Keywords

1 Introduction

The field of SMM which was first pioneered by the discovery of a dodecanuclear [Mn12O12(CH3CO2)12(H2O)4]·4H2O·2CH3COOH [1] (1) complex has sparked a worldwide research interest based on their multifarious application in the field of storage device [2], quantum computing [3], spintronic devices [4] and magnetic refrigeration [5], and others. The success (or not) of an SMM depends on two main parameters which are effective energy barrier Ueff and blocking temperature TB [6]. The value of TB is governed by the height of the anisotropy barrier which is regulated by ground-state spin S and negative anisotropy D. Later on theoretical studies have shown that large S and strong D cannot be simultaneously observed within one molecule [7]. This led to the blossoming of Ln-based SMM with the pioneering work of double-decker phthalocyanine (Pc) complexes [LnPc2]− (LnIII = Tb, Dy) [8], where Ln ions provide large magnetic anisotropy to the system. Using Ln ions remarkable energy barrier as high as 1837 K [9] and blocking temperature of 80 K [10] are reported. However, one major drawback of these LnIII-based complexes is a decrease in blocking temperature because of quantum tunneling of magnetization (QTM) [11]. The main challenge for the researcher is to overcome QTM which can be reduced by the introduction of exchange interaction in complex, thereby increasing the exchange coupling interaction (J) which enhances the degree of covalency of the metal–ligand bond [12]. For that reason, 3d-4f heterometallic architecture induces strong interaction between metal ions where magnetic exchange between the metal center occurs via super exchange mechanism through diamagnetic ligand which reduces the QTM and enhances Ueff and TB. In that respect ligand designing plays an important role. An axial ligand field is required for the ions that possess largest angular momentum belongs from oblate states like TbIII, DyIII, and on the other hand, equatorial field is required for prolate states like ErIII and YbIII [13]. So, to overcome the QTM several methods are applied; one of them includes synthesizes of heterometallic complexes. The first heterometallic metal complex SMM synthesized was a Cu2Tb2 complex whose structure is shown in Fig. 1 [14]. Later on, a variety of 3d metal ions were used owing to their individual characteristics. For example, Murray and group have reported a heterometallic Cr2IIILn2III complex where strong magnetic interaction between CrIII and LnIII metal ions, QTM was significantly quenched, thereby enhancing the blocking temperature TB [15]. Another aspect of heterometallic complexes is that by using several 3d metal ions one can elevate the crystal field present in lanthanide ions and thus can increase the energy barrier [16]. One of the main categories is cobalt lanthanide-based SMM. For CoII ion, the magnetic anisotropy does not depend on first-order orbital angular momentum, however, based on second-order orbital angular momentum [17, 18]. This mixing of ground state to excited state is regulated by the geometry of the system. Further, the D value of the cobalt ion can also be tuned by utilizing certain multidentate ligands to link CoII ions with other LnIII ions. This becomes the main focus of this chapter where recent examples of Co(II)/Ln(III) are used to depict the importance of ligand design and coordination geometry on the magnetic properties. The later sections will deal with very recent examples based on their nuclearity.

Fig. 1
A 3-D molecular structure of C u 2 T b 2 complex with elements of T b, C u, F, O, N, and C.

Ball and stick diagram of Cu2Tb2 complex

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2 Co/Ln Coordination Clusters with Various Nuclearity

2.1 Dinuclear Complexes

Ray and group have utilized a simple ligand LH = 3-methoxy-N-(2(methylsulfanyl) phenyl)salicylaldimine) to synthesize a new dinuclear CoII/LnIII complexes [LnIIICoIIL2(NO3)3]·H2O {LnIII = La(2), Gd(3), Tb(4), Dy(5), Ho(6)} whose detailed experimental and theoretical properties are studied [19]. The ligand used is chosen due to its coordination pockets which consists of coordination pockets favoring the binding of both CoII and LnIII ions simultaneously. Also, the binding of lanthanide metal ion provides a distortion to the geometry of CoII ion which is further favored due to the presence of–SM e group at axial positon which due to its large size creates a large deviation in bond parameters. Thus, this feature makes this CoII–LnIII family the first example of a dinuclear heterometallic complex having a distorted octahedral CoII metal that exists in between fac and mer geometric isomers. These complexes are formed by the action of the ligand with LiOH in acetonitrile; to which CoCl2. 6H2O was added and stirred for three hours which was filtered to give red colored crystals. The neutral complex comprises of two ligand L− bridging metal centers with two phenoxido bridges. Magnetic analysis reveals ferromagnetic interaction in compounds 3–6 between the Co–Ln ions. Theoretical calculations show 2 and 3 as weak SIM (single ion magnet) and SMM, respectively, solely based on anisotropy of CoII ion. Kou and group [20] have synthesized a CoII/LnIII [Co(H0.5L)Dy(DBM)2(H2O)](ClO4)0.5·3H2O (7) chiral complex where the enantiomer of 7 is obtained in the same pot. The complex 7 consists of a cationic structure [Co(H0.5L)Dy(DBM)2(H2O)]0.5+ with one ligand, CoII ion, DyIII ion along with two DBM− molecules. The experimental χMT value of 7 is similar to that of the theoretical value for one DyIII ion. Ac magnetic susceptibility measurement under 2000 Oe shows an energy barrier of 53.1 K and τ0 = 2.0 × 10−8 s. Further, the experimental energy barrier matches well with the theoretical value and depicts that QTM between two ground states was suppressed in fitting temperature range and the relaxation mechanism in 7 is governed by Orbach and Raman process (Fig. 2). Chandrasekhar and group [21] have used a ferrocene-based ligand to synthesize four complexes [Co(μ-L)(μ-CCl3COO)Y(NO3)2]·2CHCl3·CH3CN·2H2O (8), [Co(μ-L)(μCH3COO)Y(NO3)2]·CH3CN (9), [Co(μ-L)(μ-PhCOO)Y(NO3)2]·3CH3CN·2H2O (10), and [Co(μ-L)-(μ-tBuCOO)Y(NO3)2]·CHCl3·2H2O (11); all have them having CoIIYIII core. The complex was synthesized by the addition of ligand LH2 in CHCl3/MeOH to Co(ClO4)2 and Y(NO3)3 along with base triethylamine. For complex 9, Co(OAc)2 is used instead of Co(ClO4)2 and the solution is refluxed for three hours. The experimental χMT value of all complexes is comparable with that of theoretical ones. Ac magnetic susceptibility measurement for all complexes was studied at 1200 Oe. The data from Arrhenius plot reveals energy barrier and τ0 of 8.4(6) K and 3.2(4) × 10−6 s for 8; Ueff of 11.0(4) K and τ0 = 2.5(2) × 10−6 s for 9; Ueff of 13.7(8) K and τ0 = 2.6(4) × 10−6 s for 10 and Ueff of 18.7(6) K and τ0 = 7.4(9) × 10−7 s for 11. The Debye model reveals a single relaxation process for 8–10.

Fig. 2
Two parts. Part a, a 3-D molecular structure with elements of D y, C o, O, H, N, and C. Part b, has a graph for the temperature dependence of relaxation times. The horizontal axis represents temperature ranging from 0.15 to 0.40 in increments of 0.05 while the vertical axis represents l n ranging from minus 9 to minus 4 in increments of minus 1. Lines of experimental data, Orbach, Orbach plus Raman, and Raman have an increasing trend.

a Ball and stick diagram of 7. b Temperature dependence of relaxation time for 7

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2.2 Trinuclear Complexes

The first CoII/LnIII SMM [L2Co2Gd][NO3]·2CHCl3 (12) was synthesized by Chandrasekhar and group using a phosphorus-based tris hydrazone ligand where all metal ions Co–Gd–Co are arranged in a linear manner [22]. The structure involves two ligands holding together three metal ions along with one mole of nitrate ion. Ac magnetic susceptibility measurement shows an energy barrier of 27.2 K and τ0 = 1.7 × 10−7 s confirming the SMM behavior of the complex (Fig. 3).

Fig. 3
Two parts. Part a, has a 3-D molecular structure with elements of Gd, C o, S, P, N, O, and C. Part b, has a graph of temperature inverse versus torque plot. The horizontal axis represents temperature ranging from 0.2 to 0.55 in increments of 0.05 while the vertical axis represents torque per second ranging from 10 to the power minus 5 to 10 to the power minus 1 in increments of 10 to the power minus 1. It has a slant line drawn from the bottom left to the top right with points plotted on it.

a Ball and stick diagram of 12. b τ versus T−1 plot at Hdc = 0

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Nguyen and group have synthesized a stable trinuclear complex [LnCo2(L)2(μ1,3OOCCH3)2X] where LnIII = La (13), Ce(14), Nd(15), Sm(16), Gd(17), Dy(18), Er(19), and Yb(20) and X = κ2-CH3COO− or Cl− using one-pot reaction of 2,6-dipicolinoylbis(N, Ndiethylthiourea) with cobalt acetate and lanthanide chloride in methanol along with base trimethylamine [23]. The structure consists of two ligands with two moles of cobalt ion and one lanthanide ion along with two acetate ions acting as bridging mode between CoII and LnIII ion. Further, coordination around LnIII ion is provided by acetate ion for 13, 17, while rest of complex is coordinated by chloride ion. Magnetic studies reveal weak antiferromagnetic interaction with JCo–Co = −0.49 cm−1. Further, CoDyCo analogue shows antiferromagnetic interaction while rest of the CoLnCo analogues shows ferromagnetic interaction. Papatriantafyllopoulou and group have employed di-2-pyridyl ketone, (py)2CO to prepare four CoII/LnIII clusters [Co2Ln{(py)2C(OEt)(O)}4(NO3)(H2O)]2[M(NO3)5](ClO4)2 (LnIII = Gd (21), Dy (22), Tb (23), Y (24)) [24]. The cationic part of the complex consists of two CoII ion, one GdIII ion along with four ligands (py)2C(OEt)(O)−. Weak ferromagnetic interaction of JCo–Co =  +1.3 and +0.40 cm−1 is observed for 21 and 24, respectively. Ac magnetic susceptibility measurement reveals a weak out-of-phase signal which might depict complex 22 being a weak SMM.

2.3 Tetranuclear Complexes

Ray and group have reported two families of CoII/III–LnIII [LnIII2CoIII2L2(N-BuDEA)2(O2CCMe3)4(H2O)2] (Ln = Gd (25), Tb (26), Dy (27)) and pentanuclear LnIII2CoIICoIII2 L2(N-BuDEA)2(O2CCMe3)6(MeOH)2 (Ln = Dy (28), Ho (29)) using ligands H2L (o-vanillin oxime) and N-BuDEAH2 (N-butyldiethanolamine) (Fig. 4a) [25]. The tetranuclear series is formed by the reaction of Co2(μ-OH2)(O2CCMe3)4(HO2CCMe3)4 and Ln(NO3)3 with N-BuDEAH2 followed by addition of ligand and base in ratio 0.5:1:1:1:4 in MeOH/DCM. The tetranuclear complex consists of {LnIII2CoIII2} core with two ligands and two N-BuDEA2− anions. With the J value of −0.09 cm−1, the presence of weak antiferromagnetic exchange interaction between GdIII centers is reported. No maxima peaks are reported for 27 in Ac magnetic susceptibility measurement. Dong and group have synthesized a linear [Ln2Co2(3,4-DCB)10(2,2′bpy)2] (LnIII = Nd (30), Sm (31), Eu (32), Gd (33), Tb (34), Dy (35), and Er (36)) complex using 3,4-dichlorobenzoic acid (3,4-HDCB), 2,2-bipyridine (2,2′-bpy) as ligands (Fig. 4b) [26]. The structural analysis reveals complexes having linear arrangement CoII–LnIII–LnIII–CoII formed with two CoII and DyIIIions each, two 2,2′-bpy co-ligands and ten 3,4-DCB anions. Ac magnetic susceptibility measurements at zero dc field of 35 reveal frequency-dependent out-of-phase signal; however, no maxima peak is observed. Li and group have reported tetranuclear complexes [Ln2Co2(hfac)10(NITPhPybis)2] [LnIII = Gd (37), Tb (38), Dy (39), and Ho (40); formed using nitronyl nitroxide biradical ligands having pyridine groups which grasps CoII and LnIII ions together [27]. A centrosymmetric cyclic structure is formed using CoII and LnIII depicting a rare octaspin motif. Ac magnetic susceptibility measurement reveals 38 and 39 displaying slow relaxation of the magnetization behavior.

Fig. 4
Two parts. Part a, has a 3-D molecular structure of 27 with elements of D y, C o, O, N, and C. Part b, has a 3-D molecular structure of 35 with elements of D y, C l, C o, O, N, and C.

Ball and stick diagram of a 27 and b 35 where H atoms apart from in water molecules are omitted

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2.4 Higher Nuclearity Complexes

In this section, complexes having nuclearity higher than four are described. Only recent representative examples are discussed.

The pentanuclear complexes 28 and 29 described in the previous section consist of {LnIII2 CoIII2 CoII} core with two L2− and two N-BuDEA2− anions [25]. The Co–Ln exchange interaction is ferromagnetic for 28 while it is antiferromagnetic for 29. Magnetic analysis depicts out-of-phase susceptibility in 28. Zhao and group have synthesized a hexanuclear CoIII4LnIII2 clusters [Co4Ln2(μ3-O)2(μ-N3)2(OH)2(H2O)2(HL)4]·(CH3CO2)2·20H2O [LnIII = Dy (41), Gd (42), Tb (43), Eu (44) and Ho (45)] using 2-[Bis(pyridin-2-ylmethyl)amino]-2(hydroxymethyl)propane-1,3-diol ligand [28]. The structural arrangement consists of four CoIII ions, two DyIII ions, four HL2−, two N3− ligands, two μ3-O2−, two water molecules and two acetate ions forming a lucanidae like arrangement. Ac magnetic susceptibility measurements reveal an energy barrier of 73.51 K and τ0 = 1.68 × 10−8 s. Liang and group have recently explored the effect of solvent by synthesizing two decanuclear clusters [Dy2Co8(μ3OCH3)2(L)4(HL)2(OAc)2(NO3)2(CH3CN)2]·CH3CN·H2O (46) and [Dy4Co6(L)4(HL)2(OAc)6(OCH2CH2OH)2(HOCH2CH2OH)(H2O)]·9CH3CN (47) [29]. The only difference in the reaction process was the change of reaction solvent from methanol and acetonitrile in 46 to acetonitrile and ethylene glycol in 47. The structure of 46 is formed by two DyIII, eight CoII ions, four L3−, two HL2−, two acetate ions, two (CH3O)− and CH3CN ligand. While for 47, it contains four DyIII, six CoII ions, three L3−, three HL2−, six acetate ions, three coordinated (HOCH2CH2O)− ion along with one coordinated water molecule. Ac magnetic susceptibility measurements reveal an energy barrier of 14.89 K and τ0 = 1.68 × 10−7 s for 46 and energy barrier of 5.49 K and τ0 = 2.88 × 10−5 s for 47 at zero dc field. Dou and group have synthesized a series of butterfly shaped metallacrowns (MCs) [Dy(pyzic){Dy3Co2(pyzha)6(*pyzha)(NO3)2(H2O)(MeOH)2}]2 (LnIII = Dy (48), Ho (49) and Tm (50)) using pyrazinehydroxamic acid and pyrazinic acid ligands (Fig. 5) [30]. At zero dc field, Ueff is 1.46 K and τ0 = 2.4 × 10−5 s for 48 (Table 1).

Fig. 5
A 3-D molecular structure of 48 with elements of C o, D y, N, O, and C.

Ball and stick diagram of 48

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Table 1 Magnetic parameter of CoII/LnIII complexes
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3 Summary

The heterometallic complexes of Co/Ln represent an interesting class of molecular magnets with fascinating structures and magnetic properties. CoII provides a large spin–orbit coupling which when combined with highly anisotropic LnIII ion can lead to the formation of SMMs with better properties. The role of ligand and the coordination geometry surrounding the metal ions plays a vital part in modulating the magnetic properties. While considerable progress is made in this field, proper designing of complexes can certainly lead to the development of SMM with superior properties. Apart from this, a chemist also requires a strong theoretical understanding of the complexes which seems to be lacking. Backed by this knowledge, synthetic chemists can strategically plan designs to enhance the SMM behavior of Co/Ln complexes.