Pentanuclear [2.2] spirocyclic lanthanide(III) complexes: slow magnetic relaxation of the Dy^III analogue

Abstract

The reaction of LnCl₃·6H₂O (Ln = Dy³⁺, Tb³⁺ and Ho³⁺) with the multisite coordinating ligand N′-(2-hydroxy-3-(hydroxymethyl)-5-methylbenzylidene)acetohydrazide (LH₃) in the presence of pivalic acid (PivH) leads to the formation of three isostructural homometallic pentanuclear complexes, [Dy₅(LH)₄(η¹-Piv)(η²-Piv)₃(μ₂–η²η¹Piv)₂(H₂O)]·Cl·9·5H₂O·5MeOH (1), [Tb₅(LH)₄(η¹-Piv)(η²-Piv)₃(μ₂–η²η¹Piv)₂(H₂O)]·Cl·10.5H₂O·2MeOH·2CHCl₃ (2) and [Ho₅(LH)₄(η¹-Piv)(η²-Piv)₃(μ₂–η²η¹Piv)₂(H₂O)]·Cl·14.5H₂O·2CHCl₃ (3). 1–3 are monocationic and are comprised of four doubly deprotonated [LH]²⁻ ligands along with six pivalate ions. These complexes possess a [2.2] spirocyclic topology formed by the fusion of two triangles of Ln^III ions at a common vertex. The magneto chemical analysis reveals the presence of antiferromagnetic exchange interactions at low temperature, and the Dy^III complex 1 gives an out-of-phase signal with a small curvature in alternating current (ac) magnetic susceptibility measurement. Application of a 3000 G static field during ac measurement intensifies the signals, revealing a second slow relaxation process in the Dy^III analogue.

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Introduction

The synthesis of polynuclear clusters of 4f metal ions is of interest because of their potential utility in single-molecule magnets (SMMs), catalysis,¹ luminescence,² medical imaging,³ magnetic refrigeration,⁴ high-density data storage,⁵ spintronics⁶ and quantum computing.⁷ SMMs are characterized by a slow relaxation of magnetization below the blocking temperature, T_B. Magnetic phenomena have been thoroughly explored in polynuclear 3d metal complexes^8,9 and heterometallic 3d/4f complexes,¹⁰ and the recent years have seen the emergence of lanthanide complexes, particularly those involving Dy³⁺ or Tb³⁺. Here, the zero-field splitting of the m_J sub-states belonging to the J ground state produces the thermal energy barriers. Interest in lanthanide complex SMMs has been fueled by the seminal discovery by Winpenny and co-workers of complexes [Dy₄K₂O(O^tBu)₁₂] and [Dy₅O(OⁱPr)₁₃], possessing exceptionally high energy barriers for magnetization reversal.¹¹ Homometallic 4f complexes of varying nuclearities (Ln,¹² Ln₂,¹³ Ln₃,¹⁴ Ln₄,¹⁵ Ln₅,¹⁶ Ln₆,¹⁷ Ln₇,¹⁸ Ln₈,¹⁹ Ln₉,²⁰ Ln₁₀ ²¹ and Ln₁₁ ²²) are now documented in the literature, and we have reported new SMMs involving 3d/4f²³ and 4f²⁴ metal ions. The five examples of pentanuclear lanthanide complexes known display three distinct structural topologies: pyramidal,^16a–c butterfly^16e and trigonal bipyramidal.^16d These pentanuclear complexes evince SMM behavior including Winpenny's [Dy₅O(OⁱPr)₁₃] complex, which showed a slow relaxation of magnetization with a 528 ± 11 K thermal energy barrier.^16a To pursue new polynuclear lanthanide complexes, we have designed a new linear alkyl hydrazone-based multidentate Schiff base ligand, N′-(2-hydroxy-3-(hydroxymethyl)-5-methylbenzylidene)acetohydrazide (LH₃); upon reaction with LnCl₃·6H₂O (Ln = Dy³⁺, Tb³⁺ and Ho³⁺), this afforded homometallic pentanuclear [2.2] spirocyclic complexes 1–3, [Ln₅(LH)₄(η¹-Piv)(η²-Piv)₃(μ₂–η² η¹Piv)₂(H₂O)]·Cl·xH₂O·yMeOH·zCHCl₃ (Dy³⁺, x = 9.5, y = 5, z = 0; Tb³⁺, x = 10.5, y = 2, z = 2 and Ho³⁺, x = 14.5, y = 0, z = 2). In these complexes two triangles of Ln³⁺ ions are fused through a common vertex. The synthesis, structure and magnetic properties of these complexes are discussed herein.

Results and discussion

Synthesis

Ligand design is a crucial element in modulating the nuclearity and structural topology of lanthanide complexes. Recently, we have prepared rhombus-shaped Ln₄ complexes, [Ln₄(LH)₂(μ₂-O)₄(H₂O)₈] (Ln = Dy³⁺ and Ho³⁺),²⁵ incorporating an aroyl hydrazone-based Schiff base ligand (6-hydroxymethyl)-N′-((8-hydroxyquinolin-2-yl)-methylene)picolinohydrazide (Scheme 1a).

Scheme 1 (a) Synthesis of rhombus-shaped tetranuclear lanthanide complexes²⁵ (b) Synthesis of cubane-shaped tetranuclear lanthanide complexes.²⁷

Motivated by this work, as well as by the use of other hydrazone-based Schiff base ligands in the recent literature,^{14c,15d,17d,24,26} we have prepared a multisite coordinating semi-flexible alkyl hydrazone-based ligand, N′-(2-hydroxy-3-(hydroxymethyl)-5-methylbenzylidene)acetohydrazide (LH₃). This was produced using a two-step synthetic protocol that involved the preparation of B1, which subsequently undergoes a condensation reaction with B2 to give LH₃ (Scheme 2). The semi flexible ligand, LH₃ provides five divergent coordinating sites: an alkyl hydrazone oxygen, an imine N, an amide N, a phenolic O and a flexible –CH₂OH arm. The latter is a crucial element in the formation of the coordination geometry, as recently shown by us in the preparation of cubane-shaped tetranuclear lanthanide complexes (Scheme 1b).²⁷ The pivalic acid co-ligands help to saturate the primary coordination spheres of the metals by bridging the metal centers, and confers lipophilicity to the complexes.

Scheme 2 Synthesis of the ligand LH₃.

The reaction of LH₃, LnCl₃·6H₂O, and pivalic acid in the stoichiometric ratio of 4 : 5 : 6 in the presence of 4 equivalents of triethylamine in methanol afforded pentanuclear complexes [Ln₅(LH)₄(η¹-Piv)(η²-Piv)₃(μ₂–η² η¹Piv)₂(H₂O)]·Cl·xH₂O·yMeOH·zCHCl₃ [compound 1, Ln = Dy³⁺, x = 9.5, y = 5, z = 0; compound 2, Ln = Tb³⁺, x = 10.5, y = 2, z = 2 and compound 3, Ln = Ho³⁺, x = 14.5, y = 0, z = 2] (Scheme 3).

Scheme 3 Synthesis of [Ln₅(LH)₄(η¹-Piv)(η²-Piv)₃(μ₂–η² η¹Piv)₂(H₂O)]·Cl·xH₂O·yMeOH·zCHCl₃ (Dy, x = 9.5, y = 5, z = 0; Tb, x = 10.5, y = 2, z = 2 and Ho, x = 14.5, y = 0, z = 2).

X-ray crystal structures

Single-crystal X-ray diffraction measurements reveal that compounds 1–3 are isostructural, and crystallize in the tetragonal space group I4̄ with Z = 4. The complexes are monocationic, with their charges each balanced by a chloride anion, and possess a [2.2] spirocycle of Ln^III ions containing two fused triangular motifs. Complex 1 has been chosen as a representative example to illustrate the common structural features of these clusters. Selected bond parameters of 1 are summarized in Table 1. The molecular structure and selected bond parameters of the other complexes (2 and 3) are presented in the ESI (Fig. S1–S2† and Tables S1–S2†). A perspective view of the molecular structure of 1 is depicted in Fig. 1.

Fig. 1 Molecular structure of 1 (selected hydrogen atoms, chloride and the solvent molecules have been omitted for clarity).

Table 1 Selected bond distances (Å) and bond angles (°) parameters for 1

Selected bond length around Dy1		Dy(3)–O(11)	2.351(6)	Dy(5)–O(10)	2.409(6)
Dy(1)–O(11)	2.310(6)	Dy(3)–O(4)	2.351(6)	Dy(5)–O(18)	2.456(9)
Dy(1)–O(12)	2.396(7)	Dy(3)–O(4)	2.351(6)	Dy(5)–N(7)	2.508(9)
Dy(1)–O(20)	2.409(7)	Dy(3)–O(1)	2.353(6)
Dy(1)–O(9)	2.417(6)	Dy(3)–O(2)	2.381(7)	Selected Bond angles around Dy
Dy(1)–O(19)	2.442(7)	Dy(3)–O(5)	2.414(6)	Dy(2)–O(1)–Dy(1)	97.9(2)
Dy(1)–O(21)	2.447(6)			Dy(3)–O(1)–Dy(1)	94.3(2)
Dy(1)–O(1)	2.457(6)	Selected bond length around Dy4		Dy(5)–O(2)–Dy(3)	97.2(2)
Dy(1)–N(1)	2.519(8)	Dy(4)–O(15)	2.312(7)	Dy(3)–O(4)–Dy(5)	95.2(2)
Dy(1)–O(22)	2.521(6)	Dy(4)–O(8)	2.373(6)	Dy(3)–O(4)–Dy(4)	96.9(2)
		Dy(4)–O(7)	2.384(7)	Dy(5)–O(4)–Dy(4)	99.5(2)
Selected bond length around Dy2		Dy(4)–O(4)	2.396(6)	Dy(2)–O(5)–Dy(3)	95.3(2)
Dy(2)–O(24)	2.320(7)	Dy(4)–O(30)	2.411(7)	Dy(3)–O(8)–Dy(4)	98.4(2)
Dy(2)–O(5)	2.324(6)	Dy(4)–N(4)	2.494(8)	Dy(3)–O(9)–Dy(1)	95.9(2)
Dy(2)–O(22)	2.325(6)	Dy(4)–O(10)	2.500(6)	Dy(3)–O(9)–Dy(2)	94.5(2)
Dy(2)–O(1)	2.333(6)	Dy(4)–O(14)	2.505(8)	Dy(1)–O(9)–Dy(2)	96.18(9)
Dy(2)–O(6)	2.402(7)	Dy(4)–O(13)	2.666(9)	Dy(3)–O(10)–Dy(5)	96.0(2)
Dy(2)–O(9)	2.439(6)			Dy(3)–O(10)–Dy(4)	95.4(2)
Dy(2)–N(5)	2.450(8)	Selected bond length around Dy5		Dy(5)–O(10)–Dy(4)	96.1(2)
Dy(2)–O(23)	2.454(7)	Dy(5)–O(13)	2.258(7)	Dy(1)–O(11)–Dy(3)	98.3(2)
Selected bond length around Dy3		Dy(5)–O(2)	2.286(6)	Dy(5)–O(13)–Dy(4)	95.4(3)
Dy(3)–O(10)	2.301(6)	Dy(5)–O(17)	2.356(11)	Dy(2)–O(22)–Dy(1)	96.4(2)
Dy(3)–O(8)	2.321(6)	Dy(5)–O(3)	2.381(7)	Dy(2)–O(1)–Dy(3)	96.7(2)
Dy(3)–O(9)	2.330(6)	Dy(5)–O(4)	2.387(6)

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The pentanuclear complex 1 is formed by the concerted coordination action of the four doubly deprotonated ligands [LH]²⁻. Four out of five potential coordinating sites of the ligand are coordinated to the metal centers: a phenolate oxygen which functions as a bridging ligand between two Dy³⁺ centers, a deprotonated hydroxymethyl arm which functions as a μ₃-capping ligand among three Dy^III centers, an imine nitrogen binding a Dy³⁺ center, and an acetohydrazide oxygen coordinating a Dy³⁺ center. The framework provided by the ligand [LH]²⁻ is bolstered by the coordination of six pivalate ligands: one is η¹-coordinated to Dy4, while three others chelate Dy1, Dy2 and Dy5 in an η² fashion. The remaining two pivalate ligands exhibit both bridging and chelating modes (μ₂–η²:η¹) in their coordination that links Dy1 to Dy2 and Dy4 to Dy5. The pentanuclear [2.2] spirocyclic core is thus constructed by the coordination action of four [LH]²⁻ (μ₄–η³:η²:η¹:η¹) and six pivalate ligands. The coordination modes of the ligands are depicted in Fig. 2.

Fig. 2 Binding modes of [LH]²⁻ and the pivalate ligands with Dy³⁺ ions in 1.

The pentacationic pentanuclear core, [Dy₅(μ₃-O)₄(μ₂-O)₄(μ₂–η²η¹Piv)₂]⁵⁺ consists of two Dy₃ triangles which are interconnected by sharing a common vertex, Dy3. The remaining four vertices of the two triangles are occupied by Dy1, Dy2, Dy4 and Dy5 (Fig. 3a). The edges of the triangles are formed by the pivalate ligands as well as phenolate oxygens of the ligand [LH]²⁻ while the faces of the triangle are effectively capped by μ₃-O, emanating from the flexible pendant hydroxymethyl arms (Fig. 3a). The capping μ₃ oxygen atoms lie at an average of ∼1.219 Å away from the two triangular planes. The triangles are nearly equilateral, with Dy⋯Dy vertex lengths of 3.50–3.651 Å and Dy–Dy–Dy angles that range from 58.1°–62.2°. The two triangles are twisted at the Dy3 [2.2] spirocyclic node by a dihedral angle 55.85° (Fig. 3b).

Fig. 3 (a) [2.2] spirocyclic core of the complex 1 (b). [2.2] spirocyclic core showing the dihedral angle between the two triangular motifs along with the distance between the Dy³⁺ centers.

The five crystallographically independent lanthanide centers present in 1 can be classified into three coordination geometry types (all distorted): triangular dodecahedron (Dy1, Dy5), square antiprism (Dy3), and mono-capped square-anti-prism (Dy2, Dy4) (Fig. 4). A minor variation exists in the coordination environment around Dy2 and Dy4: unlike the rest of the dysprosium centers, Dy4 is coordinated by a water molecule, which is engaged in strong hydrogen bonding with the oxygen atom of the η¹-pivalate ligand (Fig. 1) (D–H⋯A distance 1.746 Å and angle 154.41°) accounting for the η¹ binding mode rather than the anticipated η² coordination mode of the pivalate ligand. The Dy–O_phenoxy bond lengths fall in the range of ∼2.286–2.414 Å which are slightly shorter than the Dy–O_alkoxy bond lengths (∼2.330–2.457 Å). These are in turn shorter than many Dy–O_piv bond distances that range from ∼2.258 to ∼2.660 Å. Bond lengths involving the coordinated imine nitrogens lie in the range of ∼2.449–2.519 Å. The Dy–O_phen–Dy (95.31°–98.36°) and Dy–O_piv–Dy angles (95.36–96.36°) are in a similar range.

Fig. 4 Local geometry around the three types of dysprosium centres in 1: (a) Dy1 and Dy5 possessing a distorted triangular dodecahedral geometry, (b) Dy3 possessing a distorted square antiprism geometry and (c) Dy2 and Dy4 possessing a distorted monocapped square antiprism.

The [2.2] spirocyclic topology observed in complexes 1–3 is quite distinct from the pyramidal,^16a–c butterfly^16e or trigonal bipyramidal^16d shaped homometallic Ln₅ clusters reported previously (Fig. S9†). A comparison of the geometry around the metal centers, the structural topology around the metal ions and the SMM properties of the pentanuclear lanthanide families are summarized in Table 2.

Table 2 Structural and magnetic features of pentanuclear lanthanide assemblies

Compound	Core topology	Coordination numbers (local geometries around Ln(III) centers)	Magnetic properties	Ref.
[Dy5(μ3-OH)6 (Acc)6 (H2O)10]·Cl9	Trigonal bipyramidal	Eight coordinated (distorted square-antiprism)	SMM	16d
			U eff = 1.91 K
Acc = 1-amino cyclohexane-l-carboxylic acid.			τ 0 = 1.01 × 10^−6 s
[Dy5O(O^iPr)13]	Square pyramidal	Six coordinated (octahedral)	SMM	16a
			U eff = 528 ± 11 K, 46.6 ± 0.7 K
^iPr = isopropyl
			τ 0 = 4.7 × 10^−10 s, 3.8 × 10^−6 s
[Dy5(OH)5(α-AA)4(Ph2acac)6]	Square pyramidal	Eight coordinated	SMM	16b
α-AA = D-PhGly
[Dy5 (μ3-OH)3 (opch)6 (H2O)3]	Butterfly	Eight coordinated (distorted dodecahedral), nine coordinated (distorted mono-capped square-antiprismatic) and nine coordinated (distorted triplecapped prismatic)	SMM	16e
			U eff = 8.1 ; 37.9 K
H2opch = o-vanillin pyrazine acylhydrazone			τ 0 = 1.7 × 10^−5s; 9.7 × 10^−8 s
[Dy5 (μ3-OH)3 (opch)6 (H2O)3](ClO4)2	Butterfly	Eight coordinated (distorted dodecahedral), nine coordinated (distorted mono-capped square-antiprismatic) and nine coordinated (distorted triplecapped prismatic)	SMM	16e
			U eff = 197 K ;
H2opch = o-vanillin pyrazine acylhydrazone			τ 0 = 3.2 × 10^−9 s
[Dy5(LH)4(η^1-Piv)(η^2-Piv)3(μ2–η^2 η^1Piv)2(H2O)]·Cl	[2.2] spirocycle	Eight coordinated (distorted triangulardodecahedral), eight coordinated (distorted square-antiprism) and nine coordinated (mono-capped-square-antiprism)	Slow relaxation	This work

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Thermogravimetric study of complex 1–3

Thermogravimetric analysis reveals that all the complexes 1–3 exhibit almost a similar decomposition pattern towards heat treatment involving a two-step weight loss process (Fig. S10–S12†). A small weight loss in the region of 60–120 °C was observed for all the complexes which is in part due to the loss of solvents of crystallization. In all the cases, some of the solvent molecules of crystallization are lost rapidly as the crystals are brought outside the mother liquor at room temperature. In the second step, above 300 °C, rapid weight loss of all the complexes was observed, confirming the decomposition of the complexes.

Magnetic studies

Direct current (dc) magnetic susceptibility data for 1–3 are presented as χ_mT vs. T curves at B = 0.1 Tesla, and molar magnetization M_mvs. applied field B diagrams at T = 2 K in Fig. 5. For the Dy^III analogue 1, χ_mT reaches a value of 64.9 cm³ K mol⁻¹ at 290 K which is slightly below the range 65.1–70.3 cm³ K mol⁻¹ expected^27a for five non-interacting Dy³⁺ (⁶H_15/2, J = 15/2, g_J = 4/3) centers. The decrease of χ_mT with lowering temperature is gradual down to ∼100 K, and then more precipitous (approximate slope). This behavior and the low value of χ_mT at 290 K have their origin in the ligand field effect (i.e. depopulation of the m_J sublevels) and potential antiferromagnetic exchange interactions within the compound (note that the distinct deviation from linear behavior in χ_mT is already introduced at higher temperatures T ≈ 200 K). Analysis of the dc magnetic susceptibility data for 2 and 3 reveals similar behavior, and the χ_mT values at 290 K are also slightly lower than those expected for the respective number of non-interacting lanthanide centers. For 2, χ_mT reaches 58.1 cm³ K mol⁻¹ at 290 K (expected^27a 58.2–60.1 cm³ K mol⁻¹ for five non-interacting Tb^III (⁷F₆, J = 6, g_J = 3/2) centers); for 3, χ_mT is 66.1 cm³ K mol⁻¹ at 290 K (expected^27a 66.3–69.0 cm³ K mol⁻¹ for five non-interacting Ho^III (⁵I₈, J = 8, g_J = 5/4) centers).

Fig. 5 Temperature dependence of χ_mT of 1–3 at 0.1 Tesla; inset: molar magnetization M_m as function of the applied field B at 2 K.

At 2 K, the molar magnetizations M_m of 1–3 (Fig. 5, inset) indicate saturation and thus hint roughly at the ground state of the Ln^III centers in each compound expected due to ligand field effects on the spin–orbit ground term, since the weak exchange interactions of lanthanides at zero field are eliminated by the applied maximum fields: all of the extrapolated saturation values are roughly right in the center between the minimum magnetization of the corresponding five non-interacting Ln^III centers (5g_Jm_J,minN_Aμ_B) and the maximum magnetization of such centers (5g_JJN_Aμ_B). This indicates that neither all Ln^III centers of a compound are characterized by a ground state of minimal m_J,min (±1/2 for 1, 0 for 2, 3) nor by the maximum m_J = J.

A comprehensive model of compounds 1–3 based exclusively on the magnetic susceptibility data is not feasible because the description of the compounds should include at least two different lanthanide sites (approximately D_4d and D_3h symmetric, eight- or nine-fold coordinated centers) and three different exchange pathways. Without a simpler model system such as {Gd₅}, or complementary data such as from inelastic neutron scattering, degenerate preliminary solutions modeled by the computational framework CONDON 2.0^27b,c cannot be ruled out. One feature common among all of these preliminary solutions provides qualitative insight into χ_mT and M_m behavior presented in Fig. 5: all calculations reveal very weak ferromagnetic exchange interactions between the two outer lanthanides of each triangle (i.e. Ln1⋯Ln2 and Ln4⋯Ln5, Fig. 3b), and either antiferromagnetic (≈−0.3 cm⁻¹) or almost nonexistent exchange interactions for the remaining pathways (Ln1⋯Ln3, Ln2⋯Ln3, Ln3⋯Ln4, and Ln3⋯Ln5). Note that although the bridging ligands are similar, and thus exchange interactions might be assumed to be similar as well, the various distances between the Ln pairs and the presence of three different site geometries, and thus ligand fields, explain the difference in the exchange interaction parameters.

Alternating current (ac) molar magnetic susceptibility measurements on complexes 1–3 revealed out-of-phase signals for the Dy^III analogue 1, but not for 2 and 3, at zero dc field. The very small curvatures within the Argand plane for complex 1 (Fig. S3†) reveal in-phase signals χ′_m that are almost independent from the applied frequency (Fig. 6). The application of external static fields of up to 4000 G slightly shifted χ′_m, but did not produce significantly greater curvatures for frequencies greater than 100 Hz. Similar behavior was observed by Thielemann et al.^16b who ascribed it to a blocking temperature which lies significantly below the range explored by the magnetic measurements. Optimal slow relaxation magnetic measurements of complex 1 with respect to the experimental options at hand were obtained using a 3000 G static field, and a frequency range of 0.03 Hz–111 Hz (Fig. 7, insets Fig. 6). The in-phase (χ′_m) and out-of-phase (χ′′_m′) ac susceptibility components can be fitted to a Cole–Cole equation²⁸ for each temperature (Fig. 7, solid lines; Fig. S3†). To determine the average relaxation times of the magnetization under the optimized conditions, the temperature-dependent fit parameters have been analyzed using an Arrhenius expression (τ = τ₀ exp(ΔU/k_BT)). This results in an effective energy barrier ΔU = (5.2 ± 0.5) cm⁻¹ and a time constant of τ₀ = (2.6 ± 0.6) × 10⁻² s (Fig. S5†). In the generalized Debye model, the distribution width of τ is parameterized by the scalar α. The nonzero mean value of α = 0.45 ± 0.08 reveals that several relaxation processes are active in this system. The time constant τ₀ is anomalously large for SMM behavior, i.e. the entirely phenomenological parameter τ′₀ does not fall within the typical SMM range, and indicates that rather a secondary relaxation process was observed in the presence of a bias field instead of the relaxation process indicated at zero field. This is supported by the occurrence of the minima in the Cole–Cole curves for higher frequencies (lower χ′_m values) hinting at further subsequent semi-circles which we could not enhance with the experimental set-up at hand. Since some or all exchange interactions within lanthanide compounds are overridden by the application of an external field of 3000 G, the observed process may be connected to a forced alignment of the momenta albeit further evidence is needed to prove this hypothesis.

Fig. 6 Left: in-phase magnetic susceptibility χ_m′ vs. T; right: out-of-phase magnetic susceptibility χ_m′′ vs. T of 1 at different frequencies f at zero dc bias field; insets: at 3000 G dc bias field (dashed lines are guides for the eye).

Fig. 7 Normalized Cole–Cole plot of 1 at different temperatures and 3000 G dc field (solid circles), frequencies range from 0.03–111 Hz, χ_T is the isothermal susceptibility in the limit of lowest frequencies; fit to Cole–Cole equation (solid lines).

Conclusions

In summary, a series of isostructural homometallic pentanuclear Ln₅ complexes that possess an unprecedented [2.2] spirocyclic topology were synthesized. The frameworks of these complexes are comprised of the multidentate Schiff base ligand, N′-(2-hydroxy-3-(hydroxymethyl)-5-methylbenzylidene)acetohydrazide, along with several bridging pivalate groups. The lanthanide centers present in the pentanuclear assembly can be grouped into three types based on their local coordination geometry: an eight-coordinate lanthanide in a distorted triangular dodecahedral geometry, an eight-coordinate lanthanide in a distorted square-antiprism geometry and a nine-coordinate lanthanide in a mono-capped square anti-prism geometry. Variable-temperature dc and ac magnetic susceptibility measurements reveal weak antiferromagnetic exchange interactions among the lanthanide centers of complexes 1–3. Compound 1 exhibited temperature-dependent out-of-phase ac molar magnetic susceptibility signals with small curvatures at zero dc field. The application of a static 3000 G field intensified the ac magnetic susceptibility component, and an Arrhenius analysis confirmed the slow magnetic relaxation of the Dy^III analogue.

Experimental section

Solvents and other general reagents used in this work were purified according to the standard procedures.²⁹ 2,6-Bis(hydroxymethyl)-4-methylphenol, activated manganese(IV)dioxide (MnO₂), DyCl₃·6H₂O, TbCl₃·6H₂O and HoCl₃·6H₂O were obtained from Sigma Aldrich Chemical Co. and were used as received. Acetyl hydrazide and 2-(hydroxymethyl)-6-carbaldehyde-4-methylphenol were prepared according to the literature procedure.^23e Hydrazine hydrate (80%), pivalic acid and sodium sulphate (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. Elemental analyses of the compounds were obtained from Thermoquest CE instruments CHNS-O, EA/110 model. ¹H NMR spectra were recorded in CD₃OD solutions 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). Powder X-ray diffraction (PXRD) patterns were recorded with a Bruker D8 advance diffractometer equipped with nickel-filtered Cu Kα radiation. Powder X-ray diffraction (PXRD) patterns of complexes 1–5 were in good agreement with the simulated patterns (ESI, Fig. S6–S8†). The difference in intensities could be due to the preferred orientation in the powder samples.

Magnetic measurements

Magnetic susceptibility data of 1–3 were recorded using a Quantum Design MPMS-5XL SQUID magnetometer for static field (DC) and dynamic field (AC) measurements. The polycrystalline samples were compacted and immobilized into PTFE capsules. DC susceptibility data were acquired as a function of the field (0.1–5.0 T) and temperature (2–290 K). AC susceptibility data were measured at zero field and in the presence of various static fields in the frequency range 0.03–1000 Hz (T = 1.8–50 K, B_ac = 3 G, B_dc = 0–4000 G). All data were corrected for the contribution of the sample holder (PTFE capsule) and the diamagnetic contributions of compounds 1–3 calculated from tabulated values (−1.25 × 10⁻³ cm³ mol⁻¹, −1.37 × 10⁻³ cm³ mol⁻¹ and −1.32 × 10⁻³ cm³ mol⁻¹, respectively).

X-ray crystallography

The crystal data for the compounds have been collected on a Bruker SMART CCD diffractometer (MoKα radiation, λ = 0.71073 Å). The program SMART^30a was used for collecting frames of data, indexing reflections, and determining lattice parameters, SAINT^30a for integration of the intensity of reflections and scaling, SADABS^30b for absorption correction, and SHELXTL^30c,d for space group and structure determination and least-squares refinements on F². All the structures were solved by direct methods using the program SHELXS-97^30e and refined by full-matrix least-squares methods against F² with SHELXL-97.^30e Hydrogen atoms were fixed at calculated positions and their positions were refined by a riding model. All the non-hydrogen atoms were refined with anisotropic displacement parameters. The lattice solvent molecules of the complexes 1–3 cannot be modeled satisfactorily due to the presence of very high disorder. PLATON/SQUEEZE^30f,g routine was utilized to remove the severely disordered solvent molecules. The total electron count thus squeezed is 396, 646 and 780 respectively per unit cell which corresponds to 99, 161 and 195 electrons per molecule (Z = 4). These electron counts can be assigned to 5MeOH, H₂O (expected 100) for 1, 2CHCl₃, 2MeOH, H₂O (expected 162) for 2 and 2CHCl₃, 8H₂O (expected 196) for 3. The crystallographic figures have been generated using Diamond 3.1e software.^30h The crystal data and the cell parameters for compounds 1–3 are summarized in Table 3. Crystallographic data (excluding structure factors) for the structures in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication nos. CCDC 1031235–1031237.

Table 3 Crystal data and structure refinement parameters of 1–3

	1	2	3
Formula	C148H208Cl2Dy10N16O67	C148H206Cl2N16O69Tb10	C148H208Cl2Ho10N16O63
M/g	4979.22	4967.45	4939.52
Crystal system	Tetragonal	Tetragonal	Tetragonal
Space group	I4̄	I4̄	I4̄
a/Å	40.368(2)	40.479(5)	40.446(5)
b/Å	40.368(2)	40.479(5)	40.446(5)
c/Å	11.922(7)	11.943(5)	11.940(5)
α = β = γ (°)	90	90	90
V/Å^3	19 429(2)	19 569(10)	19 532(9)
Z	4	4	4
ρ c/g cm^−3	1.702	1.688	1.680
μ/mm^−1	3.905	3.674	4.107
F(000)	9752.0	9768.0	9664.0
Cryst size (mm^3)	0.09 × 0.078 × 0.023	0.12 × 0.07 × 0.06	0.15 × 0.11 × 0.06
θ Range (°)	2.26 to 26.80	2.25 to 28.04	2.25 to 20.39
Limiting indices	−43 ≤ h ≤ 51	−51 ≤ h ≤ 50	−49 ≤ h ≤ 29
	−44 ≤ k ≤ 51	−51 ≤ k ≤ 51	−48 ≤ k ≤ 49
	−15 ≤ l ≤ 14	−15 ≤ l ≤ 12	14 ≤ l ≤ 14
Reflns collected	76 684	68 807	53 438
Ind reflns	21 174 [R(int) = 0.0722]	21 266 [R(int) = 0.1230]	18 181[R(int) = 0.0923]
Completeness to θ (%)	99.9	99.8	100.0
Refinement method	Full-matrix least-squares on F^2	Full-matrix least-squares on F^2	Full-matrix least-squares on F^2
Data/restraints/params	21 174/1/1068	21 266/22/1101	18 181/1/1081
Goodness-of-fit on F^2	1.029	1.028	1.027
Final R indices [I > 2θ(I)]	R 1 = 0.0501	R 1 = 0.0565	R 1 = 0.0676
	wR2 = 0.1065	wR2 = 0.1359	wR2 = 0.1536
R indices (all data)	R 1 = 0.0671	R 1 = 0.0828	R1 = 0.1009
	wR2 = 0.1123	wR2 = 0.1532	wR2 = 0.1799

---

Synthesis

N′-(2-Hydroxy-3-(hydroxymethyl)-5-methylbenzylidene)acetohydrazide (LH₃). To a stirred solution of 2-(hydroxymethyl)-6-carbaldehyde-4-methylphenol (1.50 g, 9.02 mmol) (B2) in 40 mL ethanol, acetyl hydrazide (0.66 g, 9.02 mmol) (B1) was added dropwise over a period of 15 minutes and the resultant yellow colored solution was refluxed for 5 h. Then, the yellow solution was concentrated in vacuo to 15 mL and kept in a refrigerator at 0 °C overnight. A light yellow colored heavy precipitate was obtained which was filtered and washed with cold ethanol as well as diethyl ether before being dried. Yield: 1.6 g (79.8%). Mp: 180 °C. FT-IR (KBr) cm⁻¹: 3398 ν(O–H); 3181 ν(N–H); 1661 ν(C=O); 1624 ν(C=N)_imine; 1516 ν(C=N)py. ¹H NMR (CD₃OD, δ, ppm, 500 MHz): 2.05 (s, 3H, –CH_3acetyl), 2.27 (s, 3H, –CH₃), 4.55 (s, 2H, –CH₂OH), 5.46 (s, 1H, –OH_phen), 7.02 (s, 1H, Ar–H), 7.22 (s, 1H, Ar–H), 8.06 (s, 1H, –NH), 8.17 (s, 1H, imine). Anal. Calcd for C₁₁H₁₄N₂O₃: C, 59.45; H, 6.35; N, 12.60. Found: C, 58.77; H, 6.01; N, 12.19. ESI-MS, m/z: (M + H)⁺. 223.09.

General synthetic procedure for the preparation of complexes 1–3

All the pentanuclear complexes (1–3) have been synthesized according to the following procedure. LH₃ (0.04 g, 0.18 mmol) was dissolved in 40 mL methanol. To this solution, under stirring, LnCl₃·6H₂O (0.23 mmol) was added and the reaction mixture was stirred for 20 minutes at room temperature. At this stage, triethylamine (0.096 mL, 0.73 mmol) was added dropwise and the stirring was continued for a further 10 minutes and pivalic acid (0.028 g, 0.27 mmol) was added dropwise to the mixture. The resulting yellow colored solution was continuously stirred for 12 hours at room temperature. Then, the solution was completely evaporated in vacuo to afford a light yellow colored solid mass which was washed 2–3 times with diethyl ether and dried. The solid mass was re-dissolved in MeOH/CHCl₃ (1 : 1) and kept for crystallization. After about 12 days, needle-shaped yellow colored crystals suitable for X-ray crystallography were obtained by slow evaporation from the solvent mixture. Specific details of each reaction and the characterization data of the products obtained are given below.

[Dy₅(LH)₄(η¹-Piv)(η²-Piv)₃(μ₂–η² η¹Piv)₂(H₂O)]·Cl·9.5H₂O·5MeOH (1). Quantities: LH₃ (0.04 g, 0.18 mmol), DyCl₃·6H₂O (0.087 g, 0.23 mmol), Et₃N (0.096 mL, 0.73 mmol), PivH (0.028 g, 0.27 mmol). Yield: 0.076 g, 63.3% (based on Dy³⁺). Mp: 200 °C (d). IR (KBr) (cm⁻¹): 3433 (b), 2978 (s), 2950 (s), 2604 (s), 2497(s), 2343 (w), 1620 (s), 1574 (s), 1482 (s), 1429 (s), 1397 (s), 1309 (w), 1262 (w), 1228 (w), 1172 (w), 1072 (w), 1037 (s), 897 (w), 851 (w), 808 (s), 608 (w). Anal. Calcd for C₇₉H₁₂₂Cl Dy₅ N₈ O_39.5 (2663.80): C, 35.62; H, 4.62 N, 4.21. Found: C, 34.96; H, 4.33 N, 4.09.

[Tb₅(LH)₄(η¹-Piv)(η²-Piv)₃(μ₂–η² η¹Piv)₂(H₂O)]·Cl·10.5H₂O·2MeOH·2CHCl₃ (2). Quantities: LH₃ (0.04 g, 0.18 mmol), TbCl₃·6H₂O (0.085 g, 0.23 mmol), Et₃N (0.096 mL, 0.73 mmol), PivH (0.028 g, 0.27 mmol). Yield: 0.058 g, 46.03% (based on Tb³⁺). Mp: 200 °C (d). IR (KBr) (cm⁻¹): 3441 (b), 2971 (s), 2930 (s), 2601 (s), 2499(s), 2339 (w), 1610 (s), 1570 (s), 1477(s), 1425 (s), 1393 (s), 1302 (w), 1252 (w), 1235 (w), 1178 (w), 1071 (w), 1032 (s), 895 (w), 854 (w), 803 (s), 610 (w). Anal. Calcd for C₇₈H₁₁₂Cl₇N₈O_37.5Tb₅ (2804.55): C, 33.40; H, 4.03; N, 4.00. Found: C, 32.93; H, 4.10 N, 4.04.

[Ho₅(LH)₄(η¹-Piv)(η²-Piv)₃(μ₂–η²η¹Piv)₂(H₂O)]·Cl·14.5H₂O·2CHCl₃ (3). Quantities: LH₃ (0.04 g, 0.18 mmol), HoCl₃·6H₂O (0.087 g, 0.23 mmol), Et₃N (0.096 mL, 0.73 mmol), PivH (0.028 g, 0.27 mmol). Yield: 0.062 g, 48.4% (based on Ho³⁺). Mp: 200 °C (d). IR (KBr) (cm⁻¹): 3447 (b), 2965 (s), 2933 (s), 2607 (s), 2497(s), 2331 (w), 1603 (s), 1579 (s), 1475(s), 1424 (s), 1390 (s), 1297 (w), 1251 (w), 1233 (w), 1172 (w), 1070 (w), 1029 (s), 899 (w), 844 (w), 798 (s), 611 (w). Anal. Calcd for C₇₆H₁₁₈Cl₇Ho₅N₈O_39.5 (2848.60): C, 32.04; H, 4.18; N, 3.93. Found: C, 31.73; H, 4.23; N, 3.61.

Acknowledgements

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. S. B. thanks the Council of Scientific and Industrial Research, India for the Senior Research Fellowship.

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Footnote

† Electronic supplementary information (ESI) available. Molecular structures of 2 and 3 (Fig. S1–S2), list of bond lengths and bond angles (Tables S1–S2), Cole–Cole plot at zero dc field (Fig. S3), χ′_mvs. T plot at 3000 G (Fig. S4), χ′′_mvs. T at 3000 G (Fig. S5), τ vs. T⁻¹ (Fig. S6), PXRD (Fig. S6–S8), reported Ln5 complexes (Fig. S9) and TGA (Fig. S10–S12). CCDC 1031235–1031237. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5dt03060a

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