Complexes: Synthetic Strategies and Magnetic Properties

Review High-Coordinate Mononuclear Ln(III) Complexes: Synthetic Strategies and Magnetic Properties

Joydev Acharya 1,2,† , Pankaj Kalita 1,† and Vadapalli Chandrasekhar 1,2,*

1 Tata Institute of Fundamental Research Hyderabad, Gopanpally, Hyderabad 500107, India; [email protected] (J.A.); [email protected] (P.K.) 2 Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India * Correspondence: [email protected] † Authors contributed equally.

Abstract: Single-molecule magnets involving monometallic 4f complexes have been investigated extensively in last two decades to understand the factors that govern the slow magnetization relaxation behavior in these complexes and to establish a magneto-structural correlation. The prime goal in this direction is to suppress the temperature independent quantum tunneling of magnetization (QTM) effect via ﬁne-tuning the coordination geometry/microenvironment. Among the various coordination geometries that have been pursued, complexes containing high coordination number around Ln(III) are sparse. Herein, we present a summary of the various synthetic strategies that were used for the assembly of 10- and 12-coordinated Ln(III) complexes. The magnetic properties of such complexes are also described.

Keywords: high-coordinate Ln(III) complexes; multi-dentate ligands; single-ion magnets; quantum tunneling of magnetization

1. Introduction Citation: Acharya, J.; Kalita, P.; Chan- The seminal discovery by Sessoli et al. that a Mn cluster can show slow relaxation of drasekhar, V.High-Coordinate Mononu- 12 magnetization reversal at very low temperatures has ignited the interest of both theoretical clear Ln(III) Complexes: Synthetic Strate- gies and Magnetic Properties. and experimental researchers from disciplines such as coordination chemistry, molecular Magnetochemistry 2021, 7, 1. https:// magnetism and molecular materials [1]. This area of work, which is now very active, dx.doi.org/10.3390/magnetochemistry includes different systems such as single-molecule magnets (SMMs), single-ion magnets 7010001 (SIMs) and single-chain magnets (SCMs). These systems are receiving attention due to the possibility of their potential to be utilized in various areas including data storage, Received: 30 November 2020 quantum computation and spintronics [2–6]. In addition, the field of molecular magnetic Accepted: 17 December 2020 refrigerants based on the principle of magneto caloric effect (MCE) also has been receiving Published: 22 December 2020 wide research interest [7]. Single-molecule magnets (SMMs), of which SIMs are a sub-set, are characterized by Publisher’s Note: MDPI stays neu- a well-defined magnetically bi-stable ground electronic state and can remain magnetized tral with regard to jurisdictional claims below certain temperatures showing a slow relaxation of magnetization. Through experi- in published maps and institutional mental and theoretical work carried out over several years it has now been established that affiliations. one of the characteristics of SMMs is the effective energy barrier (Ueff) that separates the two lowest energy spin states. This is the energy required to flip the direction of the magnetization and has been shown in polynuclear transition metal complexes to be dependent on

Copyright: © 2020 by the authors. Li- the Ising type magnetic anisotropy as characterized by the zero-ﬁeld splitting parameter, 2 2 1 censee MDPI, Basel, Switzerland. This D and the total spin: Ueff = |D|S for integer spin and |D|(S − 4 ) for half-integer spin. article is an open access article distributed In contrast to what this relationship suggests—that increasing the total spin would contin- under the terms and conditions of the uously cause an increase of Ueff—it has been shown that the magnetic anisotropy, in fact, Creative Commons Attribution (CC BY) is more important. This has led to a focus on 4f metal complexes [8–10]. The 4f orbitals license (https://creativecommons.org/ of the lanthanide ions are deeply buried and therefore do not interact strongly with an licenses/by/4.0/). external ligand ﬁeld. Thus, even in their complexes, lanthanide ions can possess large

Magnetochemistry 2021, 7, 1. https://dx.doi.org/10.3390/magnetochemistry7010001 https://www.mdpi.com/journal/magnetochemistry Magnetochemistry 2021, 7, 1 2 of 29

spin-orbit coupling due to the unquenched orbital angular momentum and therefore these systems can be good candidates for molecular magnets. Ishikawa and co-workers were the first to demonstrate slow relaxation of magnetization in Ln(III) sandwich complexes [11]. This has led to considerable research interest in Ln(III) complexes and tailoring of magnetic anisotropy through chemical and geometric modification around the paramagnetic metal ion, particularly in mononuclear complexes, has become an important focus area. Magnetochemistry 2020, 6, x FOR PEER REVIEW 2 of 29 These mononuclear complexes showing SMM properties are termed as single ion magnets lanthanide(SIMs) ions are ordeeply mononuclear buried and therefore SMMs do (MSMMs). not interact strongly All the with lanthanides, an external ligand except field. Gd(III) which is Thus, evencompletely in their complexes, isotropic, lanthanide and Eu(III) ions can which possess has large a non-magnetic spin-orbit coupling ground due stateto the (angular momen- unquenchedtum orbital quantum angular number momentumJ = and 0), theref are potentialore these systems candidates can be ingood the candidates assembly for of SMMs. Again molecularamong magnets. them, Ishikawa complexes and co-workers of Dy(III) were have the first been tothe demonstrate most widely slow studiedrelaxation becauseof Dy(III) has magnetization in Ln(III) sandwich complexes [11]. This has led to considerable research interest in a high J value of 15/2 and g value of 4/3 [12–17]. In addition, Dy(III) is a Kramers ion, Ln(III) complexes and tailoring of magnetic anisotropy through chemical and geometric modification around thehaving paramagnetic an odd metalJ value ion, whichparticularly results in inmononuclear a time-reversal complexes, symmetric has become degenerate an MJ states. importantOn focus the area. other These hand, mononuclear for the complexes non-Kramers showing ions, SMM havingproperties an are even termedJ value,as single the MJ states are ion magnetsnot (SIMs) degenerate or mononuclea [18].r ForSMMs a ligand(MSMMs). electron, All the lanthanii, thisdes, crystal except filed Gd(III) (CF) which perturbation is can be completely isotropic, and Eu(III) which has a non-magnetic ground state (angular momentum expressed by the following one electron operator, uCF (i) as: [19] quantum number J = 0), are potential candidates in the assembly of SMMs. Again among them, complexes of Dy(III) have been the most widely studied because Dy(III) has a high J value of 15/2 u (R) (1) CF (i) = −e R ρ dv and g value of 4/3 [12–17]. In addition, Dy(III) is a Kramers ion, having|R −anrj| odd J value which results in a time-reversal symmetric degenerate MJ states. On the other hand, for the non-Kramers ions, having an even InJ value, this the equation MJ states ( Rare) isnot the degenerate potential [18]. generated For a ligand by electron, means i, this of the crystal charge filed distribution and (CF) perturbation can be expressed by the following one electron operator, uCF (i) as: [19] rj is the radial distance. Thus, the overall perturbation can be calculated by summation of the one electron operators𝑢 acting on() all the electrons. However, the following Hamiltonian () (1) best describes the overall crystal field perturbation through Wybourne formalism as: [20]

In this equation (R) is the potential generated by means of the charge distribution and rj is the q q radial distance. Thus, the overall perturbation can be calculated= by summation of the one electron H\CF ∑ Bk C(i)k (2) operators acting on all the electrons. However, the following Hamiltoniani,k,q best describes the overall crystal field perturbation through Wybourne formalism as: [20] q q In the above equation, B is the crystal field (CF) coefficient and C(i) represents a 𝐻= k𝐵 𝐶(𝑖) k one-electron operator acting on ith electron. For Ln(III) ions, the k and (2)q values are limited ,, as: k ≤ 7 and −k ≤ q ≤ k and the even value of k are responsible for CF splitting, while the 𝐵 𝐶(𝑖) In theodd abovek values equation, are responsible is the crystal for field various (CF) coefficient spectroscopic and transitions. represents Thesea one- parameters can electron operator acting on ith electron. For Ln(III) ions, the k and q values are limited as: k ≤ 7 and −k ≤ q ≤ k andbe the useful even value to understandof k are responsible the naturefor CF splitting, of the while magnetic the odd anisotropy k values are responsible of the lanthanide ions in for variousa spectroscopic certain ligand transitions. field (Theseeasy planeparametersor easy can axisbe useful)[21 to]. understand Rinehart etthe al. nature [22 ]of and the Jiang et al. [23] magnetic haveanisotropy employed of the lanthanide quadruple ions approximation in a certain ligand and field electrostatic (easy plane or models easy axis respectively) [21]. for calcu- Rinehart etlating al. [22] basic and Jiang shapes et al. of [23] the have charge employed density quadruple distributions approximation corresponding and electrostatic to various MJ states models respectivelyfor all Ln(III) for calculating ions. These basic shapes studies of the allowed charge density lanthanide distributions ions to corresponding be classified to into three types: various MprolateJ states for ions all Ln(III) having ions. axially These studies elongated allowed 4f lanthanide electron ions density, to be classified oblate ionsinto three having equatorially types: prolate ions having axially elongated 4f electron density, oblate ions having equatorially expanded 4f electron density and isotropic ions of spherical electron density (see Figure1). expanded 4f electron density and isotropic ions of spherical electron density (see Figure 1).

FigureFigure 1. Electrostatic 1. Electrostatic potential surfaces potential correspondi surfacesng to corresponding the eigen states with to different the eigen MJ statesvalues for with different MJ values each Ln(III) ion. In the absence of a crystal-field all the eigen states with different MJ values are for each Ln(III) ion. In the absence of a crystal-ﬁeld all the eigen states with different MJ values are degeneratedegenerate for a particular for a particular Ln(III) ion. The Ln(III) shapes ion. are The evaluated shapes with are respect evaluated to the local with reference respect to the local reference frame for the electrostatic potential surfaces corresponding to the eigen states with the maximum MJ frame for the electrostatic potential surfaces corresponding to the eigen states with the maximum MJ values. The ﬁgure and caption are reproduced from Reference [23] with permission from the Royal Society of Chemistry. Magnetochemistry 2020, 6, x FOR PEER REVIEW 3 of 29

values. The figure and caption are reproduced from Reference [23] with permission from the Royal Society of Chemistry.

Magnetochemistry 2021, 7, 1 3 of 29 However, the MJ state with highest J value needs to be ground state, and at the same time there should be a sufficiently large separation from its excited states to have a large effective energy barrier. Ideally, in these situations, whenHowever, the relaxations the MJ state with processes highest J value such needs as to Raman, be ground state,direct and (these at the same promote time there should be a sufﬁciently large separation from its excited states to have a large relaxation of magnetization byeffective the reversal energy barrier. of Ideally,magnetization in these situations, orientation when the relaxations through processes virtual such electronic as states) and quantum tunnelingRaman, of magnetization direct (these promote (t relaxationhat enhance of magnetization the rate by the of reversal magnetization of magnetization reversal orientation through virtual electronic states) and quantum tunneling of magnetization through ground states without(that any enhance energy the rate barrier)— of magnetizationwhich reversal undercut through groundthe energy states without barrier any energy(see Figure 2)—are absent in the system andbarrier)—which there is undercuta sizable the separation energy barrier between (see Figure2 )—arethe ground absent in and the system the excited and there is a sizable separation between the ground and the excited electronic states, electronic states, the maximumthe energy maximum barrier energy barrier for magnetization for magnetization reversal reversal can becan observed. be observed.

Figure 2. All possible magneticFigure relaxation 2. All possible processes magnetic relaxation involved processes in involvedsingle-molecule in single-molecule magnets magnets (SMMs). (SMMs). The figure is reproduced from Reference [24] with permission from the Elsevier Ltd. The figure is reproduced from Reference [24] with permission from the Elsevier Ltd. This maximum separation between the MJ states can be achieved by employing an axially strong crystal field for an oblate ion and an equatorially strong ligand field for This maximum separationprolate between ions. Inthe practice, MJ states Ln(III) can ions be have achieved a very rich by coordination employing chemistry an axially with a strong crystal field for an oblate ionlarge and coordination an equatorially number variation strong that ligand ranges fromfield 3 tofor 14 [prolate25]. In order ions. to estimate In practice, the effect of coordination geometry on the magnetic anisotropy for Dy(III) an extensive Ln(III) ions have a very rich coordinationcalculation was chemistr carried outy by with Ungur a et large al. on coordination a series of empirical number complexes variation of the that 3−n ranges from 3 to 14 [25]. In ordercommon to formulaestimate [DyF nth] e effect(Figure 3of). Thiscoordination study revealed geometry that a maximum on effectivethe magnetic energy barrier can be achieved for very low coordinated Dy(III) complexes [26]. In spite of anisotropy for Dy(III) an extensivethis theoretical calculation insight, was several ca lanthaniderried out complexes by Ungur of varying et al. coordinationon a series geometry of empirical complexes of the common formulahave been [DyF studied.n]3-n This(Figure article will3). reviewThis study those complexes revealed where that thecoordination a maximum number effective around the lanthanide ions is greater than 9. energy barrier can be achieved for very low coordinated Dy(III) complexes [26]. In spite of this theoretical insight, several lanthanide complexes of varying coordination geometry have been studied. This article will review those complexes where the coordination number around the lanthanide ions is greater than 9.

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Figure 3. Energy level splitting of MJ multiplets in a different coordination number in the hypothetical 3−n complex [DyFn] . The figure is reproduced from Reference [26] with permission from the American Chemical Society copyright © 2015. Figure 3. Energy level splitting of MJ multiplets in a different coordination number in the hypothetical complex [DyF2. High-Coordinaten]3−n. The figure is reproduced Ln(III) Complexes from Reference [26] with permission from the American Chemical Society copyright © 2015. The coordination chemistry of Ln(III) ions is dominated by coordination numbers 8 or 9 although the range of coordination numbers can be from 3–14 (see Figure4). Among the 2. High-Coordinate Ln(III) Complexes high-coordinate (C.N. > 9) Ln(III) complexes coordination number 10 is the most commonly The coordinationobserved chemistry (see Figure of4 Ln(III))[ 27]. ions The is second dominated commonly by coordination observed high numbers coordination 8 or 9 although numbers the range of coordinationare 11 and12 numbers [28] whereas can be thefrom coordination 3–14 (see Figure numbers 4). Among 13 and the 14 high-coordinate are extremely rare (C.N. [29 ]. > 9) Ln(III) complexesDue to the coordination inter-ligand number repulsions 10 (“first-order”is the most commonly effects), monodentate observed (see ligands Figure could 4) [27]. lead The second commonlyto coordination observed numbers high of coordination only up to 8 andnumbers 9. Therefore, are 11 toand synthesize 12 [28] highwhereas coordinate the coordination numbers(C.N. > 9) 13 Ln(III) and 14 complexes, are extremely two different rare [29]. synthetic Due to strategiesthe inter-ligand are employed. repulsions The (‘‘first- first one is a multi-dentate ligand approach and the second one is a mixed ligand approach utilizing order’’ effects), monodentate ligands could lead to coordination numbers of only up to 8 and 9. a combination of multi-dentate ligand(s) and bidentate ligand(s). It is worth noting that Therefore, to synthesize high coordinate (C.N. > 9) Ln(III) complexes, two different synthetic bidentate ligands such as the nitrate anion, have a small “bite angle” and therefore can strategies are employed. The first one is a multi-dentate ligand approach and the second one is a minimize the inter-ligand repulsions. Broadly, the mixed ligand approach appears to be the mixed ligand approach utilizing a combination of multi-dentate ligand(s) and bidentate ligand(s). It Magnetochemistry 2020, 6, x FOR morePEER REVIEW promising synthetic strategy for assembling high-coordinate 5 of 29 lanthanide complexes. is worth noting that bidentate ligands such as the nitrate anion, have a small ‘‘bite angle’’ and therefore can minimize the inter-ligand repulsions. Broadly, the mixed ligand approach appears to be the more promising synthetic strategy for assembling high-coordinate lanthanide complexes. Unlike the reviews that have already been published, [8,24,30–34] herein, we review the magnetic properties of high-coordinate (C.N. > 9) Ln(III) complexes. Only those that have exhibited at least a field-dependent SMM behavior are discussed. Unfortunately, there seem to be no examples of zero-field SMM behavior among high-coordinate lanthanide complexes.

Figure 4. Abundance ofFigure coordination 4. Abundance numbers of in coordinationLn-complexes numbers(La–Nd, Sm–Lu, in Ln-complexes Y) based on (La–Nd, the Sm–Lu, Y) based on the analysis of 1389 crystalanalysis structures of published 1389 crystal between structures 1935 and published 1995. The between figure and 1935 captions and 1995. were The ﬁgure and captions were reproduced from Reference [24] with permission from the Elsevier Ltd. reproduced from Reference [24] with permission from the Elsevier Ltd. 2.1. Ten-Coordinate Ln(III) Complexes Both the multi-dentate ligand approach and the mixed ligand approach have been used to generate ten-coordinate Ln(III) complexes. In addition, the use of just the nitrate ligands has also allowed the assembly of ten-coordinate Ln(III) complexes. These approaches are discussed below.

2.1.1. Multi-Dentate Ligand Approach Pentadentate chelating ligands are known to generate ten-coordinate Ln(III) complexes [35–37]. Diacetylpyridine bishydrazone ligands can provide a rigid pentagonal planar structure to Ln(III) ions and therefore coordination of two such ligands orthogonally towards the Ln(III) metal center could result in a ten-coordinate structure. A bicapped square antiprism (BSA) geometry is the preferred polyhedron for a mononuclear Ln(III) complex chelated by two pentadentate ligands. Mallah and co-workers synthesized two Dy(III) compounds, [Dy(H2dapbh)2](NO3)3 (1) and [Dy(H2dapbh)(Hdapbh)](NO3)2 (2) utilizing a pentadentate hydrazone ligand (Figure 5a,b), H2dabph (H2dapbh = 2,6-diacetylpyridinebis(benzoic acid hydrazone)) to study the effect of chemical tuning on slow magnetic relaxation [38]. These complexes were isolated by the reaction of two equivalents of H2dapbh with hydrated Dy(NO3)3 in EtOH (for 1) and in EtOH-H2O (for 2). The molecular structures of the two complexes were found to be similar although the formation of 2 involves a deprotonation of an amino nitrogen in one of the dapbh ligands. Therefore, the tripositive charge of the Dy(III) ion is balanced by three NO3− counter anions in 1 while it is balanced by two NO3− counter anions in 2. In both cases, each multi-dentate ligand coordinates via the pyridyl nitrogen, two hydrazone nitrogens, and two carbonyl oxygen atoms in an interlocked fashion resulting in a ten- coordinate geometry. A C–O distance in the range 1.242–1.250 Å further confirmed the carbonyl assignment. SHAPE [39] analysis confirmed a distorted bicapped square antiprismatic geometry around the Dy(III) center with the pyridyl nitrogens being the capping atoms. The “skew angles” for the two complexes are remarkably similar: 27° for 1 and 29° for 2 (for a perfect D4 symmetry it is 45°). Both AC and DC magnetic measurements were performed to probe the differences in the magnetic properties of 1 and 2. The dynamic alternating current (AC) magnetic measurements at zero applied field did not reveal well-resolved maxima in the out-of-phase (χ″M) susceptibility component for 1 and 2 even at low temperatures. After the application of an optimum DC field of 1000 Oe strong frequency-dependent signals and maxima in both the in-phase (χ′M) and out-of-phase (χ″M) components were observed (Figure 5c,d). Both the complexes followed a thermally activated relaxation mechanism at temperatures above 4 K. Therefore, the anisotropic barrier and the pre- exponential parameter was extracted from the ln(τ) vs. 1/T plot by fitting the Arrhenius equation (τ = τ0 exp(Ueff/kBT)) which resulted in Ueff = 32.4 K for 1 and ~19 K for 2, respectively. Below 4 K, the relaxation behavior was dominated by the temperature independent QTM process. Micro-SQUID

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Unlike the reviews that have already been published, [8,24,30–34] herein, we review the magnetic properties of high-coordinate (C.N. > 9) Ln(III) complexes. Only those that have exhibited at least a ﬁeld-dependent SMM behavior are discussed. Unfortunately, there seem to be no examples of zero-ﬁeld SMM behavior among high-coordinate lanthanide complexes.

2.1. Ten-Coordinate Ln(III) Complexes Both the multi-dentate ligand approach and the mixed ligand approach have been used to generate ten-coordinate Ln(III) complexes. In addition, the use of just the nitrate ligands has also allowed the assembly of ten-coordinate Ln(III) complexes. These approaches are discussed below.

2.1.1. Multi-Dentate Ligand Approach Pentadentate chelating ligands are known to generate ten-coordinate Ln(III) complexes [35–37]. Diacetylpyridine bishydrazone ligands can provide a rigid pentagonal planar structure to Ln(III) ions and therefore coordination of two such ligands orthogonally towards the Ln(III) metal center could result in a ten-coordinate structure. A bicapped square antiprism (BSA) geometry is the preferred polyhedron for a mononuclear Ln(III) complex chelated by two pentadentate ligands. Mallah and co-workers synthesized two Dy(III) compounds, [Dy(H2dapbh)2](NO3)3 (1) and [Dy(H2dapbh)(Hdapbh)](NO3)2 (2) utilizing a pentadentate hydrazone ligand (Figure5a,b), H 2dabph (H2dapbh = 2,6-diacetylpyridinebis(benzoic acid hydrazone)) to study the effect of chemical tuning on slow magnetic relaxation [38]. These complexes were isolated by the reaction of two equivalents of H2dapbh with hydrated Dy(NO3)3 in EtOH (for 1) and in EtOH-H2O (for 2). The molecular structures of the two complexes were found to be similar although the formation of 2 involves a deprotonation of an amino nitrogen in one of the dapbh ligands. Therefore, the tripositive charge of the Dy(III) ion − − is balanced by three NO3 counter anions in 1 while it is balanced by two NO3 counter anions in 2. In both cases, each multi-dentate ligand coordinates via the pyridyl nitrogen, two hydrazone nitrogens, and two carbonyl oxygen atoms in an interlocked fashion resulting in a ten-coordinate geometry. A C–O distance in the range 1.242–1.250 Å further confirmed the carbonyl assignment. SHAPE [39] analysis confirmed a distorted bicapped square antiprismatic geometry around the Dy(III) center with the pyridyl nitrogens being the capping atoms. The “skew angles” for the two complexes are remarkably similar: ◦ ◦ ◦ 27 for 1 and 29 for 2 (for a perfect D4 symmetry it is 45 ). Both AC and DC magnetic measurements were performed to probe the differences in the magnetic properties of 1 and 2. The dynamic alternating current (AC) magnetic measurements at zero applied field did not reveal well-resolved maxima in the out-of-phase (χ”M) susceptibility component for 1 and 2 even at low temperatures. After the application of an optimum DC field 0 of 1000 Oe strong frequency-dependent signals and maxima in both the in-phase (χ M) and out-of-phase (χ”M) components were observed (Figure5c,d). Both the complexes followed a thermally activated relaxation mechanism at temperatures above 4 K. Therefore, the anisotropic barrier and the pre-exponential parameter was extracted from the ln(τ) vs. 1/T plot by fitting the Arrhenius equation (τ = τ0 exp(Ueff/kBT)) which resulted in Ueff = 32.4 K for 1 and ~19 K for 2, respectively. Below 4 K, the relaxation behavior was dominated by the temperature independent QTM process. Micro-SQUID studies confirmed the presence of an easy axis of magnetization in both 1 and 2. Both the complexes also showed hysteresis loops confirming slow relaxation of magnetization although 2 showed much faster relaxation. A larger structural distortion in 2 due to deprotonation was found to be responsible for the faster relaxation of magnetization. To gain further insight, ab initio calculations of the CASSCF/RASSI/SINGLE_ANISO type were carried out. The transverse components of the g tensor (gx and gy) were found to be greater in 2 than in 1 in the ground Kramer Doublet. This resulted in a large tunneling gap in 2 and therefore the complex showed faster relaxation in the magnetic measurements. The ab initio calculated main Magnetochemistry 2020, 6, x FOR PEER REVIEW 6 of 29

studies confirmed the presence of an easy axis of magnetization in both 1 and 2. Both the complexes also showed hysteresis loops confirming slow relaxation of magnetization although 2 showed much faster relaxation. A larger structural distortion in 2 due to deprotonation was found to be responsible for the faster relaxation of magnetization. To gain further insight, ab initio calculations of the Magnetochemistry 2021, 7, 1 6 of 29 CASSCF/RASSI/SINGLE_ANISO type were carried out. The transverse components of the g tensor (gx and gy) were found to be greater in 2 than in 1 in the ground Kramer Doublet. This resulted in a large tunneling gap in 2 and therefore the complex showed faster relaxation in the magnetic measurements. The abmagnetic initio calculated axis (gz) ofmain1 was magnetic found toaxis be ( orientedgz) of 1 was along found the to Dy–N(pyridyl) be oriented along coordination the Dy–N(pyridyl) coordinationbonds which bonds is linked which to the is link idealizeded to the four-fold idealized symmetry four-fold axis. symmetry On the other axis. hand,On depro- ◦ the other hand, deprotonationtonation in in2 causes2 causes a rotationa rotation of theof the main main magnetic magnetic axis axis atan at anglean angle of 60 of towards60° the Dy–O(carbonyl) bonds. The ab initio calculated crystal field parameters were also found to towards the Dy–O(carbonyl) bonds. The ab initio calculated crystal field parameters were also found be significantly different for the two complexes. This study revealed the sensitivity of the to be significantly different for the two complexes. This study revealed the sensitivity of the magnetic magnetic anisotropy on the ligand field around the Dy(III) sites. anisotropy on the ligand field around the Dy(III) sites.

0 Figure 5. (a,b) StructuresFigure5. of (thea,b )cationic Structures part ofof the1 and cationic 2; (c,d) part Temperature of 1 and dependence2;(c,d) Temperature of χ′M (dotted dependence of χ M lines) and χ″M (solid(dotted lines) lines) of 1 and and 2χ under”M (solid 1000 lines) Oe applied of 1 and DC2 underfield at 1000 frequencies Oe applied from DC 1 to ﬁeld 1200 at Hz frequencies from (Inset: hysteresis 1loops to 1200 taken Hz at (Inset: 0.5 K hysteresisand at variable loops scan taken rates). at 0.5 Figures K and and at variable captions scan are rates).reproduced Figures and captions from Reference [38]are with reproduced permission from from Reference the Royal [38] Society with permission of Chemistry. from the Royal Society of Chemistry.

Recently Vasiliev andRecently co-workers Vasiliev isolated and co-workers a series isolatedof neutral a series ten-coordinate of neutral ten-coordinate mononuclear mononu- lanthanide complexes,clear [Ln(Hd lanthanideapbh)(dapbh)] complexes, (Ln [Ln(Hdapbh)(dapbh)] = Dy(III), 3; Ho(III), 4;(Ln Er(III), = Dy(III), 5 and 3Tb(III),; Ho(III), 7) by4; Er(III),the 5 and Tb(III), 7) by the reaction of H2dapbh (2 equiv.) with hydrated LnCl3/Er(HCO3)3 salts in the reaction of H2dapbh (2 equiv.) with hydrated LnCl3/Er(HCO3)3 salts in the presence of 2 equivalents presence of 2 equivalents of NEt3 [40]. Following the same synthetic protocol, they isolated of NEt3.[40]. Following the same synthetic protocol, they isolated another neutral ten-coordinate another neutral ten-coordinate mononuclear Dy(III) derivative, [Dy(Hdapmbh)(dapmbh)] mononuclear Dy(III) derivative, [Dy(Hdapmbh)(dapmbh)] (6) that involved a methoxy-substituted0 (6) that involved a methoxy-substituted H2dapmbh ligand (H2dapmbh = 1,1 -(pyridine-2,6- H2dapmbh liganddiyl)bis(ethan-1-yl-1-ylidene))di-4-methoxybenzohydrazine) (H2dapmbh = 1,1′-(pyridine-2,6-diyl)bis(ethan-1-yl-1-yli [40]. Alldene))di-4- these complexes are methoxybenzohydrazine)isostructural [40]. All with thes1 ande complexes2 (Figure6 ).are Compared isostructural to 1 andwith2 (vide1 and supra), 2 (Figure the multi-dentate 6). Compared to 1 and ligand2 (vide in supra),3-7 acted the bothmulti-dentate in a dianionic ligand and in a3-7 monoanionic acted both in manner. a dianionic The free and N-H a group monoanionic manner.of theThe monoanionicfree N-H group ligand of the was monoanionic involved in hydrogen-bondingligand was involved interactions in hydrogen- with lattice bonding interactionssolvent with lattice molecules solvent (H 2moleculesO, MeOH and(H2O, EtOH). MeOH The and dihedral EtOH). angles The dihedral (Nimine–N anglespyridyl –Nimine) ◦ ◦ (Nimine–Npyridyl–Nimine)of of the the Dy(III) Dy(III) derivativesderivatives werewere foundfound to be similar (58.7° (58.7 forfor 33 andand 58.36° 58.36 forfor 6)6 ) and lie and lie between thebetween dihedral the angles dihedral of non-deprotonated angles of non-deprotonated (57.17° for(57.17 1) and◦ for singly1) and deprotonated singly deprotonated ◦ structures (59.43° forstructures 2). The coordination (59.43 for 2). polyhedra The coordination of the polyhedraLn(III) ions of in the 3– Ln(III)7 was ionsfound in 3to–7 bewas a found to bicapped square antiprismbe a bicapped with N(pyridyl) square antiprism atoms at with the capping N(pyridyl) positions. atoms at the capping positions.

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FigureFigure 6. Molecular 6. Molecular structure structure of 3 ( ofleft3) (andleft 6) and(the two6 (the H’s two at N H’s are at disordered N are disordered with 50% withoccupancy: 50% (rightoccupancy:). The structures (right). are The re-drawn structures from Refe arerence re-drawn [40] with from permission Reference from [40 MDPI.] with permission from MDPI. DC magnetic measurements on 3–6 revealed the χMT products to be close to the respective free- ion approximation values, i.e., 14.17 (Dy(III), 6H15/2), 14.07 (Ho(III), 5I8) and 11.48 (Er(III), 4I15/2) cm3 K DC magnetic measurements on 3–6 revealed the χMT products to be close to the mol−1 at 300 K. The temperature-dependent DC data was analyzed6 to extract the crystal field5 respective free-ion approximation values, i.e., 14.17 (Dy(III), H15/2), 14.07 (Ho(III), I8) parameters. All the complexes4 showed3 less−1 than 400 cm−1 of crystal field splitting energy in the and 11.48 (Er(III), I15/2) cm K mol at 300 K. The temperature-dependent DC data calculatedwas analyzed crystal to field extract energy the spectrum crystal field for parameters.the lowest J-multiplets. All the complexes In addition, showed the calculated less than g- tensors in the ground and the first excited Kramer doublets in 6 exhibited stronger Ising-type 400 cm−1 of crystal field splitting energy in the calculated crystal field energy spectrum magnetic anisotropy with gz = 19.000 (ground state) and gz = 17.310 (first excited state) respectively. for the lowest J-multiplets. In addition, the calculated g-tensors in the ground and the The transverse g components, gx = 0.082 and gy = 0.581 for 6 were also found to be smaller among all. first excited Kramer doublets in 6 exhibited stronger Ising-type magnetic anisotropy with The first excited CF state of 6 was found lying at 39 cm−1 from the ground state which is the highest g = 19.000 (ground state) and g = 17.310 (first excited state) respectively. The transverse amongz all the complexes. To probez the slow relaxation behavior in 3–6, dynamic AC magnetic g components, g = 0.082 and g = 0.581 for 6 were also found to be smaller among all. measurements werex performed (Figurey 7). At zero DC field, none of the complexes showed signals 6 −1 inThe thefirst frequency-dependent excited CF state ofout-of-phasewas found (χ″ lying) susceptibility. at 39 cm Uponfrom application the ground ofstate an external which biased is the highest among all the complexes. To probe the slow relaxation behavior in 3–6, dynamic field 3 (HDC = 800 Oe), 5 (HDC = 1500 Oe) and 6 (HDC = 300 Oe) showed frequency-dependent AC signals inAC both magnetic the in-phase measurements (χ′) and out-of-phase were performed (χ″) susceptibility (Figure 7components). At zero due DC to field, the suppression none of the of QTM.complexes The SMM-silent showed behavior signals inof th thee non-Kramer frequency-dependent Ho(III) derivative out-of-phase (4) is primarily (χ”) susceptibility.due to the non- magneticUpon application character of ofthe an ground external state. biased The temper field ature3 (HDC dependence= 800 Oe), of 5relaxation(HDC = 1500times Oe)in case and of6 4 χ0 was(HDC analyzed= 300 Oe)considering showed the frequency-dependent equation, τ−1 = τ−1QTM AC+ AT signals + BTn + in τ0 both−1exp(– theUeff in-phase/kBT) corresponding ( ) and out- to theof-phase contributions (χ”) susceptibility from quantum components tunneling dueof magnetization to the suppression (QTM) of effects QTM. (first The SMM-silentterm), direct relaxationbehavior (second of the term), non-Kramer Raman process Ho(III) (third derivative term) and (4) isthe primarily Orbach process due to(fourth the non-magnetic term). The best fitcharacter yielded C of = 1.2 the∙s− ground1, n = 6.1, state. Ueff = 87 The K and temperature τ0 = 4 × 10−12 dependence s, τQTM = 4.4 ×of 10 relaxation−3 s and A = 0. times The experimental in case of 4 −1 −1 n −1 Uwaseff = 87 analyzed K was found considering to be considerably the equation, higherτ than= thatτ QTMof the+ first AT excited + BT +CFτ state0 exp(– at 23 Ucmeff−1/k (33B T)K) whichcorresponding is close to the to second the contributions excited CF state from at 75 quantumcm−1 (108 K) tunneling as extracted of from magnetization the DC data of (QTM) 5. The χ′′effects(ν) and (first Cole–Cole term), plots direct (α1 = relaxation 0.6–0.45 and (second α2 = 0.41–0.56) term), Ramanrevealed processtwo distinct (third maxima term) atand 2–2.45 the K −1 representingOrbach process two different (fourth term).relaxation The processes best fit yieldedthat could C= be 1.2 resulting·s , n =from 6.1, Utwoeff =independent 87 K and −12 −3 moleculesτ0 = 4 × 10presents, inτ QTMthe unit= 4.4 cell× of10 3. Fittings and Aln( =τ)0. vs. The T plot experimental with the ArrheniusUeff = 87 equation K was found in the temperatureto be considerably range 4.4–5 higher K afforded than that the values of the firstof the excited effective CF energy state barrier at 23 cm and−1 the(33 time K) which constant: is −8 − Ucloseeff = 29 to K theand secondτ0 = 8.3 × excited 10 s. The CF experimental state at 75 cmvalue1 of(108 Ueff K) nicely as extracted co-related fromto the thetheoretical DC data value of 00 −1 (U5.cal The = 23χ cm(ν(first) and excited Cole–Cole state)). plots On the (α 1other= 0.6–0.45 hand, th ande temperatureα2 = 0.41–0.56) dependence revealed of relaxation two distinct times inmaxima the case at of 2–2.45 6 was Kanalyzed representing considering twodifferent combined relaxation Orbach and processes Raman processes that could and be the resulting best fit yieldedfrom two C = independent4.2 s−1, n = 4.8 (for molecules Raman process), present U ineff the = 70 unit K, τ0 cell = 2.2 of ×3 10. Fitting−11 s (forln( Orbachτ) vs. process).T plot with the Arrhenius equation in the temperature range 4.4–5 K afforded the values of the effective −8 energy barrier and the time constant: Ueff = 29 K and τ0 = 8.3 × 10 s. The experimental −1 value of Ueff nicely co-related to the theoretical value (Ucal = 23 cm (first excited state)). On the other hand, the temperature dependence of relaxation times in the case of 6 was analyzed considering combined Orbach and Raman processes and the best fit yielded −1 −11 C = 4.2 s , n = 4.8 (for Raman process), Ueff = 70 K, τ0 = 2.2 × 10 s (for Orbach process).

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Figure 7. The frequency-dependence of the χ0(ν)(top) and χ00(ν)(bottom) for 4 (a) under H = 1500 Oe, 3 (b) under Figure 7. The frequency-dependence of the χ′(ν) (top) and χ′′(ν) (bottom) for 4 (a) underDC HDC = 1500 Oe, H = 800 Oe, and 6 (c) under H = 300 Oe. The figures are reproduced from Reference [40] with permission from MDPI. DC 3 (b) under HDC = 800 DCOe, and 6 (c) under HDC = 300 Oe. The figures are reproduced from Reference [40] with permission from MDPI. Unlike the (3N2O)2 coordination environment discussed above, a flexible (N5)2 coordination resulted in a spherical LnN10 coordination geometry. Tong and co-workers Unlike the (3N2O)2 coordination environment discussed above, a flexible (N5)2 coordination synthesized two Ln(III) complexes of the formula, [Ln(N5)2](CF3SO3)3 (Ln = Dy (8) and Er resulted in a spherical LnN10 coordination geometry. Tong and co-workers synthesized two Ln(III) 0 (9)) by the reaction of Ln(CF3SO3)3 with two equivalents of N5 (2,6-diacetylpyridinebis(2 - complexes of the formula, [Ln(N5)2](CF3SO3)3 (Ln = Dy (8) and Er (9)) by the reaction of Ln(CF3SO3)3 pyridylhydrazone) in CH3CN [41]. The ten-coordinate low symmetry polyhedron in these with two equivalentscomplexes of N5 (2,6-diacetylpyridinebis(2 resulted from the coordination′-pyridylhydrazone) of two N5 ligands in CH3CN.[41] interlocked The by ten- the central coordinate low symmetrymetal ionpolyhedron (Figure8 ).in The these average complexes Ln–N resulted distances from in 8 theand coordination9 differed only of two slightly N5 (2.566 Å ligands interlocked byfor the8 and central 2.543 metal Å for ion9). (Figure SHAPE 8). analysis The average revealed Ln–N a very distances close resemblancein 8 and 9 differed between the bi- only slightly (2.566 Åcapped for 8 squareand 2.543 antiprism Å for 9 geometry). SHAPE ( Danalysis4d) and therevealed sphenocorona a very close (C2v )resemblance geometry confirming between the bicappeda spherical square coordinationantiprism geometry environment (D4d)in and both the cases. sphenocorona Quantitative (C information2v) geometry on energy- confirming a sphericallevels coordination and eigenstates environment was extracted in both cases. from theQuantitative DC magnetic information measurements. on energy- The ground levels and eigenstateseigen was extracted doublet state from of the8 was DC magnetic found to bemeasurements. mixed with theThe components ground eigen being doublet∓1/2 (79%), state of 8 was found ±to3/2 be mixed (17%) andwith∓ the5/2 components (3%) mJ states, being respectively. ∓1/2 (79%), On±3/2 the (17%) other and hand, ∓5/2 for (3%) the prolate Er(III) ion in 9 the ground doublet state was found to be mixed with the components being mJ states, respectively. On the other hand, for the prolate Er(III) ion in 9 the ground doublet state was ±15/2 (46%), ±11/2 (44%) and ±7/2 (10%) mJ states respectively. To probe the dynamics found to be mixed with the components being ±15/2 (46%), ±11/2 (44%) and ±7/2 (10%) mJ states of magnetization, AC magnetic measurements were performed under the zero DC field respectively. To probe the dynamics of magnetization, AC magnetic measurements were performed (Figure8). None of these complexes showed a clear maximum in the out-of-phase ( χ”M) under the zero DC fieldAC component(Figure 8). None in the of temperature these complexes range showed of 2–10 Ka clear (1.8–7 maximum K for 9). This in the behavior out-of- has been phase (χ″M) AC component in the temperature range of 2–10 K (1.8–7 K for 9). This behavior has been explained as due to the QTM effects resulting from the admixture of the different mJ states explained as due to inthe the QTM ground effects state. result However,ing from for the both admixture the complexes, of the different at an optimum mJ states field in the of 1200 Oe ground state. However,QTM for was both suppressed the complexes, and aat set an of optimum strong frequency-dependent field of 1200 Oe QTM peaks was suppressed in the χ”M AC com- and a set of strong ponentfrequency-dependent indicative of the peaks slow in magnetic the χ″M relaxationAC component was observed.indicative Cole–Coleof the slow plots with magnetic relaxation quasi-semi-circularwas observed. Cole–Cole features plots also wi revealedth quasi-semi-circular a narrow-to-moderate features distribution also revealed of a relaxation narrow-to-moderate timesdistribution with α =of 0.04–0.25relaxation (for times8) and with 0.04–0.18 α = 0.04–0.25 (for 9) (for respectively 8) and 0.04–0.18 (the generalized (for 9) Debye −1 respectively (the generalizedfunction wasDebye used function for fitting). was used The ln( forτ) fitting). vs. T Theplot ln( wasτ) fittedvs. T− considering1 plot was fitted theequation, −1 n −1 considering the equation,τ = τ AT−1 = + AT BT + +BTτn0 + τexp(–0−1exp(–UeffU/keff/kBT)BT) where where the the first, first, second, second, and and third third terms terms represent represent direct, Raman,direct, and Raman, Orbach and Orbachrelaxation relaxation process, process, respectively. respectively. In the In thehigh high temperature temperature regime, the Arrhenius law fit resulted in an effective anisotropic barrier of magnitude U = 79(4) eff eff regime, the Arrhenius law fit resulted−8 in an effective anisotropic barrier −of11 magnitude U = 79(4) K K(τ0 = 2.9 × 10 s) for 8 and 59(4) K (τ0 = 6.9 × 10 s) for 9 which are comparable to (τ0 = 2.9 × 10−8 s) for 8 and 59(4) K (τ0 = 6.9 × 10−11 s) for 9 which are comparable to each other. To gain each other. To gain further insight, a log(τ) vs. log(T) was plotted which showed that τ further insight, a log(τ) vs. log(T) was plotted which showed that τ obeys the T−n behavior with n = 6 obeys the T−n behavior with n = 6 for 8 and 9.2 for 9. In addition, consideration of both the for 8 and 9.2 for 9. In addition, consideration of both the direct and Raman processes to fit the τ versus direct and Raman processes to fit the τ versus T plot also verified the admixture of these T plot also verified twothe admixture types of spin-lattice of these two interaction types of mechanisms. spin-lattice interaction The magnetic mechanisms. properties The of the above magnetic properties of the above two complexes revealed that a spherically symmetric coordination geometry could be an effective strategy to develop SIMs with mixed mJ states.

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two complexes revealed that a spherically symmetric coordination geometry could be an Magnetochemistry 2020, 6, x FOReffective PEER strategyREVIEW to develop SIMs with mixed mJ states. 9 of 29

FigureFigure 8. (a,b) 8. Molecular (a,b) Molecular structures structures of the cationic of the part cationic of 8 and part9,(c ,ofd) coordination8 and 9, (c,d geometry) coordination of the Ln(III)geometry ions, of (e ,thef) Cole– Cole plotsLn(III) for 8ions,and (9eobtained,f) Cole–Cole from theplots AC for susceptibility 8 and 9 obtained data (external from DCthe fieldAC susceptibility 1200 Oe). Thesolid data lines (external correspond DC to the best fit obtained from the generalized Debye equation, and (g) Temperature-dependence of relaxation time (the red solid field 1200 Oe). The solid lines correspond to the best fit obtained from the generalized Debye equation, lines represent the Arrhenius law fit). Figures and captions are reproduced from Reference [41] with permission from the and (g) Temperature-dependence of relaxation time (the red solid lines represent the Arrhenius law Royal Society of Chemistry. fit). Figures and captions are reproduced from Reference [41] with permission from the Royal Society of Chemistry. 2.1.2. Mixed Ligand Approach 2.1.2. Mixed Ligand ApproachCrown Ether Ligands Multi-dentate crown ether ligands provide an excellent opportunity to produce Crown Ether Ligands mononuclear ten-coordinate Ln(III) complexes in association with the coordinating nitrate anions. Crown ether ligands having small cavity size such as 12-crown-4-ether Multi-dentate crown(12-crown-4 ether ligands = 1,4,7,10-tetraoxacyclododecane) provide an excellent opportunity could form to half-sandwich produce mononuclear type mononu- ten-coordinate Ln(III) clearcomplexes lanthanide in association complexes ofwith the the formula coordinating [Ln(12-crown-4)(NO nitrate anions.3)3] (Ln Crown = Dy(III) ether (10 ), ligands having smallTb(III) cavity (11), Ho(III) size (12such) Er(III) as (13 ))12-crown-4-ether [42]. These highly crystalline(12-crown-4 complexes = 1,4,7,10- were easily prepared by the reaction of an equimolar mixture of the hydrated lanthanide nitrate salts tetraoxacyclododecane)and could 12-crown-4 form half-sandwich ether in acetonitrile type solvent.mononuclear The half-sandwich lanthanide complexes structure around of the the 3 3 − formula [Ln(12-crown-4)(NOLn(III) ion) ] comprise (Ln = Dy(III) of one 12-crown-4(10), Tb(III) ether (11), ligand Ho(III) on one(12 side) Er(III) and three(13)) bidentate[42]. These NO 3 highly crystalline complexesligands onwere the othereasily side prepared giving rise by tothe a ten-coordinatereaction of an sphenocorona equimolar (SPC,mixtureC2v )of geometry the hydrated lanthanide nitrate(Figure 9salts). It isand important 12-crown-4 to note ether that the in reactionacetonitrile of an equimolarsolvent. The mixture half-sandwich of 12-crown-4 structure around the etherLn(III) and ion 15-crown-5 comprise ether of withone hydrated12-crown-4 lanthanide ether ligand perchlorate on one in acetonitrile side and resulted three in a sandwich-type complex of the formula, [Dy(12-crown-4)(15-crown-5)(CH3CN)][Dy(12- bidentate NO3− ligands on the other side giving rise to a ten-coordinate sphenocorona (SPC, C2v) crown-4)(15-crown-5)]2(CH3CN)2(ClO4)9 having mixed coordination numbers (10 and 9). geometry (Figure 9). ItDynamic is important AC magnetic to note measurementsthat the reaction were of performed an equimo onlar10 –mixture13 to assess of 12-crown-4 the SIM proper- ether and 15-crown-5ties. ether None with of thesehydrated complexes lanthani showedde perchlorate well-resolved in peaks acetonitrile in the frequency-dependent resulted in a sandwich-type complexout-of-phase of the formula, (χ”) AC [Dy(12-crown-4)(15-crown-5)(CH susceptibility measurements at zero3CN)][Dy(12-crown-4)(15- biased field. However, the ap- crown-5)]2(CH3CN)2(ClOplication4)9 having of a DCmixed biased coordination field to suppress number thes QTM(10 and partially 9). Dynamic or fully resultedAC magnetic in strong χ measurements were performedsignals in the on frequency-dependent 10–13 to assess the out-of-phase SIM properties. ( ”) AC None susceptibility of these onlycomplexes in the case of the Dy(III) derivative confirming the presence of slow relaxation of magnetization showed well-resolved(Figure peaks9). in From the thefrequency-dependent ln( τ) vs. T−1 plot at highout-of-phase temperature (χ″ the) AC following susceptibility parameters −11 measurements at zerocould biased be field. extracted: However,Ueff = 76the K application (τ0 = 6.8 × 10 of a s)DC at HbiasedDC = 500 field Oe to [42 suppress] and Ueff the= 68 K −10 QTM partially or fully( τresulted0 = 2.07 × in10 strong) at HsignDC =als 1000 in the Oe [43frequency-dependent] respectively. The non-Kramer out-of-phase Tb(III) (χ″ and) AC Ho(III) susceptibility only in the case of the Dy(III) derivative confirming the presence of slow relaxation of magnetization (Figure 9). From the ln(τ) vs. T−1 plot at high temperature the following parameters could be extracted: Ueff = 76 K (τ0 = 6.8 × 10−11 s) at HDC = 500 Oe [42] and Ueff = 68 K (τ0 = 2.07 × 10−10) at HDC = 1000 Oe [43] respectively. The non-Kramer Tb(III) and Ho(III) ions did not show this behavior because of the large energy gap between the ground and first excited states and the presence of low symmetry ligand field (sphenocorona, C2v) around the metal ion. On the other hand, the Kramer ion, Er(III) did not show non-zero AC signals probably due to the existence of strong QTM or a non Ising ground state.

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ions did not show this behavior because of the large energy gap between the ground and ﬁrst excited states and the presence of low symmetry ligand ﬁeld (sphenocorona, C2v) around the metal ion. On the other hand, the Kramer ion, Er(III) did not show non-zero AC signals probably due to the existence of strong QTM or a non Ising ground state. Magnetochemistry 2020, 6, x FOR PEER REVIEW 10 of 29

Figure 9. (a) MolecularFigure 9. structure(a) Molecular of 10, structure (b) sphenocona of 10,(b coordination) sphenocona geometry coordination around geometry the central around the central Dy(III) ion, and Dy(III)(c) frequency-dependent ion, and (c) frequency-dependent in-phase (χ’) and in-phase out-of-phase (χ’) and (χ″ out-of-phase) AC susceptibilities (χ”) AC susceptibilities from from 2.0 to 5.5 K for 10 under H = 500 Oe. Figures and captions are reproduced from Reference [42] 2.0 to 5.5 K for 10 under HDC = 500 Oe. FiguresDC and captions are reproduced from Reference [42] with permission fromwith the Royal permission Society from of Chemistry. the Royal Society of Chemistry. A larger cavity sized crown ether ligand such as 18-crown-6 ether can accommodate A larger cavity sized crown ether ligand such as 18-crown-6 ether can accommodate only one only one Ln(III) ion inside its cavity and therefore serves as a flexible equatorial ligand. Ln(III) ion inside its cavity and therefore serves as a flexible equatorial ligand. Zheng and co-workers Zheng and co-workers synthesized two isostructural ten-coordinate complexes of the synthesized two isostructural ten-coordinate complexes of the formula [Dy(18-crown-6)(NO3)2]ClO4 formula [Dy(18-crown-6)(NO3)2]ClO4 (14) and [Dy(18-crown-6)(NO3)2]BPh4 (15) by em- (14) and [Dy(18-crown-6)(NOploying two3) different2]BPh4 (15 charge) by employing balancing countertwo different anions charge [43]. The balancing cationic unitcounter in both the + + anions.[43] The cationiccomplexes, unit in i.e., both [Dy(18-C-6)(NO the complexes,3) i.e.,2] were[Dy(18-C-6)(NO similar (Figure3)2] 10were). The similar(Figure Dy–O bond lengths10). aris- The Dy–O bond lengthsing from arising the twofrom nitrate the two ligands nitrate are ligands comparatively are comparatively shorter (Dy–O shorter range (Dy–O 2.402(3)–2.439(3) range 2.402(3)–2.439(3) Å)Å) than than the the Dy–O Dy–O bond bond lengths lengths of of the the 18-crown-6 18-crown-6 ligand ligand (Dy–O (Dy–O range 2.450(5)– 2.450(5)–2.562(3) 2.562(3) Å) in both theÅ) incomplexes. both the complexes.The crown ether The crown ligand ether in 15 ligandwas found in 15 towas be more found distorted to be more than distorted that in 14. All thesethan structural that in differ14. Allences these resulted structural in different differences coordination resulted polyhedra in different in coordination 14 and 15 poly- namely sphenocoronahedra (CShM in 14 = 2.28)and for15 namely14 and bicapped sphenocorona square (CShMantiprism = 2.28)(CShM for = 3.10)14 and for bicapped15. None square of the two complexesantiprism showed (CShM out-of-phase = 3.10) for(χ″15) AC. None magnetic of the susceptibility two complexes signals showed in out-of-phasethe dynamic (χ”) AC AC magnetic measurementsmagnetic under susceptibility zero field. signals However, in the at dynamic an optimum AC magneticexternal DC measurements field of 1000 underOe, zero field. However, at an optimum external DC field of 1000 Oe, both the complexes showed both the complexes showed strong frequency-dependent in-phase (χ’) and out-of-phase (χ″) AC strong frequency-dependent in-phase (χ’) and out-of-phase (χ”) AC susceptibility signals. susceptibility signals. The analysis of this data afforded the following parameters: Ueff = 63 K (HDC = The analysis of this data afforded the following parameters: U = 63 K (H = 1000 Oe) −8 eff −6 DC 1000 Oe) and τ0 = 1.02 × 10 s for 14 and−8 Ueff = 43 K (HDC = 1000 Oe) and τ0 = 1.37 × 10 s for 15 −6 and τ0 = 1.02 × 10 s for 14 and Ueff = 43 K (HDC = 1000 Oe) and τ0 = 1.37 × 10 s respectively (Figurefor 10).15 Irrespectiverespectively of (Figure the st ructural10). Irrespective similarity of in the the structural cationic part similarity of 14 and in the 15 cationic, the part significant differenceof 14in theand anisotropic15, the significant barriers differencesolely resulted in the from anisotropic the geometrical barriers perturbations solely resulted from around the Dy(III) center.the geometrical In the low perturbations temperature regime, around the the Raman Dy(III) relaxation center. In process the low was temperature found to regime, be the dominant relaxationthe Raman process. relaxation This processstudy concludes was found that to be 18-crown-6 the dominant crown relaxation ether ligand process. could This study play an important roleconcludes as a ligand that 18-crown-6for designing crown axially ether tuned ligand high-performan could play ance important SMMs in rolethe future. as a ligand for designing axially tuned high-performance SMMs in the future.

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FigureFigure 10. 10. SchemeScheme sketch sketch (a), for (a side), for view side (b) view for 14 ( andb) for (c) 14for and15) of ( c[Dy(18-crown-6)(NO) for 15) of [Dy(18-crown-3)2]+ cation. Inset:6)(NO dihedral) ]+ cation. angles for Inset: apical dihedral bidentate anglesnitrate anions, for apical (d) and bidentate (e) Temperature-dependence nitrate anions, (d) andof the 3 2 in-phase(e) Temperature-dependence (χ’) and out-of-phase (χ″) of AC the susceptibility in-phase (χ signals’) and out-of-phaseunder 1000 Oe ( χDC”) ACfieldsusceptibility at the indicated frequencies for complexes 14 and 15. Lines are visual guides only. +Figures and captions are Figure 10. Scheme sketchsignals (a), underfor side 1000view ( Oeb) for DC 14 ﬁeldand (c at) for the 15 indicated) of [Dy(18-crown-6)(NO frequencies3 for)2] cation. complexes 14 and 15. reproduced from Reference [43] with permission from the Royal Society of Chemistry. Inset: dihedral anglesLines for apical arevisual bidentate guides nitrate only. anions, Figures (d) and and (e) captionsTemperature-dependence are reproduced of from the Reference [43] in-phase (χ’) and out-of-phasewith permission (χ″) AC fromsusceptibility the Royal signals Society under of 1000 Chemistry. Oe DC field at the indicated frequencies forBidentate complexes N,N-Donor 14 and 15 Ligands. Lines are visual guides only. Figures and captions are reproduced fromBidentate ReferenceBidentate N,N-Donor [43] N,N-donor with permis Ligands sionligands from thesuch Royal as Society phenan of Chemistry.throline (phen) and bipyridine (bipy) in combinationBidentate with N,N-donor chelating nitrate ligands ligands such are as know phenanthrolinen to produce (phen)mononuclear and bipyridine ten-coordinate (bipy) Ln(III) in Bidentate N,N-Donor Ligands complexescombination of withthe formula chelating [Ln(L) nitrate2(NO ligands3)3] (where are known L is tophen produce and mononuclearbipy). Liang ten-coordinateand co-workers synthesized a mononuclear ten-coordination complex [Dy(phen)2(NO3)3] (16) by the reaction of 1,10- Bidentate N,N-donorLn(III) complexes ligands ofsuch the as formula phenan [Ln(L)throline2(NO (phen)3)3] (where and bipyridine L is phen and(bipy) bipy). in Liang and co- phenanthroline and Dy(NO3)3∙6H2O under solvothermal conditions [44]. A perspective view of 16 is combination with workerschelating synthesized nitrate ligands amononuclear are known to produce ten-coordination mononuclear complex ten-coordinate [Dy(phen) Ln(III)2(NO 3)3](16) by the − complexes of theshown formula in Figure [Ln(L) 11.2(NO The3 )coordina3] (wheretion L ofis twophen phen and and bi threepy). NOLiang3 ligands and co-workers resulted in two symmetrical reaction of 1,10-phenanthroline and Dy(NO3)3·6H2O under solvothermal conditions [44]. synthesized a mononuclear5-membered ten-c ringsoordination (DyN2C2) complex and three [Dy(phen) 4-membered2(NO rings3)3] (16 (DyO2N)) by the reaction in 16. This of 1,10-complex did not show− A perspective view of 16 is shown in Figure 11. The coordination of two phen and three NO3 phenanthroline andanyligands Dy(NO peaks resulted 3in)3 ∙the6H 2dynamicO inunder two solvothermalAC symmetrical magnetic measurementsconditions 5-membered [44]. performed ringsA perspective (DyN2C2) under view the and of zero 16 three isDC field. 4-membered Under an applied DC field of 1500 Oe 16 showed significant− dependence on both frequency and temperature shown in Figure 11.rings The coordina (DyO2N)tion in of16 two. This phen complex and three did NO not3 ligands show resulted any peaks in two in thesymmetrical dynamic AC magnetic in the out-of-phase (χ″) AC susceptibility components in the temperature range 2–10 K revealing its 5-membered ringsmeasurements (DyN2C2) and three performed 4-membered under rings the zero (DyO2N) DC field. in 16.Under This complex an applied did not DC show field of 1500 Oe 16 field-induced slow magnetic relaxation characteristics (Figure 11). The complex 16 also exhibited a any peaks in the dynamicshowed AC significant magnetic dependence measurements on performed both frequency under the and zero temperature DC field. Under in the an out-of-phase (χ”) applied DC field redof 1500 emission Oe 16 (~630showed nm) significant corresponding dependence to the on 4F 9/2both → frequency6H13/2 transition. and temperature Investigation of such dual AC susceptibility components in the temperature range 2–10 K revealing its field-induced in the out-of-phaseproperties (χ″) AC susceptibilityin a single complex components is currently in the in temperature vogue in the range field 2–10of molecular K revealing magnetism. its slow magnetic relaxation characteristics (Figure 11). The complex 16 also exhibited a red field-induced slow magnetic relaxation characteristics (Figure 11).4 The complex6 16 also exhibited a emission (~630 nm) corresponding to the F9/2 → H13/2 transition. Investigation of such red emission (~630 nm) corresponding to the 4F9/2 → 6H13/2 transition. Investigation of such dual dual properties in a single complex is currently in vogue in the field of molecular magnetism. properties in a single complex is currently in vogue in the field of molecular magnetism.

Figure 11. (a) Molecular structure and (b) temperature dependence of the in-phase (χ’) and out-of- phase (χ″) AC susceptibilities for different frequencies at a DC field of 1500 Oe of 16. Figures and captions are reproduced from Reference [44] with permission from the Royal Society of Chemistry.

Figure 11. (a) MolecularFigure 11. structure(a) Molecular and (b) structure temperature and dependence (b) temperature of the in-phase dependence (χ’) and of theout-of- in-phase (χ’) and out-of-

phase (χ″) ACphase susceptibilities (χ”) AC for susceptibilities different frequencies for different at a DC frequencies field of 1500 at Oe a DC of 16 ﬁeld. Figures of 1500 and Oe of 16. Figures and captions are reproduced from Reference [44] with permission from the Royal Society of Chemistry. captions are reproduced from Reference [44] with permission from the Royal Society of Chemistry.

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Magnetochemistry 2020, 6, x FOR PEER REVIEW 12 of 29 Korolkov and co-workers introduced a chiral (–)-menthol fragment in the ortho Korolkov and co-workersposition of introduc phenanthrolineed a chiral resulting (–)-menthol in a new fragment chiral ligand,in the ortho 2-((1S,2R,5S)-2-isopropyl-5- position of phenanthroline resultingmethylcyclohexyl)-1,10-phenanthroline in a new chiral ligand, 2-((1S,2R,5S)-2-isopropyl- (L1). Further,5-methylcyclohexyl)-1,10-L1 was utilized to synthesize a series phenanthroline (L1). Further,of mononuclear L1 was utilized ten-coordinate to synthesize Ln(III) a complexes,series of mononuclear [Ln(L1)2(NO ten-coordinate3)3] (Ln = Eu; (17), Gd; Ln(III) complexes, [Ln((18L1),) Tb;(2(NO193)),3] Dy;((Ln 20= ))Eu; [45 (].17 The), Gd; molecular (18), Tb;( structure19), Dy;( analysis20)) [45]. revealed The molecular the presence of a structure analysis revealed2-fold the rotational presence axis of passinga 2-fold through rotational the axis metal passing center through and the N–Othe metal bond center of a coordinated and the N–O bond ofnitrate a coordinated ligand (Figure nitrate 12 ).ligand It is to (Figure be noted 12). that It the is chairto be conformation noted that the of thechair 6-membered carbocycle belonging to the menthol moiety did not change upon coordination by L1. conformation of the 6-membered carbocycle belonging to the menthol moiety did not change upon A comparison of the crystal parameters of the Tb(III) derivative with its achiral analogue, coordination by L1. A comparison of the crystal parameters of the Tb(III) derivative with its achiral [Tb(phen)2(NO3)3](21)[46] revealed that the presence of the chiral ligand changes the unit analogue, [Tb(phen)2(NO3)3] (21) [46] revealed that the presence of the chiral ligand changes the unit cell parameters and the space group (centrosymmetric (C2/c) in 21 to non-centrosymmetric cell parameters and the space group (centrosymmetric (C2/c) in 21 to non-centrosymmetric (P41212) (P41212) in 19). The temperature-dependent µeff (effective magnetic moment) values for 17– in 19). The temperature-dependent μeff (effective magnetic moment) values for 17–20 agree well with 20 agree well with the achiral analogue [Ln(phen)2(NO3)3][47] which suggests that the high the achiral analogue [Ln(phen)2(NO3)3] [47] which suggests that the high temperature μeff values are temperature µeff values are not affected by the replacement of phen by L1. None of these not affected by the replacementcomplexes of showed phen anyby L1 peaks. None in theof these frequency-dependent complexes showed out-of-phase any peaks ( χin”) the AC magnetic frequency-dependent measurementsout-of-phase (χ″ at) 2AC K evenmagnetic at high measurements frequency. at 2 K even at high frequency.

Figure 12. MolecularFigure structure 12. Molecular of 19. Hydrogen structure atoms of 19 are. Hydrogen omitted for atoms clarity. are omittedThe figure for is clarity. reproduced The ﬁgure is reproduced from Reference [45]from with Reference permission [45 from] with the permission Royal Society from of the Chemistry. Royal Society of Chemistry.

Modifying the phenanthrolineModifying backbone the phenanthroline Lau and co-workers backbone synthesized Lau and a co-workersphenanthroline- synthesized a 2 9 L2 amide ligand, N2,N9-dibutyl-1,10-phenanthrophenanthroline-amide ligand,line-2,9-dicarboxamide N ,N -dibutyl-1,10-phenanthroline-2,9-dicarboxamide (L2) [48]. In comparison to the ( )[48]. In comparison to the ligands discussed above, L2 is now a tetradentate ligand symmetrically ligands discussed above, L2 is now a tetradentate ligand symmetrically substituted by amide substituted by amide functionalities at the 2 and 9 positions of the phenanthroline moiety. functionalities at the 2 and 9 positions of the phenanthroline moiety. The reaction of L2 with The reaction of L2 with Ln(NO3)3·6H2O in MeOH in the presence of NaClO4·H2O afforded Ln(NO3)3∙6H2O in MeOH in the presence of NaClO4∙H2O afforded three isostructural mononuclear three isostructural mononuclear complexes, [Ln(L2)2(NO3)](ClO4)2 (Ln = Eu (22), Sm (23), complexes, [Ln(L2)2(NOTb3)](ClO (24) and4)2 Dy(Ln (=25 Eu)) [(4822].), TheSm ( ten-coordinate23), Tb (24) and geometry Dy (25)) of[48]. Ln(III) The ten-coordinate in 22-25 resulted from the geometry of Ln(III) in coordination22-25 resulted of from four the N atoms coordination and four ofO four atoms N atoms from twoand fourL2 ligands, O atoms and from two O atoms two L2 ligands, and fromtwo O a κatoms2O-nitrate from (Figure a κ2O -nitrate13). All (Figure the complexes 13). All exhibited the complexes sphenocorona exhibited coordination − sphenocorona coordinationgeometries geom withetries an with approximate an approximateC2v symmetry. C2v symmetry. The presence The presence of bulky of ClO bulky4 anions and ClO4− anions and thethe butyl butyl groups groups ensured ensured well-separatedwell-separated magneticmagnetic centers centers with with large large M··· M∙∙∙ separationsM in separations in the rangethe of range 10.8–11.1 of 10.8–11.1 Å. Dynami Å. Dynamicc AC magnetic AC magnetic measurements measurements were wereperformed performed to to probe probe the SIM behaviorthe in SIM the Tb(III) behavior and in Dy(III) the Tb(III) derivatives. and Dy(III) Both derivatives. did not show Both peaks did in not the show out-of- peaks in the phase(χ″) susceptibilityout-of-phase( under a zeroχ”) biased susceptibility DC field underrevealing a zero significant biased DC QTM field in revealing the ground significant state. QTM in Application of an externalthe ground DC field state. (1500 Application Oe for 24 of anand external 1000 Oe DC for field 25) (1500caused Oe the for 24QTMand to 1000 be Oe for 25) suppressed and a clearcaused maxima the QTM in tothe be suppressedfrequency-dependent and a clear maximain-phase in and the frequency-dependentout-of-phase AC in-phase and out-of-phase AC components to be observed (Figure 13). It was surprising that complex components to be observed (Figure 13). It was surprising that complex 24 showed two maxima in the 24 showed two maxima in the out-of-phase(χ”) susceptibility component below 3 K while out-of-phase(χ″) susceptibility component below 3 K while only one maximum was observed in the only one maximum was observed in the temperature range 3 to 5.5 K. The double relaxation temperature range 3 to 5.5 K. The double relaxation behavior was further confirmed in the Cole-Cole plots with two obvious semicircles. The complex relaxation behavior of 24 was analyzed considering three relaxation processes namely Raman, Orbach, and quantum tunneling, following the equation,

Magnetochemistry 2021, 7, 1 13 of 29

Magnetochemistry 2020behavior, 6, x FOR was PEER further REVIEW confirmed in the Cole-Cole plots with two obvious semicircles. The com- 13 of 29 plex relaxation behavior of 24 was analyzed considering three relaxation processes namely −1 n −1 τ−1 = CTn + τ0−1exp(–Raman,Ueff/k Orbach,BT) + τ andQTM quantum which resulted tunneling, in following Ueff = 76.0 the equation,K, τ0 = 1.35τ =× CT10−11+ s,τ 0C =exp(– 3.27 K−4.72 s−1, × −11 −4.72 −1 Ueff/kBT) + τQTM−4 which resulted in Ueff = 76.0 K, τ0 = 1.35 10 s, C = 3.27 K s , n = 4.72, and τQTM = 3.93 × 10 s respectively.−4 Although the Dy(III) derivative is isostructural with the n = 4.72, and τQTM = 3.93 × 10 s respectively. Although the Dy(III) derivative is isostruc- Tb(III) derivativetural only with on thee relaxation Tb(III) derivative process only was one relaxationobserved process in this was case observed as confirmed in this case by as the Cole- Cole plots. The temperature-dependentconfirmed by the Cole-Cole plots.relaxation The temperature-dependent times of 25 were analyzed relaxation considering times of 25 were the Raman and Orbach relaxationanalyzed mechanisms considering the (first Raman two and terms Orbach in the relaxation previous mechanisms equation) (first and two an terms energy in barrier the previous equation) and an energy barrier Ueff = 44.3 K and pre-exponential parameter Ueff = 44.3 K and pre-exponential−7 parameter τ0 = 5.17 × 10−7 s, respectively was extracted. Also, the τ0 = 5.17 × 10 s, respectively was extracted. Also, the Raman relaxation parameters C and Raman relaxationn wereparameters found to beC and 0.475 n K −were4.29 s− found1 and 4.29, to be respectively. 0.475 K−4.29 s−1 and 4.29, respectively.

Figure 13. (a,b) Perspective viewings for the cationic structures of 24 and 25,(c,d) frequency depen- Figure 13. (a,b) Perspective viewings for the cationic structures of 24 and 25, (c,d) frequency dence of in-phase (χ0) and out-of-phase (χ”) AC magnetic susceptibility for 24 and 25 under a 1.5 kOe dependence of(for in-phase24) and 1.0 (χ kOe′) and (for 25out-of-phase) DC ﬁeld, and (χ (e″,)f )AC the ln(magneticτ) vs. T− 1susceptibilityplots for 24 and for25 (red 24 linesand are25 theunder a 1.5 kOe (for 24best) and ﬁts). 1.0 Figures kOe and (for captions 25) DC are field, reproduced and (e from,f) the Reference ln(τ) vs. [48] T with−1 plots permission for 24 from and the 25 Royal (red lines are the best fits).Society Figures of Chemistry. and captions are reproduced from Reference [48] with permission from the Royal Society of Chemistry.

Magnetochemistry 2021, 7, 1 14 of 29

Terpyridyl Ligand Terpyridine ligands are known to produce stable mononuclear complexes with 4f ions. Wand and co-workers synthesized a ten-coordinate Ce(III) complex, [Ce(Fcterpy)(NO3)3(H2O)] (26) with a ferrocene-based terpyridine ligand (Figure 14), 40-ferrocenyl-2,20:60,2”-terpyridine (Fcterpy) [49]. The deca-coordination around the Ce(III) ion resulted from the coordination of one Fcterpy, three bidentate nitrate ligands and one water molecule. The polyhedral geometry Magnetochemistry 2020, 6, x FOR PEER REVIEW 14 of 29 could be described as distorted bicapped square antiprism with two nitrato O atoms in the capping positions. Static magnetic susceptibility measurement revealed a χMT value of −1 0.55 emu K mol at room temperature which is slightly lowerCe than the theoretical value of −1 The experimental susceptibility data−1 was modelled2 [50] and g = 0.686 and D = 0.04 cm was 0.80 emu K mol for a Ce(III) ( F5/2, S = 1/2; L = 3, g = 0.857) for the monomeric structure. extracted from the bestField-dependent fit. The lower magnetization value measurements of the obtained with the non-saturation zero-field of magnetizationsplitting parameter (D) even at high fields revealed the presence of magnetic anisotropy in 26. The experimental represents a small magnetic anisotropy in 26. AC magnetic measurements−1 did not show peaks in the susceptibility data was modelled [50] and gCe = 0.686 and D = 0.04 cm was extracted from out-of-phase (χ″M) susceptibilitythe best fit. The component lower value of theunder obtained zero zero-field biased splitting field. parameter Under (D an) represents applied DC field of a small magnetic anisotropy in 26. AC magnetic measurements did not show peaks in the 2000 Oe, strong frequency-dependent maxima in the out-of-phase (χ″M) susceptibility with peak out-of-phase (χ”M) susceptibility component under zero biased field. Under an applied DC maxima shifting towardsfield oflower 2000 Oe, frequencies strong frequency-dependent verified the maxima slow in the relaxation out-of-phase (ofχ”M the) susceptibility magnetization in 26. The Cole-Cole plots alsowith revealed peak maxima a narrow shifting towards distribution lower frequencies of relaxation verified the times slow relaxationwith α of= 0.05–0.11 the (2–3 K) magnetization in 26. The Cole-Cole plots also revealed a narrow distribution of relaxation (Figure 14). times with α = 0.05–0.11 (2–3 K) (Figure 14).

Figure 14. (a) Molecular structure of 26,(b) frequency dependence of the in-phase (χ0) (top) and out- Figure 14. (a) Molecular structure of 26, (b) frequency dependence of the in-phase (χ′) (top) and out- of-phase (χ”M) (bottom) magnetic susceptibility of 26 between 2 and 3 K under 2 kOe, (c) Cole–Cole of-phase (χ″M) (bottom)plots of magnetic26 between 2susceptibility K (black) and 3 K (red) of 26 (the between solid lines represent 2 and the3 K best under ﬁts as calculated 2 kOe, with (c)Cole–Cole plots of 26 between an2 extendedK (black) Debye and model). 3 K Figures(red) and(the captions solid are lines reproduced represent from Reference the best [49] withfits permissionas calculated with from Elsevier Ltd. an extended Debye model). Figures and captions are reproduced from Reference [49] with permission Schiff Base and Semicarbazone Ligands from Elsevier Ltd. Hetero donor Schiff base ligands are widely used as compartmental ligands to assemble a variety of homo- and heterometallic complexes with different nuclearity [51–53]. Schiff Base and SemicarbazoneShanmugam Ligands and co-workers synthesized a couple of ten-coordinated Ln(III) complexes of Hetero donor Schiff base ligands are widely used as compartmental ligands to assemble a variety of homo- and heterometallic complexes with different nuclearity [51–53]. Shanmugam and co- workers synthesized a couple of ten-coordinated Ln(III) complexes of the common formula [Ln(HL3)2(NO3)3] (where Ln is Dy(III) (27); Pr(III) (28) and HL3 = 2-methoxy-6-[(E)- phenyliminomethyl]phenol) (Figure 15) [54]. The central Ln(III) in both the complexes is present in a distorted bicapped square anti-prism geometry. Interestingly, in these complexes the donor N atom of the ligand does not take part in coordination. Rather, it accepts the acidic proton from the phenolic -OH group of the ligand. This deprotonated phenolic group with another -OMe group of two participating ligands provides four coordinating atoms to the Ln(III) ion. The remaining coordination sites are occupied by three chelating nitrate ions which also provide charge neutrality to the complex. Interestingly, complex 27 (Dy(III) analogue) has been found to be co-crystalized with another geometric isomer with the same molecular formula with a different orientation of the coordinated nitrate groups. The room temperature χMT values for Dy(III) analogue (14.06 cm3Kmol−1) and the Pr(III) analogue (1.61 cm3 K mol−1) are consistent with the theoretically expected values. Dynamic magnetization studies revealed that the Dy(III) analogue was a field-induced SMM at an optimum field of 2 kOe. From the Cole-Cole plot (Figure 16), two types of closely spaced relaxation dynamics were detected. These were rationalized as occurring due to the presence of two geometrical isomers in the crystal structure of 27. The extracted effective energy barriers from Arrhenius equation were found as 16.6 cm−1 (τ0 = 2.47 × 10−6 s) and 15.8 cm−1 (τ0 = 3.6 × 10−7 s).

Magnetochemistry 2021, 7, 1 15 of 29

the common formula [Ln(HL3)2(NO3)3] (where Ln is Dy(III) (27); Pr(III) (28) and HL3 = 2- methoxy-6-[(E)-phenyliminomethyl]phenol) (Figure 15)[54]. The central Ln(III) in both the complexes is present in a distorted bicapped square anti-prism geometry. Interestingly, in these complexes the donor N atom of the ligand does not take part in coordination. Rather, it accepts the acidic proton from the phenolic -OH group of the ligand. This deprotonated phenolic group with another -OMe group of two participating ligands provides four coordinating atoms to the Ln(III) ion. The remaining coordination sites are occupied by three chelating nitrate ions which also provide charge neutrality to the complex. Interestingly, complex 27 (Dy(III) analogue) has been found to be co-crystalized with another geometric isomer with the same molecular formula with a different orientation of the coordinated nitrate groups. The room temperature χMT values for Dy(III) analogue (14.06 cm3Kmol−1) and the Pr(III) analogue (1.61 cm3 K mol−1) are consistent with the theoretically expected values. Dynamic magnetization studies revealed that the Dy(III) analogue was a ﬁeld-induced SMM at an optimum ﬁeld of 2 kOe. From the Cole-Cole plot (Figure 16), two types of closely spaced relaxation dynamics were detected. These were rationalized as occurring due to the presence of two geometrical isomers in the crystal structure of 27. The extracted effective energy barriers from Arrhenius equation were found −1 −6 −1 −7 Magnetochemistryas 16.6 cm 2020, (6τ, x0 FOR= 2.47 PEER× REVIEW10 s) and 15.8 cm (τ0 = 3.6 × 10 s). 15 of 29 Magnetochemistry 2020, 6, x FOR PEER REVIEW 15 of 29

FigureFigure 15. Molecular 15. Molecular structures structures of the of thetwo two geometrical geometrical isomers isomers 2727 (left)) andand27 270 ʹ( right(right).). Figures Figures and Figure 15. Molecular structures of the two geometrical isomers 27 (left) and 27ʹ (right). Figures and captionsand are captions reproduced are reproduced from Reference from Reference [54] with [54 permission] with permission from the from Wiley-VCH. the Wiley-VCH. captions are reproduced from Reference [54] with permission from the Wiley-VCH.

Figure 16. Frequency-dependent out-of-phase susceptibility data measured for polycrystalline Figure 16. Frequency-dependentFigure 16. Frequency-dependent out-of-phase susceptibility out-of-phase datasusceptibility measured for data polycrystalline measured for polycrystalline sample of 27. Figures and captions are reproduced from Reference [54] with permission from the sample of 27. Figuressample and captions of 27. Figures are reproduced and captions from are Reference reproduced [54] from with Reference permission [54 ]from with the permission from the Wiley-VCH. Wiley-VCH. Wiley-VCH. Ab initio CASSCF SINGLE_ANISO calculations on the Dy(III) analogue were performed by Ab initio CASSCF SINGLE_ANISO calculations on the Dy(III) analogue were performed by taking the two distinct geometrical isomers separately to ascertain the origin of the magnetic taking the two distinct geometrical isomers separately to ascertain the origin of the magnetic relaxation (Figure 17). In both the isomers, although the ground Kramer’s doublet is computed to be relaxation (Figure 17). In both the isomers, although the ground Kramer’s doublet is computed to be ±15/2 a significant multiconfigurational wave nature of the ground state was observed which opened ±15/2 a significant multiconfigurational wave nature of the ground state was observed which opened the channel for ground state QTM which accounted for the absence of SMM behavior in zero field. the channel for ground state QTM which accounted for the absence of SMM behavior in zero field.

Figure 17. Ab initio computed matrix elements between the connecting pairs (ground state and first Figure 17. Ab initio computed matrix elements between the connecting pairs (ground state and first excited state) in complexes 27 (a) and 27′ (b). The thick black line indicates the Kramer’s doublets excited state) in complexes 27 (a) and 27′ (b). The thick black line indicates the Kramer’s doublets

Magnetochemistry 2020, 6, x FOR PEER REVIEW 15 of 29

Figure 15. Molecular structures of the two geometrical isomers 27 (left) and 27ʹ (right). Figures and captions are reproduced from Reference [54] with permission from the Wiley-VCH.

Magnetochemistry 7 2021, , 1 Figure 16. Frequency-dependent out-of-phase susceptibility data measured for polycrystalline16 of 29 sample of 27. Figures and captions are reproduced from Reference [54] with permission from the Wiley-VCH. Ab initio CASSCF SINGLE_ANISO calculations on the Dy(III) analogue were per- Abformed initio by CASSCF taking the SINGLE two distinct_ANISO geometrical calculations isomers on the separately Dy(III) toanalogue ascertain were the originperformed of by takingthe the magnetic two distinct relaxation geometrical (Figure 17 ).isomers In both separa the isomers,tely to although ascertain the the ground origin Kramer’s of the dou-magnetic relaxationblet is (Figure computed 17). toIn beboth±15/2 the isomers, a significant although multiconfigurational the ground Kram waveer’s naturedoublet of is the computed ground to be ±15/2state a significant was observed multiconfigurational which opened thewave channel nature for of groundthe ground state state QTM was which observed accounted which for opened the channelthe absence for ground of SMM state behavior QTM which in zero accounted field. for the absence of SMM behavior in zero field.

FigureFigure 17. Ab 17. initioAb computed initio computed matrix matrixelements elements between between the connecting the connecting pairs (ground pairs (ground state and first 0 excitedstate state) and in ﬁrst complexes excited state)27 (a) in and complexes 27′ (b). 27The(a )thick and 27black(b ).line The indicates thick black the line Kramer’s indicates doublets the Kramer’s doublets (KDs) as a function of magnetic moment. The dotted green lines show the possible pathway of the Orbach process. The zigzag lines connecting the ground state KDs represent the QTM. The dotted blues lines show the thermally activated-QTM through the ﬁrst excited state. Figures and captions are reproduced from Reference [54] with permission from the Wiley-VCH.

A subcomponent self-assembly strategy was employed by Mallah and co-workers to synthesize a series of mononuclear ten-coordinate complexes, [Ln(L4)(NO3)2](NO3) where Ln(III) = Dy, (29); Tb, (30); Ho, (31); Er, (32); Y, (33) and L4 = N,N0-bis-pyridin-2-yl- methelene-1,8-diamino-3,6-dioxaoctane (Figure 18)[55]. The Ln(III) ion in these complexes served as a template around which the ligands were organized. The deca-coordination around the Ln(III) ions resulted from the coordination of L4 with N4O2 coordination sites − in an equatorial plane and four O atoms from two NO3 ligands from above and below this plane. The equatorial plane comprising the L4 ligand was highly distorted due to the steric bulk of the pyridine rings. The LnN4O6 polyhedron in these complexes has been described as very distorted bicapped square antiprism. To probe the SMM properties in 29−32, dynamic AC magnetic measurements were performed. Under zero DC field, only the complexes 29 (Dy) and 30 (Tb) showed frequency-dependent out-of-phase (χ”) susceptibility signals, but without any clear maxima. This behavior was attributed to a fast QTM. Upon application of an optimal biased field of 1000 Oe both frequency-dependent AC signals (in-phase and out-of-phase) were observed for 29 and 30 below 10 K though only 29 (Dy) showed a clear maxima (Figure 18). The ln(τ) vs. 1/T plots were fitted with −7 the Arrhenius law which resulted in Ueff = 50 K and τ0 = 6.80 × 10 s. The Cole-Cole plots with semi-circular curves confirmed a single relaxation time with α parameter close to zero though at low temperature the appearance of a second semicircle can be seen. Both the complexes revealed hysteresis loops in the micro-SQUID measurements at varying sweep rates. The Dy(III) derivative showed step-like features signifying QTM at the level crossing while the Tb derivative did not. Magnetochemistry 2020, 6, x FOR PEER REVIEW 16 of 29

(KDs) as a function of magnetic moment. The dotted green lines show the possible pathway of the Orbach process. The zigzag lines connecting the ground state KDs represent the QTM. The dotted blues lines show the thermally activated-QTM through the first excited state. Figures and captions are reproduced from Reference [54] with permission from the Wiley-VCH.

A subcomponent self-assembly strategy was employed by Mallah and co-workers to synthesize a series of mononuclear ten-coordinate complexes, [Ln(L4)(NO3)2](NO3) where Ln(III) = Dy, (29); Tb, (30); Ho, (31); Er, (32); Y, (33) and L4 = N,N′-bis-pyridin-2-yl-methelene-1,8-diamino-3,6-dioxaoctane (Figure 18) [55]. The Ln(III) ion in these complexes served as a template around which the ligands were organized. The deca-coordination around the Ln(III) ions resulted from the coordination of L4 with N4O2 coordination sites in an equatorial plane and four O atoms from two NO3− ligands from above and below this plane. The equatorial plane comprising the L4 ligand was highly distorted due to the steric bulk of the pyridine rings. The LnN4O6 polyhedron in these complexes has been described as very distorted bicapped square antiprism. To probe the SMM properties in 29−32, dynamic AC magnetic measurements were performed. Under zero DC field, only the complexes 29 (Dy) and 30 (Tb) showed frequency-dependent out-of-phase (χ″) susceptibility signals, but without any clear maxima. This behavior was attributed to a fast QTM. Upon application of an optimal biased field of 1000 Oe both frequency-dependent AC signals (in-phase and out-of-phase) were observed for 29 and 30 below 10 K though only 29 (Dy) showed a clear maxima (Figure 18). The ln(τ) vs. 1/T plots were fitted with the Arrhenius law which resulted in Ueff = 50 K and τ0 = 6.80 × 10−7 s. The Cole-Cole plots with semi-circular curves confirmed a single relaxation time with α parameter close to zero though at low temperature the appearance of a second semicircle can be seen. Both the complexes revealed hysteresis loops in the micro-SQUID measurements at varying sweep rates. The Dy(III) Magnetochemistry 2021, 7, 1 17 of 29 derivative showed step-like features signifying QTM at the level crossing while the Tb derivative did not.

Figure 18. (a) Molecular structure of 29,(b) Cole-Cole plots for 29 obtained from the magnetic susceptibility data. The solid Figure 18. (a) Molecular structure of 29, (b) Cole-Cole plots for 29 obtained from the magnetic lines represent the ﬁt obtained with a generalized Debye model with α = 0.36, 0.35, 0.29, 0.28, 0.26, 0.21, 0.12, and 0.08 sorted by increasingsusceptibility temperatures, data. The and solid (c) Full lines sweep represent hysteresis the loopsfit obtained for 29 at with the indicated a generalized sweep ratesDebye and model temperature. with α Figures and= cations 0.36, 0.35, are reproduced 0.29, 0.28, with0.26, permission 0.21, 0.12, fromand Reference0.08 sorted [55 by] copyright increasing @ American temperatures, Chemical and Society. (c) Full sweep hysteresis loops for 29 at the indicated sweep rates and temperature. Figures and cations are reproduced with permissionSemicarbazone from Reference based [55] ligands copyright have @ beenAmerican widely Chemical used in coordinationSociety. chemistry including in the formation of lanthanide complexes. Nunes and co-workers used a semicarbazone based Schiff base ligand 1-((E)-2-pyridinylmethylidene)semicarbazone), Hscpy, to prepare Semicarbazone based ligands have been widely used in coordination chemistry including in the mononuclear complexes, [Ln(Hscpy)2 (NO3)2]NO3·MeOH (Ln(III) = Gd (34) and Tb (35)) Magnetochemistryformation 2020, 6, xof FOR lanthanide PEER REVIEW (Figurecomplexes. 19)[56 Nunes]. and co-workers used a semicarbazone based Schiff base 17 of 29 ligand 1-((E)-2-pyridinylmethylidene)semicarbazone), Hscpy, to prepare mononuclear complexes, [Ln(Hscpy)2 (NO3)2]NO3∙MeOH (Ln(III) = Gd (34) and Tb (35)) (Figure 19) [56].

Figure 19. Molecular structureFigure of complex 19. Molecular 35. structureThe figure of complex is reproduced35. The figure from is reproduced Reference from [56] Reference with [56 ] with permission from the Elsevier Ltd. permission from the Elsevier Ltd. The two ligands provided a 4N,2O coordination environment while the remaining four The two ligands provided asites 4N,2O were occupied coordination by the nitrate environment ions. In the AC while susceptibility the remaining measurements, four the complex sites 35 (Tb(III) derivative) did not show any out-of-phase susceptibility peak maxima. However, were occupied by the nitrate ions.upon In applying the AC a staticsusceptibility field of 1kOe andmeasurements, then further 1.5 kOe,the fastcomplex relaxation 35 was (Tb(III) stopped derivative) did not show any out-of-phasewhich allowed susceptibility a peak maxima topeak be observed maxima. (Figure However, 20). The extraction upon applying of the data a by fitting the extended Debye model followed by utilizing the Arrhenius equation produced static field of 1kOe and then further 1.5 kOe, fast relaxation was−6 stopped which−1 allowed a peak4 the following parameters, τ0 = 2.6(1) × 10 s, ∆ = 7.6(2) cm and τQTM = 2.60(3) × 10 Hz −7 −1 maxima to be observed (Figure 20).for the The applied extraction field of 1 kOeof the and dataτ0 = 8.3(5) by fitting× 10 s, the∆ = extended 21.9(4) cm Debyeand τQTM model= 1.92(1) × 104 Hz for applied of 1.5 kOe. followed by utilizing the Arrhenius equation produced the following parameters, τ0 = 2.6(1) × 10−6 s, ∆ = 7.6(2) cm−1 and τQTM = 2.60(3) × 104 Hz for the applied field of 1 kOe andτ0 = 8.3(5) × 10−7 s, ∆ = 21.9(4) cm−1 and τQTM = 1.92(1) × 104 Hz for applied of 1.5 kOe.

Figure 20. (a) Frequency dependence of the out-of-phase magnetic susceptibility of 35, measured for different temperatures ranging from 2.0 K (blue points) to 6.0 K (red points) measured with an applied magnetic field of 1 kOe; (b) Temperature dependence of the relaxation times measured with a static applied magnetic field of 1 kOe (empty circles) and 1.5 kOe (full circles) along with the corresponding best fitting lines. Figures and captions are reproduced from Reference [56] with permission from the Elsevier Ltd.

Tripodal Ligands Konar and co-workers utilized a tripodal ligand, tris(2-benzimidazoylmethyl)amine (ntbi) having bulky benzimidazole arms to capture a single Ln(III) ion into its cavity [57]. Therefore, the reaction of ntbi with hydrated Ln(III) nitrate salts afforded mononuclear ten-coordinate complexes of the formula [Ln(ntbi)(NO3)3] (Ln = Dy (36) and Ho (37)) (Figure 21). It is to be noted that changing the metal precursor to Ln(III) chlorides resulted in eight-coordinate mononuclear complexes where two ntbi ligands chelate one Ln(III) ion. The ten-coordinate sphenocorona polyhedron (CShM = 3.655) around the Ln(III) ions resulted from the coordination of four N atoms from the ntbi ligand and six

Magnetochemistry 2020, 6, x FOR PEER REVIEW 17 of 29

Figure 19. Molecular structure of complex 35. The figure is reproduced from Reference [56] with permission from the Elsevier Ltd.

The two ligands provided a 4N,2O coordination environment while the remaining four sites were occupied by the nitrate ions. In the AC susceptibility measurements, the complex 35 (Tb(III) derivative) did not show any out-of-phase susceptibility peak maxima. However, upon applying a Magnetochemistry 2021, 7, 1 static field of 1kOe and then further 1.5 kOe, fast relaxation was stopped which allowed a peak 18 of 29 maxima to be observed (Figure 20). The extraction of the data by fitting the extended Debye model followed by utilizing the Arrhenius equation produced the following parameters, τ0 = 2.6(1) × 10−6 s, ∆ = 7.6(2) cm−1 and τQTM = 2.60(3) × 104 Hz for the applied field of 1 kOe andτ0 = 8.3(5) × 10−7 s, ∆ = 21.9(4) cm−1 and τQTM = 1.92(1) × 104 Hz for applied of 1.5 kOe.

Figure 20. (a) Frequency dependenceFigure 20. ( ofa) Frequency the out-of-phase dependence of the magnetic out-of-phase magnetic susceptibility susceptibility of of 35,35 measured, measured for for different temperatures different temperatures ranging from 2.0 K (blue points) to 6.0 K (red points) measured with an applied ranging from 2.0 K (blue points)magnetic to 6.0 field Kof 1 (red kOe; (b points)) Temperature measured dependence of with the relaxation an applied times measured magnetic with a static field of 1 kOe; (b) Temperature applied magnetic field of 1 kOe (empty circles) and 1.5 kOe (full circles) along with the corresponding dependence of the relaxation timesbest fitting measured lines. Figures and with captions a are static reproduc applieded from Reference magnetic [56] with permission field of from 1 kOe the (empty circles) and 1.5 kOe (full circles) along with the correspondingElsevier Ltd. best fitting lines. Figures and captions are reproduced from Reference [56] with permission from the ElsevierTripodal Ltd. Ligands Konar and co-workers utilized a tripodal ligand, tris(2-benzimidazoylmethyl)amine (ntbi) having bulky benzimidazole arms to capture a single Ln(III) ion into its cavity [57]. Therefore, the reactionTripodal of ntbi Ligandswith hydrated Ln(III) nitrate salts afforded mononuclear ten-coordinate complexes of the formula [Ln(ntbi)(NO3)3] (Ln = Dy (36) and Ho (37)) (Figure 21). It is to be noted that changing the metalKonar precursor andto Ln(III) co-workers chlorides resulted utilized in eight-coordinate a tripodal mononuclear ligand, complexes tris(2-benzimidazoylmethyl)amine where two(ntbi) ntbi ligands having chelatebulky one Ln(III) benzimidazole ion. The ten-coordinate sphenocorona arms to polyhedron capture (CShM a single = 3.655) Ln(III) ion into its cavity [57]. around the Ln(III) ions resulted from the coordination of four N atoms from the ntbi ligand and six Therefore, the reaction of ntbi with hydrated Ln(III) nitrate salts afforded mononuclear ten-coordinate complexes of the formula [Ln(ntbi)(NO3)3] (Ln = Dy (36) and Ho (37)) (Figure 21). It is to be noted that changing the metal precursor to Ln(III) chlorides resulted in eight-coordinate mononuclear complexes where two ntbi ligands chelate one Ln(III) ion. The ten-coordinate sphenocorona polyhedron (CShM = 3.655) around the Ln(III) ions Magnetochemistry 2020, 6, xresulted FOR PEER from REVIEW the coordination of four N atoms from the ntbi ligand 18and of 29 six O atoms from − the three NO3 ligands. The three free NH groups corresponding to the benzimidazole − O atoms from the threeligands NO3 ligands. of the ntbi The ligandthree free were NH further groups corresponding involved in H-bonding to the benzimidazole interactions with a nitrate ligands of the ntbi ligand were further involved in H-bonding interactions with a nitrate group of group of neighboring [Ln(ntbi)(NO3)3] molecule resulting in undulating 1D chains along neighboring [Ln(ntbi)(NO3)3] molecule resulting in undulating 1D chains along the crystallographic the crystallographic c axis. Further, the N–H···O hydrogen-bonding links could be arranged c axis. Further, the N–H∙∙∙O hydrogen-bonding links could be arranged in 2D grid-like layers and 3D in 2D grid-like layers and 3D supramolecular cubic framework structures. Dynamic AC supramolecular cubic framework structures. Dynamic AC magnetic measurements were performed magnetic measurements were performed to assess the SIM properties in these complexes to assess the SIM properties in these complexes (Figure 21). Under zero applied DC field none of (Figure 21). Under zero applied DC field none of them showed clear maxima in the out-of- them showed clear maxima in the out-of-phase AC susceptibility above 2 K due to fast QTM. phase AC susceptibility above 2 K due to fast QTM. However, under an optimum biased However, under an optimum biased field of 1200 Oe distinct peaks were observed (Ueff = 53.4 K and −12 field of 1200 Oe distinct peaks were observed (U = 53.4 K and τ0 = 1.6 × 10 s). τ0 = 1.6 × 10−12 s). eff

Figure 21. (a) MolecularFigure 21. structure(a) Molecular of the structurecomplex of36, the (b) complex illustration36,( ofb) illustrationthe “sphenocorona” of the “sphenocorona” coordina- coordination geometry of the Dy(III) center, and (c) out-of-phase (χ″M) vs. T AC susceptibility plots, tion geometry of the Dy(III) center, and (c) out-of-phase (χ”M) vs. T AC susceptibility plots, (d) plot (d) plot of relaxation time (τ) as a function of temperature (T) for 36 at 1200 Oe. Figures and captions of relaxation time (τ) as a function of temperature (T) for 36 at 1200 Oe. Figures and captions are are reproduced from Reference [57] with permission from the Wiley-VCH. reproduced from Reference [57] with permission from the Wiley-VCH. Glycol Ligand

Triethylene glycol (H2TEG) ligands are rich in O donor atoms and therefore exhibit high affinity towards the Ln(III) ions. Li and co-workers synthesized a ten-coordinate mononuclear Dy(III) complex, [Dy(H2TEG)(NO3)3]∙[18-crown-6] (37) utilizing the H2TEG ligand in combination with a 18- crown-6 ligand (Figure 22) [58]. Here the crown ether ligand does not coordinate to the metal center but merely acts as a promoter in crystallizing the complex. The ten-coordinate tetradecahedron geometry of the Dy(III) ion resulted from the cumulative coordination of one H2TEG ligand and three NO3− anions. The field dependence of magnetization measurements using a sweeping magnetic field (sweep rate of 50 Oe/s) at 2 K did not reveal any hysteresis loops confirming fast magnetization relaxation in 37. To probe the relaxation dynamics, AC magnetic measurements were performed. At zero field no maximum was observed in the out-of-phase (χ″M) AC susceptibility component above 2 K. Upon application of an optimum DC field of 1000 Oe, strong χ″M signals with clear maxima were observed (Figure 22c). The frequency-dependent χ″M AC component showed two maxima indicating the possibility of two relaxation processes in 37. This was further confirmed in the Cole-Cole plots which showed two semi-circular shapes with α1 = 0.21–0.42 and α2 = 0.11–0.38. The temperature dependence of relaxation times was fitted with the Arrhenius equation and an energy barrier, Ueff = 28 K with τ0 = 2.49 × 10−9 s was obtained.

Magnetochemistry 2021, 7, 1 19 of 29

Glycol Ligand

Triethylene glycol (H2TEG) ligands are rich in O donor atoms and therefore exhibit high affinity towards the Ln(III) ions. Li and co-workers synthesized a ten-coordinate mononuclear Dy(III) complex, [Dy(H2TEG)(NO3)3]·[18-crown-6] (37) utilizing the H2TEG ligand in combination with a 18-crown-6 ligand (Figure 22)[58]. Here the crown ether ligand does not coordinate to the metal center but merely acts as a promoter in crystallizing the complex. The ten-coordinate tetradecahedron geometry of the Dy(III) ion resulted − from the cumulative coordination of one H2TEG ligand and three NO3 anions. The field dependence of magnetization measurements using a sweeping magnetic field (sweep rate of 50 Oe/s) at 2 K did not reveal any hysteresis loops confirming fast magnetization relaxation in 37. To probe the relaxation dynamics, AC magnetic measurements were performed. At zero field no maximum was observed in the out-of-phase (χ”M) AC susceptibility component above 2 K. Upon application of an optimum DC field of 1000 Oe, strong χ”M signals with clear maxima were observed (Figure 22c). The frequency-dependent χ”M AC component showed two maxima indicating the possibility of two relaxation processes in 37. This was further confirmed in the Cole-Cole plots which showed two semi-circular shapes with α1 = 0.21–0.42 and α2 = 0.11–0.38. The temperature dependence of relaxation times was fitted with the Arrhenius equation and an energy barrier, Ueff = 28 K with Magnetochemistry 2020, 6, x FOR PEER REVIEW−9 19 of 29 τ0 = 2.49 × 10 s was obtained.

FigureFigure 22. (a )22. Molecular (a) Molecular structure structure of 37 with of the 37 magnetic with the easy magnetic axes, (b) easy Frequency axes, dependence(b) Frequency of χ ”dependenceM signals under of 1000 Oe DCχ″ field,M signals and ( cunder) Cole-Cole 1000 plotsOe DC with field, the best and fits (c corresponding) Cole-Cole plots to the with sum the of two best modified fits corresponding Debye functions. to the Figures and captionssum of two are reproduced modified Debye from Reference functions. [58 ]Figures with permission and captions from are the Elsevierreproduced Ltd. from Reference [58] with permission from the Elsevier Ltd. Radical Ligand

Radical Ligand A ten-coordinate Pr(III) complex of the formula [Pr(NIT2Py)2(NO3)3](38; NIT2Py = 2-(20-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) was synthesized utilizing a A ten-coordinate Pr(III) complex of the formula [Pr(NIT2Py)2(NO3)3] (38; NIT2Py = 2-(2′-pyridyl)- chelating nitronyl nitroxide radical ligand (Figure 23)[59]. The Pr(III) was coordinated by 4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) was synthesized utilizing a chelating nitronyl two N,O-chelating NIT2Py radicals and six O atoms from the three nitrato groups in a κ2O nitroxide radical ligandchelating (Figure coordination23) [59]. The mode. Pr(III) The was ONCNO coordi fragmentnated by of two the nitronylN,O-chelating nitroxide NIT moiety2Py was radicals and six O atomsfound from to bethe almost three planar nitrato indicating groups thein a delocalization κ2O chelating of the coordination free electron mode. in this fragment.The The dihedral angles between the pyridyl and the nitronyl nitroxide moieties were found ONCNO fragment of the nitronyl◦ nitroxide◦ moiety was found to be almost planar indicating the to be 38.2 and 35.2 for the two NIT2Py ligands. Static magnetic measurements revealed delocalization of the free electron in this fragme3 nt.− 1The dihedral angles between the pyridyl and the a χMT value of 1.99 cm mol K at 300 K which is lower than the expected theoretical nitronyl nitroxide moieties were found3 −to1 be 38.2° and 35.2° for the two NIT3 2Py ligands. Static χMT = 2.36 cm mol K for two radical and one Pr(III) ion ( H4 ground state (gJ = 4/5)). 3 −1 magnetic measurementsUpon revealed cooling a theχM systemT value to of 77 1.99 K, the cmχM Tmolvalue K slowly at 300 decreased K which which is lower has been than attributed the expected theoretical χMasT = resulting 2.36 cm from3 mol the−1 K magnetic for two anisotropy radical and of the one Pr(III) Pr(III) ion. ion Dynamic (3H4 ground magnetization state (g studiesJ = 4/5)). Upon cooling the weresystem not to carried 77 K, outthe on χM thisT value system. slowly decreased which has been attributed as resulting from the magnetic anisotropy of the Pr(III) ion. Dynamic magnetization studies were not carried out on this system.

Figure 23. Molecular structure of complex 38. The structure was re-drawn from Reference [59] with permission from the Elsevier Ltd.

2.1.3. Penta-Nitrate Ln(III) Complexes In addition to the synthetic approaches discussed above, Song and co-workers pursued a spherical ligand field around the Ln(III) ions and in this pursuit they synthesized two isostructural ten-coordinate mononuclear complexes of the formula, (nBu4N)2[Ln(NO3)5] (Ln = Dy (39) and Er (40)) (Figure 24) [60]. As discussed previously, a spherical ligand field could stabilize the magnetic anisotropy of both Dy(III) (oblate) and Er(III) (prolate) ions simultaneously. The complexes were synthesized by the reactions of hydrated Ln(NO3)3 with appropriate equivalents of AgNO3 and

Magnetochemistry 2020, 6, x FOR PEER REVIEW 19 of 29

Figure 22. (a) Molecular structure of 37 with the magnetic easy axes, (b) Frequency dependence of

χ″M signals under 1000 Oe DC field, and (c) Cole-Cole plots with the best fits corresponding to the sum of two modified Debye functions. Figures and captions are reproduced from Reference [58] with permission from the Elsevier Ltd.

Radical Ligand

A ten-coordinate Pr(III) complex of the formula [Pr(NIT2Py)2(NO3)3] (38; NIT2Py = 2-(2′-pyridyl)- 4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) was synthesized utilizing a chelating nitronyl nitroxide radical ligand (Figure 23) [59]. The Pr(III) was coordinated by two N,O-chelating NIT2Py radicals and six O atoms from the three nitrato groups in a κ2O chelating coordination mode. The ONCNO fragment of the nitronyl nitroxide moiety was found to be almost planar indicating the delocalization of the free electron in this fragment. The dihedral angles between the pyridyl and the nitronyl nitroxide moieties were found to be 38.2° and 35.2° for the two NIT2Py ligands. Static magnetic measurements revealed a χMT value of 1.99 cm3 mol−1 K at 300 K which is lower than the expected theoretical χMT = 2.36 cm3 mol−1 K for two radical and one Pr(III) ion (3H4 ground state (gJ = Magnetochemistry4/5)).2021 Upon, 7, 1 cooling the system to 77 K, the χMT value slowly decreased which has been attributed as 20 of 29 resulting from the magnetic anisotropy of the Pr(III) ion. Dynamic magnetization studies were not carried out on this system.

Figure 23. MolecularFigure structure 23. Molecular of complex structure 38. The of complex structure38 was. The re-drawn structure from was Reference re-drawn [59] from with Reference [59] permission fromwith the permission Elsevier Ltd. from the Elsevier Ltd.

2.1.3. Penta-Nitrate2.1.3. Ln(III) Penta-Nitrate Complexes Ln(III) Complexes In addition to the synthetic approaches discussed above, Song and co-workers pursued In addition toa sphericalthe synthetic ligand approaches field around discussed the Ln(III) above, ions andSong in and this co-workers pursuit they pursued synthesized a two spherical ligand field around the Ln(III) ions and in this pursuit they synthesized two nisostructural isostructural ten-coordinate mononuclear complexes of the formula, ( Bu4N)2[Ln(NO3)5] ten-coordinate mononuclear(Ln = Dy (39 co)mplexes and Er ( 40of)) the (Figure formula, 24)[ (60nBu].4 AsN)2 discussed[Ln(NO3)5 previously,] (Ln = Dy ( a39 spherical) and Er ( ligand40)) field (Figure 24) [60]. Ascould discussed stabilize previously, the magnetic a anisotropyspherical ligand of both field Dy(III) could (oblate) stabilize and Er(III)the magnetic (prolate) ions anisotropy of bothsimultaneously. Dy(III) (oblate) The and complexes Er(III) (prolate) were synthesized ions simultaneously. by the reactions The complexes of hydrated were Ln(NO 3)3 n synthesized by thewith reactions appropriate of hydrated equivalents Ln(NO of3 AgNO)3 with3 appropriateand Bu4NBr equivalents in ethanolic of medium. AgNO3 and The small − “bite angle” of the NO3 anion resulted in the high coordinate geometry with a 10O coordination [61,62]. The tetradecahedron (C2v symmetry) geometry around the Ln(III) ions resulted from the simultaneous coordination of five nitrate ligands of which three nitrates formed an equatorial plane while the other two nitrates coordinated perpendicularly from above and below this equatorial plane. The Dy–O distances in the (DyO10) coordination − geometry differs only slightly (0.03 Å) suggesting symmetrical binding nature of the NO3 n + ligands. The presence of bulky [ Bu4N] cations ensured large distances between the Ln(III) ions. Both DC and AC magnetic measurements were performed on 39 and 40. The crystal field parameters were obtained from the static magnetic measurements which revealed impure eigenstates in both ground and the excited states of 39. Accordingly, the latter does not exhibit the typical Ising behavior. On the other hand, the ground doublet of 40 was found to be the mJ = ±11/2 state while the first excited state was found to be an admixture of mJ = ±9/2 and mJ = ±13/2 doublet states representing a typical Ising type anisotropy in 40. The relatively larger electron density in the equatorial plane provided by − the three NO3 ligands is particularly suited for the prolate Er(III) ion and is responsible for the Ising behavior in 40. To probe the relaxation dynamics, AC magnetic measurements were performed under zero biased field. None of the complexes showed peaks in the out-of-phase AC susceptibility (χ”M)—even at low temperature—presumably because of significant QTM in the ground state. Upon application of an optimum DC field, both the complexes showed clear maxima in the temperature- and frequency-dependent in- 0 phase (χ M) and out-of-phase (χ”M) susceptibilities (Figure 24). The Cole-Cole plots with small values of α parameters (0.15–0.22 for 39 and 0.08–0.22 for 40) implied a narrow distribution of relaxation times in both the complexes. The temperature dependence of relaxation times were fitted by the Arrhenius equation and the effective energy barriers, −9 −7 Ueff = 24.3 K (C = 3.40 × 10 s; HDC = 500 Oe) for 39 and Ueff = 22.3 K (τ0 = 3.06 × 10 s; HDC = 1000 Oe) for 40 were obtained. An analysis of the experimental data supported by theoretical calculations suggested that magnetization relaxation occurred through an intermediate state between the ground and first excited states. Magnetochemistry 2020, 6, x FOR PEER REVIEW 20 of 29

nBu4NBr in ethanolic medium. The small “bite angle” of the NO3− anion resulted in the high coordinate geometry with a 10O coordination.[61,62] The tetradecahedron (C2v symmetry) geometry around the Ln(III) ions resulted from the simultaneous coordination of five nitrate ligands of which three nitrates formed an equatorial plane while the other two nitrates coordinated perpendicularly from above and below this equatorial plane. The Dy–O distances in the (DyO10) coordination geometry differs only slightly (0.03 Å) suggesting symmetrical binding nature of the NO3− ligands. The presence of bulky [nBu4N]+ cations ensured large distances between the Ln(III) ions. Both DC and AC magnetic measurements were performed on 39 and 40. The crystal field parameters were obtained from the static magnetic measurements which revealed impure eigenstates in both ground and the excited states of 39. Accordingly, the latter does not exhibit the typical Ising behavior. On the other hand, the ground doublet of 40 was found to be the mJ = ±11/2 state while the first excited state was found to be an admixture of mJ = ±9/2 and mJ = ±13/2 doublet states representing a typical Ising type anisotropy in 40. The relatively larger electron density in the equatorial plane provided by the three NO3− ligands is particularly suited for the prolate Er(III) ion and is responsible for the Ising behavior in 40. To probe the relaxation dynamics, AC magnetic measurements were performed under zero biased field. None of the complexes showed peaks in the out-of-phase AC susceptibility (χ″M)— even at low temperature—presumably because of significant QTM in the ground state. Upon application of an optimum DC field, both the complexes showed clear maxima in the temperature- and frequency-dependent in-phase (χ′M) and out-of-phase (χ″M) susceptibilities (Figure 24). The Cole- Cole plots with small values of α parameters (0.15–0.22 for 39 and 0.08–0.22 for 40) implied a narrow distribution of relaxation times in both the complexes. The temperature dependence of relaxation times were fitted by the Arrhenius equation and the effective energy barriers, Ueff = 24.3 K (C = 3.40 × 10−9 s; HDC = 500 Oe) for 39 and Ueff = 22.3 K (τ0 = 3.06 × 10−7 s; HDC = 1000 Oe) for 40 were obtained. An Magnetochemistryanalysis2021 of, 7 ,the 1 experimental data supported by theoretical calculations suggested that21 of 29magnetization relaxation occurred through an intermediate state between the ground and first excited states.

Figure 24. MolecularFigure structure 24. Molecular of structure the anionic of the anionicpart of part (a) ofand (a) andthe the corresponding corresponding coordination coordination geom- geometry etry (b) of 39; Frequency dependence of the AC susceptibility under a DC ﬁeld of 500 Oe for 39 (b) of 39; Frequency(c) and dependence 1000 Oe for 40 (d of). Figures the AC and susceptibility captions are reproduced under from a DC Reference field [60of] with500 permissionOe for 39 (c) and 1000 Oe for 40 (dfrom). Figures the Royal and Society captions of Chemistry. are reproduced from Reference [60] with permission from the Royal Society of3. Chemistry. Twelve-Coordinate Ln(III) Complexes Although twelve-coordination is known for lanthanide ions in a few heterometallic 3d-4f complexes, such a behavior among mono nuclear Ln(III) complexes is quite sparse.

Mixed Ligand Approach Kajiwara and co-workers reported a series of mononuclear Ln(III) systems having dodeca coordination by following the mixed ligand approach [63]. The complexes, [Ln(NO3)3(18-crown-6)] (Ln = Ce (41), Pr (42), and Nd (43)) and [Ln(NO3)3(1,10-diaza-18- crown-6)] (Ln = Ce (44), Pr (45), and Nd (46)) were synthesized by employing 18-crown- 6 and 1,10-diaza-18-crown-6 systems respectively along with chelating nitrate ligands (Figure 25). For the ﬁrst set of complexes having 18-crown-6 as the main ligand, the six O donor atoms of the ligand coordinates to the metal ion equatorially while the three nitrate ions chelating the metal ion from above and below complete the coordination environment as well as provide charge neutrality to the system. On the other hand, for the other set of molecules consisting 1,10-diaza-18-crown-6 as the ligand, the coordination action of the ligand remains similar but the overall chemical environment around the Ln(III) changes to 10O,2N. The geometry around the Ln(III) ion is found to be distorted icosahedron but the Magnetochemistry 2020, 6, x FOR PEER REVIEW 21 of 29

3. Twelve-Coordinate Ln(III) Complexes Although twelve-coordination is known for lanthanide ions in a few heterometallic 3d-4f complexes, such a behavior among mono nuclear Ln(III) complexes is quite sparse.

Mixed Ligand Approach Kajiwara and co-workers reported a series of mononuclear Ln(III) systems having dodeca coordination by following the mixed ligand approach [63]. The complexes, [Ln(NO3)3(18-crown-6)] (Ln = Ce (41), Pr (42), and Nd (43)) and [Ln(NO3)3(1,10-diaza-18-crown-6)] (Ln = Ce (44), Pr (45), and Nd (46)) were synthesized by employing 18-crown-6 and 1,10-diaza-18-crown-6 systems respectively along with chelating nitrate ligands (Figure 25). For the first set of complexes having 18-crown-6 as the main ligand, the six O donor atoms of the ligand coordinates to the metal ion equatorially while the three nitrate ions chelating the metal ion from above and below complete the coordination Magnetochemistry 2021, 7, 1 22 of 29 environment as well as provide charge neutrality to the system. On the other hand, for the other set of molecules consisting 1,10-diaza-18-crown-6 as the ligand, the coordination action of the ligand remains similar but the overall chemical environment around the Ln(III) changes to 10O,2N. The geometrymagnitude around of the distortion Ln(III) ion from is found the ideal to be geometry distorted is icosahedron found to be but more the for magnitude the case ofof 12Odistortion from coordinationthe ideal geometry (vide infra is found). to be more for the case of 12O coordination (vide infra).

FigureFigure 25. Molecular 25. Molecular structure structure of 41 ( ofleft41);( Icosahedronleft); Icosahedron coordination coordination geometry geometry around around the Ce(III) the ion (middleCe(III)); and ion molecular (middle structure); and molecular of 44 (right structure). The structures of 44 (right are). There-drawn structures from areReference re-drawn [63] with permissionfrom Referencefrom the American [63] with permissionChemical Society from the© 2016. American Chemical Society © 2016.

DC susceptibilityDC susceptibility measurements measurements revealed revealed that that the the room room temperature temperature χχMMT valuesvalues forfor the − complexesthe complexes 41 to 46 were41 to 46foundwere to found be 0.76, to be1.52, 0.76, 1.41, 1.52, 0.68, 1.41, 1.48, 0.68, and 1.48, 1.60 and emu 1.60 K emu mol K−1 molrespectively1 whichrespectively are in good which agreement are in goodwith the agreement theoretical with values. the theoretical However, values. unlike However, the other unlike complexes, the at lowerother temperatures complexes, the at χM lowerT values temperatures for the two the Pr(III)χMT valuesanalogues for sharply the two decreased Pr(III) analogues and reached almostsharply to the decreaseddiamagnetic and limit. reached This almost has been to the attributed diamagnetic to the limit. rapid This depopulation has been attributed of the low to lying the rapid depopulation of the low lying Jz sublevels. Jz sublevels. In the AC susceptibility measurements, none of the complexes were found to be In the AC susceptibility measurements, none of the complexes were found to be showing out- showing out-of-phase susceptibility peak maxima. But in the presence of 1000 Oe of of-phase susceptibility peak maxima. But in the presence of 1000 Oe of applied magnetic field applied magnetic ﬁeld complexes 41, 43, 44, 46 showed both distinct in-phase and out-of- complexes 41, 43, 44, 46 showed both distinct in-phase and out-of-phase peak maxima (Figure 26). phase peak maxima (Figure 26). An analysisAn analysis of the data of the suggested data suggested the presence the presence of multiple of relaxation multiple relaxation pathways pathways(Figure 27). The best fit(Figure parameters 27). The were best extracted ﬁt parameters considering were extracted Raman and considering Orbach process Raman which and Orbach are summarized process in Tablewhich 1. are summarized in Table1.

Magnetochemistry 2020, 6, x FOR PEER REVIEW 22 of 29

Table 1. Best fitted parameters of Arrhenius equation.

Relaxation Parameters 41 43 44 46 n is fixed at 5 ΔE kB−1/K 30.3(3) 30.9(4) 45(2) 55(4) τ0/ns 220(17) 2.2(3) 26(7) 2.6(1.9) C/s−1 K−5 0.108(10) 4.1(3) 0.52(4) 0.050(12) n is fixed at 9 ΔE kB−1/K 25.6(7) 33.4(5) 23(1) 73(2) Magnetochemistry 2021, 7, 1 τ0/ns 9(2) × 102 1.6(2) 6(2) × 103 0.14(6) 23 of 29 C/s−1 K−5 16(4) 0.350(11) 22(9) 10.7(4)

Figure 26. 26.In-phase In-phase and and out-of-phase out-of-phase susceptibility susceptibility peak maxima peak for maxima41 (A); 43 (forB); 4441 ((CA);); and 4346 (B();D )44 at ( anC); applied and 46 ﬁeld (D of) 1000at an Oe. applied Figures field are reproduced of 1000 Oe. from Figures Reference are [63 reproduced] with permission from from Reference the American [63] Chemicalwith permission Society © 2016. from the American Chemical Society © 2016.

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Magnetochemistry 2020, 6, x FOR PEER REVIEW 23 of 29

Figure 27. (FigureA–D) Arrhenius 27. (A–D) plots Arrhenius for complexes plots for 41 complexes, 43, 44, and41 46, 43 respectively, 44, and 46 measuredrespectively under measured a 1000 under a Oe bias field.1000 Solid Oe biaslines ﬁeld. and Soliddotted lines curves and repres dottedent curves theoretical represent fitting theoretical taking into ﬁtting account taking the into TA- account the QTM processTA-QTM (black), process the TA-QTM (black), and the TA-QTMthe Raman and processes the Raman with processes n = 5 (blue), with and n = the 5 (blue), TA-QTM and and the TA-QTM the Ramanand processes the Raman with processesn = 9 (red). with Figures n = 9 and (red). caption Figures are and reproduced caption are from reproduced Reference from [63] Referencewith [63] permissionwith from permission the American from Chemical the American Society Chemical © 2016. Society © 2016.

4. Magneto-StructuralTable 1. Best Correlation ﬁtted parameters of Arrhenius equation.

As is evident fromRelaxation the above Parameters survey that ten- 41coordination 43is the most abundant 44 among 46 the higher coordinated Dy(III) complexes with nthree is fixed possible at 5 geometries: sphenocorona, −1 tetradecahedron and bicapped∆E kB square/K antiprism 30.3(3)(see Table 2). 30.9(4)A variety of synthetic 45(2) approaches 55(4) involving different types ofτ ligands0/ns have been empl 220(17)oyed to synthesize 2.2(3) high coordinated 26(7) complexes. 2.6(1.9) − − Although none of the highC /scoordinated1 K 5 complexes0.108(10) exhibited zero field 4.1(3) SMM behavior, 0.52(4) many revealed 0.050(12) a field-induced SMM behavior. The highest effective energyn is fixed barrier at 9 was found when the coordination −1 environment around the∆ EDy(III) kB /K was either 10N25.6(7) or 10O rather 33.4(5) than the mixed 23(1) 6N4O or 73(2)4N6O 2 3 environments. The 10N coordinationτ0/ns was achieved9(2) ×only10 from the1.6(2) multidentate6(2) ×Schiff10 base ligands0.14(6) C/s−1 K−5 16(4) 0.350(11) 22(9) 10.7(4) whereas the 10O coordination could be achieved by many ligand systems. However, the mixed ligand approach involving the crown ether ligand appeared to be most effective in generating 4. Magneto-Structural Correlation complexes that possessed high effective energy barriers. Further, it has been observed that the effective energy barrierAs is is evident not only from dependent the above on survey the coordination that ten-coordination environment is the but most also abundant seems to among be the higher coordinated Dy(III) complexes with three possible geometries: sphenocorona, dependent on the extent of distortion from regular geometry In Figure 28 a correlation of observed tetradecahedron and bicapped square antiprism (see Table2). A variety of synthetic effective energy barrier (Ueff) with geometry and the magnitude of distortion from ideal geometry approaches involving different types of ligands have been employed to synthesize high has been plotted. It is evident from this plot that in [DyO10] systems high Ueff is possible when the coordinated complexes. Although none of the high coordinated complexes exhibited geometry is bicappedzero field square SMM antiprism behavior, with many high revealed distortion. a field-inducedBut when the geometry SMM behavior. is sphenocorona The highest the distortion shouldeffective be energy low for barrier getting was high found Ueff. when the coordination environment around the Dy(III) was either 10N or 10O rather than the mixed 6N4O or 4N6O environments. The 10N coordination was achieved only from the multidentate Schiff base ligands whereas the 10O coordination could be achieved by many ligand systems. However, the mixed ligand approach involving the crown ether ligand appeared to be most effective in generating

Magnetochemistry 2021, 7, 1 25 of 29

complexes that possessed high effective energy barriers. Further, it has been observed that the effective energy barrier is not only dependent on the coordination environment but also seems to be dependent on the extent of distortion from regular geometry In Figure 28a correlation of observed effective energy barrier (Ueff) with geometry and the magnitude of distortion from ideal geometry has been plotted. It is evident from this plot that in [DyO10] systems high Ueff is possible when the geometry is bicapped square antiprism with high distortion. But when the geometry is sphenocorona the distortion should be low for getting high Ueff.

Table 2. A summary of the magnetic properties of all ten-coordinate Dy(III) and two dodeca-coordinated Ce(III)complexes.

Geometry Sl. Co-Ordination ChSM U , τ Formula (Complex Number) Around eff 0 Ref No. Environment Value (H ) Metal * dc 32.4 K 1 [Dy(H dapbh) ](NO ) (1) 4O6N BSA 2.840 [38] 2 2 3 3 (1000 Oe) 79 K, −8 2 [Dy(N5)2](CF3SO3)3 (8) 10N BSA 3.335 2.9 × 10 s, [36] 1200 Oe SPC 2.837 68 K, 2.07 × 10−10 s (1000 −11 3 [Dy(12-crown-4)(NO3)3](10) 10O Oe) 75.92 K, 6.8 × 10 s [37] BSA 20.899 (500 Oe) [Dy(18-crown-6)(NO ) ]ClO 4 3 2 4 10O BSA 3.101 63 K, 1.02 × 10−8 s (1000 Oe) [38] (14) 44.3 K, −7 5 [Dy(L2)2(NO3)](ClO4)2 (25) 6O4N SPC 3.223 5.17 × 10 s [42] (1000 Oe) 23.9 K, 2.47 × 10−6 s 6 [Dy(HL3) (NO ) ](27) 10O BSA 3.148 [48] 2 3 3 (2000 Oe) 50 K, −7 7 [Dy(L4)(NO3)2](NO3)(29) 6O4N SPC 21.104 6.80 × 10 s, [49] (1000 Oe) 53.4 K, 1.6 × 10−12 s 8 [Dy(ntbi)(NO ) ](36) 4O6N SPC 3.655 [51] 3 3 (1200 Oe)

[Dy(H2TEG)(NO3)3]·[18- 28 K, 9 crown-6] 10O TDH 3.292 2.49 × 10−9 s [52] (37) (1000 Oe) 24.3 K, n −9 10 ( Bu4N)2[Dy(NO3)5](39) 10O TDH 1.513 3.40 × 10 s, [54] (500 Oe) 30.3 K, −7 11 [CeNO3)3(18-crown-6)] (41) 12O IC 1.862 2.20 × 10 s [57] (1000 Oe)

[Ce(NO3)3(1,10-diaza-18- 44 K, 12 crown-6)] 10O2N IC 1.765 2.3 × 10−8 s [57] (44) (1000 Oe) * BSA = bicapped square antiprism; TDH = tetradecahedron; SPC = sphenocorona; IC = icosahedron.

On the other hand, when the decacoordination is fulﬁlled by mixed donor atoms (O and N) only two types of geometries are observed, sphenocorona and bicapped square antiprism (Figure 28). However, for (DyO6N4) systems only sphenocorona geometry is reported and the higher Ueff is seen when distortion is higher while for (DyO4N6) systems higher Ueff is found in sphenocorona geometry. Magnetochemistry 2021, 7, 1 26 of 29

Magnetochemistry 2020, 6, x FOR PEER REVIEW 25 of 29

Figure 28. (Figureleft) Geometry 28. (left) andGeometry magnitude and dependentmagnitude experimentallydependent experi foundmentally effective found energy effective barrier energy for (DyO10) barrier systems; (right) for (DyO6N4)for (DyO10) systems systems (magenta ; (right color) for spheres),(DyO6N4) and systems for (DyO4N6) (magenta systems color (greenspheres), color and spheres). for (DyO4N6) systems (green color spheres).

5. Summary 5. Summary In this article we have tried to summarize the status on high-coordinate lanthanide In this article wecomplexes have tried that to showsummarize a field-dependent the status on single-molecule high-coordinate magnet lanthanide behavior. complexes We considered that show a field-dependentcoordination single-molecule numbers above magnet nine behavior. for this purpose. We considered Although, coordination coordination numbers numbers 10–14 above nine for this purpose.are possible Although, for lanthanide coordinati ions,on ournumbers survey 10–14 indicated are possible that given for lanthanide the constraint ions, that such our survey indicatedcomplexes that given should the constraint show slow that relaxation such complexes of magnetization, should show only slow those relaxation possessing of coordina- magnetization, onlytion those numbers possessing 10 and coordination 12 are of interest. numbers Such 10 and complexes 12 are of have interest. been Such assembled complexes by utilizing a have been assembledvariety by utilizing of synthetic a variety methodologies of synthetic involving methodologies an array involving of ligand an systems array of including ligand mixed systems including mixedligands. ligands. Our review Our review of such of high-coordinatesuch high-coordinate complexes complexes reveals reveals that theythat they display very display very low sitelow symmetry site symmetry which enables which enables quantum quantum tunneling tunneling of magneti of magnetization.zation. Because Becauseof this of this none of the complexesnone discussed of the complexes in this review discussed have inbeen this found review to have show been zero-field found toSMM show behavior. zero-field SMM On the other hand,behavior. application On theof appropriate other hand, applicationsmall DC field of appropriate seems to smallalleviate DC some field seemsof these to alleviate some of these problems with the realization of slow relaxation of magnetization under problems with the realization of slow relaxation of magnetization under such conditions. A such conditions. A qualitative structure-property correlation reveals the dependence of qualitative structure-property correlation reveals the dependence of local coordination geometry and local coordination geometry and the magnitude of distortion from regular geometry on the the magnitude of distortionmagnetic from properties regular including geometry the on effectivethe magnetic energy properties barrier for including the reversal the effective of magnetization. energy barrier for the reversal of magnetization. 6. Future Direction 6. Future Direction There are several examples of heterometallic 3d-4f complexes featuring ten- and There are severaltwelve-coordinate examples of heterometallic lanthanide sites 3d-4f where complexes good exchange featuring interaction ten- and (between twelve- the 3d and coordinate lanthanide4f site metals where ions) good could exchange play a crucial interaction role (between to suppress the QTM.3d and Thus, 4f metal while ions) it could appears from play a crucial role theto suppress above survey QTM. that Thus, high-coordinate while it appears lanthanide from the complexes above survey by themselves that high- might not be attractive candidates for assembling molecular magnets, incorporating such sites in coordinate lanthanide complexes by themselves might not be attractive candidates for assembling heterometallic complexes might be a fruitful endeavor. molecular magnets, incorporating such sites in heterometallic complexes might be a fruitful endeavor. Author Contributions: Conceptualization, P.K. and J.A.; methodology, J.A. and P.K.; software, J.A. and P.K.; validation, P.K., J.A. and V.C.; formal analysis, V.C.; investigation, J.A.; resources, P.K.; Author Contributions:data Conceptualization, curation, V.C.; writing—original P.K. and J.A.; methodology, draft preparation, J.A. and J.A. andP.K.; P.K.; software, writing—review J.A. and P.K.; and editing, validation, P.K., J.A. andV.C.; V.C.; visualization, formal analysis, P.K.; supervision, V.C.; investigation, V.C.; project J.A.; administration, resources, P.K.; V.C.; data fundingcuration, acquisition, V.C.; V.C. writing—original draftAll preparation, authors have J.A. read and and P.K.; agreed wr toiting—review the published and version editing, of the V.C.; manuscript. visualization, P.K.; supervision, V.C.; project administration, V.C.; funding acquisition, V.C. All authors have read and agreed to Funding: This research received no external funding. the published version of the manuscript. Acknowledgments: J.A. and V.C. are thankful to Department of Science and Technology, New Delhi, Funding: This researchIndia. received All authors no external are grateful funding. to Tata Institute of Fundamental Research-Hyderabad for funding and Infrastructure. Acknowledgments: J.A. and V.C. are thankful to Department of Science and Technology, New Delhi, India. All authors are grateful toConflicts Tata Instit ofute Interest: of FundamentalThe author Research-Hyderab declares no conflictad offor interest. funding and Infrastructure.

Conflicts of Interest: The author declares no conflict of interest.

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