Functionalised β-diketonate polynuclear lanthanoid hydroxo clusters: Synthesis, characterisation, and magnetic properties

Polyhedron 28 (2009) 2123–2130

Contents lists available at ScienceDirect

Polyhedron journal homepage: www.elsevier.com/locate/poly

Functionalised b-diketonate polynuclear lanthanoid hydroxo clusters: Synthesis, characterisation, and magnetic properties Philip C. Andrews a, Tobias Beck a,b, Benjamin H. Fraser a, Peter C. Junk a,*, Massimiliano Massi a,c, Boujemaa Moubaraki a, Keith S. Murray a, Morry Silberstein d a

School of Chemistry, Monash University, Melbourne, Victoria 3800, Australia On Exchange from Faculty of Chemistry, University of Göttingen, 37077 Göttingen, Germany c Department of Applied Chemistry, Curtin University of Technology, Perth, Western Australia 6845, Australia d School of Medicine Nursing and Health Sciences, Monash University, Melbourne, Victoria 3800, Australia b

a r t i c l e

i n f o

Article history: Received 16 February 2009 Accepted 24 March 2009 Available online 12 April 2009 Keywords: Lanthanoids b-Diketones Crystal structures Magnetic susceptibility Metal-hydroxo clusters

a b s t r a c t The controlled hydrolysis of lanthanoid trichloride hexahydrate (Ln = Nd, Eu, Ho) in methanol with the bdiketone ligands dibenzoylmethane and 1,3-bis(4-ethoxyphenyl)propane-1,3-dione yielded tetranuclear and pentanuclear hydroxo clusters for Eu and Ho. In contrast, performing the reaction in the presence of 1,3-bis(4-methoxyphenyl)propane-1,3-dione yielded a mononuclear complex for Nd. The compounds were structurally characterised by means of single crystal X-ray diffraction, showing that the increased bulkiness of the ligand due to the ethoxy functionalities does not affect the capability of the diketonate to stabilize the cluster core. Variable temperature dc susceptibility magnetic measurements were made on the clusters and were indicative of very weak to zero antiferromagnetic, intra-cluster coupling. Variable frequency ac data recorded at low temperatures did not show any evidence for single molecule magnet (SMM) behaviour, unlike the recent reported case of the analogue [Dy5(OH)5(Ph2acac)10], where Ph2acac is the dibenzoylmethanide ligand. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Over the past decade, the hydrolysis of lanthanoid salts (LnX3 X = Cl, I, OTf, ClO4) performed under ‘‘controlled conditions” and in the presence of ‘‘protecting ligands”, to avoid extensive formation of insoluble hydroxide species, has allowed the isolation and characterisation of a wide variety of polynuclear oxo/hydroxo clusters [1]. Although these polynuclear cages have been envisaged for some time as precursors for the preparation of novel functional materials, due to the properties associated with the f electrons of the rare earths such as luminescence [2], magnetism [3], and Lewis acidity [1f,4], very few results in this context have so far been reported. The only relevant examples reported for their application describe their catalytic activity in polymerisation and oxidation reactions [4]. By contrast, the incorporation of polynuclear lanthanoid cages into hybrid organic/inorganic polymeric networks or molecular frameworks has been successfully attempted [5]. Despite these early results, there is still little known about the chemistry and reactivity of the rare earth clusters. In all the synthetic procedures, formation of the polynuclear cage is the ultimate step and no further reactions aimed at modifying either the metal core or the encapsulating ligands have been described. As a result, questions about the * Corresponding author. Tel.: +61 3 9905 4570; fax: +61 3 9905 4597. E-mail address: [email protected] (P.C. Junk). 0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2009.03.031

stability of the clusters under specific reaction conditions or the possibility of introducing different functional groups on the outer shell in a preformed cage remain unanswered. As part of our ongoing research into obtaining functionalisable clusters, we have pursued cluster formation using b-diketonates obtained from dibenzoylmethane (Ph2acacH) with short chain ether groups in the para position of both phenyl rings. The preparation of tetranuclear and pentanuclear lanthanoid hydroxo clusters, via hydrolysis of lanthanoid chlorides with b-diketonates as ligands, is now well established [4,6]. It is also well known that less bulky ligands promote the formation of larger clusters and vice versa [6,7]. In exploring the chemistry and reactivity of rare earth oxo/hydroxo clusters, we have also probed the relationship between the overall steric bulk of the ligand and the size of the cage (in terms of number of Ln3+ ions) which results from this synthetic procedure. This can provide valuable information on whether small modifications to the outer shell can cause a collapse of the cage as a result of changes in the steric demand of the ligands. To achieve this goal we used an ‘‘inverse approach”, i.e. firstly modifying the ligand with subsequent cluster formation. As such, methoxy and ethoxy functionalities were added in the para positions of both the phenyl rings of Ph2acacH and analogous reactions to the ones which give the unmodified Ph2acac ligated clusters were carried out. Moreover, a detailed investigation of the magnetic properties of these polynuclear cages has been conducted, in view of their possible use as single molecule magnets (SMMs) [8]. While the

2124

P.C. Andrews et al. / Polyhedron 28 (2009) 2123–2130

present work was in progress, a paper describing the SMM features of [Dy5(OH)5(Ph2acac)10], isostructural to the Ho and Eu clusters reported in this work, has just appeared [3c]. Variable frequency ac susceptibility studies have shown that the present Ho5 and Eu5 clusters are essentially flat in the out-of-phase ac v00M versus T (2–10 K) region, with the merest hint of an increase indicating they are either not SMMs or that the blocking temperature is below 2 K. In contrast, the Dy5 analogue showed characteristic maxima in v00M at 3 K, pointing to slow relaxation of the magnetisation [3c]. The static dc magnetic susceptibilities of the present compounds show Curie– Weiss behaviour, some with plateaus in susceptibilities at low temperatures, that is indicative of weak to zero antiferromagnetic intra-cluster coupling.

pension was stirred for 10 min. Ethyliodide (4.8 ml, 60 mmol) was added dropwise. After stirring for 14 h, water (100 ml) and ethyl acetate (500 ml) were added. The aqueous phase was separated from the organic layer, which was washed with water (8  250 ml), dried over MgSO4 and evaporated to dryness to yield 2 as a white solid. Yield: 6.98 g (97%). M.p. 37–38 °C. 1H NMR (300 MHz, CDCl3): d = 1.43 (t, J = 7.0 Hz, 3H, CH3CH2O), 3.88 (s, 3H, CH3O), 4.08 (q, J = 7.0 Hz, 2H, CH3CH2O), 6.88–6.92 (m, 2H, 2H, 6-H), 7.96–8.00 (m, 2H, 3-H, 5-H). 13C NMR (50 MHz, CDCl3): d = 14.8 (CH3CH2O), 51.9 (CH3O), 63.8 (CH3CH2O), 114.2 (C3, C5), 122.6 (C1), 131.7 (C2, C6), 162.9 (C4), 167.0 (COOCH3). MS (ESI): m/z = 202.9 [MNa]+, 181.2 [MH]+. 2.4. Synthesis of 4-methoxycetophenone (3)

2. Experimental 2.1. General procedures Chemicals were obtained from Sigma–Aldrich and used as received. 1H and 13C NMR spectra were recorded on a Bruker AM 200 and AM 300 spectrometers. Chemical shifts were recorded on the d scale and referenced to the solvent. Infrared spectra were recorded on a Perkin–Elmer 1600 Series FTIR using solid state pellets (KBr). Melting points were measured on a Stuart Scientific Melting Point Apparatus SMP3 in an open capillary and were not corrected. Elemental Analyses were performed by the Campbell Microanalytical Laboratory, Department of Chemistry, University of Otago, Dunedin, New Zealand. Crystallographic data were obtained on a Bruker X8 APEX CCD at the temperature of 123 K. The data for [Nd(5)3(DMF)2] were collected on an Enraf-Nonius KAPPA CCD at the temperature of 123 K. All compounds were measured with Mo Ka radiation generated by a fine-focus sealed tube using a graphite monochromator. Scans were performed in u and x and 0.5° frames were recorded. All the molecular structures were solved using the program SHELXS-97 [9]. Refinement was carried out with the program SHELXL-97 against F2 using least-square methods [9]. For the X-ray data refer to Table 3. Magnetic measurements were performed using a Quantum Design MPMS 5 Squid for dc studies and a Quantum Design PPMS instrument for ac susceptibilities, as described previously [10]. Samples of mass 20 mg were checked for crystallite torquing anomalies, due to magnetic anisotropy, by making mulls in Vaseline and comparing to the neat powder samples. The ac frequencies employed on the PPMS machine were 10, 50, 100, 250, 500, 750, 1000, 1250, 1500 Hz with the accompanying dc (static) field of about 3 Oe. 2.2. Synthesis of methyl 4-methoxybenzoate (1) Methyl 4-hydroxybenzoate (6.0 g, 40 mmol) was dissolved in dry DMF (80 ml). K2CO3 (16.6 g, 120 mmol) was added and the suspension was stirred for 10 min. Methyliodide (3.8 ml, 60 mmol) was then added dropwise. After stirring for 14 h, water (100 ml) and ethyl acetate (500 ml) were added. The aqueous phase was separated from the organic layer, which was washed with water (8  250 ml), dried over MgSO4 and evaporated to dryness to yield 1 as a yellow solid. Yield: 6.42 g (97%). M.p. 51–52 °C. 1H NMR (200 MHz, CDCl3): d = 3.84 (s, 3H, CH3O), 3.86 (s, 3H, COOCH3), 6.86–6.93 (m, 2H, 2-H, 6-H), 7.93–8.01 (m, 2H, 3-H, 5-H). 13C NMR (50 MHz, CDCl3): d = 51.9 (COOCH3), 55.5 (CH3O), 113.7 (C3, C5), 122.7 (C1), 131.7 (C2, C6), 163.4 (C4), 167.0 (COOCH3). MS (ESI): m/z = 166.8 [MH]+. 2.3. Synthesis of methyl 4-ethoxybenzoate (2) Methyl 4-hydroxybenzoate (6.0 g, 40 mmol) was dissolved in dry DMF (80 ml). K2CO3 (19.3 g, 140 mmol) was added and the sus-

4-Hydroxyacetophenone (5.5 g, 40 mmol) was dissolved in dry DMF (80 ml). K2CO3 (18 g, 130 mmol) was added and the suspension was stirred for 10 min. Methyliodide (3.8 ml, 60 mmol) was added dropwise. After stirring for 16 h, water (100 ml) and ethyl acetate (500 ml) were added. The aqueous phase was separated from the organic layer, which was washed with water (12  250 ml), dried over MgSO4 and evaporated to dryness to yield 3 as a pale yellow solid. Yield: 5.86 g (98%). M.p. 40–41 °C. 1 H NMR (200 MHz, CDCl3): d = 2.54 (s, 3H, CH3), 3.87 (s, 1H, CH3O), 6.90–6.97 (m, 2H, 3-H, 5-H), 7.90–7.97 (m, 2H, 2-H, 6-H). 13 C NMR (50 MHz, CDCl3): d = 26.5 (CH3), 55.6 (CH3O), 113.8 (C3, C5), 130.5 (C1), 130.8 (C2, C6), 163.6 (C4), 196.9 (COCH3). MS (ESI): m/z = 150.8 [MH]+. 2.5. Synthesis of 4-ethoxy acetophenone (4) 4-Hydroxyacetophenone (5.5 g, 40 mmol) was dissolved in dry DMF (80 ml). K2CO3 (18 g, 130 mmol) was added and the suspension was stirred for 10 min. Ethyliodide (4.8 ml, 60 mmol) was added dropwise. After stirring for 16 h, water (100 ml) and ethyl acetate (500 ml) were added. The aqueous phase was separated from the organic layer, which was washed with water (12  250 ml), dried over MgSO4 and evaporated to dryness to yield 4 as a beige solid. Yield: 5.86 g (99%). M.p. 38–39 °C. 1H NMR (200 MHz, CDCl3): d = 1.44 (t, J = 7.0 Hz, 3H, CH3CH2O), 2.54 (s, 3H, CH3), 4.12 (q, J = 7.0 Hz, 2H, CH3CH2O), 6.87–6.95 (m, 2H, 3H, 5-H), 7.88–7.96 (m, 2H, 2-H, 6-H). 13C NMR (50 MHz, CDCl3): d = 14.8 (CH3CH2O), 26.4 (CH3), 63.8 (CH3CH2O), 114.3 (C3, C5), 130.3 (C1), 130.7 (C2, C6), 163.0 (C4), 196.9 (COCH3). MS (ESI): m/z = 186.9 [MNa]+, 164.9 [MH]+. 2.6. Synthesis of 1,3-bis(4-methoxyphenyl)propane-1,3-dione (5H) NaH (2.0 g of 60% dispersion in mineral oil, 50 mmol) was added to a mixture of dry DMSO and anhydrous THF (1:2, 150 ml). Compounds 1 (3.3 g, 20 mmol) and 3 (2.2 g, 15 mmol) were dissolved in dry THF (20 ml) and added dropwise to the NaH-suspension while cooling with an ice bath. The brown mixture was allowed to warm up to room temperature after stirring for 1 h, and it was heated to 40 °C and stirred for 16 more h. The THF was evaporated under reduced pressure, water was added (100 ml), and the residue was neutralised with hydrochloric acid (10%) to pH 7. The mixture was then extracted with ethyl acetate (2  300 ml), and the combined organic layers were washed with water (10  250 ml), dried over MgSO4, and evaporated to dryness. The crude product was recrystallised from ethanol. Compound 5H was obtained as light brown needles. Yield: 2.22 g (52%). M.p. 119– 120 °C. 1H NMR (as mixture of keto and enol forms) (200 MHz, CDCl3): d = 3.87 (s, 6H, CH3O), 4.53 (s, CH2), 6.73 (s, 1H, CH), 6.91–7.01 (m, 4H, 3-H, 3-H, 5-H, 5-H), 7.93–8.03 (m, 4H, 2-H, 2H, 6-H, 6-H). 13C NMR (50 MHz, CDCl3): d = 55.6 (CH3O), 91.6

P.C. Andrews et al. / Polyhedron 28 (2009) 2123–2130

(COCH2CO), 114.1 (C3, C3, C5, C5), 128.4 (C1, C1), 129.2 (C2, C2, C6, C6), 163.2 (C4, C4), 184.8 (CO). IR (KBr pellet): m = 2925 (m), 2849 (m), 1604 (s), 1498 (s), 1204 (m), 120 (s), 1229 (s), 1186 (s), 1171 (s), 1116 (m), 1022 (m), 842 (m), 781 (s), 722 (m), 634 (w) cm1. MS (ESI): m/z = 307.0 [MNa]+, 285.0 [MH]+. 2.7. Synthesis of 1,3-bis(4-ethoxyphenyl)propane-1,3-dione (6H) NaH (2.0 g of 60% dispersion in mineral oil, 50 mmol) was added to a mixture of dry DMSO and anhydrous THF (1:2, 150 ml). Compounds 2 (4.3 g, 24 mmol) and 4 (2.2 g, 20 mmol) were dissolved in dry THF (20 ml) and added dropwise to the NaH-suspension while cooling with an ice bath. The brown mixture was allowed to warm up to room temperature after stirring for 1 h and it was heated to 40 °C and stirred for 16 more h. The THF was evaporated under reduced pressure, water was added (100 ml), and the residue was neutralised with hydrochloric acid (10%) to pH 7. The mixture was extracted with ethyl acetate (2  300 ml) and the combined organic layers were washed with water (8  100 ml), dried over MgSO4 and evaporated to dryness. The crude product was recrystallized from ethanol. 6H was obtained as thin yellow plates. Yield: 4.62 g (74%). M.p. 137–138 °C. 1 H NMR (300 MHz, CDCl3): d = 1.45 (t, J = 7.0 Hz, 6H, CH3CH2O), 4.11 (q, J = 7.0 Hz, 4H, CH3CH2O), 4.52 (s, CH2), 6.72 (s, 1H, CH), 6.90–6.98 (m, 4H, 3-H, 3-H, 5-H, 5-H), 7.93–8.03 (m, 4H, 2-H, 2H, 6-H, 6-H). 13C NMR (50 MHz, CDCl3): d = 14.8 (CH3CH2O), 63.9 (CH3CH2O), 91.6 (COCH2CO), 114.5 (C3, C3, C5, C5), 128.2 (C1, C1), 129.2 (C2, C2, C6, C6), 162.6 (C4, C4), 196.9 (CO). IR (KBr pellet): m = 2981 (m), 2936 (m), 2891 (m), 1604 (s), 1506 (m), 1389 (m), 1302 (m), 1254 (s), 1229 (s), 1190 (m), 1173 (s), 1120 (s), 1042 (s), 919 (m), 851 (m), 835 (w), 815 (w), 785 (s), 729 (w), 702 (w), 670 (w), 652 (w), 605 (m) cm1. MS (ESI): m/z = 335.0 [MNa]+, 313.1 [MH]+. 2.8. Synthesis of [Nd(5)3(DMF)2] NdCl3  6H2O (0.36 g, 1.0 mmol) and 5H (0.57 g, 2.0 mmol) were added to 50 ml methanol. Once both reagents were completely dissolved, NEt3 (0.84 ml, 6.0 mmol) was then added dropwise. After stirring the reaction mixture for 13 h, the solvent was evaporated under reduced pressure. The solid was washed with toluene (3  10 ml) and then dissolved in DMF (5 ml). Green block shaped crystals were obtained by slow evaporation after 6 months. Yield (not optimised): 0.17 g (15%). Anal. Calc. for C57H59N2NdO14: C, 60.04; H, 5.22; N, 2.46. Found: C, 59.78; H, 5.32; N, 2.43%. IR (KBr pellet) m: 2933 (w), 2835 (w), 1663 (w), 1641 (m), 1658 (s), 1525 (s), 148,5 (s), 1459 (s), 1430 (s), 1376 (s), 1302 (m), 1249 (s), 1217 (m), 1168 (m), 1103 (s), 1061 (w), 1022 (m), 936 (m), 842 (w), 780 (m) cm1.

2125

2.10. Synthesis of [Eu5(OH)5(6)10] Compound 6H (0.62 g, 2.0 mmol) was dissolved in a mixture of methanol/THF (1:1, 50 ml), and EuCl3  6H2O (0.37 g, 1.0 mmol) was added. Once both reagents were completely dissolved, NEt3 (0.84 ml, 6.0 mmol) was then added dropwise. After stirring for 16 h the solvents were evaporated and toluene (30 ml) was added to the residue. The mixture was stirred for 2 h and filtered. The toluene solution was left undisturbed to evaporate to yield yellow crystals after 1 week. Yield (not optimised): 0.24 g (28%). Anal. Calc. for C204H211Eu5O45: C, 59.15; H, 5.13. Found: C, 58.81; H, 5.22%. IR (KBr pellet): m 3435 (m), 2978 (m), 2928 (m), 1603 (s), 1546 (s), 1527 (s), 1490 (s), 1430 (m), 1387 (m), 1303 (m), 1255 (s), 1220 (s), 1179 (s), 1130 (m), 1115 (m), 1044 (s), 1008 (w), 922 (m), 844 (m), 782 (s), 704 (w), 632 (w) cm1. 2.11. Synthesis of [Ho5(OH)5(Ph2acac)10] HoCl3  6H2O (0.38 g, 1.0 mmol) and Ph2acacH (0.56 g, 2.5 mmol) were added to 50 ml methanol. Once both reagents were completely dissolved, NEt3 (0.84 ml, 6.0 mmol) was added dropwise. The solution was stirred for 16 h and then evaporated to dryness under reduced pressure. The residue was stirred in toluene (15 ml) and the remaining white solid was filtered off. The filtrate was allowed to stand undisturbed at room temperature to yield yellow crystals after 5 days. Yield (not optimised): 0.10 g (16%). Anal. Calc. for C150H115Ho5O25: C, 57.34; H, 3.69. Found: C, 57.67; H, 3.83%. IR (KBr pellet): m 3455 (sb), 1605 (s), 1553 (s), 1522 (s), 1480 (s), 1458 (s), 1397 (s), 1314 (m), 1281 (m), 1223 (m), 1181 (w), 1068 (w), 1051 (w), 1022 (w), 941 (w), 784 (w), 750 (m), 716 (m), 686 (m), 610 (m) cm1. 2.12. Synthesis of [Ho5(OH)5(6)10] Compound 6H (0.62 g, 2.0 mmol) was dissolved in a mixture of methanol/THF (1:1, 50 ml), and HoCl3  6H2O (0.38 g, 1.0 mmol) was added. Once both reagents were completely dissolved, NEt3 (0.84 ml, 6.0 mmol) was then added dropwise. After stirring for 16 h the solvents were evaporated and toluene (30 ml) was added to the residue. The mixture was stirred for 2 h and filtered. The filtrate was allowed to stand undisturbed at room temperature to yield yellow crystals after 5 days. Yield (not optimised): 0.16 g (18%). Anal. Calc. for C197H203Ho5O45: C, 57.49; H, 4.97. Found: C, 57.73; H, 5.08%. IR (KBr pellet): m 3456 (sb), 2977 (m), 1604 (s), 1560 (m), 1546 (m), 1528 (m), 1491 (s), 1388 (m), 1303 (s), 1254 (m), 1220 (m), 1173 (s), 1115 (w), 1044 (m), 922 (w), 844 (w), 782 (s), 632 (w) cm1.

3. Results and discussion 2.9. Synthesis of [Eu4(OH)2(Ph2acac)10] EuCl36H2O (0.37 g, 1.0 mmol) and Ph2acacH (0.45 g, 2.0 mmol) were added to 50 ml methanol. Once both reagents were completely dissolved, NEt3 (0.84 ml, 6.0 mmol) was then added dropwise. After stirring the reaction mixture for 13 h, the solvent was evaporated under reduced pressure. The residue was stirred in toluene (15 ml) and filtered. The resulting yellow solid was washed with ethanol (3  30 ml) and then dissolved in chloroform (5 ml). Bright yellow crystals were obtained by slow diffusion of hexane into this solution. Yield (not optimised) 0.21 g (15%). Anal. Calc. for C150H132Eu4O22: C, 62.24; H 4.60. Found: C, 62.32; H, 4.05%. IR (KBr pellet): m 3058 (m), 1597 (s), 1556 (s), 1520 (s), 1480 (s), 1455 (s), 1401 (s), 1312 (s), 1223 (m), 1179 (m), 1126 (w), 1069 (m), 1024 (m), 1000 (w), 940 (m), 842 (w), 812 (w), 784 (m), 752 (m), 720 (s), 686 (s), 618 (m), 604 (m) cm1.

3.1. Synthesis of the functionalised ligands and preparation of the polynuclear clusters The functionalised ligands 1,3-bis(4-methoxyphenyl)propane1,3-dione (5H) and 1,3-bis(4-ethoxyphenyl)propane-1,3-dione (6H) were prepared in two steps, as shown in Scheme 1. First, the corresponding 4-hydroxyacetophenone and methyl 4-hydroxybenzoate precursors were treated with MeI or EtI in DMF to produce either the methoxy compounds 1 and 3 or the ethoxy functionalised compounds 2 and 4. Subsequently, via Claisen condensation, 5H and 6H were obtained through coupling of 1 with 3 or 2 with 4 respectively using NaH in dry THF. Only the symmetrical versions of the b-diketones were prepared and investigated in this work. The cluster compounds were prepared according to the literature procedure [4] by treating 1 equiv. of the corresponding hy-

2126

P.C. Andrews et al. / Polyhedron 28 (2009) 2123–2130

Scheme 1. Synthesis of the functionalised b-diketonate ligands 5 and 6.

drated lanthanoid chloride (LnCl3(H2O)6, Ln = Nd, Eu, Ho) with 2 equiv. of ligand (Ph2acacH, 5H, or 6H) in the presence of an excess of triethylamine in methanol. When the ligands Ph2acacH or 6H were used, the cluster compounds were extracted with toluene or dichloromethane after complete removal of the methanol. The organic phase was then washed with water to remove any unreacted triethylamine along with residual triethylammonium chloride or lanthanoid salt. This procedure yielded the tetranuclear cluster [Eu4(OH)2(Ph2acac)8] as well as the pentanuclear clusters [Eu5(OH)5(6)10] and [Ho5(OH)5(6)10]. The reaction of the lanthanoid chlorides with 5H yielded products with poor solubility in toluene, chlorinated solvents or alcohols. Therefore, they were dissolved in DMF and allowed to stand undisturbed. After several months, single crystals suitable for X-ray diffraction were obtained starting from NdCl3 and 5H. The structure was identified as a mononuclear complex of formula [Nd(5)3(DMF)2]. At present, due to the insolubility of the product, it is still unclear whether this

mononuclear complex was the only species formed in the initial reaction mixture or resulted from deaggregation of a polynuclear cluster in DMF. However, previous studies on polynuclear oxo/hydroxo lanthanoid clusters have shown these cages are unstable in water but relatively stable in highly polar organic solvents such as DMF or DMSO [3d]. By comparison with these results it seems likely that the poor solubility of the complex prevents its further growth into an hydroxo cluster. A summary of the reactions attempted and the corresponding products is given in Scheme 2. 3.2. Crystallography The tetranuclear [Eu4(OH)2(Ph2acac)8] and the pentanuclear [Ho5(OH)5(Ph2acac)10] are isostructural with previously reported clusters and will not be described in any detail within this work. Small differences in bond lengths, primarily due to the lanthanoid contraction [11], are provided in Table 1. Interestingly though, in

Scheme 2. Summary of the attempted reactions and corresponding obtained products.

Table 1 Selected bond lengths for [Eu4(OH)2(Ph2acac)10] and [Ho5(OH)5(Ph2acac)10]. Ligand

[Eu4(OH)2(Ph2acac)10]

Bond (Å)

[Ho5(OH)5(Ph2acac)10]

Bond (Å)

OH OH g2-Diketonate g2-Diketonate (l-O)-g2-Diketonate (l-O)-g2-Diketonate (l-O)-g2-Diketonate (l-O)2-g2-Diketonate (l-O)2-g2-Diketonate (l-O)2-g2-Diketonate (l-O)2-g2-Diketonate

Eu(1)–O(1) Eu(2)–O(1) Eu(1)–O(4) Eu(1)–O(5) Eu(2)–O(8) Eu(2)–O(9) Eu(1)–O(9) Eu(1)–O(2) Eu(1)–O(3) Eu(2)–O(2)# Eu(2)–O(3)# #: x, y + 1, z + 1

2.410(4) 2.343(4) 2.325(4) 2.337(4) 2.343(4) 2.443(4) 2.469(4) 2.556(4) 2.555(4) 2.483(4) 2.489(4)

Ho(1)–O(1) (l3-OH) Ho(1)–O(2) (l4-OH) Ho(1)–O(5) Ho(1)–O(6) Ho(1)–O(4) Ho(1)–O(3) Ho(1)–O(3)#

2.312(4) 2.560(2) 2.277(4) 2.273(4) 2.343(4) 2.342(4) 2.427(4)

#: y, x + 3/2, z

P.C. Andrews et al. / Polyhedron 28 (2009) 2123–2130

2127

the case of Eu both tetranuclear and pentanuclear clusters are formed. It is known that bigger lanthanoids prefer tetranuclear cages where each metal cation is nonacoordinated, whereas smaller ones tend to form pentanuclear cages with octacoordinated metal ions [6]. In this respect, Eu represents the borderline of this trend. The Nd complex [Nd(5)3(DMF)2] (Fig. 1) crystallises in the  The metal is overall octacoordinated triclinic space group P1. by three diketonate ligands, which all adopt a chelating configuration, and two molecules of DMF. Two of the ligands lie on a plane with the third almost perpendicular, whereas the other face of the complex is occupied by the two coordinated solvent molecules. A third molecule of DMF resides in the crystal lattice. The structure does not show any appreciable intermolecular hydrogen bonding, however, the phenyl rings of the

diketonate ligands form a p-stacking network exhibiting offset face-to-face bonding with an average distance of 3.6 Å and a vertex-to-face bonding (C–H@C6H4 centroid 3.1 Å). [Eu5(OH)5(6)10], and [Ho5(OH)5(6)10] (Fig. 2, where only the Ho cage is reported) are isostructural, showing only small differences in bond lengths between the oxygen atoms of the diketonate and hydroxo ligands due to the lanthanoid contraction passing from Eu to Ho [11] (see Table 2). The structure and geometry of the pentanuclear cage is isostructural with previously published pentanuclear clusters with dibenzoylmethanide ligands. In this case however, the presence of the ethoxy groups in the ligands lowers the symmetry and the compounds crystallise in a monoclinic space group (C2/c). Four toluene molecules are present in the lattice. There is no appreciable evidence of either intermolecular hydrogen bonding or p-stacking.

Fig. 1. X-ray crystal structure of [Nd(5)3(DMF)2] (top) and coordination environment around the Nd cation (bottom) with ellipsoids at 50% probability. Hydrogen atoms are omitted for clarity.

Fig. 2. X-ray crystal structure of [Ho5(OH)5(6)10] (top) and cluster core (bottom) showing with one ligand chelating (yellow), one ligand chelating/bridging (blue) and one ligand chelating the apical holmium atom (grey). Ellipsoids at 50% probability for core atoms. Hydrogen atoms omitted for clarity, labelling only for atoms that are part of the asymmetric unit and #1 x, y, 1/2  z. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

2128

P.C. Andrews et al. / Polyhedron 28 (2009) 2123–2130

3.3. Magnetic measurements on the cluster compounds The plots of the dc leff and vM values, per Eu4 in [Eu4 (OH)2(Ph2acac)8]  2 CHCl3  hexane, are given in Fig. 3. It can be seen that the leff values decrease gradually from 6.5 lB (3.25 lB per Eu) at 300 K to 0.75 lB at 2 K (0.37 lB per Eu). The corresponding vM values show a broad maximum/plateau between 70 and 30 K, possibly due to medium strength antiferromagnetic (antiparallel) coupling of the f6 spins. This plateau region is followed by a rapid, Curie-like increase in vM that commonly (in d-block clusters) originates from traces of monomeric (more strongly) paramagnetic impurities. Single ion Eu(III) compounds display leff values at 300 K of 3.4 lB, compared to a calculated value of 3.61 lB obtained after consideration of ligand field and spin–orbit splitting of the 7F free-ion ground state (to give J levels 0 (7F0 ground level), 1, 2, 3, 4, 5, 6) and thermal population of the first excited 7F1 level following its splitting by first order Zeeman magnetic field effects (H), together with second order Zeeman contributions (H2) [12,13]. Because of large spin–orbit coupling constants in f-block ions, the separation of the 7F0 and 7F1 levels is of the order of the Boltzmann energy kT (210 K at 300 K) and thus monomeric Eu(III) compounds will show a significant temperature dependence in leff, with a zero value calculated at 0 K. Thus, in clusters of the present kind, sorting the spin–spin coupling from the single-ion ligand field/spin–orbit effects is not trivial. The plateau in vM at low temperatures might originate from a combination of second order Zeeman (TIP, temperature independent susceptibilities) single-ion effects and weak intra-cluster antiferromagnetic coupling. Very similar plots of leff (expressed as vMT) and vM, versus temperature to those given in Fig. 3 have recently been reported for tetranuclear (chain) species [Eu2(L1)2(1,10-

Fig. 3. Plots of molar magnetic susceptility, vM, per Eu4 (squares) and effective magnetic moment, leff, per Eu4 (circles) for [Eu4(OH)2(Ph2acac)8]  hexane  2chloroform. The solid lines are just guides to the eye.

phen)2(H2O)5]  3H2O, where L1 is m-sulphophenylphosphonate, (leff at 300 K = 3.39 lB, per Eu) for which antiferromagnetic coupling was suggested [2b], and in the framework structures [Eu(benzenetricarboxylate)(H2O)]n and {[Eu2(2-amino-1,4-benzene-dicarboxylate)3  (dmf)4]  2dmf}n for which the thermal population of 7F1 levels was invoked, with spin–spin coupling across the carboxylate bridges being assumed to be negligible to weak [14]. The plots of leff and vM, per Eu5, versus temperature for [Eu5 (OH)5(6)10]  4 toluene are given in Fig. 4 and generally have a similar shape to those in Fig. 3, but with a higher plateau value in vM between 60 and 10 K, because of the extra Eu(III) centre in this

Table 2 Selected bond lengths for [Eu5(OH)5(6)10] and [Ho5(OH)5(6)10]. Ligand

[Eu5(OH)5(6)10]

Bond (Å)

[Ho5(OH)5(6)10]

Bond (Å)

l3-OH l4-OH g2-Diketonate g2-Diketonate (l-O)-g2-Diketonate (l-O)-g2-Diketonate (l-O)-g2-Diketonate

Eu(1)–O(1) Eu(1)–O(3) Eu(1)–O(16) Eu(1)–O(17) Eu(2)–O(12) Eu(2)–O(13) Eu(1)–O(13)

2.360(7) 2.544(2) 2.329(6) 2.312(6) 2.377(6) 2.393(6) 2.494(7)

Ho(1)–O(2) Ho(1)–O(1) Ho(1)-O(12) Ho(1)–O(13) Ho(1)–O(8) Ho(1)–O(9) Ho(2)–O(9)

2.334(6) 2.587(2) 2.262(6) 2.285(6) 2.347(6) 2.329(6) 2.420(6)

Table 3 Crystal data and structure refinement for [Nd(5)3(DMF)2], [Eu4(OH)2(Ph2acac)10], [Eu5(OH)5(6)10], [Ho5(OH)5(Ph2acac)10], and [Ho5(OH)5(6)10].

Formula Formula weight Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) Volume (Å3) qcalcd (mg m3) Z l (mm1) Reflections collected Unique reflections Rint R1 [I > 2r(I)] wR2 [all data] Goodness-of-fit

[Nd(5)3(DMF)2]

[Eu4(OH)2(Ph2acac)10]

[Eu5(OH)5(6)10]

[Ho5(OH)5(Ph2acac)10]

[Ho5(OH)5(6)10]

C60H66N3NdO15 1213.40 triclinic  P1

C158H128C16Eu4O22 3199.14 triclinic  P1

14.297(2) 14.338(2) 16.268(3) 107.24(3) 113.85(4) 94.18(3) 2841.2(8) 1.418 2 0.984 49 423 15 687 0.1067 0.0459 0.1115 1.038

15.275(3) 16.111(3) 16.580(4) 117.22(3) 109.38(3) 91.85(2) 3339.4(12) 1.590 1 2.045 33 983 19 397 0.0356 0.0674 0.1361 1.220

C218H227Eu5O45 4326.80 monoclinic C2/c 35.217(5) 17.000(3) 33.440(5) 90 95.33(3) 90 19934(5) 1.442 4 1.628 121 698 16 964 0.0880 0.0966 0.1774 1.316

C178H147Ho5O25 3510.61 tetragonal P4/n 19.640(3) 19.640(3) 19.076(2) 90 90 90 7358.2(18) 1.584 2 2.730 163 904 10 876 0.0723 0.0626 0.1123 1.263

C218H227Ho5O45 4391.65 monoclinic C2/c 34.758(7) 17.061(3) 33.283(4) 90 94.39(3) 90 19679(6) 1.482 4 2.065 49 582 14 132 0.0691 0.0670 0.1231 1.253

P.C. Andrews et al. / Polyhedron 28 (2009) 2123–2130

Fig. 4. Plots of molar magnetic susceptility, vM, per Eu5 (squares) and effective magnetic moment, leff, per Eu5 (circles) for [Eu5(OH)5(6)10]  4 toluene.

Fig. 5. Plots of molar magnetic susceptility, vM, per Ho5 (squares) and effective magnetic moment, leff, per Ho5 (circles) for [Ho5(OH)5(6)10]  4 toluene.

cluster. The room temperature value of leff, per Eu, of 3.54 lB (7.89 lB, per Eu5), is again typical of Eu(III) compounds and the temperature variation poses the same questions that were discussed for the tetranuclear compound, above, in that the ligand field/spin–orbit effects can not easily be untangled from any antiferromagnetic coupling, although the latter appears to be very weak. The Eu. . .Eu distances in the crystal structure, of 3.6–3.9 Å, connected by hydroxide are probably longer than the cis-carboxylate bridging in the benzenecarboxylate network systems [14] and similar to one of the phosphonate bridging distances (3.81 Å) in [Eu2(L1)2(1,10-phen)2(H2O)5]  3H2O [2b]. Thus, from a chemical/ structural perspective, it is difficult to predict whether spin coupling between Eu(III) centres is significant, or whether the magnetism is largely single-ion in origin. The examples cited, and the present Eu4 and Eu5 clusters, all show rather similar magnetic moment plots irrespective of the bridging ligands employed. The plot of leff and vM, per Ho5, versus temperature for [Ho5(OH)5(6)10]  4 toluene is given in Fig. 5. The leff values gradually decrease from 22.4 lB at 300 K (10.01 lB per Ho(III)) to 15.5 lB at 4.2 K (6.93 lB per Ho(III)). The corresponding vM/T plot shows Curie–Weiss behaviour. The Ho(III) f10 configuration, with a 5I8 free-ion ground state, typically leads to observed values of 10.4 lB in monomeric systems, and this compares well to the calculated g(J(J + 1))0.5 value of 10.6 lB. Since spin–orbit coupling, second order Zeeman and ligand field effects do not influence the magnetism, as was the case in Eu(III), the decrease in leff values with decreasing temperature strongly suggests that antiferromagnetic coupling is occurring in this cluster, via the hydroxo bridges. It is not strong enough, however, to give a maximum in vM, a characteristic feature of antiferromagnetic coupling, at least in d-block

2129

Fig. 6. Plots of molar magnetic susceptility, vM, per Ho5 (squares) and effective magnetic moment, leff, per Ho5 (circles) for [Ho5(OH)5(Ph2acac)10]  4 toluene.

cluster systems. There appear to be few, if any Ho(III) clusters to make comparisons of the present data to. Some metallocrown 15-MC-5 complexes containing both Ho(III) and Cu(II), in dimeric or helical structures, show values of leff of 10.4 lB at 300 K, decreasing to 7.85 lB at 2 K, due to the constituent f10 and d1 spin centres. Exchange coupling parameters could not be obtained for such a complicated situation [15]. The magnetism plots for [Ho5(OH)5(Ph2acac)10] are given in Fig. 6, and, while showing a leff value at 300 K similar in size to that of [Ho5(OH)5(6)10]  4 toluene, viz. 10.42 lB, there is an unusual but reproducible shallow dip at 100 K, followed, below 50 K, by a rapid decrease with leff reaching 5.72 lB at 2 K. Assuming that trace impurities are not present, and that there is no phase change at 100 K, it would appear that changing the b-diketonate ligand from 6 to Ph2acac leads to small differences in the magnetism and in the electronic structure of these Ho5 clusters. To probe for SMM behaviour in the present clusters, and in view of the very recent report of this important feature occurring, below 3 K, in an isostructural Dy(III) cluster, [Dy5(OH)5 (Ph2acac)10] [3c], the latter showing dc leff versus T plots rather like those in Fig. 6, measurements of the dynamic ac in-phase, v0M , and out-ofphase, v00M , values were made on the clusters between 10 and 2 K, in frequencies of 50–1500 Hz under a close to zero dc applied field. The temperature dependence of v0M was Curie–Weiss-like, as in the dc vM plots, while the v00M values remained close to zero between 10 and 2 K, with only the merest hint of frequency dependent increases at close to 2 K. This contrasts with the clear maxima, at 3 K, observed in the v00M versus T plots for [Dy5(OH)5(Ph2acac)10]. Thus we would require facilities to measure v00M below 2 K, with a dc field applied to move any frequency dependent maxima above 2 K. 4. Conclusions The reaction of functionalised b-diketones, as analogues of dibenzoylmethane bearing para ethoxy group, with Eu and Ho trichlorides yielded tetranuclear and pentanuclear hydroxo clusters. These structures demonstrate that the increased bulkiness of the ligand has no effect on the formation of the cluster core. On the other side, the methoxy functionalised b-diketone yielded a Nd mononuclear complex. It is still unclear whether this complex does not grow into a cluster due to its limited solubility in the reaction medium or as a cause of deaggregation in polar solvents like DMF. The magnetic studies on the present clusters of Eu(III) and Ho(III) show typical temperature dependent magnetic moment data for these ions. Any antiferromagnetic coupling, within the clusters, is very weak and the decrease in magnetic moments with decreasing temperature is largely due to thermal depopulation of Zeeman lev-

2130

P.C. Andrews et al. / Polyhedron 28 (2009) 2123–2130

els that originate from the Eu(III) single ion energy levels formed after the free ion states have been split by ligand field and spin–orbit coupling effects. Low temperature plateaus in the molar susceptibilities of the Eu(III) clusters are probably due to second order Zeeman effects rather than to antiferromagnetic coupling. It is likely that antiferromagnetic coupling across the hydroxo bridges is occurring in the Ho(III) clusters since spin–orbit splittings and Zeeman population effects are much less applicable than in the Eu(III) analogues. While there are some hints of SMM behaviour in the Eu and Ho clusters, below 2 K, we do not see the typical maxima in the out-of-phase ac susceptibilities, at different frequencies, recently observed in a closely related pentanuclear Dy(III) cluster [3c]. In other work on large Ln(III) clusters containing different bridging groups than those employed here, some of us have realised, as in Ref. [3c], that to observe SMM features in homometallic Ln(III) species, Dy(III) (S = 5/2; 6H15/2 ground state) is the lanthanide ion of choice [16]. Much more work on different lanthanoids is needed, however, to test this idea.

[2]

[3]

[4] [5]

[6]

5. Supplementary data CCDC 701176–701180 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via , or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected].

[7]

[8]

Acknowledgments The authors want to thank Dr. Craig Forsyth (Monash University) and Dr. Regine Herbst-Irmer (Universität Göttingen) for the help provided regarding the crystal structures. Prof. George M. Sheldrick (Fakultät für Chemie, Universität Göttingen) is also gratefully acknowledged for Tobias Beck’s student exchange program at Monash University. This work was supported by the Australian Research Council, Monash University and Bayer Schering Pharma.

[9] [10] [11] [12]

[13] [14]

References [1] (a) Z. Zheng, Chem. Commun. (2001) 2521; (b) R. Wang, Z. Zheng, T. Jin, R.T. Staples, Angew. Chem., Int. Ed. 38 (1999) 1813; (c) R. Anwander, Angew. Chem., Int. Ed. 37 (1998) 599; (d) Z. Zák, P. Unfried, G. Gieser, J. Alloys Compd. 257 (1997) 175;

[15] [16]

(e) M.T. Gamer, P.W. Roesky, Inorg. Chem. 44 (2005) 5963; (f) D.M. Lyubov, C. Doring, G.K. Fukin, A.V. Cherkasov, A.S. Shavyrin, R. Kempe, A.A. Trifonov, Organometallics 27 (2008) 2905; (g) D.-S. Zhang, B.-Q. Ma, T.-Z. Jin, S. Gao, C.-H. Yan, T.C.W. Mak, New J. Chem. 24 (2000) 61; (h) A. Babai, A.-V. Mudring, Z. Anorg. Allg. Chem. 632 (2006) 1956; (i) R. Wang, M.D. Carducci, Z. Zheng, Inorg. Chem. 39 (2000) 1836. (a) J.-W. Cheng, J. Zhang, S.-T. Zheng, G.-Y. Yang, Chem. Eur. J. 14 (2008) 88; (b) Z.-Y. Du, H.-B. Xu, J.-G. Mao, Inorg. Chem. 45 (2006) 9780; (c) T. Kajiwara, K. Katagiri, M. Hasegawa, A. Ishii, M. Ferbinteanu, S. Takaishi, T. Ito, M. Yamashita, N. Iki, Inorg. Chem. 45 (2006) 4880. (a) C. Michaelis, W. Bauhofer, H. Buchkremer-Hermanns, R.K. Kremer, A. Simon, G.J. Miller, Z. Anorg. Allg. Chem. 618 (1992) 98; (b) J.-J. Zhang, S.-M. Hu, S.-C. Xiang, T. Sheng, X.-T. Wu, Y.-M. Li, Inorg. Chem. 45 (2006) 7173; (c) T.M. Gamer, Y. Lan, P.W. Roesky, A.K. Powell, R. Clerac, Inorg. Chem. 47 (2008) 6581; (d) N. Mahé, O. Guillou, C. Daiguebonne, Y. Gérault, A. Caneschi, C. Sangregorio, J.Y. Chane-Ching, P.E. Car, T. Roisnel, Inorg. Chem. 44 (2005) 7743. P.W. Roesky, G. Canesco-Melchor, A. Zulys, Chem. Commun. (2004) 738. (a) S. Yang, S.I. Troyanov, A.A. Popov, M. Krause, L. Dunsch, J. Am. Chem. Soc. 128 (2006) 16733; (b) X. Gu, D. Xue, Inorg. Chem. 46 (2007) 3212; (c) X.-J. Zheng, L.-P. Jin, S. Gao, Inorg. Chem. 43 (2004) 1600. (a) V. Baskar, P.W. Roesky, Z. Anorg. Allg. Chem. 631 (2005) 2782; (b) V. Baskar, P.W. Roesky, Dalton Trans. (2006) 676; (c) S. Datta, V. Baskar, H. Li, P.W. Roesky, Eur. J. Inorg. Chem. (2007) 4216. (a) G. Xu, Z.-M. Wang, Z. He, Z. Lu, C.-S. Liao, C.-H. Yan, Inorg. Chem. 41 (2002) 6802; (b) R. Wang, D. Song, S. Wang, Chem. Commun. (2002) 368. (a) M. Cavallini, M. Facchini, C. Albonetti, F. Biscarini, Phys. Chem. Chem. Phys. 10 (2008) 784; (b) J. Tang, I.J. Hewitt, N.T. Madhu, G. Chastanet, W. Wernsdorfer, C.E. Hanson, C. Benelli, R. Sessoli, A.K. Powell, Angew. Chem., Int. Ed. 45 (2006) 1729; (c) J. Luzon, K. Bernot, I.J. Hewitt, C.E. Hanson, A.K. Powell, R. Sessoli, Phys. Rev. Lett. 100 (2008) 247205; (d) L.F. Chibotaru, L. Ungur, A. Soncini, Angew. Chem., Int. Ed. 47 (2008) 4126; (e) J.-P. Costes, J.-P. Dahan, W. Wernsdorfer, Inorg. Chem. 45 (2006) 45. G.M. Sheldrick, Acta Cryst. A 64 (2008) 112. L.M. Wittick, K.S. Murray, B. Moubaraki, S.R. Batten, L. Spiccia, K.J. Berry, Dalton Trans. 1 (2004) 1003. S. Cotton, Lanthanide and Actinide Chemistry, John Wiley, England, 2006. (a) B.N. Figgis, Introduction to Ligand Fields, John Wiley, England, 1966 (Chapter 13); (b) R.L. Carlin, Magnetochemistry, Springer-Verlag, New York, 1986 (Chapter 9). M. Gerloch, E.C. Constable, Transition Metal Chemistry, Wiley-VCH, 1994 (Chapter 10). (a) J. Yang, Q. Yue, G. -D. Li, J.-J. Cao, G.-H. Li, Jie-Sheng, Inorg. Chem. 45 (2006) 2857; (b) C.A. Black, J.S. Costa, W.T. Fu, C. Massera, O. Roubeau, S.J. Teat, G. Aromi, P. Gamez, J. Reedijk, Inorg. Chem. 48 (2009) 1062. C.M. Zaleski, E.C. Depperman, J.W. Kampf, M.L. Kirk, V.L. Pecoraro, Inorg. Chem. 45 (2006) 10022. A.S.R. Chesman, D.R. Turner, B. Moubaraki, K.S. Murray, G.B. Deacon, S.R. Batten, Chem. Eur. J. 15 (2009), doi:10.1002/chem.200900400.