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Unformatted text preview: Inorg. Chem. 2010, 49, 10579–10589 10579 DOI: 10.1021/ic101594d Mn8 and Mn16 Clusters from the Use of 2-(Hydroxymethyl)pyridine, and Comparison with the Products from Bulkier Chelates: A New High Nuclearity Single-Molecule Magnet Taketo Taguchi,† Wolfgang Wernsdorfer,‡ Khalil A. Abboud,† and George Christou*,† † ‡ Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200, United States, and   Institut Neel, CNRS and Universite J. Fourier, BP 166, 38042 Grenoble Cedex 9, France Received August 6, 2010 The synthesis, crystal structures, and magnetochemical characterization of two new Mn clusters [Mn8O2(O2CPh)10(hmp)4(MeOH)2] (1; 6MnII, 2MnIII) and [Mn16O8(OH)2(O2CPh)12(hmp)10(H2O)2](O2CPh)2 (2; 6MnII, 10MnIII) are reported. They were obtained from the use of 2-(hydroxymethyl)pyridine (hmpH) under the same reaction conditions but differing in the presence or absence of added base. Thus, the reaction of hmpH with Mn(O2CPh)2 in CH2Cl2/MeOH led to isolation of octanuclear complex 1, whereas the analogous reaction in the presence of NEt3 gave hexadecanuclear complex 2. Complexes 1 and 2 possess either very rare or unprecedented core structures that are related to each other: that of 1 can be described as a linked pair of incomplete [Mn4O3] cubanes, while that of 2 consists of a linked pair of complete [Mn4O4] cubanes, on either side of which is attached a tetrahedral [Mn4(μ4-O)] unit. Solid-state direct current (dc) and alternating current (ac) magnetic susceptibility measurements on 1 and 2 establish that they possess S = 5 and 8 ground states, respectively. Complex 2 exhibits frequency-dependent out-of-phase (χM00 ) ac susceptibility signals at temperatures below 3 K suggestive of a single-molecule magnet (SMM). Magnetization versus applied dc field sweeps on single crystals of 2 3 10MeOH down to 0.04 K exhibited hysteresis, confirming 2 to be a new SMM. Comparison of the structure of 2 (Mn16) with Mn12 or Mn6 clusters previously obtained under the same reaction conditions but with two Me or two Ph groups, respectively, added next to the alkoxide O atom of hmp- indicate their influence on the nuclearity and structure of the products as being due to the overall bulk of the chelate plus the decreased ability of the O atom to bridge. Introduction Polynuclear Mn clusters have been of great interest from a variety of perspectives, including bioinorganic chemistry and materials science. In the former area, it has been found that a pentanuclear Mn4Ca cluster is an integral component of the photosystem II reaction center of green plants and cyanobacteria,1 which is responsible for the light-driven oxidation of water to O2 gas.2 From a materials point of view, Mn clusters often possess large numbers of unpaired electrons, offering the potential of acting as repeating units in moleculebased magnetic materials and even of functioning as nanoscale *To whom correspondence should be addressed. E-mail: christou@ chem.ufl.edu. Phone: þ1-352-392-8314. Fax: þ1-352-392-8757. (1) (a) Ferreira, K. N.; Iverson, T. M.; Maghlaoui, K.; Barber, J.; Iwata, S. Science 2004, 303, 1831. (b) Carrell, T. G.; Tyryshkin, A. M.; Dismukes, G. C. J. Biol. Inorg. Chem. 2002, 7, 2. (c) Cinco, R. M.; Rompel, A.; Visser, H.; Aromi, G.; Christou, G.; Sauer, K.; Klein, M. P.; Yachandra, V. K. Inorg. Chem. 1999, 38, 5988. (d) Yachandra, V. K.; Sauer, K.; Klein, M. P. Chem. Rev. 1996, 96, 2927. (2) (a) Barber, J. Inorg. Chem. 2008, 47, 1700–1710. (b) Barber, J.; Murray, J. W. Phil. Trans. R. Soc. B 2008, 363, 1129–1138. (3) (a) Bagai, R.; Christou, G. Chem. Soc. Rev. 2009, 38(4), 1011, and references cited therein. (b) Christou, G.; Gatteschi, D.; Hendrickson, D. N.; Sessoli, R. MRS Bull. 2000, 25, 66. r 2010 American Chemical Society magnetic particles themselves.3 The latter are called singlemolecule magnets (SMMs), which are individual molecules that possess a significant barrier (versus kT) to magnetization relaxation and thus exhibit the ability to function as magnets below the blocking temperature (TB). Many polynuclear clusters containing 3d transition metals have now been discovered to be SMMs,4-6 the vast majority of which are Mn complexes7 because Mn clusters often display relatively large ground state S values, as well as negative D values (easy-axis (4) For example, see: (a) Sessoli, R.; Gatteschi, D.; Caneschi, A.; Novak, M. A. Nature 1993, 365, 141. (b) Sessoli, R.; Tsai, H. L.; Schake, A. R.; Wang, S. Y.; Vincent, J. B.; Folting, K.; Gatteschi, D.; Christou, G.; Hendrickson, D. N. J. Am. Chem. Soc. 1993, 115, 1804. (5) (a) Gatteschi, D.; Sessoli, R.; Cornia, A. Chem. Commun. 2000, 725. (b) Sun, Z. M.; Grant, C. M.; Castro, S. L.; Hendrickson, D. N.; Christou, G. Chem. Commun. 1998, 721. (c) Yang, E. C.; Hendrickson, D. N.; Wernsdorfer, W.; Nakano, M.; Zakharov, L. N.; Sommer, R. D.; Rheingold, A. L.; Ledezma-Gairaud, M.; Christou, G. J. Appl. Phys. 2002, 91, 7382. (6) (a) Andres, H.; Basler, R.; Blake, A. J.; Cadiou, C.; Chaboussant, G.; Grant, C. M.; Gudel, H. U.; Murrie, M.; Parsons, S.; Paulsen, C.; Semadini, F.; Villar, V.; Wernsdorfer, W.; Winpenny, R. E. P. Chem.;Eur. J. 2002, 8, 4867. (b) Murugesu, M.; Mishra, A.; Wernsdorfer, W.; Abboud, K. A.; Christou, G. Polyhedron 2006, 25, 613. Published on Web 10/20/2010 pubs.acs.org/IC 10580 Inorganic Chemistry, Vol. 49, No. 22, 2010 Taguchi et al. Scheme 1 anisotropy) associated with the presence of Jahn-Teller axially distorted MnIII atoms. For the above reasons, there is a continuing need for new synthetic routes to metal clusters and SMMs. Our own group has contributed a variety of such methods over the last several years, from “reductive aggregation” of MnO4- as a route to new Mn12 and Mn16 clusters,8 to a methanolysis procedure to a giant Mn84 torus,9 and a number of others. A generally useful route explored by many groups has been the use of a chelate containing one or more alcohol functionalities, which on deprotonation favor bridging to at least two (7) (a) Christou, G. Polyhedron 2005, 24, 2065. (b) Gatteschi, D.; Sessoli, R.; Villain, J.; Molecular Nanomagnets; Oxford University Press: New York, 2006. (c) Aromi, G.; Brechin, E. K. Struct. Bonding (Berlin) 2006, 122, 1, and references therein. (8) (a) King, P.; Wernsdorfer, W.; Abboud, K. A.; Christou, G. Inorg. Chem. 2004, 43, 7315. (b) Tasiopoulos, A. J.; Wernsdorfer, W.; Abboud, K. A.; Christou, G. Inorg. Chem. 2005, 44, 6324. (9) Tasiopoulos, A. J.; Vinslava, A.; Wernsdorfer, W.; Abboud, K. A.; Christou, G. Angew. Chem., Int. Ed. 2004, 43, 2117. (10) (a) Foguet-Albiol, D.; O’Brien, T. A.; Wernsdorfer, W.; Moulton, B.; Zaworotko, M. J.; Abboud, K. A.; Christou, G. Angew. Chem., Int. Ed. 2005, 44, 897. (b) Bagai, R.; Abboud, K. A.; Christou, G. Inorg. Chem. 2008, 47, 621. (c) Murugesu, M.; Wernsdorfer, W.; Abboud, K. A.; Christou, G. Angew. Chem., Int. Ed. 2005, 44, 892. (11) (a) Brechin, E. K. Chem. Commun. 2005, 5141. (b) Milios, C. J.; Vinslava, A.; Wernsdorfer.; Prescimone, A.; Wood, P. A.; Parsons, S.; Perlepes, S. P.; Christou, G.; Brechin, E. K. J. Am. Chem. Soc. 2007, 129, 6547. (c) Milios, C. J.; Inglis, R.; Vinslava, A.; Bagai, R.; Wernsdorfer, W.; Parsons, S.; Perlepes, S. P.; Christou, G.; Brechin, E. K. J. Am. Chem. Soc. 2007, 129, 12505. (12) (a) Moushi, E. E.; Masello, A.; Wernsdorfer, W.; Nastopoulos, V.; Christou, G.; Tasiopoulos, A. J. Dalton Trans. 2010, 4978. (b) Moushi, E. E.; Stamatatos, T. C.; Wernsdorfer, W.; Nastopoulos, V.; Christou, G.; Tasiopoulos, A. J. Inorg. Chem. 2009, 48, 5049. (c) Tasiopoulos, A. J.; Perlepes, S. P. Dalton Trans. 2008, 5537. (13) (a) Langley, S. K.; Chilton, N. F.; Massi, M.; Moubaraki, B.; Berry, K. J.; Murray, K. S. Dalton Trans. 2010, 7236. (b) Wittick, L. M.; Jones, L. F.; Jensen, P.; Moubaraki, B.; Spiccia, L.; Berry, K. J.; Murray, K. S. Dalton Trans. 2006, 1534. (c) Wittick, L. M.; Murray, K. S.; Moubaraki, B.; Batten, S. R.; Spiccia, L.; Berry, K. J. Dalton Trans. 2004, 1003. (14) (a) Milios, C. J.; Kefalloniti, E.; Raptopoulou, C. P.; Terzis, A.; Vicente, R.; Lalioti, N.; Escuer, A.; Perlepes, S. P. Chem. Commun. 2003, 819. (b) Milios, C. J.; Kyritsis, P.; Raptopoulou, C. P.; Terzis, A.; Vicente, R.; Escuer, A.; Perlepes, S. P. Dalton Trans. 2005, 501. (c) Milios, C. J.; Raptopoulou, C. P.; Terzis, A.; Vicente, R.; Escuer, A.; Perlepes, S. P. Inorg. Chem. Commun. 2003, 6, 1056. (15) (a) Nayak, S.; Beltran, L. M. C.; Lan, Y.; Clerac, R.; Hearns, N. G. R.; Wernsdorfer, W.; Anson, C. E.; Powell, A. K. Dalton Trans. 2009, 1901. (b) Nayak, S.; Lan, Y.; Clrac, R.; Ansona, C. E.; Powell, A. K. e Chem. Commun. 2008, 5698. (c) Ako, A. M.; Hewitt, I. J.; Mereacre, V.; Clrac, e R.; Wernsdorfer, W.; Anson, C. E.; Powell, A. K. Angew. Chem., Int. Ed. 2006, 45, 4926. (16) (a) Saalfrank, R. W.; Scheurer, A.; Prakash, R.; Heinemann, F. W.; Nakajima, T.; Hampel, F.; Leppin, R.; Pilawa, B.; Rupp, H.; Muller, P. Inorg. Chem. 2007, 46, 1586. (b) Saalfrank, R. W.; Nakajima, T.; Mooren, N.; Scheurer, A.; Maid, H.; Hampel, F.; Trieflinger, C.; Daub, J. Eur. J. Inorg. Chem. 2005, 1149. (c) Saalfrank, R. W.; Bernt, I.; Chowdhry, M. M.; Hampel, F.; Vaughan, G. B. M. Chem.;Eur. J. 2001, 7, 2765. metal atoms and thus fostering formation of high nuclearity products.10-16 One challenge that we recently took on was to see whether for a particular alcohol-containing chelate type we could develop some level of even rudimentary control of the nuclearity of the obtained products by introducing controlled modifications to the chelating/bridging ligand. One favored approach that we have been investigating has involved the introduction of substituents of controllable bulk near the alkoxide functional group of a well explored ligand type to see if and how they might affect the identity of the products compared to those given by the unmodified ligand. Even if the nuclearity was not affected, this seemed at the very least to be a way of potentially perturbing the structures and resulting magnetic properties, but we were optimistic that it would also prove a source of new species with altered nuclearity and interesting structural and magnetic properties. Initial work has concentrated on 2-(hydroxymethyl)pyridine (hmpH), one of the first alcohol-containing chelates employed in Mn cluster chemistry17 and which has since been found to be the source of many products of various nuclearity.18-21 We replaced the H atoms at the CH2 position of the alcohol arm with either Me and Ph groups to give diphenyl-hmpH (dphmpH) and dimethyl-hmpH (dmhmpH) (Scheme 1) and found that the resulting deprotonated groups indeed led to very different products than did hmp-.22 In general, it was (17) Bouwman, E.; Bolcar, M. A.; Libby, E.; Huffman, J. C.; Folting, K.; Christou, G. Inorg. Chem. 1992, 31, 5185. (18) (a) Harden, N. C.; Bolcar, M. A.; Wernsdorfer, W.; A. Abboud, K.; Streib, W. E.; Christou, G. Inorg. Chem. 2003, 42, 7067. (b) Boskovic, C.; Brechin, E.; Streib, W. E.; Folting, K.; Bollinger, J. C.; Hendrickson, D. N.; Christou, G. J. Am. Chem. Soc. 2002, 124, 3725. (c) Stamatatos, T. C.; Abboud, K. A.; Wernsdorfer, W.; Christou, G. Angew. Chem., Int. Ed. 2006, 45, 4134. (19) (a) Sanudo, E. C.; Brechin, E. K.; Boskovic, C.; Wernsdorfer, W.; Yoo, J.; Yamaguchi, A.; Concolino, T. R.; Abboud, K. A.; Rheingold, A. L.; Ishimoto, H.; Hendrickson, D. N.; Christou, G. Polyhedron 2003, 22, 2267. (b) Sanudo, E. C.; Wernsdorfer, W.; Abboud, K. A.; Christou, G. Inorg. Chem. 2004, 43, 4137. (20) (a) Yoo, J.; Yamaguchi, A.; Nakano, M.; Krzystek, J.; Streib, W. E.; Brunel, L.-C.; Ishimoto, H.; Christou, G.; Hendrickson, D. N. Inorg. Chem. 2001, 40, 4604. (b) Lecren, L.; Roubeau, O.; Li, Y.-G.; Le Goff, X. F.; Miyasaka, H.; Richard, F.; Wernsdorfer, W.; Coulon, C.; Clrac, R. Dalton Trans. 2008, 755. e (c) Miyasaka, H.; Nakata, K.; Lecren, L.; Coulon, C.; Nakazawa, Y.; Fujisaki, T.; Sugiura, K.; Yamashita, M.; Clrac, R. J. Am. Chem. Soc. 2006, 128, 3770. e (21) (a) Lecren, L.; Wernsdorfer, W.; Li, Y.-G.; Roubeau, O.; Miyasaka, H.; Clerac, R. J. Am. Chem. Soc. 2005, 127, 11311. (b) Lecren, L.; Li, Y.-G.; Wernsdorfer, W.; Roubeau, O.; Miyasaka, H.; Clerac, R. Inorg. Chem. Commun. 2005, 8, 626. (c) Lecren, L.; Roubeau, O.; Coulon, C.; Li, Y.-G.; Le Goff, X. F.; Wernsdorfer, W.; Miyasaka, H.; Clerac, R. J. Am. Chem. Soc. 2005, 127, 17353. (d) Yang, E.-C.; Harden, N.; Wernsdorfer, W.; Zakharov, L. N.; Brechin, E. K.; Rheingold, A. L.; Christou, G.; Hendrickson, D. N. Polyhedron 2003, 22, 1857. (22) (a) Taguchi, T.; Daniels, M. R.; Abboud, K. A.; Christou, G. Inorg. Chem. 2009, 48, 9325. (b) Taguchi, T.; Wernsdorfer, W.; Abboud, K. A.; Christou, G. Inorg. Chem. 2010, 49, 199. Article clear that the two large Ph groups in dphmp- essentially blocked the ability of the alkoxide O atom to bridge and this group instead favors a bidentate chelating mode. However, in trying to expand these studies and accomplish more detailed comparisons between the products of hmpH, dmhmpH and dphmpH, it was imperative to compare products obtained under exactly the same reaction conditions, and we realized that some important “control” experiments with hmpH were not available for comparison with some dmhmp- and dphmp- products we had obtained. We have therefore gone back and further studied the use of hmpH in Mn cluster chemistry. In the present paper, we report (i) the use of hmpH in reactions with Mn reagents under the same conditions previously employed for dmhmpH and dphmpH reactions; (ii) the structure and magnetic properties of the new Mn8 and Mn16 clusters that have resulted from these hmpH reactions; and (iii) a structural comparison of the clusters from the three ligand types and conclusions about the influence of the added substituents and their increasing bulk on the resulting clusters and their nuclearities. This also provides important lessons for the design of new chelates for further studies in the future. Experimental Section Syntheses. All manipulations were performed under aerobic conditions using chemicals and solvents as received, unless otherwise stated. [Mn8O2(O2CPh)10(hmp)4(MeOH)2] (1). To a stirred solution of hmpH (0.055 g, 0.50 mmol) in CH2Cl2/MeOH (31 mL, 30:1 v/ v) was added solid Mn(O2CPh)2 (0.33 g, 1.0 mmol). The resulting light brown solution was stirred overnight, filtered, and the filtrate carefully layered with hexanes (60 mL). X-ray quality crystals of 1 3 CH2Cl2 slowly grew over 2 weeks, and they were collected by filtration, washed with CH2Cl2/hexanes (2  5 mL, 1:1 v/v) and Et2O (2  5 mL), and dried under vacuum; the yield was 61%. Anal. Calcd (Found) for 1 3 H2O (C96H84N4Mn8O29): C, 52.48 (52.32); H, 3.85 (3.61); N, 2.55 (2.49). Selected IR data (cm-1): 3446(wb), 3064(w), 2928(w), 2836(w), 1967(w), 1919(w), 1601(s), 1560(s), 1489(w), 1397(s), 1294(w), 1174(w), 1154(w), 1069(m), 1042(m), 1027(m), 938(w), 839(w), 759(m), 718(s), 689(m), 675(m), 637(m), 622(m), 603(m), 575(m), 508(w), 415(m). [Mn16O8(OH)2(O2CPh)12(hmp)10(H2O)2](O2CPh)2 (2). To a stirred solution of hmpH (0.11 g, 1.0 mmol) and NEt3 (0.42 mL, 3.0 mmol) in CH2Cl2/MeOH (31 mL, 30:1 v/v) was added solid Mn(O2CPh)2 (0.33 g, 1.0 mmol). The resulting dark brown solution was stirred overnight, filtered, and the filtrate carefully layered with hexanes (60 mL). X-ray quality crystals of 2 3 10MeOH formed over 3 weeks, and they were collected by filtration, washed with CH2Cl2/hexanes (2  5 mL, 1:1 v/v) and Et2O (2  5 mL), and dried under vacuum; the yield was 30%. Anal. Calcd (Found) for 2 3 H2O (C158H138N10Mn16O51): C, 49.01 (49.00); H, 3.59 (3.61); N, 3.62 (3.46). Selected IR data (cm-1): 3446(wb), 3060(w), 2904(w), 2813(w), 1604(s), 1564(s), 1486(w), 1395(s), 1289(w), 1218(w), 1175(w), 1154(w), 1078(m), 1049(m), 1025(m), 833(w), 757(m), 719(s), 677(m), 635(m), 603(m), 544(w), 509(w), 463(w), 430(w), 410(w). X-ray Crystallography. For 1 3 CH2Cl2, data were collected on a Siemens SMART PLATFORM equipped with a CCD area detector and a graphite monochromator utilizing Mo KR radia˚ tion (λ = 0.71073 A). Suitable crystals were attached to glass fibers using silicone grease and transferred to a goniostat where they were cooled to 100 K for data collection. 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 Inorganic Chemistry, Vol. 49, No. 22, 2010 10581 Table 1. Crystallographic Data for 1 3 CH2Cl2 and 2 3 10MeOH 1 a formula fw, g mol-1a crystal system space group ˚ a, A ˚ b, A ˚ c, A R, deg β, deg γ, deg ˚ V, A3 Z T, K ˚ radiation, Ab Fcalc, g cm-3 μ, mm-1 R1c,d wRe,f 2 C97H84Cl2Mn8N4O28 2264.10 triclinic P1 12.1580(18) 14.407(2) 28.249(4) 103.557(2) 93.985(2) 94.483(2) 4775.8(12) 2 100(2) 0.71073 1.574 1.159 0.0856 0.1745 C168H176Mn16N10O60 4174.24 triclinic P1 16.821(2) 17.187(2) 27.034(4) 71.970(2) 69.929(2) 68.573(2) 4618.4(11) 1 100(2) 0.71073 1.386 1.127 0.0715 0.1760 a Including P solvate molecules. b Graphite monochromator. c I > P P d 2σ(I).PR1 = ||Fo| - |Fc||/ |Fo|. e All data. f wR2 = [ w(Fo2 Fc2)2/ w(Fo2)2]1/2, w = 1/[σ2(Fo2) þ [(ap)2 þ bp], where a = 0.0220 (1) or 0.0844 (2), b = 65.7247 (1) or 0 (2), and p = [max(Fo2, 0) þ 2Fc2]/3. (maximum correction on I was <1%). Absorption corrections by integration were applied based on measured indexed crystal faces. For 2 3 10MeOH, data were collected at 100 K on a Bruker DUO system equipped with an APEX II area detector and a graphite monochromator utilizing Mo KR radiation. Cell parameters were refined using 9999 reflections. A hemisphere of data was collected using the ω-scan method (0.5° frame width). Absorption corrections by integration were applied based on measured indexed crystal faces. The structures were solved by direct methods in SHELXTL6,23 and refined on F2 using fullmatrix least-squares. The non-H atoms were refined anisotropically, whereas the H atoms were placed in calculated, ideal positions and refined as riding on their respective C atoms. For 1 3 CH2Cl2, the asymmetric unit consists of two half Mn8 clusters each located on an inversion center, and an ordered CH2Cl2 solvate molecule. A total of 1246 parameters were refined in the final cycle of refinement to yield R1 (10259 reflections with I > 2σ(I)) and wR2 (all 16560 reflections) of 8.56 and 17.45%, respectively. For 2 3 10MeOH, the asymmetric unit consists of a half Mn16 cluster dication located on an inversion center, two half PhCO2- counterions, and five MeOH solvate molecules. The latter were too disordered to be modeled properly, and thus program SQUEEZE, 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. The counterions were treated as rigid groups and constrained to maintain a geometry as close as possible to ideal. The water (O23) and hydroxyl (O21) protons were obtained from a difference Fourier map; the water protons were treated as riding on the parent O atom while the hydroxyl proton was refined without any constraints. A total of 993 parameters were refined in the final cycle of refinement to yield R1 (9205 reflections with I > 2σ(I)) and wR2 (all 16353 reflections) of 7.15 and 17.60%, respectively. Unit cell data and details of the structure refinements for the two complexes are listed in Table 1. 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 using a Quantum Design MPMS-XL SQUID magnetometer equipped with a 7 T magnet and operating in the (23) SHELXTL6; Bruker-AXS: Madison, WI, 2000. 10582 Inorganic Chemistry, Vol. 49, No. 22, 2010 Taguchi et al. Scheme 2 in 1:1:3 ratio in CH2Cl2/MeOH (30:1 v/v) gave [Mn12O7(OH)(OMe)2(O2CPh)12(dmhmp)4(H2O)] (4) and [Mn6O4(OMe)2(O2CPh)4(dphmp)4] (5), respectively (Scheme 2).22 In fact, we had never explored hmpH in this exact reaction system, and we now did so to allow a better comparison of the products of the three chelates under identical conditions (vide infra). The reaction was found to give a dark brown solution from which was subsequently obtained the new complex [Mn16O8(OH)2(O2CPh)12(hmp)10(H2O)2](O2CPh)2 (2) in 30% yield. Its formation is summarized in eq 2. Small variations in the Mn/hmpH/NEt3 ratio also gave 2, but if the NEt3 was omitted complex 1 was obtained (but in a lower yield of ∼40% than the optimized procedure 5 16MnðO2 CPhÞ2 þ 10hmpH þ O2 þ 7H2 O 2 f ½Mn16 O8 ðOHÞ2 ðO2 CPhÞ12 ðhmpÞ10 ðH2 OÞ2 ŠðO2 CPhÞ2 1.8-300 K range. Samples were embedded in solid eicosane to prevent torquing. Pascal’s constants24 were used to estimate the diamagnetic correction, which was subtracted from the experimental susceptibility to give the molar paramagnetic susceptibility (χM). Low-temperature (<1.8 K) hysteresis and dc magnetization relaxation studies were performed at Grenoble using an array of micro-SQUIDS.25 The high sensitivity of this magnetometer allows the study of single crystals of SMMs of the order of 10-500 μm. The field was applied parallel to the easy axis of the molecule. Crystals were maintained in mother liquor to avoid degradation and were covered with grease for protection during transfer to the micro-SQUID and subsequent cooling. Results and Discussion Syntheses. A variety of reaction conditions were systematically explored in arriving at the optimized procedures described below. The reaction of Mn(O2CPh)2 with hmpH in a 2:1 ratio in CH2Cl2/MeOH (30:1) afforded a light brown solution from which was subsequently isolated [Mn8O2(O2CPh)10(hmp)4(MeOH)2] (1). Its formation is summarized in eq 1, assuming atmospheric O2 is the oxidizing agent. The mixed solvent system was needed to ensure adequate solubility of all reagents. Small variations 1 8MnðO2 CPhÞ2 þ 4hmpH þ O2 þ H2 O þ 2MeOH f 2 ½Mn8 O2 ðO2 CPhÞ10 ðhmpÞ4 ðMeOHÞ2 Š þ 6PhCO2 H ð 1Þ in the MnII/hmpH ratio also gave complex 1. When the same reaction was carried out with either dmhmpH or dphmpH, the isolated product in both cases was the chelate-free complex [Mn6O2(O2CPh)10(MeOH)2(H2O)2] (3) with a known and common Mn6 topology26,27 (Scheme 2). In previous studies with dmhmpH and dphmpH, we had found that their reaction with Mn(O2CPh)2 and NEt3 (24) Weast, R. C. CRC handbook of Chemistry and Physics; CRC Press, Inc.: Boca Raton, FL1984. (25) (a) Wernsdorfer, W. Adv. Chem. Phys. 2001, 118, 99. (b) Wernsdorfer, W. Supercond. Sci. Technol. 2009, 22, 064013. (26) Blackman, A. G.; Huffman, J. C.; Lobkovsky, E. B.; Christou, G. Polyhedron 1992, 11, 251. (27) Schake, A. R.; Vincent, J. B.; Li, Q.; Boyd, P. D. W.; Folting, K.; Huffman, J. C.; Hendrickson, D. N.; Christou, G. Inorg. Chem. 1989, 28, 1915. þ 18PhCO2 H ð 2Þ discussed above). Increasing the amount of MeOH still gave complex 2, but it was contaminated with a small amount of 1, suggesting the latter might be an intermediate to the former. Although it is clear that the reactions that lead to 1 and 2 are very complicated and both clearly involve acid/base (e.g., water deprotonation to O2-/OH-) and redox (MnII oxidation) chemistry, and structural aggregation, it is of interest to note that the only difference in their reaction conditions is the presence or absence of NEt3, giving 2 and 1, respectively. In fact, the observations above are consistent with the first product of the reaction being 1 (6MnII, 2MnIII), whose oxidation level (average Mn2.25þ) is close to the MnII starting material and with a correspondingly low O2-/Mn ratio of 1:4. In the presence of NEt3, which facilitates further oxidation by O2 and deprotonation of water to generate more OH-/O2-, 1 undergoes further reaction and the product is 2 (6MnII, 10MnIII), with a higher oxidation level (average Mn2.63þ) and a correspondingly higher OH-/O2-/Mn ratio of 5:8. This interpretation of the reaction also suggests that there might be some structural similarity between 1 and 2, and this is indeed the case. Description of Structures. The partially labeled structure, a stereoview, and the labeled core of complex 1 are shown in Figure 1. Selected interatomic distances and angles are listed in Table 2. Complex 1 3 CH2Cl2 crystallizes in the triclinic space group P1 with two essentially superimposable independent molecules in the unit cell, both lying on inversion centers; only one will therefore be referred to below. The [Mn8(μ4-O)2(μ3-OR)4(μ-OR)4]6þ core can be described as consisting of a central [Mn6O2]10þ unit (Mn1, Mn2, Mn4 and their symmetry partners) with the commonly observed edge-sharing Mn6 bitetrahedral structure.26,27 On each end of this is attached a Mn atom by two μ3-hmp- and one benzoate group. The resulting core can also be described as two incomplete cubanes (Mn1, Mn20 , Mn3, Mn4 and Mn10 , Mn2, Mn30 and Mn40 ) joined together by Mn2-O120 and Mn20 -O12 linkages, with benzoate atoms O3 and O30 providing additional monatomic bridges. The peripheral ligation is provided by ten benzoate and four hmpgroups. Charge considerations, the metric parameters, Article Inorganic Chemistry, Vol. 49, No. 22, 2010 10583 Table 3. Bond Valence Sums for the Mn Atoms in Complex 1a atom MnII MnIII MnIV Mn1 Mn2 Mn3 Mn4 1.95 3.08 1.88 2.05 1.79 2.82 1.74 1.90 1.88 2.96 1.80 1.96 a The underlined value is the one closest to the charge for which it was calculated, and the nearest whole number can be taken as the oxidation state of that atom. Figure 1. Structure of complex 1 with intramolecular hydrogen-bonds shown as dashed lines (top), a stereopair (middle), and the labeled core. H atoms and Ph rings (except for the ipso C atoms) on benzoate groups have been omitted for clarity. Color code: MnII yellow; MnIII green; O red; N blue; C gray. ˚ Table 2. Selected Interatomic Distances (A) and Angles (deg) for 1 3 CH2Cl2 Mn1-O1 Mn1-O3 Mn1-O9 Mn1-O12 Mn1-O13 Mn1-O14 Mn2-O2 Mn2-O3 Mn2-O10 Mn2-O110 Mn2-O12 Mn2-O120 Mn3-O60 Mn3-O7 Mn3-O9 Mn3-O11 Mn3-O13 Mn3-N2 Mn4-O4 2.128(6) 2.248(6) 2.189(6) 2.196(6) 2.167(6) 2.163(6) 1.954(6) 2.435(6) 2.243(6) 1.957(6) 1.882(6) 1.887(6) 2.170(6) 2.079(6) 2.150(6) 2.436(6) 2.247(6) 2.260(8) 2.093(6) Mn4-O5 Mn4-O90 Mn4-O110 Mn4-O120 Mn4-N10 Mn1-O3-Mn2 Mn3-O9-Mn1 Mn3-O9-Mn40 Mn1-O9-Mn40 Mn20 -O11-Mn40 Mn20 -O11-Mn3 Mn40 -O11-Mn3 Mn2-O12-Mn20 Mn2-O12-Mn1 Mn20 -O12-Mn1 Mn2-O12-Mn40 Mn20 -O12-Mn40 Mn1-O12-Mn40 Mn1-O13-Mn3 2.144(6) 2.214(6) 2.250(6) 2.197(6) 2.190(8) 87.0(2) 102.0(2) 96.7(2) 99.6(2) 93.9(2) 124.4(3) 88.1(2) 96.7(3) 104.2(3) 115.5(3) 143.0(3) 97.6(2) 99.9(2) 99.6(2) and the presence of MnIII Jahn-Teller (JT) distortions (axial elongations) indicate a MnII6MnIII2 description with Mn2 and Mn20 being MnIII, as confirmed by bond valence sum (BVS) calculations (Table 3).28 As expected, the JT axes on Mn2 and Mn20 avoid the Mn-O2- bonds and lie on the O3-Mn2-O10 and O30 -Mn20 -O100 axes. The benzoates display three binding modes: six are in the (28) (a) Brown, I. D.; Altermatt, D. Acta Crystallogr. 1985, B41, 244. (b) Liu, W.; Thorp, H. H. Inorg. Chem. 1993, 32, 4102. Figure 2. Structure of the cation of 2 with intramolecular hydrogenbonds shown as dashed lines (top) and a stereopair (bottom). H atoms and Ph rings (except for the ipso C atoms) on benzoate groups have been omitted for clarity. Color code: MnII yellow; MnIII green; O red; C gray; N blue. common η1:η1:μ bridging mode, two are in the less common η2:η1:μ3 bridging mode, and the remaining two are terminally bound to MnII atoms Mn3 and Mn30 , with the unbound O atom (O8, O80 ) forming a H bond to the ˚ adjacent terminal MeOH (O8 3 3 3 O14 = 2.649(11) A). 10584 Inorganic Chemistry, Vol. 49, No. 22, 2010 Taguchi et al. ˚ Table 4. Selected Interatomic Distances (A) and Angles (deg) for 2 3 10MeOH Figure 3. Fully labeled core of complex 2. Color code: MnII yellow; MnIII green; O red. The four hmp- groups all bind in a η1:η3:μ3 mode. All the Mn atoms are six-coordinate with distorted octahedral geometry. While a number of other Mn8 complexes with a variety of metal topologies such as rodlike, serpentine, rectangular, linked Mn4 butterfly units, linked tetrahedral, and so forth have been are previously reported,29-32 the linked incomplete-dicubane core of 1 is very rare, having been seen only once before in Mn chemistry.33 The structure and a stereoview of the [Mn16O8(OH)2(hmp)10(O2CPh)12(H2O)]2þ cation of 2 are shown in Figure 2, and its core in Figure 3. Selected interatomic distances and angles are listed in Table 4. The centrosymmetric complex is mixed-valent MnII6MnIII10, as indicated by the metric parameters and confirmed by BVS calculations (Table 5), and contains an [Mn16(μ4-O)6(μ3O)2(μ3-OR)4(μ-O)2(μ3-OR)12] core consisting of two (29) (a) Godbole, M. D.; Roubeau, O.; Clerac, R.; Kooijman, H.; Spek, A. L.; Bouwman, E. Chem. Commun. 2005, 3715. (b) Brechin, E. K.; Soler, M.; Christou, G.; Helliwell, M.; Teat, S. J.; Wernsdorfer, W. Chem. Commun. 2003, 1276. (c) Brechin, E. K.; Christou, G.; Soler, M.; Helliwell, M.; Teat, S. J. Dalton Trans. 2003, 513. (30) (a) Jones, L. F.; Brechin, E. K.; Collison, D.; Raftery, J.; Teat, S. J. Inorg. Chem. 2003, 42, 6971. (b) Alvareza, C. S.; Bond, A. D.; Cave, D.; Mosquera, M. E. G.; Harron, E. A.; Layfield, R. A.; McPartlin, M.; Rawson, J. M.; Wood, P. T.; Wright, D. S. Chem. Commun. 2002, 2980. (c) Boskovic, C.; Huffman, J. C.; Christou, G. Chem. Commun. 2002, 2502. (31) (a) Tsai, H. L.; Wang, S. Y.; Folting, K.; Streib, W. E.; Hendrickson, D. N.; Christou, G. J. Am. Chem. Soc. 1995, 117, 2503. (b) Tanase, S.; Aromi, G.; Bouwman, E.; Kooijman, H.; Spek, A. L.; Reedijk, J. Chem. Commun. 2005, 3147. (c) Milios, C. J.; Kefalloniti, E.; Raptopoulou, C. P.; Terzis, A.; Vicente, R.; Lalioti, N.; Escuer, A.; Perlepes, S. P. Chem. Commun. 2003, 819. (32) (a) Wemple, M. W.; Tsai, H. L.; Wang, S. Y.; Claude, J. P.; Streib, W. E.; Huffman, J. C.; Hendrickson, D. N.; Christou, G. Inorg. Chem. 1996, 35, 6437. (b) Tasiopoulos, A. J.; Abboud, K. A.; Christou, G. Chem. Commun. 2003, 580. (c) Milios, C. J.; Fabbiani, F. P. A.; Parsons, S.; Murugesu, M.; Christou, G.; Brechin, E. K. Dalton Trans. 2006, 351. (d) Saalfrank, R. W.; Low, N.; Trummer, S.; Sheldrick, G. M.; Teichert, M.; Stalke, D. Eur. J. Inorg. Chem. 1998, 559. (33) Boskovic, C.; Wernsdorfer, W.; Folting, K.; Huffman, J. C.; Hendrickson, D. N.; Christou, G. Inorg. Chem. 2002, 41, 5107. Mn1-O1 Mn1-O2 Mn1-O3 Mn1-N1 Mn1-N2 Mn1-N3 Mn2-O4 Mn2-O13 Mn1-O18 Mn2-O22 Mn2-O24 Mn2-N4 Mn2-N5 Mn3-O1 Mn3-O6 Mn3-O8 Mn3-O11 Mn3-O18 Mn3-O20 Mn4-O40 Mn4-O7 Mn4-O10 Mn4-O19 Mn4-O20 Mn4-O220 Mn5-O9 Mn5-O15 Mn5-O190 Mn5-O22 Mn5-O23 Mn5-O24 Mn6-O4 Mn6-O19 Mn6-O190 Mn6-O20 Mn6-O21 Mn6-O24 Mn7-O2 Mn7-O8 Mn7-O13 Mn7-O14 2.165(4) 2.200(4) 2.168(5) 2.354(5) 2.270(6) 2.259(7) 2.189(4) 2.209(4) 2.413(4) 2.268(4) 2.140(4) 2.212(6) 2.183(6) 1.888(4) 1.954(4) 2.379(4) 2.148(4) 1.928(4) 1.874(4) 2.290(4) 1.973(4) 2.160(4) 1.882(4) 1.869(4) 1.949(4) 2.124(4) 2.122(5) 2.135(4) 2.341(4) 2.149(4) 2.158(4) 1.972(4) 1.886(4) 2.423(4) 1.900(4) 2.108(4) 1.930(4) 1.906(4) 2.221(4) 2.316(5) 1.965(4) Mn7-O18 Mn7-O24 Mn8-O3 Mn8-O11 Mn8-O12 Mn8-O16 Mn8-O18 Mn8-O21 Mn3-O1-Mn1 Mn7-O2-Mn1 Mn8-O3-Mn1 Mn6-O4-Mn2 Mn6-O4-Mn40 Mn2-O4-Mn40 Mn7-O8-Mn3 Mn2-O13-Mn7 Mn8-O18-Mn7 Mn8-O18-Mn3 Mn7-O18-Mn3 Mn8-O18-Mn1 Mn7-O18-Mn1 Mn3-O18-Mn1 Mn4-O19-Mn6 Mn4-O19-Mn50 Mn6-O19-Mn50 Mn4-O19-Mn60 Mn6-O19-Mn60 Mn50 -O19-Mn60 Mn4-O20-Mn3 Mn4-O20-Mn6 Mn3-O20-Mn6 Mn8-O21-Mn6 Mn40 -O22-Mn2 Mn40 -O22-Mn5 Mn2-O22-Mn5 Mn7-O24-Mn6 Mn7-O24-Mn2 Mn6-O24-Mn2 Mn7-O24-Mn5 Mn6-O24-Mn5 Mn2-O24-Mn5 1.917(4) 1.871(4) 1.897(4) 2.657(6) 2.174(5) 1.942(4) 1.903(4) 1.853(4) 107.39(19) 108.92(19) 107.45(19) 95.19(17) 100.92(17) 97.08(15) 86.46(15) 90.58(17) 127.4(2) 115.5(2) 110.16(19) 98.28(16) 100.56(17) 97.14(17) 96.26(18) 101.57(18) 160.9(2) 99.06(16) 95.15(15) 88.68(14) 121.88(19) 96.20(17) 125.5(2) 127.8(3) 105.28(18) 92.71(17) 96.93(15) 125.95(19) 106.44(19) 98.07(17) 114.7(2) 102.51(18) 106.79(16) Table 5. Bond Valence Sums for the Mn Atoms in Complex 2a atom MnII MnIII MnIV Mn1 Mn2 Mn3 Mn4 Mn5 Mn6 Mn7 Mn8 2.06 2.02 3.26 3.25 2.04 3.19 3.20 3.34 1.94 1.89 2.98 2.97 1.86 2.92 2.93 3.05 1.95 1.91 3.13 3.12 1.96 3.06 3.07 3.20 a See the footnote of Table 3. now-complete [Mn4(μ4-O)2(μ3-OR)2] cubanes edgelinked together at the Mn6-O190 and Mn60 -O19 edges, with μ3-O2- ions (O20 and O200 ) providing additional monatomic bridges. On each side of this central completedicubane is attached a tetrahedral [Mn4(μ4-O2-)] unit (Mn1, Mn3, Mn7, Mn8). The degree of protonation of the inorganic O atoms was established by O BVS calculations (Table 6), revealing six μ4-O2- (O18, O19, O24), two μ3-O2- (O20), two μ-OH (O21), and two terminal H2O molecules (O23). There are sixteen benzoate groups: ten are in the common η1:η1:μ mode, four in the rarer η2:η1:μ3 mode, and two are terminally bound to Mn8, with the unbound O atom (O17) forming H-bonds to the nearby  ˚ μ-OH- (O17 3 3 3 O21 = 2.652(8) A) and terminal water  ˚ 2.891(12) A). Among the ten hmp- groups, (O17 3 3 3 O230= Article Inorganic Chemistry, Vol. 49, No. 22, 2010 10585 Table 6. BVS for Selected O Atoms in 2a atom BVS assgta O18 O19 O20 O21 O23 O24 1.90 1.76 1.91 1.03 0.33 1.87 O2O2O2OHH2O O2- a The O atom is an O2- if the BVS is ∼1.8-2.0, an OH- if it is ∼1.0-1.2, and an H2O if it is ∼0.2-0.4, although the ranges can be affected slightly by H-bonding. four are bridging within the central dicubane in a η1:η3:μ3 mode, and the other six are bridging within the outer tetrahedral units in a η1:η2:μ mode. The Mn atoms are all six-coordinate with distorted octahedral geometry, except for MnII atom Mn1, which is seven-coordinate. There are several Mn16 clusters known in the literature with wheel-shaped, planar, grid, and so forth structures,10c,34,35 but none of them possess the metal topology of 2, which is thus unprecedented. Interestingly, the structure of complex 2 is similar to that of a previously reported Mn12-benzoate complex with hmp-, [Mn12O8Cl4(O2CPh)8(hmp)6] (6)18b in that both consist of two tetrahedral [MnIIMnIII3] units bridged by oxide ions to the central unit. The difference is that the central unit of 6 is a face-sharing defective dicubane (MnIII4) whereas that of 2 is a linked completedicubane (2  MnII2MnIII2); in fact, note that the latter is the former with two additional MnII atoms at each end. As stated above, the cores of 1 and 2 thus show some structural relationship in that they both contain a linked dicubane unit; in 1, each cubane is MnII3, MnIII and is incomplete in that one vertex does not contain a triply bridging monatomic bridge, whereas in 2 each cubane is MnII2, MnIII2 and as a result is now complete in this regard. With respect to the suggestion that 1 might be an intermediate to 2, it is reasonable that further oxidation of 1 and the incorporation of additional bridging OH-/O2ions would lead to enlargement to a higher nuclearity product. However, the complexity of these reactions and the absence of a discriminating spectroscopic probe make it very difficult to probe this matter further. Magnetochemistry. Magnetic Susceptibility Study of Complex 1. Variable temperature dc magnetic susceptibility measurements were performed on a microcrystalline powder sample of 1 3 H2O in a 0.1 T field and in the 5.0-300 K range. The sample was restrained in eicosane to prevent torquing. The obtained data are shown as a χMT versus T plot in Figure 4. χMT gradually decreases from 25.09 cm3 K mol-1 at 300 K to 15.11 cm3 K mol-1 at 5.0 K. The 300 K value is much less than the spin-only (g = 2) value of 29.25 cm3 K mol-1 for six MnII and two MnIII non-interacting atoms, indicating dominant antiferromagnetic exchange interactions. The plot does not (34) (a) King, P.; Wernsdorfer, W.; Abboud, K. A.; Christou, G. Inorg. Chem. 2004, 43, 7315. (b) Manoli, M.; Prescimone, A.; Mishra, A.; Parsons, S.; Christou, G.; Brechin, E. K. Dalton Trans. 2007, 532. (c) Konar, S.; Clearfield., A. Inorg. Chem. 2008, 47, 3489. (35) (a) Dey, S. K.; Abedin, T. S. M.; Dawe, L. N.; Tandon, S. S.; Collins, € J. L.; Thompson, L. k.; Postnikov, A. V.; Alam, M. S.; Muller, P. Inorg. Chem. 2007, 46, 7767. (b) Liu, W.; Lee, K.; Park, M.; John, R. P.; Moon, D.; Zou, Y.; Liu, X.; Ri, H.-C.; Kim, G. H.; Lah, M. S. Inorg. Chem. 2008, 47, 8807. (c) Lee, J.; Gorun, S. M. Angew. Chem., Int. Ed. 2003, 42, 1512. Figure 4. Plots of χMT versus T for complexes 1 and 2. appear to be heading to zero at 0 K, indicating that 1 3 H2O has an S > 0 ground state, and the 5 K value can be compared with the spin-only (g = 2) value of 15.00 cm3 K mol-1 for an S = 5 state. To determine the ground state of 1 3 H2O, as well as the magnitude and sign of D, magnetization (M) data were collected in the 1-7 T magnetic field (H) and 1.8-10.0 K temperature ranges. We attempted to fit the data, using the program MAGNET,36 by diagonalization of the spin Hamiltonian matrix assuming only the ground state is populated, incorporat^ ing axial anisotropy (D S z 2) and Zeeman terms, and employing a full powder average. The corresponding ^ spin Hamiltonian is given by eq 3, where S z is the easy-axis spin operator, g is the Land g factor, μB is the e ^2 ^ H ¼ DS z þ gμB μ0 S 3 H ð 3Þ Bohr magneton, and μ0 is the vacuum permeability. However, we could not get an acceptable fit using data collected over the whole field range, which is a common problem caused by low-lying excited states, especially if some have an S value greater than that of the ground state, as would be expected for so many weakly coupled MnII atoms. This conclusion is supported by the M versus H plot (Supporting Information, Figure S1), which does not show saturation but instead a steadily increasing magnetization with H. A common solution is to use only data collected at low fields (e1.0 T), as we showed for many mixed-valence MnII/MnIII clusters,37 but for 1 3 H2O it was still not possible to obtain a satisfactory fit, suggesting particularly low-lying excited states. Thus, we turned to ac susceptibility measurements, which are a powerful complement to dc studies for determining the ground state S of a system, because they preclude complications from an applied dc field.19b,38 The ac studies on 1 3 H2O were performed in the 1.8-15 K range using a 3.5 G ac field oscillating at frequencies in (36) Davidson, E. R. MAGNET; Indiana University: Bloomington, IN, 1999. (37) (a) Soler, M.; Wernsdorfer, W.; Folting, K.; Pink, M.; Christou, G. J. Am. Chem. Soc. 2004, 126, 2156. (b) Brechin, E. K.; Sa~udo, E. C.; n Wernsdorfer, W.; Boskovic, C.; Yoo, J.; Hendrickson, D. N.; Yamaguchi, A.; Ishimoto, H.; Concolino, T. E.; Rheingold, A. L.; Christou, G. Inorg. Chem. 2005, 44, 502. (c) King, P.; Wernsdorfer, W.; Abboud, K. A.; Christou., G. Inorg. Chem. 2005, 44, 8659. (38) (a) Murugesu, M.; Raftery, J.; Wernsdorfer, W.; Christou, G.; Brechin, E. K. Inorg. Chem. 2004, 43, 4203. (b) Scott, R. T. W.; Parsons, S.; Murugesu, M.; Wernsdorfer, W.; Christou, G.; Brechin, E. K. Angew. Chem., Int. Ed. 2005, 44, 6540. 10586 Inorganic Chemistry, Vol. 49, No. 22, 2010 Figure 5. In-phase ac susceptibility of complex 1 in a 3.5 G field oscillating at the indicated frequencies. A small spike at 2.2 K, an artifact of the liquid He triple point, has been removed. the 50-1000 Hz range. The obtained in-phase ac susceptibility data (χM0 , plotted as χM0 T) are shown in Figure 5. In the absence of slow magnetization relaxation and a resulting out-of-phase (χM00 ) signal, the ac χM0 T is equal to the dc χMT, allowing determination of the ground state S in the absence of a dc field. The χM0 T values decrease significantly with decreasing temperature, indicating depopulation of one or more excited states with S greater than the ground state S, rationalizing the problematic fits of dc magnetization data. Extrapolation of the plot from above 4 to 0 K (to avoid dips at the lowest temperature due to anisotropy, weak intermolecular interactions, etc.), where only the ground state will be populated, gives a χM0 T value of ∼15 cm3 K mol-1, which is the value expected for an S = 5 state with g ∼ 2. An S = 4 or S = 6 ground state would give a χM0 T value of ∼10 or ∼21 cm3 K mol-1, respectively, clearly very different from the experimental data. We thus feel confident in our conclusion that 1 has an S =5 ground state. Complex 1 exhibited no out-of-phase (χM00 ) ac signal down to 1.8 K, indicating that it does not exhibit a significant barrier (versus kT) to magnetization relaxation, that is, it is not an SMM. Magnetic Susceptibility Study of Complex 2. Variabletemperature dc magnetic susceptibility data for complex 2 3 H2O were collected as for 1 3 H2O, and are shown as a χMT versus T plot in Figure 4. χMT gradually decreases from 42.52 cm3 K mol-1 at 300 K to a minimum of 30.33 cm3 K mol-1 at 15.0 K, and then slightly increases to 31.17 cm3 K mol-1 at 5 K. The 300 K value is less than the spin-only (g = 2) value of 56.25 cm3 K mol-1 for six MnII and ten MnIII non-interacting ions, indicating the presence of antiferromagnetic interactions, but the χMT versus T profile suggests there may also be significant ferromagnetic interactions as well. The 5.0 K value is indicative of an S = 8 ground state (χMT = 36.00 cm3 K mol-1 for g =2). Magnetization data for 2 3 H2O were collected at different fields and temperatures, as for 1 3 H2O, and attempts were made to fit them by matrix diagonalization, but again we could not obtain a satisfactory fit using all data up to 7 T. This time, however, we were able to get an acceptable fit using only data collected at low fields, consistent with the stronger exchange interactions expected for the higher oxidation level and greater OH-/ O2- content of 2 versus 1, and a resulting bigger separation Taguchi et al. Figure 6. Plots of reduced magnetization (M/NμB) versus H/T for complex 2. The solid lines are the fit of the data; see the text for the fit parameters. Figure 7. Two-dimensional contour plot of the root-mean-square error surface for the D versus g fit of Figure 6 for complex 2. to excited states. The data are shown as a reduced magnetization (M/NμB) versus H/T plot in Figure 6, where N is Avogadro’s number, and the fit (solid lines in Figure 6) gave S = 8, D = -0.11 cm-1, and g = 1.89. Alternative fits with S = 7 or 9 were rejected because they gave unreasonable values of g. The root-mean-square D versus g error surface for the fit was generated using the program GRID,39 and is shown as a 2-D contour plot in Figure 7 for the D = -0.2 to 0.5 cm-1 and g = 1.8 to 2.0 ranges. Two fitting minima are observed with positive and negative D values, with the latter being clearly superior and supporting the D for 2 3 H2O being negative. From the shape and orientation of the contour describing the region of minimum error, we estimate the precision/uncertainty of the fit parameters as S=8, D=-0.11(1) cm-1 and g = 1.89(3). The in-phase (χ0 MT) and out-of-phase (χ00 M) ac susceptibilities for 2 are shown in Figure 8. χ M 0 T steadily increases with decreasing temperature from a value of ∼32 cm3 K mol-1 at 15 K to a value of ∼34 cm3 K mol-1 at 2.2 K before exhibiting a small decrease. The χM0 T values at the lowest temperatures clearly indicate an S = 8 ground state with g ∼ 2.0, in agreement with the dc magnetization fit. S = 7 and 9 ground states would give (39) Davidson, E. R. GRID; Indiana University: Bloomington, IN, 1999. Article Figure 8. AC susceptibility of complex 2 in a 3.5 G field oscillating at the indicated frequencies: (top) in-phase signal (χM0 ) plotted as χM0 T versus T; and (bottom) out-of-phase signal χM00 versus T. The small spike at 2.2 K is an artifact of the liquid He triple point. χM0 T values of ∼28.0 and ∼45.0 cm3 K mol-1, respectively, clearly very different from the experimental value. We thus conclude that complex 2 3 H2O has an S = 8 ground state. Frequency-dependent out-of-phase χM00 signals, which are clearly the tails of peaks whose maxima are at <1.8 K, were observed at temperatures below 3 K, indicating slow magnetization relaxation and suggesting that 2 3 H2O might be a SMM. If so, it should exhibit magnetization hysteresis, and this was therefore explored. Hysteresis Studies below 1.8 K. Magnetization versus applied dc field studies were performed on single crystals of 2 3 10MeOH (maintained in contact with mother liquor) at temperatures down to 0.04 K using a microSQUID apparatus.25 Hysteresis loops were observed below 0.7 K, whose coercivities increase with decreasing temperature and increasing field sweep rate (Figure 9), as expected for the superparamagnetic-like properties of a SMM below its blocking temperature (TB). The data thus confirm complex 2 3 10MeOH to be a new addition to the family of SMMs, but do not show the steps characteristic of quantum tunneling of magnetization (QTM). This is as expected for high nuclearity SMMs since they are more susceptible to various step-broadening effects from low lying excited states, intermolecular interactions, and/or distributions of local environments owing to ligand and solvent disorder. To estimate the effective kinetic barrier Ueff to relaxation, the magnetization was first saturated in one direction at ∼5 K with a large dc field, the temperature decreased to a chosen value, the field removed, and the magnetization decay monitored with time. The results are shown in Supporting Information, Figure S2. This provided magnetization relaxation time (τ) versus Inorganic Chemistry, Vol. 49, No. 22, 2010 10587 Figure 9. Magnetization (M) versus dc field hysteresis loops for a single crystal of 2 3 10MeOH at (top) the indicated temperatures and field sweep rate; and (bottom) the indicated field sweep rates and temperature. The magnetization is normalized to its saturation value, MS. Figure 10. Arrhenius plot of the relaxation time (τ) versus 1/T for a single crystal of 2 3 10MeOH. The dashed line is the fit of the data in the thermally activated region to the Arrhenius equation; see the text for the fit parameters. temperature data, shown as a τ versus 1/T plot in Figure 10 based on the Arrhenius relationship of eq 4, where τ0 is τ ¼ τ0 expðUeff =kT Þ ð 4Þ the pre-exponential factor, Ueff is the mean effective barrier, and k is the Boltzmann constant. The fit of the thermally activated region, shown as the dashed line in Figure 10, gave Ueff = 8.1 K and τ0 = 4  10-9 s. The Ueff is, as expected, smaller than the upper limit of the barrier, U, given by S2|D| for integer S; for the S = 8 and D = -0.11 cm-1 values obtained for 2 3 H2O from the magnetization fits, U = 7.0 cm-1 = 10.1 K. Thus, Ueff < U, consistent with QTM through upper regions of the barrier decreasing the effective barrier height. The presence of QTM in 2 is also evident in Figure 10 where the relaxation time at the lowest temperatures becomes essentially 10588 Inorganic Chemistry, Vol. 49, No. 22, 2010 Taguchi et al. Figure 11. Comparison of the cores of 2, 4, and 5. For clarity, only the C atoms connecting the O and N atoms of the chelates are shown. Color code: MnII, yellow; MnIII, green; O, red; C, gray; N, blue. temperature-independent, characteristic of relaxation only via the lowest energy MS = ( 8 levels of the S = 8 spin manifold. Comparison of hmp- (2), dmhmp- (4), and dphmp- (5) Mn Clusters. As stated in the Introduction, we sought and now have a triad of Mnx clusters prepared under exactly the same conditions but differing in the chelate employed. It was important to do so, because factors such as reagent ratio, the carboxylate employed, reaction solvent, and so forth are particularly influential in Mn cluster chemistry, and we wanted to ensure as much as possible that the identity of the product would reflect the bulk, electronic properties, and binding preferences of the chelate. The cores of the three compounds [Mn16O8(OH)2(O2CPh)12(hmp)10(H2O)2]2þ (cation of 2), [Mn12O7(OH)(OMe)2(O2CPh)12(dmhmp)4(H2O)] (4), and [Mn6O4(OMe)2(O2CPh)4(dphmp)4] (5) are shown in Figure 11. The following observations can be made: (i) The compounds show a decreasing Mnx nuclearity with increasing chelate bulk. (ii) The O atoms of hmp- adopt both μ3- and μ2bridging modes, those of bulkier dmhmp- are all μ2, and those of dphmp are all μ1, that is, terminal. (iii) The two relatively small Me groups in dmhmp- can be considered merely a perturbation of hmp-. Thus, although the nuclearity has decreased from 2 (Mn16) to 4 (Mn12) and the Mn/chelate ratio increases from 16:10 (2) to 12:4 (4), the two compounds still show some structural similarities. In particular, they both contain a [Mn4(μ4-O2-)] tetrahedron either side of a central unit: for 2, this is a linked Mn8 dicubane, whereas for 4 it is a smaller [Mn4(μ-O2-)6] face-fused incomplete dicubane (two face-sharing cubanes each missing a Mn atom). This point is consistent with the presence of μ3 modes for the hmp- O atoms in the Mn8 dicubane, which has never been seen for dmhmp- (vide infra). (iv) The much bulkier dphmp- does not bridge but instead binds as a chelate and gives a very different type of product, although it does also contain a complete dicubane, albeit face-fused rather than just linked as in 2. The above conclusions support the increasing bulk of the substituents next to the O atom of the chelate, and the Table 7. Complexes with hmp-, dmhmp-, or dphmp-, and their Alkoxide Binding Mode complexa - n (μn-O)b,c ref [Mn4O2(O2CPh)7(hmp)2] [Mn4(hmp)6X4-x(solv)x]zþ d [Mn4(6-Me-hmp)6Cl4] [Mn4(hmp)4Br2(OMe)2(dcn)2] [Mn4(hmp)4(acac)2(MeO)2]2þ [Mn7(OH)3(hmp)9Cl3]2þ [Mn8O2(O2CPh)10(hmp)4(MeOH)2] (1) [Mn10O4(OH)2(O2CMe)8(hmp)8]4þ [Mn10O4(N3)4(hmp)12]2þ [Mn12O8Cl4(O2CPh)8(hmp)6] [Mn16O8(OH)2(O2CPh)12(hmp)10(H2O)]2þ (2) [Mn21O14(OH)2(O2CMe)16(hmp)8(pic)2(py)(H2O)]4þ 1 3 3 3 2 2, 3 3 2, 3 2 2 2, 3 2 17 20, 21 20a 20c 21d 18a t.w. 18a 18c 18b t.w. 19 [Mn7O3(OH)3(O2CBut)7(dmhmp)4] [Mn12O7(OH)(OMe)2(O2CPh)12(dmhmp)4(H2O)] (4) 1, 2 2 22b 22b [Mn4O2(O2CBut)5(dphmp)3] [Mn6O4(OMe)2(O2CPh)4(dphmp)4] (5) [Mn11O7(OMe)7(O2CPh)7(dphmp)4(MeOH)2] 1, 1.5c 1 1, 1.5c 22a 22a 22a a Counterions omitted. b Bridging mode of the hmp-, dmhmp-, or dphmp- O atom; μ1-O indicates a non-bridging (terminal) mode. c n = 1.5 refers to a semi-bridging mode. d Many complexes, varying in the content of monodentate anionic ligand X- and solvent molecules (solv). t.w. = this work. resulting differing O binding preferences, as the primary cause of the differing products. This is supported by the list in Table 7 of known Mn complexes with these chelates, and a clear trend can be seen to lower μnbridging modes as the bulk of the chelate increases. The dphmp- has never displayed a μ2 mode, only the occasional semibridging (n = 1.5) situation where a weak, ˚ long (>2.4 A) contact to a second metal is formed. Conclusions We have reported two new Mn8 (1) and Mn16 (2) clusters with hmp- and shown that the latter is a new SMM with an S = 8 ground state. In addition, 2 joins 4 and 5 to give a family of clusters prepared under the same conditions except for the chelate, and they have different structures that can be rationalized on the basis of the increasing bulk of the Article substituents next to the alkoxide O atom and their influence on the resulting binding mode. Since the alkoxide O atoms of the hmp- groups each bridging two or three Mn atoms is a main reason that a high nuclearity complex is formed, it makes sense that bulkier chelates with lower μn should give lower nuclearity products; in addition, the number of chelates that can be fit around a cluster is obviously affected. However, this is not to say that, for example, dphmp- cannot give high nuclearity clusters because it cannot bridge;as can be seen in Table 7, a Mn11 cluster is known with this chelate, its many μ2 and μ3 MeO- groups replacing the dphmp- alkoxide O atoms as contributors (with the oxides) to the resulting high nuclearity. Notwithstanding such examples, the overall picture that emerges is that controlled modification of a chelate by addition of substituents of a chosen size is a valuable way to access new Mx clusters with very rare or prototypical metal topologies. Finally, we realize with Inorganic Chemistry, Vol. 49, No. 22, 2010 10589 hindsight that dmhmp- is fairly similar to hmp-, and dphmp- is very different, and as a result it would be of interest to also investigate the use of a chelate with substituents intermediate in bulk between Me and Ph, such as Pri, and this is currently under investigation. Acknowledgment. G.C. thanks the National Science Foundation for support of this work through Grant CHE-0910472. W.W. thanks the financial support from the ANR-PNANO projects MolNanoSpin ANR-08NANO-002 and the ERC Advanced Grant MolNanoSpin 226558. Supporting Information Available: X-ray crystallographic data in CIF format for complexes 1 3 2CH2Cl2 and 2 3 10MeOH. This material is available free of charge via the Internet at http:// pubs.acs.org. ...
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