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Synthesis, characterisation and computational studies on a novel one-dimensional arrangement of Schiff-base Mn3 single-molecule magnet† Po-Heng Lin,a Serge Gorelsky,b Didier Savard,a Tara J. Burchell,a Wolfgang Wernsdorfer,c Rodolphe Cl´eracd,e and Muralee Murugesu*a,b

Downloaded by University of Ottawa on 27 June 2011 Published on 13 July 2010 on http://pubs.rsc.org | doi:10.1039/C0DT00143K

Received 15th March 2010, Accepted 13th May 2010 First published as an Advance Article on the web 13th July 2010 DOI: 10.1039/c0dt00143k The syntheses, structures and magnetic properties are reported for three new manganese complexes containing the Schiff-base ((2-hydroxy-3-methoxyphenyl)methylene)isonicotinohydrazine (H2 hmi) ligand. Complex [MnII (H2 hmi)2 (MeOH)2 Cl2 ] (1) was obtained from the reaction of H2 hmi with MnCl2 in a MeOH–MeCN mixture. Addition of triethylamine to the previous reaction mixture followed by diethyl ether diffusion yielded a dinuclear manganese [MnIII 2 (hmi)2 (OMe)2 ]• ·2MeCN·2OEt2 (2) compound. Upon increasing the MnCl2 /H2 hmi ratio, the mixed valence complex [MnIII 2 MnII (hmi)2 (OMe)2 Cl2 ]• ·MeOH (3) was obtained. Dc and ac magnetic measurements were carried out on all three samples. The ac susceptibility and field dependence of the magnetisation measurements confirmed that complex 3 exhibits a single-molecule magnet behaviour with an effective energy barrier of 8.1 K and an Arrhenius pre-exponential factor of 3 ¥ 10-9 s.

Introduction Versatility of manganese chemistry continues to attract much interest in the field of molecular magnetism.1 Indeed different oxidation states in combination with a large variety of coordination environments seen for Mn ions leads to not only unique structural features but also interesting magnetic properties.2 For instance, MnIII ions not only yield interesting structural architectures due to the presence of Jahn–Teller (J–T) distortions but also afford significant uniaxial magnetic anisotropy, which generally leads to interesting magnetic behaviour when combined with a high spin ground state.3 In such cases, slow relaxation of the magnetisation, i.e. a magnet-like behaviour, can be observed. Discrete molecules with such properties are generally termed single-molecule magnets (SMMs).3 In the isolation of molecular magnetic systems, ligands play an important role by stabilising and encapsulating metal ions and promoting intramolecular interactions via superexchange pathways.4 Therefore along with appropriate metal systems careful ligand design is essential. As part of our research efforts towards new preparative routes to SMMs, we have recently turned our attention to polydentate Schiff-base ligands in manganese chemistry. In previous studies, tetradentate a Chemistry Department, University of Ottawa, 401 D’Iorio Hall, 10 Marie Curie, Ottawa, Canada K1N 6N5. E-mail: [email protected]; Fax: (613) 562 5170; Tel: (613) 562 5800 b Centre for Catalysis Research and Innovation, University of Ottawa, 30 Marie Curie, Ottawa, Canada K1N 6N5 c InstitutN´eel, CNRS &Universit´e J. Fourier, BP 166, 38042 Grenoble, Cedex 9, France d CNRS, UPR 8641, Centre de Recherche Paul Pascal (CRPP), Equipe “Mat´eriaux Mol´eculaires Magn´etiques”, 115 avenue du Dr Albert Schweitzer, Pessac, F-33600, France e Universit´e de Bordeaux, UPR 8641, Pessac, F-33600, France † Electronic supplementary information (ESI) available: Packing diagrams for all complexes along with IR spectra. CCDC reference numbers 765718– 765720. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c0dt00143k

7650 | Dalton Trans., 2010, 39, 7650–7658

Schiff-base ligands proved to be effective auxiliary ligands for stabilising the high oxidation states of manganese but usually led to low-dimensional MnIII compounds.5 Notwithstanding, the [(2-hydroxy-3-methoxyphenyl)methylene]isonicotinohydrazine ligand, H2 hmi, (Fig. 1) provides an O,N,N,O-based multichelating coordination pocket that should favour the formation of polynuclear systems via the bridging phenoxide oxygen atom. Moreover, the pyridine group in this ligand provides an avenue to promote the formation of organised extended coordination networks.6 It is noteworthy that H2 hmi can easily undergo various tautomeric forms (Fig. 1). This provides rich and versatile coordination chemistry with transition metal centres.

Fig. 1 Representation of the [(2-hydroxy-3-methoxyphenyl)methylene]isonicotinohydrazine ligand, H2 hmi, in its many tautomeric forms. The relative Gibbs free energies (kcal mol-1 ) of the tautomeric forms calculated at PBE/TZVP and, in parenthesis, B3LYP/TZVP levels of theory are shown below the structures. Structure A is most relevant for binding of the neutral H2 hmi and structure C is most relevant for binding of the dianionic form, hmi2- . Selected bond distances (calculated at the PBE/TZVP level) for structures A and C are shown.

Moreover, organisation of SMMs in an idealised packing arrangement in order to fine tune their magnetic properties continues to be an exciting challenge.3f Structure optimisation is often attempted using crystal engineering methods of solvent, This journal is © The Royal Society of Chemistry 2010

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counter ion and/or ligand modification.7 With our ligand strategy we attempt to organise SMMs in an idealised 3-D environment using the pyridine group of the ligand. Herein, we report the synthesis, structure and magnetic properties of a mononuclear MnII complex, an extended network of a dinuclear MnIII complex and a one dimensionally arranged trinuclear {MnIII 2 MnII } SMM that represent the initial products of our efforts in this area.

Experimental

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General All chemicals were used as received from either Sigma-Aldrich or Strem chemicals. The solvents were of technical grade and were used without further purification. [(2-Hydroxy-3-methoxyphenyl)methylene] isonicotinohydrazine (H2 hmi). To a solution of isonicotinic acid hydrazide (0.02 mol, 2.75 g) in methanol (10 mL) was added a solution of o-vanillin (0.02 mol, 3.04 g) in methanol (10 mL). The mixture was stirred for 24 h at room temperature. The product, a yellow powder, was filtered, washed with cold methanol and dried in vacuo for 2 h. Yield = 95%. NMR (DMSO-d 6 , 400 Mhz): 12.22 (s, 1H), 10.65 (s, 1H), 8.76 (d, 2H), 8.66 (s, 1H), 7.80 (d, 2H), 7.17 (d, 1H), 7.02 (d, 1H), 6.84 (t, 1H), 3.79 (s, 3H). IR (KBr, cm-1 ): 3449 (br), 3193 (m), 2999 (m), 1682 (s), 1602 (m), 1563 (m), 1460 (m), 1411 (m), 1350 (w), 1286 (m), 1252 (br), 1158 (m), 1106 (s), 1079 (m), 1064 (m), 972 (s), 791 (s), 740 (m), 693 (m). [MnII (H2 hmi)2 (MeOH)2 Cl2 ] (1). To a solution of MnCl2 (0.5 mmol, 0.063 g) in MeOH (5 mL) and MeCN (10 mL) was added a solution of H2 hmi (0.25 mmol, 0.068 g) in MeOH (5 mL) and MeCN (10 mL). The solution was stirred for 15 min and then filtered. After two weeks, yellow crystals of 1 were collected by filtration, washed with MeOH and dried in vacuo. Yield = 65%. IR (KBr, cm-1 ): 3455 (s), 3207 (s), 1675 (s), 1610 (m), 1579 (m), 1539 (s), 1459 (s), 1416 (s), 1388 (m), 1354 (m), 1282 (br), 1251 (br), 1146 (m), 1077 (m), 1020 (s), 852 (m), 730 (s), 699 (s). [MnIII 2 (hmi)2 (OMe)2 ]• ·2MeCN·2OEt2 (2). To a solution of MnCl2 (0.5 mmol, 0.063 g) in MeOH (5 mL) and MeCN (10 mL) was added a solution of H2 hmi (0.25 mmol, 0.068 g) and triethylamine (1 mmol, 0.14 mL) in MeOH (5 mL) and MeCN (10 mL). The resulting mixture was stirred for 15 min at room temperature and then filtered. The crystallisation was carried out in a diethyl ether bath at room temperature over two weeks, which provided dark brown crystals of 2 that were collected by filtration, washed with MeCN and dried in vacuo. Yield = 38%. IR (KBr, cm-1 ): 3401 (br), 1596 (s), 1546 (m), 1516 (m), 1499 (m), 1434 (s), 1349 (br), 1293 (w), 1246 (m), 1214 (m), 1105 (w), 970 (w), 861 (w), 740 (m), 711 (m). [MnIII 2 MnII (hmi)2 (OMe)2 Cl2 ]• ·MeOH (3). To a solution of MnCl2 (1 mmol, 0.126 g) in MeOH (5 mL) and MeCN (10 mL) was added a solution of H2 hmi (0.25 mmol, 0.068 g) and triethylamine (0.5 mmol, 0.07 mL) in MeOH (5 mL) and MeCN (10 mL). The solution was stirred for 15 min at room temperature. The solution was then filtered and crystallisation was carried over two weeks to yield brown prisms suitable for X-ray that were collected by filtration, washed with MeCN and dried in vacuo. Yield = 40%. IR (KBr, cm-1 ): 3416 (br), 1599 (s), 1551 (m), 1517 (m), 1497 (m), This journal is © The Royal Society of Chemistry 2010

Table 1 Crystallographic data of compounds 1, 2 and 3 1

2

3

Empirical formula

C30 H34 Cl2 C42 H56 Mn2 C33 H40 Cl2 MnN6 O8 N8 O10 Mn3 N6 O11 Fw 732.47 942.83 932.43 Crystal system Triclinic Tetragonal Triclinic Space group P1¯ I41/a P1¯ ˚ a/A 7.8865(15) 30.200(18) 10.0173(18) ˚ b/A 8.9804(15) 30.200(18) 10.2552(18) ˚ c/A 12.543(2) 10.016(12) 11.2365(13) a/◦ 69.358(2) 90 114.964(11) ◦ b/ 76.853(2) 90 108.935(11) g /◦ 77.743(2) 90 98.289(2) ˚3 801.0(2) 9135(13) 935.1(3) V /A Z 1 8 1 T/K 202 202 200 Radiation Mo-Ka Mo-Ka Mo-Ka Dc /g cm-3 1.519 1.371 1.659 a R1 (I > 2s(I)) 0.0408 0.0525 0.0604 wR2 (I > 2s(I))b 0.0955 0.1527 0.1383     a R1 = (F o | - |F c )/ |F o |. b wR2 = [ [w(F o 2 - F c 2 )2 ]/ [w(F o 2 )2 ]]1/2 , w = 1/[s 2 (F o 2 ) + [(ap)2 + bp], where p = [max (F o 2 , 0) + 2F c 2 ]/3.

1446 (m), 1440 (s), 1335 (br), 1284 (m), 1250 (br), 1211 (s), 1012 (w), 970 (w), 851 (w), 737 (m), 715 (m), 686 (m). X-Ray crystallography† Crystals of 1 and 3 were grown from the mother liquor and crystals of 2 were grown by slow diffusion of diethyl ether in the mixture over several days. Single yellow (1) or dark brown crystals (2 and 3) of all three complexes suitable for X-ray diffraction measurements were mounted on a glass fibre. To prevent solvent loss in 3, a single crystal was mounted on greased covered glass fibre while it was in its mother liquor and then transferred quickly to the cold stream of the diffractometer. Unit cell measurements and intensity data collections were performed on a Bruker-AXS SMART 1 k CCD diffractometer at 202 K using graphite monochromatized ˚ ) (Table 1). The data reduction Mo-Ka radiation (l = 0.71073 A included a correction for Lorentz and polarization effects with an applied multi-scan absorption correction (SADABS).8 The crystal structure was solved and refined using the SHELXTL9 program suite. Direct methods yielded all non-hydrogen atoms. All hydrogen atom positions were calculated geometrically and were located on their respective atoms. For 2, a diethyl ether solvent molecule was disordered over two positions and modelled as a 50 : 50 isotropic mixture. Magnetic measurements Magnetic susceptibility measurements were obtained with the use of a Quantum Design SQUID magnetometer MPMS-XL operating between 1.8 and 400 K for dc applied fields ranging from -7 to 7 T. Measurements were performed on ground polycrystalline samples of approximately 15 mg. The data sets were corrected for the sample holder and the intrinsic diamagnetic contributions. When the sample was measured directly in its mother liquor (as crystals), the sample mass was measured after drying the compound at the end of the measurement. In this case the molecular weight of the compound used is calculated without the solvent molecules. It is worth mentioning that the Dalton Trans., 2010, 39, 7650–7658 | 7651

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mother liquor has been measured separately and found to be a negligible correction to the sample magnetisation. Magnetisation measurements on oriented single crystals were carried out with an array of micro-SQUIDs.10 This magnetometer works in the temperature range of 0.04 to ca. 7 K and in fields of up to 0.8 T with sweeping rates as high as 0.28 T s-1 , and exhibits field stability of better than mT. The time resolution is approximately 1 ms. The field can be applied in any direction of the micro-SQUID plane with precision greater than 0.1◦ by separately driving three orthogonal coils. In order to ensure good thermalisation, a single crystal was fixed with apiezon grease. IR and NMR spectroscopy Infrared spectra were recorded in the solid state (KBr pellets) on a Nicolet Nexus 550 FTIR spectrometer in the 600–4000 cm-1 range. Nuclear magnetic resonance analyses were conducted on a BrukerAvance 400 equipped with an automatic sample charger and a 5 mm auto-tuning broadband probe with Z gradient. DFT calculations Density functional theory (DFT) calculations were performed using the Gaussian 03 program.11 Optimised molecular geometries were calculated using the PBE12,13 exchange–correlation functional for all Mn complexes. The DZVP14 basis set and tight SCF convergence criteria were used for calculations. The Gibbs free energies (kcal mol-1 ) of the tautomeric forms of the H2 hmi ligand were calculated at PBE/TZVP and B3LYP/TZVP levels of theory.15 Wave function stability calculations were performed to confirm that the calculated wave functions corresponded to the ground state. The analysis of molecular orbitals (MOs) in terms of atomic orbital contributions and the calculation of twocentres Mayer bond orders16 were carried out using the AOMix program17,18 and the Mulliken population analysis.19 Atomic charges were calculated by natural population analysis (NPA)20 as implemented in Gaussian 03.

Results and discussion Synthesis of the ligand The Schiff-base ligand, H2 hmi, was prepared using the Schiff base reaction of isonicotinic acid hydrazide with o-vanillin in methanol (Scheme 1). The ligand was carefully designed for the coordination of MnIII ions in the inner O,N,N,O coordination pocket upon deprotonation of H2 hmi. In such systems, the coordinated atoms generally occupy the equatorial plane of the MnIII ion with two elongated Jahn–Teller axial positions coordinated with linking/terminal groups. Such an environment was recently observed by Ge et al. in an end-on azide bridged dinuclear MnIII complex of the similar Schiff-base ligand, N-

isonicotinamidosalicylaidimine7b,21 that displays single-molecule magnet properties. Reaction of the Schiff-base ligand H2 hmi with MnCl2 under varying reaction conditions yielded three novel manganese complexes with different structural topologies. Syntheses of the complexes Reaction of H2 hmi, with two equivalents of MnCl2 in a MeCN–MeOH (1 : 2) mixture gave the mononuclear complex, [MnII (H2 hmi)2 (MeOH)2 Cl2 ] (1). Large pale yellow crystals of 1 were grown from the mother liquor over a period of two weeks at room temperature. The reaction mixture contained an excess of MnII ions since the 2 : 1 metal-to-ligand ratio provided a higher yield and better quality crystals than a 1 : 2 metalto-ligand ratio. Further variations of the metal-to-ligand ratio and solvent conditions still afforded 1 in similar crystal quality and yields. On the other hand, treatment of H2 hmi with four equivalents of triethylamine prior to the addition of the metal ions in similar solvent conditions yielded a dinuclear complex, [MnIII 2 (hmi)2 (OMe)2 ]• ·2MeCN·2OEt2 (2). The dark brown crystals of the latter complex were grown by slow diethyl ether diffusion over several days. By increasing the metal-to-ligand ratio from 2 : 1 to 4 : 1, under the same reaction conditions, the trinuclear complex, [MnIII 2 MnII (hmi)2 (OMe)2 Cl2 ]• ·MeOH (3), was obtained. The dark brown crystals of 3 were grown from the mother liquor over a period of two weeks. As opposed to the previous reaction, the base-to-ligand ratio had to be lowered from 2 : 1 to 1 : 1 to allow a slower growth of suitable crystals for single-crystal XRD. When an excess of base was used, a dark brown polycrystalline product of 3 was obtained in a shorter period. Structural description of 1 Complex 1 (Fig. 2, top) is composed of one MnII ion located on an inversion centre with two H2 hmi ligands coordinated to a single manganese ion by its pyridine group. In addition, two Cl atoms and two MeOH molecules fill the remaining octahedral coordination sphere of the metal centre. In the coordination pocket, an intramolecular H-bond between O2 and N1 [O– ˚ ] is present instead of the expected chelated H ◊ ◊ ◊ N = 2.580(3) A metal centre. Furthermore, as shown in its packing diagram (Fig. 3, S1†), the molecules of 1 link through hydrogen bonding; specifically ˚ , N2– through the NH ◊ ◊ ◊ Cl interaction [N2 ◊ ◊ ◊ Cl1¢ = 3.368(2) A H ◊ ◊ ◊ Cl1¢ = 167.7◦ ] and the OH ◊ ◊ ◊ O interaction [O2 ◊ ◊ ◊ O4¢ = ˚ , O2 ◊ ◊ ◊ H–O4¢ = 152.5◦ ] between the coordinated 3.007(3) A MeOH molecules and the ligand phenoxide groups of adjacent molecules to form an extended two dimensional sheet. Compar˚ ] and C9–N2 [1.360(3) A ˚ ] bond ison of the C9–O3 [1.216(3) A distances in 1 (Table 2) and the H2 hmi ligand (Fig. 1) point to the fact that C9–O3 and C9–N2 bonds are double and single bonds, respectively, confirming the ligand’s neutral state (H2 hmi). The Mn oxidation state of +2 was established by bond valence sum calculations (Table 3) and charge consideration. Structural description of 2

Scheme 1 The reaction of isonicotinic acid hydrazide and o-vanillin in methanol yielding the ligand H2 hmi.

7652 | Dalton Trans., 2010, 39, 7650–7658

Complex 2 (Fig. 1, middle) crystallises in the tetragonal space group I41/a with the asymmetric unit containing half of a dinuclear MnIII unit, one MeCN and one OEt2 molecule. The This journal is © The Royal Society of Chemistry 2010

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˚ ) and angles (◦ ) for 1, 2 and 3 Table 2 Selected bond distances (A Complex 1 Mn1–O4 O3–C9 C9–N2 N2–N1 N1–C8 Mn1–N3a Mn1–Cl1 Complex 2 Mn1–O4 Mn1–O4¢ Mn1–Mn1¢ Mn1–O3 O3–C9 C9–N2 N2–N1 N1–C8 Mn1–O2 Mn1–N1 Mn1–N3a Mn1–O4–Mn1¢ Complex 3 Mn1–O4 Mn2–O4 Mn2–O3 Mn2–O5 O3–C9 C9–N2 N2–N1 N1–C8 Mn2–O2 Mn1–N3a Mn2–Cl1 Mn1–Cl1 Mn1–O4–Mn2 Mn1–Cl–Mn2 O4–Mn1–O4¢ Cl1–Mn1–Cl1¢

Fig. 2 Molecular structure of 1 (top), 2 (middle) and 3 (bottom). The H atoms and solvent molecules are omitted for clarity. Symmetrically equivalent positions [-x + 2, -y + 1, -z + 1] are denoted by a prime in their respective labels. For 2 and 3, the Jahn–Teller elongated axes are represented as dark bonds. Colour code: turquoise (MnII ), purple (MnIII ), red (O), blue (N), gray (C), green (Cl), black (H).

Fig. 3 Packing arrangement of 1. Most hydrogen atoms and solvent molecules are omitted for clarity. The black dotted lines represent hydrogen bonds between the molecules. Colour code: turquoise (MnII ), red (O), blue (N), green (Cl), black (H).

complex consists of a dinuclear core, [Mn2 (m-OMe)2 (hmi)2 ], in which two MnIII ions are bound in the coordination pockets of two hmi2- ligands and are bridged by two methoxide anions. The O2, O3 and N1 atoms from the hmi2- ligands, as well as the O4 This journal is © The Royal Society of Chemistry 2010

2.252(2) 1.216(3) 1.360(3) 1.368(3) 1.276(3) 2.275(2) 2.499(6) 1.885(3) 2.195(3) 3.122(2) 1.949(3) 1.305(5) 1.294(5) 1.414(4) 1.285(5) 1.910(3) 1.976(4) 2.360(3) 99.6(1) 2.185(4) 1.888(4) 1.920(4) 2.310(4) 1.313(6) 1.297(7) 1.404(6) 1.293(7) 1.903(4) 2.334(4) 2.688(2) 2.562(1) 113.6(2) 81.00(4) 180.0 180.0

Table 3 Bond-valence sum (BVS) calculations for complexes 1,2 and 3

II

Mn MnIII MnIV Assignment

1

2

3

1.90 1.84 1.84 MnII

3.28 3.05 3.12 MnIII

3.22 3.00 3.08 MnIII

1.80 1.73 1.74 MnII

atom of a bridging methoxide anion, coordinate to MnIII in the equatorial plane. The axial coordination sites are occupied by a methoxide oxygen atom (O4¢) and a pyridine nitrogen atom (N3a). Bond valence sum calculations (Table 3) confirm the +3 oxidation state of the metal centres. Thus, the MnIII ions are in the J–T elongated octahedral coordination environment with distances of ˚ for Mn1–O4 and 2.360(4) A ˚ for Mn1–N3a. The J–T 2.195(3) A axes of the two MnIII sites within the dinuclear unit are parallel to ˚ each other. The intramolecular Mn ◊ ◊ ◊ Mn separation is 3.123(2) A ◦ and the bridging Mn1–O4–Mn1¢ angle is 99.6(1) . Each [Mn2 (mOMe)2 (hmi)2 ] unit is connected to neighbouring dinuclear units via the pyridine groups (N3a) of the hmi2- ligand, forming an extensive two-dimensional network (Fig. 4, S2†). The 2-D layers are arranged on top of each other along the c-axis to form two types of channels. The channels are filled with diethyl ether and ˚ acetonitrile solvent molecules and their edges span are of 11.9 A ˚. and 9.2 A Dalton Trans., 2010, 39, 7650–7658 | 7653

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Fig. 4 Perspective view of the packing arrangement of 2 viewed along the c-axis. The hydrogen atoms and solvent molecules are omitted for clarity. Colour code: purple (MnIII ), red (O), blue (N), gray (C).

Structural description of 3 Complex 3 (Fig. 1, bottom) crystallises in the triclinic P1¯ space group. It is a trinuclear complex composed of two MnIII ions and one MnII ion, linked into a one-dimensional chain. As in 2, the MnIII ions are located in the O,N,N,O pockets of the hmi2- ligands. The MnII ion sits on an inversion centre which links two terminal MnIII sites using the methoxide and chloride bridging ligands (the MnIII –O4–MnII and MnIII –Cl–MnII angles are 113.6(2)◦ and 81.00(4)◦ , respectively), producing the linear trinuclear complex (the MnIII –MnII –MnIII angle is 180◦ ). Due to the J–T distortion, the MnIII sites are axially elongated along the Mn2–Cl and Mn2–CH3 OH bonds (the Mn2–Cl and Mn2– ˚ , respectively). CH3 OH bond distances are 2.688(2) and 2.310(4) A Bond valence sum calculations (Table 3) and charge consideration confirm the +3 oxidation state of Mn2 and the +2 oxidation state of Mn1. In addition to two m-Cl and two m-OMe ligands, the MnII ion is coordinated by two pyridine nitrogen atoms from the neighbouring trinuclear complexes as can be seen from the packing diagram of 3 (Fig. 5, S3, S4†). Since each trinuclear Mn complex is connected to its neighbours by the pyridine groups (N3a, N3a¢), these complexes form 1-D polymeric chains in the crystal structure of 3. These chains are running along the b-axis ˚ (Fig. 5). Two methanol with an inter-chain distance of 4.7(1) A molecules coordinated to the MnIII ions occupy the space between these chains. Furthermore, a short contact between Cl1 and C8 ˚ ] arising from the crystal packing may also [Cl1 ◊ ◊ ◊ C8 = 3.775(1) A contribute to the close proximity of the neighbouring chains. Infrared spectroscopy Infrared (IR) spectra were recorded in the 650–4000 cm-1 range for complexes using KBr pellets (Fig. S5 and S6†). The IR spectra for the free H2 hmi ligand and complex 2 were simulated using the harmonic-frequency DFT calculations at the PBE/DZVP level (Fig. S7 and S8†), allowing the assignment of the observed IR 7654 | Dalton Trans., 2010, 39, 7650–7658

Fig. 5 Packing arrangement of 3 viewed along the c-axis. The hydrogen atoms and solvent molecules are omitted for clarity. The black dashed ˚ lines demonstrate the relatively short intermolecular distance of 4.7(1) A between Mn1 and O1 from a neighbouring unit. Colour code: turquoise (MnII ), purple (MnIII ), red (O), blue (N), gray (C), green (Cl).

bands. The broad band due to the OH stretching mode is located at 3455, 3401 and 3416 cm-1 for 1, 2 and 3, respectively. Smaller bands between 3300 and 2800 cm-1 for 2 and 3 are due to C–H stretches attributable to the hmi2- ligand. For 1, those bands are at 3207 cm-1 . The IR spectrum of 1 is very similar to the IR spectrum of the H2 hmi ligand. The 1675 cm-1 band in the IR spectrum of 1 and the 1690 cm-1 band in the IR spectrum of H2 hmi are due to the stretching mode of the uncoordinated carbonyl group. This band is absent in the IR spectra of both 2 and 3 indicating the presence of the dianionic form of the hydrazine ligand (hmi2- ). Instead these complexes have a band near 1300 cm-1 due to the stretching vibration of the Mn-coordinated C9–O3 bond. A strong band near 1600 cm-1 is due to the C2=N9 bond stretching mode and the deformation modes of the aromatic rings of the hmi2- ligand. Below 1600 cm-1 , 2 and 3 exhibit similar vibrational bands. Discussion The differences in the ligand coordination and metal oxidation states between 1 and 2 arise from the addition of a base in the reaction mixture. The chosen organic base, triethylamine, deprotonates the H2 hmi ligand, promoting the metal ion coordination in the O,N,N,O pocket of the negatively charged hmi2- ligand. Without addition of a base to the reaction mixture, the ligand remains in its neutral state and its coordination to the MnII ions is only observed via the N atom of the pyridine group. The hmi2binding to the metal promotes the aerial oxidation of the MnII ions into MnIII ions as well as the deprotonation of MeOH into MeO- , which subsequently becomes the bridging ligand between the Mn ions. Noteworthy that the addition of triethylamine to a solution of 1 in a MeOH–MeCN mixture gives a dark brown solution from which no crystalline material is isolated. On the other hand, the reaction of 1 with MnCl2 and triethylamine in a MeOH–MeCN mixture converts 1 into 2. The increase of the metal-to-ligand ratio from 1 : 1 to 4 : 1 results in the formation of the trinuclear complex 3. The addition of a base allowed the deprotonation of the ligand into hmi2- and, in a similar way as for complex 2, promoted the coordination of metal ions in This journal is © The Royal Society of Chemistry 2010

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the O,N,N,O pocket. An excess of MnCl2 in the reaction mixture enabled the coordination of a MnII ion between the two MnIII sites, forming the polymeric MnIII 2 MnII compound.

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Magnetic properties of 1 Magnetic susceptibility measurements of 1 (Fig. 6) were performed under an applied dc field of 0.1 T and at temperatures ranging between 1.8 K and 300 K. At room temperature, the cT product is 4.79 cm3 K mol-1 which is close to the expected value of 4.375 cm3 K mol-1 (S = 5/2; g = 2.00) for a mononuclear noninteracting MnII unit. Upon cooling, the cT product decreases slowly down to a value of 4.17 cm3 K mol-1 at 1.8 K. The data could be fitted to the Curie–Weiss law (C = 4.76(2) cm3 K mol-1 , g = 2.09(2), and q = -0.40(4) K) suggesting weak intermolecular antiferromagnetic interactions between the molecules. This behaviour might be caused by the proximity of the mononuclear units and the O–H ◊ ◊ ◊ O and N–H ◊ ◊ ◊ Cl H-bonds between the molecules (Fig. 3). The field dependence of the magnetisation, M, of 1 (Fig. 6, inset) at 1.8 K reaches a maximum non-saturating value of 5.36 mb at 7 T. That value is slightly higher than 5 mb as expected for one S = 5/2 MnII ion with a g factor slightly higher than 2.

Fig. 7 cT vs. T plot of 2 measured between 1.8 K and 300 K under an applied dc field of 0.1 T (with c defined as the molar susceptibility: M/H). The solid red line is the best fit obtained with the dimer model described in the text.

leads to an excellent data/theory agreement as shown Fig. 7 with J/kB = -4.1(1) K and g = 2.10(5). Magnetic properties of 3 Magnetic properties of 3, studied between 1.8 K and 250 K on polycrystalline samples, were (i) freshly filtered and dried in air for a few minutes and (ii) kept in solution during the measurements (Fig. 8). At 250 K, the cT product is 10.2 cm3 K mol-1 , which is close to the expected value of 10.37 cm3 K mol-1 for non-interacting spins: two MnIII ions (S = 2; g = 2.00, C = 3.00 cm3 K mol-1 ) and one MnII ion (S = 5/2; g = 2.00, C = 4.375 cm3 K mol-1 ).

Fig. 6 cT vs. T plot of 1 measured between 1.8 K and 300 K under a dc applied field of 0.1 T. The solid red line is the best fit obtained with a Curie–Weiss law. Inset: M vs. H/T plot measured between 1.8 K and 8 K. The solid lines are guides for the eye.

Magnetic properties of 2 The magnetic measurements of complex 2 (Fig. 7) were carried out in the 1.8–300 K range under 0.1 T. At room temperature, the cT product is 6.26 cm3 K mol-1 in good agreement with the presence of two non-interacting high-spin MnIII ions (S = 2; g = 2, C = 3 cm3 K mol-1 ). Upon cooling, cT decreases gradually down to 0.21 cm3 K mol-1 indicating strong antiferromagnetic interactions between the two MnIII spins yielding a singlet spin ground state. In order to analyse the magnetic properties of 2, an isotropic Heisenberg dimer model has been employed using the following definition of the spin Hamiltonian: H = -2JS1 S2 , with J being the intramolecular Mn ◊ ◊ ◊ Mn interactions. In the low field approximation, the expression of the susceptibility can be easily found in the literature, for example in reference 8. This model This journal is © The Royal Society of Chemistry 2010

Fig. 8 cT vs. T plot of 3 measured after simple filtration (red dots) and in mother solution (black dots) between 1.8 K and 250 K under a dc applied field of 0.1 T. Inset: M vs. H/T plot measured in mother solution between 1.8 K and 8 K. The solid lines are guides for the eye.

Lowering the temperature, the cT product remains first fairly constant and then increases slightly at lower temperatures to reach a maximum value of 10.5 cm3 K mol-1 at 13 K for the compound kept in solution while the sample measured after filtration exhibits a maximum cT value of 12.4 cm3 K mol-1 at 2.5 K. These thermal behaviours indicate the presence of weak ferromagnetic interactions between the two peripheral MnIII and the central MnII ions that has been estimated from a trinuclear Si = 2, 5/2, 2 Heisenberg model to about +0.03(1) K (g = 1.99) for the crystals kept in solution, suggesting an ST = 13/2 spin ground state for the trinuclear complex at T = 0 K. Below 13 and Dalton Trans., 2010, 39, 7650–7658 | 7655

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2.5 K respectively, both samples display a decrease of their cT product (with a value of 9.8 and 11.5 cm3 K mol-1 at 1.8 K) that may be caused by the presence of intra-trimer antiferromagnetic interactions. Comparing these two sets of data and thus two sample preparations, it is obvious that the magnetic properties of 3 are strongly affected by a modification of the compound as soon as it is removed from it mother liquor. Because infrared spectroscopy did not indicate significant changes in the molecular structure upon filtration, leading us to the belief that a loss of coordinated or interstitial solvent may occur during the procedure. The loss of terminal CH3 OH solvent molecules is most likely to change the magnetic interactions between the linear chains (Fig. 5) and thus to modify the inter-trimer antiferromagnetic interactions. The field dependence of the magnetisation of 3 measured in solution (Fig. 8, inset) was performed between 1.8 K and 8 K under an applied dc field ranging between 0 and 7 T. The measurement revealed that the magnetisation of 3 quickly increases below 1 T and then slowly augments to reach a maximum value of 11.7 mb at 1.8 K and 7 T. The lack of saturation at 7 T and also the non-superposition of the M vs. H/T data (Fig. 8, inset) on a single master curve (as expected for isotropic systems with a well defined spin ground state) is indicative of either (i) strong magnetic anisotropy that may result from the two Jahn–Teller elongated peripheral MnIII ions and/or (ii) the presence of low-lying excited states even at 1.8 K. For 3, ac susceptibility measurements were performed between 1.8 and 5 K (Fig. 9). At low temperature typically below 3 K, complex 3 exhibits a very weak out-of-phase ac signal that indicates a slow relaxation of magnetisation. This behaviour might

be the signature of either an SMM behaviour or a magnetic three-dimensional order below 1.8 K. Although the signal is rather weak, the frequency dependency in the signal generally indicates SMM behaviour rather than magnetic ordering. In order to further investigate the low temperature behaviour, single crystals dc magnetisation relaxation measurements were performed using a micro-SQUID in the temperature range 1.0–0.04 K (Fig. 10) and under dc applied fields ranging from -1 T to 1 T.10 To prevent solvent loss, crystals were covered in Paratone oil while removing from its mother liquor then transferred to the micro-SQUID and cooled rapidly. Below 1 K, hysteresis loops were observed with an opening of the loops at a temperature of ~ 1 K (Fig. 10). The coercive field increases continuously upon lowering of the temperature and reaches a maximum around 0.1 K. Below this temperature, the hysteresis loops become temperature independent as expected in the pure quantum regime for SMMs. Similarly, sweep rate dependent hysteresis loops were also observed at 0.04 K. Such behaviour strongly suggests the SMM nature of 3. The characteristic relaxation of this system was extracted from the time decay of the magnetisation measured below 1.8 K (Fig. 11, inset), and plotted in Fig. 11. The experimental data have been fitted to an Arrhenius law above 0.5 K that leads to an effective energy barrier (U eff ) of 8.1 K and a pre-exponential factor (t 0 )

Fig. 10 Field dependence of the normalised magnetisation (the magnetisation is normalised at saturation) measured in the easy direction of an oriented single crystal of 3. The measurements were carried out between 0.04 and 1.0 K under fields ranging from -1 T to 1 T.

Fig. 9 Ac susceptibility measurements of 3 kept in mother solution at temperatures ranging from 1.8 K to 5 K with ac frequencies between 1 Hz and 1500 Hz (H ac = 3 Oe and H dc = 0 Oe).

7656 | Dalton Trans., 2010, 39, 7650–7658

Fig. 11 Relaxation time, t, vs. 1/T plot for 3 derived from the time dependence of the magnetisation shown in inset. Inset: time dependence of the normalized magnetization of 3 between 0.04 K and 1 K.

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of 3 ¥ 10-9 s. Such t 0 value also implies that 3 behaves as a SMM rather than the three dimensionally ordered magnet. It is worth mentioning that below 0.1 K, the relaxation time becomes temperature independent as expected when the slow relaxation of magnetisation is dominating by quantum tunnelling.

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DFT calculations In order to look into the electronic structure of 3 and to identify possible causes for the observed changes in the magnetic behaviour upon filtration, DFT calculations at the PBE/DZVP level were performed. The crystal structure of 3 was taken as an initial geometry and the geometry optimisations in the gas phase were performed using a trinuclear cluster version of the structure where the two axial ligands coordinated to the central MnII ion were replaced by two pyridine ligands (Fig. 12 and 13). The calculated structure of 3 was in agreement with the X-ray structure. The valence description obtained after the optimisation matched the one derived experimentally from the magnetic susceptibility measurements: [(hmi2- )MnIII (m-X)2 MnII (m-X)2 MnIII (hmi2- )] where the Mn ions are ferromagnetically coupled to produce the ground electronic state with the net spin of ST = 13/2 (Fig. 13). The calculated atomic spin densities (4.75 for Mn1 and 3.81 for Mn2 from MPA calculations; 4.32 for Mn1 and 3.42 for Mn2 from NPA calculations) are consistent with this valence description.‡ The bridging chloride and methoxide ligands also carry significant spin density (the Cl1 and O4 NPA-derived spin densities are 0.27 and 0.18, respectively). The analysis of metal–ligand interactions in 3 indicates that the Mn2–CH3 OH interaction is weakest among the all Mn–ligand interactions present in 3. The Mayer bond order for the Mn2–O(CH3 OH) interaction is only 0.21.

Fig. 13 The two minima of the [(hmi2- )MnIII (X2 MnII X2 )MnIII (hmi2- )] complexes where the Mn2 ion can be either hexa-coordinated with the coordinated CH3 OH (top) or penta-coordinated (no CH3 OH ligand, bottom). The Mn2–Cl1 distances are written in blue and the atomic charges on Mn2 and Cl1 are written in dark red.

Fig. 12 Spin density (isosurface value of 0.002) of the ground electronic state of 3 producing a net spin ground state of ST = 13/2. H atoms are omitted for clarity.

Removal of two CH3 OH ligands from 3 and re-optimisation of the remaining structure results in complex 3¢ (Fig. 13, bottom) where each Mn2 is now penta-coordinate. The calculated atomic ‡ The Mn spin densities deviate from the ideal (localised) values of 5 for MnII and 4 for MnIII because of the spin delocalisation into the bridging chloro and methoxy anions.

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spin densities (4.78 for Mn1 and 3.76 for Mn2 from MPA calculations; 4.35 for Mn2 and 3.42 for Mn1 from NPA calculations) indicate that the loss of CH3 OH from 3 did not change the valence description of the complex. When going from 3 to 3¢, the bridging OCH3 ligands change their orientation with the Cl1¢–Mn1–O4– C dihedral angle increasing from -29.2◦ in 3 to 34.3◦ in 3¢, the Mn2–hmi and Mn2–OCH3 bond lengths decrease slightly (0.000– ˚ ). The Mn2–Cl bond (that was in the trans position to 0.046 A ˚ ) in length Mn2–CH3 OH) shows the largest decrease (0.077 A (Fig. 13). The Mayer bond order for Mn2–Cl interaction increases from 0.53 in 3 to 0.59 in 3¢. Thus, the increased Cl-to-Mn2 charge Dalton Trans., 2010, 39, 7650–7658 | 7657

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donation in 3¢ compensates for the loss of the Mn2–CH3 OH interaction. The electronic energy that is required for a loss of the CH3 OH ligand from 3 is only 6.6 kcal mol-1 . These computational results indicate that complex 3 can easily lose coordinated CH3 OH ligands. This loss of the CH3 OH ligands and the subsequent change in the structure of complex 3 is likely responsible for the observed changes in the magnetic behaviour.

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Conclusions In conclusion, by combining versatile H2 hmi ligand with the fruitfulness of manganese chemistry a mononuclear MnII complex and two unique coordination networks based on [MnIII 2 ] and [MnIII MnII MnIII ] units were isolated. Synthetically, complex 1 can be converted to complex 2 by simple addition of base and an additional amount of Mn ion in the reaction mixture, whereas complex 3 can be obtained via a slight modification of the reaction conditions. Complex 3 exhibits SMM behaviour at low temperatures with an effective energy barrier of 8.1 K. Complex 3 represents one of the rare examples of a SMM linked covalently to form a polymeric system.

Acknowledgements We thank the University of Ottawa, the Canada Foundation for Innovation (CFI), FFCR, NSERC (Discovery and RTI grants), the University of Bordeaux, the CNRS, the ANR (NT09_469563, AC-MAGnets project), the Region Aquitaine, the GIS Advanced Materials in Aquitaine (COMET Project), and MAGMANet (NMP3-CT-2005-515767) for their financial support.

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