Heterometallic Heptanuclear [Cu5Ln2] (Ln = Tb, Dy, and Ho) Single

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Heterometallic Heptanuclear [Cu5Ln2] (Ln = Tb, Dy, and Ho) SingleMolecule Magnets Organized in One-Dimensional Coordination Polymeric Network Atanu Dey,†,‡ Sourav Das,†,§ Subrata Kundu,† Abhishake Mondal,∥,⊥,# Mathieu Rouzières,∥,⊥ Corine Mathonière,∇,○ Rodolphe Clérac,*,∥,⊥ Ramakirushnan Suriya Narayanan,‡ and Vadapalli Chandrasekhar*,†,‡ Downloaded via UNIV OF CALIFORNIA SANTA BARBARA on June 26, 2018 at 20:05:20 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India Tata Institute of Fundamental Research Hyderabad, Survey # 36/P, Gopanpally, Serilingampally, Hyderabad 500107, India § Department of Chemistry, Institute of Infrastructure Technology Research and Management, Near Khokhara Circle, Maninagar East, Ahmedabad 380026, India ∥ CNRS, CRPP, UPR 8641, F-33600 Pessac, France ⊥ Univ. Bordeaux, CRPP, UPR 8641, 33600 Pessac, France # Solid State and Structural Chemistry Unit, Indian Institute of Science, C. V. Raman Road, Bangalore 560012, India ∇ CNRS, ICMCB, UPR 9048, F-33600 Pessac, France ○ Univ. Bordeaux, ICMCB, UPR 9048, 33600 Pessac, France ‡

S Supporting Information *

ABSTRACT: The reaction of a multisite coordination ligand, LH3, with Cu(II) salts and Ln(NO3)3·nH2O in a 1:2:1 stoichiometric ratio in the presence of triethylamine was found to afford a series of one-dimensional heterometallic [{Cu5Ln2(L)2(μ3−OH)4(ClO4)(NO3)3(OH2)5}(ClO4)2(H2O)x]∞ [Ln = Tb(1), Dy(2) and Ho(3), x = 4.25, 5.5, and 5 for 1−3, respectively] coordination polymers. Complexes 1−3 have been characterized by single crystal X-ray crystallography. The detailed study of the magnetic properties has also been performed and compared with the parent [Cu5Ln2] molecular analogues. The ac susceptibility measurements for complexes 1−3 confirm their SMM behavior with the following characteristics: Δeff/kB = 23.4 K, τ0 = 1.1 × 10−6 s and Δeff/kB = 27.9 K, τ0 = 6.6 × 10−7 s under 0 and 1200 Oe dc fields, respectively for 1; Δeff/kB = 8.3 K, τ0 = 3.1 × 10−6 s for 2 under 0 dc field. For 3, the fast QTM below 4 K prevents the estimation of the SMM energy barrier. Remarkably, the magnetic and SMM properties of the previously reported molecular [Cu5Ln2] analogues are preserved after their assembly in these coordination networks.

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INTRODUCTION Polymetallic assemblies containing 3d, 4f, or 3d−4f mixed metal ions are receiving a great deal of attention in the past few years due to their potential applications as magnetic materials.1 The discovery of the first single-molecule magnet (SMM) at the beginning of the 90s has motivated chemists to prepare polynuclear homo- and heterometallic assemblies, which can also display SMM properties.2 SMMs are molecular objects exhibiting slow relaxation of their magnetization that is often induced from the combined effect of a high-spin ground state (ST) and uniaxial anisotropy (D, defined by the following Hamiltonian: H = DST,z2), which create an energy barrier to reverse their total spin.3 In this approximation, the theoretical energy barrier Δ is equal to |D|ST2 for integer ST spin and |D| (ST2 − 1/4) for half-integer ST spin.3 The potential applications of SMMs include high density data storage,4 quantum computing,5 spintronics,6 or magnetic refrigeration.7 © 2017 American Chemical Society

Initially, most of the SMMs were polynuclear homometallic complexes containing often Mn(III) ions that possess a significant magnetic anisotropy in their high-spin S = 2 state due to Jahn−Teller distortion.8 Realizing that most of the trivalent lanthanide ions also contain a significant magnetic anisotropy (due to unquenched orbital angular moment) in addition to a high magnetic moment, much research activity has been developed toward the synthesis of 4f and 3d−4f based SMMs.1a,b,e The first 3d−4f based SMM, a [Cu2Tb2] complex, was reported in 2004 by Osa and co-workers.9 In the subsequent years, a large variety of Cu−Ln complexes of different nuclearities were described.10 However, accommodating both the 3d and 4f metal ions in the same molecule is a challenging synthesis task. In particular, this requires the choice Received: September 25, 2017 Published: November 21, 2017 14612

DOI: 10.1021/acs.inorgchem.7b02450 Inorg. Chem. 2017, 56, 14612−14623

Article

Inorganic Chemistry

function of the temperature at 1000 Oe. The magnetic data were corrected for the sample holder and the diamagnetic contribution. Synthesis. Preparation of Complexes 1−3. A general procedure for the preparation of the metal complexes is as follows: the LH3 ligand (0.07 g, 0.2 mmol) was dissolved in a mixture of methanol (20 mL) and chloroform (20 mL). Cu(ClO4)2·6H2O (0.15 g, 0.40 mmol) and triethylamine (0.11 mL, 0.80 mmol) were added to this solution. The reaction mixture was stirred at room temperature for 1 h to afford a clear deep green solution. At this stage, Ln(NO3)3·nH2O (0.20 mmol) was added, and the reaction mixture was refluxed for a further period of 20 h to afford a clear deep green solution. The deep greencolored solution was evaporated to dryness, and the residue was washed with Et2O. The solid was dissolved in a methanol/chloroform mixture (1:1), and few drops of toluene were added and kept for crystallization. After about a week, block-shaped bluish-green crystals suitable for X-ray crystallography were obtained. The characterization data for these complexes are given below. [{Cu5Tb2(L)2(μ3-OH)4(ClO4)(NO3)3(OH2)5}(ClO4)2(H2O)4.25]∞ (1). Yield: 0.078 g, 47.7% (based on Cu). Mp: >220 °C. IR (KBr) cm−1: 3419 (b), 2954 (w), 1649 (s), 1609 (m), 1562 (w), 1464 (s), 1442 (w), 1293 (m), 1242 (w), 1225 (s), 1096 (s), 1069 (s), 1041 (w), 985 (s), 947 (w), 855 (w), 740 (w). Anal. Calcd for C 38 H 60 Cl 3 Cu 5 N 7 O 44 Tb 2 [corresponds to {Cu 5 Tb 2 (L) 2 (μ 3 OH)4(ClO4)(NO3)3(OH2)5}(ClO4)2(H2O)4]: C, 22.10; H, 2.97; N, 4.75. Found: C, 22.15; H, 2.71; N, 4.86. [{Cu5Dy2(L)2(μ3-OH)4(ClO4)(NO3)3(OH 2)5}(ClO4)2(H2O)5.5]∞ (2). Yield: 0.082 g, 49% (based on Cu). Mp: >220 °C. IR (KBr) cm−1: 3418 (b), 2940 (w), 2739 (w), 2679 (m), 2492 (w), 1649 (s), 1609 (m), 1562 (w), 1472 (s), 1398 (w), 1296 (m), 1243 (w), 1225 (s), 1092 (s), 948 (w), 856 (w), 741 (w). Anal. Calcd for C 38 H 63 Cl 3 Cu 5 Dy 2 N 7 O 45 [corresponds to {Cu 5 Dy 2 (L) 2 (μ 3 OH)4(ClO4)(NO3)3(OH2)5}(ClO4)2(H2O)5]: C, 21.79; H, 3.03; N, 4.68. Found: C, 21.66; H, 2.88; N, 4.77. [{Cu 5 Ho 2 (L) 2 (μ 3 -OH) 4 (ClO 4 )(NO 3 ) 3 (OH 2 ) 5 }(ClO 4 ) 2 (H 2 O) 5 ] ∞ (3). Yield: 0.076 g, 45.8% (based on Cu). Mp: >220 °C. IR (KBr) cm−1: 3428 (b), 2953 (w), 1650 (s), 1609 (m), 1563 (w), 1465(s), 1294 (m), 1242 (w), 1225 (s), 1095 (s), 1070 (s), 947 (w), 855 (w), 740 (m). Anal. Calcd for C38H62Cl3Cu5Ho2N7O45 [corresponds to {Cu5Ho2(L)2(μ3-OH)4(ClO4)(NO3)3(OH2)5}(ClO4)2(H2O)5]: C, 21.83; H, 2.99; N, 4.69. Found: C, 21.71; H, 2.89; N, 4.78. X-ray Crystallography. The crystal data for 1−3 have been collected on a Bruker SMART CCD diffractometer using a graphitemonochromated MoKα radiation (λ = 0.71073 Å). The crystal data was collected at 100 K for all the complexes. The programs used were as follows: SMART15a for collecting frames of data, indexing reflections, and determining lattice parameters, SAINT15a for integration of the intensity of reflections and scaling, SADABS15b for absorption correction, and SHELXTL15c,d for space group and structure determination and least-squares refinements on F2. All the structures were solved by direct methods using the programs SHELXS-9715e and refined by full-matrix least-squares methods against F2 with SHELXL-2014.15e SHELXL-2014 using Olex-215f was used to solve the crystal structures and for refinement by full-matrix least-squares methods against F2. Hydrogen atoms were fixed at calculated positions, and their positions were refined by a riding model. The crystallographic figures shown in this manuscript have been generated using Diamond 3.1e software.15g Complexes 1−3 crystallize in the triclinic P1̅ space group. The asymmetric unit of complexes 1−3 contains one-half of unit A and one-half of unit B; both units are bridged by a nitrate anion and two perchlorates as counteranions. All the non-hydrogen atoms (except one of the noncoordinating perchlorate anion in complexes 1−3) were refined with anisotropic displacement parameters. Some of the noncoordinating water molecules with abnormal thermal ellipsoids are refined with fractional occupancies and ISOR restraints. Nitrate anion in all these Complexes 1−3 are fixed using restraints FLAT, DFIX, ISOR. The disorder in complexes 1−3 has been systematically observed for all the crystals tested. The crystal data and the refinement parameters for compounds 1−3 (CCDC reference numbers: 1477690−1477692

of an appropriate ligand system that can associate both 3d and 4f metal ions by utilizing specific coordinating sites.1d Over the years, serendipity appears to be the most common and successful route to construct polynuclear architectures containing 3d and 4f metal ions.11 In this synthetic approach, the ligand and the metal ions of varying ratio are mixed and left to self-assemble. An alternative strategy is to choose a compartmental ligand that has the ability to accommodate both 3d and 4f metal ions in its different coordination pockets.10,12 For this purpose, our research group has designed different phosphorus-based tris-hydrazone ligands that led to the synthesis of several heterometallic trinuclear complexes including series of [Co2Ln] and [Ni2Ln] complexes exhibiting SMM properties.13 Recently, in a radically different approach, a series of heterometallic heptanuclear [Cu5Ln2] complexes were prepared using N,N′-bis(3-methoxysalicylidene)-1,3-diamino-2propanol (H3L) as a compartmental ligand. It is worth emphasizing that the Dy and Ho analogues of this series exhibited SMM behavior.14 The efficiency of the LH3 ligand to associate 3d and 4f metal ions prompted us to investigate different reaction conditions, which could lead to changes in the nuclearity of the final heterometallic complexes. Accordingly, we describe in this work the synthesis and structural characterization of new materials containing a heterometallic heptanuclear [Cu5Ln2] repeating units linked by nitrate anions to form one-dimensional [Ln = Tb (1), Dy (2), and Ho (3)] coordination polymers, structurally related to their molecular analogues reported in ref 13. The detailed magnetic measurements of these three complexes were performed revealing that the SMM behavior of the heptanuclear repeating units is preserved in the 1D coordination polymers 1−3.

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EXPERIMENTAL SECTION

Reagents and General Procedures. All the reagents and the chemicals were purchased from commercial sources and were used without further purification. N,N′-Bis(3-methoxysalicylidene)-1,3diamino-2-propanol (H3L) was prepared following a literature procedure.14 1,3-Diaminopropan-2-ol, Cu(ClO4)2·6H2O, and 3methoxysalicylaldehyde (Fluka, Switzerland) were used as purchased. Tb(NO3)3·5H2O, Dy(NO 3)3·H2O, and Ho(NO3)3·5H2O were obtained from Sigma-Aldrich Chemical Co. and were used as received. Instrumentation. Melting points were measured using a JSGW melting point apparatus and are uncorrected. IR spectra were recorded as KBr pellets on a Bruker Vector 22 FT-IR spectrophotometer operating at 400−4000 cm−1. Elemental analyses of the compounds were obtained from Thermoquest CE instruments CHNS-O, EA/110 model. Powder X-ray diffraction (PXRD) data were collected with a Bruker D8 Advance diffractometer equipped with a nickel-filtered Cu Kα radiation source. Thermogravimetric analyses (TGA) (heating rate of 5 °C/min under nitrogen atmosphere) were carried out with a Mettler Toledo Star System. Magnetic Measurements. The magnetic susceptibility measurements were obtained with the use of MPMS-XL Quantum Design SQUID magnetometer and PPMS-9 susceptometer. These magnetometer and susceptometer work between 1.8 and 400 K for dc applied fields ranging from −7 to 7 T (MPMS-XL). Measurements were performed on polycrystalline samples of 15.30, 14.68, and 10.45 mg, for 1, 2 and 3 respectively, introduced in a polyethylene bag (3 × 0.5 × 0.02 cm). The ac susceptibility measurements were measured with an oscillating ac field of 1 to 6 Oe with frequency between 10 and 10000 Hz (PPMS-9) and with an oscillating ac field of 3 Oe with frequency between 1 to 1500 Hz (MPMS-XL). M versus H measurements has been performed at 100 K to check for the presence of ferromagnetic impurities that has been found absent (Figure S14). The susceptibility at 100 K is thus 0.22, 0.24, and 0.28 cm3 mol−1 for 1−3 respectively, in good agreement with all the susceptibility measurements done as a 14613


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Inorganic Chemistry Table 1. Details of the Data Collection and Refinement Parameters for 1−3 formula MW/g mol−1 crystal system space group unit cell dimensions (Å, deg)

V/Å3 Z ρc/g cm−3 μ/mm−1 F(000) crystal size (mm3) θ range (deg) limit hkl indices

reflns. collected ind. reflns. completeness to θ (%) refinement method data/restraints/params. goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) largest diff. peak and hole (e·Å−3)

1

2

3

C38H56Cl3Cu5N7 O44.25Tb2 2060.78 triclinic P1̅ a = 14.701(9) b = 15.191(9) c = 17.127(10) α = 92.802(12) β = 102.775(12) γ = 113.793(11) 3372(3) 2 2.030 3.842 2026 0.24 × 0.19 × 0.17 4.11−5.02 −19 ≤ h ≤ 19 −19 ≤ k ≤ 18 −22 ≤ l ≤ 14 21324 11688 [Rint = 0.0453] 98.43 full-matrix-block least-squares on F2 11688/135/900 1.007 R1 = 0.0767 wR2 = 0.1954 R1 = 0.1337 wR2 = 0.2386 2.88 and −1.71

C38H62Cl3Cu5Dy2N7O48 2133.99 triclinic P1̅ a = 14.509(5) b = 15.262(5) c = 16.916(5) α = 93.812(5) β = 100.88(5) γ = 113.709(5) 3325.5(19) 2 2.131 4.024 2102 0.31 × 0.26 × 0.22 5.101−6.02 −17 ≤ h ≤ 16 −8 ≤ k ≤ 18 −20 ≤ l ≤ 20 18768 12696 [Rint = 0.0541] 98 full-matrix-block least-squares on F2 12696/57/911 1.027 R1 = 0.0867 wR2 = 0.2076 R1 = 0.1596 wR2 = 0.2807 2.32/−-1.48

C38H61Cl3Cu5Ho2 N7O45 2089.84 triclinic P1̅ a = 14.581(13) b = 15.162(13) c = 17.278(15) α = 92.579(2) β = 103.472(2) γ = 113.193(2) 3374.3(5) 2 2.057 4.090 2056 0.29 × 0.21 × 0.16 4.10−4.99 −17 ≤ h ≤ 15 −18 ≤ k ≤ 17 −20 ≤ l ≤ 20 17503 11617 [Rint = 0.0499] 97.8 full-matrix-block least-squares on F2 11617/73/863 1.028 R1 = 0.0837 wR2 = 0.2140 R1 = 0.1376 wR2 = 0.2601 2.92 and −2.32

respectively) are tabulated in Table 1. Additional crystallographic information are available in Tables S1−S4.

for 1−3, respectively). Views of the structures of 1−3 are given in Figures 1, S1, and S2, respectively. Selected bond distances and angles for these compounds are given in the Tables S1−S4. The heptanuclear moieties in 1−3 are assembled together by the cumulative coordination action of two fully deprotonated [L]3−, four [OH]−, and three [NO3]− ligands. While the iminonitrogen atoms of the [L]3− ligand bind each of the four peripheral copper centers (Cu−N = 1.894−1.949 Å), the deprotonated [L]3− alcoholic and phenolato oxygens are in μcoordination modes respectively between two Cu(II) centers (Cu−O = 1.906−1.966 Å) and between a Cu(II) and a lanthanide ion (Cu−O = 1.907−1.971 Å and Ln−O = 2.272− 2.357 Å). In contrast, the [L]3− methoxy oxygen coordinates selectively to the lanthanide ion (Ln−O = 2.460−2.609 Å). The central copper site is connected with the six peripheral metal ions by four μ3-OH and two μ:η2-NO3 ligands (Scheme 1) with Cu−O = 1.929−2.005 Å and Ln−O = 2.319−2.449 Å bond distances associated with the μ3-OH groups and Cu−O = 2.482−2.567 Å bond distances related to the μ:η2-NO3 ligands. The presence of μ3-OH groups is confirmed by BVS calculations (Table 2). Apart from these coordinating groups, μ:η1:η1-NO3 ligands bridge additionally copper and lanthanide centers (Cu−O = 2.611−2.694 Å and Ln−O = 2.360−2.402 Å). The coordination sphere of the lanthanide ions is fulfilled by the presence of terminal water molecules (Ln−O = 2.291− 2.431 Å). The coordination environments around the copper sites are completed by the presence of a terminal perchlorate

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RESULTS AND DISCUSSION Previously we have reported the efficiency of the LH3 ligand to react sequentially with Cu(OAc)2·H2O and Ln(NO3)3·nH2O in an 1:2:1 stoichiometric ratio in the presence of triethylamine to afford a series of molecular heterometallic [Cu5Ln2] complexes (Ln = Y, Lu, Dy, Ho, Er, and Yb).14 The modification of the reaction conditions as well as the use of different Cu(II) salt leads to new materials involving analogous [Cu5Ln2] units that assemble into one-dimensional coordination polymers. Accordingly, the reaction of LH3 with Cu(II) salts and Ln(NO3)3· nH2O in a 1:2:1 stoichiometric ratio in the presence of triethylamine was found to afford one-dimensional [{Cu5Ln2(L)2(μ3-OH)4(ClO4)(NO3)3(OH2)5}(ClO4)2(H2O)x]∞ [Ln = Tb(1), Dy(2) and Ho(3), x = 4.25, 5.5, and 5 for 1−3, respectively] complexes. As can be seen in Scheme 1, the coordination polymeric systems arise as a result of the bridging coordination of nitrate ions, which link successive [Cu5Ln2] units. Molecular Structures of 1−3. X-ray crystallographic analysis reveals that the one-dimensional assembly in 1−3 is composed of dicationic units, {Cu5Ln2(L)2(μ3-OH)4(ClO4)(NO3)3(OH2)5}2+, which are compensated in charge by two perchlorate anions. These compounds are isomorphous with a different number of interstitial water molecules (4.25, 5.5, and 5 14614


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Inorganic Chemistry

Scheme 1. Schematic Synthesis of the One-Dimensional Coordination Polymers (1−3) Containing the [Cu5Ln2] Repeating Unita

a

Ln = Tb (1), Dy (2), and Ho (3).

Figure 1. Ball and stick view of the tetra-cationic part of the repeating unit in the 1D coordination assembly containing two [Cu5Tb2] units in 1. All the hydrogen atoms, counteranions, and water molecules (except the coordinated ones) have been omitted for clarity.

Table 2. BVS Calculation Values for Selected Oxygen Atoms in complex 1.16 atom

BVS value

assignment

O6 O7 O18 O19

1.30 1.19 1.30 1.29

OH− OH− OH− OH−

and a water molecule (Cu−OClO4 = 2.478−2.609 Å and Cu− OOH2 = 2.582−2.662 Å). The presence of μ3:η2:η1-NO3 coordinating anions that bridge only copper centers of the different heptanuclear units, helps the formation of the onedimensional polymeric structure. The Cu−O bond distances associated with the μ3:η2:η1-NO3 bridges lie in the 2.439−2.660 Å range. Overall, three different types of nitrate ligands are present in these compounds with different coordination modes (Scheme 2). 14615


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Inorganic Chemistry

dron and distorted capped square antiprismatic geometries, respectively (Figure 2). The phase purity of complexes 1−3 was checked by powder X-ray diffraction studies (Figure S15). Thermogravimetric analysis (TGA) shows that complexes 1−3 show similar kind of decomposition pattern and are stable up to ∼240 °C (Figure S16). Magnetic Properties of Complexes 1−3. Static Magnetic Properties. The dc magnetic susceptibility was measured on polycrystalline samples of 1−3 in the 1.8−300 K temperature range at 1000 Oe. The room-temperature χT products estimated as 22.4 (1), 24.6 (2), and 30.2 (3) cm3 K mol−1 are in relative good agreement (see Table S5) with the presence of five antiferromagnetically coupled S = 1/2 CuII ions (on the basis of the experimental data of the molecular [Cu5Y2] and [Cu5Lu2] complexes, i.e., 1.3−1.5 cm3 K mol−1)13 and two lanthanide metal ions: two TbIII metal ions (S = 3, L = 3, 7F6, g = 3/2: χT = 11.81 cm3 K mol−1) for 1, two DyIII metal ions (S = 5/2, L = 5, 6H15/2, g = 4/3: χT = 14.17 cm3 K mol−1) for 2, and two HoIII metal ions (S = 2, L = 6, 5I8, g = 5/4: χT = 14.06 cm3 K mol−1) for 3.17 The slight differences between the experimental and predicted χT products are likely the result of

Scheme 2. Various Coordination Modes of the Nitrate Anions in the Formation of the 1D Coordination Assembly in Complexes 1−3

Six structurally independent Cu(II) centers are found in the crystal structure (three from unit A and remaining three are from unit B) (Scheme 1). Four of them adopt a distorted octahedral geometry (Cu2, Cu3, Cu4, and Cu6), while the two remaining ones display a distorted square pyramidal coordination sphere for Cu1, τ5 = 0.11, and a nearly perfect square pyramidal geometry for Cu5, τ5 = 0.002 (Figure 2). The two lanthanide ions (from unit A and unit B) (Scheme 1) also possess different coordination geometries; Tb1 and Tb2 are eight- and nine-coordinated in a distorted trigonal dodecahe-

Figure 2. Coordination environments of the different Cu and Tb crystallographic sites in complex 1. 14616


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studied [Cu5Y2] or [Cu5Lu2] complexes),14 these thermal behaviors could involve either antiferromagnetic or ferromagnetic Ln(III)···Cu(II) interactions combined with the thermal depopulation of the excited states of the lanthanide ions17 (i.e., the low temperature increase of the χT product could be either due to the ferromagnetic or ferrimagnetic arrangement of the local magnetic moments). The field dependence of the magnetization (Figures S4−S6) is very similar for these three 1D systems and is very close (for [Cu5Ho2]14 and 3) or identical (for [Cu5Dy2]14 and 2) to the molecular parent complexes: (i) At low field, a rapid increase of the magnetization without indication of antiferromagnetic interactions (inflection point or “S” shape curve) supports the presence of ferromagnetic Ln(III)···Cu(II) interactions; (ii) the linear increase of magnetization at high fields without clear saturation, even at 1.8 K and 7 T for 1−3, suggests the presence of magnetic anisotropy in these systems. As in the molecular [Cu5Ln2] complexes,14 the effect of the anisotropy on the magnetization might be superposed with the field induced population of the low lying excited states. At 1.8 K, the magnetization reaches 9.7, 9.9, and 11.9 μB at 7 T for 1−3, respectively (Figures S4−S6). It is worth noting that no hysteresis effect on the M versus H data has been observed above 1.8 K with sweep-rate used in a traditional SQUID magnetometer (100−600 Oe/min or 1.5− 10 × 10−4 T/s). Nevertheless, the ac susceptibility of these compounds (1−3) has been measured and revealed that these complexes display an out-of-phase ac signal at low temperatures, indicating the slow relaxation of their magnetization and thus their SMM properties. Dynamic Magnetic Properties. As shown in Figure 4, the temperature and frequency dependence of the in-phase and out-of phase ac susceptibility have been measured independ-

subtle modulations of the Cu···Cu antiferromagnetic interactions in these new materials in comparison to the [Cu5Y2] and [Cu5Lu2] complexes.14 As shown in the χT versus T plot (Figure 3), the lack of a χT saturation at low temperatures indicates the absence of a well-

Figure 3. Temperature dependence of χT product at 1000 Oe (where χ is the molar magnetic susceptibility equal to M/H per [Cu5Ln2] unit) between 1.8 and 300 K for polycrystalline samples of 1−3.

defined ground state for these complexes. For all the complexes and as already observed for the molecular analogous complexes ([Cu5Dy2] and [Cu5Ho2]),14 the χT product exhibits a clear minimum at ca. 10−20 K, around 20.2, 20.4, and 25.1 cm3 K mol−1 for 1−3, respectively, before increasing at lower temperatures up to 24.5, 23.6, and 29.9 cm3 K mol−1 at 1.8 K respectively. For these systems, it is not possible to make a straightforward conclusion on the sign of the magnetic interactions between Ln(III) and Cu(II) spin carriers. Even knowing the presence of dominating antiferromagnetic interactions among the CuII magnetic sites (from the previously

Figure 4. Temperature (left) and frequency (right) dependence of the real (χ′, top) and imaginary (χ″, bottom) components of the ac susceptibility at different ac frequencies between 10 and 10 000 Hz and different temperatures between 1.8 and 20 K, respectively, with a 1 Oe ac field for a polycrystalline sample of 1 in a zero dc field. Inset: Temperature dependence of the magnetization relaxation time as τ versus T−1 plot for 1 in zero dc field. 14617


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Inorganic Chemistry ently in zero-dc field for the one-dimensional [Cu5Tb2] complex, 1. Both in- and out-of-phase components of the ac susceptibility are strongly frequency dependent below 12 K for ac frequencies up to 10 kHz as expected in the presence of the slow relaxation of the magnetization. The blocking temperature estimated at 10 kHz is 6.7 K. At 1.8 K, two relaxation modes around 20 Hz and above 10 kHz are clearly observed on the χ″versus ν data. This observation is not surprising considering the presence of two different [Cu5Tb2] units along the one-dimensional coordination network (Figure S3) and the absence of well-defined ground state. The temperature dependence of the relaxation time, τ, has been estimated for the low frequency mode (the only one accessible with our experimental set up). From all the collected ac data (Figure 4), the Arrhenius plot has been built as shown in inset of Figure 4. Above 3 K, the relaxation time follows a thermally activated behavior, τ = τ0 exp(Δeff/kBT), with an energy gap around 23.4 K and a pre-exponential factor, τ0, of about 1.1 × 10−6 s. At lower temperatures, the relaxation time becomes temperature independent as expected for a SMM in a relaxation regime dominated by the quantum tunneling of the magnetization (QTM). Below 2 K, the QTM relaxation time can be estimated at about 8 × 10−3 s. Even if the crystal structure of this compound exhibits a one-dimensional coordination network, no sign of significant magnetic interaction between the [Cu5Tb2] complexes along the chain has been observed above 1.8 K, i.e., ln(χT) does not increase exponentially when plotted against 1/T. The Cole−Cole plots were fitted using generalized Debye model18 (Figure S9, Table S6), details on this fitting are given in Supporting Information. Therefore, the observed relaxation of the magnetization appears to be of molecular origin from the individual [Cu5Tb2] moieties and is thus the signature of their SMM properties as already observed for the [Cu5Dy2] and [Cu5Ho2] molecular analogues.14 In order to minimize the probability of the quantum relaxation pathway in zero dc field and to estimate more accurately the energy barrier associated with the thermally activated relaxation above 1.8 K, the ac susceptibility has been measured at 4 K under different dc fields (Figure 5). From these ac measurements and the field variation of the characteristic frequency (inset of Figure 5), quantum effects are clearly minimized around 1200 Oe as shown by the minimum of the characteristic frequency of the system at this dc field. Therefore, the ac susceptibility has been remeasured at 1200 Oe at different temperatures and ac frequencies (Figure 6). As in zero dc field, two relaxation modes are clearly observed (especially at 2.5 K) but our experimental window of ac frequency did not allow us to study the high frequency mode (>10 kHz) even at 1200 Oe. In these experimental conditions, the temperature dependence of the low frequency relaxation time has been extracted above 3 K (below this temperature, the relaxation mode is situated at frequencies lower than 1 Hz) and reported as an Arrhenius plot shown in Figure 7. Above 3 K, the relaxation displays a thermally activated behavior with an energy barrier around 27.9 K and a preexponential factor, τ0, of about 6.6 × 10−7 s. As seen in Figure 7, the effect of the applied dc field is clear on the relaxation time below 6 K, i.e., the relaxation is slowed down, as expected in a temperature domain for which the quantum pathway influences the relaxation process. Therefore, the energy barrier estimated under 1200 Oe (∼28 K) is certainly closer to the value of the

Figure 5. Frequency dependence of the real (χ′, top) and imaginary (χ″, bottom) components of the ac susceptibility for a polycrystalline sample of 1 at different dc fields between 0 and 10000 Oe at 4 K. Solid lines are guides. Inset: Field dependence of the characteristic relaxation frequency for 1 at 4 K.

Figure 6. Frequency dependence of the real (χ′, top) and imaginary (χ″, bottom) components of the ac susceptibility for a polycrystalline sample of 1 at different temperatures between 1.8 and 10 K in an 1200 Oe dc field. Solid lines are guides.

purely thermally activated regime of relaxation in 1. From the Cole−Cole plots (Figure S10, Table S7), we obtained different values of α, which falls in the range of 0.18−0.39, revealing a weak distribution of relaxation time of the complex 1 was not unique. 14618


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estimated at 8.3 K and a pre-exponential factor, τ0, of about 3.1 × 10−6 s. At lower temperatures, the nonlinearity of the τ versus 1/T plot (inset Figure 8) reveals the influence of the quantum relaxation pathway. It is important to mention that these SMM properties are extremely similar to the ones reported for the [Cu5Dy2] (Δeff/kB = 6 K and τ0 = 3.1 × 10−6 s) and [Cu5Ho2] (relaxation mode above 10 kHz at 1.8 K) molecular analogues,14 further supporting the absence of significant intrachain magnetic interactions as already been discussed for compound 1. The dynamics of the magnetization has also been studied under dc fields (Figures S7 and S8) to probe the possibility to reduce the influence of quantum relaxation pathway in these two one-dimensional compounds (2 and 3). Unfortunately, unlike for 1, the relaxation time is not significantly increased by the applied magnetic field, and instead, the characteristic relaxation mode moves to higher frequency. A similar conclusion can be drawn after fitting the Cole−Cole plots (Figure S11 and S12; Tables S8 and S9). The α values fall in the ranges of 0.34−0.43 and 0.45−0.73 for complexes 2 and 3, respectively, which is probably due to the presence of different relaxation pathways of these complexes.

Figure 7. Temperature dependence of the magnetization relaxation time as τ versus T−1 plot for 1 in zero (red) and 1200 Oe (blue) dc field.

As shown in Figures 8 and 9, the ac susceptibility of onedimensional compounds 2 and 3 has also been measured in zero dc field. For both, slow relaxation of the magnetization is evidenced by frequency dependence of the in- and out-of-phase ac susceptibilities below 10 and 5 K, respectively, for ac frequencies of about 10 kHz. Unlike for 1, one relaxation mode is observed in this experimental ac frequency window. It is worth noting that these ac data highlight a common case for which a clear relaxation mode is revealed by the χac versus ν plots while no maximum of the χ″ versus T data is observed. In order to estimate the relaxation time of a SMM, these examples underline the absolute necessity to study systematically the ac susceptibility as a function of the ac frequency and not only as a function of the temperature as commonly reported. From these ac susceptibility data, the characteristic relaxation times of 2 and 3 were determined. In the case of 3, the relaxation time is temperature-independent at about 2.6 × 10−5 s, as expected when the dynamics of the magnetization is controlled by quantum tunneling. For 2, the relaxation time follows a thermally activated behavior above 3 K with an energy barrier

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CONCLUSION The investigation of the different reaction conditions to assemble Cu(II) and Ln(III) metal ions with the compartmental N,N′-bis(3-methoxysalicylidene)-1,3-diamino-2-propanol (H3L) ligand allowed us to synthesize a new series of heterometallic complexes containing heptanuclear [Cu5Ln2] units linked by nitrate anions to form one-dimensional [Ln = Tb (1), Dy (2), and Ho (3)] coordination polymers. The magnetic and SMM properties of the previously reported molecular [Cu5Ln2] analogues14 are remarkably preserved after their assembly in these coordination networks. More work is currently underway in our laboratory to use designed linkers between [Cu5Ln2] units, which should promote strong magnetic coupling between the individual SMM building

Figure 8. Temperature (left) and frequency (right) dependence of the real (χ′, top) and imaginary (χ″, bottom) components of the ac susceptibility at different ac frequencies between 1 and 10000 Hz and different temperatures between 1.85 and 5.7 K, respectively, for a polycrystalline sample of 2 in a zero dc field. Inset: Temperature dependence of the magnetization relaxation time as τ versus T−1 plot for 2 in zero dc field. 14619


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Figure 9. Temperature (left) and frequency (right) dependence of the real (χ′, top) and imaginary (χ″, bottom) components of the ac susceptibility at different ac frequencies between 10 and 10000 Hz and different temperatures between 1.85 and 4 K, respectively, with a 1 Oe ac field for a polycrystalline sample of 3 in a zero dc field.

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ACKNOWLEDGMENTS We thank the CEFIPRA for the collaborative project No.51053. V.C. is thankful to the Department of Science and Technology, for a J. C. Bose fellowship. A.D. thanks the Science and Engineering Research Board (SERB) of India for the National Postdoctoral Fellowship (PDF/2016/000612). S.D. acknowledges support from Early Career Research Award Grant from Science and Engineering Research Board (SERB), India (Project File no. ECR/2016/001746). A.M., M.R., R.C., and C.M. thank the University of Bordeaux, the Région Nouvelle-Aquitaine, CEFIPRA, the MOLSPIN COST action CA15128, the GdR MCM-2, and the CNRS for financial support. R.S.N thanks CEFIPRA for the research associate fellowship.

blocks and thus should lead to single-chain magnet properties or related behaviors.19

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02450. Molecular structures of complexes 2 and 3, bond lengths and bond angles of complexes 1−3, χT values of 1−3, field-dependent magnetization curves for 1−3, field dependent ac measurements for 2 and 3, Cole−Cole plots and extracted parameters for 1−3, Arrhenius plot for Raman process for 1, powder X-ray diffraction data and thermogravimetic analysis data for 1−3 (PDF)

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DEDICATION The authors would like to dedicate this paper to Prof. U. Maitra, Indian Institute of Science on the occasion of his 60th birthday.

Accession Codes

CCDC 1477690−1477692 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Atanu Dey: 0000-0002-3174-4059 Abhishake Mondal: 0000-0002-5061-2326 Rodolphe Clérac: 0000-0001-5429-7418 Vadapalli Chandrasekhar: 0000-0003-1968-2980 Notes

The authors declare no competing financial interest. 14620


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Heterometallic Heptanuclear [Cu5Ln2] (Ln = Tb, Dy, and Ho) Single

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