = 2.00, J = 3.7 cm1, H = 1.43 K for 2. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Polynuclear manganese complexes are attracting significant interest for several reasons. Firstly, they play key roles in metallobiosites such as the oxygen evolving complex (OEC) of photosystem II or the active center of manganese catalase (Mn-CAT) [1–3]. Secondly, polynuclear manganese(III) complexes are very prominent in the field of molecular magnetism, due to the large single-ion anisotropy that is usually associated with this Jahn–Teller d4 metal ion [4]. Unravelling the magnetic coupling between manganese(III) ions that are linked by commonly used bridging units is therefore of general importance. It is well established that pyrazoles, 1,2,4-triazoles, pyridazines and phthalazines are able to span two metal ions through the diazine or diazole (@N–N@) groups, respectively [5]. Such binuclear metal arrangements are particularly interesting because these bridging moieties provide a metal–metal separation of 3.5–4.5 Å, which is reminiscent of the situation in bimetallic biosites. Pyrazole derived binucleating ligands that possess two chelating arms attached to the 3- and 5-positions of the heterocyclic ring allow to further tune metal metal separations and other characteristics * Corresponding author. Fax: +49 551 393063. ** Corresponding author. Tel./fax: +380 44 2393393. E-mail addresses: [email protected] (F. Meyer), [email protected] (I.O. Fritsky). 0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2010.04.038
of the bimetallic core [6–8]. However, only very few manganese(III) complexes of this ligand class have yet been reported [9,10]. Furthermore, relatively few unsymmetric pyrazole-centered ligands with different chelate arms in the 3- and 5-positions have been investigated so far [11]. We here introduce a new pyrazole-derived ligand L (Chart 1) that features two bidentate coordination compartments with relatively hard donor set, namely with a carboxylate and an oximic chelate arm, which are well suited for manganese(III). Crystal structures and some physico-chemical properties of two bis(pyrazolato)-bridged dimanganese(III) complexes are reported. 2. Experimental Solvents were purified by established procedures. All other chemicals were purchased from commercial sources and were used as received. Microanalyses were performed by the Analytical Laboratory at the Institute of Inorganic Chemistry at GeorgAugust-Universität Göttingen. IR spectra (as KBr pellets) were recorded with a Digilab Excalibur, and mass spectra were measured with a Finnigan MAT 8200 (ESI-MS). NMR spectra were recorded with a Bruker Avance 500, and chemical shifts were calibrated to the residual proton and carbon signal of the solvent (DMSO-d6: dH = 2.49, dC = 39.7 ppm). Magnetic data were measured with a Quantum-Design MPMS-5S SQUID magnetometer at 0.2 and 0.5 T
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O HO
N
OH
NH
N
L Chart 1.
in the range from 2 to 295 K. The powdered samples were contained in a gel bucket and fixed in a nonmagnetic sample holder. X-Band EPR spectra were 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. Caution! Although no problems were encountered in this work, transition metal perchlorate complexes are potentially explosive and should be handled with proper precautions. Synthesis of 3-[(1E)-N-hydroxyethanimidoyl]-4-methyl-1H-pyrazole-5-carboxylic acid (L): 5-Acetyl-4-methyl-1H-pyrazole-3-carboxylic acid [12] (3 g, 0.018 mol), NH2OHHCl (1.86 g, 0.027 mol), and CH3COONa (2.80 g, 0.027 mol) were dissolved in water (50 ml). The mixture was stirred for 2 h, and the pH was adjusted to 4 by slow addition of aqueous HCl (1:1). A white precipitate formed, which was left to stand for 10–12 h, separated by filtration and dried under vacuum. Yield: 2.2 g (67%); white solid, m.p. 220 °C. IR (KBr, cm1) 1690 (s), 1640 (m), 1590 (s), 1400 (m), 1370 (m), 1300 (m), 1260 (m), 1240 (s), 1200 (m), 1130 (w), 1070 (w), 1005 (m), 960 (w), 925 (w), 790 (m), 725 (w), 650 (w). 1 H NMR (500 MHz, DMSO-d6): d = 2.13 (s, 3H, CH3), 2.34 (s, 3H, CH3), 11.08 (br, 1H, NOH). 13C NMR (500 MHz, DMSO-d6): d = 29.9 (CH3); 30.6 (CH3); 121.1; 125.6; 129.2 (3CPz). MS (EI, 70 eV): m/z (%) = 182 (75) [MH]+. Anal. Calc. for C7H9N3O3: C, 45.90; H, 4.95; N, 22.94. Found: C, 45.23; H, 5.36; N, 22.44%. [MnIII2(L–3H)2(CH3OH)4]2CH3OH (1) and [MnIII2(L–3H)2(Py)4] 2Py (2): To a solution of L (46 mg, 0.25 mmol) in methanol (20 ml) was added Mn(CH3COO)32H2O (67 mg, 0.25 mmol). The mixture was kept in a sonifier at room temperature for 5 min.
Table 1 Crystal data and structure refinement for 1 and 2.
Empirical formula Formula weight (g/ mol) Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dcalc (g cm3) l (mm1) F(0 0 0) T (K) h range (°) Reflections collected Reflections unique Goodness-of-fit (GoF) R1/wR2* [I > 2r(I)] R1/wR2* (all data)
*
[MnIII2(L– 3H)2(CH3OH)4]2CH3OH (1)
[MnIII2(L– 3H)2(Py)4]2Py (2)
C20H36N6O12Mn2 662.43
C44H42N12O6Mn2 944.78
triclinic P1 8.0217(7) 9.1712(8) 10.6930(9) 86.768(7) 73.846(7) 66.642(6) 692.39(10) 1 1.589 0.982 344 133(2) 1.99–27.32 13 544
triclinic P1 10.0581(10) 10.9889(11) 11.0912(11) 91.285(8) 113.080(8) 106.438(8) 1069.24(18) 1 1.467 0.655 488 133(2) 1.95–24.65 15 713
3123 1.058
3584 1.041
0.0294/0.0752 0.0434/0.1114 0.0376/0.0776 0.0603/0.1181 n h i h io1=2 R1 ¼ RðF o F c Þ=RF o ; wR2 ¼ R wðF 2o F 2c Þ2 =R wðF 2o Þ2 .
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The resulting green solution was filtered and salted-out with ether (in case of 1) or with pyridine (in case of 2) to gradually yield green crystals of 1 (77 mg, 93%) or 2 (113 mg, 96%), respectively. 1: IR (KBr, cm1) 3460 (br, s), mC@N 1650 (vs), mC@O 1600 (m), 1524 (m), 1431 (m), 1381 (m), 1321 (m), 1278 (vs), 1231 (s), 1138 (w), mN–O 1092 (s), 1055 (s), 964 (m), 855 (m), 801 (m), 732 (m), 651 (m), 542 (m), 462 (m), 430 (w). ESI–MS (DMF) m/z (%) = 639 (100) [Mn2(L–3H)2(CH3OH)3DMF–1H]. Anal. Calc. for C20H36N6O12Mn2: C, 36.26; H, 5.48; N, 12.69. Found: C, 35.79; H, 4.63; N, 13.18%. 2: IR (KBr, cm1) 3450 (br, s), 1881 (w), mC@N 1653 (vs), mC@O 1597 (m), 1518 (w), 1441 (s), 1403 (m), 1376 (w), 1314 (m), 1262 (vs), 1219 (s), 1152 (w), mN–O 1088 (m), 1036 (m), 1007 (m), 941 (w), 841 (s), 804 (w), 752 (m), 725 (m), 702 (s), 640 (m), 527 (w), 458 (m), 424 (m). Anal. Calc. for C44H42N12O6Mn2: C, 55.94; H, 4.48; N, 17.79. Found: C, 55.81; H, 4.36; N, 17.63%. Crystal structure determination: Details of the data collection and processing, structure analysis and refinement are summarized in Table 1. Diffraction data were collected on a Xcalibur-3 diffractometer (x and u scans) equipped with graphite-monochromated Mo Ka radiation (k = 0.71073 Å). The data were corrected for Lorentz-polarization effects and for the effects of absorption (analytical method using a multifaceted crystal model). The structures were solved by direct methods and refined by full-matrix leastsquares methods on F2 using the SHELX 97 set of programs [13]. Assignment of each of the metal atoms is based on refinement and on comparison of the bond lengths with the literature values.
3. Results and discussion 3.1. Synthesis and structural characterization of complexes The new ligand L is readily obtained from 5-acetyl-4-methyl1H-pyrazole-3-carboxylic acid and hydroxylamine, and manganese(III) complexes are best obtained from the reaction of the ligand with Mn(CH3COO)32H2O in a sonifier. Green crystals of 1 and 2 were grown by slow diffusion of diethyl ether (1) or pyridine vapour (2) into methanol solutions of the crude complexes, and the crystal structures were determined by X-ray crystallography (Fig. 1, Table 2). The crystals of the complexes consist of the centrosymmetric neutral molecules [Mn2(L–3H)2(CH3OH)4] (1) or [Mn2(L–3H)2(Py)4] (2) and two solvate methanol or pyridine molecules for 1 or 2, respectively. The complex ions are composed of two manganese(III) ions (the distance Mn(1) Mn(10 ) (x + 1,y + 1,z + 1) is 3.9544(6) Å and 3.9932(9) Å for 1 and 2, respectively), two ligands in their triply deprotonated form ([L– 3H]3) and four metal-bound solvent molecules. All manganese(III) ions are found six-coordinate with the typical Jahn–Teller situation of four relatively short and two significantly longer bonds. Each of the triply deprotonated ligands [L– 3H]3provides both manganese center with two donors, namely the pyrazolate nitrogen and the carboxylate oxygen donors [d(Mn–N) = 2.020(1)–2.022(2) Å and d(Mn–O) = 1.973(1)– 1.942(2) Å] or the pyrazolate nitrogen and the oxime oxygen donors [d(Mn–N) = 1.978(1)–1.972(2) Å and d(Mn–O) = 1.833(1)– 1.852(2) Å], which occupy the basal plane of the axially elongated octahedron. Apical sites are occupied by solvent molecules (viz. methanol [2.216(1)–2.235(1) Å] or pyridine [2.340(2)–2.367(3) Å] for 1 and 2, respectively). The manganese atom forms a five-membered chelate ring [Mn(1)–O(20 /40 )–C(70 )–C(30 )–N(20 )] with the [L– 3H]3 scaffold when the coordination involves the oxygen atom of the carboxylate group, but a six-membered chelate ring [Mn(1)– N(1)–C(2)–C(5)–N(3)–O(1)] when the oxygen atom of the oxime group is the coordinating atom. As a result of the double pyrazolate bridging a six-membered central bimetallic ring [Mn(1)–N(1)–
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Fig. 1. View of the molecular structures of 1 and 2. All protons have been omitted for clarity.
Table 2 Selected interatomic distances (Å) and angles (°) for 1 and 2. 1 Mn(1)–O(1) Mn(1)–O(40 ) Mn(1)–O(6) Mn(1)–O(7) Mn(1)–N(1) Mn(1)–N(20 ) Mn(1) Mn(10 ) N(1)–N(2) N(3)–O(1) O(1)–Mn(1)–O(40 ) O(1)–Mn(1)–O(6) O(1)–Mn(1)–O(7) O(1)–Mn(1)–N(1) O(1)–Mn(1)–N(20 ) O(40 )–Mn(1)–O(7) O(40 )–Mn(1)–O(6) O(40 )–Mn(1)–N(1) O(40 )–Mn(1)–N(20 ) O(7)–Mn(1)–O(6) N(1)–Mn(1)–O(6) N(1)–Mn(1)–O(7) N(1)–Mn(1)–N(20 ) N(20 )–Mn(1)–O(6) N(20 )–Mn(1)–O(7)
2 1.833(1) 1.973(1) 2.235(1) 2.216(1) 1.978(1) 2.020(1) 3.9544(6) 1.338(2) 1.355(2) 92.45(6) 89.18(6) 92.25(6) 89.11(6) 172.87(6) 90.55(5) 89.71(5) 178.43(6) 80.42(5) 178.53(5) 90.16(6) 89.54(5) 98.02(6) 90.56(6) 88.06(6)
Mn(1)–O(1) Mn(1)–O(20 ) Mn(1)–N(1) Mn(1)–N(20 ) Mn(1)–N(4) Mn(1)–N(5) Mn(1) Mn(10 ) N(1)–N(2) N(3)–O(1) O(1)–Mn(1)–O(20 ) O(1)–Mn(1)–N(1) O(1)–Mn(1)–N(20 ) O(1)–Mn(1)–N(4) O(1)–Mn(1)–N(5) O(20 )–Mn(1)–N(1) O(20 )–Mn(1)–N(20 ) O(20 )–Mn(1)–N(4) O(20 )–Mn(1)–N(5) N(1)–Mn(1)–N(20 ) N(1)–Mn(1)–N(4) N(1)–Mn(1)–N(5) N(20 )–Mn(1)–N(4) N(20 )–Mn(1)–N(5) N(4)–Mn(1)–N(5)
1.852(2) 1.942(2) 1.972(2) 2.022(2) 2.340(2) 2.367(3) 3.9932(9) 1.340(3) 1.391(4) 92.63(9) 89.12(9) 174.23(9) 89.77(9) 88.31(9) 178.16(1) 81.60(9) 89.99(9) 89.68(9) 96.64(9) 90.58(9) 89.82(9) 90.48(9) 91.39(9) 178.03(9)
Symmetry transformations used to generate equivalent atoms: 0 x + 1, y + 1, z + 1.
N(2)–Mn(10 )–N(10 )–N(20 )] is present in the complexes. The mean planes of the {MnN2O2} and {Mn2N4} motifs are rigorously coplanar, and the manganese(III) atoms lie in the mean-square planes of the complexes by symmetry. Interestingly, in both complexes 1 and 2 the ligands strands are oriented antiparallel with respect to each other, i.e., each Mn ion is coordinated by one carboxylate and one oxime oxygen atom. Formation of only this isomer that has an inversion center is evidently dictated by thermodynamic reasons. In the case of the unobserved C2v isomer with parallel ligand strands, the two metal ions would have different coordination environments, with one of them sur-
rounded by three six-membered chelate rings. Such situation causes significant conformational tension, and consequently the C2v isomer is disfavored both in solution and in the solid state. Xray check of the unit cell parameters of randomly chosen single crystals from the synthetic crops of both 1 and 2 confirmed the presence of only one crystal phase corresponding to the Ci isomer. While the planar central [Mn2(L–3H)2] cores in 1 and 2 are basically identical, crystal packing is significantly different and is obviously determined by the solvent molecules that are bound in axial positions. In 1 individual complex molecules are associated via intermolecular hydrogen bonds that involve the OH of the coordinated methanol molecules and the non-coordinating carboxylateO and oxime-N atoms (Fig. 2, Table 3). In contrast, the neutral complex molecules of 2 are united into chains along the crystallographic b -axis by means of offset p-stacking interactions between the pyridine rings (interplanar spacing 3.396 Å) (Fig. 3) [14]. Diffuse reflectance spectra of both complexes show an intense charge transfer at 280 nm. The broad bands observed at ca. 415 nm and 600 nm can be assigned to 5B1g ? 5Eg, 5B1g ? 5B2g or 5 B1g ? 5A1g ligand field transitions, respectively, in accordance with a distorted octahedral arrangement around the manganese(III) ions [15]. Solid-state magnetic susceptibilities of powdered samples of the complexes were measured in the temperature range 2–295 K. The temperature dependence of vMT is depicted in Fig. 4. The room temperature values for vMT are 6.42 and 5.98 cm3 K mol1 for 1 and 2, respectively, which is close to the expected value for two uncoupled high-spin Mn(III) (S = 2) ions (Calc. vMT = 6.00 cm3 K mol1). vMT decreases upon lowering the temperature, very gradually in the high-temperature region but more abruptly at low temperature, down to 0.48 and 0.69 cm3 K mol1 at 2 K for 1 and 2, respectively. Such behavior suggests the presence of antiferromagnetic coupling between the manganese(III) ions. Experimental data for complexes 1 and 2 were modelled by using a fitting procedure to the appropriate Heisenberg–Dirac– van-Vleck (HDvV) spin Hamiltonian for isotropic exchange coupling and Zeeman splitting, equation (1) [16]
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Fig. 2. Fragment of the crystal structure of 1.
Table 3 Hydrogen bond distances (Å) and angles (°) for 1. D–H A 00
O(6)–H(6) N(3 ) O(7)–H(7) O(8) O(8)–H(8) O(5000 )
d(D–H)
d(H A)
d(D A)
\(DHA)
0.81(3) 0.71(3) 0.75(3)
1.91(3) 1.95(3) 2.01(3)
2.717(2) 2.647(2) 2.756(2)
174(3) 166(3) 173(3)
Symmetry transformations used to generate equivalent atoms: 00 x + 1, y, z + 1; x + 1, y 1, z.
000
^ ¼ 2J^S1 ^S2 þ g l ð^S1 þ ^S2 ÞB H B
ð1Þ
Temperature-independent paramagnetism (TIP) and a Curie-behaved paramagnetic impurity (PI) were included according to vcalc = (1 PI)v + PIvmono + TIP [17]. For both complexes slightly better agreement with experimental data was obtained when intermolecular interactions were considered in a mean field approach by using a Weiss temperature H [16,18]. Best simulation parameters are g = 1.99, J = 3.6 cm1, H = 2.02 K for 1 and g = 2.00, J = 3.7 cm1, H = 1.43 K for 2 [19]. The Weiss temperature H relates to intermolecular interactions zJ0 of 0.7 cm1 for 1 and 0.5 cm1 for 2, where J0 is the interaction parameter between two nearest neighbour magnetic centers and z is the number of nearest neighbours [20]. Given the very similar, planar {Mn2N4} cores in 1 and 2, it is not surprising that intramolecular antiferro-
magnetic coupling in both complexes is basically identical. On the other hand, mutual orientation of the {Mn2N4} cores in the crystals is very different and is obviously dictated by the intermolecular Hbonding contacts (in 1) or p-stacking (in 2), which leads to fundamentally different intermolecular magnetic interactions, either antiferromagnetic (in 1) or ferromagnetic (in 2). These two systems thus provide valuable benchmark data for magnetic coupling in planar bis(pyrazolato)-bridged dimanganese(III) cores and at the same time demonstrate the significant effect of weak intermolecular contacts on the bulk magnetic properties. It should be noted that magnetic coupling in a bis(pyrazolato)-bridged dimanganese(II) complex is much weaker (J = 0.40 cm1) [9]. 4. Conclusions In summary, two new pyrazolato-bridged dinuclear manganese(III) complexes of an unsymmetrical ligand have been prepared. The ligand contains two distinct compartments that are formed by the pyrazolate-N and the oxime or the carboxylic groups. The complexes were characterized by X-ray single crystal diffraction, revealing both 1 and 2 consist of dinuclear units in which the two metal ions are doubly linked by pyrazolate bridges with a planar {Mn2N4} core. The magnetic behavior of these manganese(III) complexes is characterized by intradinuclear superexchange interactions that give rise to moderate antiferromagnetic
Fig. 3. Fragment of the crystal structure of 2.
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Fig. 4. Plots of vMT vs. temperature for 1 (left) and 2 (right) at 0.2 T; the solid lines represent the calculated curve fits.
coupling between the dimanganese(III) units, whereas distinct crystal packing is reflected by different intermolecular interactions transmitted by weak H-bonding or p-stacking contacts.
[8]
Supplementary material CCDC 766011 and 766012 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.
[9]
Acknowledgements
[10]
Financial support by the Deutsche Forschungsgemeinschaft (SFB 602, project A16) is gratefully acknowledged. We also thank the State Fund for Fundamental Research of Ukraine for financial support (Grant No. F28/241-2009).
[11]
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