Molecular Magnets

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Molecular Magnets Guest Editor Euan Brechin University of Edinburgh, UK Published in issue 20, 2010 of Dalton Transactions

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Novel mixed-valent CoII 2 CoIII 4 LnIII 4 aggregates with ligands derived from tris-(hydroxymethyl)aminomethane (Tris)†‡ Hua Xiang,a,b Yanhua Lan,a Huan-Yong Li,b Long Jiang,b Tong-Bu Lu,*b Christopher E. Ansona and Annie K. Powell*a Received 6th January 2010, Accepted 17th February 2010 First published as an Advance Article on the web 2nd March 2010 DOI: 10.1039/c000008f Isostructural CoII 2 CoIII 4 LnIII 4 (Ln = Y (1), Gd (2) and Dy (3)) coordination clusters formed using the ligand Tris are the first examples of 3d-4f complexes involving this ligand and show weak ferromagnetic coupling between the CoII ions and slow relaxation (SMM) behaviour for 3. The discovery of single-molecule magnetism in Mn12 -acetate has spurred intense efforts on the quest for new Single-Molecule Magnets (SMMs).1 SMMs require both significant spin ground state (S) and uniaxial magnetic anisotropy. Recently, the incorporation of lanthanide ions into 3d systems has attracted great interest because lanthanide ions bring large and, in most cases, anisotropic magnetic moments.2 A number of 3d-4f clusters behaving as SMMs has been reported,3,4,5 showing diverse structures, from simple linear arrays,3a-3c semicircles3d and butterflies,3e,3f to doublepropeller,4a dicubanes,4b tricubanes4c and bells,4d to beautifully unusual and irregular high-nuclearity clusters.5 The vast majority of these have Cu, Mn or Fe as 3d metal. Although many Co-Ln heterometallic clusters have been reported,6 only the linear trinuclear complexes CoII 2 LnIII (Ln = Gd, Tb, Dy, Ho) have been reported to be SMMs.3a,3b We recently obtained a new CoII 2 DyIII 2 heterocubane complex showing slow relaxation of the magnetisation indicative of SMM behaviour.7 Since higher nuclearity systems might lead to higher ground spin states (S), we decided to investigate the use of the flexible multidentate ligand tris-(hydroxymethyl)aminomethane (commonly known as the Tris buffer = H3 tris) in place of the rigid ligands used in the CoII 2 LnIII and CoII 2 DyIII 2 complexes. Tris has been widely used with transition metal ions,8 and a Tris-based Co7 complex has been reported to be a SMM.8d Our use of Tris in 3d4f chemistry has resulted in the formation of three isostructural decanuclear CoII 2 CoIII 4 LnIII 4 (Ln = Y (1), Gd (2) and Dy (3)) complexes,§ which are the first Tris-based 3d-4f complexes as well as the first mixed-valent CoII -CoIII -Ln aggregates.

a Institute of Inorganic Chemistry, Karlsruhe Institute of Technology, Engesserstr 15, D-76131 Karlsruhe, Germany. E-mail: [email protected]; Fax: (+49) 721-608-8142 b MOE Key Laboratory of Bioinorganic and Synthetic Chemistry/State Key Laboratory of Optoelectronic Materials and Technologies/School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou, 510275, China. E-mail: [email protected]; Fax: (+86) 2084112921 † This paper is dedicated to the memory of Dr Ian J. Hewitt. ‡ Electronic supplementary information (ESI) available: Experimental details, selected bond lengths and angles, binding modes of (Htris)2- ligand, additional magnetic data. CCDC reference numbers 758590–758592. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c000008f

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The reaction of Co(OAc)2 ·4H2 O, Ln(NO3 )3 ·6H2 O and H3 tris in the presence of triethylamine in methanol produces [CoII 2 CoIII 4 LnIII 4 (Htris)8 (OAc)6 (NO3 )4 (L)2 ](NO3 )2 ·xMeOH·yH2 O: Ln = Y, L = MeOH, x = 6, y = 7 (1), Ln = Gd, (L)2 = (H2 O)(MeOH), x = 5, y = 4 (2) and Ln = Dy, L = MeOH, x = 4, y = 6 (3).9 Single-crystal X-ray diffraction studies reveal that 1-3 crystallise isotypically in P1¯ with Z = 1 and are isostructural except that one of the two methanol ligands in 1 and 3 has been ˚ ) and replaced by an aqua ligand in 2. Selected bond lengths (A angles (◦ ) for 1-3 are listed in Table S1‡ and the structure for 1 is described in detail here. The centrosymmetric molecular structure of 1 (Fig. 1) is based on a {CoII 2 CoIII 4 YIII 4 }28+ core. Each of the CoIII centres are ligated by the amino nitrogens of two doubly deprotonated (Htris)2- ligands. The deprotonated oxygens of these form either m2 - or m3 -alkoxo bridges that link the metal ions together with the third, protonated hydroxyl group of each ligand not coordinated. The three coordination modes of the (Htris)2ligands are shown in Fig. S1, ESI.‡

Fig. 1 The molecular structure of 1: CoII blue, CoIII pale green, Y purple, O red, N blue, C black, organic H atoms omitted for clarity.

The core of the complex can be visualised as an almost planar ˚ ) CoIII 2 CoII 2 Y2 unit built up of edge-sharing (to within 0.0112(6) A Co2 Y and Co3 triangles (involving Co(1), Co(2), Y(1) and their symmetry-equivalents). This shares its Y(1) vertices with two CoIII Y2 triangles (involving Co(3), Y(1) and Y(2)). Each triangular face is m3 -bridged by an alkoxo oxygen, O(1), O(4) or O(7), and each outer edge of the resulting decanuclear unit is bridged either by a m-alkoxo group or a chelating-bridging acetate. The CoIII 2 CoII 2 Y2 unit is nearly perpendicular to the planes of the two CoIII Y2 triangles, with dihedral angles of 85.47(2)◦ . Peripheral ligation is provided by four syn,syn-acetate ligands, with two nitrates and a methanol coordinated to Y(2). Charge balance is afforded by two nitrate anions in the lattice. All the cobalt ions have distorted octahedral coordination spheres, cis-N2 O4 for CoIII and O6 for CoII . The CoIII -O, CoIII -N ˚ , 1.927(5)–1.950(5) A ˚ and CoII -O distances, 1.886(3)–1.919(4) A ˚ , respectively, are typical for the assigned and 1.996(4)–2.227(3) A Dalton Trans., 2010, 39, 4737–4739 | 4737

oxidation states.3a,3b,6,7 Y(1) is eight-coordinate with a distorted dodecahedral geometry; Y(2) is nine-coordinate with a distorted capped square-antiprismatic geometry. The decanuclear units are linked through intermolecular hydrogen bonds involving the -NH2 and -OH groups of Htris- ligands and oxygen atoms from ligands, nitrates and lattice waters and methanols to form a 3D supramolecular framework. Magnetic studies were carried out on powder samples dispersed in Apiezon grease. The dc magnetic susceptibility data for 1-3 in the 1.8–300 K temperature range at 1000 Oe are plotted in Fig. 2. At 300 K, the experimental cT value of 1 (5.89 cm3 K mol-1 ) is much larger than the expected value (3.75 cm3 K mol-1 ) for two spinonly HS CoII ions (S = 3/2, g = 2, C = 1.875 cm3 K mol-1 ), four diamagnetic CoIII and YIII ions, probably as a result of significant orbital contributions for the CoII ions. However, the experimental cT values for 2 (35.48 cm3 K mol-1 ) and 3 (60.36 cm3 K mol-1 ) are close to those expected: 35.25 cm3 K mol-1 for 2 (containing GdIII : S = 7/2, L = 0, 8 S7/2 , g = 2: C = 7.875 cm3 K mol-1 ) and 60.43 cm3 K mol-1 for 3 (containing DyIII : S = 5/2, L = 5, 6 H15/2 , g = 4/3: C = 14.17 cm3 K mol-1 )11 for 3.

Fig. 2

Temperature dependence of cT at 1000 Oe for complexes 1-3.

Ignoring the diamagnetic CoIII ions, the magnetic core in 2 and 3 can be regarded as a CoII 2 LnIII 4 oxygen-bridged chain (Fig. 3). In the case of 1, the presence of the diamagnetic YIII ions allows us to view 1 as a CoII dimer. The cT value for 1 decreases gradually with decreasing temperature and reaches a minimum of 5.05 cm3 K mol-1 at 30 K (Fig. 2). On cooling further the cT value increases rapidly to come to a maximum at 4 K and then falls sharply to 5.65 cm3 K mol-1 at 1.8 K. The decrease of cT above 30 K is apparently due to the spin-orbital coupling of CoII ions, while the behaviour below 30 K indicates ferromagnetic coupling between the CoII ions. Similar interactions have been observed in a pure CoII dimer with similar double oxo-bridges reported recently,12 and between pairs of CoII ions in several tetranuclear butterfly Co4 complexes (ref. 3 in ESI‡).10 These ferromagnetic interactions probably result from the small CoII –O–CoII angles of 93.8◦ in 1

Fig. 3 The magnetic core of 1 (defined by orange bonds to O(1)) and 2-3 (orange and yellow bonds).

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which can be compared with 89.8-101.3◦ in the previously reported complexes.10,12 The final decrease of cT below 4 K could arise from intermolecular antiferromagnetic interactions and/or magnetic anisotropy. On lowering the temperature, the cT values for 2 and 3 decrease slowly to reach 30.86 and 49.61 cm3 K mol-1 , respectively, at ca. 10 K with a sharp decrease to 23.93 cm3 K mol-1 at 1.8 K for 2. However, the cT value of 3 decreases below 10 K to a minimum of 48.75 cm3 K mol-1 at 3 K before slightly increasing to 51.63 cm3 K mol-1 at 1.8 K. The overall magnetic behaviour of the compounds results from the CoII -CoII , CoII -LnIII and LnIII -LnIII magnetic interactions along with the spin–orbit coupling effects and magnetic anisotropy of the CoII and LnIII ions. The diamagnetic Y analogue 1 allows us to judge the magnetic contribution from the 6 Co ions, and more importantly, the central CoII dinuclear portion. We can subtract the molar susceptibility from 2 and 3 to give suitably adjusted plots (Fig. S2) to assess contributions from the CoII -LnIII interactions, assuming, as seems likely, that the LnIII -LnIII interactions are much weaker than the CoII -LnIII interactions,11 as well as the intrinsic contribution from the LnIII ions. Since the GdIII ions in 2 have no orbital contribution, the continuous decrease of its adjusted cT vs T curve suggests antiferromagnetic CoII -GdIII coupling. Curie– Weiss fits for each of the adjusted cT vs T curves above 20 K allow us to assess the extent of the orbital contribution of the Dy ions (Fig. S3) and lead to C = 26.4 and 47.6 cm3 K mol-1 and q = -0.75 and -1.06 K for 2 and 3, respectively. The negative q value of 2 supports the assignment of an antiferromagnetic CoII -GdIII interaction. The q value obtained for 3 is more negative than that for 2, but this could result from thermal depopulation of the Stark levels (from the splitting of the free-ion ground state of DyIII , 6 H15/2 , by the crystal field). The increase of cT below 3 K may result from the ferrimagnetic arrangement of spins in 3. The field dependence of the magnetisation at low temperature for compounds 1-3 shows that these increase gradually at low fields (Fig. S4‡) followed by a lack of saturation even at 7 T. This is indicative of the presence of magnetic anisotropy, also supported by the non-superposition of the M vs. H/T curves. The magnetisation value of 1 at 5 T (4.29 mB ) is consistent with double that expected for single CoII ions, 2.15 mB ,13 confirming weak coupling between the CoII ions. Variable-temperature ac susceptibility measurements were carried out under zero dc fields. No ac signals were observed for 1 and 2, but frequency-dependent out-of-phase signals were detected for 3 below 10 K (Fig. 4, S5‡), indicative of the slow relaxation of the

Fig. 4 Plot of the out-of-phase (c¢¢) ac susceptibility signals versus temperature for complexes 3 in zero dc field.


magnetisation behaviour characteristic of SMMs. Given that the 1-3 are closely isostructural, it appears that the necessary magnetic anisotropy for the observation of this is contributed by the DyIII ions. In conclusion, three novel mixed-valent Co-Ln complexes have been presented, representing the first Tris-based 3d-4f complexes and the first mixed-valent Co-Ln clusters so far reported. These isostructural compounds have allowed us to gauge the contributions of the 3d and 4f ions to the magnetic behaviour of each. The magnetic measurements on the analogue with diamagnetic YIII indicate there is weak ferromagnetic coupling between the two CoII ions. These results were used to show that there is a somewhat stronger antiferromagnetic CoII -LnIII interaction in 3 than in 2 and compound 3 also shows slow relaxation of its magnetisation. The compounds add interesting new members to the growing family of 3d-4f coordination clusters showing magnetic relaxation phenomena. We acknowledge the DFG (Center for Functional Nanostructures), MAGMANet (NMP3-CT-2005-515767), State-Sponsored Scholarship Program for “985 Project”, Natural Science Foundation of China (20625103, 20831005), and 973 Program of China (2007CB815305).

Notes and references ¯ a = 12.7727(14), § 1: C52 H136 Co6 N14 O69 Y4 , 2770.97 g mol-1 , triclinic, P1, ˚ , a = 72.353(2), b = 70.913(2), g = b = 13.6246(15), c = 16.6712(18) A ˚ 3 , T = 100(2) K, rc = 1.803 g cm-3 , 72.982(2)◦ , Z = 1, V = 2551.6(5) A F(000) = 1416, m(Mo-Ka) = 3.310 mm-1 ; 19033 data, 11005 unique (Rint = 0.0264), 639 parameters, final wR2 = 0.1634, S = 1.033 (all data), R1 (8185 data with I > 2s(I)) = 0.0553. ¯ a = 12.8831(6), b = 2: C50 H124 Co6 Gd4 N14 O65 , 2944.21 g mol-1 , triclinic, P1, ˚ , a = 72.486(3), b = 71.881(4), g = 73.986(4)◦ , 13.6555(5), c = 16.0874(6) A ˚ 3 , T = 123(2) K, rc = 1.946 g cm-3 , F(000) = Z = 1, V = 2512.73(18) A 1460, m(Cu-Ka) = 25.278 mm-1 ; 20277 data, 10863 unique (Rint = 0.0536), 618 parameters, final wR2 = 0.1268, S = 0.972 (all data), R1 (6337 data with I > 2s(I)) = 0.0594. ¯ a = 12.8569(15), 3: C50 H126 Co6 Dy4 N14 O66 , 2983.29 g mol-1 , triclinic, P1, ˚ , a = 72.386(2), b = 71.502(2), g = b = 13.6424(16), c = 16.3608(19) A ˚ 3 , T = 100(2) K, rc = 1.951 g cm-3 , 73.644(2)◦ , Z = 1, V = 2538.5(5) A F(000) = 1478, m(Mo-Ka) = 3.968 mm-1 ; 17186 data, 10754 unique (Rint = 0.0270), 642 parameters, final wR2 = 0.1899, S = 1.070 (all data), R1 (8149 data with I > 2s(I)) = 0.0615. CCDC 758590–758592.‡ 1 (a) R. Sessoli, H.-L. Tsai, A. R. Schake, S. Wang, J. B. Vincent, K. Folting, D. Gatteschi, G. Christou and D. N. Hendrickson, J. Am. Chem. Soc., 1993, 115, 1804; (b) E. K. Brechin, Chem. Commun., 2005, 5141; (c) L. M. C. Beltran and J. R. Long, Acc. Chem. Res., 2005, 38, 325; (d) G. Arom´ı and E. K. Brechin, in Struct. Bonding, ed. D. M. P. Mingos, and R. Winpenny, Springer-Verlag, Berlin Heidelberg, 2006, vol. 122, pp. 1-67, and references therein; (e) E. Cremades, J. Cano, E. Ruiz, G. Rajaraman, C. J. Milios and E. K. Brechin, Inorg. Chem., 2009, 48, 8012.


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