Article pubs.acs.org/IC
Comproportionation Reactions to Manganese(III/IV) Pivalate Clusters: A New Half-Integer Spin Single-Molecule Magnet Shreya Mukherjee,† Khalil A. Abboud,† Wolfgang Wernsdorfer,‡ and George Christou*,† †
Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200, United States Institut Néel, CNRS/UJF, BP 166, 25 Avenue des Martyrs, 38042, Grenoble, Cedex 9, France
‡
S Supporting Information *
ABSTRACT: The comproportionation reaction between MnII and MnVII reagents under acidic conditions has been investigated in the presence of pivalic acid as a route to new high oxidation state manganese pivalate clusters containing some MnIV. The reaction of Mn(O2CBut)2 and NBun4MnO4 with an excess of pivalic acid in the presence of Mn(ClO4)2 and NBu n4Cl in hot MeCN led to the isolation of [Mn8O6(OH)(O2CBut)9Cl3(ButCO2H)0.5(MeCN)0.5] (1). In contrast, the reaction of Mn(NO3)2 and NBun4MnO4 in hot MeCN with an excess of pivalic acid gave a different octanuclear complex, [Mn8O9(O2CBut)12] (2). The latter reaction but with Mn(O 2 CBu t ) 2 in place of Mn(NO 3 ) 2 , and in a MeCN/THF solvent medium, gave [Mn9O7(O2CBut)13(THF)2] (3). Complexes 1−3 possess rare or unprecedented Mnx topologies: 1 possesses a [MnIII7MnIV(μ3-O)4(μ4-O)2(μ3-OH)(μ4-Cl)(μ2-Cl)]8+ core consisting of two body-fused Mn4 butterfly units attached to the remaining Mn atoms via bridging O2−, OH−, and Cl− ions. In contrast, 2 possesses a [Mn6IVMn2III(μ3-O)6(μ-O)3]12+ core consisting of two [Mn3O4] incomplete cubanes linked by their O2− ions to two MnIII atoms. The cores of 1 and 2 are unprecedented in Mn chemistry. The [MnIII9(μ3-O)7]13+ core of 3 also contains two body-fused Mn4 butterfly units, but they are linked to the remaining Mn atoms in a different manner than in 1. Solid-state direct current (dc) and/or alternating current (ac) magnetic susceptibility data established S = 15/2, S = 2, and S = 1 ground states for 1·MeCN, 2·1/4MeCN, and 3, respectively. The ac susceptibility data also revealed nonzero, frequency-dependent out-of-phase (χ″M) signals for 1·MeCN at temperatures below 3 K, suggesting possible single-molecule magnet behavior, which was confirmed by single-crystal magnetization vs dc field scans that exhibited hysteresis loops. The combined work thus demonstrates the continuing potential of comproportionation reactions for isolating high oxidation state Mnx clusters, and the sensitivity of the product identity to minor changes in the reaction conditions.
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agents for a large variety of organic compounds.3,4 In the arena of nanoscale magnetic materials, Mnx clusters containing MnIII often possess a large ground state spin (S) because of the presence of ferromagnetic interactions and/or spin frustration effects,5 and in combination with a sufficiently large easy-axis magnetic anisotropy (i.e., negative zero-field splitting parameter D), such clusters can be single-molecule magnets (SMMs).6 These are molecules that function as nanoscale magnets below their blocking temperature (TB),6,7 and they also display interesting quantum properties such as quantum tunneling of the magnetization (QTM)6,8 and quantum phase interference (QPI).9 The presence of octahedral MnIII is pivotal for making Mn SMMs, since Jahn−Teller elongated MnIII has significant easy-axis anisotropy (negative D), in contrast to essentially isotropic MnII and MnIV. There is thus a continuing interest in developing synthetic strategies to new MnIII-containing SMMs of various metal
INTRODUCTION Paramagnetic 3d metal clusters continue to attract a great deal of attention for a variety of reasons, such as their relevance to disparate areas such as bioinorganic chemistry and nanoscale magnetic materials. In Mn chemistry, one unifying theme between these areas is that both almost always involve the higher Mn oxidation states (MnIII/MnIV), either wholly or in combination with the lower MnII state. In bioinorganic chemistry, the oxidizing power of the higher Mn oxidation states is at the heart of the evolution of Mn-containing redox enzymes, such as the oxygen-evolving complex (OEC), also known as the water-oxidizing complex (WOC), near photosystem II (PSII) of plants and cyanobacteria.1,2 This is responsible for the sunlight-driven oxidation of water to oxygen gas, which is thermodynamically an extremely challenging transformation. The OEC is now known to comprise a [Mn3CaO4] cubane unit to which is attached a fourth, external Mn ion. This oxidizing strength of MnIII and MnIV, as well as the even stronger oxidant MnVII, also form the foundation of the long history of the use of Mn compounds as oxidizing © XXXX American Chemical Society
Received: September 17, 2012
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dx.doi.org/10.1021/ic302021a | Inorg. Chem. XXXX, XXX, XXX−XXX
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Table 1. Crystallographic Data for 1·3MeCN, 2·MeCN, and 3·1/3THF·2/3MeCN parameter a
formula fw,a g mol−1 crystal system space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z T, °C radiation, Åb ρcalc, mg/m3 μ, mm−1 R1c,d wR2e
1·3MeCN
2·MeCN
3·1/3THF·2/3MeCN
C54.5H97N3.5O26Cl3Mn8 1763.23 triclinic P1̅ 14.045(4) 14.666(4) 22.284(6) 80.803(4) 88.478(4) 64.989(4) 4101.8(19) 2 173(2) 0.71073 1.428 1.355 0.0577 0.1570
C60H108O33Mn8 1796.98 monoclinic P21/n 15.010(2) 21.876(4) 26.275(4) 90 98.750(8) 90 8527(2) 4 173(2) 0.71073 1.400 1.219 0.0369 0.0947
C73H133Mn9O35 2065.25 orthorhombic Aba2 20.555(3) 68.895(9) 21.549(3) 90 90 90 30516(7) 12 173(2) 0.71073 1.349 1.150 0.0388 0.0846
a Including solvate molecules. bGraphite monochromator. cI > 2σ(I). dR1 = [∑||Fo| − |Fc||]/∑|Fo|. ewR2 = [∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]]1/2, w = 1/[σ2(Fo2) + [(ap)2 +bp], where p = [max (Fo2, O) + 2Fc2]/3.
a lower than normal MnII:MnVII ratio to target formation of MnIV-containing products. This work has successfully led to two Mn8 and one Mn9 pivalate products, with the former two being of unprecedented structural types. The syntheses, structures and magnetochemical characterizations of these complexes are described in this paper.
topologies, nuclearities, and ground state S values. In fact, it is also important to avoid the presence of MnII if possible, because MnII leads to weak exchange coupling and low-lying excited states, which are deleterious to good SMM behavior. Unfortunately, there are few commercially available MnIII starting materials, and most MnIII-containing SMMs and other clusters have been prepared from aerial oxidation of MnII under basic conditions, which often gives mixed MnII/ MnIII products. For this reason, we have in the past occasionally used a comproportionation approach to MnIII-containing products, namely, the reaction of MnII with MnVII (MnO4−), such as for the preparation of [Mn4O2(O2CPh)7(H2O)2]−10 and [Mn4O2(O2CR)7(pic)2]− salts (picH = piclinic acid), shown in eq 1.11 4Mn 2 + + Mn 7 + → 5Mn 3 +
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EXPERIMENTAL SECTION
Syntheses. All manipulations were performed under aerobic conditions using chemicals (reagent grade) and solvents as received. NBun4MnO417 and Mn(O2CBut)218 were prepared as previously reported. Caution! Perchlorate salts are potentially explosive; such compounds should be synthesized and used in small quantities, and treated with utmost care at all times. [Mn8O6(OH)(O2CBut)9Cl3(ButCO2H)0.5(MeCN)0.5] (1). To a stirred solution of Mn(O2CBut)2·2H2O (0.16 g, 0.50 mmol) and ButCO2H (2.83 mL, 24.6 mmol) in hot MeCN (20 mL, ∼80 °C) was added solid Mn(ClO4)2·6H2O (0.18 g, 0.50 mmol) and NBun4Cl (0.14 g, 0.50 mmol). The light pink slurry was stirred for 5 min, and then solid NBun4MnO4 (0.18 g, 0.50 mmol) was slowly added in small portions, resulting in the formation of a dark brown-black solution. This was stirred for a further 15 min, allowed to cool to ambient temperature, filtered, and the filtrate was allowed to stand undisturbed at room temperature. After 2 weeks, the resulting X-ray quality dark brown crystals of 1·3MeCN were collected by filtration and dried in vacuo. The yield was 25%. Anal. Calcd (Found) for 1·MeCN (C50.5H91.5N1.5Mn8O26Cl3): C, 36.06 (35.77); H, 5.48 (5.33); N, 1.25 (1.31). Selected IR data (cm−1): 3392 (w), 2971 (m), 2874 (s), 1671 (s), 1533 (s), 1484 (s), 1458 (m), 1421 (s), 1378 (s), 1364 (s), 1226 (s), 1032 (m), 938 (m), 896 (m), 656 (s), 621 (s), 597 (s), 480 (m), 458 (m). [Mn8O9(O2CBut)12] (2). To a stirred solution of Mn(NO3)2 (0.09 g, 0.50 mmol) and ButCO2H (1.89 mL, 16.4 mmol) in hot MeCN (25 mL, ∼80 °C) was slowly added NBun4MnO4 (0.27 g, 0.75 mmol). The solution was stirred for 15 min during which time the pink slurry changed to a dark red solution. The solution was allowed to cool to ambient temperature, filtered, and the filtrate allowed to stand undisturbed at room temperature. X-ray quality deep red crystals of 2·MeCN slowly grew over a week, and they were collected by filtration and dried in vacuo. The yield was 36%. Anal. Calcd (Found) for 2·1/4MeCN (C60.5H108.75N0.25Mn8O33): C, 40.21 (40.37); H, 6.07
(1)
The precedent for this approach in MnIII carboxylate cluster chemistry was provided, of course, by the synthesis of {[MnIII3O(O2CMe)6](O2CMe)(HO2CMe)}n from the reaction of Mn(O2CMe)2·4H2O with KMnO4 in acetic acid,12 and Lis’ synthesis of [MnIII/IV12O12(O2CMe)16(H2O)4]·2MeCO2H·4H2O from a lower MnII:MnVII ratio.13 These also emphasize that control of the MnII:MnVII ratio can provide a convenient means to target mixed MnIII/MnIV products, which can often help prevent complications from low-lying excited states compared with MnII/MnIII and MnIII/ MnIII couplings.14 Indeed, it should be noted that the comproportionation reaction of MnII with MnVII to give MnIV−containing products has much more commonly been employed for the preparation of small nuclearity products with N-based chelating ligands such as 2,2′-bipyridine, 1,10phenanthroline, and many others.15 In the present work, we have explored comproportionation reactions between MnII salts and MnVII in the presence of pivalic acid as a potential route to new Mnx pivalate clusters. It was anticipated that the difference in bulk and the higher pKa of pivalic acid (pKa = 5.03) compared with acetic acid (pKa = 4.75) and benzoic acid (pKa = 4.20) would allow access to distinctly different manganese complexes.16 We also employed B
dx.doi.org/10.1021/ic302021a | Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 1. Structure of complex 1 (top), a stereopair (middle), and the labeled core (bottom). H atoms have been omitted for clarity. Color code: MnIV blue; MnIII green; O red; Cl purple; C gray. (6.14); N, 0.19 (0.21). Selected IR data (cm−1): 3442 (w), 2965 (m), 2930 (m), 1599 (s), 1529 (m), 1482 (s), 1416 (s), 1362 (m), 1223 (s), 1031 (m), 937 (m), 893 (m), 785(m), 721 (m), 621 (s), 570 (m), 453 (m). [Mn9O7(O2CBut)13(THF)2] (3). To a stirred solution of Mn(O2CBut)2·2H2O (0.16 g, 0.50 mmol) and ButCO2H (1.89 mL, 16.4 mmol) in hot MeCN/THF (20 mL, 2:1 v/v, ∼80 °C) was slowly added solid NBun4MnO4 (0.18 g, 0.50 mmol) in small portions. The solution was stirred for 15 min during which time the pink slurry
changed to a dark brown solution. This was allowed to cool to ambient temperature, filtered, and the filtrate allowed to stand undisturbed at room temperature. X-ray quality black crystals of 3·1/3THF·2/3MeCN slowly grew over 3 days and were collected by filtration and dried in vacuo. The yield was 35%. Dried solid analyzed as solvent-free. Anal. Calcd (Found) for 3 (C73H133Mn9O35): C, 42.45 (42.81); H, 6.50 (6.90); N, 0.00 (0.00). Selected IR data (cm−1): 2964 (m), 2928 (m), 1565 (s), 1484 (s), 1424 (s), 1375 (m), 1360 (m), 1228 (s), 1045 (m), 893 (m), 787 (m), 699 (m), 674 (m), 625 (s), 453 (m), 471 (m). C
dx.doi.org/10.1021/ic302021a | Inorg. Chem. XXXX, XXX, XXX−XXX
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X-ray Crystallography. Data were collected on a Siemens SMART PLATFORM equipped with a CCD area detector and a graphite monochromator utilizing Mo Kα radiation (λ = 0.71073 Å). Suitable crystals of 1·3MeCN, 2·MeCN, and 3·1/3THF·2/3MeCN were attached to glass fibers using silicone grease and transferred to a goniostat where they were cooled to 173 K for data collection. Data collection parameters are listed in Table 1. Cell parameters were refined using 8192 reflections. A full sphere of data (1850 frames) was collected using the ω-scan method (0.3° frame-width). The first 50 frames were remeasured at the end of data collection to monitor instrument and crystal stability (maximum correction on I was 2σ(I) to yield R1 and wR2 of 5.77 and 15.70%, respectively. For 2·MeCN, the asymmetric unit consists of a Mn8 cluster and a MeCN solvent molecule. The latter was too disordered to be modeled properly, thus program SQUEEZE,20 a part of the PLATON package of crystallographic software, was used to calculate the solvent disorder area and remove its contribution to the overall intensity data. A total of 950 parameters were included in the final cycle of refinement using 15351 reflections with I > 2σ(I) to yield R1 and wR2 of 3.69 and 9.47%, respectively. For 3·1/3THF·2/3MeCN, the asymmetric unit consists of a Mn9 cluster in a general position, a half Mn9 cluster located on a 2-fold rotation axis, a THF solvent molecule disordered around a 2-fold rotation axis, and an MeCN solvent molecule in a general position. The solvent molecules were again too disordered to be modeled properly, and SQUEEZE was used to remove their contribution to the overall intensity data. One pivalate ligand (C102) has the Me groups disordered because of symmetry, and each disordered atom was given a 0.5 occupancy. The cluster in a general position has five disordered pivalates (C11, C20, C39, C44, and C65), and each was refined in two parts with their site occupancies independently refined. A total of 1586 parameters were included in the final cycle of refinement using 28402 reflections with I > 2σ(I) to yield R1 and wR2 of 3.88 and 8.46%, respectively. Other Studies. Infrared spectra were recorded in the solid state (KBr pellets) on a Nicolet Nexus 670 FTIR spectrometer in the 400− 4000 cm−1 range. Elemental analyses (C, H, and N) were performed by the in-house facilities of the University of Florida, Chemistry Department. Variable-temperature direct current (DC) and alternating current (AC) magnetic susceptibility data were collected on a Quantum Design MPMS-XL SQUID susceptometer equipped with a 7 T magnet and operating in the 1.8−300 K range. Pascal’s constants21 were used to estimate the diamagnetic corrections, which were subtracted from the experimental susceptibilities to give the molar paramagnetic susceptibilities (χM). Low temperature (
