Exploring Tuning of Structural and Magnetic Properties by Modification

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Exploring Tuning of Structural and Magnetic Properties by Modification of Ancillary #-Diketonate Co-ligands in a Family of Near-Linear Tetranuclear DyIII Complexes Joydev Acharya, Sourav Biswas, jan van Leusen, Pawan Kumar, Vierandra Kumar, Ramakirushnan Suriya Narayanan, Paul Kögerler, and Vadapalli Chandrasekhar Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00358 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018

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Crystal Growth & Design

Exploring Tuning of Structural and Magnetic Properties by Modification of Ancillary β-Diketonate Co-ligands in a Family of Near-Linear Tetranuclear DyIII Complexes Joydev Acharya,a Sourav Biswas,a,b Jan van Leusen,c Pawan Kumar,a Vierandra Kumar,a Ramakirushnan Suriya Narayanan,d Paul Kögerler*c and Vadapalli Chandrasekhar*a,d

a

Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur-208016, India. E-

mail: [email protected], http://www.iitk.ac.in b

Department of Chemistry, Jadavpur University, 188, Raja S.C. Mallick Rd, Kolkata 700032,

India. c

Institut für Anorganische Chemie, RWTH Aachen University, D-52074 Aachen, Germany.

E-mail: [email protected], http://www.ac.rwth-aachen.de d

Tata Institute of Fundamental Research, 36/P, Gopanpally Village, Serilingampally Mandal,

Ranga Reddy District, Hyderabad 500107, India.

ACS Paragon Plus Environment

Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract Three

DyIII

tetranuclear

complexes,

[Dy4(LH)2(CH3OH)4(acac)6]

[Dy4(LH)2(CH3OH)4(hmacac)6]·2CH3OH

(2)

(1), and

[Dy4(LH)2(CH3OH)4(dpacac)6]·2CHCl3·2CH3OH·2H2O (3) have been synthesized and characterized

[LH4

=

(2E,N'E)-N'-(2,3-dihydroxybenzylidene)-2-

(hydroxyimino)propanehydrazide; acacH = acetylacetone; hmacacH = 2,2,6,6-tetramethyl3,5-heptanedione; dpacacH = dibenzoylmethane]. The structural elucidation of these complexes reveals two types of DyIII centers in terms of the number of ancillary β-diketonate co-ligands coordinated to the metal centers. Detailed magnetic studies have been carried out on 1-3 which reveal a slow relaxation of magnetization at low temperatures. The relaxation of complexes 2 and 3 are distributed in three temperature ranges: lower temperature process, transition range and higher temperature process. In the higher temperature range the best fitting of the data for 2 yields τ0 = (6.3±3.6)×10–6 s and Ueff = (23.8±4.0) K and for 3 τ0 = (9.4±5.9)×10–6 s, Ueff = (29.0±6.3) K.

Keywords β-diketonate complexes, clusters, Schiff base, oxime ligand, linear Dy4 tetramer, single molecule magnets, ac susceptibility, anisotropy, slow relaxation of magnetization.


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Introduction The discovery of single molecule magnet (SMM) character in [MnIV4MnIII8(µ3– O)12(CH3COO)16(OH2)4]·2MeCO2H·4H2O has ignited significant interest in molecular complexes that can operate as molecular magnets1. This interest has transcended the boundaries of traditional subjects and has attracted chemists, physicists and materials scientists alike. While the potential applications of these molecular materials in futuristic applications such as high-density data storage, quantum computing2, spintronics3, molecular refrigeration4, etc., is one of the reasons for the interest another important reason is that many interesting fundamental questions in physics can be addressed through these systems. From the chemist’s point of view, in addition to the above interests, there is also a synthetic challenge of designing and assembling molecular complexes which can show the most favorable properties. From studies on exchange-coupled transition metal clusters it has been shown that the essential criterion for a molecule to show SMM properties includes a large ground state spin S and a negative magnetic anisotropy of the Ising-type characterized by the zero-field splitting parameter, D, which allows a blocking of the magnetization below a certain critical temperatures, the energy of barrier being Ueff = DS2 for integral spin values and Ueff = DS2-1/4 for non-integer spins values. However, it was soon realized that the complexes, [MnIII12MnII7(µ4-O)8(µ3,η1-N3)8(HL)12-(MeCN)6]Cl2·10MeOH·MeCN5 [MnIII12MnII6DyIII(µ4-O)8(µ3-Cl)6.5(µ3-N3)1.5(HL)12(MeOH)6]Cl3·25MeOH6

and (H3L

=

2,6-

bis(hydroxy-methyl)-4-methylphenol)having S=86/2 and 83/2 are not good SMMs indicating that a mere increase in S does not lead to an increase in the Ueff.7 In fact, in exchange-coupled transition metal clusters showing ferromagnetic exchange there is an inverse relationship between S and D and for this a recent insightful review points out the need to increase the



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magnetic anisotropy over S.8 Unlike transition metal ions, many lanthanide ions have substantial unquenched orbital angular momentum values and hence large single-ion anisotropy. Ishikawa, through his seminal discovery of [TbPc2]−,9 has demonstrated the efficacy of lanthanide-ion complexes as single molecule magnets. This discovery has stimulated substantial interest in lanthanide complexes, in particular those involving DyIII, TbIII, HoIII and ErIII.10 Among these, DyIII, by virtue of being a Kramers ion would always have a bistable electronic ground state and therefore most of the lanthanide single molecule magnets involve this ion. We have been interested in polynuclear lanthanide complexes both from the point of view of the synthetic challenge in modulating the nuclearity and in studying their structure and magnetic properties.11 We have been intrigued by the possibility of studying a family of near isostructural complexes possessing the same nuclearity where one can modulate, in a subtle manner, the electronic/steric effects of the ancillary ligands and can tune the SMM properties as seen in few of the previously reported examples where the effective

energy

barrier

of

[Dy(tmhd)3]·2bpm

(tmhd

=

2,2,6,6-tetramethyl-3,5-

heptanedionate, bpm = 2,2’-bipyrimidine)12 sharply increases when ‘tmhd’ is replaced by ‘dbm’(dbm = dibenzoyl methane)13. A similar behaviour was observed in [DyL3]·CH3OH (HL = 2-(tetrazole-5-yl)-1,10-phenanthroline)14 where the introduction of 2,2,6,6tetramethylheptane-3,5-dionate (tmh) or 2-thenoyltri-fluoroacetonate (tta) distinctly altered the effective energy barriers. Such selective replacement of ligands while retaining the overall structural features is challenging from the point of view synthesis. In view of the fact that β-diketonate ligands can be modified without changing their basic coordination behaviour, we decided to use these ligands for our purpose. Our objective was to retain a robust molecular core containing essentially similar coordination numbers and geometry around the lanthanide ions, but changing the electronic/steric nature of the ancillary ligands.


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Keeping the above objective in mind, herein, we report the synthesis, structural characterization

and

magnetic

properties

[Dy4(LH)2(CH3OH)4(hmacac)6]·2CH3OH 2CHCl3·2CH3OH·2H2O

(3),

of

(2)

[Dy4(LH)2(CH3OH)4(acac)6]

and

(1),

[Dy4(LH)2(CH3OH)4(dpacac)6]·

[(LH4=(2E,N'E)-N'-(2,3-dihydroxybenzylidene)-2-

(hydroxyimino)propanehydrazide)]. The main structural Dy4 core in 1-3 is similar, except that the nature of the acetylacetonate ligand is varied (Scheme 1); (acetylacetone for 1, hmacac: hexamethylacetylacetone for 2, dpacac: diphenylacetylacetone for 3).

O Dy(NO3)3 H N

N

HO

O

R

R1

OH

N

OH O

MeOH/CHCl3 rt, 12hr

R R R1

O1 O2

O7H R HO O9

R1

O8 R

N1

* N3

O3 Dy2 O10

O5*H

N2

* *Dy2 O3

*Dy1 O4*

*O6

*N2

O11 HO5 R1

*O1 R

O11* R *O8

O6

O4

Dy1

R1 *O10

N3 N1 * O2*

O9 * OH

R1

H*O7

R1

For 1: R=CH3, R1= CH3 For 2: R= tBu, R1= tBu For 3: R= Ph, R1= Ph

Scheme 1.Syntheses of 1-3. (Note the changes in the substituents on the acetylacetonate ligand.)



Experimental Section Solvents used in this work were purified according to standard procedures.15 Dy(NO3)3·xH2O, 2,2,6,6-tetramethyl-3,5-heptanedione, acetylacetone; dibenzoylmethane and 2,3-dihydroxybenzaldehyde were obtained from Sigma Aldrich Chemical Co. and were used as received. Hydrazine hydrate (80%), ethyl pyruvate, hydroxylamine hydrochloride, sodium acetate and sodium sulfate (anhydrous) were obtained from S.D. Fine Chemicals, Mumbai, India and were used as such. 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. 1H NMR spectra were recorded on a JEOL JNM LAMBDA 400 model spectrometer operating at 500·0 MHz. Chemical shifts are reported in parts per million (ppm) and are referenced with respect to internal tetramethylsilane (1H). Elemental analyses of the compounds were obtained from Thermoquest CE instruments CHNS-O, EA/110 model. Electrospray ionization mass spectrometry (ESI-MS) spectra were recorded on a Micromass Quattro II triple quadrupole mass spectrometer. Methanol was used as the solvent for the electrospray ionization (positive ion, full scan mode). Capillary voltage was maintained at 2 kV, and cone voltage was kept at 31 kV. Magnetic Measurements Magnetic data of 1–3 were recorded using a Quantum Design MPMS-5XL SQUID magnetometer. The polycrystalline samples were compacted and immobilized into cylindrical PTFE capsules. DC data were acquired as a function of the magnetic field (0.1−5.0 T at 2.0 K) and temperature (2.0–290 K at 0.1 T). AC data were measured in the absence of a static bias field in the frequency range 1−1000 Hz (T = 1.9−50 K, Bac = 3 G). Data were


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corrected for the diamagnetic contributions of the sample holders and compounds (χdia / 10–3 cm3 mol–1: –0.938 (1), –1.43 (2), –1.31 (3)). X-ray Crystallography The SCXRD data for the compounds have been collected on a Bruker SMART CCD diffractometer (MoKα radiation, λ = 0.71073 Å). Collecting frames of data, indexing reflections, and determining lattice parameters was done by the program SMART, integrating the intensity of reflections and scaling was done by SAINT16, SADABS17 for absorption correction, and SHELXTL18 for space group and structure determination and least-squares refinements on F2. The crystal structures were solved and refined by full-matrix least-squares methods against F2 by using the program SHELXL-201419 using Olex-2 software20. In spite of our best efforts to get the best quality data a few atoms in 2 and 3 have thermal disorder. In 2 one C atom of a coordinated CH3OH group experiences thermal disorder. In addition to these, some interstitial solvent molecules are highly disordered in 3. So we could not assign all the interstitial solvent molecules properly due to the disorder and weak residual Q peaks hence, the Olex-2 mask program was used to discard the disordered solvents molecules which gave electron density of around 56 leading to the presence of two CH3OH and two H2O molecules. The void volumes and the possible masked electron counts have been included in the corresponding CIFs. In the cases of very large thermal displacements in 2, we partitioned the electron densities of the corresponding atoms into two positions and some of these atoms were refined isotropically. All other non-hydrogen atoms were refined with anisotropic displacement parameters. The position of the hydrogen atoms were fixed at calculated positions and refined isotropically thoroughly. The crystallographic figures have been generated using Diamond 3.1e program21 and in the case of the positionally disordered solvents/main residue atoms were partitioned at two positions, only the positions of highest occupancy are shown in the diagrams for clarity. The crystal data and the cell parameters for



Page 8 of 37

compounds 1−3 are summarized in Table 1. Crystallographic data (excluding structure factors) for the structures of compounds 1-3 have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication nos. CCDC 1824857-1824859. Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge

CB2

1EZ,

U.K.:

http://www.ccdc.cam.ac.uk/cgi-bin/catreq.cgi,

[email protected], or fax: +44 1223 336033.

Table 1. Crystal data and structure refinement parameters of 1−3

Formula

g/mol Crystal system Space group a/Å b/Å c/Å α (°) β (°) γ(°) V/Å3 Z ρc/g cm-3 µ/mm-1 F(000) Crystal size (mm3) θ range (deg) Limiting indices

Reflns collected Indreflns

1 2 3 C54 H76 Dy4 N6 C92 H150 Dy4 N6 C116H100Cl6Dy4 O24 O26 N6O24 1843.21 Monoclinic P21/c 17.8188(1) 12.7954(6) 15.4601(8) 90.000 111.488(1) 90.000 3279.9(3) 2 1.866 4.583 1796.0 0.02 × 0.02 × 0.02 4.02 to 50.24 -21 ≤ h ≤ 21 -15 ≤ k ≤ 15 -18 ≤ l ≤ 18 38133 5834 [Rint = 0.0704]

Completeness to θ 100 % (%)

2406.19 Triclinic P-1 13.7651(1) 14.0737(1) 16.3050(1) 66.736(2) 84.547(2) 64.669(2) 2612.4(4) 1 1.529 2.898 1216.0 0.02 × 0.02 × 0.02 4.082 to 50.116 -16 ≤ h ≤ 16, -16 ≤ k ≤ 16, -19 ≤ l ≤ 19 31584 9237 [Rint = 0.0462] 100%


2824.71 Triclinic P-1 14.4014(3) 14.4195(4) 15.3744(4) 82.259(5)

69.208(5) 64.059(5) 2885.2(2) 1 1.626 2.772 1396.0 0.02 × 0.02 × 0.02 4.984 to 56.73 -19 ≤ h ≤ 19, -19 ≤ k ≤ 19, -21 ≤ l ≤ 21 43310 14389 [Rint = 0.0558] 100%

e-mail:

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Refinement method

Data/restraints/ parameters Goodness-of-fit on F2 Final R indices [I > 2θ(I)] R indices (all data) CCDC Number

Full-matrix Full-matrix Full-matrix least-squares on least-squares on least-squares on F2 F2 F2 5834/6/419 9237/47/602 14389/16/671 1.071 R1 = 0.0504 wR2 = 0.1170 R1 = 0.0799 wR2 = 0.1345 1824857

1.081

1.029

R1 = 0.0451, wR2 = 0.1104 R1 = 0.0641, wR2 = 0.1231 1824858

R1 = 0.0563, wR2 = 0.1273 R1 = 0.0962, wR2 = 0.1438 1824859

Synthesis Ethyl 2-(hydroxyimino)propanoate (A1) To a solution of hydroxylamine hydrochloride (3.18 g, 45.8 mmol) and sodium acetate (3.0 g, 36.6 mmol) in 20 mL distilled water was added a 20 mL ethanolic solution of ethyl pyruvate (4.24 g, 36.6 mmol) and was stirred at room temperature for 3 hrs. The organic part was separated by using dichloromethane (4 × 30 mL) followed by drying over anhydrous sodium sulfate. The resulting filtrate was evaporated to give an amorphous white solid. Yield: 4.02g, (84.0%). Anal. Calcd. for C5H9NO3 (131.13) C, 45.8; H, 6.92; N, 10.68. Found: C, 45.5; H, 6.86; N, 10.62. 1H NMR (CDCl3): δ = 1.29 (t, 3H, CH3,-ester), 3.09 (s, 3H, CH3), 4.3 (q, 2H, CH2, -ester). 2-(Hydroxyimino)propanehydrazide (A2) To a solution of the ethyl 2-(hydroxyimino) propanoate (1.31 g, 10 mmol) (A1) in methanol (20 mL) was added hydrazine hydrate (2.5 mL) at room temperature. After 1 h, the white precipitate which had formed was filtered off, washed with cold methanol, and air-dried. Yield: 1.04 g (88.9%). Anal. Calcd. for C3H7N3O2(117.11) C, 30.8; H, 6.0; N, 35.8. Found: C, 30.51; H, 6.20; N, 35.55%. 1H NMR: (DMSO-D6): δ 1.872 (s, 3H, CH3), 4.314 (s, 2H, NH2), 9.106 (s, 1H, NH) and 11.580 (s, 1H, OH), EI-MS, m/z(M+H)+ = 244.



(2E,N'E)-N'-(2,3-dihydroxybenzylidene)-2-(hydroxyimino)propanehydrazide (LH4) To a solution of 2-(hydroxyimino)propanehydrazide (A2) (1.00 g, 8.54 mmol) in dry methanol (20 mL), 2,3-dihydroxybenzaldehyde (1.17 g, 8.54 mmol) was added dropwise in dry methanol (20 mL). The resulting pale yellow colored solution was then refluxed for 4 hrs. Subsequently the solution was concentrated in vacuo and stored overnight at 0 °C. A light yellow precipitate was obtained, filtered and washed carefully with cold methanol followed by diethyl ether and finally air dried. Yield: 1.27 g (74%). M.P: 146 °C. FT-IR (KBr) cm−1: 3390 (b), 3209 (b), 3069 (m), 2924 (w), 2852(m), 1914 (s), 1653 (s), 1629 (s), 1604 (s), 1554 (s), 1485(s), 1259(s), 1021(s), 733(s). 1H NMR (400 MHz, DMSO-D6,δ, ppm): 1.92 (s, 1H, oxime OH), 2.47 (s, 3H, CH3), 6.69 (t, 1H, –Ar-H ), 6.81 (m, 1H, Ar–H), 6.84 (m, 1H, Ar– H), 8.54 (s, 1H, imine–H). Anal. Calcd. for C10H11N3O4 (237.22): C, 50.63; H, 4.67; N, 17.71. Found: C, 50.85; H, 4.32; N, 17.43.ESI-MS, m/z: (M+H)+: 238.08. Common Synthetic Procedure for the Preparation of the Complexes 1−3 The complexes 1-3 were synthesized following a common synthetic procedure as follows. A CH3OH/CHCl3(1:1) (30 mL) solution of LH4 (0.168 mmol) was prepared to which Dy(NO3).xH2O (0.337 mmol) and stirred for 10-15 minutes. To the metal-ligand reaction mixture triethylamine (1.012 mmol) was added causing an immediate precipitation which dissolved by the addition of RR1acac (0.516 mmol) (R, R1= CH3 for 1; R, R1 = tBu for 2; R, R1= Ph for 3). Subsequently, the solution was stirred for another 12 hrs at room temperature before complete stripping off the solvent in vacuo resulting in a yellow powder. Suitable crystals for X–ray diffraction were obtained by slow evaporation of the re-dissolved solution in a 1:1 MeOH/CHCl3within 10 days. Specific details of each reaction, yield and the characterization data of the complexes are outlined below.


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[Dy4(LH)2(CH3OH)4(acac)6](1) Quantities: LH4 (0.04g), Dy(NO3).xH2O (0.148 g), acetyl acetone (acac) (0.0516g,), Et3N (0.140 µL), : 0.139 g, 44.7% (based on Dy3+). M.P: 260 °C. FT-IR (KBr) cm−1: 3431 (b), 2976 (b), 2940 (s), 2678 (s), 2491 (s), 1610 (s), 1384 (s), 1266 (s), 1212 (w), 1036 (w), 911 (s), 825 (s), 735 (w). ESI-MS m/z, ion: 1034.97, [C27H36Dy2N3O12+ 3 MeOH+ H2O – H+]–. Anal. Calcd. For C54H74Dy4N6O24 (1841.20): C, 35.23; H, 4.05; N, 4.56. Found: C, 35.55; H, 4.25; N, 4.75. [Dy4(LH)2(CH3OH)4(hmacac)6]·2CH3OH (2) Quantities: LH4 (0.04g), Dy(NO3).xH2O (0.148 g), 2,2,6,6-tetramethyl-3,5-heptanedione (hmacac) (0.0932 g), Et3N (0.140 µL), yield: 0.248 g, 61.0% (based on Dy3+). M.P: >260 °C. FT-IR (KBr) cm−1: 3436 (b), 2962 (s), 2867 (w), 1619 (w), 1603 (w), 1582 (s), 1384 (s), 1211 (s), 1141 (s), 1055 (s), 867(s). Anal. Calcd. For C92 H150 Dy4 N6 O26 (2406.19): C, 45.92; H, 6.28; N, N, 3.49. Found: C, 45.48; H, 6.57; N, 3.34. [Dy4(LH)2(CH3OH)4(dpacac)6]·2CHCl3·2CH3OH·2H2O (3) Quantities: LH4 (0.04g), Dy(NO3).xH2O (0.148 g), dibenzoylmethane (dpacac) (0.114g), Et3N (0.140 µL), yield: 0.313 g, 63.4% (based on Dy3+ ). M.P: > 260 °C. FT-IR (KBr) cm−1: 3435 (b), 2738 (w), 2678 (w), 2491 (w), 1603 (s), 1552 (s), 1519 (s), 1478 (s), 1478 (s), 1384 (s), 1210(w), 1023(w). Anal. Calcd. for C118H112Cl6Dy4N6O28 (2926.282): C, 48.46; H, 3.86; N, 2.87. Found: C, 48.68; H, 3.57; N, 2.34.

Results and Discussion Synthetic Aspects Literature survey has revealed that aroylhydrazone-based Schiff base ligands are very useful in assembling homometallic lanthanide complexes.22 These ligands possess many favourable features. First, because of the possibility of keto-enol tautomerism, depending on the



coordination requirements, one or both of the isomers may be involved in binding to the metal ion. Second, the possibility of C-C bond rotation lends additional flexibility to the ligand. We had previously utilized these features in the formation of Dy4 and Dy6 assemblies where

the

enol

form

of

the

ligand,

6-((bis(2-hydroxyethyl)amino)methyl)-N'-((8-

hydroxyquinolin-2-yl)methylene)picolinohydrazide (L’H4) enabled the formation of former while the assembly of the latter involved the keto form of the ligand (Figure 1).11b

Figure 1. Formation of Dy4 and Dy6 assemblies through the keto-enol tautomerisation of the ligand,

6-((bis

(2-hydroxyethyl)

amino)

methyl)-N'-((8-hydroxyquinolin-2-yl)

methylene)picolinohydrazide (L’H4)11b


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Inspired by the aforementioned result, we have synthesized a new hydrazine-based Schiff base ligand (LH4) that contains N and OH binding sites from an oxime part; O, N, N binding sites from a hydrazone part; two O sites from the phenol motif. The terminal position of the oxime motif allows in modulating nuclearity, if needed, by controlling the deprotonation level. The ligand LH4 was prepared as shown in the Scheme 2.

Scheme 2. Outline of the synthesis of the ligand, LH4

Reaction of LH4 with Dy(NO3).xH2O in the presence of triethylamine as the base and acetyl acetone as a co-ligand in a molar ratio of (1:2:6:3) afforded a tetranuclear complex, [Dy4(LH)2(CH3OH)4(acac)6] (1). In a similar manner, we have accomplished the synthesis of the

other

two

complexes,

[Dy4(LH)2(CH3OH)4(hmacac)6]·2CH3OH

(2)

and[Dy4(LH)2(CH3OH)4(dpacac)6]·2CHCl3·2CH3OH·2H2O (3) by employing hmacac and dpacac respectively instead of acetyl acetone. To investigate the structural integrity of the 1–3 in solution, we have carried out ESI–MS studies that revealed a peak at m/z 1034.97 corresponding to a monoanionic species [C27H36Dy2N3O12+ 3 MeOH+ H2O – H+]– that represents one-half half of the complex 1



(Figure 2). However we were unable to detect similar molecular ion peaks for 2 and 3, indicating that the latter dissociate in solution under the ESI-MS conditions. a)

b)

Figure 2. a) Experimental and b) simulated mass spectral pattern of the species [C27H36Dy2N3O12+ 3 MeOH+ H2O – H+]–

X-ray Crystallography Suitable single crystals of the DyIII complexes were obtained by slow evaporation of their solutions in methanol/chloroform mixture (1:1) over a week. Complex 1 crystallized in the monoclinic system in the space group P21/c (Z = 2) whereas 2 and 3 crystallized in the triclinic system in the space group of P-1 (Z = 1). X-ray crystallographic analysis revealed that all the three complexes are neutral. The asymmetric unit of all the complexes contains one-half of the entire molecule (Figure 3a) and the full molecules are generated as a result of center of inversion present in all of these complexes. In view of their structural similarity, complex 1 (Figure 3b) is chosen for elucidating the salient structural features of these complexes. Selected bond lengths and bond angles of 1 are given in Table 2 while those of 2– 3 are given in the Supporting Information (Tables S1 and S2).


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a)

b)

c)

Figure 3. a) Asymmetric unit of 1 (atoms labeled with an asterisk are not the part of the asymmetric unit, they are shown to clarify the coordination environment of the corresponding metal center); b) Full molecular structure of 1 (selected hydrogen atoms and all solvent molecules are not shown for clarity). Bluish-green bonds represent the intramolecular hydogen bonds; c) crystallographic packing diagram of the molecule 1.



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Table 2. Selected bond lengths (Å) and bond angles (°) of 1. Bond lengths around Dy1

Bond angles around Dy1

Dy(1)-O(1)

2.280(8)

O(1)-Dy(1)-O(2)

67.54(2)

Dy(1)-O(2)

2.363(5)

O(1)-Dy(1)-O(3)

80.90(2)

Dy(1)-O(3)

2.294(4)

O(4)-Dy(1)-O(2)

98.85(2)

Dy(1)-O(4)

2.253(4)

O(4)-Dy(1)-O(5)

75.85(2)

Dy(1)-O(5)

2.446(5)

N(2)-Dy(1)-O(5)

75.54(2)

Dy(1)-O*(6)

2.315(6)

O(4)-Dy(1)-O*(4)

71.82(2)

Dy(1)-N(2)

2.486(8)

O(3)-Dy(1)-O*(6)

70.71(2)

O(1)-Dy(1)-O(2)

69.54(2)

Bond lengths around Dy2 Dy(2)-O(8)

2.336(7)

Bond angles around Dy2

Dy(2)-O(9)

2.322(3)

O(8)-Dy(2)-O(9)

71.82(2)

Dy(2)-O(10)

2.269(5)

O(10)-Dy(2)-O(11)

72.21(2)

Dy(2)-O(11)

2.271(5)

N*(3)-Dy(2)-O(7)

74.06(2)

Dy(2)-O(3)

2.365(6)

O(3)-Dy(2)-O*(6)

68.35(2)

Dy(2)-O*(6)

2.347(4)

O(9)-Dy(2)-O(8)

71.88(3)

Dy(2)-O(7)

2.440(7)

O(10)-Dy(2)-O(11)

Dy(1)-N*(3)

2.572(8)

Bridging angles Dy(1)-O(3)-Dy(2)

111.61(2)

Dy(1)-O*(6)-Dy(2)

109.26(2)

Dy(1)-O(4)-Dy*(1)

108.17(2)


Complex 1 is formed by the concerted coordination action of two triply deprotonated ligands, each of which utilized five coordinating centers out of seven. Each ligand in it’s triply deprotonated form is seen to partition into three coordination pockets: two (pocket 1 and pocket 3) of these are bidentate providing a coordination environment of ON and OO while the other is tetradentate providing a OONN coordination environment (Figure 4). It is noted that the phenolate and the enolate oxygen atoms are utilized to bridge adjacent DyIII centers. In addition to the binding provided by the ligand, the tetranuclear framework is further bolstered by six acetyl acetonate ligands. While the terminal DyIII centres are bound with two acetylacetonate ligands, the central DyIIIs are bound with one acetyl acetonate ligand. It is of interest to note that the ligand has utilized the enol form exclusively in the assembly of the complex. Also, the terminal oxime motif C=N-OH remains unutilized.

1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Po ck et

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Figure4. The three coordination pockets of [LH]3- (top) and its binding mode (µ4– η2: η2: η1: η2: η1). The binding mode of β-diketonate co-ligands (η2) is also shown.



The hexacationic core of 1, [Dy4(O)6]6+ is made up of three contiguous four-membered rings (Dy2O2) containing the two central DyIII ions as the nodal points of the spirocyclic rings. (Scheme 1; Figure 5). Each of these four-membered rings is approximately orthogonal to each other (Figure 5a). The four DyIII ions themselves are organized in a zig-zag manner (Figure 5b). The average distance between adjacent DyIII ions is ~ 3.82Å.

a)

b)

Figure 5. a) The arrangement of three contiguous Dy2O2 rings in 1. b) Zigzag type arrangement of the four DyIII ions in 1.

Over all, the tetranuclear assembly contains two types of eight-coordinated Dy3+ centers, the eighth coordination being provided by neutral methanol for all the dysprosium centers. Although the coordination environment around the two metal centers is same (7O, 1N) they are structurally nonequivalent since the central DyIII centers are chelated by only one acetylacetonate ligand while the terminal DyIII is chelated by two. Another point of nonequivalence arises from the fact that while the central DyIII is bound by the hydrazone nitrogen, the terminal DyIII is coordinated by the oxime nitrogen (Scheme 1). The local


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geometry around both the non-equivalent DyIII centers is biaugmented trigonal prism (Figure 6) which is confirmed by continuous shape analysis 23.

a)

b)

Figure 6. Biaugmented trigonal prism geometry around a) Dy1, b) Dy2.

The following are some comments on the important metric parameters found in 1. The longest Dy-O bond distances in the molecule are observed with coordinated methanol molecule (Dy2-O7, 2.440 (7) Å; Dy1-O5, 2.486(8) Å) while the shortest bond distance is found in the central four membered Dy2O2 ring involving the phenoxide center (Dy1-O4, 2.253(4) Å). The bond distances involving the enolate bridge are slightly longer (Dy1-O3, 2.294 (4) Å; Dy2-O3, 2.365(6) Å) than the Dy-Ophenoxide bonds. The Dy-O distances involving the acetylacetonate chelating ligand vary from 2.269 (5) Å (Dy2-O10) to 2.363 (5) Å (Dy2-O2). Among the Dy-N distances, the Dy1–Nhydrazone distance (2.429(7) Å) is somewhat shorter than the Dy2–Noxime bond distance (2.586(8) Å). Overall, it can be seen



from the above data that the Dy-O and Dy-N bond distances vary over a very narrow range, indicating the tight binding around each of the DyIII centers. Although, the molecular structures of 1-3 are grossly similar a closer inspection reveals that, probably because of the variation of the steric/electronic factors of the β-diketonate coligands that have been used, the local chemical and geometrical environment of the corresponding Dy3+ centers are not equal in all the complexes (Figure 9). Hence it is meaningful to examine and compare the asymmetric units of all the three complexes in view of the crystallographic aspects.

a)

` b)

c)


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Figure 7. a) Asymmetric unit of 1, b) Asymmetric unit of 2, b) Asymmetric unit of 3 (atoms labeled with an asterisk are not the part of the asymmetric unit, they are shown to clarify the coordination environment of the corresponding metal center).

a)

b)

c)



Figure 8. a) Orientation of only co-ligands around the metal centers of complex 1, b) around complex 2, c) around complex 3.

Due to the difference in steric properties of the β-diketonate co-ligands the orientation of the coordinated co-ligand at the Dy1 center is not the same for complexes 1-3. This discord in orientation leads to an inequality in distortion from the perfect triangular dodecahedral geometry (Figure 9). The inequality in distortion can easily be supported by calculating the variation in the C(8)-O(3)-Dy(1)-O(1) and C(8)-O(3)-Dy(1)-O(2) torsional angles in 1-3. The variation of the torsion angle involving C(8)-O(3)-Dy(1)-O(1) is: 75.19(71)⁰(1), 62.81(58)⁰ (2) and 61.58(54)⁰ (3). On the other hand, the variation in the torsion angle involving C(8)O(3)-Dy(1)-O(2) is more severe: 51.00(81)⁰ (1); 121.04(54)⁰ (2); 113.39(53)⁰ (3). This difference in orientations of the β-diketonate co-ligand in 1-3 around Dy1 can also be corroborated by observing the sharp change in O(1)-Dy(1)-O(4) and O(2)-Dy(1)-O(4) bond angles along the series 1-3 (Table 2). Along with the steric bulk, the distinction in electron donating ability by the β-diketonate co-ligands towards metal center in 1-3 controls the Dy(1)-O(1) and Dy(1)-O(2) bond lengths in 1-3 also. The increasing bond lengths for the coordinated β-diketonate co-ligands in 1-3 are as follows: acac>dpacac>hmacac. Because of the strong electron donating effect of the two tBu groups, in the case of hmacac, in 2 the Dy(1)-O(1) and the Dy(1)-O(2) bonds are found to be shortest in all the examples studied, herein. Two Ph groups in dpacac have both electron withdrawing as well as accepting properties in complex 3. To balance these opposing trends the electron donating ability of


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dpacac becomes diminished and results in an increased bond length in 3 compared to 2. The acac with the lowest electron donating ability leads to longest Dy (1)-O(1) and Dy (1)-O(2) bonds.

a)

b)

c)

Figure 9. a) For 1 local geometry around Dy1is biaugmented trigonal prism having minimal CShM value 0.896 and having C2v symmetry, b) for 2 the local geometry around Dy1 is biaugmented trigonal prism having minimal CShM value 1.663 and having C2v symmetry and c) for 3 the local geometry around Dy1is triangular dodecahedron, having a minimal CShM value 2.123 with D2d symmetry.

In spite of the variation of the steric environment of the co-ligands, the bite angle O(1)Dy(1)-O(2) remains almost the same for 1-3. Representative bond lengths and angles around Dy1 are summarized in Table 3.



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Table 3: Comparison of bond lengths (Å) and bond angles (⁰) in the complexes 1-3 around Dy1 center Description of bond or angle

Complex 1

Complex 2

Complex 3

Dy(1)-Ophenoxo(4)

2.343(52)

2.253(45)

2.267(56)

Dy(1)-Nhydrazone(2)

2.429(76)

2.487(82)

2.490(68)

Dy(1)-Ohydrazone(3)

2.315(55)

2.294(58)

2.294(42)

Dy(1)-ORR1acac(1)

2.407(78)

2.363(58)

2.339(46)

Dy(1)-ORR1acac(2)

2.350(58)

2.280(85)

2.322(66)

Dy(1)-Omethanol(5)

2.446(71)

2.446(50)

2.438(51)

ORR1acac(1)-Dy(1)-Ophenoxo(4)

134.39(23)

95.67(24)

105.34(22)

ORR1acac(2)-Dy(1)-Ophenoxo(4)

73.41(22)

98.85(17)

97.58(19)

ORR1acac(1)-Dy(1)-Ohydrazone(3)

69.30(25)

80.90(22)

74.33(21)

ORR1acac(1)-Dy(1)-Nhydrazone(2)

81.39(23)

70.12(26)

71.49(24)

ORR1acac(2)-Dy(1)-Omethanol(5)

147.78(19)

146.95(21)

146.39(19)

Nhydrazone(2)-Dy(1)-Omethanol(5)

85.57(23)

75.54(21)

75.49(19)

ORR1acac(1)-Dy(1)-ORR1acac(2)

69.55(21)

67.54(54)

69.62(19)

Unlike the Dy1 center, the Dy2 center in all the complexes is chelated by two β-diketonate co-ligands each. Other than two β-diketonate co-ligands, Dy2 is coordinated to one N atom from hydrazone part, one O atom from methanol and two O sites: one from phenoxide group of the ligand and another from enolate oxygen. Continuous shape measurement reveals the local geometry around Dy2 centers as follows: for 1 the geometry is biaugmented trigonal prism, for 2 the geometry is triangular dodecahedron and for 3 it is square antiprism (Figure S2). Interestingly, the β-diketonate co-ligands do not play any role in controlling the Dy(2)-O bond lengths and O(1)-Dy(2)-O(4) and O(2)-Dy(1)-O(4) bond angles (See Supporting


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Information, Table S3). As Dy2 is the terminal metal center, the co-ligands have more room to accommodate themselves as per steric demand and hence do not perturb the bond parameters in any significant way. Finally, the Dy(1)-Dy(2) distances in 1-3 are similar: 3.784(8) Å (1); 3.85(5) Å (2); 3.84(1) Å (3). The distance between Dy1 and Dy1* for all the complexes are also similar (on an average of 3.85 Å). It is interesting to note that among the known tetranuclear complexes,24 two examples, have close structural features in terms of the linear zig-zag arrangement of the Dy4 core (Figure 10). a)

b)

Figure 10. Molecular structures of a previously reported near linear Dy4 complexes.24b, 25

Magnetic Studies According to the structural data, the magnetic centers of compounds 1-3 are characterized by small but distinctly different distortions, essentially due to the varying steric demands of the diverse peripheral groups as well the electronic situations. Therefore, the magnetic properties are expected to be similar, too, revealing relevant differences only at lower temperatures and small magnetic fields. The measurements of the magnetic properties in static fields (dc measurements) are shown for all three compounds in Figure 11 as χmT vs. T curves at 0.1 T, and Mm vs. B curves at 2.0 K. At 290 K, the χmT values are 53.35 cm3 K mol–1 (1),



53.55 cm3 K mol–1 (2), and 53.61 cm3 K mol–1 (3), well within the expected26 range 52.0– 56.2 cm3 K mol–1 for four non-interacting Dy3+ centers. Upon lowering the temperature, χmT continuously decreases, revealing deviations at T = 12.0–120 K (where 3 noticeably differs from 1 and 2) and 2.0–12.0 K (where 2 differs from 1 and 3). At 2.0 K, the χmT reaches 27.46 cm3 K mol–1 (1), 32.43 cm3 K mol–1 (2), and 26.89 cm3 K mol–1 (3). The general features of the χmT vs. T curves – an almost constant value at T > 100 K, and a sharp decrease for T < 50 K – predominately arise from the individual single-ion effects of each Dy3+ center: Mainly due to the ligand-field and the spin-orbit coupling, the mJ sub states mix and split, yielding decreasing χmT values. Additionally, weak exchange interactions may be present between the four Dy3+ centers of a compound. These should be predominately antiferromagnetic since the χmT curves reveal no shoulders or maxima, even though also very weak ferromagnetic interactions (Jex up to +0.01 cm–1) could be present, albeit effectively hidden by the application of a small magnetic field. This can also be concluded from the magnetization curves at 2.0 K (inset, Figure 11), which slightly differ for 1–3 at B < 1.5 T, and match in good approximation in the field range 1.5–5.0 T, for which the magnetizations reach 21.1 NA µB (1) and 21.0 NA µB (2 and 3). These non-saturated values are slightly larger than half of the saturation value of four Dy3+ centers (4×gJ J NA µB = 40 NA µB). This is in agreement with very weak ferromagnetic, no or weak antiferromagnetic exchange interactions between the centers, since single Dy3+ centers of similar symmetries (powder samples) reach magnetization values of roughly half of their saturation value in the range 5.0–7.0 T at 2.0 K.


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Figure 11. Temperature dependence of χmT at 0.1 T of 1 (blue circles), 2 (red diamonds), and 3 (open black circles); inset: molar magnetization Mm vs. applied magnetic field B at 2.0 K.

We further analyzed the compounds in dynamic (ac) magnetic fields to study the impact of the functionalization of the ligands at the peripheral positions in more detail. The corresponding data at zero bias field are presented as Cole-Cole plots (out-of-phase ac susceptibility χm’’ vs. in-phase ac susceptibility χm’) in Figures S5-S7. For 1, the data reveal slow relaxation up to 10.0 K (Figure S5), indicated by finite values of χm’’. However, the curvatures of the curves at each temperature in the Cole-Cole plot are not sufficient to be analyzed in more detail. Applying further static fields up to 1000 Oe did not yield any improvement with respect to the frequency range of our experimental set-up (1–1000 Hz). We thus focus on the analysis of the ac susceptibility data of compounds 2 and 3.



Figure 12. Relaxation time τ vs. inverted temperature T–1 at zero bias field of 2 (left), and 3 (right); data derived from magnetic ac susceptibility (open circles), fits to Orbach and/or quantum tunneling processes (lines).

For 2 and 3, the Cole-Cole plots (Figures S6 and S7) are characterized by circle segments including maxima. There are, however, noticeable changes of the rate at which the segments get smaller with rising temperature at about 4.5 K (2) and 6.5 K (3), respectively. This behavior hints at relaxation processes that are activated at different temperatures. Fitting of the data (χm’ vs. f and χm’’ vs. f, simultaneously, Figures S8 and S9) to a generalized Debye expression27 results in the Arrhenius plots shown in Figure 12. In this representation, the data are divided into three temperature ranges: higher temperature processes (indicated by the blue line), lower temperature processes (indicated by the red line), and a transition range, in which the dominating set of processes changes to the other. A possible cause for the two different sets of processes might be that at low temperatures single-ion magnet characteristics are present while at higher temperatures single-molecule magnet characteristics related to exchange interactions between the Dy3+ centers dominate the ac data. Alternatively, maybe in one range the single molecules, and in the other inter-molecular interactions or lattice interactions determine the nature of the relaxation processes. The distributions of relaxation


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times α are similar for both compounds, but very different for the temperature ranges. At the higher temperatures α = 0.413±0.035 (2) and 0.415±0.010 (3), while at the lower temperatures α = 0.188±0.023 (2) and 0.164±0.054 (3). Such comparably high values indicate the coexistence of multiple relaxation processes. Due to the small amount of data in the higher temperature range for 2, we restrict the fit, however, to a single process (Orbach relaxation process) to avoid over-parametrization, while we consider a different Orbach process and quantum tunneling of magnetization (QTM) for the lower temperature range. Since the amount of data with respect to the temperature ranges is inverted for 3, we consider an Orbach process and QTM for the higher temperature range, and only QTM at the lower range. The general equation for fitting the data is thus τ–1 = B + τ0–1×exp[–Ueff/(kBT)], where B is the measure of QTM at zero bias field, τ0 the attempt time, and Ueff the effective barrier of the Orbach process. The fits yield τ0 = (6.3±3.6)×10–6 s and Ueff = (16.6±2.8) cm–1 (= (23.8±4.0) K) for the higher temperature range of 2 (blue line), and for the lower temperature range τ0 = (4.4±1.5)×10–8 s, Ueff = (27.0±1.0) cm–1 (= (38.8±1.4) K), and B = (1589±20) s–1 (red line). For 3, the fits yield τ0 = (9.4±5.9)×10–6 s, Ueff = (20.1±4.4) cm–1 (= (29.0±6.3) K), and B = (1736±454) s–1 for the higher temperature range (blue line), and for the lower temperature range B = (3020±27) s–1 (red line). We, therefore, identify a slower Orbach relaxation process as well as a smaller QTM rate B characterized by the same parameters within the errors. At lower temperatures, an additional faster Orbach process is observed for 2, and a different larger QTM rate B for 3. Comparing the ac magnetic data of 1, 2 and 3, the different steric demand of the peripheral groups, and thus the higher degree of distortion introduced to the Dy3+ centers from 1 to 3, are primarily responsible for the variation of the slow relaxation processes of the compounds. The different groups affect the phase and magnitude of the out-of-phase susceptibility and shift the transition range to higher (or



respectively lower) temperatures. They do not (or only marginally) change the temperature range at which these processes are observable (for all compounds at T < 10 K).

Conclusion We presented three near-linear, tetranuclear Dy(III) complexes with different peripheral steric demands of the ligands, and characterized their variations on structural and magnetic properties. Due to the varying distortions of the Dy3+ centers the magnetic properties of the compounds slightly change with respect to the behavior in static magnetic fields, and distinctly change with respect to slow relaxation processes observed in dynamic fields. Such processes can be suppressed or enhanced by the introduction of different groups in the periphery of the ligands, which thus allows for the modification or tuning of otherwise known compounds. This is because the presented peripheral groups induce different distortions of the magnetic centers instead of (drastically) changing the electronic situation by the introduction of other coordinating atoms. Supporting Information Supporting Information: full molecular structure of 2 and 3; list of bond lengths and angles of the complexes 2, 3; full range ESI-MS of 1; details of SHAPE measurement; Cole–Cole plots of all the complexes; in-phase and out-of-phase magnetic susceptibilities vs. frequency plots of 2 and 3.

Conflicts of interest There are no conflicts to declare.

Acknowledgements


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We thank the Department of Science and Technology (DST), India, for financial support, including support for a Single Crystal CCD X-ray Diffractometer facility at IIT-Kanpur. V.C. is grateful to the DST for a J. C. Bose fellowship. J.A. thanks Department of Science and Technology (DST), India for INSPIRE Junior Research Fellowship. J.A. also thankful to Mr. Chinmay Das, IIT Bombay for helping in SHAPE calculations.

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“For Table of Contents Use Only”

Exploring Tuning of Structural and Magnetic Properties by Modification of Ancillary β-Diketonate Co-ligands in a Family of Near-Linear Tetranuclear DyIII Complexes Joydev Acharya,a Sourav Biswas,a,b Jan van Leusen,c Pawan Kumar,a Vierandra Kumar,a Ramakirushnan Suriya Narayanan,d Paul Kögerler*c and Vadapalli Chandrasekhar*a,d

Three, near-linear tetranuclear DyIII complexes have been synthesized and characterized. By varying the electronic and steric demand of the ancillary β-diketonate co-ligands subtle changes in the geometry around the DyIII centers have been achieved. Correlation of these structural changes with the magnetic behavior of the complexes has been studied.


Exploring Tuning of Structural and Magnetic Properties by Modification

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