Inorganic Chemistry Communications 37 (2013) 1720

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Inorganic Chemistry Communications

j ourna l homepage: www.e lsev ie r .com/ locate / inocheTwo tetranuclear 3d4f complexes: Syntheses, structures andmagnetic propertiesXin Tan, Yun-Xia Che, Ji-Min Zheng Department of Chemistry, Nankai University, Tianjin 300071, PR China Corresponding author. Tel./fax: +86 22 23508056.E-mail address: jmzheng@nankai.edu.cn (J.-M. Zheng)

1387-7003/$ see front matter. Crown Copyright 2013http://dx.doi.org/10.1016/j.inoche.2013.09.019a b s t r a c ta r t i c l e i n f oArticle history:Received 22 July 2013Accepted 6 September 2013Available online 18 September 2013

Keywords:Crystal structures3d4fTetranuclearMagnetic propertiesThe solvothermal reactions of 3,5-bis(trifluoromethyl)benzoic acid (HL) and 2,2-bipyridine (bipy) with thecorresponding metal salts afforded two novel 3d4f tetranuclear complexes M2Dy2(L)10(bipy)2 (M = Mn 1,Cu 2), which have been characterized by single-crystal X-ray diffraction, infrared spectroscopy, element analyses,powder X-ray diffraction and thermal gravimetric analyses. Complexes 1 crystallizes in the triclinic space groupP-1, and the completely deprotonated L ligands possess 2: 1, 1 and 2: 2, 1 coordination modes to bridgeadjacent metal ions which further connect into 2D layers by hydrogen bonds. Complex 2 crystallizes in the dis-tinct monoclinic space group P21/c, and adjacent metal ions were linked by L ligands with the same coordinatedpatternswith 1, and supramolecular sheetswere formed in the abplane. Dynamicmagnetic susceptibility studiesreveal slow magnetic relaxation processes for 2, suggesting SMM type behavior.

Crown Copyright 2013 Published by Elsevier B.V. All rights reserved.The rational design and construction of novel functional polynuclearheterometallic compounds have drawn remarkable attention in recentyears due to their unique electronic, magnetic, magneto-electric, struc-tural, reactivity and catalytic properties [1]. The 3d4f heterometallicpolynuclear complexes have been proved to be promising materialsfor potential applications as luminescence, catalysis and magneticmaterials [2]. Nowadays, many efforts have been devoted to the rationaldesign and synthesis of 3d4f heterometallic polynuclear complexes[3]. However, because of competitive reactions between lanthanideand transition metals with the organic ligands, the assembly of 3d4fpolynuclear complexes is still a great challenge for chemists. Selectingappropriate acids as organic linkers is one of the best viable approachesfor the construction of diverse 3d4f polynuclear complexes.

Carboxylate ligands are extensively employed in the preparation of3d4f complexes, which is a result of the ligand system being prone tocoordinate to both transition and lanthanidemetal ions acting as chelat-ing and/or bridging agent. Taking into account all the above and giventhe on-going interest of our group in the construction of coordinationcomplexes from various carboxylate ligandswith interesting topologiesand magnetic properties [4], we have chosen 3,5-bis(trifluoromethyl)benzoic acid (HL) as the target ligand. Plenty of homometallic com-plexes have been successfully synthesized due to the variety of coordi-nation modes of HL [5], while no article concerning heterometalliccomplexes of HL with both transition and lanthanide metal ions hasbeen documented.

Hereinwe report on the synthesis [6] and characterization [7] of twonovel 3d4f complexes M2Dy2(L)10(bipy)2 (M = Mn 1, Cu 2) based on.

Published by Elsevier B.V. All rightsHL in the presence of auxiliary N-donor co-ligand 2,2-bipyridine (bipy).Complexes 1 and 2 are first series of 3d4f heterometallic complexes forHL. They both exhibit tetranuclear structure feature, whereas crystallizein different space group and have various magnetic behavior. Themagnetic studies reveal slow magnetic relaxation processes for 2.

Complex 1 crystallizes in the triclinic P-1 space group and the asym-metric unit of 1 contains one crystallographically independent Mn(II)center, one crystallographically independent Dy(III) center, five L

ligands and one bipy ligand. The Mn1 is located at the center of adistorted trigonal bipyramid with three oxygen atoms from threedistinct L ligands and two nitrogen atoms from one bipy ligand. TheDy1 center is bonded with eight oxygen atoms from seven distinct L li-gands which exhibit a slightly twisted bicapped trigonal prism coordi-nation environment. The Dy\O and Mn\O/N bond lengths are in thenormal range [8a,b]. The completely deprotonated L ligands possesstwo different coordination modes, 2: 1, 1 and 2: 2, 1 (Scheme 1).The Mn1Dy1, Dy1Dy1A distances and Mn1Dy1Dy1A angles are4.0645(18), 3.9457(16) and 166.770(27), respectively. A 2D supramo-lecular layer was formed via hydrogen-bonding interactions (C(9)\H(9)O(6), 3.176(9) , 126; C(33)\H(33)F(28), 3.238(14) , 124)(Fig. 1) [9a].

Complex 2 is also tetranuclear bridged by L ligands, which is similarto the structure of 1, while it crystallizes in the distinctmonoclinic spacegroup P21/c. The asymmetric unit of 2 contains one crystallographicallyindependent Cu(II) center, one crystallographically independent Dy(III)center, five L ligands and one bipy ligand. The Cu1 site is coordinatedby three oxygen atoms from three different L ligands and two nitrogenatoms from one bipy ligand creating a tetragonal pyramid geometry.The Dy1 site is connected to eight oxygen atoms from seven distinct L li-gands defining a distorted square antiprism geometry. The Dy\O andreserved.

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Scheme 1. Coordination modes of L ligand in 1 and 2.

18 X. Tan et al. / Inorganic Chemistry Communications 37 (2013) 1720Cu\O/N bond lengths are in the normal range [8a,c]. The completelydeprotonated L ligands adopt two different coordination styles, 2: 1,1 and 2: 2, 1 (Scheme 1). The Cu1Dy1, Dy1Dy1A distances andCu1Dy1Dy1Aangles are 4.0393(13), 4.0622(9) and 169.682(18), re-spectively. The discrete tetranuclear molecules aggregate into 2D sheetby hydrogen bonds in the ab plane (C(8)\H(8)O(4) 3.110(12) ,144; C(24)\H(24)F(29) 3.531(17) , 158) (Fig. 2) [9b].

Variable-temperaturemagnetizationmeasurements of 1 and 2wereperformed in the 1.8300 K range at 1000 Oe applied field (Fig. 3). TheMT values for 1 and 2 are 37.71 and 30.04 cm3 mol1 K at room tem-perature, which are comparable with the calculated value of37.09 cm3 mol1 K (per Mn2Dy2, SMn = 5/2, gMn = 2.0, JDy = 15/2,gDy = 4/3) and 29.10 cm3 mol1 K (per Cu2Dy2, SCu = 1/2, gCu =2.0, JDy = 15/2, gDy = 4/3) in the free-ion approximation [10]. For 1,the MT value decreases slowly with lowering temperature down to36.65 cm3 mol1 K at 104.8 K, then increases gradually to36.97 cm3 mol1 K at 63.9 K, finally it drops monotonously to a mini-mum of 22.94 cm3 mol1 K, The data above 1.8 K obey the CurieFig. 1. (a) Coordinated environments of themetal centers in 1. (b) Coordination polyhedron of Dfluorine atoms omitted for clarity).Weiss law with C = 37.48 cm3 mol1 K and = 1.05 K (Fig. S1).However, it is difficult to concludewhether the negativeWeiss temper-ature indicates antiferromagnetic interactions between adjacent metalions because of the presence of strong spinorbital coupling effects inDy3+ ions. Lowering the temperature induces a continuous increaseof the MT product of 2 to a value of 59.01 cm3 mol1 K at about1.8 K indicating the onset of ferromagnetic interactions. The datacan be fitted by the CurieWeiss law above 37.8 K with C =28.55 cm3 mol1 K, = +15.86 K (Fig. S1). The positive value of in-dicates ferromagnetic interactions between the adjacent metal ions,which further support the conclusion above. Further magnetic charac-terization of 2 was performed by field-dependent magnetizationunder a magnetic field up to 80 kOe at T = 2 K. For 2, the M value in-creases laggardly at low field and sharply increase at high field, finallyreaching values of 20.10 N at 80 kOe, but without a clear saturation(Fig. S2). This value is close to the expected saturation value of expectedferromagnetic value of 22.0 N for two Dy(III) (20.0 N) and two Cu(II)(2.0 N) ions. The temperature dependence of the alternating-current(ac) susceptibility studies of 2 has been undertaken at zero dc fieldwith an ac field of 3 Oe in the 218.5 K temperature range. The signalsin both in-phase componentM and out-of-phaseM exhibit frequencydependence (Fig. 4), which indicates the presence of slowmagnetic relax-ation of 2 at a low temperature, but there is nomaxima visible above 2 K,therefore it is difficult to quantify the energy barrier. The observations offrequency dependence forM in 2 are strong evidences of a slow relaxa-tion of the magnetization and strongly support SMM-type behavior [11].Contrasting the two complexeswith other tetranuclear 3d4f compounds[12], complexes 1 and 2 are first series of 3d4f heterometallic complexesfor HL with SMM-type behavior. This work may provide a promisingaccess to new complexes from HL ligands with SMM properties.

The PXRDpatterns of complexes 1 and 2 are in good agreementwiththe simulated one (Fig. S3), which further indicated the phase puritiesof complexes 1 and 2. To study the thermal stability of 1 and 2,thermogravimetric analyses (TGA)were performed on the single crystaly(III) ion in complex 1. (c) The 2D supramolecular layer of 1 (some hydrogen, carbon and

image of Scheme1

Fig. 2. (a) Coordinated environments of themetal centers in 2. (b) Coordination polyhedron of Dy(III) ion in complex 2. (c) The 2D supramolecular layer of 2 (some hydrogen, carbon andfluorine atoms omitted for clarity).

19X. Tan et al. / Inorganic Chemistry Communications 37 (2013) 1720samples of 1 and 2 under N2 atmosphere in the temperature range of25800 C with a heating rate of 5 C/min (Fig. S4). For complex 1, theTGA curve reveals that the framework could be stable up to 255 C.Then,weight losses occur in the temperature range of 255645 C, indi-cating decomposition of the organic components. Finally the remnantsare about 15.69% which should be Dy2O3 and MnO (calcd. 15.52%). Forcomplex 2, above 278 C, the frameworks began to collapse accompa-nying with the decomposition of the organic ligands. The remainingresidue is presumed to beDy2O3 and CuO (calcd. 15.95%; found 16.03%).

In conclusion, two novel 3d4f tetranuclear complexes from mixed3,5-bis(trifluoromethyl)benzoic acid and 2,2-bipyridine have beenobtained. As is known to us, complexes 1 and 2 are first series of 3d4f heterometallic complexes of 3,5-bis(trifluoromethyl)benzoic acid.They both display tetranuclear structure feature, whereas crystallize indifferent space group and exhibit diverse magnetic behavior. The mag-netic studies demonstrate slow magnetic relaxation processes for 2.Fig. 3. Plot of MT versus T for 1 and 2. The field strength used was Hc = 1000 Oe.Further studies on syntheses and characterization of polynuclearheterometallic complexes with SMM properties are ongoing in ourlaboratory.Fig. 4. The ac magnetic susceptibilities of 2 in zero applied static field with an oscillatingfield 3.0 Oe.

image of Fig.2image of Fig.3image of Fig.4

20 X. Tan et al. / Inorganic Chemistry Communications 37 (2013) 1720Acknowledgments

The authors are thankful for the financial support from the NationalNatural Science Foundation of China (Grant No. 50872057).

Appendix A. Supplementary material

Additional bond length, angle data and figures for 1 and 2 are avail-able as electronic supplementary information in the online version, athttp://dx.doi.org/10.1016/j.inoche.2013.09.019.

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Two tetranuclear 3d4f complexes: Syntheses, structures and magnetic propertiesAcknowledgmentsAppendix A. Supplementary materialReferences