Keywords

1 Introduction

In the previous chapter while discussing the complexes containing Co(II)-based SMMs/SIMs, it was noted that the ground state S value is fixed, and the D value is the sole parameter to fine-tune the magnetic behavior. Because of factors such as (1) the small and fixed “S” value associated with Co(II) ions, (2) quenching of orbital angular momentum due to the ligand field, (3) ligand-induced structural distortion, and (4) nuclear hyperfine interaction, faster relaxation mechanism such as QTM can become operative in homometallic Co(II) complexes [1,2,3]. To some extent, these factors can be overcome by employing multidentate ligand or compartmental ligand to link Co(II) along with other suitable lanthanide ions simultaneously in a heterometallic ensemble. This will be the focus of this chapter.

The first lanthanide-based SMM in 2003, a mononuclear [Pc2Tb] complex, phthalocyanine (Pc), has attracted a great interest toward the use of lanthanide ions in SMMs [4]. Accordingly, the first heterometallic SMM, a Cu2Tb2 complex, was reported in 2004 [5]. The heterometallic tetrameric complex was isolated by the reaction of K[CuL] and [TbIII(hfac)3(H2O)2] (1) where H3L = 1-(2-hydroxybenzamido)-2-(2-hydroxy-3-methoxy-benzylideneamino)-ethane. The crystal structure of the complex with molecular formula [CuIILTbIII(hfac)2]2 is shown in Fig. 1. Instead of the [CuL]− precursor, if the analogous [NiL]− precursor [NiIILTbIII(hfac)2]2 (2) is used, where the paramagnetic Cu(II) ion was replaced with diamagnetic NiII affording an opportunity to compare the role of Cu(II) ion in 1.

Fig. 1
figure 1

Line diagram of 1

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Complex 1 shows ferromagnetic interaction between the CuII and TbIII ions with a positive Weiss constant (θ = +14.3 K) as originally proposed by Gatteschi and co-workers [6]. Complex 1 showed SMM behavior [(τ 0) = 2.7 × 10−8 s; U eff = Δ/k B) = 21 K; T B = 1.2 K). However, hysteresis was not observed at the measured temperatures, viz., above 2 K.

Under similar condition, complex 2 reveals a simple paramagnetic behavior that may be due to the magnetic anisotropy and/or intermolecular antiferromagnetic interaction and/or dipolar interaction. AC susceptibility measurement of 2 does not display χ M″ signal which may be due to the fast QTM at zero magnetic field. Possibly the presence of ferromagnetic exchange interaction between Cu(II) and Tb(III) ion is likely the reason for the observed SMM behavior in 1 (H DC = 0).

Based on these early forays, the advantage of using 3d–4f heterometallic complexes were reasoned as: (1) relatively high spin ground state can be achieved using less number of metal ions compared to larger polynuclear 3d metal complexes, and (2) anisotropy can be harvested through the lanthanide ions by exploiting its unquenched orbital angular momentum.

Presence of QTM is a major problem in incorporating lanthanide ion although the single-ion magnetic anisotropy of these ions is generally large as compared to the 3d metal ions. Due to this fact, the blocking temperature remains well below 5 K in majority of the 3d–4f metal complexes [7]. However, this disadvantage can be minimized by enhancing the exchange interaction between 3d and 4f ions. This phenomenon was first reported by Murray and co-workers by enhancing the exchange interaction between the Cr(III) and Dy(III) ion in a heterometallic [CrIII 2LnIII 2(OMe)2(mdea)2(O2CPh)4(NO3)2] (3), LnIII = Pr, Nd, Gd, Tb, Ho, and Er and mdea = N-methyl diethanolaminato(2−) butterfly complex where QTM is significantly reduced/quenched which facilitate in enhancing the blocking temperature [8, 9]. Due to the arrest/quenching of magnetization, opening of a hysteresis loop is generally observed unlike in transition metal clusters (Fig. 2). Similarly, heterometallic Ni2Dy2(4) complex is found to show a similar behavior, where QTM is found to be suppressed completely resulting in a zero-field SMM [10]. The anisotropic barrier extracted for the later complex (19 cm−1) in zero applied DC magnetic field, and the one estimated in the presence of external magnetic field (18.9 cm−1) is found out to be similar indicating that QTM is efficiently suppressed. In both cases (Cr2Dy2 and Ni2Dy2), quenching of QTM is attributed to the presence of enhanced exchange interaction compared to the other 3d–4f complexes reported in the literature. Further, it has been proposed that a larger ∠Ni–O–Dy angle and smaller distortion in the dihedral plane formed by Ni–O–Dy–O are the recipe for increasing the ferromagnetic exchange.

Fig. 2
figure 2

(a) Ball and stick presentation of 3. (b) Magnetization vs field plot with a sweep rate of 0.003 Ts−1. Adapted from Angew. Chem. Int. Ed. 2013, 52, 12014 with permission from John Wiley and Sons. (c). Ball and stick presentation of 4. (d) Frequency-dependent AC susceptibility measurements performed on polycrystalline sample of 4. Adapted from Chem. Eur. J, 2014, 20, 14235 with permission from John Wiley and Sons

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The presence of 3d ion in near vicinity of Ln(III) ion environment is not the only option, but paramagnetic bridging ligands can play a crucial role in increasing the exchange interaction. This has been elegantly proven in a series of Ln2 dimers linked through unusual N2 3− radical ligand (with a blocking temperature of 14 K for the Tb2 analogue) [11, 12].

Since several 3d–4f metal complexes are known in the literature, we will restrict to Co(II)/4f SMM reported in the literature in this chapter. We will also discuss some examples of Co(III)/4f complexes. Before this a brief introduction on the nature of interaction between the 3d and 4f metal ions is in order.

To ascertain qualitatively the nature of exchange interaction between the 3d and the 4f metal ion, Andruh et al. proposed an empirical approach by considering Ni–Ln (Ln = Dy or Pr) dimeric complexes [13]. In such complexes, the total magnetic moment experimentally observed is the combination of magnetic moment contribution from individual metal ions (e.g., nickel and Ln(III) ion) along with the exchange couple state. Hence, by subtracting the individual metal ion contribution from the total magnetic moment, the masked nature of interaction will be clearly reflected by plotting the temperature-dependent Δχ MT value.

The empirical equation is

$$ \Delta {\chi}_{\mathrm{M}}T={\chi}_{\mathrm{M}}{T}_{\mathrm{Ni},\mathrm{Dy}}-{\chi}_{\mathrm{M}}{T}_{\mathrm{Zn},\mathrm{Dy}}-{\chi}_{\mathrm{M}}{T}_{\mathrm{Ni},\mathrm{Lu}}=\sim {J}_{\mathrm{Ni}-\mathrm{Dy}} $$

For example, the presence of ferromagnetic exchange interaction observed between Ni(II) and Dy(III) complexes in Ni2Dy2 tetrameter is revealed using the empirical equation shown above.

For a system with ferromagnetic interaction, the Δχ MT plot will raise at low temperature in positive direction, while for an antiferromagnetic interaction, the plot will plunge into negative Δχ MT value. The general trend noticed in case of Cu(II)–Ln or Ni(II)–Ln complexes are: (1) a ferromagnetic exchange interaction is observed if Ln(III) valence shell contains ≥f7 electrons, and (2) an antiferromagnetic coupling exists if Ln(III) valence shell electron become less than 7. This scenario is witnessed in many such complexes, which is very well exemplified [13]. We have noticed recently that a similar trend is also observed in Co(II) containing 4f complexes. Hence, targeting Co(II)–Ln(III) (where LnIII≥f7) is an ideal approach to reveal a new generation of SMMs. Accordingly, various Co(II)/(III)–Ln(III) SMMs reported in literature have been overviewed below.

2 Hybrid Co–4f Complexes as SMMs

This section deals with various examples on heterometallic Co(II)/Ln(III) and Co(III)/Ln(III) complexes. In the case of Co(III)/Ln(III) complexes, the magnetic properties are entirely due to the lanthanide ion.

Based on the above insight, several heterometallic 3d/4f complexes were investigated [7, 14,15,16,17,18,19,20,21,22,23]. The first Co/Ln SMM, [L2CoII 2Gd][NO3] (5), was reported by Chandrasekhar and co-workers. The complex was assembled using a phosphorus-based tris-hydrazone ligand (LH3) and contains a linear array of metal ions [24] (Fig. 3).

Fig. 3
figure 3

Line diagram of 5 along with the ligand

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The zero-field SMM behavior of this complex was confirmed by AC susceptibility measurements (Fig. 4): U eff = 27.2 K and τ 0 = 1.7 × 10−7 s.

Fig. 4
figure 4

Temperature (top) and frequency (bottom) dependence of the in-phase and out-of-phase AC susceptibility measurements under zero applied DC field. Reprinted with permission from (Inorg Chem. 2009, 48, 1148–1157), Copyright (2009) American Chemical Society

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Several other structurally analogous trinuclear complexes {[L2CoII 2Ln][X]} [Ln = Eu, X = Cl; Ln = Tb, Dy and Ho, X = NO3] were also prepared, all of which except the EuIII analogue were shown to be SMMs [25]. Table 1 summarizes the magnetic data for all of these complexes.

Table 1 Magnetic data for [L2CoII 2Ln]+ SMMs
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Following these first examples, there have been several studies on such heterometallic Co(II)/Ln(III) and Co(III)/Ln(III) complexes. In the subsequent sections, we will discuss these based on the nuclearity of the complexes. Only such complexes will be discussed where there has been a demonstration of SMM behavior.

2.1 Dinuclear Complexes

The preparation of the heterometallic complexes discussed in this and subsequent sections is dependent on the use of the so-called compartmental ligands which have specificity toward either the transition metal ion or the lanthanide metal ion.

A cyanido-bridged complex, [{DyIII(3-OHpy)2(H2O)4][CoIII(CN)6}] (9), was reported by the self-assembly reaction involving DyIII–3-hydroxypyridine (3-OHpy) complexes with hexacyanidocobaltate(III). This complex, which can be considered as single-ion magnet, shows SMM behavior with a high U eff of 266 cm−1(≈385 K) and a τ 0 = 3.2 × 10−11 s above 23 K at H DC = 0 Oe. Moreover, magnetization hysteresis loops are observed below 6 K with a field sweep rate of 10 Oe s−1 [26].

In contrast to the above, a CoII/YIII complex, [CoII(μ-L)(μ-OAc)Y(NO3)2] (10), was prepared using a compartmental ligand N,N′,N″-trimethyl-N,N″-bis(2-hydroxy-3-methoxy-5-methylbenzyl)diethylenetriamine (H2L) [27] (Fig. 5).

Fig. 5
figure 5

Line diagram of the complex 10 along with the ligand

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Although these complexes do not show zero-field SMM behavior, AC measurements at H DC = 1,000 Oe revealed them to be SMMs. An effort was made to modulate the structural features by varying the bridging ligand which did not result in any significant change in the magnetic properties. An interesting aspect of these complexes is that all of them have been shown to have a positive D and in spite of this they exhibit a field-induced SMM behavior, rather intriguingly [28]. Rationale for the observation of field-induced slow relaxation of magnetization with easy plane anisotropy was explained in the previous chapter.

Another family of dinuclear Co–Ln complexes, [CoIILnIII(L)(DBM)3] [Ln = Y (11), Dy (12) and Gd (13)], is known; the ligands used were N,N′-dimethyl-N,N′-(2-hydroxy-3-methoxy-5-methyl-benzyl)ethylenediamine (LH2) and the anion of 1,3-diphenyl-propane-1,3-dione (DBM−) [29] (Fig. 6).

Fig. 6
figure 6

Line diagram of complexes 11−13 along with the ligand

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These complexes also reveal a positive D (S = 3/2, g = 2.39, D = 10.3 cm−1 and E = 4 × 10−4 cm−1 for CoII−Y analogue); the latter reveals a field-induced single-molecule magnet (SMM) behavior (Fig. 7).

Fig. 7
figure 7

Line diagram of complexes 14 and 15 and the corresponding ligand

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MII−Ln binuclear complexes, [MII(3-MeOsaltn)(MeOH)(OAc)Ln(hfac)2] (MII = Co, Ni, Cu and Zn; Ln = GdIII, TbIII, DyIII, LaIII) were prepared by using N,N′-bis(3-methoxy-2-oxybenzylidene)-1,3-propanediaminato (3-MeOsaltn) and hexafluoroacetylacetonato (hfac) [30]. The MII−Ln magnetic interactions are ferromagnetic when MII = (CuII, NiII, and CoII) and Ln = (GdIII, TbIII, and DyIII). The D value was found to be positive for the CoII/La analogue. These complexes however did not display zero-field SMM behavior.

Table 2 summarizes the magnetic data for some dinuclear Co(II)/Ln(III) complexes.

Table 2 Magnetic properties of dinuclear [Co–Ln] SMMs
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2.2 Trinuclear Cobalt–Lanthanide SMMs

In contrast to the trinuclear complexes described above involving a phosphorus-supported ligand, another series, [CoIII 2Dy(L)2(μ-O2CCH3)2(H2O)3](NO3) (22) (LH3 = 2-methoxy-6-[{2-(2-hydroxyethylamino)ethylimino}-methyl]phenol), is known. This complex showed slow relaxation of magnetization at 1,000 Oe applied DC field [(U/k B) = 88 K; (τ 0) = 1.0 × 10−8 s) [35] (Fig. 8).

Fig. 8
figure 8

Line diagram of 22 and the corresponding ligand

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In these examples, the analogous Tb(III) complex (23) has a lower U eff = 15.6 K. It has been suggested that this may be due to the fact that while Dy(III) is a Kramers ion, the integer m j level of Tb(III) is likely to trigger the ground state tunneling [36].

[CoIILn2 III] complexes, [LnIII 2CoII(C7H5O2)8] [Ln = Dy (24) and Tb (25)] containing an in situ generated salicylaldehyde as the ligand, have been prepared [37] (Fig. 9).

Fig. 9
figure 9

Line diagram of complexes 24 and 25

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Both 24 and 25 display SMM behavior at zero DC field, although 25 does not show a clear maxima in the χ′′ vs T plot. For 24, two relaxation processes could be delineated: relaxation at the higher temperature region (above 5 K) being suggested as being associated with the excited Kramer doublets of individual DyIII ions, while at the low temperature region (below 5 K), the weak coupling between CoII and DyIII appears to predominate [38].

Complexes containing Co(III), [CoIII 2Dy(hmb)2(CH3O)2(OAc)3] [Ln = Dy (26) and Lu (27)], could be prepared using 2-hydroxy-3-methoxybenzylidene benzohydrazide (H2hmb) [39] (Fig. 10).

Fig. 10
figure 10

Line diagram of complexes 26 and 27 along with the ligand

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Frequency-dependent AC susceptibility measurements for 26 at 500 Oe applied DC field provide the energy barrier (U eff) = 5.5 K and τ 0 = 2.7 × 10−5 s.

The magnetic properties of trinuclear Co(II)/Ln(III) and Co(III)/Ln(III) SMMs are summarized in Table 3.

Table 3 Magnetic data of trinuclear Co(II)−Ln(III) SMMs
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2.3 Tetranuclear Cobalt–Lanthanide SMMs

A [CoII 2Dy2(L)4(NO3)2(THF)2] (39) complex having a butterfly/defect-dicubane topology was assembled using 2-[(2-hydroxy-phenylimino)-methyl]-6-methoxyphenol) (H2L) [45] (Fig. 11).

Fig. 11
figure 11

Line diagram of complex 39 along with the ligand

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Analysis of the frequency-dependent AC measurements in zero DC field revealed the presence of two thermally activated relaxation regimes [(U eff) of 11.0 cm−1 (15.8 K); τ 0 = 7.7 × 10−4 s in the temperature range 1.6–8 K and (U eff) of 82.1 cm−1 (118.12 K); τ 0 = 6.2 × 10−7 s between 18 and 22 K]. Interestingly, this complex shows hysteresis below 3 K at a sweep rate of 235 mTs−1 (Fig. 12). The coercivity of the hysteresis loops increases with decreasing temperature and increasing field sweep rate. The loops display steplike features below 1.5 K, indicating the possibility of resonant QTM below this temperature.

Fig. 12
figure 12

Temperature-dependent magnetic hysteresis loops for 39 below 4 K with a sweep rate of 235 mTs−1. Adapted from Angew. Chem. Int. Ed. 2012, 51, 7550–7554 with permission from John Wiley and Sons

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Replacement of the solvent molecules coordinated with the Co2+ centers to form [CoII 2Dy2(L)4(NO3)2(MeOH)2] (40) and [CoII 2Dy2(L)4(NO3)2(DMF)2] (41) did not affect the compounds from being SMMs [46]. An analogous Zn2Dy2 (42) complex has also been assembled. A comparison of the magnetic properties in the complexes 39–42 is given in Table 4 (Fig. 13).

Table 4 Comparison of energy barriers for complexes [Co2Dy2] (39–41) with the analogous [Zn2Dy2] (42)
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Fig. 13
figure 13

Line diagram of complexes 40 and 41

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A tetranuclear complex [CoII 2Dy2(L)4(NO3)2(DMF)2] (43) possessing a butterfly/defect-dicubane topology such as described above could be obtained by the use of (E )-2-ethoxy-6-(((2-hydroxyphenyl)imino)methyl)phenol (H2L) [47] (Fig. 14).

Fig. 14
figure 14

Molecular structure complex 43 along with the ligand. Adapted from Ref. [47] with permission from The Royal Society of Chemistry

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The magnetic properties of the [CoII 2Dy2] analogue and the analogous [Dy2Zn2(L)4(NO3)2(CH3OH)2] (44) and [Dy2MnIII 2(L)4(NO3)2(DMF)2] (45) reveal that they are SMMs (Table 5).

Table 5 Comparison of the AC magnetic data for [CoII 2Dy2] with analogous [MII 2Dy2] (M = Mn and Zn)
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The range of ligands that can afford tetranuclear complexes seem to be quite large. Thus, the complexes [CoII 2Ln2(Hhms)2(CH3COO)6(CH3OH)2(H2O)2](NO3)2[Ln = DyIII (46), GdIII (47), and YIII (48)] could be prepared by using (2-hydroxy-3-methoxybenzylidene)-semicarbazide (H2hms) [48] (Fig. 15).

Fig. 15
figure 15

Line diagram of complexes 46−48 along with the ligand

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Complex 46 shows temperature as well as frequency-dependent out-of-phase (χ″) signals (τ 0 = 6.4 × 10−6 s; U eff = 6.7 K at zero DC field; τ 0 = 3.2 × 10−6 s and U eff = 13.8 K at H DC = 800 Oe in the range 2.0–5.5 K). Theoretical CASSCF calculation studies revealed that the Dy–Dy interactions are largely ferromagnetic and dominant, while the exchange coupling (J exch) of Dy–Co in {CoII 2DyIII 2} is antiferromagnetic. Interestingly, in the analogous {NiII 2DyIII 2}(49) complex, ferromagnetic exchange between NiII and DyIII ions is found which is more conducive to zero-field single-molecule magnet behavior. The magnetic properties of tetranuclear complexes are summarized in Table 6.

Table 6 Magnetic properties of representative tetranuclear [Co2Ln2] SMMs
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Many tetranuclear complexes could also be assembled by the use of ethanolamine ligands. Thus, the complexes, [CoIII 2LnIII 2(OH)2(bdea)2(acac)2(NO3)4] [Ln = Tb (59) and Dy(60)] and bdeaH2 = n-butyldiethanolamine) containing two Co(III) ions, were prepared [57] (Fig. 16).

Fig. 16
figure 16

Line diagram of complexes 59 and 60 along with the ligand

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Analysis of the AC susceptibility data for 60 allowed the extraction of the following parameters: U eff = 169 K and τ o = 1.47 × 10−7 s above 20 K where the relaxation is thermally activated. As the temperature is decreased, a slight curvature appears in the Arrhenius plot of ln(τ) vs 1/T but does not become temperature independent at any point, indicating that a pure quantum regime is not observed within the timescale and temperature range of experiment. In contrast to complex 60, 59 does not show SMM characteristics at zero DC field. However, upon application of 5,000 Oe DC field, a frequency-dependent maxima in the plot of χ M′′ vs T is seen. This phenomenon is a common feature for non-Kramers TbIII-based complexes and is due to fast zero-field quantum tunneling of the magnetization between the sublevels. The non-Kramers ion generally allows the direct mixing of opposing projections of the ground state angular momentum/spin projections by the crystal field, so that tunneling pathways become readily accessible [58,59,60,61,62,63].

Other examples of tetranuclear heterometallic complexes [{LnIII 2CoIII 2(OMe)2 (teaH)2(O2CPh)4(MeOH)4}(NO3)2][LnIII 2CoIII 2(OMe)2(teaH)2(O2CPh)4(MeOH)2 (NO3)2] [Ln = Gd (61), Tb (62) and Dy (63)] were prepared using triethanolamine (teaH3). Interestingly two tetranuclear units containing [LnIII 2CoIII 2(OMe)2 (teaH)2(O2CPh)4(MeOH)4](NO3)2 and [LnIII 2CoIII 2(OMe)2(teaH)2(O2CPh)4(MeOH)2 (NO3)2] are present within the same crystal [64] (Fig. 17).

Fig. 17
figure 17

Line diagrams of complexes 61−63 and the corresponding ligand

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AC susceptibility measurements in a zero DC field reveal the SMM behavior for the DyIII analogue with the following characteristics above 8.5 K: (U eff) of 88.8 K (~61 cm−1) and τ 0 = 5.64 × 10−8 s. But below 8.5 K, the Arrhenius plot deviates slightly from linear behavior indicating the existence of QTM. However, applying field up to 1,000 Oe does not change significantly the peak maxima in the χ M″ vs T plot, indicating that QTM is inefficient in this system.

Among other tetranuclear complexes assembled using triethanolamine as the ligand, containing two Co(III), are [DyIII 2CoIII 2(OMe)2(teaH)2(O2CPh)4(MeOH)4](NO3)2 and [DyIII 2CoIII 2(OMe)2(teaH)2(O2CPh)4(MeOH)2(NO3)2] (63), [DyIII 2CoIII 2 (OMe)2(dea)2(O2CPh)4(MeOH)4](NO3)2 (64), [DyIII 2CoIII 2(OMe)2(mdea)2(O2CPh)4 (NO3)2] (65), [DyIII 2CoIII 2(OMe)2(bdea)2(O2CPh)4(MeOH)4](NO3)2, and [DyIII 2CoIII 2 (OMe)2(bdea)2(O2CPh)4(MeOH)2(NO3)2] (66) (teaH3 = triethanolamine, deaH2 = diethanolamine, mdeaH2 = N-methyldiethanolamine, and bdeaH2 = N-n-butyldiethanolamine). The extracted magnetic parameters, from the AC measurements of these complexes, are summarized in Table 7 [65].

Table 7 Magnetic data for 63–66
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In addition to the aforementioned complexes, complex [CoIII 2DyIII 2(OMe)2(teaH)2(Piv)6] (67) can also be prepared using triethanolamine ligand. This complex displays SMM behavior with U eff = 51 K; τ 0 = 6.1 × 10−7 s and τ QT = 7.3 s in the range 4.5–7.5 K [U eff = 127 K; τ 0 = 1.2 × 10−9 s; C Ram = 1.7 × 10−3 in the range of 7.5–9.5 K] [66] (Fig. 18).

Fig. 18
figure 18

Line diagram of complex 67

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For this complex, the energy level splitting under crystal field of the DyIII ground J = 15/2 state was determined (Fig. 19). The thermal barrier for the fast relaxation pathways through m J = ±13/2 and m J = ±11/2 from ground state should be 39 and 104 cm−1. These values compare quite well with the experimental U eff = 35 cm−1 (51 K) and 88 cm−1 (127 K) values obtained from AC data (Fig. 20).

Fig. 19
figure 19

Energy level splitting under crystal field of the DyIII ground J = 15/2 state, with crystal field parameters, B 0 2 = −2.4 cm−1 B 0 4 = 2.9 × 10−3 cm−1. Arrows indicate the suggested relaxation pathways across the barrier. Adapted from Ref. [66] with permission from The Royal Society of Chemistry

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Fig. 20
figure 20

Energy level splitting under crystal field of the DyIII ground J = 15/2 state, with crystal field parameters, B 0 2 = −2.4 cm−1 B 0 4 = 2.9 × 10−3 cm−1 and exchange interaction J exc = −0.046 cm−1. Arrows indicate the suggested relaxation pathways across the barrier. Doublets g z eff values between parentheses. Adapted from Ref. [66] with permission from The Royal Society of Chemistry

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A summary of magnetization relaxation dynamics for this [CoIII 2LnIII 2] family (67−70) is shown in Table 8 [67].

Table 8 Magnetization dynamics data of complexes [CoIII 2LnIII 2] [67]
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N-n-butyldiethanolamine (bdeaH2) and N-methyldiethanolamine (mdeaH2) were used as ligands for preparing [Co2 IIIDy2 III(OMe)2(O2CPh-2-Cl)4(bdea)2(NO3)2] (71), [Co2 IIIDy2 III(OMe)2(O2CPh-4-tBu)4(bdea)2(NO3)(MeOH)3](NO3) (72), [Co2 IIICoIILnIII(OH)(O2CPh-4-OH)(bdea)3(NO3)3(MeOH)] [Ln = Dy (73), Gd (74)], [Co2 IIIDy2 III(OMe)(OH)(O2CPh-2-CF3)4(bdea)2(NO3)2] (75), and [Co2 IIIDy2 III(mdea)4(hfacac)3(O2CCF3)(H2O)] (76) [68]. A summary of magnetization relaxation dynamics of these complexes (71−76) is enlisted in Table 9.

Table 9 Magnetization relaxation parameters for complexes 71–76
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Similarly a series of SMMs [CoIII 2LnIII 2(μ 3-OH)2(o-tol)4(mdea)2(NO3)2] [Ln = Dy (77), Tb (78), Ho (79)] [69], [DyIII 2CoIII 2(OH)2(teaH)2(acac)6] (80), [DyIII 2CoIII 2(OH)2(bdea)2(acac)6](81), and [DyIII 2CoIII 2(OH)2(edea)2(acac)6] (82) (teaH3 = triethanolamine, bdeaH2 = N-n-butyldiethanolamine, edeaH2 = N-ethyldiethanolamine and acacH = acetylacetone) [70] are reported. The detailed parameters associated with their SMM behavior are summarized in Table 10 (Fig. 21).

Table 10 Magnetization relaxation parameters on heterometallic {Co2 IIIDy2 III} butterfly SMMs, with the DyIII ions in the body position, constructed with various ethanolamine-based ligands
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Fig. 21
figure 21

Line diagram of complexes 77−79 along with the ligand

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2.4 Higher Nuclearity Cobalt−Lanthanide SMMs

In this section we will deal with complexes whose nuclearity is greater than 4. Only representative examples will be discussed. The magnetic data for these complexes are tabulated in Table 11. A hexanuclear complex [Dy4CoIII 2(HL2)2(μ 3-OH)2(piv)10(OH2)2] complex (86) was prepared by the use of 2-(2,3 dihydroxpropyliminomethyl)-6-methoxyphenol(H3L2) and pivalic acid as ligands. The molecule contains two dimeric Dy(III) sub-units on either side of a dimeric Co(III) motif. Each of the Co(III) centers along with a Dy(III) is involved in a defect cubane structural motif [83] (Fig. 22).

Table 11 Magnetic properties of high nuclearity CoII/LnIII SMMs
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Fig. 22
figure 22

Molecular structure of complex 86 along with the ligand. Adapted from Ref. [83] with permission from The Royal Society of Chemistry

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The field dependence of magnetization shows a rapid increase of M values at lower DC field, indicating the presence of intramolecular ferromagnetic interactions between spin carriers. The Arrhenius plot obtained from the frequency-dependent AC susceptibility measurements provides the signature of SMM with an energy gap (U eff) of 18.4 cm−1 (26.47 K) and a pre-exponential factor τ 0 = 8.7 × 10−6 s at H DC = 0. The Cole–Cole plot provides the α value within the 0.19–0.13, indicating a single relaxation time is mainly involved and is independent of the temperature.

Two octanuclear complexes, [CoIII 4Dy4(μ-OH)4(μ 3-OMe)4{O2CC(CH3)3}4 (tea)4(H2O)4] (87) and [CoIII 4DyIII 4(μ-F)4(μ 3-OH)4(o-tol)8(mdea)4] (88) (tea3− =  triply deprotonated triethanolamine; mdea2− = doubly deprotonated N-methyldiethanolamine; o-tol = o-toluate), have been recently reported. The central core of the octanuclear ensemble consists of a [Dy(III)]4 motif and is surrounded by four Co(III) ions. Like in the previous case, each of the Co(III) along with two Dy(III) centers is involved in a defect cubane motif [84] (Fig. 23).

Fig. 23
figure 23

Molecular structure of complex 88 along with the ligand. Adapted from Chem. Eur. J. 2017, 23, 1654–1666 with permission from John Wiley and Sons

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Complex 87 reveals frequency-dependent “tails” in the out-of-phase susceptibility against temperature plots below 3 K at H DC = 0 Oe. This behavior does not improve even after application of fields up to 5,000 Oe. But for complex 88, at H DC = 5,000 Oe, the corresponding energy barrier U eff = 39 cm−1 and pre-exponential factor τ 0 = 1.0 × 10−6 s can be obtained between 8 and 10.5 K.

A dodecanuclear complex [CoII 2Dy10(L)4(OAc)16(SCN)2(CH3CN)2(H2O)4 (OH)2(μ 3-OH)4][Co(SCN)4(H2O)]2 (89) was assembled by using the multifunctional ligand, 1,2-bis(2-hydroxy-3-methoxybenzylidene) hydrazine (H2L). In contrast to the examples discussed above, this complex contains Co(II) [85] (Fig. 24).

Fig. 24
figure 24

Molecular structure of complex 89 along with the ligand. Adapted from Ref. [85] with permission from The Royal Society of Chemistry

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The nature of the CoII–Dy and Dy–Dy interactions could not be delineated with certainty. However, the authors, based on the AC susceptibility measurements, suggest that this complex has a SMM behavior.

3 Summary

Co(II) is a promising 3d metal ion with first-order orbital contribution that has been investigated for its interesting magnetic properties. The combination of Co(II) and lanthanide ions in the form of heterometallic complexes leads to an interesting array of complexes where the role of the ligand seems to be extremely crucial in modulating the nuclearity and the coordination geometry. While there has been considerable progress in this field, it is anticipated that appropriate design of complexes can lead to SIMs and SMMs with even better properties. One crucial element that is missing from the studies carried out so far seems to be a strong theoretical input. Once such an understanding is in place, it becomes easier for synthetic chemists to make appropriate designs for assembling SMMs with superior properties.